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  • 1. Recommended Practicefor Planning, Designing and Constructing Fixed Offshore Platforms-Working Stress Design API RECOMMENDED PRACTICE 2A-WSD (RP 2A-WSD) TWENTY-FIRST EDITION, DECEMBER 2000 American Petroleum Institute Helping You Get The Job Done Right?COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 2. COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 3. Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms-Working Stress Design Upstream Segment API RECOMMENDED PRACTICE 2A-WSD (RP 2A-WSD) TWENTY-FIRST EDITION, DECEMBER2000 American Petroleum Institute Helping You Get The Job Done Right?COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 4. SPECIAL NOTES API publications necessarily address problems a general nature. With respect to partic- of ular circumstances, local, state, and federal laws and regulations should be reviewed. API is not undertaking to meet the duties of employers, manufacturers, or suppliers to warn and properly train and equip their employees, and others exposed, concerning health and safety risks and precautions, nor undertaking their obligations under local, state, fed- or eral laws. Information concerning safety and health risks and proper precautions with respect to par- ticular materials and conditions should obtained from the employer, the manufacturer or be supplier of that material, or the material safety data sheet. Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the manufacture, sale,use of any method, apparatus, prod- or or uct covered by letters patent. Neither should anything contained in the publication be con- strued as insuring anyone against liability for infringementletters patent. of Generally, API standards are reviewed and revised, reaffirmed, or withdrawn every at least five years. Sometimes a one-time extension up to two years will be added to this review of cycle. This publication will no longer be in effect five years after publication date as an its operative API standard or, where an extension has been granted, upon republication. Status of the publication can be ascertained from the API Standards Manager [telephone (202) 682-8000]. A catalog of API publications and materials is published annually and updated quarterly by API, 1220L Street, N.W., Washington, D.C. 20005. This document was produced under API standardization procedures that ensure appropri- ate notification and participation in the developmental process and is designated an API as standard. Questions concerning the interpretation of the content of this standard or com- ments and questions concerning the procedures under which this standard was developed should be directed in writing to the Standards Manager, American Petroleum Institute, 1220 L Street, N.W., Washington, D.C. 20005. Requests for permission to reproduce or translate all or any part of the material published herein should alsoaddressed to the general man- be ager. API standards are published to facilitate the broad availability of proven, sound engineer- ing and operating practices. These standards are not intended to obviate the need for apply- ingsoundengineering judgment regardingwhenandwherethesestandardsshould be utilized. The formulation and publication of API standards is not intended in any way to inhibit anyone from using any other practices. Any manufacturer marking equipment or materials in conformance with the marking requirements of an API standard is solely responsible for complying with all the applicable requirements of that standard. API does not represent, warrant, or guarantee that such prod- ucts do in fact conform to the applicable API standard. All rights reserved. No part o this work may be reproduced, stored inretrieval system, or f a transmitted by any means, electronic, mechanical, photocopying, recording, otherwise, or without prior written permission from the publisher. Contact the Publisher; API Publishing Services, 1220 L Street, N. W , Washington, D.C. 20005. Copyright O 2000 American Petroleum InstituteCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 5. FOREWORD This Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms contains engineeringdesignprinciplesandgoodpracticesthathaveevolvedduringthedevelopment of offshoreoil resources. Good practice is based on good engineering; therefore, this recommended practice consists essentially of good engineering recommendations. In no case is any specific recommendation included which could notbe accomplished by presently available techniques and equipment. Consideration is given in all cases to the safety of personnel, compliance with existing regulations, and antipollutionwater bodies. of Metric conversionsof customary English units provided throughout the text this publication in parenthe- are of ses, e.g., 6in. (152 mm). Mostof the converted values have been rounded for most practical usefulness; however, precise conversions have been used where safety and technical considerations dictate. case of dispute, the cus- In tomary English values should govern. Offshore technology is growing rapidly. In those areas where the committee felt that adequate were avail- data able, specific and detailed recommendations are given. In other areas general statementsare used to indicate that consideration shouldbe given to those particular points. Designers encouraged to utilize research advances are all available to them. As offshore knowledge continues to grow, this recommended practice will be is hopedIt revised. that the general statements contained herein will gradually be replaced by detailed recommendations. Reference in this practice is made to the latest edition AISCSpecijicationfor the Design, Fabrication and of the Erection of Structural Steel Buildings (see Section 2.5. While the use latest editionof this specification is for la). of still endorsed, the use the new AISC of Load & Resistance Factor Design (LRFD), First Edition is specifically not recommended for design offshore platforms.The load and resistance factors in this new code are based on cali- of bration with building design practices and are therefore not applicable to offshore platforms. Research work is now in progress to incorporate the strength provisions the new AISCLRFD code into offshore design practices. of In this practice, reference is made to ANSIIAWS Dl.1-92 Structural Welding Code-Steel. While use of this edition is endorsed, the primary intent that the AWS code be followed for the welding and fabrication Fixed is of Offshore Platforms. Chapters 8, 9, and 10 of the AWS Code give guidance that maybe relevant to the design of Fixed Offshore Platforms.This Recommended Practice makes specific referenceto Chapter 9 and 10 for certain as in Sections4 and 5 , this guidance design considerations. Where specific guidance is given in this API document, should take precedence. This standard shall become effective on the printed on the cover but may be used voluntarily from the date date of distribution. Attention Users: Portions of this publication have been changed from the previous edition. The locations of changes have been marked with a bar in the margin, as shown to the left of this paragraph. In some cases the changes are significant, while in other cases the changes reflect minor editorial adjustments. bar notations in The the margins are provided as an aid to usersas to those parts of this publication that have been changed from the previous edition, but API makes no warranty the accuracy such bar notations. as to of Note: This edition supersedes the 20th Edition dated July 1, 1993. This Recommended Practice is under jurisdiction of the API Subcommittee on Offshore Structures and was authorized for publication the 1969 standardization conference. first edition was issued October 1969. at The API publications may used by anyone desiring to so. Every effort has been made by the Institute to assure be do the accuracy and reliability the data contained in them; however, the Institute makes no representation, warranty, of or guarantee in connection with this publication and hereby expressly disclaims any liability responsibility for or loss or damage resulting from its use for the violationof any federal, state, or municipal regulation with which or this publication may conflict. Suggested revisions are invited and shouldsubmitted to the Standards Manager, American Petroleum Institute, be 1220 L Street, N.W., Washington, D.C. 20005. iiiCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 6. COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 7. CONTENTS Page O D E ~ I T I O N S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 PLANNWG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2OperationalConsiderations .............................................. 1 1.3EnvironmentalConsiderations ............................................ 2 1.4Site Investigation-Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.5 Selecting the Design Environmental Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.6PlatformTypes ........................................................ 7 1.7Exposure Categories ................................................... 8 1.8 PlatformReuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.9PlatformAssessment ................................................... 9 1.10 SafetyConsiderations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.11 Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2 DESIGN CRITERIA AND PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.1 General ............................................................. 10 2.2LoadingConditions ................................................... 11 2.3 DesignLoads ........................................................ 12 2.4 Fabrication and Installation Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 STRUCTURALSTEELDESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3.1 General ............................................................. 38 3.2 Allowable Stresses for Cylindrical Members ............................... 39 3.3 Combined Stresses for Cylindrical Members ............................... 41 3.4 Conical Transitions .................................................. 44 CONNECTlONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 4.1 Connections of Tension and Compression Members . . . . . . . . . . . . . . . . . . . . . . . . . 46 4.2 Restraint and Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 4.3 Tubular Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 FATlGUE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 5.1 FatigueDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 5.2 Fatigue Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 5.3 S-N Curves for All Members and Connections. Except Tubular Connections . . . . . 53 5.4 S-N Curves for Tubular Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 5.5 StressConcentrationFactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 6 FOUNDATlONDESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 6.1 General ............................................................. 55 6.2PileFoundations ...................................................... 55 6.3PileDesign .......................................................... 56 6.4 Pile Capacity for Axial Bearing Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 6.5 Pile Capacity for Axial Pullout Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 6.6AxialPilePerformance ................................................ 60 6.7 Soil Reaction for Axially Loaded Piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 6.8 Soil Reaction for Laterally-Loaded Piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.9 Pile Group Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 6.10 Pile Wall Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 6.11 Length of Pile Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 6.12 Shallow Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 6.13 Stability of Shallow Foundations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 6.14 Static Deformation of Shallow Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 6.15 Dynamic Behavior of Shallow Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 6.16 Hydraulic Instability of Shallow Foundations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 6.17 Installation and Removal of Shall Foundations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 7 OTHER STRUCTURAL COMPONENTS AND SYSTEMS . . . . . . . . . . . . . . . . . . . . ... 71 7.1SuperstructureDesign ................................................. 71 7.2 Plate Girder Design ................................................... 72 7.3 Crane Supporting Structure ............................................. 72 7.4 Grouted Pile to Structure Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 7.5 Guyline System Design ................................................ 74 VCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 8. 15 REUSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 15.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 15.2 Reuse Considerations ................................................. 104 16MINlMUMSTRUCTURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 16.1General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 16.2 Design Loads and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 16.3Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 16.4 Material and Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 viCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 9. Page 17 ASSESSMENT OF EXISTING PLATFORMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 17.1General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 17.2 Platform Assessment Initiators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 17.3 Platform Exposure Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 17.4 Platform Assessment InformationSurveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 17.5 Assessment Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 17.6 Metocean, Seismic, and Ice CriteridLoads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 17.7 Structural Analysis For Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 17.8 Mitigation Alternatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 17.9References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 18 FIRE, BLAST, AND ACCIDENTAL LOADING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 18.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 18.2 Assessment Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 18.3 Platform Exposure Category . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 18.4 Probability of Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 18.5 Risk Assessment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 18.6Fire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 18.7Blast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 18.8 Fire and Blast Interaction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 18.9AccidentalLoading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 COMMENTARY ON SECTION 1.7-EXPOSURE CATEGORIES . . . . . . . . . . . . . . . . . . . 128 COMMENTARY ON WAVE FORCES. SECTION 2.3.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 COMMENTARY ON HYDRODYNAMIC FORCE GUIDELINES. SECTION2.3.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 COMMENTARY ON EARTHQUAKE CRITERIA. SECTION 2.3.6. . . . . . . . . . . . . . . . . . 144 . COMMENTARY ON ALLOWABLE STRESSES AND COMBINED STRESSES. SECTIONS 3.2 AND 3.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 COMMENTARY ON MINIMUM CAPACITY REQUIREMENT. . . . . . . . . . . . . . . . . . . 165 .. COMMENTARY ON TUBULAR JOINTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 COMMENTARY ON FATIGUE. SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 5 COMMENTARY ON AXIAL PILE CAPACITY IN CLAY. SECTION 6.4 . . . . . . . . . . . . 188 . COMMENTARY ON CARBONATESOILS. SECTION 6.4.3. . . . . . . . . . . . . . . . . . . . . . . . 189 COMMENTARY ON PILE CAPACITY FOR AXIAL CYCLIC LOADINGS. SECTION 6.6.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 COMMENTARY ON FOUNDATIONS SECTIONS 6.14 THROUGH 6.17. . . . . . . . . . . . . 195 COMMENTARY ON GROUTED PILE TO STRUCTURE CONNECTIONS. SECTION7.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 COMMENTARY ON MATERIAL. SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 8 COMMENTARY ON WELDING. SECTION10.2.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 COMMENTARY ON MINIMUM STRUCTURES. SECTION 16 . . . . . . . . . . . . . . . . . . . . 205 . COMMENTARY ON SECTION 17-ASSESSMENT OF EXISTING PLATFORMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 COMMENTARY ON SECTIONS 18.6-18.9-FIRE, BLAST. AND ACCIDENTAL LOADING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 Figures 2.3.1-1 Procedure for Calculation of Wave Plus Current Forces for Static Analysis . . . . 13 . 2.3.1-2DopplerShiftDuetoSteadyCurrent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3.1-3 Regions of Applicability of Stream Function, Stokes V, and Linear Wave Theory . 14 2.3.1-4 Shielding Factor for Wave Loads on Conductor Arrays as a Function of ConductorSpacing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.3.4-1 AreaLocationMap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.3.4-2 Region of Applicability of Extreme Metocean Criteria in Section 2.3.4.C . . . . . . 24COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 10. Page 2.3.4-3 Guideline Omnidirectional Design Wave Height. MLLW. Gulf of Mexico. vs North of 27" N and West of 86" W. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 . 2.3.4-4 Guideline Design Wave Directions and Factors to Apply to the Omnidirectional Wave Heights (Figure 2.3.4-3) for and L-2 Structures. L-l Gulf of Mexico. North of 27" N and West of 86" .W. . . . . . . . . . . . . . . . . . . . 25 .. 2.3.4-5 Guideline Design Current Direction (Towards) with Respect to North in < Shallow Water (Depth 150 ft) forL-l and L-2 Structures. Gulf of Mexico. North of 27"N and West of 86"W. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 . 2.3.4-6 Guideline Design Current Profile for L.2. and L-3 Structures. Gulf of L.1. Mexico. North of 27"N and West of 86"W . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 ... 2.3.4-7 Guideline Storm Tide .vs MLLW and Platform Category. Gulf of Mexico. North of 27"N and West of 86"W. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 . 2.3.4-8 Elevation of Underside of Deck (Above MLLW) . vs MLLW. Gulf of Mexico. North of 27"N and West of 86"W. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 . 3.4.1-1 Example Conical Transition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 4.1-1 Terminology and Geometric Parameters for Simple Tubular Connections. . . . 47 .. 4.3.1-1 Example of Joint Classification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 . 4.3.1-2 Detail of Simple Joint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.3.2-1 Detail of Overlapping Joint. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.3.2-2 SecondaryBracing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.3.4-1 Definition of Effective Cord Length. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 . 5.4-1 Fatigues-NCurves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 6.7.2-1 Typical Axial Pile Load Transfer-Displacement Curves . . . . . . . . . . . . . . . . . 62 (r-z) 6.731 Pile Tip-Load-Displacement(Q-Z) curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.8.6-1 Coefficients as Function off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 . 6.8.7-1 Relative Density. % . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 7.4.4-1 Grouted Pile to Structure Connection with Shear Keys . . . . . . . . . . . . . . . . . 73 .... 7.4.4-2 Recommended Shear Key Details. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 . 11.1.3 Welded Tubular ConnectionsShielded Metal Arc Welding. . . . . . . . . . . . . . . . 85 17.5.2 Platform Assessment Process-Metocean Loading . . . . . . . . . . . . . . . . . . . . . . . 112 17.6.2-1 Base Shear for a Vertical Cylinder BasedP on A I Recommended Practice 2A. 9th Edition Reference Level Forces. . . . . . . . . . . . . . . . . . . . . . . . 117 . 17.6.2-2a Full Population Hurricane Wave Height and Storm Tide Criteria. . . . . . . . . . 119 .. 17.6.2-2b Full Population Hurricane Deck Height Criteria. . . . . . . . . . . . . . . . . . . . . . . . 119 . 17.6.2-3a Sudden Hurricane Wave Height and Storm Tide Criteria. . . . . . . . . . . . . . . . 120 ... 17.6.2-3b Sudden Hurricane Deck Height Criteria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 .. 17.6.2-4 Sudden Hurricane Wave Directions and Factors to Apply to the Omnidirectional Wave Heights in Figure 17.6.2-3a for Ultimate Strength Analysis . . . . . . . . 122 ... 17.6.2-5a Winter Storm Wave Height and Storm Tide Criteria . . . . . . . . . . . . . . . . . . . . 123 .. 17.6.2-5b Winter Storm Deck Height Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 ... 18.2-1 Assessment Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 18.5-1 RiskMatrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 C2.3.1-1 Current Vectors Computed from Doppler Measurements at 60 ft on the Bullwinkle Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 . C2.3.1-2 Comparison of Linear and Nonlinear Stretching of Current Profiles. . . . . . . 133 ... C2.3.1-3 Definition of Surface Roughness Height and Thickness . . . . . . . . . . . . . . . . . 133 .. C2.3.1-4 Dependence of Steady Flow Drag Coefficient on Relative Surface Roughness . 135 C2.3.1-5 WakeAmplificationFactorforDragCoefficientasaFunctionof . . . . . . . 135 C2.3.1-6 Wake Amplification Factor for Drag Coefficient as a Function .of. . . . . . . . 137 K . C2.3.1-7 Inertia Coefficient as a Function K . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 of C2.3.1-8 Inertia Coefficient a asFunction Of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 C2.3.1-9 Shielding Factor for Wave Loads on Conductor Arrays as a Function of ConductorSpacing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 C2.3.4-1 Example Calculation of Current Magnitude. Direction. and Profile in the Intermediate Depth Zone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 . C2.3.6-1 Seismic Riskof United States Coastal Waters . . . . . . . . . . . . . . . . . . . . . . . . . . 149 . C2.3.6-2 Response SpectraSpectra Normalized to 1.0 Gravity . . . . . . . . . . . . . . . . . . . . 150 C2.3.6-3 Examplestructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 C2.3.6-4 Vertical Frame Configuration Not Meeting Guidelines. . . . . . . . . . . . . . . . . . 153 .. C2.3.6-5 Vertical Frame Configurations Meeting Guidelines. . . . . . . . . . . . . . . . . . . . . 153 .. C3.2.2-1 Elastic Coefficients for Local Buckling of Steel Cylinders Under Axial Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 C3.2.2-2 Comparison of Test Data with Design Equation for Fabricated Steel Cylinders Under Axial Compression. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 .. C3.2.3-1 Design Equation for Fabricated Steel Cylinders Under Bending. . . . . . . . . 160 .... viiiCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 11. Page C3.2.5-1 Comparison of Test Data with Elastic Design Equations for Local Buckling of Cylinders Under Hydrostatic Pressure (M 0.825 D/t) . . . . . . . . . . . . . . . . . . 162 > C3.2.5-2 Comparison of Test Data with Elastic Design Equations for Local Buckling of Cylinders Under Hydrostatic Pressure (M 0.825 D/t) . . . . . . . . . . . . . . . . . . 162 < C3.2.5-3 Comparison of Test Data with Design Equations for Ring Buckling and Inelastic Local Buckling Cylinders Under Hydrostatic Pressure. . . . . . . . . . 163 of . C3.3.3-1 Comparison of Test Data with Interaction Equation for Cylinders Under Combined Axial Tension and Hydrostatic Pressure . . . . . . . . . . . . . . . . . . . . 164 ... C3.3.3-2 Comparison of Interaction Equations for Various Stress Conditions for Cylinders Under Combined Axial Compressive Load and Hydrostatic Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 c3.3.3-3 Comparison of Test Data with Elastic Interaction Curve for Cylinders Under Combined Axial Compressive Load and Hydrostatic Pressure . . . . . . . . . . . 167 ... c3.3.3-4 Comparison of Test Data on Fabricated Cylinders with Elastic Interaction Curve for Cylinders Under Combined Axial Load and Hydrostatic Pressure. . . . . 167 ... c3.3.3-5 Comparison of Test Data with Interaction Equations for Cylinders Under Combined Axial Compressive Load and Hydrostatic Pressure (Combination Elastic and Yield Type Failures.) . . . . . . . . . . . . . . . . . . . . . . . 168 ... C431 Brace Load Interaction Equations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 . C4.3-2 Variation of K Joint Axial Load Capacity with Chord Flexibility . . . . . . . . . 170 ... c433 Chord Stress Reduction Effects for All Branch Load Types with Safety FactorRemoved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 C4.3.1-1 Adverse Load Patterns with a Up to 3.8 (a) False Leg Termination, (b) Skirt Pile Bracing. (c) Hub Connection . . . . . . . . . . . . . . . . . . . . . . . . . 171 ..... C4.3.1-2 Computed a (a) Equation. (b) Definitions. (c) Influence Surface . . . . . . . . . 172 .... C5.1-1 Allowable Peak Hot Spot Stress. (S-N CurveX) . . . . . . . . . . . . . . . . . . . . . . . 176 S C5.1-2 C5.1-3 4 Allowable Peak Hot Spot Stress. (S-N CurveX? . . . . . . . . . . . . . . . . . . . . . . 176 Example Wave Height Distribution Over Time. . . . . . . . . . . . . . . . . . . . . . . 178 T.. C5.2-1 Selection of Frequencies for Detailed Analyses. . . . . . . . . . . . . . . . . . . . . . . . . 178 . C5.4-1 Weld Profile Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 . C5.4.2 Size and Profile Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 . C551 WRC Data Base for Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 K C552 Illustrations of Branch Member Stresses Corresponding to Mode of Loading . . 185 c553 Corrosion-Fatigue Data Notched or Welded Specimens in Sea .Water . . . 186 ..... c554 Tests in Sea Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 C6.13.1-1 Recommended Bearing Capacity Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 .. C6.13.1-2 Eccentrically Loaded Footings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 . C6.13.1-3 Area Reduction Factors Eccentrically Loaded Footings. . . . . . . . . . . . . . . . 197 .... C6.13.1-4 Definitions for Inclined Base and Ground Surface (After . . . . . . . . . . . . . 198 Vesic) C7.4.4a-1 Measured Bond Strength . Cube Compressive Strength. . . . . . . . . . . . . . . . .202 vs . C7.4.4a-2 Measured Bond Strength vs . Cube Compressive Strength Times the Height to Spacing Ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 . C7.4.4a-3 Number of Tests for Safety Factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 . C7.4.4a-4 Cumulative Histogram of Safety Factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 . . C17.6.2-la Silhouette AreaDefinition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 C17.6.2-lb Wave Heading and Direction Convention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 .. C18.6.2-1 Strength Reduction Factors for Steel at Elevated Temperatures (Reference . . 215 1) C18.6.3-1 Maximum Allowable Temperature of Steel as a Function of Analysis Method) . 216 C18.6.3-2 Effect of Choice of Strain in the Linearization of the Stress/Strain Characteristicsof Steel at Elevated Temperatures . . . . . . . . . . . . . . . . . . . . . . . 217 . C 18.7.2- Example Pressure Time Curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 1 C18.9.2-1 DRRatio versus Reduction in Ultimate Capacity, 48,54, and 60 InchLegsStraight with L = 60 Feet,K = 1.0, and Fy = 35 ksi. . . . . . . . . . . 224 C18.9.2-2 DRRatio versus Reduction in Ultimate Capacity, 48,54, and 60 InchLegsStraight with L = 60 Feet,K = 1.0, and Fy = 50 ksi . . . . . . . . . . . 224 C18.9.2-3 DRRatio versus Reduction in Ultimate Capacity, 48,54, and 60 Inch Legs-Bent with L = 60 Feet,K = 1.0, and Fy = 35 ksi . . . . . . . . . . . . . 225 C18.9.2-4 DRRatio versus Reduction in Ultimate Capacity, 48,54, and 60 Inch Legs-Bent with L = 60 Feet,K = 1.0, and Fy = 50 ksi . . . . . . . . . . . . . 225 Tables 2.3.4-1 U.S. Gulf of Mexico Guideline Design Metocean Criteria. . . . . . . . . . . . . . . . . . . 23 2.3.4-2 Guideline Extreme Wave, Current, and Storm Tide Values for Twenty Areas in United States Waters (Water depth> 300 ft. (91 m) except as noted). . . . . . . . 29COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 12. Page 2.3.4-3 Guideline Extreme Wind Speeds for Twenty Areas in United States Waters. . . . 30 . 4.3.1-1 Values Qq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 for 4.3.1-2 Valuesfor Qu( 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 6.431 DesignParametersforCohesionlessSiliceousSoil . . . . . . . . . . . . . . . . . . . . . . . . 59 8.1.4-1Structural Plates Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 8.1.4-2StructuralSteelShapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 8.2.1-1StructuralSteelPipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 8.3.1-1Input Testing Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 10.2.2 ImpactTesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 12.5.7Guideline Thickness Wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 13.4.3 Recommended Minimum Extent of NDE Inspection . . . . . . . . . . . . . . . . . . . . . . . 99 14.4.2-1 Guideline Survey Intervals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Recommended Extent of NDE Inspection-Reused Structure . . . . . . . . . . . . . . 106 17.6.2-1 U.S. Gulf of Mexico Metocean Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 17.6.2-2 100-Year Metocean Criteria for Platform Assessment U S . Waters (Other Than Gulf of Mexico), Depth 300 feet . . . . . . . . . . . . . . . . . . . . . . . . . 117 > C5.1-1 Selected SCF Formulas for Simple Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 C10.2.2Average H M Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 cd, C17.6.2-1 Drag Coefficient, for WaveKurrent Platform Deck Forces . . . . . . . . . . . . 209 .. C18.6.2-1 Yield Strength Reduction Factors for Steel at Elevated Temperatures (ASTM A-36 and A-633 GR and D). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 .C . C18.6.3-1 Maximum Allowable Steel Temperature as a Function of Strain for Use With the “Zone” Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 . C18.6.3-2 Maximum Allowable Steel Temperature as a Function of Utilization Ratio(UR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 C18.6.4-1 Summaryof Fire Ratings and Performance for Fire Walls. . . . . . . . . . . . . . .218 ... C18.9.2-1 Required Tubular Thickness to Locally Absorb Vessel Impact Broadside Vessel Impact Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 XCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 13. Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms-Working Stress Design O Definitions Adequate planning shouldbe done before actual design is started in order to obtain a workable and economical offshore fixed platform: A platform extending above and supported structure to perform a given function. The initial planning by the sea bed by means of piling, spread footings or other should include the determination of all criteria upon which means with the intended purposeremaining stationary over of the designof the platform is based. an extended period. manned platform: A platform which is actually and con- 1.1.2 Design Criteria tinuouslyoccupied by personsaccommodatedandliving Design criteria as used herein include operational all thereon. requirements and environmental data which could affect the detailed designof the platform. unmanned platform: A platform upon which persons may beemployed at anyonetime,butuponwhichnoliving 1.1.3 Codes and Standards accommodations or quarters provided. are This publication has also incorporated and made maximum operator: The person, firm, corporation or other organiza- use of existingcodesandstandardsthathavebeenfound tion employedby the owners to conduct operations. acceptableforengineeringdesignandpracticesfromthe ACI: American Concrete Institute. standpoint of public safety. AIEE: American Instituteof Electrical Engineers. 1.2 OPERATIONAL CONSIDERATIONS AISC: American Instituteof Steel Construction. 1.2.1 Function API: American Petroleum Institute. The function for which a platform is be designed is usu- to ally categorizedas drilling, producing, storage, materials han- ASCE: American Society of Civil Engineers. dling, living quarters, or a combination of these. platform The configuration should be determined by a study of layouts of ASME: American Society of Mechanical Engineers. equipment to be located on the decks. Careful consideration ASTM: American Society for Testing and Materials. should be given to the clearances and spacing of equipment before the final dimensions are decided upon. AWS: American Welding Society. 1.2.2 Location IADC: International Associationof Drilling Contractors. The location of the platform should be specific before the NACE: National Associationof Corrosion Engineers. designiscompleted.Environmentalconditionsvarywith NFPA: National Fire Protection Association. geographiclocation;withinagivengeographicarea,the foundation conditions will vary as will such parameters as OTC: Offshore Technology Conference. design wave heights, periods, and tides. 1.2.3 Orientation 1 Planning The orientation of the platform refers to its position in the 1.1 GENERAL plan referenced to a fixed direction such true north. Orien- as 1.1.1 Planning tation is usually governed by the directionof prevailing seas, winds, currents, and operational requirements. This publication serves as a guide for those who are con- cerned with the design and construction of new platforms 1.2.4 Water Depth and for the relocation existing platforms used for the of drill- ing, development, and storage of hydrocarbons in offshore Information on water depth and tides is needed to select areas. In addition, guidelinesare provided for the assessment appropriate oceanographic designparameters. The water of existing platforms in the event that it becomes necessary depth should be determined as accurately as possible so that to make a determination of the “fitness for purpose” of the elevations be can established boat for landings, fenders, structure. decks, and corrosion protection. 1COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 14. 2 1.2.5 Access and Auxiliary Systems type and size of supply vessels, and the anchorage system required to hold them in position platform.The number, at the The locationandnumber of stairwaysandaccessboat size, and location of the boat landings should be determined landings on the platform should be governed by safety con- as well. siderations. A minimum of two accesses to each manned level should be installed and should be located so that escape is The type, capacity, number and location the deck cranes of possibleundervaryingconditions.Operatingrequirements should also be determined. equipment or materials are to be If should alsobe considered in stairway locations. placed on a lower deck, then adequately sized and conve- nientlylocatedhatchesshouldbeprovidedontheupper 1.2.6 Fire Protection decks as appropriate for operational requirements. possi- The ble use of helicopters shouldbe established and facilities pro- The safety of personnel and possible destructionof equip- vided for their use. ment requires attention to fire protection methods. selec- The tion of the system depends upon the function the platform. of 1.2.1 1 Spillage and Contamination Procedures should conform to all federal, state, and local reg- ulations where they exist. Provision for handling spills and potential contaminants should be provided. A deck drainage system that collects and 1.2.7Deck Elevation stores liquids for subsequent handling should be provided. The drainage and collection system should meet appropriate Large forces and overturning moments result when waves governmental regulations. strikeaplatform’slowerdeckandequipment.Unlessthe platform has been designed to resist these forces, the eleva- tion of thedeckshouldbesufficienttoprovideadequate 1.2.1 2 Exposure clearance above the crest the design wave. In addition, con- of Design of all systems and components should anticipate sideration should be given to providing an “air gap” to allow extremes in environmental phenomena that may be experi- passage ofwaveslargerthanthedesignwave.Guidelines enced at the site. concerning thear gap are provided in 2.3.4d.3 and 2.3.48. i 1.3 ENVIRONMENTAL CONSIDERATIONS 1.2.8 Wells 1.3.1 General Meteorological and Oceanographic Exposed well conductors add environmental forces to a Considerations platform and require support. Their number, size, and spacing should be known early in the planning stage. Conductor pipes Experienced specialists should be consulted when defining may or may not assist in resisting the wave force. If the plat- thepertinentmeteorologicalandoceanographicconditions form is to be set over an existing well with the wellhead affecting a platform site. The following sections present a above water, information is needed on the dimensions of the general summary of the information that could be required. tree, size of conductor pipe, and the elevations of the casing Selection of information neededat a site should be made after head flange and top wellhead above mean low water. If the of consultation with both the platform designer and a meteoro- existing well is a temporary subsea completion, plans should logical-oceanographic specialist. Measured and/or model- be made for locating the well and setting the platform prop- generated data should be statistically analyzed to develop the erly so that the well can later be extended above the surface of descriptions of normal and extreme environmental conditions the water. Planning should consider the need for future wells. as follows: 1.2.9 Equipment and Material Layouts 1. Normal environmentalconditions (conditions are that expected to occur frequently during the life of the structure) Layouts and weights of drilling equipment and material and production equipment are needed in the development of are important both during the construction and the servicelife the design. Heavy concentrated loads on the platform should of a platform. be located so that proper framing for supporting these loads 2. Extreme conditions (conditions that occur quite rarely dur- can be planned.Whenpossible,considerationshouldbe ingthe life of thestructure)areimportantinformulating given to future operations. platform design loadings. All data used should be carefully documented. The esti- 1.2.10Personnel and Material Handling mated reliability and the source of all data should be noted, Plans handling for personnel materials and shouldbe and the methods employed in developing availabledata into developed at the start of the platform design, along with the the desired environmental values should defined. beCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 15. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 3 1 3 2 Winds .. forms, experienced specialists knowledgeable in the fields of meteorology,oceanography,andhydrodynamicsshould be Wind forces are exerted upon that portion of the structure consulted. that is above the water, as well as on any equipment, deck In those areas where prior knowledge of oceanographic houses, and derricks that are located on the platform. The conditions is insufficient, the development of wave-dependent wind speed may be classified as: (a) gusts that average less design parameters should include least the following steps: at than one minute in duration, and (b) sustained wind speeds thataverage one minute or longer induration.Wind data 1. Development of all necessary meteorological data. should be adjusted to a standard elevation, such feet (10 as 33 2. Projection of surface wind fields. meters) above mean water level, with a specified averaging 3.Prediction of deepwater general sea-statesalongstorm time, such as one hour. Wind data may be adjusted to any tracks using an analytical model. specified averaging time or elevation using standard profiles 4. Definition of maximum possible sea-statesconsistent with and gust factors (see2.3.2). geographical limitations. The spectrum of wind speed fluctuations about the average 5. Delineation of bathymetric effects deepwater on sea- should be specified in some instances. For example, compli- states. ant structures like guyed towers and tension leg platforms in 6. Introduction of probabilistictechniquestopredictsea- deep water may have natural sway periods the range of one in state occurrences at the platform site against varioustime minute, in which thereis significant energy in the wind speed bases. fluctuations. The following shouldbe considered in determining appro- 7. Development of design wave parameters through physical priate design wind speeds: and economic risk evaluation. For normal conditions: In areas where considerable previous knowledge and expe- rience withoceanographic conditions exist,theforegoing 1. The frequency of occurrence of specified sustained wind sequence may be shortened to those steps needed to project speeds from various directions each monthor season. for this past knowledge the required design parameters. into 2. The persistence of sustained wind speeds above specified It is the responsibility of the platform owner to select the thresholds for each month orseason. design sea-state, after considering all of the factors listed in 3. The probablespeed of gusts associatedwithsustained Section 1.5. In developing sea-state consideration data, wind speeds. should be given to the following: For extreme conditions: For normal conditions (for both seas and swells): Projected extreme wind speeds of specified directions and 1. For each month and/or season, the probabilityof occurrence averagingtimes as afunction of their recurrence interval andaveragepersistence of varioussea-states(forexample, should be developed. Data should be given concerning the waves higher than 10 feet [3 meters]) from specified directions following: in termsof general sea-state description parameters (for exam- 1. The measurement site, date of occurrence, magnitude of ple, the significant wave height and the average wave period). measured gusts and sustained wind speeds, and wind direc- 2. The windspeeds,tides,andcurrentsoccurringsimulta- tions for the recorded wind data used duringthe development neously with the sea-states of Section 1 above. of the projected extreme winds. For extreme conditions: 2. The projected number of occasions during the specified life of the structure when sustained wind speeds from speci- Definition of the extremesea-statesshouldprovidean fied directions should exceed a specific lower bound wind insight as to the number, height, and crest elevations of all speed. waves above a certain height that might approach the plat- form site from any direction during the life of the struc- entire 1 3 3 Waves .. ture. Projected extreme wave from heights specified directions should be developed presented as a functionof and Wind-driven waves are a major source of environmental their expected average recurrence intervals. Other data which forces onoffshoreplatforms. Such waves are irregularin should be developed include: shape, vary in height and length, and may approach a plat- form from one or more directions simultaneously. For these 1. The probable range and distribution of wave periods asso- reasons the intensity and distributionof the forces applied by ciated with extreme wave heights. waves are difficultto determine. Because of thecomplex 2. The projected distribution of other wave heights, maxi- nature of thetechnicalfactorsthatmustbeconsideredin mum crest elevations, and the wave energy spectrum in the developing wave-dependent criteria for the design of plat- sea-state producing anextreme wave height(s).COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 16. 4 2A-WSD PRACTICE RECOMMENDED API 3. The tides,currents,andwindslikelytooccursimulta- Research in ice mechanics is being conducted by individ- neously withthe sea-state producing the extreme waves. ual companies and joint industry groups to develop design 4. The nature, date, and place of the events which produced criteria for arctic andsubarcticoffshore areas. Global ice the historicalsea-states(forexample,HurricaneCamille, forces vary depending on such factors as size and configura- August 1969, U.S. Gulf of Mexico) used in the development tion of platform, locationof platform, modeof ice failure, and of the projected values. unit ice strength. Unit ice strength depends on the ice feature, temperature, salinity, speedof load application, and ice com- 1.3.4 Tides position. Forces to be used in design should be determined in consultation with qualified experts. Tides are importantconsiderationsinplatform design. API Recommended Practice 2N outlines the conditions Tides may be classified as: (a) astronomical tide, (b) wind that should be addressed in the design and construction of tide, and (c) pressure differential tide. The latter two are fre- structures inarctic and subarctic offshore regions. quently combined and called storm surge; the sum of the three tides is called the storm tide. In the design of a fixed 1.3.7Active Geologic Processes platform, the storm tide elevation is the datum upon which storm waves are superimposed. The variations in elevations 1.3.7.a General of the daily astronomical tides, however, determine the eleva- In many offshore areas, geologic processes associated with tions of the boatlandings,bargefenders,thesplash zone movement of the near-surface sediments occur within time treatment of the steel membersof the structure, and the upper periods that are relevant to fixed platformdesign. The nature, limits of marine growth. magnitude, and return intervals of potential seafloor move- ments should be evaluated site investigations and judicious by 1.3.5 Currents analytical modeling to provideinput for determination of the Currents are important in the design of fixed platforms. resulting effects on structures and foundations. Due to uncer- They affect: (a) the location and orientation of boat landings tainties definition with of theseprocesses,parametric a and barge bumpers, and the forces on the platform. Where (b) approach to studies may be helpful in the development of possible, boat landingsand barge bumpers should located, be design criteria. to allow the boat to engage the platform as it moves against the current. 1.3.7.b Earthquakes The most common categoriesof currents are: (a) tidal cur- Seismic forces should be considered in platform design rents (associated with astronomical tides), (b) circulatory cur- for areas that are determined to be seismically active. Areas rents (associated withoceanic-scale circulation patterns),and are considered seismically active on the basis of previous (c) storm-generated currents. The vector sum of these three records of earthquake activity, both in frequency of occur- currents is the total current,and the speed and direction of the rence and in magnitude. Seismic activity of an area for pur- current at specified elevationsis the current projile. The total poses of design of offshore structures is rated in terms of current profile associated with the sea-state producingthe possible severity of damage to these structures. Seismic risk extreme waves should be specified for platform design. The for United States coastal areas is detailed in Figure C2.3.6-1. frequency of occurrence of total current of total current speed Seismicity of an area may also be determined on the basis of and direction at different depths for each monthand/or season detailed investigation. may be usefulfor planning operations. Seismic considerations should include investigation of the subsurface soils at the platform site for instability due to liq- 1.3.6 Ice uefaction, submarine slides triggered by earthquake activity, proximity of the site tofaults, the characteristics of the In some areas where petroleum development is being car- ground motion expected during the life of the platform, and ried out, subfreezing temperatures can prevail a major portion the acceptable seismic risk the type of operation intended. for of the year, causing the formation of sea-ice. Sea-ice may exist Platforms in shallow water that may subjected to tsunamis be in these areas first-year sheet ice, multi-year floes, first-year as should be investigated the effectsof resulting forces. for and multi-year pressure ridges, and/or islands. Loads pro- ice duced by ice features could constitute a dominant design fac- 1.3.7.c Faults tor for offshore platforms inthe most severe ice areas such as the Alaskan Beaufort and Chukchi Seas, Norton Sound. In and In some offshore areas, planes may extend the seaf- fault to milder climates, such as the southern Bering Sea and Cook loor with the potential for either vertical or horizontal move- Inlet, the governing design factor may be seismic- or wave- ment. Fault movementcanoccur as a result of seismic induced,but ice featureswouldnonethelessinfluencethe activity, removalof fluids from deep reservoirs, or long-term design and construction the platforms considered. of creep related to large-scale sedimentation or erosion. of SitingCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 17. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 5 facilities in close proximity to fault planes intersecting the multiple interaction structure or wave/soil/structure seafloor should be avoided if possible. If circumstances dic- interaction. tate siting structures near potentially active features, the mag- 3. Overall seabedmovement:Movement of sandwaves, nitudeandtimescale of expected movement should be ridges, and shoals that would occur in the absence a struc- of estimated on the basis of geologic study for use in the plat- ture. movement be This can caused by lowering or form design. accumulation. The presence of mobile seabed sandwaves, sandhills, and 1.3.7.d Seafloor Instability sand ribbons indicates a vigorous natural scour regime. Past Movement of the seafloor can occur as a result of loads bed movement may evidenced by geophysical contrasts, be or imposed on the soil mass by ocean wave pressures, earth- by variation in density, grading, color, or biological indicators quakes, soil self-weight, combination of these phenomena. or in seabed samples and soundings. Sand or silt soils in water Weak, underconsolidated sediments occurring in areas where depths less than about 130 feet (40 meters) are particularly wave pressures are significant the seafloorare most suscep- at susceptible to scour, but scour has been observed in cobbles, tible to wave induced movement and may be unstable under gravelsandclays;indeeperwater,thepresence of scour negligible angles. slope Earthquakeinduced forces can depends on the vigor currents and waves. of inducefailure of seafloorslopesthat are otherwisestable Scour can result in removalof vertical and lateral support under the existing self-weight forces and wave conditions. for foundations, causing undesirable settlements mat foun- of dationsandoverstressing of foundationelements.Where In areas of rapid sedimentation, such as actively growing scour is a possibility,it should be accounted for in design and/ deltas, low soil strength, soil self-weight, and wave-induced or its mitigation should be considered. Offshore scour phe- pressures are believed to be the controlling factors for the nomena are described in “Seafloor Scour, Design Guidelines geologic processes that continually move sediment down- for Ocean Founded Structures,” by Herbich 1984,No. 4 et al., slope. Important platformdesign considerations under these in Marcel Dekker Inc., Ocean Engineering Series; and “Scour conditions include the effects of large-scale movement of PreventionTechniquesAroundOffshoreStructures.” SUT sediment in areas subjected to strong wave pressures, Seminars, London, December 1980. downslope creep movements in areas not directly affected bywave-seafloor interaction, andthe effects of sediment 1.3.7.f ShallowGas erosion and/ordeposition on platform performance. The scope of site investigations in areasof potential insta- The presence of either biogenic or petrogenic gas in the bility should focus on identification of metastable geologic porewater of near-surface soils is an engineering consider- features surrounding the site and definition of the soil engi- ation in offshore areas. addition to being a potential drilling In neering properties required for modeling and estimating sea- hazard for both site investigation soil borings and oil well floor movements. drilling, the effects of shallow gas may be important to engi- Analyticalestimates of soilmovement as afunction of neering of the foundation. The importance of assumptions depth below the mudline can be used with soil engineering regarding shallow gas effects on interpreted soil engineering properties to establish expected forces on platform members. properties and analytical models geologic processes should of Geologic studies employing historical bathymetric data may be established during initial stages the design. of be useful for quantifying deposition rates during the design life of the facility. 1.3.8 Marine Growth Offshorestructuresaccumulatemarinegrowthtosome 1.3.7.e Scour degree in all the world‘s oceans. Marine growth is generally greatest near the mean water level but in some areas may be Scour is removal of seafloor soils caused by currents and significant 200 feet or morebelowthemeanwaterlevel. waves. Such erosion canbe a natural geologic process or can Marine growth increases wave forces (by increasing member be caused by structural elements interrupting the natural flow diameter and surface roughness) and mass of the structure, regime near the seafloor. and should be considered in design. From observation, scour can usually be characterized as some combinationof the following: 1.3.9OtherEnvironmentalInformation 1. Local scour: Steep-sided scour pits around such structure Depending on the platform site, other environmental infor- elements as piles and pile groups, generallyas seen in flume mation of importanceincludesrecordsand/orpredictions models. with respect to precipitation, fog, wind chill, and sea tem- air, 2. Globalscour:Shallowscouredbasins of largeextent peratures. General information on the various types storms of around a structure, possibly due to overall structure effects, that might affect the platform site should be used to supple-COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 18. 6 2A-WSD PRACTICE RECOMMENDED API ment other data developed for normal conditions. Statistics voir studies. Shallowsampling of near-surfacesediments can be compiled giving the expected occurrence storms by of using drop, piston, grab samplers, or vibrocoring along geo- season, directionof approach, etc. Of special interest for con- physical tracklines may be useful for calibration of results struction planning are the duration, the speed of movement and improved definitionof the shallow geology. and development, and the extent of these conditions. Also of For more detailed description of commonly used sea-bot- major importanceis the ability toforecast storms inthe vicin- tom survey systems, refer the paper “Analysisof High Res- to ity of a platform. olution Seismic Data” by H. C. Sieck and G. W. Self (AAPG), Memoir 26: Seismic Stratigraphy-Applications to 1.4 SITE INVESTIGATION-FOUNDATIONS Hydrocarbon Exploration, 1977, pp.353-385. 1.4.1 Site Investigation Objectives 1.4.3 Soil Investigation and Testing Knowledge of the soil conditions existing at the site of con- If practical, the soil sampling and testing program should struction on any sizable structure is necessary to permit asafe and economical design. On-site soil investigations should be be defined after a review of the geophysical results. On-site performed to define the various soil strata and their corre- soil investigation should include one or more soil borings to sponding physical and engineering properties. Previous site provide samples suitable for engineering property testing, and investigations and experience at the site maypermitthe a means to perform in-situ testing, if required. The number installation of additional structures without additional studies. and depth of borings will depend on the soil variability in the vicinity of the site and the platform configuration. Likewise, The initial step for a site investigation is reconnaissance. the degree of sophistication of soil sampling and preservation Information may be collected through a review of available techniques, required laboratory testing, and the need for in- geophysical data and soil boring data available in engineering situ property testing are a function of the platform design files,literature, or governmentfiles. The purpose of this requirements andthe adopted design philosophy. review is to identify potential problems to aid in planning and subsequent data acquisition phasesof the site investigation. As a minimum requirement, the foundation investigation for pile-supported structures should provide soil engineer- the Soundings and any required geophysical surveys should be ing property data needed to determine the following parame- part of the on-site studies, and generally should be performed ters: (a) axial capacity of piles in tension and compression, before borings. These data shouldbecombinedwithan (b) load-deflection characteristics of axially and laterally understanding of the shallow geologyof the region to develop loaded piles, (c) driveability characteristics, and (d) mud- pile the required foundation design parameters. The on-site stud- mat bearing capacity. ies should extend throughout the depth and areal extent of soils that will affect or be affected by installation foun- of the The required scope of the soil sampling, in-situ testing, dation elements. and laboratory testing programs is a function of the platform design requirements and the need to characterize active geo- 1.4.2 Sea-Bottom Surveys logic processes that may affect the facility. For novel plat- form concepts, deepwater applications, platforms in areas of The primary purpose of a geophysical survey inthe vicin- potential slope instability, and gravity-base structures, the ity of the site is to provide data for a geologic assessment of geotechnical program should be tailored to provide the data foundation soilsand the surroundingarea that could affect the necessary for pertinent soil-structure interaction and pile site. Geophysical data provide evidence of slumps, scarps, capacity analyses. irregular or rough topography, mud volcanoes, mud lumps, Whenperforming site investigationsinfrontier areas or collapse features, sand waves, slides, faults,diapirs, erosional areas known to contain carbonate material, the investigation surfaces,gasbubblesinthesediments, gas seeps, buried should include diagnostic methods to determine the existence channels, andlateral variations in strata thicknesses. areal The of carbonate soils. Typically, carbonate deposits are variably extent of shallow soil layers may sometimes be mapped if cemented and range from lightly cemented with sometimes good correspondencecan be established between soil bor- the significant void spaces to extremely well-cemented. In plan- ing information and the results fromthe sea-bottom surveys. ninga site investigationprogram,thereshouldbeenough The geophysical equipment used includes: (a) subbottom flexibility in the program to switch between soil sampling, profiler (tuned transducer) for definition of bathymetry and rotary coring, and in-situ testing as appropriate. Qualitative structural features within near-surface sediments,(b) side- the tests should be performed to establish thecarbonate content. scan sonar to define surface features, (c) boomer or mini- In a soil profile which contains carbonate material (usually in sparker for definition of structure to depths up to a few hun- excess of 15 to 20 percent of the soil fraction) engineering dred feet below the seafloor, and (d) sparker, air gun, water behavior of the soil could be adversely affected. these soils In gun, or sleeve exploder for definition of structure at deeper additional field and laboratory testing and engineering may data depths, and to tie together with deep seismic from reser- be warranted.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 19. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 7 1.5 SELECTING THE DESIGN ENVIRONMENTAL Where sufficient information is available, the designer may CONDITIONS take into account the variation in environmental conditions expected to occur from different directions. When this is con- Selection of the environmental conditions to which plat- sidered, an adequate tolerance in platform orientation should forms are designed should be the responsibility the owner. of be used in the design of the platform and measures must be The design environmental criteria should be developed from employed during installation to ensure the platform is posi- the environmental information described in Section 1.3, and tioned within the allowed tolerance. For the assessment of may also include a risk analysis where prior experience is limited. The risk analysis may include the following: existing structures, the application a reduced criteria is nor- of mally justified. Recommendations for the development an of 1. Historical experience. oceanographiccriteriafortheassessment of existingplat- 2. The planned life and intendeduse of the platform. forms is provided in Section 17. 3. The possible loss of human life. Structures should be designed for the combination of wind, 4. Prevention of pollution. wave, current and conditionscausing extreme the load, 5. The estimated cost of the platform designed to environ- accountingfortheir joint probability of occurrence(both mentalconditionsforseveralaverageexpectedrecurrence magnitude and direction). For most template, tower, gravity, intervals. and caisson typesof platforms, the design fluid dynamic load 6. The probability of platformdamageorlosswhensub- is predominantlydue to waves, with currents and winds play- jected to environmental conditions with various recurrence ing a secondary role. design conditions, therefore, consist The intervals. of the wave conditions and the currents and winds likely to 7. The financial loss due to platform damage or loss includ- coexistwiththedesignwaves.Forcompliantstructures, ing production, lost cleanup,replacing platform the and responsetowavesisreduced, so thatwindsandcurrents redrilling wells, etc. becomerelativelymoreimportant.Also,forstructuresin As a guide, recurrence the interval oceanographic for shallow water and structures with a large deck and/or super- design criteria should be several times the planned of the life structure, the wind load may a more significant portion be of platform. Experience with major platforms inU.S. Gulf of the the total environmental force. This may lead to multiple sets Mexico supports the use of 100-year oceanographic design of design conditions including; as an example, for LevelL-1 criteria. This is applicable only to new and relocated plat- structures (a) the 100-year waves with associated winds and forms that are manned during the design event, or are struc- currents, and (b) the 100-year winds with associated waves tureswherethelossof,orseveredamagetothestructure and currents. could result in a high consequence of failure. Consideration Twolevelsofearthquakeenvironmentalconditionsare may be given a reduced design requirements for the design to needed to address the risk damage or structure collapse: (1) of or relocationof other structures, that unmanned or evacu- are ground motion which has a reasonable likelihood being of not ated during the design event, and have either a shorter design exceeded at the site during the platform life, and (2) ground life than the typical 20 years, or where the loss of or severe motion for a rare, intense earthquake. damage to the structure would not result in a high conse- Consideration of the foregoing factors has led the estab- to quence of failure. Guidelines to assist in the establishment of lishment of the hydrodynamic force guideline of 2.3.4, and the exposure category to be used in the selection of criteria the guidelines for earthquake design 2.3.6. of for the design of new platforms and the assessmentof exist- ing platforms are provided in Section 1.7. Risk analyses may 1.6 PLATFORMTYPES justify either longer or shorter recurrence intervals for design 1.6.1 Fixed Platforms criteria.However, less 100-year not than oceanographic design criteria should be considered where the design event A fixed platform is definedas a platform extending above may occur without warning while the platform is manned the water surface and supported at the sea bed by means of and/or when there are restrictions on the speed of personnel piling, spread footing(s), or other means with the intended removal (for example, great flying distances). purpose of remaining stationary over an extended period. Sectionprovides 2 guidelines developing for oceano- graphic design criteria that are appropriate for use with the 1.6.1 .a Jacket or Template ExposureCategoryLevelsdefinedinSection1.7.Forall These type platforms generally consist the following: of Level 1 Category new structures located in U.S. waters, the use of nominal 100-year return period is recommended. For 1. Completely braced, redundant welded tubular space frame Level 2 and Level 3 Category new structures located in the extending from an elevation at or near the sea bed to above U.S. Gulf of Mexico north of 27" N latitude and westof 86" thewatersurface,which is designed to serve as themain W longitude, guidelines for reducing design wave, wind, and structural element of the platform, transmitting lateral and current forcesare provided. vertical forces to the foundation.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 20. 8 2A-WSD PRACTICE RECOMMENDED API 2. or foundation Piles other elements permanently that reduction in forces transmitted to the platform and the sup- anchor the platform to the ocean floor, and carry both lateral portingfoundation.Guyedtowersarenormallycompliant, and vertical loads. unless the guying system is very stiff. Compliant platforms 3. A superstructure providing space supporting deck for are covered in this practice only to the extent that the provi- operational and other loads. sions are applicable. 1.6.1 .b Tower 1.6.2 Floating Production Systems A tower platform is a modification of the jacket platform A number of different floating structures are being devel- that has relatively few large diameter [for example, feet (5 15 oped and used as floating production systems (e.g., Tension meters)legs]. The towermay be floatedtolocationand Leg Platforms, Spars, Semisubmersibles). Many aspects of placed in position by selective flooding. Tower platforms may this Recommended Practice are applicable to certain aspects or may not be supported by piling. Where piles used, they are of the design of these structures. are driven through sleeves inside or attached the outsideof to API RP 2T provides specific advice for TLPs. the legs. The piling may also serve as well conductors. If the 1.6.3 Related Structures tower’s support is furnished by spread footings instead of by piling, the well conductors may be installed either inside or Other structures include undenvaer storage oil tanks, outside the legs. bridges connecting platforms, flare booms, etc. 1.6.1.cGravity Structures 1.7 EXPOSURE CATEGORIES A gravity structure is one that relies on the weight of the Structures can be categorized various levels of exposure by structure rather than piling to resist environmental loads. to determine criteria for the design new platforms and the of assessment of existing platforms that are appropriate for the intended serviceof the structure. 1.6.1 .d Minimum Non-Jacket and Special The levels are determined by consideration of life-safety Structures and consequences of failure. Life-safety considers the maxi- Many structures have been installed and are serving satis- mum anticipated environmental that event would be factorily that do not meet the definition for jacket type plat- expected to occur while personnel are on the platform. Con- forms as defined above. In general, these structures do not sequences of failure should consider the factors listed in have reserve strength or redundancy equal to conventional Section 1.5 and discussed in the Commentary for this sec- jacket type structures. For this reason, special recommenda- tion. Such factors include anticipated losses to the owner tions regarding design and installation are provided in Section(platform and equipment repair or replacement, lost produc- 16. Minimum structures are definedas structures which have tion, cleanup), anticipated losses to other operators (lost one or moreof the following attributes: production through trunklines), and anticipated losses to industry and government. 1. Structural framing, which provides less reserve strength and redundancy than a typical well braced, three-leg template Categories for life-safety are: type platform. L-1 = manned-nonevacuated 2. Free-standing and guyed caisson platforms which consist L-2 = manned-evacuated of one large tubular member supporting or more wells. one L-3 = unmanned 3. Well conductor(s) or free-standing caisson(s), which are Categories for consequences failure are: of utilized as structuraland/oraxialfoundationelements by means of attachment using welded, nonwelded, or noncon- L-1 = high consequence of failure ventional welded connections. L-2 = medium consequence of failure 4. Threaded, pinned, or clamped connections to foundation L-3 = low consequence of failure elements (piles or pile sleeves). The level to be used for platform categorization is the more 5. Braced caissons and other structures where a single ele- restrictive level for either life-safety or consequence of fail- ment structural system a major componentof the platform, is ure. Platform categorization may be revised over the life of such as a deck supported by a single deck let or caisson. the structure as a result of changes in factors affecting life- safety or consequence failure. of 1.6.1 .e Compliant Platform 1.7.1 Life Safety A compliant platform is a bottom-founded structure having substantial flexibility. It is flexible enough that applied forces The determination oftheapplicablelevelforlife-safety are resisted in significant part by inertial forces. result is a The should be based on the following descriptions:COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 21. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 9 1.7.1.aL-1Manned-Nonevacuated event. All wells that could flow on their own in the event of platformfailuremustcontainfullyfunctional,subsurface The manned-nonevacuated category refers to a platform safety valves, which are manufactured and tested in accor- that is continuously occupiedby persons accommodated and dance with the applicable API specifications. Oil storage is living thereon, and personnel evacuation prior to the design limited to process inventory and “surge” tanks for pipeline environmental event is either not intended or impractical. transfer. 1.7.1.b L-2 Manned-Evacuated 1.7.2.cL-3Low Consequence The manned-evacuated category refers to a platform that is normally manned except during a forecast design environmen- The low consequence of failure category refers to minimal tal event. For categorization purposes, a platform should be platformswhereproductionwould be shut-induringthe classified as a manned-evacuated platform if, prior to a design design event. All wells that could flow on their own in the environmental event, evacuation is planned and sufficient time event of platform failure must contain fully functional, sub- exists to safely evacuate all personnel from the platform. surface safety valves, which are manufactured and tested in accordancewithapplicableAPIspecifications.Theseplat- 1.7.1.cL-3Unmanned forms may support production departing from the platform and low volume infield pipelines. Oil storage is limited to pro- The unmanned category refers to a platform that is not nor-cess inventory. New Gulf of Mexico platforms in this cat- U.S. mally manned, or a platform that is not classified as either egory include caissons and small well protectors with no more manned-nonevacuated or manned-evacuated. Platformsin than five well completions either located on or connected to this classification may include emergency shelters. However, the platform and with no more than two pieces production of platforms permanent with quarters are notdefined as equipment. In addition, platforms in this category are defined unmanned and should be classified manned-nonevacuated as as structures in water depths not exceeding 100 feet. or as manned-evacuated as defined above. An occasionally manned platform could be categorized as unmanned in cer- 1.8PLATFORM REUSE tain conditions (see Commentary C1.7.1~). Existing platforms may be removed and relocated for con- 1.7.2 Consequence of Failure tinued use at a new site. When this is to be considered, the platform should be inspected to ensure that is in (or can be it As stated above, consequences of failure should include returned to) an acceptable condition. In addition,it should be consideration of anticipatedlossestotheowner,theother reanalyzed and reevaluated for the use, conditions, and load- operators, and the industry in general. following descrip- The ing anticipated at the new site. In general, this inspection, tions of relevant factors serve as a basis for determining the reevaluation, and any required repairs or modification should appropriate level for consequence failure. of follow the procedures and provisions for new platforms that are stated in this recommended practice. Additional special 1.7.2.a L-1 High Consequence provisions regarding reuse listed in Section 15. are The high consequence of failure category refers to major platforms and/or those platforms that have the potential for 1.9PLATFORM ASSESSMENT well flow ofeither oil or sour gas in the event of platform fail- An assessment to determine fitness for purpose may be ure. In addition, it includes platforms where the shut-in theof required during the life of a platform. This procedure is nor- oil or sourgas production is not planned, not practical prior or mally initiated by a change in the platform usage such as to the occurrence the design event (such areas with high of as revised manning or loading, modifications to the condition by seismic activity). Platforms that support major oil transport of theplatformsuch as damage or deterioration, or by a lines(seeCommentary C l .7.2-Pipelines)and/orstorage reevaluation of the environmental loading or the strength of facilities for intermittent oil shipment are also considered to thefoundation.Generalindustrypracticesrecognizethat be in the high consequence category. All new U.S. Gulf of older, existing structures may not meet current design stan- Mexico platforms which are designed for installation in water dards.However, many these of platforms are an that in depthsgreaterthan 400 feet are includedinthiscategory acceptable condition can be shownbe structurally adequate to unless a lower consequence failure canbe demonstrated to of using a risk-based assessment criteria that considers platform justify a reduced classification. use, location, and the consequences failure. of For platforms which were designed in accordance with the 1.7.2.b L-2 Medium Consequence provisions of the 20th and earlier editions, as well as plat- The medium consequence of failure category refers to plat- forms designed prior to the first edition of this publication, forms where production would be shut-in during the design recommendations regarding the developmentof reduced cri-COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 22. 10 2A-WSD PRACTICE RECOMMENDED API teria for assessment considering life-safety and consequences fabrication, installation, operation, and removal offshore of of failure as well as for assessment procedures are included in structures and related appurtenances. Section 17. These fitness for purpose provisions shall not be 4. 29 Code o Federal Regulations Part 1910, Occupational f used to circumvent normaldesignpracticerequirements Safety and Health Act 1970. This act specifies require- of when designing new platforms. The reduced environmental ments for safe design floors, handrails, stairways, ladders, of criteria as defined in Section 17 should not be utilized to jus- etc. Some of its requirements may apply to components of tify modifications or additions to a platform that will result in offshore structures that located in state waters. are an increased loading on the structure for platforms that have been in service less than five years. 5. 33 Code of Federal Regulations Part 330, “Permits for Assessment of platforms designed in accordance with pro- Work in Navigable Waters,”U.S. Corps of Engineers. Nation- visions of the 21st Edition and later editionsof this publica- wide permits describes requirements for making application tionshouldbeperformedusingtheenvironmentalcriteria for permits for work (for example, platform installation) in originally used for the design, unless a special study canjus- navigable waters. Section of the River and Harbor Act 10 of tify a reduction in Exposure Category defined in Section 1. as 1899 and Section of the Clean Water Act apply state 404 to waters. 1.10 SAFETY CONSIDERATIONS 6. Obstruction Marking and Lighting, Federal Aviation The safety of life and property depends upon the ability of Administration. This booklet sets forth requirements for the structure to support the loads for which it was designed, marking towers, poles, and similar obstructions. Platforms and to survive the environmental conditions that may occur. with derricks, antennae, etc., governed by the rules set are Over and above this overall concept, good practice dictates forth in this booklet. Additional guidance is provided by API use of certain structural additions, equipment and operating 2L, Recommended Practice Recommended Practice Plan- for procedures on a platformso that injuries to personnel will be ning, Designing,and Constructing Heliports Fixed Off- for minimized and the risk of fire, blast and accidental loading shore Platforms. (for example,collision ships, from dropped objects) is 7. Various state and local agencies (for example, U.S. reduced. Governmental regulations listed in Section 1.1 1 and Department of Wildlife and Fisheries) require notification of all other applicable regulations should be met. any operations that may take place under jurisdiction. their 1.1 1 REGULATIONS Other regulations concerning offshore pipelines, facilities, drillingoperations,etc.,maybeapplicableandshouldbe Each country has own set of regulations concerning off- its consulted. shore operations. Listed below are some of the typical rules and regulations that, applicable, should be considered when 2 Design Criteria and Procedures if designing and installing offshore platforms in U.S. territorial waters.Otherregulationsmayalsobeineffect. It isthe 2.1 GENERAL responsibility of the operator to determine which rules and 2.1.1 Dimensional System regulations are applicable and should be followed, depending upon the location and type of operations be conducted. to All drawings, calculations, etc., should consistent inone be dimensional system, such as the English dimensional system 1. 33 Code o Federal Regulations Chapter N, Parts 140 to f or the SI metric system. U.S. 147, “Outer Continental Shelf Activities,” Coast Guard, Department of Transportation. These regulations stipulate 2.1.2 Definition of Loads requirements for identification marks for platforms, means of escape, guard rails, fire extinguishers, preservers, ring life 2.1.2.a General buoys, first aid kits, etc. The followingloadsandanydynamiceffectsresulting 2. 33 Code o Federal Regulations Part 67, “Aids to Naviga- f from them should be considered in the development of the U.S. tion on Artificial Islands and Fixed Structures,” Coast design loading conditions in 2.2.1. Guard, Departmentof Transportation. These regulations pre- scribe in detail the requirements for installation of lights and 2.1.2.b Dead Loads foghorns on offshore structures in various zones. Dead loads are the weights of the platform structure and 3. 30 Code of Federal Regulations Part 250, Minerals Man- any permanent equipment and appurtenant structures which agement Service (formerly U.S. Geological Service), OCS do not change with the mode operation. Dead loads should of Regulations. These regulations govern the marking, design, include the following:COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 23. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 11 1. Weight of the platform structure in air, including where ing to impact. Excitation of a platform may be caused by appropriate the weight piles, grout and ballast. of waves, wind, earthquake or machinery. Impact may be caused 2. Weight of equipment and appurtenant structures perma- by a barge or boat berthing against the platform or by drilling nently mounted on the platform. operations. 3. Hydrostatic forces acting on the structure below the water- line including external pressure and buoyancy. 2.2 LOADING CONDITIONS 2.2.1 General 2.1.2.c Live Loads Live loads are the loads imposed on the platform during its Designenvironmentalloadconditions are thoseforces use and which may change either during a mode operation of imposed the on platforms the by selected designevent; or from one mode of operation to another. Live loads should whereas, operating environmental load conditions are those include the following: forces imposed on the structure a lesser event which is not by severe enough to restrict normal operations, as specified by 1. The weight of drilling and production equipment which the operator. can be added or removed from the platform. 2. The weight of living quarters, heliport and other life sup- 2.2.2 Design Loading Conditions port equipment, life saving equipment, diving equipment and utilities equipment which can be added or removed from the The platform should be designed for the appropriate load- platform. ing conditions which will produce the most severe effects on 3. The weight of consumable supplies and liquids in storage the structure. The loading conditions should include environ- tanks. mental conditions combined with appropriate dead and live 4. The forces exerted on the structure from operations such loads in the following manner. as drilling, material handling, vessel mooring and helicopter1. Operating environmental conditions combined with dead loadings. loads and maximum live loads appropriate to normal opera- 5. The forces exerted on the structure from deck crane usage.tions of the platform. These forcesare derived from consideration of the suspended 2. Operating environmental conditions combined with dead load and its movement well as dead load. as loads and minimum live loads appropriate normal oper- to the ations of the platform. 2.1.2.d Environmental Loads 3. Designenvironmentalconditionswithdeadloadsand Environmental loads are loads imposed on the platform by maximum live loads appropriate for combining with extreme naturalphenomena including wind,current,wave,earth- conditions. quake, snow, ice and earth movement. Environmental loads 4. Designenvironmentalconditionswithdeadloadsand also include the variation in hydrostatic pressure and buoy- minimum live loads appropriate for combining with extreme ancy on members caused by changes in the water level to due conditions. waves and tides. Environmental loads should be anticipated Environmentalloads,withtheexception of earthquake from any direction unless knowledge of specific conditions load, should be combined in a manner consistent with the makes a different assumption more reasonable. probability of their simultaneous occurrence during the load- ing condition beingconsidered. Earthquake where load, 2.1.2.e Construction Loads applicable, should be imposed on the platform as a separate Loadsresultingfromfabrication,loadout,transportation environmental loading condition. and installation should be considered in design and are fur- The operating environmental conditions should be repre- ther defined in Section 2.4. sentative of moderatelysevereconditions at theplatform. They should not necessarily be limiting conditions which, if 2.1.2.f Removal and Reinstallation Loads exceeded, require the cessationof platform operations. Typi- cally, a l-year to 5-year winter storm is usedas an operating For platforms which are to relocated to new sites, loads be condition in the Gulf Mexico. of resulting from removal, onloading, transportation, upgrading Maximum live loads for drilling and production platforms andreinstallationshould be consideredinadditiontothe shouldconsiderdrilling,productionandworkovermode above construction loads. loadings,andanyappropriatecombinations of drilling or workover operations with production. 2.1.2.g Dynamic Loads Variations in supply weights and the locations of movable Dynamic loads are the loads imposed on the platform due equipment such as a drilling derrick should be considered to to response to an excitation a cyclic natureor due to react- of maximize design stress in the platform members.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 24. 12 2A-WSD PRACTICE RECOMMENDED API 2.2.3Temporary Loading Conditions The effective local current profile is combined vectori- ally with the wave kinematics to determine locally inci- Temporaryloadingconditionsoccurringduringfabrica- dent velocities accelerations fluid and for use in tion, transportation, installation or removal and reinstallation Morison’s equation. of the structure should be considered. For these conditions a Member dimensions are increasedaccount to for combination of the appropriate dead loads, maximum tempo- marine growth. rary loads, and the appropriate environmental loads should be considered. Drag and inertia force coefficients are determined as functions of wave and current parameters; and member shape, roughness (marine growth), size, and orientation. 2.2.4 Member Loadings Wave force coefficientsfortheconductorarray are Each platform member should be designed for the loading reduced by the conductor shielding factor. condition which produces the maximum stress in the mem- Hydrodynamic models for risers and appurtenances are ber,takingintoconsiderationtheallowablestressforthe developed. loading condition producing this stress. Local wave/current forces are calculated for all plat- form members, conductors, risers, and appurtenances 2.3 DESIGN LOADS using Morison’s equation. 2.3.1 Waves The global force is computed as the vector sum of all the local forces. 2.3.1.a General The discussion in the remainder of this section is in the The wave loads on a platform are dynamic in nature. For same order as the steps listed above. There is also some dis- most design water depths presently encountered, these loads cussion on local forces (such as slam and lift) that are not may be adequately representedby their static equivalents. For included in the global force. deeper waters or where platforms tendbe more flexible, the to 1. Apparent Wave Period. A current in the wave direction static analysis may not adequately describe the true dynamic tends to stretch the wave length, while an opposing current loads induced in the platform. Correct analysis of such plat- shortens it. For the simple case of a wave propagating on a forms requires a load analysis involving the dynamic action uniform in-line current, the apparent wave period seen anby of the structure. observer moving with the current can be estimated from Fig- ure 2.3.1-2, in which is the actual wave period (as seen by a T 2.3.1 .b Static Wave Analysis stationary observer). VI is the current component in the wave The sequence of steps in the calculation of deterministic direction, d, is storm water depth (including storm surge and tide), andg is the acceleration gravity. This figure provides of static design wave forces on a fixed platform (neglecting plat- estimates for d/gT2 > 0.01. For smaller values of d/gT2, the form dynamic response and distortion the incident wave by of equation (Tupp/q= 1 + V I G d can be used. While strictly the platform) is shown graphically in Figure 2.3.1- 1.The pro- applicable only to a current that is uniform over the full water cedure, for a given wave direction, begins with the specifica- depth, Figure 2.3.1-2 provides acceptable estimates of Tupp tion of the design wave height and associated wave period, for “slab” current profiles that are uniform over the top 165 ft storm water depth, and current profile. Values of these param- (50m) or more of the water column. For other current pro- eters for U.S. waters are specified in 2.3.4. The wave force files, a system of simultaneous nonlinear equations must be calculation procedure follows these steps: solved interactively to determine (see Commentary).The Tupp An apparent wave period is determined, accounting for current used to determine Tuppshould be the free-stream cur- the Doppler effect the current on the wave. of rent (not reduced structure blockage). by The two-dimensional wave kinematics are determined 2. Two-Dimensional Wave Kinematics. Fortheapparent from an appropriate wave theory for the specified wave wave period Tupp,specified wave height H , and storm water height, storm water depth, and apparent period. depth, d , two-dimensional regular wave kinematics can be calculated using the appropriate order of Stream Function The horizontalcomponents of wave-inducedparticle wave theory. In many cases, Stokes V wave theory will pro- velocities and accelerations are reduced by the wave duce acceptable accuracy.Figure2.3.1-3 Atkins (1990) kinematics factor, which accounts primarily for wave shows the regions of applicability of Stokes V and various directional spreading. orders of Stream Function solutions in the H/gTupp2,&gTupp2 The effectivelocalcurrentprofileisdetermined by plane. Other wave theories, such Extended Velocity Poten- as multiplying the specified current profile by the current tial and Chappelear, may be used if an appropriate order of blockage factor. solution is selected.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 25. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHORE PLATFORMS-WORKINGSTRESS DESIGN 13 Smooth 0.65 1.6 Wave Period 2-D Wave Theory - (Including a Doppler Effect) t Kinematics I I I Appurtenance Growth Current Hydrodynamic Models Global Forces W Figure 2.3.1 -1-Procedure for Calculation of Wave Plus Current Forces for Static Analysis Figure 2.3.1 -2-Doppler Shift Due to Steady CurrentCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 26. 14 PRACTICE RECOMMENDED API 2A-WSD 0.05 Deep Water Breaking Limit H/L = 0.1 4 0.02 0.01 0.005 Shallow Water Breaking Limit / / / H/d = 0.78 A 0.002 0.001 0.0005 0.0002 0.0001 Deep 0.00005 0.002 0.001 0.005 0.01 0.02 0.2 0.05 0.1 d - gTapp2 H/gTapp2: Dimensionless wave steepness d:Mean water depth d/gTapp2: Dimensionless relative depth Tapp:Waveperiod Wave H: height g: Acceleration of gravity H Breaking wave height , : V, Figure 2.3.1 -3-Regions of Applicability of Stream Function, Stokes and Linear Wave Theory (From Atkins, 1990; Modified by API Task Group Wave Force Commentary) onCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 27. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONS1 XUCTING FIXED AND OFFSHORE PLATFORMS-WORKING STRESS DESIGN 15 3. Wave Kinematics Factor. The two-dimensional regular blockage. Since the current profile is specified only to storm wave kinematics from Stream FunctionStokes V wave the- or mean water level in the design criteria, some way to stretch ory do not account for wave directional spreading or (or compress) it to the local wave surface must be used. As irregularity in wave profile shape. These “real world” wave discussed in the Commentary, “nonlinear stretching” is the characteristics can be approximately modeled in determinis- preferredmethod.For slab current profilessuch as those tic waveanalyses by multiplying the horizontal velocities and specified forU.S. waters in 2.3.4, simple vertical extensionof accelerations from the two-dimensional regular wave solution the current profile from storm mean wear level to the wave by a wave kinematics factor. Wave kinematics measurements surface is a good approximation to nonlinear stretching. For support a factor in the range 0.85 to 0.95 for tropical storms other current profiles, linearstretching is an acceptable and 0.95 to 1.00 for extra-tropical storms. Particular values approximation.In linear stretching,the current at a point with within these ranges that should be usedfor calculating guide- elevation z, abovewhich the wavesurfaceelevation is q line wave forces are specified for the Gulf of Mexicoin (where z and q are both positive above storm mean water 2.3.4d.1 and for other U.S. waters in 2.3.4f.1. The Commen- level and negative below), is computed from the specified tary provides additional guidance for calculating the wave current profile at elevation z’ The elevations z and z’ are lin- kinematics factor for particular sea states whose directional early related,as follows: spreading characteristics are known from measurements or hindcasts. (z’ + 4 = (z + 4 d/(d + q) 4. Current Blockage Factor. The current speed in the vicin- where ity of the platformis reduced fromthe specified “free stream” value by blockage.In other words, the presence of the struc- d = storm water depth. ture causes the incident flow to diverge; some of the incident flow goes around the structure rather than through it, andthe 6. Marine Growth. All structural members, conductors, ris- current speed within the structure is reduced. Since global ers, and appurtenances should be increased in cross-sectional platform loads are determined by summing local loads from area to account for marine growth thickness. Also, elements Morison’s equation, the appropriate local current speed with circular cross-sectionsshould be classified as either should beused. “smooth” or “rough” depending on the amount of marine growth expected to have accumulated them at the time of on Approximate current blockage factors for typical Gulf of the loading event. Specific marine growth profiles are pro- Mexico jacket-type structures are as follows: vided forU.S. waters in 2.3.4. # of Legs Heading Factor 7. Drag and Inertia Coefficients. Drag and inertia coeffi- 3 all 0.90 cients are discussed in detail in the Commentary. For typical 4 end-on 0.80 design situations, global platform wave forces can be calcu- diagonal 0.85 lated using the following values for unshielded circular broadside 0.80 cylinders: 6 end-on 0.75 smooth c d = 0.65, c, = 1.6 diagonal 0.85 broadside 0.80 8 end-on 0.70 diagonal 0.85 These values are appropriate for the case of a steady cur- broadside 0.80 rent with negligible waves or the case of large waves with U,, TupJD > 30. Here,U,, is the maximum horizontal parti- cle velocity at storm mean water level under the wave crest For structures with other configurations or structures with a from the two-dimensional wave kinematics theory, is the Tupp typical number of conductors, a current blockage factor can apparent wave period,and D is platform leg diameter storm at be calculated with the method described in the Commentary. mean water level. Calculated factors less than 0.7 should not be used without empiricalevidence support to them. freestanding For or For wave-dominant cases with U,, Tup/D < 30, guidance braced caissonsthe current blockage factor should be.O. 1 on how c d and C, for nearly vertical members are modified by “wake encounter” is provided in the Commentary. Such 5. Combined WavdCurrent Kinematics. Wave kinematics, situations may arise with large-diameter caissons in extreme adjusted for directional spreading and irregularity, should be seas or ordinary platform members in lower states consid- sea combined vectorially with the current profile, adjusted for ered in fatigue analyses.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 28. 16 2A-WSD PRACTICE RECOMMENDED API For members that are not circular cylinders, appropriate can significantly increase the global wave forces. addition, In coefficients canbe found inDet norske Veritas’ “Rules for the forcesonsomeappurtenancesmaybeimportantforlocal Design, Construction, and Inspectionof Offshore Structures; memberdesign.Appurtenances are generallymodeledby Appendix B-Loads,’’ 1977. non-structuralmemberswhichcontributeequivalentwave forces. For appurtenances suchas boat landings, wave forces 8. Conductor Shielding Factor. Depending upon the con- are highly dependent on wave direction because shielding of figuration of the structure and the number well conductors, of effects.Additionalguidanceonthemodeling of appurte- the wave forces on the conductors can a significant portion be nances is provided in the Commentary. of the total wave forces. If the conductors closely spaced, are the forces on them may be reduced due tohydrodynamic 10. Morison Equation. The computation of force the shielding. A wave force reduction factor, tobe applied to the exerted by waves on a cylindrical object depends on the ratio drag and inertia coefficients for the conductor array, can be of the wavelength to the member diameter. When this ratio is estimated from Figure 2.3.1-4, in which S is the center-to- large (>5), the member does not significantly modify the inci- center spacing of the conductors in the wave direction and D dent wave. The wave force can then be computedas the sum is the diameter of the conductors, including marine growth. of a dragforce and an inertia force, follows: as This shielding factor is appropriate for either (a) steady cur- rent with negligible waves or (b) extreme waves, with U,, W W 6U Tap# > 57c.For less extreme waves with TapJS < 57c,as U,, F = F , + F , = CD - A U l U l + C,,, - V - (2.3.1-1) 2g ¿? in fatigue analyses, there may be less shielding. The Com- mentary providessome guidance conductor on shielding where factors for fatigue analyses. F = hydrodynamic force vector per unit length acting 9. HydrodynamicModelsforAppurtenances. Appurte- normal to the axis the member, lb/ft (N/m), of nances such as boat landings, fendersor bumpers, walkways, stairways, grout lines, and anodes should be considered for F 0 = drag force vector per unit length acting axis to the inclusionthe in hydrodynamic model of thestructure. of the member in the plane the member axis of Depending upon the type and number appurtenances, they of and U, lb/ft (N/m), 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 Figure 2.3.1 -4-Shielding Factor for Wave Loads on Conductor Arrays as a Function of Conductor SpacingCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 29. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONS1 XUCTING FIXED AND OFFSHORE PLATFORMS-WORKING STRESS DESIGN 17 FI = inertia force vector per unit length acting normal forces (see 2.3.lc), and (c) wind forces on the exposed por- to the axis the member in the plane of the of tions of the structure (see 2.3.2). Slam forces be neglected can member axis andaU/at, lb/ft (N/m), because they are nearly vertical. Lift forces can be neglected for jacket-type structures because they are not correlated from cd = dragcoefficient, member to member. Axial Froude-Krylov forces can also be W = weight density of water, lb/ft3 (N/m3), neglected. The wave crest should be positioned relative to the structure so that the total base shear and overturning moment g = gravitational acceleration, ft/sec2 (m/sec2), have their maximum values. It should be kept in mind that: (a) maximum base shear may not occur at the same wave A = projected area normal to the cylinder axis per unit position as maximumoverturningmoment;(b)inspecial length (= D for circular cylinders), (m), ft cases of waves with low steepness and an opposing current, V = displaced volume of the cylinder per unit length maximum global structure force may occur near the wave (= 7cD2/4 for circular cylinders), (m2), ft2 troughratherthannearthewavecrest;and(c)maximum local member stresses may occur for a wave position other D = effective diameter of circular cylindrical member than that causing the maximum global structure force. including marine growth, (m), ft 12. Local Member Design. Local member stresses are due U = component of the velocity vector (due to wave to both local hydrodynamic forces and loads transferred from and/or current)of the water normalto the axisof the rest of the structure.Locally generatedforces include not the member, ft/sec (m/sec), only the drag and inertia forces modeled Morison’s equa- by I U I = absolute value of U, ft/sec (m/sec), tion (Eq. 2.3.1-l), but also lift forces, axial Froude-Krylov forces, and buoyancy and weight. Horizontal members near C, = inertiacoefficient, stormmeanwaterlevelwillalsoexperienceverticalslam - = component of the local acceleration vector of the forces as a wave passes. Both and slam forces can dynam- lift water normalto the axisof the member, ft/sec2 ically excite individual members, thereby increasing stresses (m/sec2). (see Commentary). Transferred loads are due to the global fluid-dynamicforcesanddynamicresponse of theentire Note that the Morison equation,as stated here, ignores the structure. The fraction of total stress due to locally generated convective acceleration component in the inertia force calcu- forces is generally greater for members higher in the struc- lation(seeCommentary). It alsoignores lift forces,slam ture;therefore,local lift andslamforcesmayneedtobe forces, and axial Froude-Krylov forces. considered in designing these members. The maximum local Whenthesize of a structural body or member is suffi- member stresses may occur a different position the wave at of ciently large to span a significant portiona wavelength, the of crest relative to the structure centerline than that which causes incident waves are scattered, or diffracted. This diffraction the greatest global waveforce on the platform. For example, regimeisusuallyconsideredtooccurwhenthemember some members of conductor guide frames may experience width exceeds a fifthof the incident wave length. Diffraction their greatest stresses due to vertical drag and inertia forces, theory, which computes the pressure acting on the structure which generally peak when the wave crest is far away from due to both the incident wave and the scattered wave, should the structure centerline. be used, instead of the Morison equation, to determine the wave forces. Depending on their diameters, caissons may be 2.3.1 .c Dynamic Wave Analysis in the diffraction regime, particularly for the lower sea states associated fatigue with conditions. Diffractiontheory is 1. General. A dynamic analysis of a fixed platform is indi- reviewed in “Mechanics of Wave Forces on Offshore Struc- catedwhenthedesignseastatecontainssignificantwave tures” by T. Sarpkaya and M. Isaacson, Van Nostrand Rein- energy at frequencies near the platform’s natural frequencies. hold Co., 1981. A solution of the linear diffraction problem The wave energy content versus frequency can be described for a vertical cylinder extending from the sea bottom through by wave (energy) spectra as determined from measured data the free surface (caisson) can be found in “Wave Forces on orpredictionsappropriatefortheplatformsite.Dynamic Piles: A Diffraction Theory,” by R. C. MacCamy and R. A. analyses should be performed for guyed towers and tension Fuchs, U.S. Army Corpsof Engineers, Beach Erosion Board, leg platforms. Tech. Memo No. 69, 1954. 2. Waves. Use of a random linear wave theory with modified 11. Global Structure Forces. Total bas shear and overtum- crest kinematics is appropriate for dynamic analysisof fixed ing moment are calculated by a vector summation of (a) local platforms. Wave spreading (three-dimensionality) should be drag inertia and forces due to waves currents and (see considered. Wave groupeffectsmayalsocauseimportant 2.3.lb20), (b) dynamicamplification ofwaveandcurrent dynamic responses in compliant structures.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 30. 18 2A-WSD PRACTICE RECOMMENDED API 3. Currents. Currents associated with the design sea state includegeometric stiffness displacement (large effects). can affect dynamic loading through the nonlinear drag force Forcesaffectinggeometricstiffnessincludegravityloads, term in Morison’s Equation 2.3.1-1, and therefore should be buoyancy, the vertical component of the guyline system reac- considered in dynamic analysis. tion, and the weight conductors including their contents. of 4. Winds. For analysis of template, tower, gravity, or mini- 7. AnalysisMethods. Timehistorymethods of dynamic mum platforms, global loads due to the sustained wind may analysis preferred predicting extreme are for the wave be superimposed on the global wave and current load. response of templateplatforms,minimumstructures,and For guyed towers and tension leg platforms, the analysis guyedtowersbecausethesestructures are generallydrag should include the simultaneous action of wind, waves, and forcedominated. The nonlinearsystemstiffnessalsoindi- current. It may be appropriate to consider wind dynamics. catestimedomainanalysisforguyedtowers.Frequency domain methods may be used for extreme wave response 5. Fluid Forceon a Member. Equation 2.3.1-1 may be used analysis to calculate the dynamic amplification factor to com- to compute forces on members template, tower, gravity,or of bine with the static load, provided linearization of the drag minimum structure platforms. Guidance on selectionof drag force can be justified; for guyed towers, both the drag force and inertia coefficients for dynamic analysis is provided in and non-linear guyline stiffness would require linearization. the Commentary on Wave Forces, C2.3.lb7. For guyed tow- Frequencydomainmethodsaregenerallyappropriatefor ers andtensionlegplatforms,Equation2.3.1-1shouldbe small wave fatigue analysis. modified to account for relative velocity by making the fol- Formember design, stresses may bedeterminedfrom lowing substitution in the drag force term: static analyses which include in an appropriate manner the significant effects of dynamicresponsedeterminedfrom replace UIUI by (U- i ) l U - iI separate analyses made according to the provisions of this Section. where 2.3.2 Wind i = component of structural velocity normal to the axis of the member, ft/sec( d s ) , 2.3.2.a General U = as defined for Equation 2.3.1- 1. The windcriteriafordesignshouldbedeterminedby proper analysis of wind data collected in accordance with Fluid forces associated with the platform acceleration are 1.3.2. As with wave loads, wind loads dynamic in nature, are accounted forby added mass. but some structures will respond to them in a nearly static fashion. For conventional fixed steel templates in relatively 6. Structural Modeling. The dynamic model of fixed plat- shallow water, winds are a minor contributor to global loads forms should reflect the key analytical parameters of mass, (typically less than 10 percent). Sustained wind speeds should damping, and stiffness. The mass should include that of the be used to compute global platform wind loads, and gust platform steel, all appurtenances, conductors, and deck loads, speeds should be used for the designof individual structural the mass of water enclosed in submerged tubular members, elements. the mass of marine growth expected to accumulate on the In deeper water and for compliant designs, wind loads can structure the and added mass of submerged members, besignificantandshouldbestudiedindetail. A dynamic accountingforincreasedmemberdiameter due tomarine analysis of the platform is indicated when the wind field con- growth. tains energyat frequencies near the natural frequencies theof Equivalent viscous damping values may used in lieu of be platform. Such analyses may require knowledge of the wind anexplicitdetermination of dampingcomponents.Inthe turbulenceintensity,spectra,andspatialcoherence.These absence of substantiating information for damping values for items are addressed below. a specific structure, a damping value of two to three percent of critical for extreme wave analyses and two percent of criti- 2.3.2.b Wind Properties cal for fatigue analyses may used. be The analytical model should include the elastic stiffness of Wind speed and direction vary in space and time. On length the platform and reflect the structure/foundation interaction. scalestypical of even largeoffshorestructures,statistical It maybeappropriate to considerastifferfoundationfor wind properties (e.g., mean and standard deviation of speed) I fatigue analyses than for extreme wave response analyses. taken over durations of the order an hour do not vary hori- of For guyed towers, these stiffnesses should be augmented to zontally, but no change with elevation (profile factor). Within account for the guyline system. Analysis procedures may be longdurations,therewillbeshorterdurationswithhigher required that account for the dynamic interactionthe tower of mean speeds (gusts factor). Therefore, a wind speed value is and guyline system. Guyed tower analytical models should onlymeaningful if qualified by its elevationandduration. ICOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 31. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 19 1. Wind profiles and Gusts. For strong wind conditions the for the maximum total static wind load on larger structures. design wind speedU (z,t ) (ft/s) at height z (ft) above sea level The one minute sustained wind is appropriate for total static and corresponding to an averaging time period [where t I t(s) superstructurewindloadsassociatedwithmaximumwave to; to = 3600 sec] is given by: forces for structures that respond dynamically to wind excita- tion but which do not require a full dynamic wind analysis. uz t ) = Uz (, () x [l - 0.41 x Z) x Zn(; )] (2.3.2-1) ,( z For structures with negligible dynamic response winds, the to o one-hour sustained wind is appropriate for total static super- structure wind forces associated with maximum wave forces. where the 1 hour mean wind speed() (ft/s) at level z (ft) is Uz In frequency domain analysesof dynamic wind loading,it given by: can be conservatively assumed that all scales of turbulence are fully coherent over the entire superstructure. For dynamic U Z = U , x 11 + C x z ~ ( & ) I (> (2.3.2-2) analysis of some substructures, it may beneficial be to account for the less-than-full coherent at higher frequencies. C = 5.73 x x (1 + 0.0457 x U) ,12 The squared correlation between the spectral energy densities of the longitudinal wind speed fluctuations of frequency f and where the turbulence intensity at level z is given by: Zz ,) ( between 2 points in space is described in termsthe 2 point of coherence spectrum. Zz ,) ( = 0.06 x [l + 0.0131 x U,] x (&)-o.22 (2.3.2-3) The recommended coherence spectrum between 2 points at levels z and z2above the sea surface, 1 where U, (ft/s) is the 1 hour mean wind speed 32.8 ft. at with across-wind positions y1 and y2, 2. Wind Spectra. For structures and structural elements for with along-wind positions x1 and x2. which the dynamic wind behavior is of importance, the fol- is given by lowing 1 point wind spectrum may be used for the energy density of the longitudinal wind speed fluctuations. 320~ - X - (2.3.2-6) S(f> = ( 2 3 ;1. 8 r (8 (2.3.2-4) (1 +r)% 7= 172 X (sr5 (ir3 (32.8) X 32.8 (2.3.2-5) where Ai=aIxf y. xA; 4; XZ, -P; (2.3.2-7) where 1/2 n = 0.468, z = (z1- , z2) 32.8 S m (ft2/s2/Hz) = spectral energy density frequencyf at (Hz), and where the coefficients a, , q, r and the distances A are p z (ft) = height above sea level, given below: U , (ft/s) = 1 hour mean wind speed 32.8 ft above at sea level. 1 -x11 1x2 1.00 0.4 0.92 2.9 3. Spatial Coherence. Wind gusts have three dimensional 2 ly2-yll 1.00 0.4 0.92 45.0 spatial scales related to their durations. For example, 3 second gusts are coherent over shorter distances and therefore affect 3 Iz2 -zll 1.25 0.5 0.85 9.66 similar elements of a platform superstructure than 15 second gusts. The wind in a 3 second gust is appropriate for deter- 2.3.2.c Wind Speed and Force Relationship miningmaximum wind on the static load individual members; 5 second gusts are appropriate for maximum total The wind drag force on an object should be calculated as: loads on structures whose maximum horizontal dimension is (50 less than 164 feet m); and 15 second gusts are appropriate F = (p/2)u2 C d (2.3.2-8)COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 32. 20 2.3.3 Current 2.3.3.a General p = mass density of air, (slug/ft3, 0.0023668 slugs/ft3 As described in 1.3.5, the total current is the vector sum of for standard temperatureand pressure), the tidal, circulational, and storm-generated currents. The rel- ative magnitude of these components, and thus their impor- p = wind speed (ft/s), tance for computing loads, varies with offshore location. C, = shape coefficient, Tidal currents are generally weak in deep water past the shelf break. They are generally stronger on broad continen- A = area of object (ft2). tal shelves than on steep shelves, but rarely exceed 1 ft/s (0.3 m / s ) along any open coastline. Tidal currents can be 2.3.2.d Local Wind Force Considerations strengthened by shoreline or bottom configurations such For all angles of wind approach to the structure,forces on that strong tidal currents can exist in many inlet and coastal flat surfaces should be assumed to act normal to the surface regions; e.g., surface values of about 10 ft/s (3 m / s ) can and forces on verticalcylindricaltanks,pipes, and other occur in Cook Inlet. cylindrical objects should be assumed to act in the direction Circulational currents are relatively steady,large scale fea- of thewind.Forces on cylindricaltanks,pipes, and other tures of the general oceanic circulation. Examples include the cylindrical objects which are not in a vertical attitude should Gulf Stream in the Atlantic Ocean and the Loop Current in be calculated using appropriate formulas that take into the Gulf of Mexico where surface velocities can be in the account the direction of the wind in relation to theattitude of range of about 3 4 ft/s (1-2 m/s). While relatively steady, the object. Forces on sides buildings and other flat surfaces of thesecirculation features can meander and intermittently that are not perpendicular to the direction of the wind shall break off from the main circulation feature to become large also be calculated usingappropriate formulas that accountfor scale eddies or rings which then drift a few miles per day. the skewness betweenthe direction of the windand the plane Velocities in such eddies or rings can approach that of the of the surface. Where applicable, local wind effects such as main circulation feature. These circulation features and asso- pressure concentrations andinternal pressures should be con- ciate eddies occur in deep water beyond the shelf break and sidered by the designer. These local effects should be deter- generally do not affect sites with depths less than about 1000 mined using appropriate means such as the analytical ft (300 m). guidelines set forth in Section 6, ANSI A58.1-82; Building Storm generated currents caused by the wind stress are and Code Requirementsfor Minimum Design Loads in Buildings atmospheric pressure gradient throughout the storm. Current and Other Structures. speeds are acomplexfunction of the stormstrength and meteorological characteristics, bathymetry and shoreline con- 2.3.2.e Shape Coefficients figuration, and waterdensityprofile.In deep wateralong open coastlines, surface storm current can be roughly esti- In the absence of data indicating otherwise, the following mated to have speeds up to2-3 percent of the one-hour sus- shape coefficients (C,> are recommended for perpendicular tained wind speed during tropical storms and hurricanesand wind approachangles with respect to each projected area. up to 1% of the one-hour sustained wind speed during winter Beams ........................................................ 1.5 stormsorextratropicalcyclones.Asthestormapproaches Sides of buildings ...................................... 1.5 shallower waterand the coastline, the storm surge and current Cylindrical sections................................... 0.5 can increase. Overall projectedarea of platform............ 1.0 2.3.3.b Current Profile 2.3.2.f Shielding Coefficients A qualified oceanographer should determine the variation Shielding coefficients may used when, inthe judgment be of current speedanddirectionwithdepth. The profile of of the designer, the second object lies close enough behind storm-generated currents in upper layer of the ocean is the the the first to warrant use of the coefficient. the subject of active research. 2.3.2.g WindTunnelData 2.3.3.c Current Force Only I Wind pressures and resulting forces may be determined from wind tunnel tests on a representative model. Where current is acting alone (i.e., no waves) the drag force should be determined Equation 2.3.1-1 with dU/dt = O. byCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 33. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 21 2.3.3.d Current Associated with Waves forms designed by different practices. Historical experience, loading, and strength characteristics of these structures may Due consideration should be given to the possible super- be used for such evaluations. The provisions of this section position of currentandwaves. In thosecaseswherethis are intended to accommodate such considerations. actual The superpositionisnecessarythecurrentvelocityshouldbe platform experience and exposure and the amountdetailed of added vectorially to the wave particle velocity before the total oceanographic data available vary widely among the areas force is computed described in2.3.lb. Where thereis suffi- as shown in Figure 2.3.4-1. Gulf of Mexico is characterized The cient knowledge of wavekurrent joint probability, it may be by a substantial amount of experience, exposure, and data. used to advantage. For other areas, there less experience and data. amount is The of wave information available for the different areas is indi- 2.3.3.e Vortex-Induced-Vibration cated by the quality rating in Table 2.3.4- 1. The guidelines All slender members exposed to the current should be presented herein represent the best information available at investigated for the possibility of vibration due to periodic this time, andare subject to revision from time to timefur- as vortex shedding as discussed in the Commentary on Wave ther work is completed. Forces C2.3.lb12. 2.3.4.c Guideline Design Metocean Criteria for the 2.3.4 Hydrodynamic Force Guidelines for U.S. Gulf of Mexico North of 27" N Latitude and Waters West of 86"W Longitude 2.3.4.a General The Criteria is suitable for the design of new L-1 Struc- tures and are based on the 100-year wave height and associ- Designparametersforhydrodynamicloadingshouldbe ated variables that result from hurricanes. Additional criteria selected based on life safety and consequence of failure in the recommendation for the design new L-2 and L-3 structures of manner described in Section 1.5, using environmental data are also provided.The criteria are defined in terms of the fol- collected and presented as outlined in Section 1.3. This sec- lowing results: tion presents guideline design hydrodynamic force parame- ters which should be used if the special site specific studies Omnidirectional wave height vs. water depth. described in Sections1.3 and 1.5 are not performed. Principal direction associated with the omnidirectional wave height. 2.3.4.b Intent Wave height vs. direction. The provisions of this section provide for the analysis of Currents associated with the wave height by direction. static wave loads for platforms in the areas designated in Fig- ure 2.3.4-1. Depending upon the natural frequencies of the Associatedwaveperiod. platform and the predominant frequenciesof wave energy in Associatedstormtide. the area, it may be necessary to perform dynamic analyses. Associatedwindspeed. Further, the general wave conditions in certain of these areas are such that consideration fatigue loads may necessary. of be For locations affected by strong tidal and/or general circu- As described in Section 1.5, the selection of the environ- lation currents, such as the Loop current and its associated mental criteria should be based on risk considering safety life detached eddies, special metocean criteria need to defined be and consequences of failure. Using successful industry expe- to take into account the possibility large forces caused by a of rienceintheGulfofMexico,guidelinesforselectingthe combination of extreme currents and smaller (than the 100- hydrodynamic force criteria are recommended for the three year hurricane wave) waves. platform exposure categories determined by the definitions as The metocean criteria are intended tobe applied in combi- inSection1.7. The use of conditionsassociatedwiththe nation with other provisions of 2.3.4 to result in a guideline nominal100-yearreturnperiodarerecommendedforthe design levelof total base shear and overturning moment on a design of new L-1 platforms.Recommendationsarealso structure. included for the design newL-2 and L-3 platforms. of The criteria apply for Mean Lower Low Water (MLLW) Use of the guidelines should result in safe but not necessar-greater than 25ft and outside of barrier islands, except in the ily optimal structures. Platform owners may find jurisdiction immediate vicinity of the Mississippi Delta (denoted by the for designing structures for conditions more or less severe cross-hatched area in Figure 2.3.4-2). In this area the guide- than indicated by these guidelines. As discussed in Section lines may not apply because the Delta may block waves from 1.5 design criteria depend upon the overall loading, strength, some directions, and there are some very soft seafloor areas and exposure characteristics of the installed platform. The that may partially absorb waves. Wave heights lower than the guidelines should not be taken as a condemnation of plat- guideline values may be justified in these areas.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 34. 22 API RECOMMENDED PRACTICE 2A-WSD LOWER COOK INLET INSET I Inlet Inset United States See California Inset CALIFORNIA INSET San Clemente Figure 2.3.4-1-Area Location MapCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 35. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 23 Table 2.3.4-1-U.S. Gulf of Mexico Guideline Design MetoceanCriteria Parameter L-1 High Consequence L-2 Medium Consequence L-3 Low Consequence Wave height, ft 2.3.4-3 Figure Figure 2.3.4-3 Figure 2.3.4-32.3.4-4 Figure direction Wave Figure 2.3.4-4 Omnidirectionaligure direction Current Figure 2.3.4-5 Omnidirectional Storm tide, ft 2.3.4-7 Figure Figure 2.3.4-7 Figure 2.3.4-7 elevation, Deck ft 2.3.4-8Figure Figure 2.3.4-8 Figure 2.3.4-8 speed, Current 1.8 1.4 sec Wave period, 11.6 13.0 12.4 Wind speed (1-hr 6 10 m), 3 knots 80 70 58 1. OmnidirectionalWaveHeightvs.WaterDepth. The tion, the associated current inline withthe wave (there is MLLW for all three levels guideline omnidirectional wave vs. is no transverse component) and proportional to wave of platform exposure categories given in Figure 2.3.4-3. is height. The magnitudeassociatedwiththeprincipal wave direction (towards 290”) is given in Table 2.3.4-1. 2. PrincipalDirectionAssociatedwiththeOmnidirec- The magnitudesforotherdirections are obtained by tional Height. Wave The principal direction is 290” multiplying the surface current value by the same fac- (towards, clockwise from north) for L-1 and L-2 structures. tors thatare used to define wave heights by direction. For L-3 structures, the waves omnidirectional. are o Intermediate zone: This region is in between the shal- 3. Wave height vs. Direction.Wave heights for L- 1 and L-2 low and deep water zones, i.e., depth less than 300 ft. structures are defined for eight directions as shown in Figure and greater than 150 ft. The currents associated with 2.3.4-4. each wave direction a given water depth in this for zone The factors should be applied to the omnidirectional wave are obtained by linear interpolation of the currents for height of Figure 2.3.4-3 to obtain wave height by directionfor depths of 150 ft. and 300ft. For each wave direction the a given waterdepth. The factors are asymmetric with respect interpolation should bedone on both theinline and the to the principal direction, they apply for water depths greater transverse component. The end result will be an associ- than 40 ft., and to the given direction k22.5”. Regardless of ated current vector for each wavedirection. how the platform is oriented, the omnidirectional wave Before applying the current vector for force calcula- height, in the principal wavedirection, must be considered in tions in either the shallow water zone or the intermedi- at least one design load case. For L-3 and L-3 structures the ate zone, the component of the current that is in-line waves are omnidirectional. with the wave shouldbe checked to makesure that it is greater than 0.2 knots. If it is less, the in-line compo- 4. Currents Associated with the Wave Height by Direc- nent should be set to 0.2 knots for calculating design tion. The associated hurricane-generatedcurrent for the Gulf guideline forces. of Mexico depends primarily on water depth. The current profile is givenin Figure 2.3.4-6. The a. L-1 and L-2 Criteria storm water level (swl) is the O-ft. level. The profile for Shallow water zone: The water depth for this zone is shallower waterdepths should be developed by truncat- less than 150 ft. The extreme currents in thiszone flow ing the bottom part of the profile. from east towest and followsmoothedbathymetric To combine the wave kinematics with current above the contours. Consequently, combined the when with the swl, the current must be “stretched” up to the wave waves, the resulting base shears will vary with respect crest. See 2.3.1b.5 for “stretching” procedures. to geographical location. current magnitudes at the The surface are given in Table 2.3.4-1. The direction of the b. L-3 Criteria current (towards, clockwise from north)is given inFig- The surface current magnitude is given in Table 2.3.4-1. ure 2.3.4-5 vs.longitude. The current is to be taken inline with the wave. The same Deepwater zone: The waterdepth for this zone is magnitude is to be used for all directions. The profile is the greater than 300 ft. In this zone, for each wave direc- same as for L- 1 and L-2. COPYRIGHT American Petroleum Institute Licensed by Information Handling Services
  • 36. 24 PRACTICE RECOMMENDED API 2A-WSD I Provisions in Section I 2.3.4.c not valid ........... :Water depth, ft. 98 96 94 92 86 90 88 Longitude Figure 2.3.4-2-Region of Applicability of Extreme MetoceanCriteria in Section 2.3.4.C 70 . .. .. .. .. .. ... ... ... .. .. .. .. .. .. . .. . .. ,/ .. .. .. .. - . . . . .. .. .. .. / . . . . .. .. .. .. .. .. .. .. . . . . . . .. . . . . .. 7L . . 60 .. .. .. .. . . . . /" . . . . . . . . . .. .. .. . .. .. .. . .. .. .. . .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-2 (only for depths < 400 ft.) .. .. ... . . .. .. ... . . . .. .. ... . .. . .. .. ... ... ... . .. .. .. .. .. .. .. .. .. . . . . . . . . .. . . .. . . . . . . . . . . . . $ 50 / . / . / . / . . . . . . . . . . . . . .. .. .. .. . . . . . .. .. .. .. . . . . . . . . . . . . . . . . . . . ... ... ... ... . . . . . . .. . ... ... . .. o .. . . .. .. .. .. .. 40 . . . . . . . . . . . . . . . . .. .. .. .. .. .. .. .. . . . . . . . . . . . . .. .. .. .. ... ... ... ... .. .. .. .. .. .. .. .. . . . . . . . . . . . . .. . .. . . .. . . .. .. . . . . . . . . . . ... ... . .. .. .. .. .. .. .. .. .. . . . . . . . . .. . . .. .. .. .. .. .. .. .. .. . . . . . . . . .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 30 .. .. .. .. .. .. .. .. . . . . . . . . .. .. .. .. . . . . . . . . * For depths > 400 ft., . . . . .. .. .. .. . . . . . . . . the L-1 wave height .. .. .. .. . . . . .. .. .. .. .. .. .. .. . . . . . . . . increases linearly to . . . . . . . . . . . . . . . . . . . . . . . . . 70.5 ft. at 1,000 ft. . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 350 O 300 50250 1O0 200 150 400 MLLW, ft. Figure 2.3.4-3-Guideline Omnidirectional Design Wave Height vs. MLLW, Gulf of Mexico, North of 27" N and Westof 86" WCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 37. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 25 tN n Wave direction (towards, clockwise from N) Factor Figure 2.3.4-4"Guideline Design Wave Directions and Factors to Apply to the Omnidirectional Wave Heights (Figure 2.3.4-3) for L-1 and L-2Structures, Gulf of Mexico, North of 27" N and West of 86" W I I V 98 96 9490 92 86 88 W Longitude, deg Figure 2.3.4-54uideline Design Current Direction (Towards) with for L-1 and L-2 Structures, Gulf of Mexico, Respect to Northin Shallow Water (Depth North of 27"N andWest of 86"W <150 ft) ICOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 38. 26 Current, U SWI -200 -300 Elevation, ft. -600 Figure 2.3.4-6-Guideline Design Current Profile for L-1, L-2, and L-3 Structures, Gulf of Mexico, North of 27"N and West of 86"W 5. AssociatedWavePeriod. The wave period is given in 2.3.4.d Guideline Design Wave, Wind, and Current I Table2.3.4-1andappliesforallwaterdepthsandwave directions. Forces for the Gulf of Mexico, North of 27"N Latitude and West of 86"W Longitude 6. Associate Storm Tide. The associated storm tide (storm The guideline design forces for static analysis should be cal- surge plus astronomical tide) is given in Figure 2.3.4-7. culated using (a) the metocean criteria given in 2.3.4c, (b) the wave and current force calculation procedures given 2.3.lb,in I 7. Associated Wind Speed. The associate 1-hr. wind speed, as listed in Table 2.3.4-1, occurs an elevation of33 feet and at applies to all water depths and wave directions. use of the The (c) other applicable provisions of 2.3.1, 2.3.2, and 2.3.3, and (d) specific provisions in this section as given below. same speed for all directions is conservative; lower speeds for 1. WaveKinematicsFactor. The extremeforceswillbe directions away from the principal wave direction may be dominated by hurricanes and consequently a wave kinematics jus- tified by special studies. factor of 0.88 should be used. The associated wind speed is intended to be applicable for 2. Marine Growth. Use 1.5 inches from Mean Higher High the design ofnew structures where the wind force and/or Water (MHHW)to -150 ft. unless a smaller or larger value of overturningmomentislessthan 30%of thetotalapplied thickness is appropriate from site specific studies. MHHW is environmental load. Ifthetotalwindforceoroverturning one foot higher than MLLW. moment on the structure exceeds this amount, then the struc- Structural members can be considered hydrodynamically ture shall also be designed for the 1 minute wind speed con- smooth iftheyareeitheraboveMHHWordeepenough currently with a wave of 65% of the height of the design (lower than about -150 ft.) where marine growth might be wave, acting with the design tide and current. light enough to ignore the effect roughness. However, cau- of As an alternate, the use of wave and current information tion should be used because it takes very little roughness to likely to be associated with the 1 minute wind may bejusti- cause a c d of 1.05 (see Commentary, Section C2.3.lb.7 for fiedbysitespecificstudies.However,innocasecanthe relationship of c d to relative roughness). the zone between In resulting total force and/or overturning moment used for the MHHW and -150 ft. structural members should be consid- design of the platform be less than that calculated using the 1 eredto be hydrodynamically rough. Marine growthcan hour wind with the guideline wave, current and tide provided extend to elevations below -150ft. Site specific data may be in 2.3.4~. used to establish smooth and rough zones more precisely.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 39. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 27 12 10 8 6 4 2 L-2 (for depths < 400 ft.) depths < 1O0 ft.) . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. .. .. . , . . O . . . . . O 1 50 O0 350 300 250 MLLW. ft. Figure 2.3.4-7"Guideline Storm Tide vs. MLLW and Platform Category, Gulf Mexico, of North of 27"N and Westof 86"W 3. Elevation of Underside of Deck. Deck elevations for new nominal 100-year wave heights. Except as noted, the guide- platforms should satisfy all requirements of 2.3.48. For new line wavesandstormtidesareapplicabletowaterdepths L-1 and L-2 platforms, the elevation for the underside the of greater than 300feet. deck should not be lower than the height given in Figure The ranges of wave heights, currents, and storm tides in 2.3.4-8. Additional elevation should allowed to account for be Table 2.3.4.-2 reflect reasonable variations in interpretationof I structures which experience significant structural rotation or the data in the references cited in 2.3.4h, quality rating, and "set down". the spatial variability within the areas. The ranges in wave For new L-3 platforms, the deck may located below the be steepnessreflectthevariabilityinwaveperiodassociated calculated crest elevation of thewavedesignatedfor L-3 with a given wave height. Significant wave eight, H,, can be Structures. In this case, the full wave and current forces on determined from the relationship HJH, = 1.7 to 1.9. Spectral the deck must be considered. However, the deck is located if peak period, Tp, can be determined from the relationship TJ above thecrest elevation of theL-3 wave, then the deck must T,,, = 1.05 to 1.20. be located above the calculated crest elevation of the wave designated for the L-1 structures. Section C17.6.2 provides 2. Winds. Guideline wind speeds (one-hour average at 33 guidance for predicting the wavekurrent forces on the deck. feet elevation) are provided in Table The first column I gives the wind speed to use to compute global wind load to 2.3.4.e Guideline Design Metocean Criteria for combine with global wave and current load on a platform. Other U.S. Waters This wind is assumed to act simultaneously and co-direction- 1. Waves, Currents, and Storm Tides. Guideline omnidi- ally guideline with 100-yearextremewaves Table from rectional wave heights with a nominal return period of 100 The second column gives 100-year wind speeds with-I years are given in Table 2.3.4-2 for the 20 geographical areas outregard the to coexisting waveconditions: are these showninFigure2.3.4-1.Alsogiven are deepwaterwave appropriate for calculating local wind loads, per the provi- as steepnesses,currents,andstormtidesassociatedwiththe sions of 2.3.2.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 40. 28 10 1O0 1O00 MLLW, ft. I Figure 2.3.4-8“Elevation of Underside of Deck (Above MLLW)MLLW, vs. Gulf of Mexico, North of 27”N and Westof 86”W I 3. CurrentProfile. The currents, Ui,inTable2.3.4-2 are (b) the wave and current force calculation procedures given in near-surface values. For the GulfMexico the guideline cur- of 2.3.lb, (c)otherapplicableprovisions of 2.3.1,2.3.2,and rent profile given in Figure 2.3.4-6 should be used. Outside 2.3.3, and (d) specific provisions in this section as given below. the Gulf of Mexico there is no unique profile; site specific measured data should be used in defining the current profile. 1. Wave Kinematics Factor. Extreme wave forces for some In lieu of data, the current profile may be crudely approxi- of the areas in Table 2.3.4-2 are produced by hurricanes, for I matedbytheGulfof Mexico guideline current profile of some by extratropical storms, and for others both hurricane Figure 2.3.4-6 with U = U in the mixed layer, and U = U - i i and extratropical stormsare important. The appropriate wave 1.9 knots in the bottom layer. kinematics factor depends on the type of storm system that will govern design. 4. Local Site Effects. The “open shelf’ wave heights shown Areas # 1 and are dominated by hurricanes; a wave kine- #2 I in Table 2.3.4-2 are generalized to apply to open, broad, con- matics factor of 0.88 should be used. Areas #3 through #17 tinental shelf areas where such generalization is meaningful. are dominated by extratropical storms; the wave kinematics Coastalconfigurations, exposure wave to generation by factor should be taken 1.0, unless a lower factor can as be jus- severe storms, or bottom topography may cause variations in tified on the basis of reliable and applicable measured data. wave heights for different sites within an area; especially, the Areas #18 through #20 are impacted by both hurricanes Lower CookInlet, the Santa Barbara Channel, Norton Sound, and extratropical storms. The “open shelf’ wave heights in North Aleutian Shelf, Beaufort Sea, Chukchi Sea, and Geor- Table 2.3.4-2 for these three areas correspond the 100-year I to gia Embayment areas. Thus, wave heights which are greater return period values taking into consideration both storm pop- than or less than the guideline “openshelf’ wave heights may ulations. Consequently, the wave kinematics factor will be be appropriate for a particular site. Reasonable ranges for between 0.88 and 1.0. Based on the results on the relative I such locations are incorporated in Table 2.3.4-2. importance of hurricanes vs. extratropical storms in the paper “Extreme Wave HeightsAlongtheAtlanticCoast ofthe 2.3.4.f Guideline Design Wave, Wind, and Current United States,” by E. G. Ward, D. J. Evans, and J. A. Pompa, Forces for Other U.S. Waters 1977 OTC Paper 2846, pp. 315-324, the following wave kine- ~~ The guideline design forces for static analysis should be cal- matics factors are recommended: 1.0 for Area #IS, 0.94 for I culated using (a) the metocean criteria given in Table2.3.4-2,Area#19,and0.88forArea#20.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 41. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 29 Table 2.3.4-2-Guideline Extreme Wave, Current, and Storm Tide Values* for Twenty Areas in United States Waters (Water depth > 300 fi. (91 m) exceptas noted) Ui,kt H,, ft. S x,ft. "Open "Open Shelf Range Shelf Range Range Shelf Quality Range 1. Gulf of Mexico (N of 27"N & W of 86"W) (See Section 2.3.4~ L-1, L-2, L-3 criteria) for and I 2. (E of of Mexico Gulf 86"W) 2 70 (6CL80) 11 1 - 115 3 (2-5) 2Southern 3. 45 (35-55) I1 1 - I30 6 (5-7) 1 (Santa Barbara& San Pedro Ch) lifornia 4. 60 (5CL65) 113 - 125 5 (4-6) 2Central 5. 60 (5CL70) I13 - I25 7 (6-8) 2 2 6. WashingtodOregon 85 (7CL100) I13 - I19 8 (7-10) 3Bay) (Icy of Gulf 7. Alaska 100 (9CL120) 113 - 111 11 (10-13) 2 3 Alaska of (Kodiak) 8. Gulf 90 (8CL110) 113 - 111 10 (9-12) 2k Lower 9. Inlet 4 60 (45-70) l h o - l/11 16 (13-20) 2 10. Northern Aleutian (6-12) 3Shelf 70 (6CL90) I12 - I16 8 (6-12) 1 3 Basin George 11. St. 85 (75-95) 112 - 116 5 (3-7) 1 Basin 12. Navarin 85 (75-95) 112 - 116 4 (3-5) 1 13. Norton Sound (d 3 ft.) 60 = 45 (35-50) I1 1 - I18 11 (8-14) 2 14. Chukchi Sea (d > 2 ft.) 60 50 (4CL60) I1 1 - I15 6 (4-8) 3 15. Chukchi Sea (d c 3 ft.) 60 0.78 @+X) ** 9 (6-12) 3 16. Beaufort Sea (d > 2 ft.) 50 40 (35-50) 113 - 111 4 (2-7) 2 17. Beaufort Sea (d c 4 ft.) 50 0.78 @+X) ** 8 ([-21-12) 2 18. Georges Bank 2 85 (75-95) I10 - I16 5 (4-6) 2 3 Canyon Baltimore 19. 90 (SCLlOO) 110 - 114 5 (4-6) 2 ment Georgia 20. 75 (65-85) 11 1 - 115 5 (3-7) 2 U = inline current at storm water level. i H, = 100-year maximum individual wave height. S = deep water wave steepness from linear theory = (2zHm)/(gTm2). g = acceleration gravity. of T, = zero-crossing period associated with H,, which can be calculated from S. X = storm tide (Section 1.3.4) associated with H, (mean higher high water plus storm surge). d = datum depth. water Quality 1 = based on comprehensive hindcast study verified with measurements. 2 = basedonlimitedhindcastsand/ormeasurements. 3 = preliminary guide. * H, Wind speeds, significant wave height, and spectral peak period associated with are discussed in Sections 2.3.4e.1 and 2.3.4e.2. ** Use the same range for T, as in deeper water. COPYRIGHT American Petroleum Institute Licensed by Information Handling Services
  • 42. 30 2A-WSD PRACTICE RECOMMENDED API Table 2.3.4-3"Guideline Extreme Wind Speeds* for 2. Marine Growth. For many of the areas in table 2.3.4-2 I Twenty Areas in United States Waters the thickness can be much greater than the1.5 inch guideline value for the Gulf of Mexico. For example, offshore Southern Wind with Extreme Wind and Central California thicknesses of 8 inches are common. Waves, Alone, Site specificstudiesshouldbeconductedtoestablish the mph ( d s ) mph ( d s ) thickness variation vs. depth. I Gulf of Mexico (N of27" N & W of 86"W) 92 (41) 97 (43) Structural members can be considered hydrodynamically smooth ifthey are either aboveMHHWordeepenough 2. Gulf of Mexico (E of 86"W) 98 (44) 109 (49) where marine growth might be light enough to ignore the effect of roughness. However, caution should used because be 3. Southern California 58 (26) 69 (31) it takes very little roughness to cause a c d of 1.05 (see Com- (Santa Barbara and San Pedro mentary, Section C2.3.lb.7 for relationship of c d to relative Channels) roughness). Site specific data should be used to establish the 4. California Outer Bank 58 (26) 69 (31) extent of the hydrodynamically rough zones; otherwise the structural members should be considered rough down to the 5. Central California 69 (31) 81 (36) mudline. 6. WashingtodOregon 69 (31) 92 (41) 3. Elevation of Underside of Deck. Deck elevations should 7. Gulf of Alaska (Icy Bay) 69 (31) 104 (46) satisfy all requirements of 2.3.48. Crest heights should be based on the guideline omnidirectional wave heights, wave 8. Gulf of Alaska (Kodiak) 69 (31) 104 (46) periods, and stormtide given in Table 2.3.4-2,and calculated using anappropriate wave theoryas discussed in 2.3.1b.2. 9. Lower Cook Inlet 69 (31) 104 (46) 10. North Aleutian Shelf 69 (31) 104 (46) 2.3.4.g Deck Clearance 11. St. George Basin 69 (31) 104 (46) Large forces result when waves strike a platforms deck and equipment. To avoid this, the bottom of the lowest deck 12. Navarin Basin 69 (31) 104 (46) should be located at an elevation which will clear the calcu- lated crest of the design wave with adequate allowance for 13. Norton Sound (d = 90 ft.) 69 (31) 104 (46) safety. Omnidirectional guideline wave heights with a nom- (d = 27 m) inal return period of 100 years, together with the applicable wave theories and wave steepnesses should be used to com- 14. Chukchi Sea (d > 60ft.) 69 (31) 92 (41) pute wave crest elevations above storm water level, includ- ing guideline storm tide. A safety margin, or air gap, of at (d > 18 m) least 5 feet should be added to the crest elevation to allow 15. Chukchi Sea (d c 60ft.) 69 (31) 92 (41) for platform settlement, water depth uncertainty, and for the possibility of extreme waves in order to determine the mini- (d c 18 m) mum acceptable elevation of the bottom beam of the lowest 16. Beaufort Sea (d > 50 ft.) 69 (31) 81 (36) deck to avoid wavesstriking the deck. An additional air gap should be provided for any known or predicted long term (d > 15 m) seafloor subsidence. In general, no platform components, piping or equipment 17. Beaufort Sea (d c 50 ft.) 69 (31) 81 (36) should be located belowthe lower deck in the designatedair (d c 15 m) gap. However, when it is unavoidable to position such items as minor subcellars, sumps, drains or production piping in the 18. Georges Bank 69 (31) 104 (41) ar gap, provisions should be made the wave forces devel- i for oped on these items. These wave forces may be calculated 19. Baltimore Canyon 104 (46) 115 (51) using the crest pressure of the design wave appliedagainst the 20. Georgia Embayment 104 (46) 115 (51) projected area. These forces may be considered on a "local" basis in the design of the item.These provisions do not apply *Reference one-hour average speed 10%) at 33 feet (10 meters) (k to vertical members such deck legs, conductors, risers, etc., as elevation. which normallypenetrate the ar gap. iCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 43. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 31 2.3.4.h References “Gulf of Alaska Wave and Wind Measurement Program,” Intersea Research Corporation,1974-1976. The following list of references represents some studies of “A DataCollection,Analysis,andSimulationProgram to I wave conditions used to support values in Tables 2.3.4-1 and 2.3.4-2 and Sections 2.3.4.c and 2.3.4.e. Although some these studies are proprietary cooperative studies, all may be of Investigate OceanCurrents, Northeast Gulf Intersea Research Corporation, 1975. of Alaska,” obtained.Additionally,numerousotherstudieshavebeen “Climatic Atlas of the Outer Continental Shelf Waters and made by individual companies for specific locations within Coastal Regions of Alaska, Vol, I, Gulf of Alaska,” W. A. these areas. Brower et al., National Oceanic and Atmospheric Adminis- Gulf of Mexico tration, 1977.** “Consequence-Based Criteria for the Gulf Mexico: Philos- of “Gulf of Alaska Hindcast Evaluation,” Intersea Research Cor- ophy & Results,” E. G. Ward, G. C. Lee, D. Botelho, J. W. L. poration, 1975-1976. Turner, F. Dyhrkopp, and R. A. Hall, Offshore Technology Lower Cook Inlet Conference, OTC Paper 11885, May 2000. “A Meteorological and Oceanographic Study Extreme and of “Consequence-Based Criteria for the Gulf Mexico: Devel- of Operational Criteria in Lower Cook Inlet,” Evans-Hamilton, opment and Calibration of Criteria,” E. G. Ward, G. C. Lee, Inc. 1977. D. L. Botelho, J. W. Turner, F. Dyhrkopp, andR. A. Hall, Off- shore Technology Conference, OTC Paper 11886, May 2000. “Oceanographic Conditions and Extreme Factors in Lower “Gulf of Mexico Hurricane Wave Heights,” R. G. Bea, Off- Cook Inlet,Alaska,’’ Intersea Research Corporation, 1976. shore Technology Conference, OTC Paper 21 10, 1974. “Oceanographic ConditionsOffshore for Operations in “An EnvironmentalDesignStudyfortheEasternGulf of Lower Cook Inlet, Alaska,’’ Intersea Research Corporation, Mexico Outer Continental Shelf,” Evans-Hamilton, Inc., 1973. 1975. “Gulf of Mexico Rare Wave Return Periods,” R. E. Haring Bering Sea and J. C. Heideman, Journal of Petroleum Technology, Janu- “Climatic Atlas of the Outer Continental Shelf Waters and ary, 1980. Coastal Regions of Alaska, Vol. II, Bering Sea,” W. A. Broer “Statistics of Hurricane Waves in the Gulf of Mexico,” G. E. et al., NationalOceanicandAtmosphericAdministration, Ward, L. E. Borgman, and V. J. Cardone, Journal of Petro- 1977.** leum Technology, May 1979. “The Eastern Bering Sea Shelf;Oceanographyand “Wind and Wave Model for Hurricane Wave Spectra Hind- Resources,” D. W. Hood andJ. A. Calder, Eds., National Oce- casting,” M. M. Kolpak. Offshore Technology Conference, anic and Atmospheric Administration, 1982. OTC Paper 2850, 1977. “Bering Sea PhaseOceanographic 1 Study-Bering Sea “Texas Shelf Hurricane Hindcast Study,” ARCTEC and Off- Storm SpecificationStudy,” V. J. Cardone et al.,Ocean- shore and Coastal Technologies, Inc., 1985. weather, Inc., 1980. “GUMSHOE, Gulf of Mexico Storm Hindcast of Oceano- “Bering Sea ComprehensiveOceanographicMeasurement graphic Extremes,” August, 1990. Program, “Brown and Caldwell, 1981-1983. West Coast “Bering Sea Oceanographic Measurement Program,” Intersea “Santa Barbara Channel Wave Hindcast Study,” Ocean- Research Corporation, 1976-1978. weather, Inc., 1982. “BristolBayEnvironmentalReport,”OceanScienceand “An Environmental Study for the Southern California Outer Engineering, Inc., 1970. Continental Shelf,” Evans-Hamilton, 1976. “St. GeorgeBasinExtreme Wave ClimateStudy,”Ocean- “Storm Wave Study, Santa Barbara Channel,” Oceanographic Services, Inc., 1969. weather, Inc., 1983. “Informal Final Report-Pt. Conception Hindcast Area, Beaufort/Chukchi Study,” Oceanweather, Inc., 1980. “Climatic Atlas of the Outer Continental Shelf Waters and “Final Report-Wave Hindcast Pt. Conception Area, North- Coastal Regions of Alaska, Vol. III, Chukchi-Beaufort Seas,” west Type Storms, “Oceanweather, Inc., 1982. W. A.Brower et al., NationalOceanicandAtmospheric Administration, 1977.** Gulf of Alaska “GroupOceanographic Survey4ulf ofAlaska,”Marine **Estimates extreme heights these of wave in references are Advisors, Inc., 1970. xroneous.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 44. 32 2A-WSD PRACTICE RECOMMENDED API “Beaufort Sea Wave Hindcast Study: Prudhoe Bay/Sag Delta It should be recognized that these provisions represent the and Harrison Bay,” Oceanweather, Inc., 1982. state-of-the-art, and that a structure adequately sized and pro- portioned for overall stiffness, ductility, and adequate strength “Arctic Development Project, Task 1/10, Part I, Meteorologi- at the joints, and which incorporates good detailing and weld- cal Oceanographic and Conditions,Part II, Summary of ing practices, is the best assurance good performance dur- of Beaufort Sea Storm WaveStudy,” E. G. WardandA. M. ing earthquake shaking. Reece, Shell Development Company, 1979. The guidelines in the following paragraphs of this section “Reconnaissance EnvironmentalStudy of Chukchi Sea,” are intended to apply to the design of majorsteelframed Ocean Science and Engineering, Inc., 1970. structures. Only vibratory ground motion is addressed in this section. Other major concerns such those identified in Sec- as “Alaska Beaufort Sea Gravel Island Design,” Exxon Com- tions 1.3.7 and 1.3.8 (e.g., large soil deformations or instabil- pany, U.S.A., 1979. ity) should be resolved special studies. by “Beaufort Sea SummerOceanographicMeasurementPro- grams,” Oceanographic Services, Inc., 1979-1983. 2.3.6.b Preliminary Considerations 1. Evaluation of SeismicActivity. Forseismicallyactive East Coast areas it is intended that the intensity and characteristics of “A Preliminary Environmental Study for the East Coast of the seismic ground motion used for design be determined by a United States,” Evans-Hamilton, Inc., 1976. site specific study. Evaluation of the intensity and characteris- tics of ground motion should consider the active faults within “Extreme Wave HeightsAlongtheAtlanticCoast ofthe the region, the type of faulting, the maximum magnitude of United States,” E. G. Ward, D. J. Evans, and J. A. Pompa, earthquake which can be generated each fault, the regional by Offshore Technology Conference, OTC paper 2846, 1977. seismic activity rate, the proximity the site to the potential of “Characterization of Currents over Chevron Tract #510 off source faults, the attenuation of the ground motion between Cape Hatteras, North Carolina,” Science Applications, Inc., these faults and the platform site, and the soil conditions at 1982. the site. To satisfy the strength requirements a platform should be “An Interpretation of Measured Gulf Stream Current Veloci- designed for ground motions having an average recurrence tiesoffCapeHatteras,NorthCarolina,”Evans-Hamilton, interval determined in accordance with Section 1.5. Inc., 1982. The intensity of ground motion which may occur during a rareintenseearthquakeshould be determinedinorderto “Final Report-Mante0 Block 510 Hurricane Hindcast decide whether a special analysis is required to meet the duc- Study,” Oceanweather, Inc., 1983. tilityrequirements.Ifrequired,thecharacteristics of such motion should be determined to provide the criteria for such 2.3.5 Ice an analysis. In areas where ice is expected to be a consideration in the 2. Evaluation for Zones of Low Seismic Activity. In areas planning, designing or constructing of fixed offshore plat- of low seismic activity, platform design would normally be forms,usersarereferredtoAPIBulletin 2 N “Planning, controlled by stormorotherenvironmentalloadingrather Designing, and Constructing Fixed Offshore Platforms Ice in than earthquake. For areas where the strength level design Environments,” latest edition. horizontalgroundaccelerationislessthan O.O5g, e.g.,the Gulf of Mexico, no earthquake analysis is required, since the 2.3.6 Earthquake design for environmental loading other than earthquake will 2.3.6.a General providesufficientresistanceagainstpotentialeffectsfrom seismically active zones. For areas where the strength level This section presents guidelines for the design of a plat- design horizontal ground acceleration is in the range of 0.05g form for earthquake ground motion. Strength requirements to O. log, inclusive, allof the earthquake requirements, except are intended to provide a platform which is adequately sized those for deck appurtenances, may be considered satisfiedif for strength and stiffness to ensure no significant structural the strength requirements (Section 2.3.6~) met using the are damage for the level of earthquake shaking which has a rea- ground motionintensity characteristics and of therare, sonable likelihood not being exceeded during the of the of life intense earthquake in lieu the strength level earthquake. of In structure. The ductility requirements are intended to ensure this event, the deck appurtenances should be designed for the that the platform has sufficient reserve capacity prevent its to strength level earthquake in accordance with 2.3.6e2, but the collapseduringrareintenseearthquakemotions,although ductility requirements (Section 2.3.6d) are waived, and tubu- structural damage may occur. lar joints need be designed for allowable stresses specified inCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 45. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 33 Section 2.3.6e1 using the computed joint loads insteadof the 4. Response Assessment. In calculation the of member tensile load or compressive buckling load the member. of stresses, stresses to the due earthquake induced loading should be combined with those due to gravity, hydrostatic 2.3.6.c Strength Requirements pressure, and buoyancy.Forthestrengthrequirement, the basic AISC allowable stresses and those presented in Section 1. Design Basis. The platform should be designed to resist 3.2 may be increased by 70 percent. Pile-soil performance the inertially induced loads produced by the strength level and pile design requirements should be determined on the ground motion determined in accordance with 2.3.6b1 using basis of special studies. These studies should consider the dynamic analysis procedures suchas response spectrumanal- designloadings of 2.3.6~3,installationprocedures,earth- ysis or time historyanalysis. quake effects on soil properties and characteristics of the soils as appropriate to the axial or lateral capacity algorithm being 2. StructuralModeling. The mass used in the dynamic used. Both the stiffness and capacity of the pile foundation analysis shouldconsist of the mass of the platform associated should be addressed in a compatible manner for calculating with gravity loading defined in 2.3.6~3, mass of the fluids the the axial andlateral response. enclosedinthe structure andtheappurtenances,andthe added mass.The added mass may be estimated the massof as 2.3.6.d Ductility Requirements the displaced water for motion transverse to the longitudinal axis of the individual structural framing and appurtenances. 1. The intent of these requirementsis to ensure that platforms For motions along the longitudinal of the structural fram- axis to be located in seismically active have adequate reserve areas ing and appurtenances, the added mass may neglected. be capacity to preventcollapse under a rare,intense earthquake. The analytical model should includethe three dimensional Provisions are recommended hereinwhich,whenimple- distribution of platform stiffness and mass. Asymmetry in mented in the strength design of certain platforms, will not platform stiffnessor mass distribution may lead significant to require an explicit analytical demonstrationof adequate duc- torsional response which should be considered. tility. These structure-foundation systemsare similar to those In computing the dynamic characteristics of braced, pile for which adequate ductility has already been demonstrated supported steel structures, uniform model damping ratios of analytically in seismically active regions where the intensity five percent of critical should be used for an elasticanalysis. ratio of the rare, intense earthquake ground motions to Where substantiating data exist, other damping ratios may be strength level earthquake ground motions is 2 or less. used. 2. No ductilityanalysis of conventional jacket-type struc- 3. Response Analysis.It is intended that the design response tures with 8 or more legs is required if the structure is to be should be comparable for any analysis method used. When located in an area where the intensity ratio of rare, intense the response spectrum method is used and one design spec- earthquakegroundmotionstrength earthquake to level trum is appliedequallyinbothhorizontaldirections, the ground motionis 2 or less,the piles are to be founded soils in complete quadratic combination(CQC) method may beused that are stable under ground motions imposed by the rare, for combining modal responses and the square root of the intense earthquake and the following conditions are adhered sum of the squares (SRSS) may be used for combining the to in configuring structure and proportioning members: the directional responses. If other methods are used for combin- a. Jacket legs, including any enclosed piles, are designed to ing modal responses, such the squareroot of the sumof the as meet the requirements of 2.3.6~4,using twice the strength squares, care should be taken to underestimate comer pile not level seismicloads. and leg loads. For the response spectrum method, as many modes should be considered as required for an adequate rep- b. Diagonalbracinginthevertical frames are configured resentation of the response. At least two modes having the such that shear forces between horizontalframes or in vertical highest overall response should be included for each of the runs between legs are distributed approximately equally to three principal directions plus significant torsional modes. both tension and compression diagonal braces, and that “ K ’ Where the time history method used, the design response is bracing is not used where the ability of a panel to transmit should be calculated as the average of the maximum values shear is lost if the compression brace buckles. Where these for each of the time histories considered. conditions are not met,including areas such as the portal frame between the jacket and the deck,the structural compo- Earthquake loading should be combined with other simul- nents shouldbe designed to meetthe requirements of Section taneous loadings such as gravity, buoyancy, and hydrostatic 2.3.6~4 using twicethe strength level seismicloads. pressure. Gravity loading should include the platform dead weight (comprised of the weight of the structure, equipment, c. Horizontalmembers are providedbetween all adjacent appurtenances), actual live loads and 75 percent of the maxi- legs at horizontal framing levels in vertical frames and that mum supplyand storage loads. these members have sufficient compression capacity to sup-COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 46. 34 2A-WSD PRACTICE RECOMMENDED API port the redistributionof loads resulting from the buckling of 2. Deck Appurtenances and Equipment. Equipment, pip- adjacent diagonalbraces. ing, and other deck appurtenances should be supportedso that induced seismic forces can be resisted and induced displace- d. The slenderness ratio (KZUr)of primary diagonal bracing in ments can be restrained that such no damage to the vertical frames is limited to 80 and their ratio of diameter to equipment, piping, appurtenances, and supporting structure thickness is limited to 1900/Fy where Fy is in ksi (13100/Fy occurs. Equipment should be restrained by means of welded for Fy in MPa). All non-tubular members at connections in connections, anchor bolts, clamps, lateral bracing, or other vertical frames are designed as compact sections in accor- appropriate tie-downs. The design of restraints should include dance with the AISC Specifications or designed to meet the both strength considerationsas well as their ability to accom- requirements of 2.3.6~4 using twicethe strength level seismic modate imposed deflections. loads. Specialconsiderationshouldbegiventothedesign of 3. Structure-foundation systems which do not meet the con- restraints for critical piping and equipmentwhose failure ditions listed in 2.3.6d2 should be analyzed to demonstrate could result in injury to personnel, hazardous material spill- their ability to withstand the rare, intense earthquake without age, pollution, or hindrance to emergency response. collapsing. The characteristics of the rare, intense earthquake Design accelerationlevels should include the effects of glo- should be developed from site-specific studies of the local balplatform dynamicresponse; and, if appropriate,local seismicity following the provisions of 2.3.6bl. Demonstra- dynamic responseof the deck and appurtenance itself. Due to tion of the stability of the structure-foundation system should the platform’s dynamic response, these design acceleration lev- be by analytical procedures that are rational and reasonably els are typically much greater than those commonly associated representative of the expected response of the structural and with the seismic design similar onshore processing facilities. of soil components of the system to intense ground shaking. In general, most typesof properly anchored deck appurte- Models of the structural and soil elements should include nances are sufficiently stiff so that their lateral and vertical their characteristic degradation strength and stiffness under of responsescanbecalculateddirectlyfrommaximum com- extreme load reversals and the interaction of axial forces and puted deck accelerations, since local dynamic amplification is bendingmoments,hydrostaticpressures and local inertial negligible. forces, as appropriate. The P-delta effect of loads acting Forceson deck equipmentthat do notmeetthis“rigid through elastic and inelastic deflections of the structure and body” criterion shouldbe derived by dynamic analysis using foundation should be considered. either: 1) uncoupled analysis with deck level floor response spectra or 2) coupled analysis methods. Appurtenances that typically do not meet the “rigid body” criterion are drilling 2.3.6.e Additional Guidelines rigs, flare booms, deck cantilevers, tall vessels, large unbaf- 1. Tubular Joints. Where the strength level design horizon- fled tanks,and cranes. tal ground motion is 0.05g or greater (except as provided in Coupled analyses that properly include the dynamic inter- 2.3.6b2 when inthe range of 0.05g to O.log, inclusive,), joints actions between the appurtenance and deck result in more for primary structural members should be sized for either the accurate andoftenlowerdesignaccelerationsthanthose tensile yield load or the compressive buckling load of the derived using uncoupled floor response spectra. members framing into the joint, as appropriate for the ulti- Drilling and well servicing structures should be designed mate behaviorof the structure. for earthquake loads in accordance with API Specification 4F. Joint capacity may be determined on the basis of punch- It is important that these movable structures their associ- and ing shear or nominal loads in the brace in accordance with ated setback and piperack tubulars be tied downrestrained or Section 4.3 except that the allowable punching shear stress at all times except when structures are being moved. the in the chord wall, vp, and the allowable joint capacities, P, Deck-supported structures, equipment and tie-downs, and M,, may be increased by 70 percent in lieu of a l/3 should be designed with a one-third increase in basic allow- increase. The factor A used in determining vp should be able stresses,unless the framingpattern,consequences of computed as follows: failure, metallurgy, and/or site-specific ground motion inten- sities suggestotherwise. 2.3.7 Accidental Loads A = % k+ t c . 9 + 7. 29 (2.3.6-1) F, Offshore platforms may be subject to various accidental loads such as: collision from boats and barges; impact from WherefM,JpB, andfopBare stresses in the chorddue to twice dropped objects; explosion or fire. Considerations should be the strength level seismic loads in combination with gravity, given in the design of the structure and in the layout and hydrostatic pressure buoyancy loadsor to the full capacity and arrangement of facilities andequipmenttominimize the of the chord away from the can, whicheveris less. joint effects of these loads.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 47. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 35 Potential impact from operational boat or barge traffic for struction. The magnitude of such forces should determined be jacket waterline members, risers, and external wells should be through consideration the of static dynamic and forces considered. Barge bumpers, boat landings, and other external applied to the structure during lifting and from the action of fendering may be used as protection. Certain locationsof the the structure itself. Lifting forces on padeyes and on other deck, such as crane loading areas and areas near the drilling members of the structure should include both vertical and rig, are more likely to be subject dropped objects.The loca- to horizontal components, the latter occurring when lift slings tion of equipment and facilities below these areas should be are otherthanvertical.Verticalforcesonthe lift should considered to minimize damage from dropped objects. include buoyancy as well as forces imposed by the lifting Typically, an offshore structure is constructed of an open equipment. framework of structural shapes and tubular members which is To compensate for any side loading on lifting eyes which relatively resistant to blast and explosion. When it is neces- may occur, in addition to the calculated horizontal and verti- sary to enclose portions of a platform in locations where the cal components of the static load for the equilibrium lifting potentialforgasexplosionexists,theprotectivesidingor condition, lifting eyes and the connections to the supporting walls should include blowout panels should be designed or to structural members should be designed for a horizontal force collapse at low uniform pressureto minimize the load on pri- of 5% of the static sling load, applied simultaneously with the mary members. Fire protection precautions are coveredin static sling load. This horizontal force should applied per- be other API and industry codes and specifications. pendicular to the padeye the centerof the pinhole. at It ispossiblethataccidentsorequipmentfailuresmay cause significant structural damage. Inspection of this dam- 2.4.2.b Static Loads age in accordance with Section 14 of RP2A can provide the information for analytical work to determine the need for When suspended, the lift will occupy a position such that immediate or eventual repair. Such analysis will also identify the centerof gravity of the lift and the centroidof all upward under what conditions the installation should be shut-in a n d acting forces on the lift in static equilibrium. are The position or evacuated. It is not anticipated that the accidental event of the lift in this state of static equilibrium should be usedto will occur simultaneously with design environmental loads. determine forces in the structure and in the slings. move- The ment of the lift as it is pickedup and set down should be taken into account in determining critical combinations of vertical 2.4 FABRICATION AND INSTALLATION FORCES and horizontal forces at all points, including those to which 2.4.1 General lifting slings are attached. Fabrication forces are those forces imposed upon individ- 2.4.2.c Dynamic Load Factors ual members, component parts of the structure, or complete units during the unloading, handling and assembly in the fab- For lifts where either the lifting derrick or the structure to ricationyard.Installationforcesarethoseforcesimposed be lifted is on a floating vessel, the selection design lift- of the upon the component parts of the structure during the opera- ing forces should consider the impact from vessel motion. tions of moving the components from their fabrication or site Load factors shouldbe applied to the design forcesas devel- prioroffshorelocationtothefinaloffshorelocation,and oped from considerations 2.4.2a and 2.4.2b. of installing the component parts to form the completed plat- For lifts to be made at open, exposed sea (i.e., offshore form. Since installation forces involve the motion of heavy locations), padeyes and other internal members (and both end weights, the dynamic loading involved should be considered connections)framingintothe joint wherethepadeye is and the static forces increased by appropriate impact factors attached and transmitting lifting forces within the structure to arrive at adequate equivalent loads for design the mem- of should be designed for a minimum load factor 2.0 applied of bersaffected.Forthoseinstallationforcesthatareexperi- to the calculated static loads. All other structural members enced during only transportation launch, which and and transmitting lifting forces should be designed using a mini- includeenvironmentaleffects,basicallowablestressesfor mum load factorof 1.35. member design may be increased by l/3 in keeping with pro- For other marine situations (i.e., loadoutat sheltered loca- visions of 3. l .2. Also see Section 12, “Installation,” for com- tions), the selectionof load factors should meet the expected ments complementary to this section. local conditions but should not be less than a minimum of 1.5 and 1.15 for the two conditions previously listed. 2.4.2 Lifting Forces For typical fabrication yard operations where both the lift- 2.4.2.a General ing derrick and the structure or components to be lifted are land-based, dynamic load factors are not required. For special Lifting forcesare imposed on the structure by erection lifts procedures where unusual dynamic loads possible, appro- are during the fabrication and installation stages platform con- of priate load factors may be considered.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 48. 36 2A-WSD PRACTICE RECOMMENDED API 2.4.2.d Allowable Stresses installation, since lifting in open water will impose more severe conditions. so The lift should be designed that all structural steel mem- bers are proportioned for basic allowable stresses specified as 2.4.3.b Horizontal MovementOnto Barge in Section 3.1. The AISC increase in allowable stresses for short-term loads should not be used. In addition, all critical Structures skidded onto transportation barges are subject to structural connections primary and members shouldbe load conditions resulting from movement the barge due to of designed to have adequate reserve strength to ensure struc- tidalfluctuations,nearbymarinetrafficand/orchangein tural integrity during lifting. draft;andalsofromloadconditionsimposed by location, slope and/or settlement of supports at all stages of the skid- 2.4.2.eEffectof Tolerances dingoperation. Since movementisnormallyslow,impact need not be considered. Fabricationtolerancesandslinglengthtolerancesboth contribute to the distribution of forces and stresses in thelift 2.4.4 Transportation Forces system which are different from that normally used for con- ventional design purposes.The load factors recommended in 2.4.4.a General 2.4.2~ intended to apply to situations where fabrication are Transportation forces acting on templates, towers, guyed tolerances do notexceedtherequirements of 11.1.5,and towers, minimum structures and platform deck components where the variation in length slings does not exceed plus of or should be considered in their design, whether transported on minus l/4 of 1% of nominal sling length, 1l/2 inches. or barges or self-floating. These forces result from the way in The total variation from the longest to the shortest sling which the structure is supported, either by barge buoyancy, or should not be greater than l/2 of 1% of the sling length or 3 and from the response the tow to environmental conditions of inches. If either fabrication tolerance or sling length toler- encountered enroute to the site. In the subsequent paragraphs, ance exceedstheselimits,a detailed analysis taking into the structure and supporting barge and the self-floating tower account these tolerances should be performed to determine are referred toas the tow. theredistribution of forcesonbothslingsandstructural members. This same type analysis should also be performed 2.4.4.b Environmental Criteria in any instances where it is anticipated that unusual deflec- tions of particularly stiff structural systems may also affect The selection of environmental conditions to be used in load distribution. determining the motionsof the tow and the resulting gravita- tional and inertial forces acting on the tow should consider 2.4.2.f Slings, Shackles and Fittings the following: For normal offshore conditions, slings should be selected 1. Previous experience along the tow route. to have a factor of safety of 4 for the manufacturer’s rated 2. Exposure and time reliability of predicted “weather minimum breaking strength of the cable compared to static windows.’’ sling load.The static sling load should be the maximum load 3. Accessibility of safe havens. onanyindividualsling, as calculatedin2.4.2a,b,ande 4. Seasonal weather system. above, by taking into account all components of loading and 5. Appropriateness of the recurrence interval used in deter- theequilibriumposition of the lift. Thisfactor of safety mining maximum design wind, wave and current conditions should be increasedwhenunusuallysevereconditionsare and considering the characteristics of the tow, such as size, anticipated, and maybe reduced to a minimumof 3 for care- structure, sensitivity, and cost. fully controlled conditions. Shackles and fittings should be selectedso that the manu- 2.4.4.c Determination of Forces facturer’s rated working load is equal to or greater than the The tow including the structure, sea fastenings and barge static sling load, provided the manufacturer’s specifications should be analyzed for the gravitational, inertial and hydro- include a minimum factor safety of 3 compared to the min- of dynamic loads resulting from the application of the environ- imum breaking strength. mental criteria in 2.4.4b. The analysis should be based on model basin test results or appropriate analytical methods. 2.4.3 Loadout Forces Beam, head and quartering wind and seas should be consid- 2.4.3.a Direct Lift ered to determine maximum transportation forces in the tow structural elements. In the case of large barge-transported Lifting forces for a structure loaded out by direct lift structures, the relative stiffnesses of the structure and barge onto the transportation barge should be evaluated only if are significant and should be considered inthe structural the lifting arrangement differs from that to be used in the analysis.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 49. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 37 Where relative size of barge and jacket, magnitude of the 2.4.5.c Hook Load sea states, andexperience make such assumptions reasonable, towsmay be analyzed basedongravitationaland inertial Floating jackets for which lifting equipment is employed forces resultingfrom the tow’s rigid bodymotionsusing for turning to a vertical positionshould be designed to resist appropriate periodand amplitude by combining roll with the gravitational and inertial forces required to upright the heave and pitch with heave. jacket. 2.4.4.d Other Considerations 2.4.5.d Submergence Pressures Large jackets for templates and guyed towers will extend The submerged, non-flooded or partially flooded members beyond the barge and will usually subjected to submersion be of the structure should be designed to resist pressure-induced during tow. Submerged members should be investigated for hoop stresses during launching and uprighting. slamming, buoyancy and collapse forces. Large buoyant A member may be exposed to different values of hydro- overhanging membersalso may affect motions should be and staticpressureduringinstallationandwhilein place. The considered. The effects on long slender members of wind- integrity of the member may be determined usingthe guide- induced vortex shedding vibrations should be investigated. lines of 3.2.5 and 3.4.2. This condition maybe avoided by the use of simple wirerope spoilers helically wrapped around the member. 2.4.6 Installation Foundation Loads For long transocean tows, repetitive member stresses may 2.4.6.a General become significant tothe fatigue life of certain member con- nections or details and should be investigated. Calculated foundation loads during installation should be conservativeenough to give reasonable assurance thatthe 2.4.5 Launching Forces and Uprighting Forces structure will remain at the planned elevation and attitude until piles canbeinstalled. Reference shouldbemadeto 2.4.5.a Guyed Tower and Template Type appropriate paragraphs in Sections 2 and 13. Guyed tower and template type structures which trans- are ported by barge are usually launched at or near the installa- 2.4.6.b Environmental Conditions tion location. The jacket is generallymovedalongways, Considerationshouldbegiventoeffects of anticipated which terminate in rocker arms, the deck of the barge. As on storm conditions during thisstage of installation. the position of the jacket reaches a pointof unstable equilib- rium, the jacket rotates, causingthe rocker arms at the end of 2.4.6.c Structure Loads the ways to rotate as the jacket continues to slide from the rocker arms. Forces supporting the jacket on the ways should Vertical and horizontal loads should be considered taking be evaluatedfor the full travel of the jacket. Deflection of the into account changes in configuratiodexposure, construction rocker beam and the effect on loads throughout the jacket equipment, and required additional ballastfor stability during should be considered. In general, the most severe forces will storms. occur at the instant rotation starts. Consideration should be given to the development of dynamicallyinducedforces 2.4.7 Hydrostatic Pressure resulting from launching. Horizontal forces required to ini- tiate movement of the jacket should also be evaluated. Con- Unfloodedorpartiallyfloodedmembers of a structure siderationshould givenwind, be to wave, current and should beable to withstand the hydrostatic pressure acting on dynamic forces expected on the structure and barge during them caused by theirlocationbelowthewatersurface. A launching and uprighting. member may be exposed to different values pressure dur- of ing installation and while in place. The integrity of the mem- 2.4.5.bTowerType ber maybedeterminedusingtheguidelines of 3.2.5 and 3.4.2. Tower type structuresare generally launched from the fab- rication yard to float with their own buoyancyfor tow to the 2.4.8 Removal Forces installation site. The last portion of such a tower leaving the launching ways may have localized forces imposed on it as Due consideration should be takenof removal forces such the first portion the tower toenter the water gains buoyancy of as blast loads, sudden transfer of pile weight to jacket and and causes the tower to rotate from the slope of the ways. mudmats, lifting forces, concentrated loadsduringbarge Forces should be evaluated for the full travel of the tower loading, increased weight, reduced buoyancy and other forces down the ways. which may occur.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 50. 38 2A-WSD PRACTICE RECOMMENDED API 3 StructuralSteelDesign 3.1.3.a Directional Environmental Forces 3.1 GENERAL Figure 2.3.4-4 provides wave directions and factors to be applied to the omnidirectional wave heights to be used in the 3.1.1 Basic Stresses determination of in-place environmental forces. When these Unless otherwise recommendedtheplatformshouldbe directional factors are used, the environmental forces should designed so that all members are proportioned for basic be calculated forall directions which are likely to control the allowable stresses specified the AISC Specijication for the by design of any structural memberor pile. As a minimum, this Design,FabricationandErection o StructuralSteel for f should include environmental forces in both directions paral- Buildings, latest edition. Where the structural element or type lel and perpendicular to each jacket face as well as all diago- of loading is not covered by this recommended practice or by nal directions, if applicable. These directions are to be AISC, a rational analysis should be used to determine the determined bythe base of the jacket. basic allowable stresses with factors of safety equal to those A minimum of 8 directions are required for symmetrical, given by this recommended practice or by AISC. Allowable rectangular and square platforms a minimumof 12 direc- and pile stresses are discussed in Section 6.9. Members subjected tions are required for tripod jackets. For unsymmetrical plat- to combined compression and flexure should be proportioned forms or structureswith skirt piles,thecalculation of the to satisfy both strength stability criteriaat all points along and environmental forces from additional directions may also be their length. required. As stated in 2.3.4~-3, one of these directions is not if The AISC Load and Resistance Factor Design, First Edi- the principal direction, then the omnidirectional wave from tion code is not recommended for design of offshore plat- the principal direction must also be considered. The maxi- forms. mum force should becalculated with the crest of the wave at several locations as the crest of the wave passes through the 3.1.2 Increased Allowable Stresses platform. Where stresses are due in part to the lateral and vertical 3.1.3.b Platform Orientation forces imposed by design environmental conditions, basic the AISC allowable stresses may be increased by one-third. For Due to difficulties in orienting jacket during installation the earthquake loadings, design levels should be in accordance it is not alwayspossibletoposition the jacket exactly as with 2.3.6.c4 and 2.3.6e. The requiredsectionproperties planned. When platforms are to be installed on a relatively computed on this basis should not be less than required for flat bottom with no obstructions and with no more than one design dead and live loads computed without the one-third existing well conductor, in addition to the directions stated increase. above, thejacket should be designed wave conditions that for would result if the jacket were positioned 5.0" in either direc- 3.1.3 Design Considerations tion from the intendedorientation. When ajacket is to be installed over two ormore existing Industry experience to date has indicated that existing, well conductors or in an area where obstructions on the bot- conventional, jacket type,fixed offshore platforms have tom suchan uneven sea floor resulting from previous drilling demonstrated good reliability and reserve strength not only by mobile drilling rigs, are likely, the condition of the site for the design environmental loads but for general usage as must be determined prior to the design of the platform. The well. For these structures, the design environmental loading probability of the jacket being installed out of alignment has been more or less equal from all directions. This has should be considered and the 5.0" tolerance increased resulted in platform designs that are reasonably symmetrical accordingly. from a structural standpoint and which have proven to be adequate for historical operational and storm conditions as 3.1.3.c Pile Design well as for loads not normally anticipated in conventional in-place analysis. Piling shall bedesigned in accordance with Sections 3 and Withrecentimprovements Metocean in technologyin 6 and may be designed for the specific loading for each pile some operational areas, is now possible to specify the varia- individually as predicted considering directionality of design it tion in design conditions from different directions. allows This conditions. This will likelyresult in non symmetrical founda- the designer to take advantage platform orientationand the of tionswithpileshavingdifferentpenetration,strength and directional aspects of storm forces. However, application of stiffness. Industry experience to date, based on symmetrical the predicted directional loads may result in a structure which foundations with piles having the same wall thickness, mate- is designed for lower forces in one direction than another. In rial grades and penetration has demonstrated good reliability order to provide minimum acceptable platform strength all in and reserve strength. For the design of non symmetricalfoun- directions, the following recommendations made. are dations, the different stiffnessof each pile shall be consideredCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 51. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 39 as well as the redistributionof loads throughjacket bracing to 3.2.2.b Local Buckling stifferpilemembers by modelingtherelativestiffness of Unstiffened cylindrical members fabricated from structural foundation members interacting with the jacket stiffness. steels specified in Section8.1 should be investigated for local buckling due toaxialcompressionwhenthe D/t ratiois 3.2ALLOWABLESTRESSES FOR CYLINDRICAL greater than60. When theD/t ratio is greater than and less 60 MEMBERS than 300, with wall thickness t 2 0.25 in. (6 mm), both the 3.2.1 Axial Tension elastic (Fxe) and inelastic local buckling stress (Fxc) due to axial compression should determined from Eq. 3.2.2-3 and be The allowable tensile stress, F,, for cylindrical members Eq. 3.2.2-4. Overall column buckling should be determined subjected to axial tensile loads should be determined from: by substituting the critical local buckling stress (Fxe or Fxc, whichever is smaller) forFy in Eq. 3.2.2-1 and in the equation F, = 0.6 Fy (3.2.1-1) for C,. 1. Elastic Local Buckling Stress. where The elastic local buckling stress, should be determined Fxe, Fy = yield strength, ksi (MPa). from: F,, = 2CE t/D (3.2.2-3) 3.2.2 Axial Compression where 3.2.2.a Column Buckling C = critical elastic buckling coefficient, The allowableaxialcompressivestress, F,, should be D = outside diameter, in. (m), determined from the following AISC formulas for members with aD/t ratio equal to or less than 60: t = wall thickness, in. (m). F, = [1 - y’] F, for Kl/r < C, (3.2.2-1) The theoretical valueof C is 0.6. However, a reduced value of C = 0.3 is recommended for use inEq. 3.2.2-3 to account for the effect of initial geometric imperfections within API 5/3+--- 3(Kl/r) KU^)^ Spec 2B tolerance limits. 8c c 8 c: 2. Inelastic Local Buckling Stress. The inelastic local buckling stress, F,,, should be deter- mined from: F, = l2 for Kl/r 2 C, (3.2.2-1) 23(Kl/r)’ F,, = F , x [ 1.64 - 0.23(D/t)1/4] F,, 5 (3.2.2-4) where F,, = F , for ( D / t ) 5 60 3.2.3 Bending c, = [12r:‘E11/’ - The allowable bending stress, Fb, should be determined E = Young’s Modulus of elasticity, ksi (MPa), from: K = effective length factor, Section 3.3.ld, D 1500 Fb = 0.75 Fy for - 5 - (3.2.3-la) 1 = unbraced length, in. (m), t F, r = radius of gyration, in. (m). For members with a ratio greater than 60, substitute the D/t critical local buckling stress or F,,, whichever is smaller) (Fxe for Fy in determiningC, and F,. Equation1.5-3intheAISCSpecificationshouldnotbe y1 0.84- 1.74 - F for -<D5 - 3000 (3.2.3-1b) l5Oo Fy - t Fy usedfordesignofprimarybracingmembersinoffshore structures. This equationmay be usedonlyforsecondary 10,340 <D5 - 20 680 , - members such as boat landings, stairways, etc. Units)COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 52. 40 2A-WSD PRACTICE RECOMMENDED API F ”1 0.72 - 0 . 5 8 Y F, for Et F, t ’ < 5 300 (3.2.3-1~) and the allowable torsional shear stress, F,,,, should be deter- mined from: F,,, = 0.4 F, (3.2.4-4) 20’680 < 5 300 , SI Units 3.2.5 Hydrostatic Pressure* (Stiffened and Unstiffened Cylinders) For D/t ratios greater than 300, refer API Bulletin 2U. to For tubular platform members satisfying Spec 2B out- API of-roundness tolerances,the acting membrane stress, j , in ksi f 3.2.4 Sheart (MPa), should not exceed the critical hoop buckling stress, 3.2.4.a Beam Shear Fhc, divided by the appropriate safety factor: The maximum beam shear stress,f,,, cylindrical mem- for bers is: f h 5 FhC/SFh (3.2.5-1) fj = pDI2t (3.2.5-2) V f,, = - (3.2.4-1) where OSA fj = hoop stress due to hydrostatic pressure, ksi (MPa), where p = hydrostatic pressure, ksi (MPa), f,, = the maximum shear stress, ksi (MPa), SF, = safety factor against hydrostatic collapse (see V = the transverse shear force, kips (MN), Section 3.3.5). A = the cross sectional area, in.2 (m2). 3.2.5.a Design Hydrostatic Head The allowable beam shear stress, should be determined F,,, The hydrostatic pressure (P = y H,) to be used should be from: determined from the design head, defined as follows: H,, F,, = 0.4 Fy (3.2.4-2) 3.2.4.bTorsional Shear where The maximumtorsionalshearstress, F,,, for cylindrical members causedby torsion is: z = depth below still water surface including tide, ft (m). z is positive measured downward from the still water surface.For installation, z should bethe max- (3.2.4-3) imum submergence during launch or differential the head during the upending sequence, plus a reason- able increase in head toaccount for structural where weight tolerances and deviations from the for fvt = maximum torsional shear stress, ksi (MPa), planned installation sequence. H , = wave height, ft(m), M, = torsional moment, kips-in. (MN-m), 27c = polar moment of inertia, in.4 (m4), k = - with L equal to wave length, ft-l (m-l), L d = still water depth, ft. (m), tWhile the shear yield stress of structural steel has been variously y = seawater density, 64 lbs/ft3 (0.01005 MNlm3). estimated as between l/2 and 5/8 of the tension and compression yield stress and is frequentlytakenas Fy /& , its permissible working stress value is given by AISC as 2/3 the recommended *For large diameter cylinders of length, a more rigorous anal- finite basic allowable tensile stress. For cylindrical members when local ysis may be used to justify fewer or smaller ringstiffeners provided shear deformations may be substantial due to cylinder geometry, a the effects of geometrical imperfections and plasticity are properly reduced yield stress may be need to be substituted for Fy in Eq. considered. API Bulletin 2U and the fourth edition of the Guide to 3.2.4-4. Further treatment of this subject appears in Reference 1, Stability Design Criteria Metal Structuresby the Structural for Sta- Section C3.2. bility Research Council provides detailed analysis methods.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 53. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 41 3.2.5.bHoop Buckling Stress 3.2.5.c Ring Design The elastic hoop buckling stress,Fhe, and the critical hoop Circumferential stiffening ringsize may be selected on the buckling stress, Fhc, are determined from the following for- following approximate basis. mulas. 1. Elastic Hoop Buckling Stress. elastic hoop buckling The tLD2 z, = -F,, 8E (3.2.5-7) stress determinationis based on a linear stress-strain relation- ship from: where Fhe = 2 Ch E(3.2.5-4) t/D Zc = required moment of inertia for ring composite where section, in.4 (m4), The critical hoopbucklingcoefficient Ch includes the L = ring spacing, in. (m), effect of initial geometric imperfections withinAPI Spec 2B tolerance limits. D = diameter, in. (m) see note 2 for external rings. C h = 0.44 t/D @M 2. 1.6 D/t Note1:Aneffectivewidthofshellequalto 1.1 (Dt)12may be assumed as the flangefor the composite ring section. C h = 0.44 (UD) + o21 (30.825 D/t 5 M < 1.6 D/t M4 Note 2: For external rings, D in Eq. 3.2.5-7 should be taken to the C h = 0.736/(M - 0.636) @3.5 I M < 0.825 D/t centroid of the composite ring. C h = 0.755/(M - 0.559) @1.51M< 3.5 Note 3: Where out-of-roundness in excess API Spec 2B is permit- of ted, larger stiffeners may be required. The bending due to out-of- C h = 0.8 @M < 1.5 roundness should be specifically investigated. The geometric parameter,M , is defined as: Note 4: The width-to-thickness ratios of stiffening rings should be selected in accordance with AISC requirements so as to preclude L local buckling of the rings. M = - (2D/t)12 (3.2.5-5) D Note 5: For flat bar stiffeners, the minimum dimensions should be 3/8 x 3 in. (10 x 76 mm) for internal rings and l/2 x 4 in. (13 x 102 where mm) for external rings. L = length of cylinder between stiffeningrings, dia- Note 6: Eq. 3.2.5-7 assumes that the cylinder and stiffening rings phragms, or end connections, in. (m). have the same yield strength. Note: For M L. 1.6D/t, the elastic buckling stress is approximately equal to that of a long unstiffened cylinder. Thus, stiffening rings, if 3.3COMBINEDSTRESSES FOR CYLINDRICAL required, should be spaced such thatM c 1.6D/t in order to be bene- MEMBERS ficial. Sections 3.3.1 and 3.3.2 apply to overall member behavior 2. Critical Hoop Buckling Stress. The material yield while Sections 3.3.3 and 3.3.4 apply tolocal buckling. strength relative to the elastic hoop bucklingstress determines whether elastic or inelastic hoop buckling occurs the crit- and 3.3.1Combined Axial Compression and Bending ical hoop buckling stress, Fhc, in ksi (MPa) is defined by the appropriate formula. 3.3.1.a Cylindrical Members Cylindrical members subjected to combined compression Elastic Buckling and flexure shouldbe proportioned to satisfy both follow- the ing requirements at all points along theirlength. Fhc = Fhe @ Fhe I 0.55 Fy Inelastic Buckling: 5 1.0 (3.3.1-1) Fhc = 0.45Fy + 0.18Fhe @0.55Fy < Fhe 5 1.6 Fy I (3.2.5-6) 5 1.0 (3.3.1-2) Fhc = Fy > 6.2 @ Fhe Fy J 0.6Fa FbCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 54. 42 where the undefined terms used are as defined by the AISC E M = axial loading and bending moment computed from Specijication for theDesign,Fabrication,andErection of a nonlinear analysis, including the - A) effect, (P Structural Steel Buildings. for F,, = critical local buckling stress from 3.2.2-4 with a Eq. When f- 5 0.15, the following formula may a be used in limiting valueof 1.2 Fy considering the effect of Fa strain hardening, lieu of the foregoing two formulas. Load redistribution between piles and along a pile may be considered. (3.3.1-3) 3.3.1.dMember Slenderness Determination of the slenderness ratio Kl/r for cylindrical Eq. 3.3.1-1 assumes that the same values C, and F, are of compressionmembersshould be inaccordancewiththe appropriate for fb, and fbY If different values are applicable, AISC. A rational analysis for defining effective length factors the following formula or other rational analysis should be should considerjoint fixity andjoint movement. Moreover, a used insteadof Eq. 3.3.1-1: rational definition of the reduction factor should consider the character of thecross-sectionandtheloadsactingonthe member. In lieu of such an analysis, the following values may be used Effective Reduction Length Factor Factor Situation K C$) 3.3.1.b Cylindrical Piles Superstructure Legs Column buckling tendencies should be considered for pil- Braced 1.0 ing below the mudline. Overall column buckling is normally Portal (unbraced) 852) not a problem in pile design, because even soft soils help to inhibitoverallcolumnbuckling.However,whenlaterally Jacket Legs and Piling loaded pilingsare subjected to significant axial loads, the load Grouted Composite Section 1.0 Ungrouted Jacket Legs 1.0 deflection (P - A) effect should be considered in stress com- Ungrouted Piling Between 1.0 putations. An effective methodof analysis isto model the pile Shim Points as a beam column on an inelastic foundation. When such an analysis is utilized, the following interaction check, with the Deck TrussWeb Members one-third increase where applicable, should be used: In-Plane Action 0.8 Out-of-plane Action 1.0 Jacket Braces Face-to-face length of Main 0.8 where F,, is given byEq. 3.2.3-4. Diagonals Face of leg to Centerline of Joint 0.8 3.3.1 .c Pile Overload Analysis Length ofK Braced3) For overload analysis of the structural foundation system under lateral loads (Ref. Section 6.7.1), the following interac- Longer Segment Length of X Braced3) 0.9 tion equation may be used to check piling members: Secondary Horizontals 0.7 5 1.0 (3.3.1-6) Deck Truss Chord Members 1.0 (1) Defined in Section3.3.le. (2) Use Effective Length Alignment Chart in Commentary of AISC. where the arc sin term is in radians and This maybe modified to account for conditions different from those assumed in developing the chart. A = cross-sectional area, in.2 (m2), (3) At least one pair of members framing into a joint must be in ten- sion if thejoint is not braced out of plane. Z = plastic section modulus, in3 (m3), (4) Whichever is more applicable to a specific situation.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 55. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 43 3.3.1 .e Reduction Factor fj = absolute value of hoop compression stress,ksi Values of the reduction factor C, referred to in the above table areas follows (with terms defined by AISC): as Fhc = critical hoop stress (see Eq. 3.2.5-6), (a) 0.85 SF, = safety factor for axial tension (see3.3.3, M, (b) 0.6 - 0.4 (- 0.85 ), but not less than 0.4, nor more than SFh = safety factor for hoop compression (see 3.3.5). M2 3.3.4 Axial Compression and Hydrostatic (c) 1 - 0.4 (-f a ) or 0.85, whichever is less Pressure F,’ ’ Whenlongitudinalcompressivestressesandhoopcom- 3.3.2 Combined Axial Tension and Bending pressive stresses occur simultaneously, the following equa- tions should besatisfied Cylindrical members subjected to combined tension and bending should be proportioned to satisfy Eq. 3.3.1-2 at all points along their length, where and fby are the computed fb, ( S F , ) +fb ( S F , ) 5 1.0 (3.3.4-1) bending tensile stresses. F, F,, 3.3.3 Axial Tension and Hydrostatic Pressure fh - + ( S F , ) 5 1.0 (3.3.4-2) F,c When member longitudinal tensile stresses and hoop com- pressive stresses (collapse) occur simultaneously, the follow- Eq. 3.3.4-1 should reflect the maximum compressive stress ing interaction equation should satisfied be combination. The following equation should alsobe satisfied whenf, > A2+B2+2vlAlB51.0 (3.3.3-1) 0.5 F h where A = where the term “A” should reflect the maximum tensile stress combination, B = SF, = safety factor for axial compression (see Section v = Poisson’s ratio = 0.3, 3.33, Fy = yield strength, ksi (MPa), SFb = safety factor for bending (see Section3.3.3, fa = absolute valueof acting axial stress, ksi (MPa), f, = fa +f b + (0.5f j ) * ; f, should reflect the maxi- mum compressive stress combination. fb = absolute valueof acting resultant bending stress, ksi (MPa), where F , , F , , Fhe, and Fhc are given by Equations 3.2.2-3, 3.2.2-4, 3.2.5-4, 3.2.5-6, and respectively. The remaining terms are defined in Section 3.3.3. tThis implies that the entire closed end force due to hydrostatic pressure is takenbythetubularmember.In reality, this forceNote:If f b > f a + 0.5 f h , bothEq.3.3.3-1andEq.3.3.4-1mustbe depends on therestraintprovidedbythe rest ofthestructureonthe satisfied. member and the stress may be more or less than 0.5fj. The stress computed from a more rigorous analysis may be substantiatedfor 0.5fj. 3.3.3. Section to * See footnoteCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 56. 44 2A-WSD PRACTICE RECOMMENDED API 3.3.5 Safety Factors W 0.5D To compute allowable stresses within Sections 3.3.3 and 3.3.4, the following safety factors should be used with the local buckling interaction equations. Loading Hoop Axial*** Axial Design Tension Compr. Condition Bending Compr. 1. the allow- Wherebasic 1.67 F/Fb** 1.67 2.0 to 2.0 able stresses would be used, e.g., pressures which will definitely be encountered during the installation or life of the structure. External junction ring 2. Where the one-third 1.25 Fy/1.33Fb 1.25 1.5 to 1.5 increase in allowable stresses is appropriate, Figure 3.4.1 -1-Example Conical Transition e.g., when considering interaction with storm Limiting Anglea, Deg. loads. Normal Extreme Condition Condition 3.4 CONICALTRANSITIONS D/t (fa + . h ) = 0.6 F v v + . h ) = 0.8 F v a 10.5 60 5.8 3.4.1AxialCompression and Bending 48 11.7 6.5 The recommendations in this paragraph may applied to be 13.5 36 7.5 a concentric cone frustum between two cylindrical tubular sections. In addition, the 16.4 rules may be applied to conical tran- 24 9.1 sitions at brace ends, with the cone-cylinder junction ring 18 18.7 10.5 rules applicable only the brace end the transition. to of 12 22.5 12.8 3.4.1 .a Cone Section Properties A cone-cylinder junction that does not satisfy the abovecriteria may be strengthened either by increasing the cylinder and cone wall The cone section properties should chosen to satisfy the be thicknesses at the junction, or by providing a stiffening ring at the axial and bending stresses each end of the cone.The nomi- at junction. nal axial and bending stressesat any section in a cone transi- tion are givenapproximatelyby v ,+ fb)/cos a,where a under consideration. This diameter is used in Eq. 3.2.2-4 to determine Fxc. cones of constantthickness,usingthe For equals one-half the projected apex angle the cone (see Fig- of f, ure 3.4.1- 1) and and fb are the nominal axial and bending diameter at the small end the cone would be conservative. of stresses computed using the section properties of an equiva- lent cylinder with diameter and thickness equal to the cone 3.4.1 .c Unstiffened Cone-Cylinder Junctions diameter and thickness the section. at Cone-cylinder junctions are subject to unbalanced radial forces due tolongitudinalaxialandbendingloadsandto 3.4.1 .b Local Buckling localized bending stresses caused by the angle change. The longitudinal and hoop stresses at the junction may be evalu- For local buckling under axial compression and bending, ated as follows: conical transitions with an apex angle less than 60 degrees maybeconsidered as equivalentcylinderswithdiameter 1. Longitudinal Stress equal to Dlcos a,where D is the cone diameter at the point In lieu of detailed analysis, the localized bending stressat anunstiffened cone-cylinderjunction be may estimated, based on results presented in Reference 3, Section from: C3.2 **The safety factor with respect to the ultimate stress is equal to 1.67 and illustrated on Figure C3.2.3-1. ***The value used should notbe less than the AISC safety factor 0.6t,/D(t + t,) fb = (fa+ f b ) tana (3.4.1-1) for column buckling under axial compression. tfCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 57. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONS1 XUCTING FIXED AND OFFSHORE PLATFORMS-WORKING STRESS DESIGN 45 where ultimate tensile strength, limit analysis indicates that plastic sectionmodulusandloadredistributionprovidesufficient D = cylinder diameter at junction, in. (m), reservestrength so thattransitionswiththeseanglescan t = cylinder thickness, in. (m), develop the full yield capacity of the cylinder. If the steels used at the transition have sufficient ductility to develop this t, = cone thickness, in. (m), reserve strength, similar joint cans, these same angles may be te = t for stress in cylinder section, appliedload in allowable to caseswhich stresses are increased by one third. = t, for stress in cone section, The limiting angles for the extreme condition have been fa = acting axial stress in cylinder section junc- at derived on the more conservative basis that the allowable hot tion, ksi (MPa), spot stressat the transition continues to the ultimate tensile be strength, while allowable stresses in the cylinder have been fb = acting resultant bending stress in cylinder sec- increased by one-third. This also reduces the stress concentra- tion at junction, ksi (MPa), tion factor from 2.22 to 1.67, which less than the minimum is a = one-half the apex angle of the cone, degrees. brace SCF at nodes (Table 5.1.1-1) and would thus rarely govern the design. The fatigue strength of the cone-cylinder For strength requirements, the total stress vu+ fb + f’b) junction should be checked in accordance with the require- ments of Section 5. should be limited to the minimum tensile strength cone of the vu and cylinder material, with + fb) limited to the appropriate allowable stress. For fatigue considerations, the cone-cylinder 3.4.1.d Cone-Cylinder Junction Rings junction should satisfy the requirements of Section 5 with a If stiffening are rings required, section the properties stress concentration factor equal to [ 1 + fb’/vu + fb)], where should be chosen satisfy both the following requirements: to fb’ is given by Eq. 3.4.1- 1. For equal cylinder and cone wall thicknesses, the stress concentration factor is equal(1 + 0.6 to (3.4.1-3) A J m t tana). 2. Hoop Stress (3.4.1-4) The hoop stress caused by the unbalanced radial line load may be estimated from: where D = cylinder diameter at junction, in. (m), fj’ = 0.45 vu+fb) tn a a (3.4.1-2) D, = diameter to centroidof composite ring section,in. (m). See note3, where the termsare as defined in Subparagraph (1). For hoop A, = cross-sectional area of composite ring section, in.2 tension, fj’ should be limited to 0.6 F, For hoop compres- sion,fj’ should be limited to 0.5 Fh,, where Fh, is computed (m2L using Eq. 3.2.5-6 with Fhe = 0.4 Et/D. This suggested value Z = moment of inertia of composite ring section, in.4 , of Fhe is based on results presented in ReferenceCommen- 4, tary on Allowable Stresses, Par.C3.2. BasedonthestrengthrequirementsofEqs.3.4.1-1and In computing A, and Z,, the effective width of shell wall 3.4.1-2, limiting cone transition angles canbe derived below acting as a flange for the composite ring section may be com- which no stiffening is required to withstand the cone-cylinder puted from: junction stresses. For example, the following table limiting of cone transition angels is derived for equal cone and cylinder be= 0.55 ( f i t + f i c ) (3.4.1-5) wall thicknesses, Fy 5 60 ksi, and the corresponding mini- mum tensile strengths given in Table 8.1.4- 1. The limiting Note 1: Where the one-third increaseis applicable, the required sec- angles in the table represent the smaller of the two angles tion propertiesA, and I, may be reduced by 25%. evaluatedbysatisfyingthestrengthrequirements of Eqs. Note 2: For flat barstiffeners,the minimum dimensions should be 3/8 3.4.1-1 and 3.4.1-2. limiting angles in the table were gov- The x 3 in. (10 x 76 mm) for internal rings andl/2 x 4 in. (13 x 102 mm) erned by Eq. 3.4.1- The limiting angles for the normal con- 1. for external rings. dition apply to design cases where basic allowable stresses Note 3: For internal rings, D should be used instead of D, in Eq. are used. While elastic hot spot stresses are notionallyat the 3.4.1-4.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 58. 46 2A-WSD PRACTICE RECOMMENDED API 3.4.2 Hydrostatic Pressure Note 1: A junction ring is not required for hydrostatic collapse if Eq. 3.2.5-1 is satisfied with Fhe computed using c = 0.44 @/D)cos a in h The recommendations in this paragraph may applied to be Eq. 3.2.5-4, whereD is the cylinder diameterat the junction. a concentric cone frustum between two cylindrical tubular sections. In addition, therules may be applied to conical tran- Note 2: For external rings, D in Eq. 3.4.2-1 should be taken to the centroid of the composite ring. sitions at brace ends, with the cone-cylinder junction ring rules applicable only to the brace end of the transition. 4 Connections 3.4.2.a Cone Design 4.1 CONNECTIONS OFTENSION AND Unstiffened conical transitions or cone sections between COMPRESSION MEMBERS rings of stiffened cones with a projected apex angle less than The connections at the ends of tension and compression 60 degrees may be designedfor local buckling under hydro- members should developthestrengthrequiredbydesign static pressure as equivalent cylinders with a length equal to loads, but not less than 50% of the effective strength of the the slant height of the cone between rings and a diameter member. The effective strengthis defined as the buckling load equal to Dlcos a,where D is the diameter at the large end of for members loaded in either tension or compression, and as the cone section and a equals one-half the apex angle of the the yield load for members loaded primarily in tension. cone (see Figure 3.4.1-1). The above rule can be considered satisfiedfor simple tubu- lar joints when the following conditionis obtained. 3.4.2.b Intermediate Stiffening Rings If required, circumferential stiffeningringswithin cone (4.1-1) transitions may be sized using Eq. 3.2.5-7 with an equivalent diameter equal to Dlcos a,where D is the cone diameter at the ring, t is the cone thickness, L is the average distance to where adjacent rings along the cone axis and Fhe is the average of the elastic hoop buckling stress values computed for the two Fyc = the yield strength of the chord member at the adjacent bays. joint (or 2/3 of the tensile strength less), ksi if (MW, 3.4.2.cCone-Cylinder Junction Rings Fyb = the yield strength of the brace member, Circumferential stiffening rings required the cone-cylin- at der junctions should be sized such thatthe moment of inertia ß, ,T, 8 = joint geometry parameters(see Figure 4.1-l), T y of the composite ring section satisfies following equation: the and Fyb should be based on thenominal brace member, not the brace stub should one exist. 16E { I , = - tL,F,, + tCc L Fhec cos2a (3.4.2-1) Weldsin connections at the ends of tubularmembers should bein accordance with 11.1.3 or should not less than be where required to develop a capacity of equal to the lesser r, = moment of inertia of composite ring section with 1. Strength of the branch member based on yield, or effective widthof flange computed from 3.4.1- Eq. 2.Strength of the chord based onpunching shear (where 5 , in.4 (m4), applicable). D = diameter of cylinder at junction, in. (m). See Note 2, t = cylinder thickness,in. (m), 4.2 RESTRAINT AND SHRINKAGE t, = cone thickness, in. (m), Details should be such as to minimize constraint against ductile behavior, to avoid undue concentration of welding, L, = distance to first stiffening ring cone section along in and to afford simple access for the placing of weld metal. cone axis, in. (m), Joints should be designed so as to minimize, insofar as LI = distance to first stiffening ring in cylinder section, practicable, stresses due to the contraction of the weld metal in. (m), and adjacentbasemetalupon cooling. Particular care is requiredwhereshrinkage strains inthethrough-thickness Fhe = elastic hoop buckling stress cylinder, ksi (MPa), for direction may lead to lamellar tearing in highly restrained Fhec = Fhe for cone section treatedas an equivalent cylin- joints. See 10.5.4 ofAWS D1.2 Commentary on the Struc- der, ksi (MPa). tural Welding Code.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 59. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHORE PLATFORMS-WORKINGSTRESS DESIGN 47 chord) O = Brace angle (measured from g = Gap, in. (mm) t = Brace thickness,in. (mm) T= Chord thickness, in. (mm) d = Brace diameter,in. (mm) D= Chord diameter, in. (mm) T = -t T ß = -d D D y = - 2T Figure 4.1 -1-Terminology and Geometric Parametersfor Simple Tubular Connections 4.3 TUBULAR JOINTS a. Punching shear The acting punching shear should be calculated by 4.3.1 Simple Joints Simple tubular joints, without overlap of principal braces Vp = T f sin0 (4.3.1-1) and having no gussets, diaphragms, or stiffeners should use the following guidelines. Terminology is defined in Figure where 4.1-1. f = nominal axial, in-plane bending,or out-of- Joint classification as K, T & Y, or cross should apply to plane bending stress in the brace (punching individual braces according to their load pattern for each load shear for each kept separate). case. To be considered a K-joint, the punching load in a brace should be essentially balanced loads on other braces in the by The allowable punching shear stress in the chord is the wall same plane on the same sidethe joint. In T andY joints the of lesser of the AISC shear allowable or punching load is reached beam shear in the chord. cross as In joints the punching load is carried through the chord to braces on the opposite side. For braces which carry of their load part as K-joints, and part as T & Y or cross joints, interpolate based on the portion of each in total. Examples are shown in (plus l/3 increase applicable) where (4.3.1-2) Figure 4.3.1-1.See Commentary on Joint Classification. Many properly designed tubular joints, especially those Capacity vpa must be evaluated separately for each compo- with brace to chord diameter ratios approaching 1.0, will nent of brace loading, utilizing the appropriate Qq and Qffac- exhibit different failure mechanisms and strength properties tors. Qq is a factor to account for the effects of loading of type than the empirically based formulas contained herein. At and geometry, as given in Table 4.3.1-1. Qf is a factor to present, insufficient experimentalevidence exists topre- account for the presence nominal longitudinal stress in the of cisely quantify the degree of increased strength. Therefore, chord. inlieu of the recommendations contained in Section 4.3 herein, reasonable alternative methods may be used for the Qf= 1.0-hyA2 design of such joints. The adequacy of the joint may be determinedon thebasis where of (a) punching shear or (b) nominal loads in the brace. These approaches are intended to give equivalent results. h = 0.030 for brace axial stress, Brace axial loads and bending moments essential to the - 0.045 for brace in-plane bending stress, - integrity of the structure** should be included in the - 0.021 for brace out-of-plane bending stress, - calculations. A = **Reductions in secondary (deflection-induced) bending moments due tojoint flexibility or inelastic relaxation maybe considered. (l/3 increase applicable to denominator).COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 60. 48 2A-WSD PRACTICE RECOMMENDED API 50% K, 50% T&Y 1O00 Figure 4.3.1 -1"Example of Joint ClassificationCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 61. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 49 F, m are the nominal in bending, axial, plane and &T2 chord. bending out-of-plane the stresses in P a = QuQf 0- Set Qf= 1.O when all extreme fiber stresses in the chord are (plus l/3 increase applicable) where (4.3.1-4a) tensile. For combined axial and bending stresses in the brace, the following equations interaction should be satisfied M a = QuQf F.ycT2 (0.8d) (plus l/3 increase applicable) where (4.3.1-4b) 5 1.0 (4.3.1-3a) where P, = allowable capacity for brace axial load, + 2. arcsin 5 1.0 V p a IPB V p a OPB M, = allowable moment. capacity brace bending for where term (4.3.1-3b) the radians arcsinis in Other terms, except are defined in4.3.l(a) Qu, b. Nominal Loads Allowable joint capacitiesinterms of nominalbraceloads Qu is theultimatestrengthfactorwhichvarieswiththe are type, load Table 4.3.1-2. and joint in as given Table 4.3.1 -1-Values for Qq Qg=1.8-0.1g/Tfory<20 for ß > 0.6 Qp = ß( 1 - 0.8338) Q, = 1.8 - 4 g/D for y > 20 Qp = 1.0 for ß < 0.6 but in no case shallQ, be taken as less than 1.0. Type of Load in Brace Member Axial Axial In-Plane Out-of-Plane Compression Tension Bending Bending 4hl + overlap 1.8 plus see 4.3.2 B K 8 a @P (1.10 + 0.2043) Q, 8 + T&Y (1.10 + 0.2043) (3.72 + 0.67/ß) (1.37 + 0.67/ß)Qp .- E O w/o diaphragms (1.10 + 0.2043) (0.75 + 0.20/ß)Qp h U cross w/diaphragms per 2.5.5c.4 (1.10 + 0.2043) Table 4.3.1 -2-Values for Qu() Type of Load in Brace Member Axial Axial In-Plane Out-of-Plane Compression Tension Bending Bending K (3.4 + 19ß)Qg 4 h -4 T&Y (3.4 + 19ß) (3.4 + 19ß) g € w o (3.4 + 19ß) :c3 cross w/o diaphragms (3.4 + 13ß)Qp $2 w/diaphragms per 2.5.5c.4 (3.4 + 19ß) 1) Terms are defined in Figure 4.1-1 and Table 4.3.1-1.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 62. 50 2A-WSD PRACTICE RECOMMENDED API For combined axial and bending loads in the brace, the fol- lowing interaction equations should satisfied be (4.3.1-5a) r "i where term (4.3.1-5b) the radians arcsinis in c. Design Practice " " I " If an increased wall thickness in the chord at the joint is required, it should be extended past the outside edge of the Separation bracing a minimum one quarter of the chord diameter 12 of or 2" minimum inches (305 mm) including taper, whichever is greater. See Figure 4.3.1-2. The effect of joint can length on the capacity of cross joints is discussed in Section 4.3.4. Where increased wall thickness or special steel is used for braces in the tubular joint area, it should extend a minimum of one brace diameter or 24 inches (610 mm) from the joint, I including taper, whichever greater. is Nominally concentric joints may be detailed with the i workingpoints (intersections of brace and chord center- lines) offset in either direction by as much as one quarter of the chord diameter in order to obtain a minimum clear dis- t Stub of heavy wall or special steel in brace (optional) tance of 2 inches (51 mm) between nonoverlapping braces or to reduce the required length of heavy wall in the chord. Figure 4.3.1 -2"Detail of Simple Joint See Figure4.3.1-2. For joints having a continuous chordof diameter substantially greater than the brace members (e.g., jacket leg joints), the moments caused by this minor eccen- or for punching shear format tricity may be neglected. For K and X joints where all mem- bers are of similar diameter, the moments caused by I, eccentricity may be importantand should beassessed by the P I = (Pusin@ ) + (2vw,twZ2) - (4.3.2-2) I designer. Simple joints which cannot be detailed to provide the 2 for nominal load format. inch (5 1 mm) minimum clear distance between braces within the limits of allowable offset of the working point, as estab- where lished above, should be designed for stress transfer as dis- vp = allowable punching shear stress in ksi( m a ) as cussed in 4.3.2 below and specially detailed on the drawings. defined in4.3.l(a) for axial stress, 4.3.2 Overlapping Joints P, = Allowable axial load in kips (N) as defined in 4.3.l(b), Overlapping joints, in which brace moments are insignifi- cant and part the axial load is transferred directly from of one Vw, = AISC allowable shear stress in ksi (MPa) for braceanother to through common may their weld, be weld between braces, designed as follows: tw = the lesser of the weld throat thickness or the The allowable axial load component perpendicular to the thickness tof the thinner brace,in. (mm), chord. P I in kips (N), should be taken as I1 = circumference for that portion the brace which of P I = (vp TI1) + (2VW,tWZ2) (4.3.2-1) contacts the chord (actual length) (mm), in.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 63. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 51 I = circumference of brace contact with chord, presenceneglecting of overlap, "I T 12 = the projected chord length (one side) the over- of lapping weld, measured perpendicular to the chord, in. (mm). These terms are illustrated in Figure 4.3.2- 1. The overlap should preferablybe proportioned for at leastacting 50% of the PI. Section A-A Where thebraces carry substantially different loads and/or one brace is thicker than the other, the heavierbrace should preferablybethethrough brace (asillustratedin Figure 4.3.2-1) with its full circumference welded to the chord. In no case should the brace wall thickness exceed the chord wall thickness. Moments causedby eccentricity of the brace working lines and exceeding that in 4.3.l(c) may be important and should Figure 4.3.2-1-Detail of Overlapping Joint be assessed bythe designer. 4.3.3 Congested Joints to Where bracing members in adjacent planes tendoverlap in congestedjoints, the following corrective measures may be considered by the designer. Where primary braces are substantially thicker than the secondary braces, they may be made the through member, with the secondary braces designed overlapping braces per as Section 4.3.2.See Figure 4.3.2-2, Detail A. An enlarged portion of the through member may be used as indicated in Figure 4.3.2-2, DetailB designed as a simple joint per Section4.3.1. Primary braces L Overlapping secondary A spherical joint, Figure 4.3.2-2, Detail C may be used, brace with t < t, , designed on the basis of punching shear per Section 4.3.1, assuming: Enlarged joint y = D/4T 8 = arccos (P) Qq = 1.0 L Intersection lines for braces Qf = 1.0 Detail C Secondary braces causing interference may be spread out as indicated in Figure 4.3.2-2, Det. D, provided the moments caused by the eccentricity of their working lines are consid- ered in the design analysis. 4.3.4 Load Transfer Across Chords Detail D Cross joints, launch leg joints, and other joints in which load is transferred across the chord should be designed to resist general collapse. However, for such joints reinforced V O f f s e t secondary braces only by a joint can having increased thickness and lengthL T, (for cases where joint cans are centered on the brace inter- of est L is definedas shown in Figure 4.3.4- and having brace la) Figure 4.3.2-2-Secondary Bracing COPYRIGHT American Petroleum Institute Licensed by Information Handling Services
  • 64. 52 2A-WSD PRACTICE RECOMMENDED API Any reinforcement within this dimension (e.g., diaphragms, Pl P3 rings, gussets or the stiffening effect of plane members) of out may be considered in the analysis, although its effectiveness decreases with distance from the branch footprint. Joints havingtwo or more appropriately located dia- phragms at each branch need only be checked for local capac- ity. The diaphragmsshallbe at least as thick as thewall thickness of the corresponding branch member. capacity The may be calculated using Table 4.3.1-1 or 4.3.1-2 for cross joints with diaphragms. 4.3.5 OtherComplex Joints Joints not coveredby Sections 4.3.1 through 4.3.4 may be designed on the basis of appropriate experimental or in ser- vice evidence.In lieu of such evidence, an appropriate analyt- H icalcheckshouldbemade. This checkmaybedoneby cutting sections which isolate groupsof members, individual members, and separate elements of the joint (e.g., gussets, diaphragms, stiffeners, welds in shear, surfaces subjected to punching shear), and verifying that a distribution of stress can be assumed that satisfies equilibrium without exceeding the allowable stressof the material. -P 5 Fatigue 5.1 FATIGUE DESIGN Inthedesign of tubularconnections, due consideration should be given to fatigue problems as related to local cyclic stresses. A detailed fatigue analysis should be performed for tem- Figure 4.3.4-1-Definition of Effective Cord Length plate type structures. is recommended that a spectral analy- It sis technique be used. Other rational methods may be used 0.9, chord diameter ratio less than the allowable axial branch provided adequate representation of the forces and member load shall be taken as: responses can be shown. In lieu of detailed fatigue analysis, simplified fatigue anal- yses, which have been calibrated for the design wave climate, P = P(l) + - [P(2) - P(1)] for L < 2.50 L (4.3.4-la) may be applied to tubular joints in template type platforms 2.5 D that: 1. Are in less than 400 feet (122 m) water. of P = P(2) for L > 2.50 (4.3.4-1b) 2. Are constructed of ductile steels. 3. Have redundant structural framing. where 4. Have natural periods less than 3 seconds. P(l) = P from Eq. 4.3.1-4a using the nominal chord , member thickness, 5.2 FATIGUEANALYSIS P(2) = P from Eq. 4.3.1-4a using thicknessT,. , A detailed analysis of cumulative fatigue damage, when required, shouldbe performed as follows: Special consideration is required for more complex joints. 5.2.1 The wave climate should be derived as theaggre- For multiple branches in the same plane, dominantly loaded gate of all sea states to be expectedover the long term. This in the same sense, the relevant crushing load is Ci Pi Sin 0i. may be condensed for purposes of structural analysis into An approximateclosedringanalysismay be employed, representative sea states characterized by wave energy spec- includingplasticanalysiswithappropriatesafetyfactors, tra and physical parameters together with a probability of using an effective chord length as shown in Figure 4.3.4-1b. occurrence.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 65. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 53 5 2 2 A space frame analysis should be performed .. to When fatigue damage can occur due to other cyclic load- obtain the structural response in terms of nominal member ings, such as transportation, the following equation should be stress for given wave forces applied to the structure. In gen- satisfied eral, wave force calculations should follow the procedures describedin Section However, 2.3.1. current may be C SFiDi < 1.O (5.2.5-1) neglected and, therefore, considerations for apparent wave i period and current blockage are not required. In addition, wave kinematics factor equal to 1.0 and conductor shielding Where Di is the fatigue damage ratio for each type of load- factor equal to 1.0 should be applied for fatigue waves. The ing and SFi is the associated safety factor. For transportation drag and inertia coefficients depend on the sea state level, as where long term wave distributions are used to predict short parameterized by the Keulegan-Carpenter Number K (see term damage a larger safety factor should be considered. Commentary C2.3.lb7). For small waves (1.0 < K < 6.0 for platform legs at mean water level), values of C, = 2.0, c d = 5 3 S-N CURVES FOR ALL MEMBERS AND . 0.8 for rough members and c d = 0.5 for smooth members CONNECTIONS, EXCEPTTUBULAR should be used.Guidelinesforconsideringdirectionality, CONNECTIONS spreading, tides and marine growth provided in the com- are Non-tubular members and connections in deck structures, mentary for this section. appurtenances equipment; tubular and and members and A spectral analysis technique should be used determine to attachments to them, including ring stiffeners, may be subject the stress response for each sea state. Dynamic effects should to variations of stress due to environmental loads or opera- be considered for sea states having significant energy near a tional loads. Operational loads would include those associ- platform’s natural period. atedwithmachinevibration,craneusageandfillingand emptying of tanks. Where variations of stress are applied to 5 2 3 Local stresses that occur within tubular connections .. conventional weld details, identified in ANSI/AWSD 1.1-92 should be considered in terms of hot spot stresses located Figure 9.1, the associated S-N curves provided in Figure 9.2 immediately adjacent to the joint intersection using suitable or 9.3 should be used, dependent on degree of redundancy. stress concentration factors.The microscale effects occurring Where such variationsof stress are applied to situations iden- at the toeof the weld are reflected in the appropriate choice of tified in ANSI/AWS Dl.1-92 Table 10.3, the associated S-N the S-N curve. curves provided in Figure 10.6 should be used. For service 5 2 4 Foreachlocationaroundeachmemberintersection .. conditionswheredetailsmaybeexposedtocorrosion, no of interest in the structure, the stress response for each sea endurance limit should be considered. For submerged service, state should be computed, giving adequate consideration to where effective cathodic protectionis present, the endurance both global and local stress effects. limit should be considered to occur at 2*108 cycles. Stress Categories DT, ET, F T , K1, K2, X1, and X2 refer to tubular The stressresponsesshouldbecombinedintothelong connections and are covered by Section 5.4 of this Recom- term stress distribution, which should then be used to calcu- mended Practice which takes precedence. late the cumulative fatigue damage ratio, where D, The referenced S-N curves in ANSIIAWS Dl. 1 .-92 Figure 9.2,9.3, and 10.6 are Class curves. For such curves, the nom- D = C(nnv) (5.2.4-1) inal stress range in the vicinity of the detail should be used. and II = number of cycles applied at a given stress range, Due to load attraction, shell bending, etc., not present in the class type test specimens, the appropriate stress may be larger N = number of cycles for which the given stress thanthenominalstressinthegrossmember.Geometrical range would be allowed by the appropriate S-N stressconcentrationandnotcheffectsassociatedwiththe curve. detail itself are included in the curves. Alternatively, the damage ratio may be computed for each 5 4 S-N CURVES FORTUBULAR CONNECTIONS . seastateandcombinedtoobtainthecumulativedamage ratio. For tubular connections exposed to variations stress due of to environmental or operational loads, the S-N curves shown 5 2 5 In general the design fatigue .. life ofeach joint and in Figure 5.4-1 should be used. These curves are applicable to member should be at least twice the intended service life of random loading and presume effective cathodic protection. the structure (i.e., Safety Factor 2.0). For the design fatigue = For splash zone,free corrosion, or excessive corrosion condi- life, D should not exceed unity. For critical elements whose tions no endurance limit should be considered. Connections sole failure could catastrophic, use of a larger safety factor be in the splash zone should generally be avoided. For tubular should be considered. connections subject to regular cyclic loading in atmosphericCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 66. 54 PRACTICE RECOMMENDED API 2A-WSD service the endurance limits for the and X curves may be X where assumed tobe at lo7 and 2*lo7 respectively. So = allowable stress from the S-N curve, For welds without profile control, but conforming to a basic standard flatprofile (ANSUAWS D l . 1.-92 Figure t = branchmemberthickness, 10.12) and having a branch thickness less than 0.625-in. (16 mm) the X curve is applicable. For the same flat pro- to = limitingbranchthickness. file at greater wall thicknesses the scale effect correction (Eq. 5.4.-1) should be used. However, the X-curve may be The X-curve is applicable for welds with profile control,as used for unlimited branch thicknesses provided the profile defined in 11.1.3d, and having a branch thickness less than 1 control requirements of 11.1.3d are satisfied. in. (25 mm). For the same controlled profile at greater wall The scale effect correction is: thicknesses, the scale effect correction (Eq. 5.4-1) should be used.However,reductions the below X-curve not are required. allowable stress = So. (5.4-1) For branch thicknesses greater than 1 in., the X-curve may be used without scale effect provided the profile is ground 1O0 "" x i 50 I= s g) 20 c 2 u) 10 ." 5 ._ -V K i 5 ." Q 0 c I 2 "" 1 .... 0.5 103 104 108 10 5 107 106 109 Permissible Cyclesof Load N Note:These curvesmay be represented mathematically as where N is the permissiblenumber of cycles for applied cyclic stress rangeAo, with Aoref and m as listedbelow. &ref m Stress Range at Inverse Endurance Limit at Cu we 2 Million Cycles Log-Log Slope 200 Million Cycles X MPa) ksi 14.5 (100 MPa) ksi5.07 (35 4.38 X MPa) ksi 11.4 (79 MPa) ksi3.33 (23 3.74 Figure 5.4-1-Fatigue S-N CurvesCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 67. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 55 smooth to a radius greater than or equal to half the branch or hydraulic power as the source of energy. The pile wall thickness. Final grinding marks should be transverse to the thickness should be adequate to resist axial and lateral loads weld axis and the entire finished profile should pass magnetic as well as the stresses during pile driving.is possible to pre- It particle inspection. dict approximately the stresses during pile driving using the principles of one-dimensional elastic stress wave transmis- 5.5 STRESS CONCENTRATION FACTORS sionbycarefullyselectingtheparametersthatgovernthe behavior of soil, pile, cushions, capblock and hammer. For a The X and X’ curves should be used with hot spot stress more detailed study of theseprinciples, to refer E.A.L. ranges based on suitable stress concentration factors. Stress Smith’s paper, “Pile Driving Analysis by the Wave Equation,” concentrationfactorsmay be derivedfromfiniteelement Transactions ASCE, Vol. 127, 1962, Part 1, Paper No. 3306, analyses, model tests or empirical equations based on such pp,1145-1193. The aboveapproachmayalso be usedto methods. optimize the pile hammer cushion and capblock with the aid For joints not meeting the requirements of Section 4.3.1, of computer analyses (commonly known as the Wave Equa- e.g., connections in which load transfer is accomplished by tion Analyses).The design penetration of driven piles should overlap (Section 4.3.2), or by gusset plates, ring stiffeners, be determined in accordance with the principles outlined in etc. (Section 4.3.3, a minimum stress concentration factor Sections 6.3 through 6.7 and 6.9 rather than upon any correla- of 6.0 should be used in the brace member, in lieu of addi- tion of pile capacity with the number of blows required to tional analysis. Where the chord and/or other joint rein- drive the pile a certain distance into the seafloor. forcement are not designed to develop the full static capacity of the members joined, these elements should also When a pile refuses before it reaches design penetration, be checked separately. one or moreof the following actions can taken: be 6 Foundation Design a. Review of hammer performance.A review of all aspects of hammer performance, possibly with the aid of hammer and The recommended criteria of Section 6.1 through Section pile head instrumentation, may identify problems which can 6.1 1 are devoted to pile foundations, and more specifically to be solved by improved hammer operation and maintenance, steel cylindrical (pipe) pile foundations. The recommended or by theuse of a more powerful hammer. criteria of Section 6.12 through Section 6.17 are devoted to shallow foundations. b.Reevaluation of designpenetration.Reconsideration of loads, deformations and required capacities, of both individ- 6.1 GENERAL ual piles and other foundation elements, and the foundationas a whole, may identify reserve capacity available. An interpre- The foundation should be designed to carry static, cyclic tation of driving records in conjunction with instrumentation and transient loads without excessive deformationsor vibra- mentioned above may allow design soil parameters or stratifi- tions in the platform. Special attention shouldbe given to the cation to be revised and pile capacity to be increased. effects of cyclic and transient loading on the strength of the supporting soilsas well as on the structural response piles. of c. Modifications to piling procedures, usually the last course Guidance provided in Sections 6.3,6.4, and 6.5is based upon of action, may includeone of the following: static, monotonic loadings. Furthermore, this guidance does Plug Removal. The soil plug inside the pile is removed not necessarily apply to so called problem soils such as car- by jetting and air lifting or by drilling to reduce pile bonate material or volcanic sands or highly sensitive clays. driving resistance.If plug removal results in inadequate The possibility of movement of the seafloor against the foun- pile capacities, the removed soil plug should be replaced dation members should be investigated and the forces caused by a gravel grout or concrete plug having sufficient load- by such movements, if anticipated, should be considered in carryingcapacitytoreplacethat oftheremovedsoil the design. plug. Attention should be paid to plug/pile load transfer characteristics.Plugremovalmaynotbeeffectivein 6.2 PILE FOUNDATIONS some circumstances particularly in cohesive soils. Soil Removal BelowPile Tip. Soil below the pile tip is Types of pile foundations used to support offshore struc- tures areas follows: removed either by drilling an undersized hole jetting or equipment is lowered through the pile which as the acts casing pipe for the operation. The effect on pile capac- 6.2.1 Driven Piles ity of drilling an undersized hole is unpredictable unless Open ended piles are commonly used in foundations for there has been previous experience under similar condi- offshore platforms. These piles are usually driven into the tions. Jetting below the pile tip should in general be sea-floor with impact hammers which use steam, diesel fuel, avoided becauseof the unpredictabilityof the results.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 68. 56 2A-WSD PRACTICE RECOMMENDED API Two-State Driven Piles. A first stage or outer pile is spiral reinforcement meeting the requirements ofAC1 318. driven a topredetermineddepth, soil the plug is Load transfer between the concrete and the pile should be removed,andasecondstage or inner pile is driven designed in accordance with Sections 7.4.2,7.4.3, and 7.4.4. inside the first stage pile. annulus between the two The pilesisgroutedtopermitloadtransferanddevelop 6.3 PILE DESIGN composite action. 6.3.1 Foundation Size Drilled and grouted insert piles as described in 6.2.2(b) below. When sizing a pile foundation, the following items should be considered: diameter, penetration, wall thickness, type of 6.2.2 Drilled and Grouted Piles tip,spacing,number of piles,geometry,location,mudline restraint,materialstrength,installationmethod,andother Drilled and grouted piles can be used in soils which will parameters as may be considered appropriate. hold an open hole with without drilling mud. Load transfer or betweengroutandpileshould be designedinaccordance 6.3.2 Foundation Response with Sections 7.4.2, 7.4.3, and 7.4.4. There are two types of drilled and grouted piles, follows: as A number of different analysis procedures may be utilized to determine the requirements of a foundation. At a mini- a. Single-Stage.Forthesingle-staged,drilledandgrouted mum, the procedure used should properly stimulate the non- pile, an oversized hole is drilled to the required penetration, a linear response behavior the soil and assure load-deflection of pile is lowered into the hole and the annulus between the pile compatibility between the structure and the pile-soil system. and the soil is grouted.This type pile can be installed only in soils which will hold an open hole to the surface. As an alter- 6.3.3 Deflections and Rotations native method, the pile with expendable cutting tools attached to the tip can be usedpart of the drill stem to avoid the time as Deflections and rotations of individual piles and the total required to remove the drill bit and insert a pile. foundation system should be checked at all critical locations b. Two-Stage. The two-staged, drilled and grouted pile con- which may include pile tops, points of contraflecture, mud- sists of two concentrically placed piles grouted to become a line, etc. Deflections and rotations should not exceed service- composite section.A pile is driven to a penetration which has ability limits which would render the structure inadequate for been determined to be achievable with the available equip- its intended function. ment and below which an open hole can be maintained. This outer pile becomes the casing for the next operation which is 6.3.4 Pile Penetration to drill through it to the required penetration for the inneror The design pile penetration should sufficient to develop be “insert” pile. The insert pile is then lowered into the drilled adequatecapacity to resistthemaximumcomputedaxial hole and the annuli between the insert pile and the soil an bearing and pullout loads with an appropriate factor of safety. between the two piles are grouted. Under certain oil condi- The ultimate pile capacities can be computed in accordance tions, the drilled hole is stopped above required penetration, with Sections6.4 and 6.5 or by other methods which sup- are and the insert pile is driven to required penetration. The diam- ported by reliable comprehensive data. The allowable pile eter of the drilled hole should be at least 6 inches (150 mm) capacities are determined by dividing the ultimate pile capac- larger than the pile diameter. ities by appropriate factorsof safety which should not less be than the following values: 6.2.3 Belled Piles Factors of Bells may be constructedat the tipof piles to give increased Condition Load Safety bearing and uplift capacity through direct bearing on the soil. Drilling of the bell is carried out through the pile by under- 1. Design environmental conditions with reaming with an expander tool. A pilot hole may be drilled 1.5 loads drilling appropriate below the bell to act a sump for unrecoverable cuttings. as The 2. Operating environmental conditions during bell and pile are filled with concrete to a height sufficient to operations drilling 2.0 develop necessary load transfer between the bell and the pile. 3. Design environmental conditions with Bells are connected to the pile to transfer full uplift and bear- 1.5 loads appropriate producing ing loads using steel reinforcing such as structural members with adequate shear lugs, deformed reinforcement or pre- bars 4. Operating environmental conditions during stressed tendons. Load transfer into the concrete should be operations producing 2.0 designed in accordance with AC1 318. The steel reinforcing 5. Design environmental conditions with should be enclosed for their full length below the pile with 1.5 (for pullout) minimum loadsCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 69. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 57 6.3.5 Alternative Design Methods and the pile well as the compressibilityof the soil pile sys- as tem. Eq. 6.4.1-1 assumesthatthemaximumskinfriction The provisions of this recommended practice for sizing the along the pile and the maximum and bearing are mobilized foundation pile are based on an allowable stress (working simultaneously.However,theultimateskinfrictionincre- stress) method except for pile penetration per Section 6.3.4. ments along the pile are not necessarily directly additive, nor In this method, the foundation piles should conform to the is the ultimate end bearing necessarily additive to the ultimate requirements of Sections 3.2 and 6.10 in addition to the provi- skin friction. In some circumstances this effect may result in sions of Section 6.3. Any alternative method supported by the capacity being less than that givenEq. 6.4.1 1. In such by - sound engineering methods and empirical evidence may also cases a more explicit consideration axial pile performance of be utilized. Such alternative methods include the limit state effects on pile capacity may be warranted. For additional dis- design approachor ultimate strength design the total foun- of cussion of these effects refer to Section6.6 and ASCE Jour- dation system. nal o the Soil Mechanics and Foundations DivisionLoad f for Transferfor Axially Loaded Piles in Clay, H.M. Coyle and by 6.3.6 Scour L.C. Reese,Vol. 92, No. 1052, March 1966, Murff, J.D., “Pile Seabedscouraffectsbothlateralandaxialpileperfor- Capacity ain SofteningSoil,” InternationalJournal for mance and capacity. Scour prediction remains an uncertain Numerical and Analytical Methods in Geomechanics (1980), art. Sediment transport studies may assist in defining scour Vol. 4,No.2,pp.185-189,andRandolph,H.F.,“Design design criteria but local experience is the best guide. The Considerations for Offshore Piles,” Geotechnical Practice in uncertainty on design criteria should be handled by robust Offshore Engineering,ASCE, Austin 1983, pp. 422439. design, or by an operating strategy of monitoring and remedi- The foundation configurations should be based on those ation as needed.Typicalremediationexperienceisdocu- that experience has shown can be installed consistently, prac- mented in “Erosion Protection of Production Structures,” by tically and economically under similar conditions with the Posey, and C.J., Sybert, J.H., Proc. 9th Conv. I.A.H.R., pile size and installation equipment being used. Alternatives Dobrovnik, 1961, pp. 1157-1162, and “Scour Repair Meth- for possible remedial action in the event design objectives ods in the Southern North Sea,” by Angus, N.M., and Moore, cannot be obtained during installation should also investi- be R.L., OTC 4410, May 1982. Scour design criteria will usually gated and defined prior to construction. be a combination local and global scour. of For the pile-bell system, the factors of safety should be those given in Section 6.3.4. The allowable skin friction val- 6.4 PILE CAPACITY FOR AXIAL BEARING LOADS ues on the pile section should be those given in this section 6.4.1 Ultimate Bearing Capacity and in Section6.5. Skin friction on the upper bell surface and possibly above the bell on the pile should be discounted in The ultimate bearing capacity of piles, including belled computing skin friction resistance, Qf The end bearing area piles, Qd should be determined by the equation: of a pilot hole, if drilled, should be discounted in computing total bearingarea of the bell. Q d = Q f + Qp=fAs + (6.4.1-1) 6.4.2 Skin Friction and End Bearing in Cohesive where Soils Qf = skin friction resistance,lb (kN), For pipe piles in cohesive soils, the shaft friction,f, in lb/ft2 (kPa) at any point along the pile may be calculated by the Q, = total end bearing, lb (W), equation. f = unit skin friction capacity, lb/ft2 (kPa), f=ac (6.4.2-1) As = side surface area of pile, ft2 (m2), where q = unit end bearing capacity,lb/ft2 (kPa), a = a dimensionless factor, A, = gross end area of pile, ft2 (m2). c = undrained shear strength of the soil at the point Total end bearing, Q,, should not exceed the capacity of in question. the internal plug. In computing pile loading and capacity the The factor, a, be computed by the equations: can weight of thepile-soilplugsystemandhydrostaticuplift should be considered. a = 0.5 W 5 1.0 (6.4.2-2) In determining the load capacity of a pile, consideration should be given to the relative deformations between the soil a = 0.5 W > 1.0COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 70. 58 2A-WSD PRACTICE RECOMMENDED API with the constraint that, 5 1.O, a tial effects of drilling mud, should be considered. A further check should be made of the allowable bond stress between where the pile steel and the grout recommended in Section 7.4.3. as = c/pb for the point in question, For further discussion refer to “State oftheArt:Ultimate Axial Capacity of Grouted Piles” by Kraft and Lyons, OTC pb = effective overburden pressure at the point in 2081, May, 1974. question lb/ft2(Wa). In layered soils, shaft friction values,& in the cohesive lay- ers should be as given in Eq. (6.4.2-1). bearing values for End A discussion of appropriate methods for determining the piles tipped in cohesive layers with adjacent weaker layers undrained shear strength, c, and effective overburden pres- maybe as givenin Eq. (6.4.2-3),assumingthatthepile sure, pb, including the effects various sampling and testing of achieves penetration of two to three diameters or more into procedures is included in the commentary. For underconsoli- the layer in question and the tip is approximately three diame- datedclays(clayswithexcess pore pressuresundergoing ters above the bottom the layer to preclude punch through. of active consolidation), a,can usually be taken as 1.0. Due to Where these distances are not achieved, some modification in the lack of pile load tests in soils having c/pb ratios greater the end bearing resistance may be necessary. Where adjacent thanthree,equation6.4.2-2shouldbeappliedwithsome layers are of comparable strength to the layerof interest, the engineering judgment for high c/pb values. Similarjudgment proximity of the pile tip to the interface is not a concern. should be applied for deep penetrating piles in soils with high undrained shear strength, c, where the computed shaft fric- 6.4.3 Shaft Friction and End Bearing in tions, f , using equation 6.4.2-1 above, are generally higher Cohesionless Soils than previously specified in 2A. RP For pipe piles in cohesionless soils, the shaft friction,, in f For very long piles some reduction in capacity may be war- lb/ft2 (Wa) may be calculated the equation: by ranted, particularly where the shaft friction may degrade to some lesser residual value on continued displacement. This effect is discussed in more detail in the commentary. f = Kp, tan 6 (6.4.3-1) Alternativemeans of determiningpilecapacitythatare based on sound engineering principles and consistent with are where industry experience are permissible. A more detailed discus- K = coefficient of lateral earth pressure (ratio hori- of sion of alternate prediction methods is included in the com- zontal to vertical normal effective stress), mentary. For piles end bearing in cohesive soils, the unit end bearing Po = effective overburden pressure lb/ft2 (Wa) at the q, in lbs/ft2(Wa), may be computed the equation by point in question, 6 = friction angle between the soil and pile wall. (6.4.2-3) q = 9c For open-ended pipe piles driven unplugged, it is usually The shaft friction,f , acts on both the inside and outsideof appropriate to assume Kas 0.8 for both tension and compres- the pile. The total resistance is the sum of: the external shaft sionloadings.Valuesof K for displacement full piles friction; the end bearing on the pile wall annulus; the total (plugged or closed end) may be assumed to be 1.0. Table internal shaft friction or the end bearing of the plug, which- 6.4.3-1 may be used for selection of 6 if other data are not ever is less. For piles considered to be plugged, the bearing available. For long piles f may not indefinitely increase lin- pressure may be assumed to act over the entire cross section early with the overburden pressure implied by Eq. 6.4.3-1. as of the pile. For unplugged piles, the bearing pressure on acts In such cases it may be appropriate to limit f to the values thepilewallannulusonly.Whetherapile is considered given in Table 6.4.3-1. plugged or unplugged may be based on static calculations. For piles end bearing in cohesionless soils the unit end For example, a pile coulddriven in an unplugged condition be bearing q in lb/ft2(Wa) may be computed by the equation but act plugged under static loading. For piles driven in undersized drilled holes, piles jetted in place, or piles drilled and grouted in place the selection of (6.4.3-2) 4 = Pdvq shaft friction values should take into account the soil distur- banceresultingfrominstallation.Ingeneral f shouldnot where exceed values for driven piles; however, in some cases for po = effective overburden pressure lb/ft2 (Wa) at the drilledandgroutedpilesinoverconsolidatedclay, f may pile tip, exceed these values. In determining for drilled and grouted f piles, the strengthof the soil-grout interface, including poten- Nq = dimensionless bearing capacity factor.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 71. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 59 Table 6.4.3-1-Design Parameters for Cohesionless Siliceous Soil* End Unit Limiting Skin Limiting Soil-Pile Values Bearing Values Friction Angle, Friction Density Soil Description 6 Degrees kips/ft2 (Wal N4 kips/ft2 ( m a ) Very Loose Sand 15 1.0 (47.8) 8 40 (1.9) Loose Sand-Silt** Medium Silt Loose Sand 20 1.4 (67.0) 12 60 (2.9) Medium Sand-Silt** Dense Silt Medium Sand 25 1.7 (81.3) 20 100 (4.8) Dense Sand-Silt** Dense Sand 30 2.0 (95.7) 40 200 (9.6) Very Dense Sand-Silt** Dense Gravel 35 2.4 (114.8) 50 250 (12.0) Very Dense Sand *The parameters listed in this table are intended as guidelines only. Where detailed information such as situ cone tests, strength tests on high in quality samples, model tests, pile driving performanceis available, other values may be or justified. **Sand-Silt includes those soils with significant fractions of both sand and silt. Strength values generally increase with increasing sand fractions and decrease with increasing fractions. silt Recommended valuesof Nq are presented in Table 6.4.3-1. this uncertainty through a selection of conservative design The shaft friction, , acts on both the inside and outside the f of parametersand/orsafetyfactors. This maybeespecially piles. However, the total resistance in excess of the external importantwhereloadsheddingsubsequent to peak load shaft friction plus annular end bearing is the total internal development leading to an abrupt (brittle) failure may occur shaft frictionor the end bearing of the plug, whichever is less. such as the case for short piles under tension loading. For piles considered to be plugged the bearing pressure may For soils that do not fall within the ranges of soil density be assumed to act over the entire cross section pile. For of the and description given in Table 6.4.3-1, or for materials with unplugged piles the bearing pressure on the pile annulus acts unusually weak grains or compressible structures,Table only. Whether a pile is considered to be plugged or unplugged 6.4.3-1 may not be appropriate for selection design param- of may be based on static calculations. For example, a pile could eters. For example, very loose silts or soils containing large be driven in an unplugged condition but act plugged under amounts of mica or volcanic grains may require special labo- static loading. ratory or field tests for selectiondesign parameters.Of par- of Load test data for piles in sand (see “Comparison Mea- of ticularimportancearesandscontainingcalciumcarbonate sured and Axial Load Capacitiesof Steel Pipe Piles in Sand whicharefoundextensivelyinmanyareas of the oceans. with Capacities Calculated Using the 1986 API RP2A Stan- Available data suggest that driven piles in these soils may dard,” Final Report to API, Dec. 1987, by R. E. Olson) indi- havesubstantiallylowerdesign strengthparameters than cate that variability in capacity predictions may exceed those given in Table 6.4.3-1. Drilled and grouted piles in carbonate for piles in clay. Other data (see Toolan and Ims (1988)) (1) sands, however, may have significantly higher capacities than suggest that for piles in loose sands and long (> 50 m) inpiles driven piles and have been used successfully in many carbon- tension the method may be less conservative than for com- ate areas. The characteristics of carbonate sands are highly pression piles in medium dense to dense sands. Therefore, in variable localand experience should dictate the design unfamiliar situations, the designer may want to account for parameters selected. For example, available qualitative dataCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 72. 60 2A-WSD PRACTICE RECOMMENDED API suggest that capacity is improved in carbonate soils of high For rock, should be the same stated in Section 6.4.4. f as densities higher and quartzcontents. Cementation may The allowable pullout capacity should be determined by increase end bearing capacity, but result in a loss of lateral applying the factors of safety in 6.3.4 to the ultimate pullout pressure and a corresponding decrease in frictional capacity. capacity. These materialsare discussed further in the Commentary. For piles driven in undersized drilled or jetted holes in 6.6 AXIAL PILE PERFORMANCE cohesionless soils the values and q should be determined off by some reliable method that accounts for the amount soil of 6.6.1 Static Load-Deflection Behavior disturbance due to installation but they should not exceed val- uesfordrivenpiles.Exceptinunusualsoiltypessuch as Piling axial deflections should be within acceptable ser- described above, the values and q in Table6.4.3-1 may be off viceability limits and these deflections should be compatible used for drilled and grouted piles, with consideration given to with structural the forces movements. and An analytical the strengthof the soil grout interface. method for determining axial pile performance provided in is In layered soils shaft friction values, , in the cohesionless f Computer Predictions o Axially Loaded Piles with Non-lin- f layers shouldbe as outlined in Table 6.4.3- End bearing val- 1. ear Supports, by P. T. Meyer, et al., OTC 2186, May 1975. ues for piles tipped in cohesionless layers with adjacent soft This method makes useof axial pile shear transition vs. local layers may also be taken from Table 6.4.3-1, assuming that pile deflection (t-z) curves to model the axial support pro- the pile achieves penetration of two to three diameters or vided by the soil along the size of the pile. An additional (Q- more into the cohesionless layer, and the tip is approximately z ) curve is used to model the tip and bearing vs. the deflection three diameters above the bottom of the layer to preclude response. Methods for constructing t-z and Q-z curves are punch through. Where these distances not achieved, some are given in Section 6.7. Pile response is affected by load direc- modification in the tabulated values may be necessary. Where tions,loadtypes,loadrates,loadingsequenceinstallation adjacentlayersare of comparablestrength to thelayer of technique, soil type, axial pile stiffness and other parameters. interest, the proximity of the pile tip to the interface is not a Some of these effects cohesive have for soils been concern. observed in both laboratory and field tests. In some circumstances, i.e., for soils that exhibit strain- 6.4.4 Skin Friction and End Bearing of Grouted softening behavior and/or where the piles axially flexible, are Piles in Rock the actual capacityof the pile may be less than that given by Eq. 6.4.1-1. In these cases an explicit consideration of these The unit skin friction of grouted piles in jetted or drilled effects on ultimate axial capacity may be warranted. Note that holes in rock should not exceed the triaxial shear strength of other factors such as increased axial capacity under loading the rock or grout, but in general should be much less than this rates associated with storm waves may counteract the above value based on the amount of reduced shear strength from effects. For more information see Section 6.2.2, its commen- installation. For example the strength dry compacted shale of tary, as well as “Effects of Cyclic Loading and Flexibility Pile may be greatly reduced when exposed to water from jetting on Axial Pile Capacities in Clay” by T. W. Dunnavant, E. C. or drilling. The sidewall of the hole may develop a layer of Clukey and J. D. Murff, OTC 6374, May 1990. slaked mud or clay which will never regain the strength of the rock. The limiting value for this type pile maybe the allow- able bond stress between the pile steel and the grout as rec- 6.6.2 Cyclic Response ommended in 7.4.3. Unusual pile loading conditions or limitations on design The end bearing capacity the rock should determined of be pile penetrations may warrant detailed consideration of cyclic from the triaxial shear strength the rock and appropriate of an loading effects. bearing capacity factor based on sound engineering practice Cyclic loadings (including inertial loadings) developed by for the rock materials but should not exceed 100 tons per environmentalconditionssuch as stormwavesandearth- square foot (9.58 MPa). quakes can have two potentially counteractive effects on the static axial capacity. Repetitive loadings can cause a tempo- 6.5 PILE CAPACITY FOR AXIAL PULLOUT LOADS rary or permanent decrease in load-carrying resistance, and/or The ultimate pile pullout capacity maybe equal to or less anaccumulation of deformation.Rapidlyappliedloadings than but should not exceed Q$ the total skin friction resis- can cause an increase in load-carrying resistance and/or stiff- tance. The effective weight of the pile including hydrostatic ness of the pile. Very slowly applied loadings can cause a uplift and the soil plug shall be considered in the analysis to decrease in load-carrying resistance and/or stiffness ofthe determine the ultimate pullout capacity. For clay, f should be pile. The resultant influenceof cyclic loadings will a func-be the sameas stated in 6.4.2. For sand and fsilt, should be com- tion of the combined effects of the magnitudes, cycles, and puted according to 6.4.3. rates of applied pile loads, the structural characteristics the ofCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 73. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 61 pile, the types soils, and the factors safety used in design of of Loaddeflectionrelationshipsforgroutedpilesaredis- of the piles. cussedin Criteria for Design o AxiallyLoaded Drilled f The design pile penetration should sufficient to develop be Shafts, by L. C. Reese and M. O’Neill, Center for Highway an effective pile capacity to resist the design static and cyclic Research Report, University of Texas, August 1971. Curves loadings as discussed in6.3.4. developed from pile load tests in representative soil profiles The design pile penetration can be confirmed by perform- or based on laboratory soil tests that model pile installation ing pile response analyses the pile-soil system subjected to of may also be justified. Other information may be used, pro- staticandcyclicloadings.Analyticalmethodstoperform vided such information can be shown to result in adequate such analyses are described in the commentary to this Sec- safeguards against excessive deflection and rotation. tion. The pile-soil resistance-displacement t-z, Q-z character- In the absenceof more definitive criteria, the following t-z izations are discussed in Section 6.7. curves are recommended for non-carbonate soils. recom- The mended curves are shown in Figure 6.7.2-1. 6.6.3 Overall Pile Response Analyses When any of the above effects are explicitly considered in Clays t/D tltrnax pile response analysis, the design static and cyclic loadings 0.0016 0.30 should be imposed on the pile top and the resistance-displace- 0.003 1 0.50 ments of the pile determined. the completionof the design At 0.0057 0.75 loadings, maximum resistance displacement the pile and should be determined. Pile deformations should meet struc- 0.0080 0.90 ture serviceability requirements. total pile resistance after The 0.0100 1.o0 the design loadings should meet the requirements of 6.3.4. 0.0200 0.70 to 0.90 6.7 SOIL REACTION FOR AXIALLY LOADED W 0.70 to 0.90 PILES 6.7.1 General Sands t (in.) tltrnax 0.00 0.000 The pile foundation should be designed to resist the static and cyclic axial loads.The axial resistance of the soil is pro- 0.100 1.o0 vided by a combination of axial soil-pile adhesion or load W 1.o0 transfer along the sides the pile and end bearing resistance of at the pile tip. The plotted relationship between mobilized soil-pile shear transfer and local pile deflection depth is at any where described a using t-z curve.Similarly, relationship the t = local pile deflection, in. (mm), between mobilized bearing end resistance axial and tip deflection is described using a curve. Q-z D = pile diameter, in. (mm), 6.7.2 Axial Load Transfer (f-z)Curves t = mobilized soil pile adhesion, lb/ft2 (kPa), Various empirical and theoretical methods are available tmm = maximum soil pile adhesion or unit skin friction for developing curves for axial load transfer and pile dis- capacity computed according to Section 6.4, lb/ft2 placement, (t-z) curves. Theoretical curves described by Wal. Kraft, et al. (198 1) maybe constructed. Empirical t-z curves based on the results of model and full-scale pile load tests The shape of the t-z curve at displacements greater than may follow the procedures in clay soils described by Cole zmm as shown in Figure 6.7.2-1 should be carefully consid- and Reese (1966) or granular soils byCoyle,H.M.and ered. Values of the residual adhesion ratio treitmmat the axial Suliaman, I.H. Skin Friction for Steel Piles in Sand, Journal pile displacement at which it occurs (z,) are a function of of the Soil Mechanics and Foundation Division, Proceedings soilstress-strainbehavior, stresshistory, installation pipe of the American Society of Civil Engineers, Vol. 93, No. method, pile load sequence and other factors. SM6, November,1967,p.261-278.Additionalcurvesfor The value of can range 0.70 0.90. from to Labora- clays and sands are provided by Vijayvergiya, V.N., Load tory, in situ or model pile tests can provide valuable informa- Movement Characteristics o Piles, Proceedings of the Ports f tion for determining values of treitmm and zres for various ‘77 Conference, American Society of Civil Engineers, Vol. soils. For additional information see the listed references at II, p. 269-284. the beginningof 6.7.2.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 74. 62 2A-WSD PRACTICE RECOMMENDED API 1.0 . 1 0.8 . 0.6 . I: Clay: I I Sand: I %ax UD %ax I Z, inch %ax I - 0.00 0.00 0.00 I 0.00 0.001 6 0.30 I 0.1 o 1.o0 0.4 . 0.0031 0.50 I 1.o0 II 0.0057 0.75 I m 0.0080 0.90 I 0.0100 I 1.o0 I 0.0200 0.70 to 0.90 I m 0.70 to 0.90 I 0.2 . I I I r 0- Y 0.02 I 0.01 0.04 0.05 m ZD ~""1""~""~""~"" +-+4 0.02 O 0.01 0.04 0.05 m Z, inches Figure 6.7.2-1-Typical Axial Pile Load Transfer-Displacement (t-z)CurvesCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 75. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 63 6.7.3 Tip Load-Displacement Curve 6.8SOILREACTION FOR LATERALLY-LOADED PILES The end bearing or tip load capacity should be determined as described in 6.4.2 and 6.4.3. However, relatively large pile 6.8.1 General tip movements are required to mobilize the full end bearing resistance. A pile tip displacement up to percent of the pile 10 The pile foundation should be designed to sustain lateral diameter may be required for full mobilization in both sand loads,whetherstaticorcyclic.Additionally,thedesigner and clay soils. In the absence of more definitive criteria the should consider overload cases in which the design lateral following curve is recommended for both sands and clays. loads on the platform foundation are increased by an appro- priate safety factor. The designer should satisfy himself that í/D e/e, the overall structural foundation system will fail under the not overloads. The lateral resistanceof the soil near the surface is 0.002 0.25 significant to pile design, and the effects on this resistanceof 0.50 0.013 scour and soil disturbance during pile installation should be 0.042 0.75 considered. Generally, under lateral loading, clay soils behave 0.073 0.90 as a plastic material which makes it necessary to relate pile- 1 0.100 .o0 soil deformation to soil resistance. To facilitate this proce- dure, lateral soil resistance deflection @-y) curves should be where constructed using stress-straindata from laboratory soil sam- ples. The ordinate for these curves is soil resistance, p , and z = axial tip deflection, in. (mm), the abscissa is soil deflection, y. By iterative procedures, a D = pile diameter, in. (mm), compatible set of load-deflection values for the pile-soil sys- Q = mobilized end bearing capacity, lb (KN). tem can be developed. For a more detailed study of the construction p-y curves of Q, = total end bearing,lb (W, computed according refer to the following publications. to Section 6.4. Soft Clay: OTC 1204, Correlations for Design of Laterally The recommended curve is shown in Figure 6.7.3-1. Loaded Piles in Soft Clay, by H. Matlock, April 1970. Q/Q, = 1.O z, = 0.10 x Pile Diameter (D) z/D Figure 6.7.3-1"Pile Tip-Load-Displacement (Q-z)curveCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 76. 64 2A-WSD PRACTICE RECOMMENDED API Stiff Clay: OTC 2312, Field Testing and Analysis of Later- Wherethestrengthvarieswithdepth,Equa- ally Loaded Piles in Stiff Clay,L. C. Reese andW. R. Cox, by tions 6.8.2-1 and 6.8.2-2 may be solved by plot- April 1975. ting the two equations, i.e., pu vs. depth. The point of first intersectionof the two equations is Sand: “Fan Evaluationof p-y Relationships in Sands,” M. by taken to be X,. These empirical relationships W. O’Neill and J. M. Murchinson. A report to the American Petroleum Institute, May 1983. maynotapplywherestrengthvariationsare erratic. general, In minimum values of XR In the absence of more definitive criteria, procedures rec- should be about 2.5 pile diameters. ommended in 6.8.2 and 6.8.3 may be used for constructing ultimate lateral bearing capacity curvesandp-y curves. 6.8.3 Load-Deflection @-y) Curves for Soft Clay 6.8.2 Lateral Bearing Capacity for Soft Clay Lateral soil resistance-deflection relationships for piles in softclay are generallynon-linear. The P-y curvesforthe Forstaticlateralloadstheultimateunitlateralbearing short-term static load case may be generated from the follow- capacity of soft clay pu has been found to vary between Sc ing table: and 12c except at shallow depths where failure occurs in a different mode due to minimum overburden pressure. Cyclic Phu Y/Yc loads cause deterioration of lateral bearing capacity below 0.00 0.0 that for static loads. In the absencemore definitive criteria, of the following is recommended 1.0 0.50 pu increases from 3c to 9c as X increases from O to XR 0.72 3.0 according to: 1.o0 8.0 1.o0 W cx pu= 3 c + ~ X + - J (6.8.2-1) D where p = actual lateral resistance, psi (kPa), and y = actual lateral deflection, in. (mm), pu = 9c f o r x 2 x R (6.8.2-2) y , = 2.5 E, D, in. (mm), E, = strain which occurs at one-halfthemaximum where stress laboratory on undrainedcompression tests of undisturbed soil samples. Pu = ultimate resistance,psi (kPa), c = undrained shear strength for undisturbed clay For the case where equilibrium has been reached under soil samples, psi (kPa), cyclic loading, the curves may be generated from the fol- P-y lowing table: D = pile diameter, in. (mm), x< X>xR XR Y= effective unit weight soil, lb/in2 (MN/m3), of Phu Y/Y c Phu Y/Y c J = dimensionlessempiricalconstantwithvalues 0.00 0.00 0.0 0.0 rangingfrom0.25to 0.5 havingbeendeter- mined by field testing. A value of 0.5 is appro- 0.50 0.50 1.0 1.0 priate for Gulf of Mexico clays, 0.72 3.0 0.72 3.0 depth below soil surface, in. (mm), 0.72 W 0.72 x / x , 15.0 0.72x/x~ depth below soil surface to bottom of reduced resistance zone in in. (mm). For a condition of 6.8.4 Lateral Bearing Capacity for Stiff Clay constant strength with depth, Equations6.8.2-1 and 6.8.2-2 are solved simultaneously to give: For static lateral loads the ultimate bearing capacity pu of stiff clay (c > 1 Tsf or 96 kPa) as for soft clay would vary 60 between Sc and 12c. Due to rapid deterioration under cyclic YJ+J loadings the ultimate resistance will be reduced to something C so considerably less and should be considered in cyclic design.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 77. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHORE PLATFORMS-WORKINGSTRESS DESIGN 65 6.8.5 Load-Deflection (P-y) Curves for Stiff Clay 1O0 While stiff clays also have non-linear stress-strain relation- 90 ships, they are generally more brittle than soft clays. In devel- opingstress-straincurvesandsubsequent p - y curvesfor 80 cyclic loads, goodjudgment should reflect the rapid deterio- ration of load capacityat large deflections for stiff clays. 70 f c S 60 6.8.6 Lateral Bearing Capacity for Sand 5 The ultimatelateralbearingcapacityforsandhasbeen 50 5 + found to vary from a value at shallow depths determined by 40 ; Eq. 6.8.6-1 to a valueat deep depths determined Eq. 6.8.6- by - 3 2. At a given depth the equation giving the smallest value of 30 2 pu should be used as the ultimate bearing capacity. 20 puS=(C1xH+C2xD)xyxH (6.8.6-1) 10 p&=C3xDxyxH (6.8.6-2) n 20 25 30 35 40- where Angle of Internal Friction,q, deg pu = ultimate resistance (forcehnit length), lbs/in. (kN/m) (s = shallow, d = deep), Figure 6.8.6-1 -Coefficients as Function of @ y = effective soil weight, lb/in.3 (KN/m3), H = depth, in. (m), @ angle = of internal friction of sand, deg., C l , C,, C3 = Coefficients determined from Figure 6.8.6-1 as function of @, q, Angle of Internal Friction D = average pile diameter from surfaceto depth, 29" 28" 36" 30" 45" 40" in. (m). Medium Very Loose Dense Dense Dense 6.8.7 Load-Deflection (P-y) Curves for Sand The lateral soil resistance-deflection@-y) relationships for sand are also non-linear and in the absence more definitive of information may be approximated at any specific depthH , by the following expression: P = A X p ux tanh [AXPU x y ] (6.8.7-1) where A = factor to account for cyclic or static loading condi- tion. Evaluated by: , /i/ Sand below thya;ter A = 0.9 for cyclic loading. 3 A = 3.0 - 0.8 - 2 0.9 for static loading. - n d k pu = ultimate bearing capacity depth H , lbs/in. (kN/m), at o 20 40 80 60 100 k = initial modulus of subgrade reaction, lb/in? (kN/ Relative Density, % m3). Determine from Figure 6.8.7-1 as function of angle of internal friction,@. Figure 6.8.7-1-Relative Density, %COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 78. 66 2A-WSD PRACTICE RECOMMENDED API y = lateral deflection, inches (m). the variations in the shear strength and stiffness modulus of H = depth, inches (m) the soil with depth. O’Neill and Dunnavant(1983, in arecent API-sponsored 6.9 PILE GROUP ACTION project, [An Evaluation of the Behavior and Analysis Lat- of erally Loaded Pile Groups, API, PRAC 84-52, University of 6.9.1 General Houston, University Park, Department of Civil Engineering, Considerationshouldbegiventotheeffects of closely Research Report No. UHCE 85- 111 found of the four group spaced adjacent piles on the load and deflection characteris- analysis methods examined in study, this the following tics of pile groups. Generally, for pile spacing less than eight methodsto be themost appropriate for use in designing (8) diameters, group effects may have to be evaluated. For group pile foundations for the given loading conditions: (a) more detailed discussions refer to the following four papers: advancedmethods,such as PILGP2R, for defining initial “Group Action in Offshore Piles,” by O’Neill, M. W., Pro- group stiffness; (b) the Focht-Koch (1973) method [“Ratio- ceedings, Conference on Geotechnical Practice in Offshore nal Analysis of the Lateral Performance of Offshore Pile Engineering, ASCE, Austin, Texas, pp. 25-64; “An Approach Groups,”OTC18961 as modifiedby Reese et al. (1984) for the Analysis of Offshore Pile Groups,” by Poulos, H. G., [“Analysis of a Pile Group Under Lateral Loading,” Later- Proceedings, Ist International Conference on Numerical ally Loaded Deep Foundations: Analysis and Pe$ormance, Methods in Offshore Piling, Institution of Civil Engineers, ASTM, STP 835, pp. 56-71] for defining group deflections London,pp.119-126;“TheAnalysis of Flexible Raft-Pile and average maximum pile moments for design event System” by Han, S. J., and Lee, I. K., Geotechnique 28, No. loads-deflections are probably underpredicted at loads giv- 1, 1978; and Offshore Technology Conference paper number ing deflections of 20 percentor more of the diameter of the OTC 2838, Analysis of Three-Dimensional Pile Groups with individual piles in the group; (c) largest value obtained from Non-Linear Soil Response and Pile-Soil Interaction W.by M. the Focht-Koch and b methods evaluating maximumpile for O’Neill, et al., 1977. load ata given group deflection. Past experience and the results of the study by O’Neill 6.9.2Axial Behavior and Dunnavant (1985) confirm that the available tools for For piles embedded in clays, the group capacity may be analysis of laterally loaded pile groups provide approxi- lessthanasingleisolatedpilecapacitymultipliedbythe mateanswers that sometimes deviate significantly from number of piles in the group; conversely, for piles embedded observed behavior, particularly with regard todeflection in sands the group capacity may be higher than the sum of the calculations. Also, limitations in site investigation proce- capacities in the isolated piles. group settlement in either The dures and in the ability to predict single-pile soil-pile inter- clay or sand would normally be larger than that of a single action behavior produce uncertainty regarding proper soil pile subjectedto the average pile load the pile group. of input to group analyses. Therefore multiple analyses should Ingeneral,groupeffects dependconsiderably pile on be performed for pile groups, using two or more appropri- group geometry and penetrations, and thickness any bear- of ate methods of analysis and upper-bound and lower-bound ing strata underneath the pile tips. Refer to “Group Action in values of soil properties inthe analyses. By performing Offshore Piles” by O’Neill, M. W., Proceedings, Conference such analyses, the designer will obtain an appreciation for on Geotechnical Practice in Offshore Engineering, ASCE, the uncertainty involved in his predictions of foundation Austin, Texas, pp. 25-64: “Pile Group Analysis: A Study of performance and can make more informed decisions Two Methods,” by Poulos, H. G., and Randolph, F., Jour- M. regarding the structural design of the foundation and super- nal Geotechnical Engineering Division, ASCE, Vol, 109, No. structure elements. 3, pp. 355-372. 6.9.4 Pile Group Stiff ness and Structure Dynamics 6.9.3 Lateral Behavior When the dynamic behaviorof a structure is determined to Forpileswiththesamepileheadfixityconditionsand be sensitive to variations in foundation stiffness, parametric embedded in either cohesive or cohesionless soils, the pile analyses suchas those described in 6.9.3 should be performed group would normally experience greater lateral deflection to bound the vertical and lateral foundation stiffness values to than that of a single pile under the average pile load of the be used in the dynamic structural analyses. For insight regard- corresponding group.The major factors influencing the group ing how changes in foundation stiffness can impact the natu- deflections and load distribution among the piles the pile are ral frequencies of tall steel jacket platforms, see K. A. Digre spacing, the ratio of pile penetration to the diameter, the pile et al. (1989), “The Designof the Bullwinkle Platform,”OTC flexibility relativeto the soil the dimensions the group, and of 6060.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 79. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 67 6.10 PILE WALLTHICKNESS 1. The pile projecting section should be considered as a freestanding column with minimum effective length fac- a 6.10.1 General tor K of 2.1 and a minimum Reduction Factor of 1.0. C, The wall thickness of the pile may vary along its length 2. Bending moments and axial loads should be calculated and may be controlled a particular pointby any one of sev- at using the full weight of the pile hammer, cap, and leads eral loading conditions or requirements which are discussed acting through the center of gravity of their combined in the paragraphs below. masses, and the weight the pile add-on section with of due considerationtopilebattereccentricities. The bending 6.10.2Allowable Pile Stresses moment so determined should not be less than that corre- sponding to a load equal to 2 percent of the combined The allowable pile stresses should be the same as those weight of the hammer, cap, and leads applied at the pile permitted by the AISC specification fora compact hot rolled head and perpendicular to centerline. its section, giving due consideration to Sections 3.1 and 3.3. A rational analysis considering the restraints placed upon the 3. Allowable stresses in the pile should be calculated in pile by the structure and the soil should be used determine to accordancewithSections3.2and3.3. Theone third the allowable stresses for the portion the pile whichis not of increase in stress should not allowed. be laterally restrained by the soil. General column buckling of the portionof the pile below the mudline need not be consid- 6.10.5 Stresses During Driving ered unless the pile is believed to be laterally unsupported because of extremely low soil shear strengths, large computed Consideration should also be given to the stresses that lateral deflections, or for some other reason. occur in the freestanding pile section during driving. The sum of the stresses due to the impact of the hammer (the dynamic stress) and the stresses due to axial load and bend- 6.10.3 Design Pile Stresses ing (the static stresses) should not exceed the minimum The pile wall thickness in the vicinity of the mudline, and yield stress of the steel. A method of analysis based on possibly at other points, is normally controlled by the com- wave propagation theory should be used to determine the bined axial load and bending moment which results from the dynamic stresses, (see 6.2.1). In general it may be assumed designloadingconditionsfortheplatform. The moment that columnbuckling will notoccurasa result of the curve for the pile may be computed with soil reactions deter- dynamic portion of the driving stresses. The dynamic mined in accordance with Section 6.8 giving due consider- stresses should exceed not 80 to 90 percent of yield depending onspecific circumstances such as the location of ation to possible soil removal by scour. It may be assumed the maximum stresses down the length of pile, the number that the axial load is removed from the pile by the soil at a of blows, previous experience with the pile-hammer combi- rate equal to the ultimate soil-pile adhesion divided by the nation and the confidence level in the analyses. Separate appropriate pile safety factor from 6.3.4. When lateral deflec- considerations apply when significant driving stresses may tions associated with cyclic loads at or near the mudline are be transmitted into the structure and damage to appurte- relatively large (e.g., exceeding yc as defined in 6.8.3 for soft nances must be avoided. The static stress during driving clay), consideration should be given to reducing or neglecting may be taken to be the stress resulting from the weight of the soil-pile adhesion through this zone. the pile above the point of evaluation plus the pile hammer components actually supported by the pile during the ham- 6.10.4 Stresses Due to Weight of Hammer During mer blows, including any bending stresses resulting there- Hammer Placement from.Allowable static stresses in the pile should be calculated in accordance with Sections 3.2 and 3.3. The one Each pile or conductor section on which a pile hammer third increases in stress should not be allowed. The pile (pile top drilling rig, etc.) will be placed should be checked hammers evaluated for use during driving should be noted for stressesdue to placing the equipment. These loads may be by the designer on the installation drawings or specifica- the limiting factors in establishing maximum length of add- tions. When using hydraulic hammers is possible that the it on sections.This is particularly true in cases where piling will driving energy may exceed the rated energy andthis should be driven or drilled on a batter. The most frequent effects beconsidered in the analyses. Also the static stresses include: static bending, axial loads, and arresting lateral loads induced by hydraulic hammers need to be computed with generated during initial hammer placement. special care due to the possible variations in driving config- Experience indicates that reasonable protection from fail- urations, for example when driving vertical piles without ure of the pile wall due to the above loads is providedif the lateral restraint and exposed to environmental forces, see static stressesare calculated as follows: also 12.5.7(a).COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 80. 68 2A-WSD PRACTICE RECOMMENDED API 6.10.6 Minimum Wall Thickness heavy wall material in the vicinity of the mudline so the pile will not be overstressedat this point if the design penetration The D/t ratio of the entire lengthof a pile should be small is not reached. amount of underdrive allowance provided The enough to preclude local buckling at stresses up to the yield in the design will depend on the degreeuncertainty regard- of strength of the pile material. Consideration should given to be ing the penetration that can be obtained.some instances an In the different loading situations occurring during the installa- overdrive allowance should be provided in a similar manner tion and the service life of a piling. For in-service conditions, in the event an expected bearing stratumnot encounteredat is and for those installation situations where normal pile-driving the anticipated depth. is anticipated or where piling installation will be by means other than driving, the limitations of Section 3.2 should be considered to be the minimum requirements. For piles that 6.10.8 Driving Shoe are to be installed by driving where sustained hard driving The purpose of driving shoes is toassist piles to penetrate (250 blows per foot [820 blows per meter] with the largest through hard layers or to reduce driving resistances allowing size hammer to be used) is anticipated, the minimum piling greater penetrations to be achieved than would otherwise be wall thickness used should not less than be the case. Different design considerations apply for each use. D If an internal driving shoe is provided to drive through a hard = + 1 layer it should be designed to ensure that unacceptably high driving stresses do not occurat and above the transition point Metric Formula i (6.10.6-1) between the normal and the thickened section the pile tip. at Also it should be checked that the shoe does not reduce the t = 6.35 + - 1O 0 endbearing capacity of the plug soil below value the assumed in the design. External shoes are not normally used where as they tend to reduce the skin friction along the length of t = wall thickness, in. (mm), pile above them. D = diameter,in.(mm). 6.10.9 Driving Head Minimum thickness normally pile wall for used sizes should beas listed in the following table: Any driving head at the top of the pile shouldbe designed in association with the installation contractor to ensure that it is fully compatible with the proposed installation procedures Minimum Pile Wall Thickness and equipment. Pile Diameter Nominal Thickness, Wall t in. mm mm in. 6.1 1 LENGTH OF PILE SECTIONS 24 610 l/2 13 In selecting pile section lengths consideration should be762 30 9/16 14 given to: 1) the capability of the equipment to raise, lower lift 36 914 5/8 16 and stab the sections; 2) the capability lift equipment to of the 42 1067 "116 17 place the pile driving hammer on the sections to be driven; 3) 48 1219 3/4 19 the possibilityof a large amount downward pile movement of 60 1524 18 22 immediately following the penetration a jacket leg closure; of 72 1829 1 25 4) stresses developed in the pile section while lifting; 5 ) the 84 2134 11 1 ~ 28 wallthicknessandmaterialproperties at fieldwelds; 6) 96 2438 1 31 avoiding interference with the planned concurrent drivingof 108 2743 131~ 34 neighboring piles; and7) the type of soil in which the pile tip 120 3048 1112 37 is positioned during driving interruptions for field welding to attachadditionalsections. In addition,staticanddynamic The preceding requirement for a lesser ratio when hard D/t stresses due to the hammer weight and operation should be driving is expected may be relaxed when it can be shown by considered as discussed in 6.10.4 and 6.10.5. past experienceor by detailed analysis that the pile will not be Each pile section on which driving is required should con- damaged during its installation. tain a cutoff allowance to permit the removal of material dam- aged by the impact of the pile driving hammer. The normal 6.10.7 Allowance for Underdrive and Overdrive allowance is 2 to 5 ft. (0.5 to 1.5 meters) per section. Where With piles having thickened sections at the mudline, con- possiblethe cut fortheremoval of thecutoffallowance sideration should be given to providing an extra length of should be made a conveniently accessible elevation. at COPYRIGHT American Petroleum Institute Licensed by Information Handling Services
  • 81. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONS1 XUCTING FIXED AND OFFSHORE PLATFORMS-WORKING STRESS DESIGN 69 6.12 SHALLOW FOUNDATIONS D = depth of embedment of foundation, Shallow foundations are those foundations for which the A’ = effective area of the foundation depending on the depth of embedment is less than the minimum lateral dimen- load eccentricity, sion of the foundation element. The design of shallow foun- dationsshouldinclude,whereappropriatetotheintended Kc = correction factor which accounts for load inclina- application, considerationof the following: tion, footing shape, depth embedment, inclina- of tion of base, and inclination the ground surface. of 1. Stability, including failure due to overturning, bearing, sliding or combinations thereof. A method for determining the correction factor and the 2. foundation Static deformations, including possible effective area is given in the Commentary. Two special cases damage to components of the structure andits foundation of Eq. 6.13.1- are frequently encountered. For a vertical con- 1 or attached facilities. centric load applied to a foundation at ground level where boththefoundationbaseandgroundarehorizontal,Eq. 3. Dynamic foundation characteristics, including the 6.13.1- 1 is reduced below for two foundation shapes. influence of the foundation on structural response and the performance of foundation under the itself dynamic 1. Infinitely Long Strip Footing. loading. (6.13.1-2) Q, = 5.14cAO 4.Hydraulicinstabilitysuch as scourorpiping due to where wave pressures, including the potential for damage to the structure and for foundation instability. Q, = maximum vertical load per unit length of 5. Installationandremoval,includingpenetrationand footing pull outof shear skirts or the foundation base itself and the A, = actual foundation area per unit length effects of pressure build up draw down of trapped water or underneath the base. 2. Circularor Square Footing. Recommendations pertaining to these aspects of shallow Q = 6.17cA (6.13.1-3) foundation design are given in through 6.17. 6.13 where 6.13 STABILITY OF SHALLOW FOUNDATIONS A = actual foundation area The equations of this paragraph should be considered in evaluating the stability of shallow foundations. These equa- 6.13.2 Drained Bearing Capacity tions are applicable to idealized conditions, and a discussion The maximum net vertical load which a footing can sup- of the limitations and of alternate approaches is given in the port under drained conditions is Commentary. Where use of these equations is not justified, a morerefinedanalysisorspecialconsiderationsshouldbe considered. Q’ = (c WCKc+ qNqKq + ‘/21/BNFy)A’ (6.13.2-1) 6.13.1 = Undrained Bearing Capacity (@ O) where The maximum gross vertical load which a footing can sup- Q’ = maximum net verticalload at failure, port under undrained conditions is c’ = effective cohesion intercept of Mohr Enve- lope, Q = (cNcKc + y D)A’ (6.13.1-1) Nq = (Exp [x tan@])(tan2(45” + @’/2)), a dimen- where sionless functionof @’, Q = maximum vertical load at failure, Nc = (Nq - 1)cot@’,adimensionlessfunction of @’, c = undrained shear strength of soil, Ny = an empirical dimensionless function of @’ Nc = a dimensionless constant, 5.14 for @ = O, that can be approximated by + 1) t n , 2(Nq a@ @ = undrained friction angle = O, = @’ effectivefrictionangle of Mohr Envelope, y = total unit weight of soil, y’ = effective weight, unitCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 82. 70 2A-WSD PRACTICE RECOMMENDED API y’D, where D = depth of embedment of foun- These values should be used after cyclic loading effects dation, have been taken into account. Where geotechnical data are sparse or site conditions are particularly uncertain, increases minimum lateral foundation dimension, in these values may be warranted. See the Commentary for effective area of the foundation depending on further discussionof safety factors. the load eccentricity, 6.14 STATIC DEFORMATION OF SHALLOW correctionfactorswhichaccountforload FOUNDATIONS inclination, footing shape, depth of embed- ment, inclination of base, and inclination of The maximumfoundationdeformationunderstatic or thegroundsurface,respectively. The sub- equivalent static loading affects the structural integrity the of scripts c, q, and y refer to the particular term platform, its serviceability, and its components. Equations for in the equation. evaluating the static deformation of shallow foundations are given in 6.14.1 and 6.14.2 below. These equations are appli- A complete description of the K factors, as well as curves cable to idealized conditions. A discussion of the limitations showing the numerical valuesof Nq, Nc, Ny as a function and and of alternate approaches is given in the Commentary. of @’ given in the Commentary. are Two special cases of Eq. 6.13.2-1 for c’ = O (usually sand) 6.14.1 Short Term Deformation are frequentlyencountered. a For vertical, centric load For foundation materials which canbe assumedto be iso- applied to a foundationat ground level where both the foun- tropic and homogeneous and for the condition where the dation and base ground are horizontal,Eq. 6.13.2-1 is structure base is circular, rigid, and rests on the soil surface, reduced below for two foundation shapes. the deformations of the base under various loads are as fol- 1. Infinitely Long Strip Footing. lows: Q , = 0 . 5 ~ BNyAo ’ (6.13.2-2) 1-v 2. Circularor Square Footing. Vertical: U,, = (=)e (6.14.1-1) Q = 0.3v’BNyA (6.13.2-3) 6.13.3 Sliding Stability The limiting conditions of the bearing capacity equations in 6.13.1 and 6.13.2, with respect to inclined loading, repre- sent sliding failure and result in the following equations: Rocking: (6.14.1-3) 1. Undrained Analysis: H = CA (6.13.3-1) Torsion: 0, = - (6.14.1-4) (16iR3IT where H = horizontal load at failure. where 2. Drained Analysis: U, U, = vertical and horizontal displacements, H = c’A+Qe@’ (6.13.3-2) Q, H = vertical and horizontal loads, 6.13.4SafetyFactors 0,, = overturning torsional and rotations, Foundationsshouldhaveanadequatemargin of safety M, T = overturning and torsional moments, against failure under the design loading conditions. The fol- lowing factorsof safety should be used for the specific failure G = elastic shear modulus of the soil. modes indicated: v = poisson’s ratio of the soil, Failure Mode Factor Safety R = radius of the base. Failure Bearing 2.0 These solutions can also be used for approximating the Failure Sliding 1.5 response of a square base equal area. ofCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 83. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 71 6.1 4.2 Long Term Deformation 6.17 INSTALLATION AND REMOVAL OF SHALL FOUNDATIONS An estimate of the vertical settlement of a soil layer under an imposed verticalload can be determined by the following Installation should be planned ensure the foundation can to equation: be properly seated at the intended site without excessive dis- turbance to the supportingsoil. Where removal is anticipated hC an analysis should be made of the forces generated during U,>= - log,, " 1 + e, - 40 + " (6.14.2-1) removal toensure that removal can accomplished with the be means available. where Reference U, = vertical settlement, h = layer thickness, Toolan, F. E., and Ims. B. W., "Impact of Recent Changes in the API Recommended Practice for Offshore Piles in eo = initial void ratio of the soil, andSandClays,UnderwaterTechnology, V. 14, No. 1 C = compression indexof the soil over the load (Spring 1988) pp. 9-13.29. range considered, Other Structural Components and 40 = initial effective vertical stress, Systems Aq = added effective vertical stress. 7.1 SUPERSTRUCTURE DESIGN Where the vertical stress varies within a thin layer, as in The superstructure may be modeled in a simplified form the case of a diminishing stress, estimates may bedetermined for the analysis of the platform jacket, or substructure; how- by using the stress at the midpoint of the layer. Thick homo- ever, recognition should be given the vertical and horizon- to geneouslayers should besubdivided for analysis. Where talstiffnesses of the system and thelikelyeffect on the more than one layer is involved, the estimate is simply the substructure. This modeling should consider the overturning sum of the settlement of the layers. Compression characteris- effects of wind load for environmental loading conditions, the tics of the soil are determined from one-dimensionalconsoli- proper location of superstructure and equipment masses for dation tests. seismicloadingconditions,andthealternatelocations of heavy gravity loads such the derrick. as 6.15 DYNAMIC BEHAVIOR OF SHALLOW The superstructure itself may be analyzed as one or more FOUNDATIONS independent structures depending its upon configuration; Dynamic loads are imposed on a structure-foundation sys- however, consideration should be given tothe effect of deflec- tem by current, waves, ice, wind, and earthquakes. Both the tions of the substructure in modeling the boundary supports. influence of the foundation on the structural response and the Differential deflections of the support points of heavy deck integrity of the foundation itself should considered. be modules placedon skid beams or trussesat the top of the sub- structure may result in a significant redistributionof the sup- 6.16 HYDRAULIC INSTABILITY OF SHALLOW port reactions. In suchacase,theanalysismodelshould FOUNDATIONS include the deck modules and the top bay or two of the sub- structure to facilitate accurate simulation of support condi- 6.16.1 Scour tions. This model should beanalyzedtodevelopsupport reaction conditions which reflect these effects. Positive measures should be taken to prevent erosion and Depending upon the configuration of a platform designed undercutting of the soil beneath or near the structure base due with a modular superstructure, consideration should be given to scour. Examples such measuresare (1) scour skirts pene- of to connectingadjacent deck modules to resist lateral environ- trating through erodible layers into scour resistant materials or mental forces. Connection may also have the advantage of to such depths as to eliminate the scour hazard, or (2) riprap providing additional redundancy to the platform in the event emplaced around the edges of the foundation. Sediment trans- of damage to a member supporting the modules. deck port studies may of value in planning and design. be In areas where seismic forces may govern the design of superstructuremembers,apseudo-static analysis may be 6.16.2 Piping used. The analysis should be based on peak deck accelera- The foundation should be so designed to prevent the cre- tions determined from the overall platform seismic analysis. ation of excessive hydraulic gradients (piping conditions) in The height at which the acceleration is selected should be the soil due to environmental loadings or operations carried based upon the structural configuration the location of the and out during or subsequent tostructure installation. dominant superstructure masses.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 84. 72 2A-WSD PRACTICE RECOMMENDED API 7.2PLATEGIRDER DESIGN 7.4GROUTEDPILETOSTRUCTURE CONNECTIONS Plate girders should be designed in accordance with the AISC Spec$cations for the Design, Fabrication and Erec- 7.4.1 General tion o Structural Steelfor Buildings, latest edition and Sec- f Platform loads may be transferred to steel by grouting piles tion 9 of the AWS Structural Welding Code, AWS D l . 1, the annulus between the jacket leg (or sleeve) and the pile. latest edition. Where stress concentrations such as abrupt The load is transferred tothe pile from the structure across the changes in section, penetrations, jacking slots, etc., occur, grout. Experimental work indicates that the mechanism of their effect on fatigue and fracture should be considered. load transfer is a combination of bond and confinementfric- Steel for plate girders should have sufficient notch tough- tion between the grout and the steel surfaces andthe bearing ness to prevent brittle fracture at the lowest anticipated of the groutagainst mechanical aids such as shear keys. ambient temperature. Centralizers shouldbe used to maintain a uniform annulus or space between the pile and the surrounding structure. A 7.3CRANESUPPORTINGSTRUCTURE minimum annulus width of 11/2 in. (38 mm) should be pro- 7.3.1 Static Design vided where grout is the only means of load transfer. Ade- quate clearance between pile and sleeve should be provided, The supporting structure should be designed for the dead taking into account the shear keys’ outstand dimension, h. load of the crane plus a minimum 2.0 times the static rated of Packers should be used as necessary to confine the grout. load as defined in API Spec2C and the stresses compared to Proper means for the introduction of grout into the annulus the Par. 3.1.1 allowables with increase. no should be provided so that the possibility of dilution of the The loading conditions to be investigated should include grout or formation of voids in the grout will be minimized. the following. The use of wipers or other means of minimizing mud intru- sion into the spaces to be occupied bypiles should beconsid- 1.Maximumoverturningmomentwithcorresponding ered at sites having soft mud bottoms. vertical load plus a side load, equal to 4% of the maxi- mum vertical load, applied simultaneously to the boom 7.4.2 Factors Affecting the Connection Strength head sheave. Many factors affect the strength of a grouted connection. 2. Maximum vertical load with corresponding overtum- These include, but are not limited to, the unconfined com- ing moment plus a load, equal to 4% of the maximum side pressive strength of the grout; size and spacing of the shear vertical load, applied simultaneously to the boom head keys; type of admixture; method of placing grout; condition sheave. of the steel surfaces, presence surface materials that would of prevent bonding of grout to steel; and the amount of distur- 7.3.2 Dynamic Design bance from platform movement while the is setting. For grout high D/t ratios the hoop flexibility the sleeve and the pile is of No increase for dynamic load is required in the design of also known tobe a factor. supporting structures for cranes with ratings in accordance with API Spec2C. 7.4.3 Computation of Applied Axial Force. 7.3.3 Fatigue Design In computing the axial force applied to a grouted pile to structure connection, due account should be takenof the dis- The crane supporting structure should be designed to resist tribution of overall structural loads among various piles in a fatigue, in compliance with Section 5.3, during life of the the group or cluster. The design loadfor the connection should be structure. The followingmaybe used inlieu of detailed the highest computedload with due consideration given to the fatigue analysis. range of axial pile and in-situ soil stiffnesses. A minimumof 25,000 cycles should be assumed under the following conditions: 7.4.4 Computation of Allowable Axial Force a. A load of 1.33 times the static rated load at the boom posi- In the absence of reliable comprehensive data which would tion and crane orientation producing maximumstress in each support the use of other values of connection strength, the component of the supporting structure. allowable axial load transfer should be taken as the smaller value (pile or sleeve) of the force calculated by a multiplica- b. The stress range used should be difference between the the tion of the contact area between the grout and steel surfaces stress caused by the above loadingand stress with the boom and the allowable axial load transfer stress f j u , where f j u is in thesame position butunloaded. computed by the appropriate value in 7.4.4a or 7.4.4b for theCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 85. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHORE PLATFORMS-WORKINGSTRESS DESIGN 73 grouthteel interface. This allowableaxialforceshouldbe Grout O.D. = Dg+ greaterthanorequaltotheappliedaxialforcecomputed according to 7.4.3. 7.4.4.a Plain pipe connections The value of the allowable axial load transfer stress, fba, should be takenas 20 psi (0.138 MPa) for loading conditions 1 and 2, Section 2.2.2: and 26.7 psi (0.184 MPa) for loading conditions 3 and 4, Section 2.2.2. 7.4.4.bShearkey connections Where shear keys are used at the interface between steel and grout, the value the nominal allowable axial load trans- of fer stress,fba, should be taken as: fba = 20 psi (0.138MPa) + 0.5 fcu h x - S for loading conditions 1 and of Section 2.2.2, and should be 2 (7.4.4-1) ” I Pile O.D. = D t s , Sleeve O.D. = D, t I taken as: Figure 7.4.4-1-Grouted Pile to Structure Connection h with Shear Keys fba = 26.7 psi (0.184 MPa) + 0.67 fcu x - (7.4.4-2) S for loading conditions 3 and of Section 2.2.2, where: 4 U U U f,, = unconfined grout compressive strength (psi, MPa) as per Section 8.4.1, h = shear key outstand dimension (inches, mm) (See Figures7.4.4-1 and 7.4.4-2), s = shear key spacing (inches, mm) (See Figures 7.4.4-1 and 7.4.4-2). (A) Weld bead (B) Flat bar with (C) Round bar with Shear keys designed according to Equations 7.4.4-1 and fillet welds fillet welds 7.4.4-2 should be detailed in accordance with the following requirements: Figure 7.4.4-2-Recommended Shear Key Details 1. Shear keys may be circular hoops at spacing “s” or a continuous helix with a pitch “S.” See Section7.4.4~ of for tions1and2,Section2.2.2. The shearkeyandweld limitations. shouldbedesigned at basicallowablesteelandweld stresses to transmit an average force equal to the shear key 2. Shear keys shouldbe oneof the types indicated in Fig- bearing area multiplied 1.7f,,, except for a distance o by f ure 7.4.4-2. 2 pile diameters from the top and the bottom end of the 3.Fordrivenpiles,shearkeysonthepileshould be connections where2.5f,,should be used. applied to sufficient length to ensure that, after driving, the length of the pile in contact with the grout has the required 7.4.4.c Limitations number of shear keys. The following limitations shouldobserved be when 4. Eachshear key crosssectionandweldshouldbe designing a connection according to Section 7.4.4a or 7.4.4b. designed to transmit that part of the connection capacity which is attributable to the shear key for loading condi- 2,500 psi (17.25 MPa) fcu 5 16,000 psi (1 10 MPa) 5COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 86. 74 2A-WSD PRACTICE RECOMMENDED API The following limitations shouldobserved be when 7.5.2 Components designing a connection according to Section7.4.4b (see Fig- A guyline system may be composed the following com- of ure 7.4.4-1 and 2): ponents: a. Lead Lines. The lead line extends from the tower to a geometry Sleeve 3580 clumpweight. If steel rope or strand is used API Specification ts 9A and API RP 9B establish standards for procurement and usage. Other materials may used if sufficient design infor- be Pile geometry E! 540 mation is available. t!J Design consideration should include mechanical proper- ties, fatigue characteristics, corrosion protection, and abrasion resistance. Grout annulus geometry 7 5 55 45 b. Clumpweights. The clumpweight is a heavy mass inter- t, mediate betweenlead the line andanchor line. The D clumpweights serve to soften the stiffnessof the guyline sys- Shear key spacing ratio 2.5 * 5 35 8 S tem during extreme seastates to allow larger tower deflection withoutincreasing line tensionsexcessively.Clumpweight variables include weight, location, dimensions, and construc- tion details. The configuration of the clumpweight should be Shear key ratio 5 5 0.10 S chosen to minimize soil suction and break-out forces. Since settlement or “mudding in”of the clumpweights might occur, W the increased resistance lift-off shouldbe considered. to Shearkeyshapefactor1.5 5 -<3 h- c. Anchor Lines. The anchor line extends from the clump- weight to the anchor. API Specification API RP 9B, and 9A, h API Specification 2F establish standards steel for rope, Product offcu and - ; 5 800 psi (5.5 m a ) strand, and chain respectively. The design considerations for S anchor lines are similar to those for lead lines. In addition, abrasion of the line caused by contactwiththeseafloor 7.4.4.dOther Design Methods should be considered. Other methods whichare based on testing and verification d. Anchor. The anchor transmits guyline loads to the soil. may be used for calculating the allowable load transfer stress The anchor system design should consider both horizontal fba. One such method is included and described in the Com- and vertical components the anchor load. of mentary Section C.7.4.4d. An anchor system may consist of a single pile (Ref. l), a piled template, or other anchoring devices. The pile compo- 7.4.5 Loadings other than Axial Load nents of an anchor should be designed using the criteria rec- ommended in Section 6, except that the ultimate capacity of Groutedpiletosleeveconnectionswillbesubjectedto the anchor system should be twice the anchor line load during loading conditions other than axial load, such as transverse loading condition 1. (See Section 7.5.5.) shear and bending moment or torque. effect of such load- The Other anchoring methods may be employed if these tech- ings, if significant, should be considered in the designcon- of niques can be substantiated by sufficient analysis or experi- nections by appropriate analyticalor testing procedures. mentation. 7.5GUYLINESYSTEM DESIGN e. Tower Terminations. The tower terminations system transmits guyline forces into the tower framework. Specific 7.5.1 General hardware should be chosen with consideration for bending A guyline system provides lateral restoring force and sta- fatigue of the lead line, limitations on bend radius, tolerance of lead line azimuth, capacity of the hardware to support the bility to a guyed tower. The guyline system consists of an mooring loads, and operational requirements. array of guylines, each attached to the tower and anchored on the seafloor. f. Terminations at Clump or Anchor. Resin or hot metal sockets for used guyline terminationsshould include a method of bending strain relief to reduce the stress concentra- *For helical shear keys only. tion factor and minimize the mass discontinuity.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 87. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 75 7.5.3 Configuration 8 Material The guyline system should provide the desired strength, 8.1STRUCTURAL STEEL stiffness,andredundancy to supportthetowerunderthe action of the environmental forces. Tower response should be 8.1.1 General evaluated and shown to remain stable with one or more criti- Steel should conform to a definite specification andto the cally loaded guylines out of service for the design environ- minimumstrengthlevel,groupandclassspecifiedbythe mental conditions. Major design variables include the number designer. Certified mill test reports or certified reports of tests and sizeof individual guylines, the distance from the tower to made by the fabricator or a testing laboratory in accordance the clumpweight and anchor, the size and configuration of the withASTMA6orA20, as applicable to the specification clumpweight, and the guyline preload and connections. listedinTable8.1.4-1,constitutesevidenceofconformity with the specification. Unidentified steel should be used. not 7.5.4 Analysis Generally,theloadsinaguylineshould be determined 8.1.2 Steel Groups from a specific dynamic analysis a detailed guyline model. of The modelshouldconsiderhydrodynamicandstructural Steel may be grouped according to strength level and weld- damping, inertia and drag characteristics of the guyline and ing characteristicsas follows: clumpweight, and interaction with the seafloor. The guyline 8.1.2a Group I designates mild steels with specified mini- may be excited at the tower termination with a displacement mum yield strengths of 40 ksi (280 MPa) or less. Carbon input determined according to the provisions 2.3.1~. of Other equivalent is generally 0.40% or less*, and these steels may design considerations are local vibration of the guyline and be welded by any of the welding processes as described in overall current force on the guyline system. AWS D1.l. 7.5.5RecommendedFactorsofSafety 8.1.2b Group II designates intermediate strengthsteels with specified minimum yield strengths of over 40 ksi (280 The ultimate guyline capacities can be assumed to be the MPa) through 52 ksi (360 MPa). Carbon equivalent ranges of rated breaking strengths. allowable guyline capacities are The up to 0.45% and higher, and these steels require the use of determinedbydividingtheultimateguylinecapacityby low hydrogen welding processes. appropriate factorsof safety which should not be less than the following values: 8 . 1 . 2 ~ Group III designates high strength steels with speci- fied minimum yield strengths in excess of52 ksi (360 MPa). Safety Such steels may be used provided that each application is Factor Loading Conditions investigated with regard to: 1) environmental Design conditions 2.0 with appropriate deck loads, including appropri- 1.Weldability special and weldingprocedures which ate dynamic amplification guyline forces. of may be required. Operating 2) environmental 3.0 conditions 2. Fatigue problems which may result from the use of higher working stresses, and These safety factors are based on the redundancy found in 3. Notch toughness in relation to other elements of frac- typical guyline configurations. turecontrol,such as fabrication,inspectionprocedures, service stress, and temperature environment. 7.5.6 Fatigue The axial and bending fatigue of the guylines should be life 8.1.3 Steel Classes evaluated. The loading history should be developed in accor- Consideration should be given for the selection of steels dancewith3.3.2.Discussions of fatigueforsteelrope or strand are given in References 2 and 3. with notch toughness characteristics suitable for the condi- tions of service. For this purpose, steels may be classified as References follows: Reese, L. D., “A Design Method for an Anchor in a Pile 8.1.3a Class C steels are those which have a historyof suc- Mooring System”; OTC 1745 (May, 1973). cessful application in welded structures at service tempera- Stonsifer, F. R., Smith, H. L., “Tensile Fatigue in Wire turesabovefreezing,butforwhichimpacttests are not Rope.” OTC 3419 (May, 1979). Ronson, K. T., “Ropes for Deep Water Mooring,” OTC Mn N i + C u + C r + M o + V *Carbon equivalent CE = C + -+ ~ 3850 (May, 1980). 6 15 5COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 88. 76 2A-WSD PRACTICE RECOMMENDED API Table 8.1.4-1-Structural Steel Plates Yield Strength Tensile Strength Specification Groupand Class Grade ksi MPa ksi MPa I ASTM(to C A36 2 in. thick) 36 25O 58-80 400-550 ASTM A131 Grade A (to l/2 in. thick) 34 5 23 5 8-7 1 400490 ASTM A285 Grade C (to in. thick) 3/4 30 205 55-75 380-515 I B ASTMA131 Grades B, D 34 5 23 5 8-7 1 400490 ASTM A5 16 Grade 65 35 240 65-85 450-585 ASTM A573 Grade 65 35 240 65-77 450-530 ASTM A709 Grade 36T2 36 25O 58-80 400-550 I ASTMGrades A A131 CS, E 34 5 23 58-71 400490 II C ASTM Grade A572 42 (to in. 2 thick)* 42 290 60 min. 415 min. ASTM A572 Grade 50 (to 2 in. thick; S91 required over l/2in.)* 50 345 65 min. 450 min. I II B API Spec 2MTI 50 345 7CL90 483-620 ASTM A709 Grades 5OT2,5OT3 50 345 65 min. 450 min. ASTM A13 1 Grade AH32 45.5 315 68-85 470-585 350 ASTM A131 Grade AH36 51 7 1-90 490-620 II API A Spec 2H Grade 42 42 290 62-80 430-550 Grade 50 (to 2l/2 in. thick) 50 345 7CL90 483-620 (over 2l/2 in. thick) 47 325 7CL90 483-620 API Spec 2W Grade 42 (to 1 in. thick) 42-67 29W62 62 min. 427 min. (over 1 in. thick) 42-62 29W27 62 min. 427 min. Grade 50 (to 1 in. thick) 50-75 345-5 17 65 min. 448 min. (over 1 in. thick) 50-70 345483 65 min. 448 min. Grade 50T (to 1 in. thick) 50-80 345-522 70 min. 483 min. (over 1 in. thick) 50-75 345-5 17 70 min. 483 min. Grade 60 (to 1 in. thick) 60-90 414-621 75 min. 5 17 min. (over 1 in. thick) 60-85 414-586 75 min. 5 17 min. API Spec 2Y Grade 42 (to 1 in. thick) 42-67 29W62 62 min. 427 min. (over 1 in. thick) 42-62 29W27 62 min. 427 min. Grade 50 (to 1 in. thick) 345-5 50-75 17 65 min. 448 min. (over 1 in. thick) 50-70 345483 65 min. 448 min. 345-572 Grade 50T (to 1 in. thick) 50-80 70 min. 483 min. (over 1 in. thick) 345-5 50-75 17 70 min. 483 min. ASTMA131 Grades DH32, EH32 45.5 315 68-85 470-585 Grades DH36, EH3 6 350 51 7 1-90 490-620 ASTM A537 Class I (to 2l/2 in. thick) 50 345 7CL90 485-620 A Grade ASTM A633 42 290 63-83 435-570 Grades C, D 50 345 7CL90 485-620 ASTM A678 Grade A 50 345 7CL90 485-620COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 89. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 77 Table 8.1.4-1-Structural Steel Plates (Continued) Strength Tensile Strength Yieldication Class Group ksi MPa MPa III A ASTM Class A537 II (to 2l/2 in. thick) 60 415 8CL100 550-690 ASTM A678 GradeB 60 415 8CL100 550-690 API Spec 2W Grade 60 (to 1 in. thick) 60-90 414-621 75 min. 5 17 min. (over 1 in. thick) 60-85 414-586 75 min. 5 17 min. API Spec 2Y Grade 60 (to 1 in. thick) 60-90 414-621 75 min. 5 17 min. (over 1 in. thick) 60-85 414-586 75 min. 5 17 min. ASTM A710 Grade A Class 3 (quenched and precipitation heat treated) through 2 in. 75 515 85 5 85 2 in. to 4 in. 65 45O 75 5 15 over 4 in. 60 415 70 485 *Maximum Vanadium Level Permitted 0.10% V. = specified.Suchsteelsareapplicabletoprimarystructural Impact testingfrequency for Class A steels should be in members involving limited thickness, moderate forming, low accordance with the specification under which the steel is restraint,modeststressconcentration,quasi-staticloading ordered; in the absence of other requirements, heat lot test- (rise time 1 second or longer) and structural redundancy such ing may be used. that an isolated fracture would not catastrophic. Examples be of such applications are piling, jacket braces and legs, and 8.1.4 Unless otherwisespecified the by designer,plates deck beams and legs. should conform to one of the specifications listed in Table 8.1.4-1.Structuralshapespecifications are listedinTable 8.1.3b Class B steels are suitable for usewhere thick- 8.1.4-2. Steels above the thickness limits stated may be used, ness, coldwork, restraint, stress concentration, impact provided applicable provisions of 8.1.2~ considered by are loading, and/or lack of redundancy indicate the need for the designer. improved notch toughness. Where impact tests are speci- fied, Class B steels should exhibit Charpy V-notch energy 8.2STRUCTURALSTEEL PIPE of 15 ft-lbs (20 J) for Group I, and 25 ft-lbs (34 J) for GroupII,at the lowest anticipated service temperature. 8.2.1 Specifications Steels enumerated herein as Class B can generally meet Unlessotherwisespecified,seamless or weldedpipe** these Charpy requirements at temperatures ranging from should conform to one of the specifications listed in Table 50" to 32°F (10" to 0°C). When impact tests are specified 8.2.1- 1.Pipe should be prime quality unless use of limited the for Class B steel, testing in accordance withASTM A 673, service, structural or pipe grade, reject is specifically Frequency H , is suggested. approved by the designer. 8 . 1 . 3 ~ Class A steels are suitable for use at subfreezing temperatures and for critical applications involving adverse 8.2.2 Fabrication combinations of the factors cited above. Critical applications may warrant Charpy testing 36-54°F (2&30"C) below the at Structuralpipeshouldbefabricatedinaccordancewith API Spec. 2B,ASTMA139**,ASTMA252**,ASTMA381, lowest anticipated service temperature. This extra margin of notch toughness prevents the propagation brittle fractures of or ASTM A671 using grades structural plate listed in Table of from large flaws, and provides for crack arrest in thicknesses 8.1.4- 1 except that hydrostatic testing may be omitted. of several inches. Steels enumerated herein as Class A can generally meet the Charpy requirements stated abovetem- at peratures ranging from 4" to 4 0 ° F (-20" to 40°C). **With longitudinal welds and circumferential butt welds. COPYRIGHT American Petroleum Institute Licensed by Information Handling Services
  • 90. 78 2A-WSD PRACTICE RECOMMENDED API Table 8.1.4-2-Structural Steel Shapes Strength Tensile Strength Yield ASTMon Class Group ksi & Grade MPa ksi MPa (to I A36 ASTM C 36 in. thick) 2 25O 400-550 58-80 Grade A131 ASTM A (to l/2 in. thick) 34 235 400-550 58-80 I Grade B A709 ASTM 36T2 36 25O 400-550 58-80 II A572 C Grade ASTM 42 (to 2 in. thick)* 42 290 60 min. 415 min. ASTM A572 Grade 50 (to 2 in. thick; S91 required over l 2 in.)* / 50 345 min. 65 450 min. II GradesA709ASTM 5OT2,5OT3B 50 345 min. 65 450 min.Grade 1 A13 ASTM AH32 45.5 315 470-585 68-85 AH36 Grade A131 ASTM 350 51 490-620 7 1-90 *Maximum Vanadium Level Permitted 0.10% V. = Table 8.2.1 -1-Structural Steel Pipe StrengthTensileStrength Yieldon Class Group & Grade MPa ksi ksi MPa I C API Grade 5L B* 35 240 60 min. 415 min. ASTM A53 GradeB 35 240 60 min. 415 min. ASTM A135 GradeB 35 240 60 min. 415 min. ASTM A139 GradeB 35 240 60 min. 415 min. ASTM A500 Grade A (round) 33 230 45 min. 3 10 min. (shaped) 39 270 45 min. 3 10 min. ASTM A501 36 250 58 min. 400 min. I B ASTM A106 Grade B (normalized) 35 240 60 min. 415 min. ASTM A524 Grade I (through 3/g in. w.t.) 35 240 60 min. 415 min. Grade II (over 3/g in. w.t.) 30 205 55-80 38CL550 I A ASTM A333 Grade 6 35 240 60 min. 415 min. ASTM A334 Grade 6 35 240 60 min. 415 min. II C API 5L Grade X42 2% max. expansion cold 42 290 60 min. 415 min. API 5L GradeX52 2% max. cold expansion 52 3 60 66 min. 455 min. ASTM A500 Grade B (round) 42 290 58 min. 400 min. (shaped) 46 320 58 min. 400 min. ASTMA618 50 345 70 min. 485 min. II B API 5L Grade X52 with SR5 or SR6 52 3 60 66 min. 455 min. II See A Section 8.2.2 *Seamless or with longitudinal seam welds. COPYRIGHT American Petroleum Institute Licensed by Information Handling Services
  • 91. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 79 8.2.3 Selections for Conditions of Service Table 8.3.1 -1-Input Testing Conditions Consideration should be given for the selection of steels D/t Test Temperature Test Condition with toughness characteristics suitable for the conditions of over 30 36°F (20°C) below LAST* Flat plate service (see Section 8.1.3). For tubes cold-formed to less D/t than30,andnotsubsequentlyheat-treated, due allowance 20-30 54°F (30°C) below LAST Flat plate should be made for possible degradation notch toughness, of under 20 18°F ( 10°C) below LAST As fabricated e.g., by specifying a higher class of steel or by specifying notch toughness tests run reduced temperature. at *LAST = Lowest Anticipated Service Temperature 8.3STEELFORTUBULAR JOINTS 8.3.4 Brace Ends Tubular joints are subjecttolocalstressconcentrations Although the brace ends at tubular connections are also which may lead to local yielding and plastic strains at the subject to stress concentration, the conditions of service are design load. During the service life, cyclic loading may ini- not quite as severe as for joint-cans. For critical braces, for tiate fatigue cracks, making additional demands on the ductil- whichbrittlefracturewouldbecatastrophic,consideration ity of the steel, particularlyunderdynamic These load. should be given to the use of stub-ends in the braces having demands particularly are severe heavywall in joint-cans the same classas the joint-can, or one class lower.This provi- designed for punching shear. sion need not apply to the body of braces (between joints). 8.3.1 Underwater Joints 8.4 CEMENT GROUT AND CONCRETE For underwater portions of redundant template-type plat- 8.4.1 Cement Grout forms,steelfor joint cans(such as jacket leg joint cans, chordsinmajor X and K joints, and through-members in If required by the design, the space between the piles and joints designed as overlapping) should meet one of the fol- thesurrounding structureshould carefully be filledwith lowing notch toughness criteria at the temperature given in grout. Prior to installation, the compressive strength of the Table 8.3.1-1. grout mix design should be confirmed on a representative number of laboratory specimens curedunder conditions 1. NRL Drop-Weight Test no-break performance. which simulate the field conditions. Laboratory test proce- 2. Charpy V-notch energy: 15 ft-lbs (20 Joules) for Group dures should be in accordance with ASTM 109. The uncon- I steels and 25 ft-lbs (34 Joules) for Group II steels, and 35 fined compressive strength of 28dayoldgroutspecimens ft-lbs for Group steels (transverse test). III computed as described in AC1 214-77 but equatingf’, tofcu, should not be less than either 2500 psi (17.25 MPa) or the Forwatertemperature of 40°F (4°C) or higher, these specified design strength. requirementsmaynormallybemet by usingtheClass A A representative number of specimens taken from random steels listed in Table 8.1.4- 1. batches during grouting operations should be tested to con- firm that the design grout strength has been achieved. Test 8.3.2 Above Water Joints procedures should be in accordance with ASTM 109. The For above waterjoints exposed to lower temperatures and specimens taken from the field should be subjected, until test, possible impact from boats,or for critical connectionsat any to a curing regime representativethe situ curing conditions, of location in which it is desired to prevent all brittle fractures, i.e., underwater and with appropriate seawater salinity and the tougher Class A steels should be considered, e.g., API temperature. Spec. 2H, Grade42 or Grade 50. For 50 ksi yield and higher strength steels, special attention should be given to welding 8.4.2 Concrete procedures. The concrete mix used in belled piles shouldselected on be the basis of shear strength, bond strength and workability for 8.3.3 Critical Joints underwater placement including cohesiveness and flowabil- For critical connections involving high restraint (including ity. The concrete mix may be made with aggregate and sand, adverse geometry, high yield strength and/or thick sections), or with sand only. water-cement ratio should be less than The through-thickness shrinkage strains, and subsequent through- 0.45. If aggregate is used, the aggregates should be small and thicknesstensileloadsinservice,considerationshouldbe rounded,thesandcontentshouldbe45% or greater,the given to the use of steel having improved through-thickness cement content should be not less than lb. per cubic yard 750 (Z-direction) properties, e.g., API Spec 2H, Supplements S4 (445 kg/m3), and the workability as measured by the slump and S5. test should be 7 to 9 inches (180 to 230 mm). To obtain theCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 92. 80 2A-WSD PRACTICE RECOMMENDED API properties required for proper placement, a suitable water- Equipment layoutdrawingsshouldbeincludedfor all reducing and plasticizing admixture may be necessary. decks. Sufficiently detailed process, mechanical and utility flow diagrams andelectrical one-line diagrams shouldbe 8.5 CORROSION PROTECTION included for all systems whichare covered by the bid. Specifications for equipment, machinery, and other engi- Unless specifiedotherwise by the designer, the systems for neered components should include an itemized list and corrosion protection should be designed in accordance with description of all items not shown on the drawings but which NACE W-01-76. are to be included in the bid, even such items as lighting and cathodic protection. Specifications for materials and fabrica- 9 DrawingsandSpecifications tion should include all types of material allowed for use and 9.1 GENERAL anyparticular requirements for dimensional tolerances, inspection, testingand welding. For use in connection with fixed offshore platforms and related facilities, the drawings and specifications are defined 9.4 DESIGN DRAWINGSANDSPECIFICATIONS as follows: Designdrawingsgivedescriptiveinformation about the 9.2 CONCEPTUAL DRAWINGS major components of the facility. Emphasis in these drawings is placed on overall layouts and definition of critical items, Conceptual drawingsare intended to supply a general idea supplemental by essential details. They should indicate all of the facility under consideration. These drawings should appurtenances and should include all dimensions wherestrict include preliminary layouts and elevation viewsof the overall adherence is required. facility showingthe number, typeof construction and approx- Design drawings should include a layout of the location imate size of each platform, as well as the more important and orientation of the structure or structures in the field, as auxiliary features, such as heliports and boat landings. well as the location of equipment on the desks of each struc- Simplified process or mechanical flow diagrams and elec- ture. Structural drawings showing member sizes of all major trical one-line diagrams should be included all production for structural members and all controlling dimensions should be or utility systems. A generalized equipment layout drawing included. General locations and preliminary or typical details should be included whichalso indicates buildings, storage of of miscellaneous structuralitems, such as joints, cover plates, supplies, etc. web plate stiffeners,etc., should be indicated. Also any other Allinformationwhichcontributestoclarifytheoverall typical structural details should be included which are not intent of the facility should be shown. Specificationsare not normally standard to this type construction. generally required. However, if included, they should be of Design drawings should also include all items necessary general descriptive nature to supplementthe drawings toade- for installation purposes, such as lifting eyes and launching quately describethe facility. trusses, which are critical to the structural designof the plat- form. 9.3 BID DRAWINGSANDSPECIFICATIONS Mechanical and utility flow diagrams showing size of all Bid drawingsare intended to show the total facility with its equipment,pipingandvalves,andelectrical one-line dia- configuration and dimension in sufficientdetail to accurately gramsshowing rating and sizes of feedersand controls define the scope of the project. With supplemental specifica- should be included.Equipment layout drawings of all equip- tions, bid drawings are suitable for submittal by the contrac- ment shown onthe flow diagrams or one-line diagrams, man- tor to generally define the of the proposal,or suitable to scope ifolds and major instrumentation items, suchas large control be furnished by the owner requesting a quotation where the valves, meter runs, control valve stations and control panels design is to be part of the contractor’s bid. In the latter case, should be shown. Piping plan and elevation drawings should all essential information needed by the designer should be show major piping only and indicate adequate space reserved included. for minor piping and conduit and cable runs. for Structural drawingsshouldshowmajoroveralldimen- Design drawings shouldbe supplemented by all specifica- sions, deck arrangements, operational loading requirements tions necessary to convey the intent of the design. Standard and any preferred type of construction and materials. Struc- specifications for material and fabrication whichare referred tural details and member sizes are not necessarily furnished to in thisW can be properly referenced on appropriate draw- since these are considered as “Design” drawings. All auxil- ings. However, any deviations fromthese specifications must iary items which are to be included in the bid, such as boat be detailed. Specifications should be included for equipment, landings, barge bumpers, stairs, walks, fence, handrail, etc., machinery and other engineereditems. should be shown on these drawings. Typical preferred con- Design drawings and specifications are often used as part struction details of the terms should be included. of the solicitation package or as part of the contract docu-COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 93. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 81 ment. As such, they need to be sufficiently detailed and suit- ences in methods and proceduresof various fabricators, shop able to be furnished by the owner to the contractor be used to drawings may vary in appearance. for making accurate material take-Offs for bidding purposes Shop drawings may include typical shop details to supple- when no design is required on the part of the contractor, or ment details and dimensionsshownon either fabrication suitable for submittal by the contractor to the owner to com- drawings or patterns for coping the ends of members, detailed pletely define the proposal. When design drawings are used piece-markeddrawings for eachmember and pipe spool for bid or contract purposes, all auxiliary items suchas stairs, drawings. boat landing, walkways, etc., should be shown in sufficient Shop drawings are the responsibility of the fabricator. detail for estimating purposes. Approvalorreview of shop drawings by the designer or owner shouldnot relieve the fabricator of his responsibility to 9.5 FABRICATIONDRAWINGSAND complete the work in accordance with the contractor fabrica- SPECIFICATIONS tion drawings and specifications. Fabrication drawings are intended to supply sufficient 9.7 INSTALLATIONDRAWINGSAND information that fabrication can be performed directly from SPECIFICATIONS thesedrawings.They should contain all design data fully detailed and dimensioned.At the fabricator’soption,they Installation drawings furnishall pertinent informationnec- may be supplemented by shop drawings. essary for the construction of the total facility on location at A set of fabrication drawingsincludes completely detailed sea. They contain relevant information included on fabri- not designdrawingswithdescriptions,exactlocations,sizes, cation drawings. thicknesses and dimensions of all structuralmembers and If specialprocedures are required,a set of installation stiffeners. This informationshould also be shown for all drawings may include installation sequence drawings. Details structural items, such as brackets, stiffeners, cover plates, etc., of all installation aids such as lifting eyes, launching runners and for all auxiliary items, such as stairs, walkways, fence, or trusses, jacket brackets, stabbing points, etc., should be handrail, etc. Connections and joints should be completely included if these are not shown on fabrication drawings. For detailed, including welding symbols, unless standard proce- jackets or towers installedby flotation or launching, drawings dures apply. Methods of attaching timber, grating and plate showing launching,upending, and flotationprocedures should be included. should be provided. Details should also be provided for pip- In addition to complete piping plan and elevation draw- ing,valvingandcontrols of the flotationsystem, closure ings, a set of fabrication drawings should include piping iso- plates, etc. metric drawings and details for all pipe supports, if required Erection of temporarystrutsorsupport should be indi- by thecomplexity of the facility.Instrumentationlocation cated. All rigging, cables, hoses, etc., which to be installed are plans and supports, electrical location diagrams showing gen- prior to loadout should detailed. Barge arrangement, load- be eral routing, and wire cable tie-ins toelectrical equipment and out and tie-down details should be provided. should be included. Installation drawingsare intended to be used in connection Fabrication drawings should clearly indicate the compo- with fabrication drawings. They should be supplemented by nents or “packages” scheduled for assembly as units in the detailed installation specifications, installation procedures,or fabrication yard. Welds and connections to be performed in special instructions as requiredtoprovide all information the “field” should beindicated. required to complete the field installation. Detailed specifications should be included for all work to be done by the fabricator such as welding, fabrication, test- 9.8 AS-BUILTDRAWINGSANDSPECIFICATIONS ing, etc., and for all materials, equipment or machinery to be As-built drawings show in detail the manner in which the furnished by the fabricator. However, for standard specifica- facility was actually constructed. These drawings are usually tions covered under the recommendations this W, no cop- of made by revising the original fabrication drawings, supple- ies need to be furnished provided reference is made on key mented by additional drawings if necessary. As-built draw- drawings. Specifications for equipment and other engineered ings are intended to reflectall changes, additions, corrections items not purchased by the fabricator may also be included or revisions made during the course of construction. They are with fabrication drawings general information. for prepared for use by the owner to provide information related to the operation, servicing, maintenance, and future expan- 9.6 SHOP DRAWINGS sion of the facility. Shop drawings or sketches prepared by or forthe fabri- are When the preparation of as-built drawings has been autho- cator, at his option,to facilitate the fabrication of parts and/or rized by the owner, it is the responsibilityof the fabricator and components of platforms. They are intended to provide all the field erector to furnish to the owner the designer or to ade- information and instructions for that purpose. Due to differ- quate information regarding all variations between the draw-COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 94. 82 2A-WSD PRACTICE RECOMMENDED API ings and the facility as actually constructed. This is usually quality, prior to any production welding. In general, the short- furnished as corrections from the yard, the shop and the field, circuiting mode GMAW should be limited to secondary or marked on prints of the original drawings or by supplemen- minor structural welds, and to root passes in welding proce- tary sketches, if required. This information should be suffi- dures qualifiedby tests. ciently complete that the owner or the designer can correct and revise the original drawings without additional data or 10.1.3d Downhill progression deposition of cover passes, fieldmeasurements. Since the fabricator erector and are using any welding procedure where heat of the cover pass responsible for the accuracy the corrections, a review n d of a deposition is less than 25 kilojoules per inch, should be pro- or approvalof the corrected drawings should be madeboth by hibitedunlessqualified hardness by testing of theheat the fabricator and erector. affected zones. A macro-section for hardness testing should be prepared from a weldthe maximum thickness and the of of Minor deviations from the original drawingsare generally maximum carbon equivalent steel to welded by the proce- be numerous.Differencesbetweentheactualdimensionsand dure; with the cover pass deposited at a preheat no higher those shown on the drawings need not be reported if they are than the minimum preheat specified on the welding procedure within the specified allowable tolerances. specification. The maximum hardness acceptable in the heat Specificationsshould be also corrected reflect to any affected zones, at any point of sampling, should not exceed changes made during the purchase material, equipment or of 325 HV10. machinery. 10.1.4 Welders and Welding Operators 10 Welding Welders should be qualified for the type of work assigned 10.1 GENERAL and should be issued certificates of qualification describing 10.1.1 Specifications thematerials,processes,electrodeclassifications,positions and any restrictions qualification. of Welding and weld procedure qualifications should be done in accordance with applicable provisions of the AWS Struc- 10.2 QUALIFICATION tural WeldingCode AWS Dl.1.-88 as follows: 10.2.1 General 1. Sections 1 through 6 constitute a body of rules which apply for the construction any welded steel structure. of Welding procedures, welders and welding operators should be qualified in accordance with Dl. 1-88 as further qual- AWS 2.Section8appliesforgeneralstructuralwelding of ified herein. plates structural and shapes, portions e.g., of deck sections. 10.2.2 Impact Requirements 3. Section 10 applies to Tubular Structures. When welding procedure qualification by test is required 10.1.2 Welding Procedures (i.e., when the procedure is not pre-qualified, when compara- ble impactperformancehasnotbeenpreviouslydemon- Writtenweldingproceduresshould be requiredforall strated, or when the welding consumables are to be employed work, even where prequalified. essential variables should The outside the rangeof essential variables covered by prior test- be specified in the welding procedure and adhered to in pro- ing), qualifications should include Charpy V-notch testing of duction welding. the as-deposited weld metal. Specimens should be removed fromthetestweld,andimpacttested,inaccordancewith 10.1.3Welding Procedure Limitations AppendixIII,RequirementsforImpactTesting, of AWS Dl. 1-88. The followingtesttemperaturesandminimum 10.1.3a Excluding the root pass, all weldingof steel with a energyvaluesarerecommended,formatchingtheperfor- nominal yield strength of 40 Ksi or more, or a weld throat mance of thevarioussteelgrades as listedinAPITables thickness in excess of l/2 inch, should be accomplished with 8.1.4-1 and 8.2.1-1. Single specimen energy values (one of low hydrogen processes (i.e., less than 15 m1/100g). three) may be 5 ft-lbs (75) lower without requiring retest. 10.1.3b Allweldingbyprocessesemployinganexternal gas shield of the arc area should be accomplished with wind 10.2.3 MechanicalTesting in Procedure protection. Qualification 10.1.3~ Any procedure requiring the Gas Metal Arc Weld- The mechanical testingof procedure qualification test cou- ing (GMAW) process should be proven by tests, per AWS pons should be performed by a competent independent test- Dl. 1-88, Section 5, to produce the desired properties and ing laboratory.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 95. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 83 Table 10.2.2-Impact Testing 10.3.5 Stress Relief Weld Metal Avg. In general, thermal stress relieving should not be required Steel Steel Impact Test for the weldable structural steels listed in Tables8.1.4-1 and Group Class Temperature (Joules) Ft-Lbs 8.2.1-1 for the range of wall thickness normally used in off- shore platforms. However, where postweld heat treatment is I C 27 0°F (-18°C) 20 to be used, it should be included in the procedure qualifica- I B 27 0°F (-18°C) 20 tion tests. I A 27 -20°F (-29°C) 20 10.3.6 InstallationWelding Welding machines should properly grounded to prevent be II C 27 0°F (-18°C) 20 underwater corrosion damage. Recommended procedures are II B 27 -20°F (-29°C) 20 presented in Section 12.7.1 through 12.7.3. II A 4 0 ° F (40OC) 25 34 10.3.7 Arc Strikes m A 40 4 0 30 (40OC) °F Arc strikes should be made only in the weld groove. A pro- cedure should be established for determining the extent of See Commentary for further discussion of prequalification, CTOD any methods for repairing damage to materials resulting from testing, and heat affected zones. inadvertent arc strikes outside the weld groove.The meth- of ods of defining the hardened zone, presence of cracks, and 10.2.4 Prior Qualifications surface integrity restoration should be detailed. New qualifications maybe waived by owner if prior quali- fications are deemed suitable. 10.3.8 Air-Arc Gouging Surfacesandcavitiesproducedbygougingoperations 10.3 WELDING using the air carbon arc cutting process should be thoroughly 10.3.1 General cleaned to remove all traces residual carbon and oxidation of prior to commencement welding in the affected area. of Welding should conform to sizes of welds and notes on drawings as well as qualified welding procedures; otherwise 10.3.9Temporary Attachments weldingshouldconformtothe AWS specificationslisted The same care and procedures used in permanent welds under 10.1.1 above and further qualified herein. should be used in welding temporary attachments. 10.3.2 Specified Welds 10.4RECORDSANDDOCUMENTATION Intersecting and abutting parts should be joined by com- Before construction begins, the fabricator should compile plete joint penetration groove welds, unless otherwise speci- all owner approved welding procedures well as a weld pro- as fied. This includes “hidden” intersections, such as may occur cedure matrix identifying where each welding procedure is to in overlapped braces and pass-through stiffeners. beused. This documentationshouldbeforwardedtothe owner for permanent record. 10.3.3 Groove Welds Made From One Side 11 Fabrication At intersecting tubular members, where access to the root side of theweldisprevented,complete joint penetration 11.1 ASSEMBLY groove welds conforming toFigure 11.1.3 may be used. The 11.1.1 General procedure, methods, as well as the acceptability of in-place weld build-up of wide root opening should be evaluated and Fabrication, other than welding, should be in accordance approved by the owner’s engineer or inspector. with the Specification for the Design, Fabrication and Erec- tion of Structural Steel for Buildings, AISC, eighth edition, 10.3.4 Welds Seal unless otherwise specified herein. Unless specified otherwise, all faying surfaces should be 11.1.2 Splices sealedagainstcorrosionbycontinuousfilletwelds.Seal 11.1.2.aPipe welds should not be less thaninch but need not exceed l/8 3/16 inch regardless of base metal thickness. Minimum preheat Pipe splices should be in accordance with the requirements temperatures of AWS Table 4.2 should be applied. of Section 3.8, API Spec. 2B, Fabricates Structural Pipe. SteelCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 96. 84 2A-WSD PRACTICE RECOMMENDED API Pipe used as beams should also be subject to the requirements access from one side only,edgepreparationandwelding of the following Section 11.1.2b. should be as shown inFigure 11.1.3. Bevelsshould be feather edged without a root face, the root opening should be as and 11.1.2.b Beams detailed. Tolerance on bevel angles should be +5". Grooves which are too tight after fit-up may be opened by arc goug- up Segments of beams with the same cross-sections may be ing to the dimensions as shown in Figure 11.1.3. If the gap is spliced. Splices should be penetration in accordance with full too wide, it may be built up as per AWS Dl.1-88, Section AWS Dl. 1-88. The use of the beam should determine the 3.3.4 and API RP 2A,Section 10.3.3. locationandfrequency of splicing. Splices should not be located closer together than twice the depth of the beam, or 11.1.3.dWeld Profile Control three feet (1 m) whichever is smaller. In areas critical to the integrity of the structure, splice locations should be specified Where controlled weld profiling has been consideredthe in by the designer. fatigue analysis by the use of the S-N curve X, Section 5.4, a capping layer shouldbe applied so that the as-welded surface 11.1.2.c Joint Cans merges smoothly with the adjoining base metal andapproxi- mates the concave profiles shown in Figure 11.1.3. In addi- In orderto avoid bracing members falling on a longitudinal tiontoconsidering the weldqualityprovisions of Section weld of a can, the longitudinal welds for joint cans may be 13.4, deviations in the weld profile should no deeper than be staggered a minimum of 12 inches to avoid the interference. 0.04 in. (1 mm) relative to a think disk withdiameter equal a Otherwise the longitudinal welds should be staggered a mini- to or greater than thebrace thickness at the weld. Every effort mum of 90 degrees. should be made to achieve the profile in the as-welded condi- tion. However, the weld surface maybe ground to the profile 11.1.3WeldedTubular Connections shown inFigure 11.1.3. Final grinding marks should be trans- 11.1.3.a General verse to the weld axis. For tubular joints requiring weld pro- file control, the weld toes on both the brace and chord side The intersection of two or more tubular membersforms a shouldreceive100%magnetic particle inspection(Section connection with stress concentrations at and near the joining 13.4) for surface and near surface defects. weld.Properfabrication is essential; in particular,welds should achieveas full a joint penetration as is practicable, and 11.1.3.eSpecial Details the externalweldprofileshouldmergesmoothlywiththe base metal oneither side. Special details should be prepared whenthe local dihedral angel is less than 30". These should be of a manner and type 11.1.3.b Fabrication Sequence to develop adequate welds, as demonstrated on sample joints or mock-ups. When two or more tubulars join in an X joint, the large diameter member should continue through the joint, and the 11.1.3.f Slotted Members other should frame onto the through member and be consid- ered the minor member. Unless specified otherwise on the When membersare slotted to receive gusset plates, the slot drawings, when two ore more minor members intersect or should be 12 in. (305 mm) or twelve times the member wall overlap at a joint, the order in which each member frames thickness,whichever is greater,fromanycircumferential into the joint should be determined by wall thickness and/or weld. To avoid notches the slotted member should be drilled diameter. The member with the thickest wall should be the or cut and ground smooth the end of the slot with a diame- at continuous or through member, andthe sequence for framing ter of at least l/8 in. (3 mm) greater than the width of slot. the the remainingmembers should bebased on theorder of Where the gusset plate passes throughthe slot, the edge of the decreasing wall thickness. If two or more members have the gusset plate should be ground to an approximately round half same wall thickness, the larger diameter member should be shape to provide a better fit-up and welding condition. the continuous or through member.If two or more members have the same diameter and wall thickness, either member 11.1.4 Plate Girder Fabrication and Welding may be the through member unless a through member has Fabrication tolerances should be governed AWS Dl.1-88 by been designatedby the designer. except where specific service requirements dictate the use of more severe control over the deviations from the theoretical 11.1.3.c Joint Details dimensions assumed in the design. localized heating is pro- If Any member framing into or overlapping onto any other posed for the straightening or repairout of tolerance, consid- of member should be beveled for a complete joint penetration eration should begiven to its effect on the material properties grooveweld. Where member size orconfigurationallows and the procedure should be approvedthe Owner. byCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  • 97. RECOMMENDED PRACTICE FOR PLANNING, DESIGNING CONSTRUCTING FIXED AND OFFSHOREPLATFORMS-WORKINGSTRESS DESIGN 85 Note: Includes tolerance I~ I 1 Build out to full thickness except "T" need not exceed 1.75 t. Section A-A Section B-B Min. 1/16 in. (1.6 mm) Back-up weld not subject to I 4 Section C-C Section C-C (Alternative) Figure 11.1.3-Welded Tubular Connections-Shielded Metal Arc WeldingCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services