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- 1. RECOMMENDED PRACTICE DNV-RP-C203 FATIGUE DESIGN OFOFFSHORE STEEL STRUCTURES APRIL 2008 DET NORSKE VERITAS
- 2. FOREWORDDET NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life, prop-erty and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification and consultancyservices relating to quality of ships, offshore units and installations, and onshore industries worldwide, and carries out researchin relation to these functions.DNV Offshore Codes consist of a three level hierarchy of documents:— Offshore Service Specifications. Provide principles and procedures of DNV classification, certification, verification and con- sultancy services.— Offshore Standards. Provide technical provisions and acceptance criteria for general use by the offshore industry as well as the technical basis for DNV offshore services.— Recommended Practices. Provide proven technology and sound engineering practice as well as guidance for the higher level Offshore Service Specifications and Offshore Standards.DNV Offshore Codes are offered within the following areas:A) Qualification, Quality and Safety MethodologyB) Materials TechnologyC) StructuresD) SystemsE) Special FacilitiesF) Pipelines and RisersG) Asset OperationH) Marine OperationsJ) Wind TurbinesO) Subsea SystemsAmendments and CorrectionsThis document is valid until superseded by a new revision. Minor amendments and corrections will be published in a separatedocument normally updated twice per year (April and October).For a complete listing of the changes, see the “Amendments and Corrections” document located at:http://webshop.dnv.com/global/, under category “Offshore Codes”.The electronic web-versions of the DNV Offshore Codes will be regularly updated to include these amendments and corrections.Comments may be sent by e-mail to rules@dnv.comFor subscription orders or information about subscription terms, please use distribution@dnv.comComprehensive information about DNV services, research and publications can be found at http://www.dnv.com, or can be obtained from DNV, Veritas-veien 1, NO-1322 Høvik, Norway; Tel +47 67 57 99 00, Fax +47 67 57 99 11.© Det Norske Veritas. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including pho-tocopying and recording, without the prior written consent of Det Norske Veritas.Computer Typesetting (FM+SGML) by Det Norske Veritas.Printed in NorwayIf any person suffers loss or damage which is proved to have been caused by any negligent act or omission of Det Norske Veritas, then Det Norske Veritas shall pay compensation to such personfor his proved direct loss or damage. However, the compensation shall not exceed an amount equal to ten times the fee charged for the service in question, provided that the maximum compen-sation shall never exceed USD 2 million.In this provision "Det Norske Veritas" shall mean the Foundation Det Norske Veritas as well as all its subsidiaries, directors, officers, employees, agents and any other acting on behalf of DetNorske Veritas.
- 3. Recommended Practice DNV-RP-C203, April 2008 Changes – Page 3Main changes April 2008— The principal stress direction in Figure 2-2 is changed — A commentary section on stress concentration factors for from 60° to equations combining stress normal to the weld details in pipelines and cylindrical tanks with stress and shear stress when fatigue cracking along the weld toe cycling mainly due to internal pressure is included. This is the most likely failure mode and a function of maximum includes circumferential welds and longitudinal welds in principal stress when the main stress direction is more par- pipes. allel to the weld. — The section on grouted joints is extended to include joints— Some guidance on how to consider principal stress direc- with the annulus between tubular members filled with tion relative to the weld toe with respect to selection of grout such as joints in jacket legs with insert piles. S-N curve is included in the commentary section. — Stress concentration factors at circumferential butt welds— Guidance on derivation of an effective thickness to be in tubulars subjected to axial load are included for thick- used together with the S-N curve for cast joints subjected ness transitions on inside and for welds made from outside to some bending moment over the thickness is given. only.— The δ0 in equations for stress concentration factors for butt welds in pipelines is removed due to rather strict toler- — Some printing errors have been corrected and some of the ances used in pipeline fabrication and it can not be docu- texts have been revised to improve readability. mented that a large tolerance δ0 is embedded in the S-N data used in design. DET NORSKE VERITAS
- 4. Recommended Practice DNV-RP-C203, April 2008Page 4 – Changes DET NORSKE VERITAS
- 5. Recommended Practice DNV-RP-C203, April 2008 Page 5 CONTENTS1. INTRODUCTION .................................................. 7 3.3 Tubular joints and members................................ 23 3.3.1 Stress concentration factors for simple tubular joints ...... 231.1 General .....................................................................7 3.3.2 Superposition of stresses in tubular joints ........................ 231.2 Validity of standard.................................................7 3.3.3 Tubular joints welded from one side ................................ 241.2.1 Material............................................................................... 7 3.3.4 Stiffened tubular joints ..................................................... 241.2.2 Temperature........................................................................ 7 3.3.5 Grouted tubular joints ....................................................... 251.2.3 Low cycle and high cycle fatigue ....................................... 7 3.3.6 Cast nodes......................................................................... 25 3.3.7 Stress concentration factors for tubular butt weld1.3 Methods for fatigue analysis...................................7 connections ....................................................................... 251.4 Definitions ................................................................7 3.3.8 Stress concentration factors for stiffened shells ............... 27 3.3.9 Stress concentration factors for conical transitions .......... 271.5 Symbols.....................................................................8 3.3.10 Stress concentration factors for tubulars subjected to axial force ......................................................................... 292. FATIGUE ANALYSIS BASED ON S-N DATA . 9 3.3.11 Stress concentration factors for joints with square sections.................................................................. 292.1 Introduction .............................................................9 3.3.12 Stress concentration factors for joints with gusset plates . 302.2 Fatigue damage accumulation..............................10 4. CALCULATION OF HOT SPOT STRESS BY2.3 Fatigue analysis methodology and FINITE ELEMENT ANALYSIS........................ 30 calculation of Stresses ...........................................102.3.1 General.............................................................................. 10 4.1 General................................................................... 302.3.2 Plated structures using nominal stress S-N curves ........... 10 4.2 Tubular joints........................................................ 302.3.3 Plated structures using hot spot stress S-N curves............ 112.3.4 Tubular joints ................................................................... 11 4.3 Welded connections other than tubular joints ... 312.3.5 Fillet welds........................................................................ 12 4.3.1 Stress field at a welded detail ........................................... 312.3.6 Fillet welded bearing supports.......................................... 12 4.3.2 FE modelling .................................................................... 31 4.3.3 Derivation of stress at read out points 0.5 t and 1.5 t ....... 312.4 S-N curves ..............................................................12 4.3.4 Derivation of hot spot stress ............................................. 312.4.1 General.............................................................................. 12 4.3.5 Hot spot S-N curve ........................................................... 322.4.2 Failure criterion inherent the S-N curves.......................... 12 4.3.6 Derivation of effective hot spot stress from FE analysis.. 322.4.3 S-N curves and joint classification ................................... 12 4.3.7 Limitations for simple connections .................................. 322.4.4 S-N curves in air ............................................................... 13 4.3.8 Verification of analysis methodology............................... 332.4.5 S-N curves in seawater with cathodic protection ............. 142.4.6 S-N curves for tubular joints............................................. 15 5. SIMPLIFIED FATIGUE ANALYSIS ............... 342.4.7 S-N curves for cast nodes ................................................. 162.4.8 S-N curves for forged nodes ............................................. 16 5.1 General................................................................... 342.4.9 S-N curves for free corrosion ........................................... 16 5.2 Fatigue design charts ............................................ 342.4.10 S-N curves for base material of high strength steel.......... 162.4.11 S-N curves for stainless steel............................................ 16 5.3 Example of use of design charts........................... 382.4.12 S-N curves for small diameter umbilicals ........................ 162.4.13 Qualification of new S-N curves based on 6. FATIGUE ANALYSIS BASED ON fatigue test data ................................................................. 17 FRACTURE MECHANICS................................ 392.5 Mean stress influence for non welded structures ...........................................17 7. IMPROVEMENT OF FATIGUE LIFE BY FABRICATION ................................................... 392.6 Effect of fabrication tolerances ............................18 7.1 General................................................................... 392.7 Design chart for fillet and partial penetration welds .......................................................................18 7.2 Weld profiling by machining and grinding ........ 392.8 Bolts ........................................................................18 7.3 Weld toe grinding.................................................. 402.8.1 General.............................................................................. 18 7.4 TIG dressing .......................................................... 402.8.2 Bolts subjected to tension loading .................................... 182.8.3 Bolts subjected to shear loading ...................................... 18 7.5 Hammer peening ................................................... 402.9 Pipelines and risers................................................18 8. EXTENDED FATIGUE LIFE............................ 412.9.1 General.............................................................................. 182.9.2 Combined eccentricity for fatigue analysis of 9. UNCERTAINTIES IN FATIGUE LIFE seamless pipes................................................................... 19 PREDICTION ...................................................... 412.9.3 SCFs for pipes with internal pressure............................... 192.10 Guidance to when a detailed fatigue analysis can 9.1 General................................................................... 41 be omitted ...............................................................20 9.2 Requirements to in-service inspection for fatigue cracks......................................................... 443. STRESS CONCENTRATION FACTORS ........ 203.1 Stress concentration factors for 10. REFERENCES..................................................... 44 plated structures ....................................................20 APP. A CLASSIFICATION OF STRUCTURAL3.1.1 General.............................................................................. 203.1.2 Stress concentration factors for butt welds....................... 20 DETAILS............................................................................ 473.1.3 Stress concentration factors for cruciform joints.............. 203.1.4 Stress concentration factors for A.1 Non-welded details................................................... 47 rounded rectangular holes ................................................ 21 A.2 Bolted connections ................................................... 483.1.5 Stress concentration factors for holes with edge A.3 Continuous welds essentially parallel to the reinforcement.................................................................... 22 direction of applied stress......................................... 493.1.6 Stress concentration factors for scallops........................... 22 A.4 Intermittent welds and welds at cope holes.............. 513.2 Stress concentration factors for ship details .......23 A.5 Transverse butt welds, welded from both sides ....... 52 DET NORSKE VERITAS
- 6. Recommended Practice DNV-RP-C203, April 2008Page 6A.6 Transverse butt welds, welded from one side .......... 55 APP. D COMMENTARY ............................................. 115A.7 Welded attachments on the surface or the edge of a D.1 Comm. 1.2.3 Low cycle and high cycle fatigue..... 115 stressed member ....................................................... 56 D.2 Comm. 1.3 Methods for fatigue analysis ............... 115A.8 Welded joints with load carrying welds ................... 60 D.3 Comm. 2.2 Combination of fatigue damages fromA.9 Hollow sections ........................................................ 63 two dynamic processes .......................................... 115A.10 Details relating to tubular members ......................... 66 D.4 Comm. 2.3.2 Plated structures using nominal stress S-N curves...................................... 116APP. B SCF’S FOR TUBULAR JOINTS ..................... 68 D.5 Comm. 2.4.3 S-N curves........................................ 117 D.6 Comm. 2.4.9 S-N curves and efficiency ofB.1 Stress concentration factors for simple tubular joints corrosion protection ............................................... 119 and overlap joints ..................................................... 68 D.7 Comm. 2.9.3 SCFs for pipes with internal pressure ..................................................... 119APP. C SCF’S FOR PENETRATIONS WITH D.8 Comm. 3.3 Stress concentration factors ................ 121REINFORCEMENTS ....................................................... 78 D.9 Comm. 3.3.3 Tubular joints welded from one side 121 D.10 Comm. 4.1 The application of the effectiveC.1 SCF’s for small circular penetrations with notch stress method for fatigue assessment of structural details ..................................................... 121 reinforcement............................................................ 78 D.11 Comm. 4.3.8 Verification of analysis methodologyC.2 SCF’s at man-hole penetrations ............................. 100 for FE hot spot stress analysis................................ 123C.3 Results .................................................................... 101 D.12 Comm. 5 Simplified fatigue analysis..................... 129 DET NORSKE VERITAS
- 7. Recommended Practice DNV-RP-C203, April 2008 Page 71. Introduction establishing the stress history. A fatigue analysis may be based on an expected stress history, which can be defined as expected1.1 General number of cycles at each stress range level during the predicted life span. A practical application of this is to establish a longThis Recommended Practice presents recommendations in term stress range history that is on the safe side. The part of therelation to fatigue analyses based on fatigue tests and fracture stress range history contributing most significantly to themechanics. Conditions for the validity of the Recommended fatigue damage should be most carefully evaluated. See alsoPractice are given in section 1.2. Appendix D, Commentary, for guidance.The aim of fatigue design is to ensure that the structure has an It should be noted that the shape parameter h in the Weibulladequate fatigue life. Calculated fatigue lives also form the distribution has a significant impact on calculated fatigue dam-basis for efficient inspection programmes during fabrication age. For effect of the shape parameter on fatigue damage seeand the operational life of the structure. also design charts in Figure 5-1 and Figure 5-2. Thus, when theTo ensure that the structure will fulfil its intended function, a fatigue damage is calculated based on closed form solutionsfatigue assessment, supported where appropriate by a detailed with an assumption of a Weibull long term stress range distri-fatigue analysis, should be carried out for each individual bution, a shape parameter to the safe side should be used.member, which is subjected to fatigue loading. See also section2.10. It should be noted that any element or member of the 1.4 Definitionsstructure, every welded joint and attachment or other form of Classified structural detail: A structural detail containing astress concentration, is potentially a source of fatigue cracking structural discontinuity including a weld or welds, for whichand should be individually considered. the nominal stress approach is applicable, and which appear in1.2 Validity of standard the tables of this Recommended Practice. Also referred to as standard structural detail.1.2.1 Material Constant amplitude loading: A type of loading causing a reg-This Recommended Practice is valid for steel materials in air ular stress fluctuation with constant magnitudes of stresswith yield strength less than 960 MPa. For steel materials in maxima and minima.seawater with cathodic protection or steel with free corrosion Crack propagation rate: Amount of crack propagation duringthe Recommended Practice is valid up to 550 MPa. one stress cycle.This Recommended Practice is also valid for bolts in air envi- Crack propagation threshold: Limiting value of stress inten-ronment or with protection corresponding to that condition of sity factor range below which the stress cycles are consideredgrades up to 10.9, ASTM A490 or equivalent. to be non-damaging.This Recommended Practice may be used for stainless steel. Eccentricity: Misalignment of plates at welded connections measured transverse to the plates.1.2.2 Temperature Effective notch stress: Notch stress calculated for a notch withThis Recommended Practice is valid for material temperatures a certain effective notch radius.of up to 100°C. For higher temperatures the fatigue resistancedata may be modified with a reduction factor given as: Fatigue deterioration of a component caused by crack initia- tion and/or by the growth of cracks. R T = 1.0376 − 0.239 ⋅ 10 −3 T − 1.372 ⋅ 10 −6 T 2 (1.2.1) Fatigue action: Load effect causing fatigue. Fatigue damage ratio: Ratio of fatigue damage at consideredwhere T is given in °C (Derived from figure in IIW document number of cycles and the corresponding fatigue life at constantXII-1965-03/XV-1127-03). Fatigue resistance is understood to amplitude loading.mean strength capacity. The reduced resistance in the S-Ncurves can be derived by a modification of the log as: Fatigue life: Number of stress cycles at a particular magnitude required to cause fatigue failure of the component.Log a RT = Log a + m Log RT (1.2.2) Fatigue limit: Fatigue strength under constant amplitude load- ing corresponding to a high number of cycles large enough to be considered as infinite by a design code.1.2.3 Low cycle and high cycle fatigueThis Recommended Practice has been produced with the pur- Fatigue resistance: Structural detail’s resistance against fatigue actions in terms of S-N curve or crack propagationpose of assessing fatigue damage in the high cycle region. See properties.also Appendix D, Commentary. Fatigue strength: Magnitude of stress range leading to partic-1.3 Methods for fatigue analysis ular fatigue life.The fatigue analysis should be based on S-N data, determined Fracture mechanics: A branch of mechanics dealing with theby fatigue testing of the considered welded detail, and the lin- behaviour and strength of components containing cracks.ear damage hypothesis. When appropriate, the fatigue analysismay alternatively be based on fracture mechanics. If the Design Fatigue Factor: Factor on fatigue life to be used forfatigue life estimate based on S-N data is short for a component design.where a failure may lead to severe consequences, a more accu- Geometric stress: See “hot spot stress”.rate investigation considering a larger portion of the structure,or a fracture mechanics analysis, should be performed. For cal- Hot spot: A point in structure where a fatigue crack may initi-culations based on fracture mechanics, it should be docu- ate due to the combined effect of structural stress fluctuationmented that there is a sufficient time interval between time of and the weld geometry or a similar notch.crack detection during in-service inspection and the time of Hot spot stress: The value of structural stress on the surface atunstable fracture. the hot spot (also known as geometric stress or structuralAll significant stress ranges, which contribute to fatigue dam- stress).age, should be considered. The long term distribution of stress Local nominal stress: Nominal stress including macro-geo-ranges may be found by deterministic or spectral analysis, see metric effects, concentrated load effects and misalignments,also ref. /1/. Dynamic effects shall be duly accounted for when disregarding the stress raising effects of the welded joint itself. DET NORSKE VERITAS
- 8. Recommended Practice DNV-RP-C203, April 2008Page 8Local notch: A notch such as the local geometry of the weld account the effects of a structural discontinuity, and consistingtoe, including the toe radius and the angle between the base of membrane and shell bending stress components. Alsoplate surface and weld reinforcement. The local notch does not referred to as geometric stress or hot spot stress.alter the structural stress but generates non-linear stress peaks. Structural stress concentration factor: The ratio of hot spotMacro-geometric discontinuity: A global discontinuity, the (structural) stress to local nominal stress. In this RP the shortereffect of which is usually not taken into account in the collec-tion of standard structural details, such as large opening, a notation: “Stress concentration factor” (SCF) is used.curved part in a beam, a bend in flange not supported by dia- Variable amplitude loading: A type of loading causing irregu-phragms or stiffeners, discontinuities in pressure containing lar stress fluctuation with stress ranges (and amplitudes) ofshells, eccentricity in lap joints. variable magnitude.Macro-geometric effect: A stress raising effect due to macro-geometry in the vicinity of the welded joint, but not due to the 1.5 Symbolswelded joint itself.Membrane stress: Average normal stress across the thickness C material parameterof a plate or shell. D accumulated fatigue damage, diameter of chordMiner sum: Summation of individual fatigue damage ratios DFF Design Fatigue Factorcaused by each stress cycle or stress range block according to Dj cylinder diameter at junctionPalmgren-Miner rule. E Young’s modulusMisalignment: Axial and angular misalignments caused either F fatigue lifeby detail design or by fabrication. I moment of inertia of tubularsNominal stress: A stress in a component, resolved, using gen- Kmax maximum and minimum stress intensity factorseral theories such as beam theory. Kmin respectivelyNonlinear stress peak: The stress component of a notch stress Kw stress concentration factor due to weld geometrywhich exceeds the linearly distributed structural stress at alocal notch. ΔK Kmax - KminNotch stress: Total stress at the root of a notch taking into L length of chord, length of thickness transitionaccount the stress concentration caused by the local notch. N number of cycles to failureThus the notch stress consists of the sum of structural stress Ni number of cycles to failure at constant stress rangeand non-linear stress peak. Δσ iNotch stress concentration factor: The ratio of notch stress to N axial force in tubularstructural stress. R outer radius of considered chord, reduction factorParis’ law: An experimentally determined relation between on fatigue lifecrack growth rate and stress intensity factor range. SCF stress concentration factorPalmgren-Miner rule: Fatigue failure is expected when the SCFAS stress concentration factor at the saddle for axialMiner sum reaches unity. Reference is also made to Chapter 9 loadon uncertainties). SCFAC stress concentration factor at the crown for axialRainflow counting: A standardised procedure for stress range loadcounting. SCFMIP stress concentration factor for in plane momentShell bending stress: Bending stress in a shell or plate like part SCFMOP stress concentration factor for out of planeof a component, linearly distributed across the thickness as momentassumed in the theory of shells. Ra surface roughnessS-N curve: Graphical presentation of the dependence of fatigue RT reduction factor on fatigue resistancelife (N) on fatigue strength (S). T thickness of chordStress cycle: A part of a stress history containing a stress max-imum and a stress minimum. Te equivalent thickness of chordStress intensity factor: Factor used in fracture mechanics to Td design life in secondscharacterise the stress at the vicinity of a crack tip. Q probability for exceedance of the stress range ΔσStress range: The difference between stress maximum and A crack depthstress minimum in a stress cycle. ai half crack depth for internal cracksStress range block: A part of a total spectrum of stress ranges a intercept of the design S-N curve with the log Nwhich is discretized in a certain number of blocks. axis e -α exp(-α)Stress range exceedances: A tabular or graphical presentationof the cumulative frequency of stress range exceedances, i. e. g gap = a/D; factor depending on the geometry ofthe number of ranges exceeding a particular magnitude of the member and the crack.stress range in stress history. Here frequency is the number of h Weibull shape parameter, weld sizeoccurrences. k number of stress blocks, exponent on thicknessStress ratio: Ratio of minimum to maximum value of the stressin a cycle. l segment lengths of the tubularStructural discontinuity: A geometric discontinuity due to the m negative inverse slope of the S-N curve; cracktype of welded joint, usually found in tables of classified struc- growth parametertural details. The effects of a structural discontinuity are (i) ni number of stress cycles in stress block iconcentration of the membrane stress and (ii) formation of sec- no is the number of cycles over the time period forondary bending stress. which the stress range level Δσo is definedStructural stress: A stress in a component, resolved taking into tref reference thickness DET NORSKE VERITAS
- 9. Recommended Practice DNV-RP-C203, April 2008 Page 9T plate thickness, thickness of brace member the purpose of achieving a reliable design with respect to this failure mode.tc cone thickness — Fatigue crack growth from the weld root through the fillettp plate thickness weld.Q Weibull scale parameter Fatigue cracking from root of fillet welds with a crackΓ gamma function growth through the weld is a failure mode that can lead to significant consequences. Use of fillet welds should beη usage factor sought avoided in connections where the failure conse-α the slope angle of the cone; α = L/D quences are large due to less reliable NDE of this type ofβ d/D connection compared with a full penetration weld. How-δ eccentricity ever, in some welded connections use of fillet welds can hardly be avoided and it is also efficient for fabrication.δ0 eccentricity inherent in the S-N curve The specified design procedure in this document is consid-γ R/T ered to provide reliable connections also for fillet welds.νo average zero-up-crossing frequency — Fatigue crack growth from the weld root into the sectionν Poisson’s ratio under the weld. Fatigue crack growth from the weld root into the sectionσlocal local stress under the weld is observed during service life of structuresσnominal nominal stress in laboratory fatigue testing. The number of cycles to fail-σhot spot hot spot stress or geometric stress ure for this failure mode is of a similar magnitude as fatigue cracking from the weld toe in as welded condition.σx maximum nominal stresses due to axial force There is no methodology that can be recommended used toσmy maximum nominal stresses due to bending about avoid this failure mode except from using alternative typesσmz the y-axis and the z-axis of welds locally. This means that if fatigue life improve-Δσ stress range ment of the weld toe is required the connection will become more highly utilised and it is also required to makeΔσ 0 stress range exceeded once out of n0 cycles improvement for the root. This can be performed using aτ t/T, shear stress full penetration weld along some distance of the stiffener nose. — Fatigue crack growth from a surface irregularity or notch2. Fatigue Analysis Based on S-N Data into the base material. Fatigue cracking in the base material is a failure mode that2.1 Introduction is of concern in components with high stress cycles. Then the fatigue cracks often initiate from notches or grooves inThe main principles for fatigue analysis based on fatigue tests the components or from small surface defects/irregulari-are described in this section. The fatigue analysis may be based ties. The specified design procedure in this document ison nominal S-N curves for plated structures when appropriate. considered to provide reliable connections also withAdditional stresses resulting from fabrication tolerances for respect to this failure mode.butt welds and cruciform joints should be considered when thefabrication tolerances exceed that inherent the S-N data. Ref-erence is made to sections 3.1 and 3.3.When performing finite element analysis for design of platedstructures it is often found more convenient to extract hot spotstress from the analysis than that of a nominal stress. Guidanceon finite element modelling and hot spot stress derivation ispresented in section 4.3. The calculated hot spot stress is thenentered a hot spot S-N curve for derivation of cycles to failure.Also here additional stresses resulting from fabrication toler-ances for butt welds and cruciform joints should be considered.For design of simple tubular joints it is standard practice to useparametric equations for derivation of stress concentration fac-tors to obtain hot spot stress for the actual geometry. Then thishot spot stress is entered a relevant hot spot stress S-N curvefor tubular joints. a) Fatigue crack growth from the weld toe into the baseResults from performed fatigue analyses are presented in sec- materialtion 5 in terms of design charts that present allowable stressesas function of the Weibull shape parameter. The basis for thedesign charts is that long term stress ranges can be describedby a two parameter Weibull distribution. The procedure can beused for different design lives, different Design Fatigue Fac-tors and different plate thickness.The following fatigue cracking failure modes are considered inthis document (see also Figure 2-1):— Fatigue crack growth from the weld toe into the base material. In welded structures fatigue cracking from weld toes into the base material is a frequent failure mode. The fatigue crack is initiated at small defects or undercuts at the weld toe where the stress is highest due to the weld notch geom- b) Fatigue crack growth from the weld root through the fillet etry. A large amount of the content in this RP is made with weld DET NORSKE VERITAS
- 10. Recommended Practice DNV-RP-C203, April 2008Page 10 method as the position of the integration points may have a sig- nificant influence on the calculated fatigue life dependent on integration method. See also section 5 for calculation of fatigue damage using design charts. Reference is made to commentary section for derivation of fatigue damage calculated from different processes. 2.3 Fatigue analysis methodology and calculation of Stresses 2.3.1 General Fatigue analysis may be based on different methodologies depending on what is found most efficient for the considered structural detail. Different concepts of S-N curves are devel-c) Fatigue crack growth from the weld root into the section oped and referred to in the literature and in this RP. It is thusunder the weld important that the stresses are calculated in agreement with the definition of the stresses to be used together with a particular S-N curve. Three different concepts of S-N curves are defined: — Nominal stress S-N curve that is described in section 2.3.2. — Hot spot stress S-N curve that is described in section 2.3.3 for plated structures and in section 2.3.4 for tubular joints. — Notch stress S-N curve that is not used in the main part of this RP. (A notch stress S-N curve is listed in the commen- tary that can be used together with finite element analysis where the local notch is modelled by an equivalent radius. This approach is foreseen used only in special cases where it is found difficult to reliably assess the fatigue life using other methods).d) Fatigue crack growth from a surface irregularity or notch Nominal stress is understood to be a stress in a component thatinto the base material can be derived by classical theory such as beam theory. In aFigure 2-1 simple plate specimen with an attachment as shown in FigureExplanation of different fatigue failure modes 4-1 the nominal stress is simply the membrane stress that is used for plotting of the S-N data from the fatigue testing. An example of fatigue design using this procedure is shown in the2.2 Fatigue damage accumulation commentary section (Example with fatigue analysis of a drum).The fatigue life may be calculated based on the S-N fatigueapproach under the assumption of linear cumulative damage Hot spot stress is understood to be the geometric stress created(Palmgren-Miner rule). by the considered detail. (The notch stress due to the local weld geometry is excluded from the stress calculation as it isWhen the long-term stress range distribution is expressed by a assumed to be accounted for in the corresponding hot spot S-Nstress histogram, consisting of a convenient number of con- curve. The notch stress is defined as the total stress resultingstant stress range blocks Δσi each with a number of stress rep- from the geometry of the detail and the non-linear stress fieldetitions ni the fatigue criterion reads: due to the notch at the weld toe). Derivation of stresses to be used together with the different S- ni 1 k m N curves are described in more detail in the following section. = ∑ ni ⋅ (Δσ i ) ≤ η k D=∑ (2.2.1) i =1 N i a i =1 The procedure for the fatigue analysis is based on the assump- tion that it is only necessary to consider the ranges of cyclicwhere stresses in determining the fatigue endurance (i. e. mean stresses are neglected for fatigue assessment of welded con-D = accumulated fatigue damage nections).a = intercept of the design S-N curve with the log N axis 2.3.2 Plated structures using nominal stress S-N curvesm = negative inverse slope of the S-N curve The joint classification and corresponding S-N curves takesk = number of stress blocks into account the local stress concentrations created by theni = number of stress cycles in stress block i joints themselves and by the weld profile. The design stressNi = number of cycles to failure at constant stress range Δσi can therefore be regarded as the nominal stress, adjacent to the weld under consideration. However, if the joint is situated in aη = usage factor region of stress concentration resulting from the gross shape of = 1 / Design Fatigue Factor from OS-C101 Section 6 the structure, this must be taken into account. As an example, Fatigue Limit States. for the weld shown in Figure 2-2 a), the relevant local stress for fatigue design would be the tensile stress, σnominal. For theApplying a histogram to express the stress distribution, the weld shown in Figure 2-2 b), the stress concentration factor fornumber of stress blocks, k, should be large enough to ensure the global geometry must in addition be accounted for, givingreasonable numerical accuracy, and should not be less than 20. the relevant local stress equal to SCF σnominal, where SCF isDue consideration should be given to selection of integration the stress concentration factor due to the hole. Thus the local DET NORSKE VERITAS
- 11. Recommended Practice DNV-RP-C203, April 2008 Page 11stress is derived as stress and hot spot stress may be defined as σ local = SCF σ nominal (2.3.1) σ hot spot = SCF σ nominal (2.3.2)σlocal shall be used together with the relevant S-N curves D where SCF is structural stress concentration factor normallythrough G, dependent on joint classification. denoted as stress concentration factor.The maximum principal stress is considered to be a significant The effect of stress direction relative to the weld toe as shownparameter for analysis of fatigue crack growth. When the prin- in Figures 2-3 and 2-4 when using finite element analysis andcipal stress direction is different from that of the normal to theweld toe, it becomes conservative to use the principle stress hot spot stress S-N curve is presented in section 4.3.4.range together with a classification of the connection for stressrange normal to the weld toe as shown in Figure 2-3. As the 2.3.4 Tubular jointsangle between the principal stress direction and the normal to For a tubular joint, i. e. brace to chord connection, the stress tothe weld, ϕ, is increased further, fatigue cracking may no be used for design purpose is the range of idealised hot spotlonger initiate along the weld toe, but may initiate in the weld stress defined by: the greatest value of the extrapolation of theand grow normal to the principal stress direction as shown in maximum principal stress distribution immediately outside theFigure 2-4. This means that the notch at the weld toe does no region effected by the geometry of the weld. The hot spot stresslonger significantly influence the fatigue capacity and a higherS-N curve applies for this stress direction. to be used in combination with the T-curve is calculated asMore guidance on this for use of nominal S-N curves is pre-sented in commentary D.4 Comm. 2.3.2 Plated structures σ hot spot = SCF σ nominal (2.3.3)using nominal stress S-N curves.2.3.3 Plated structures using hot spot stress S-N curves whereFor detailed finite element analysis of welded plate connec-tions other than tubular joints it may also be convenient to use SCF = stress concentration factor as given in section 3.3.the alternative hot spot stress for fatigue life assessment, seesection 4.3 for further guidance. A relation between nominalFigure 2-2Explanation of local stresses DET NORSKE VERITAS
- 12. Recommended Practice DNV-RP-C203, April 2008Page 12 2.3.6 Fillet welded bearing supports ϕ Where support plating below bearings are designed with fillet Δσ ⊥ Principal stress welded connection, it should be verified that fatigue cracking Δτ // direction Δσ // of the weld will not occur. Even though the joint may be Weld required to carry wholly compressive stresses and the plate toe surfaces may be machined to fit, the total stress fluctuation Fatigue crack should be considered to be transmitted through the welds for fatigue assessment. If it is assumed that compressive loading is transferred through contact, it should be verified that the contact will not be lost during the welding. The actual installation condition including maximum construction tolerances should be accounted for. 2.4 S-N curves Section 2.4.1 GeneralFigure 2-3Fatigue cracking along weld toe The fatigue design is based on use of S-N curves, which are obtained from fatigue tests. The design S-N curves which fol- lows are based on the mean-minus-two-standard-deviation curves for relevant experimental data. The S-N curves are thus associated with a 97.6% probability of survival. Δσ ⊥ 2.4.2 Failure criterion inherent the S-N curves Δτ // Δσ // Principal stress Most of the S-N data are derived by fatigue testing of small ϕ direction Weld specimens in test laboratories. For simple test specimens the toe testing is performed until the specimens have failed. In these specimens there is no possibility for redistribution of stresses during crack growth. This means that most of the fatigue life is associated with growth of a small crack that grows faster as the Fatigue crack crack size increases until fracture. For details with the same calculated damage, the initiation period of a fatigue crack takes longer time for a notch in base material than at a weld toe or weld root. This also means that with a higher fatigue resistance of the base material as com- Section pared with welded details, the crack growth will be faster inFigure 2-4 base material when fatigue cracks are growing.Fatigue cracking when principal stress direction is more parallelwith weld toe For practical purpose one defines these failures as being crack growth through the thickness. When this failure criterion is transferred into a crack size in a2.3.5 Fillet welds real structure where some redistribution of stress is moreThe relevant stress range for potential cracks in the weld throat likely, this means that this failure criterion corresponds to aof load-carrying fillet-welded joints and partial penetration crack size that is somewhat less than the plate thickness.welded joints may be calculated as: The tests with tubular joints are normally of a larger size. These joints also show larger possibility for redistribution ofΔσ w = Δσ ⊥ + Δτ ⊥ + 0.2 Δτ 2 2 2 // (2.3.4) stresses as a crack is growing. Thus a crack can grow through the thickness and also along a part of the joint before a fracturewhere the stress components are explained in Figure 2-5. occur during the testing. The number of cycles at a crack size through the thickness is used when the S-N curves are derived.The total stress fluctuation (i.e. maximum compression and As these tests are not very different from that of the actualmaximum tension) should be considered to be transmitted behaviour in a structure, this failure criterion for S-N curvesthrough the welds for fatigue assessments. for tubular corresponds approximately to the thickness at the hot spot (chord or brace as relevant). 2.4.3 S-N curves and joint classification For practical fatigue design, welded joints are divided into sev- eral classes, each with a corresponding design S-N curve. All tubular joints are assumed to be class T. Other types of joint, including tube to plate, may fall in one of the 14 classes spec- τ ified in Table 2-1, Table 2-2 and Table 2-3, depending upon: σ τ — the geometrical arrangement of the detail — the direction of the fluctuating stress relative to the detail — the method of fabrication and inspection of the detail. Throat section Each construction detail at which fatigue cracks may poten- tially develop should, where possible, be placed in its relevant joint class in accordance with criteria given in Appendix A. It should be noted that, in any welded joint, there are severalFigure 2-5 locations at which fatigue cracks may develop, e. g. at the weldExplanation of stresses on the throat section of a fillet weld toe in each of the parts joined, at the weld ends, and in the weld DET NORSKE VERITAS
- 13. Recommended Practice DNV-RP-C203, April 2008 Page 13itself. Each location should be classified separately. t = thickness through which a crack will most likely grow.The basic design S-N curve is given as t = tref is used for thickness less than tref k = thickness exponent on fatigue strength as given in log N = log a − m log Δσ (2.4.1) Table 2-1, Table 2-2 and Table 2-3. k = 0.10 for tubular butt welds made from one sideN = predicted number of cycles to failure for stress k = 0.25 for threaded bolts subjected to stress variation in range Δσ the axial direction.Δσ = stress rangem = negative inverse slope of S-N curve In general the thickness exponent is included in the design equation to account for a situation that the actual size of theloga = intercept of log N-axis by S-N curve structural component considered is different in geometry from that the S-N data are based on. The thickness exponent is con-loga = log a − 2 s (2.4.2) sidered to account for different size of plate through which a crack will most likely grow. To some extent it also accounts forwhere size of weld and attachment. However, it does not account for weld length or length of component different from that testeda = constant relating to mean S-N curve such as e. g. design of mooring systems with a significant larger number of chain links in the actual mooring line thans = standard deviation of log N. what the test data are based on. Then the size effect should beThe fatigue strength of welded joints is to some extent depend- carefully considered using probabilistic theory to achieve aent on plate thickness. This effect is due to the local geometry reliable design, see Appendix D, Commentary.of the weld toe in relation to thickness of the adjoining plates. 2.4.4 S-N curves in airSee also effect of profiling on thickness effect in section 7.2. Itis also dependent on the stress gradient over the thickness. Ref- S-N curves for air environment are given in Table 2-1 and Fig-erence is made to Appendix D, Commentary. The thickness ure 2-6. The T curve is shown in Figure 2-8. In the low cycleeffect is accounted for by a modification on stress such that the region the maximum stress range is that of the B1 curve asdesign S-N curve for thickness larger than the reference thick- shown in Figure 2-6. However, for offshore structures sub-ness reads: jected to typical wave and wind loading the main contribution to fatigue damage is in the region N > 106 cycles and the bilin- ear S-N curves defined in Table 2-1 can be used. ⎛ ⎛ t ⎞ k ⎞log N = log a − m log⎜ Δσ ⎜ ⎟ ⎟ (2.4.3) ⎜ ⎜ t ref ⎟ ⎟ ⎝ ⎝ ⎠ ⎠wherem = negative inverse slope of the S - N curvelog a = intercept of log N axistref = reference thickness equal 25 mm for welded connec- tions other than tubular joints. For tubular joints the reference thickness is 32 mm. For bolts tref = 25 mmTable 2-1 S-N curves in air S-N curve N ≤ 10 7 cycles N > 10 7 cycles Fatigue limit at 10 7 Thickness exponent k Structural stress log a 2 cycles *) concentration embedded in m1 the detail (S-N class), log a1 m2 = 5.0 ref. also equation (2.3.2) B1 4.0 15.117 17.146 106.97 0 B2 4.0 14.885 16.856 93.59 0 C 3.0 12.592 16.320 73.10 0.15 C1 3.0 12.449 16.081 65.50 0.15 C2 3.0 12.301 15.835 58.48 0.15 D 3.0 12.164 15.606 52.63 0.20 1.00 E 3.0 12.010 15.350 46.78 0.20 1.13 F 3.0 11.855 15.091 41.52 0.25 1.27 F1 3.0 11.699 14.832 36.84 0.25 1.43 F3 3.0 11.546 14.576 32.75 0.25 1.61 G 3.0 11.398 14.330 29.24 0.25 1.80 W1 3.0 11.261 14.101 26.32 0.25 2.00 W2 3.0 11.107 13.845 23.39 0.25 2.25 W3 3.0 10.970 13.617 21.05 0.25 2.50 T 3.0 12.164 15.606 52.63 0.25 for SCF ≤ 10.0 1.00 0.30 for SCF >10.0*) see also section 2.10 DET NORSKE VERITAS
- 14. Recommended Practice DNV-RP-C203, April 2008Page 14 1000 B1 B2 Stress range (MPa) C C1 C2 100 D E F F1 F3 G W1 W2 W3 10 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 Number of cyclesFigure 2-6S-N curves in air2.4.5 S-N curves in seawater with cathodic protectionS-N curves for seawater environment with cathodic protectionare given in Table 2-2 and Figure 2-7. The T curve is shown inFigure 2-8. For shape of S-N curves see also comment in 2.4.4.Table 2-2 S-N curves in seawater with cathodic protection S-N curve N ≤ 10 6 cycles N > 10 6 cycles Fatigue limit at 10 7 Thickness exponent k Stress concentration in the S- loga2 cycles*) N detail as derived by the hot m1 spot method loga1 m2= 5.0 B1 4.0 14.917 17.146 106.97 0 B2 4.0 14.685 16.856 93.59 0 C 3.0 12.192 16.320 73.10 0.15 C1 3.0 12.049 16.081 65.50 0.15 C2 3.0 11.901 15.835 58.48 0.15 D 3.0 11.764 15.606 52.63 0.20 1.00 E 3.0 11.610 15.350 46.78 0.20 1.13 F 3.0 11.455 15.091 41.52 0.25 1.27 F1 3.0 11.299 14.832 36.84 0.25 1.43 F3 3.0 11.146 14.576 32.75 0.25 1.61 G 3.0 10.998 14.330 29.24 0.25 1.80 W1 3.0 10.861 14.101 26.32 0.25 2.00 W2 3.0 10.707 13.845 23.39 0.25 2.25 W3 3.0 10.570 13.617 21.05 0.25 2.50 T 3.0 11.764 15.606 52.63 0.25 for SCF ≤ 10.0 1.00 0.30 for SCF >10.0*) see also 2.10 DET NORSKE VERITAS
- 15. Recommended Practice DNV-RP-C203, April 2008 Page 15 1000 B1 Stress range (MPa) B2 C 100 C1 C2 D E F F1 F3 G W1 W2 W3 10 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 Number of cyclesFigure 2-7S-N curves in seawater with cathodic protection2.4.6 S-N curves for tubular jointsS-N curves for tubular joints in air environment and in seawa-ter with cathodic protection are given in Table 2-1, Table 2-2and Table 2-3. 1000 In air 100 Seawater with Stress range (MPa) cathodic protection 10 1 1,00E+04 1,00E+05 1,00E+06 1,00E+07 1,00E+08 1,00E+09 Number of cyclesFigure 2-8S-N curves for tubular joints in air and in seawater with cathodic protection DET NORSKE VERITAS
- 16. Recommended Practice DNV-RP-C203, April 2008Page 162.4.7 S-N curves for cast nodes For high strength steel with yield strength above 500 MPa andIt is recommended to use the C curve for cast nodes. Tests may a surface roughness equal Ra = 3.2 or better the followinggive a more optimistic curve. However, the C curve is recom- design S-N curve can be used for fatigue assessment of themended in order to allow for weld repairs after possible casting base materialdefects and possible fatigue cracks after some service life. Theprobability of a repair during service life depends on accumu- Log N = 17.446 − 4.70 LogS (2.4.5)lated fatigue damage. Reference is made to section 9.1 and Fig-ure 9-3 which indicates fatigue failure probability as function In air a fatigue limit at 2·106 cycles at a stress range equal 235of Design Fatigue Factor. MPa can be used. For variable amplitude loading with one stress range larger than this fatigue limit a constant slope S-NFor cast nodes a reference thickness tref = 38 mm may be used curve should be used. Reference is also made to section 2.10.provided that any possible repair welds have been ground to asmooth surface. (The mean S-N curve is given by Log N = 17.770 – 4.70 LogS).For cast nodes with a stress gradient over the thickness a For seawater with cathodic protection a constant slope S-Nreduced effective thickness may be used for assessment of curve should be used. (The same as for air to the left of 2·106thickness effect. The effective thickness to be used in equation cycles, see Figure 2-9). If requirements to yield strength, sur-(2.4.3) can be calculated as: face finish and corrosion protection are not met the S-N curves presented in sections 2.4.4, 2.4.5 and 2.4.9 should be used. The 1/ k thickness exponent k = 0 for this S-N curve. ⎛S ⎞t e = t actual ⎜ i ⎜S ⎟ ⎟ (2.4.4) 1000 ⎝ 0 ⎠Where AirS0 = hot spot stress on surface Stress range (MPa)Si = stress 38 mm below the surface, under the hot spottactual = thickness of cast piece at considered hot spot meas- Seawater with ured normal to the surface 100 cathodic protectionte = effective thickness. te shall not be less than 38 mm.k = thickness exponent = 0.152.4.8 S-N curves for forged nodesFor forged nodes the B1 curve may be used for nodes designedwith a Design Fatigue Factor equal to 10. For designs with DFFless than 10 it is recommended to use the C-curve to allow for 10weld repair if fatigue cracks should occur during service life. 10000 100000 1000000 10000000 100000000 Number of cycles2.4.9 S-N curves for free corrosionS-N curves for free corrosion, i.e. without corrosion protec- Figure 2-9tion, are given in Table 2-3. S-N curve for high strength steel (HS – curve)See also Commentary section for consideration of corrosionprotection of connections in the splash zone and inside tanks in 2.4.11 S-N curves for stainless steelFPSOs. For Duplex and for Super Duplex steel one may use the sameTable 2-3 S-N curves in seawater for free corrosion classification as for C-Mn steels. S-N curve log a Thickness exponent k Also for austenitic steel one may use the same classification as For all cycles m = 3.0 for C-Mn steels. B1 12.436 0 2.4.12 S-N curves for small diameter umbilicals B2 12.262 0 C 12.115 0.15 For fatigue design of small diameter pipe umbilicals (outer diameter in the range 10 -100 mm) made of super duplex steel C1 11.972 0.15 with a yield strength larger than 500 MPa with thicknesses in C2 11.824 0.15 the range 1.0 to 10 mm the following S-N curve can be used D 11.687 0.20 for fatigue assessment E 11.533 0.20 F 11.378 0.25 For N ≤ 10 7 : F1 11.222 0.25 ⎧ 0.25 ⎫ ⎪ ⎛ t ⎞ ⎪ F3 11.068 0.25 Log N = 14.100 − 3.5 * Log ⎨S ⎜ ⎟ ⎬ ⎜t ⎟ G 10.921 0.25 ⎪ ⎩ ⎝ ref ⎠ ⎪ ⎭ W1 10.784 0.25 (2.4.6) and for N > 10 7 W2 10.630 0.25 ⎧ ⎛ t ⎞ 0.25 ⎫ W3 10.493 0.25 ⎪ ⎜ ⎟ ⎪ Log N = 17.143 − 5.0 * Log ⎨S ⎬ T 11.687 0.25 for SCF ≤ 10.0 ⎜t ⎟ ⎪ ⎩ ⎝ ref ⎠ ⎪ ⎭ 0.30 for SCF >10.0 where2.4.10 S-N curves for base material of high strength steelThe fatigue capacity of the base material is depending on the t = actual thickness of the umbilicalsurface finish of the material and the yield strength. tref = 1.0 mm DET NORSKE VERITAS
- 17. Recommended Practice DNV-RP-C203, April 2008 Page 17A normal good fabrication of the umbilicals is assumed as It is recommended to perform fatigue testing of at least 15basis for this design S-N curve. The welds on the inside and specimens in order to establish a new S-N curve. At least threeoutside of the pipes should show a smooth transition from the different stress ranges should be selected in the relevant S-Nweld to the base material without notches and/or undercuts. A region such that a representative slope of the S-N curve can bedetailed NDE inspection for each connection is assumed. determined.The NDE methods are visual inspection and X-ray. For single Reference is made to IIW document no IIW-XIII-WG1-114-pass welds, no indications are acceptable. For multipass welds 03 for statistical analysis of the fatigue test data. Normallythe acceptance criteria shall be according to ASME B31.3, fatigue test data are derived for number of cycles less than 107.chapter IX high pressure service girth groove. Dye penetrant It should be noted that for offshore structures significantshall be used as a surface test in addition to visual inspection fatigue damage occurs for N ≥ 107 cycles. Thus how to extrap-when relevant indications, as defined by ASME VIII div. 1, olate the fatigue test data into this high cycle region is impor-app.4. are found by X-ray. tant in order to achieve a reliable assessment procedure. InThe S-N curve is based on fatigue testing of specimens sub- addition to statistical analysis one should use engineeringjected to a mean stress up to 450 MPa. judgement based on experience for derivation of the S-N data in this region. It is well known that good details where fatigueThe given S-N curve is established from test specimens that are initiation contribute significantly to the fatigue life show anot prestrained from reeling. However, based on a few test data more horizontal S-N curve than for less good details where thewith prestrained specimens it is considered acceptable to use fatigue life consists mainly of crack growth. Reference is alsothe S-N curve also for umbilicals that have been reeled. Thus made to S-N curves with different slopes shown in this chapter.this S-N curve applies also when number of cycles under reel-ing is less than 10 and strain range during reeling is less than It should also be remembered that for N ≥ 107 cycles there is2%. additional uncertainty due to variable amplitude loading. This is an issue that should be kept in mind if less conservative S-N 1000 curves than given in this RP are aimed for by qualifying a new S-N curve. Also the probability of detecting defects during a production should be kept in mind in this respect. The defects that nor- mally can be detected by an acceptable probability are nor- Stres s range (M P mally larger than that inherent in the test specimens that are produced to establish test data for a new S-N curve. 100 2.5 Mean stress influence for non welded structures For fatigue analysis of regions in the base material not signifi- cantly affected by residual stresses due to welding, the stress range may be reduced if part of the stress cycle is in compres- sion. 10 This reduction may e.g. be carried out for cut-outs in the base 10000 100000 1000000 10000000 100000000 1000000000 material. The calculated stress range obtained may be multi- Num b e r o f cycle s plied by the reduction factor fm as obtained from Figure 2-11 before entering the S-N curve.Figure 2-10 The reduction factor can be derived from the following equa-S-N curves for small diameter pipe for umbilicals tion2.4.13 Qualification of new S-N curves based on fatigue σ t + 0.6σ c fm = (2.5.1)test data σt + σcFor qualification of new S-N data to be used in a project it is whereimportant that the test specimens are representative for theactual fabrication and construction. This includes possibilityfor relevant production defects as well as fabrication toler- σt = maximum tension stressances. The sensitivity to defects may also be assessed by frac- σc = maximum compression stressture mechanics.Figure 2-11Stress range reduction factor to be used with the S-N curve for base material DET NORSKE VERITAS

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