Machine Foundation Design


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Machine Foundation Design

  1. 1. MACHINE FOUNDATIONS IN OIL AND GAS INDUSTRYChennai Office By Varanasi Rama Rao B.E, M.S.(I.I.Sc.)
  2. 2. CONTENTS1. INTRODUCTION2. INPUT3. ANALYSIS&DESIGN4. REFERENCESEngineering Services – Chennai office January 2011
  3. 3. 1. INTRODUCTION The analysis and design of structure or foundation subjected to vibratory loading is a complex problem as it involves interaction of three domains viz. Structural Engineering, Geotechnical Engineering and Structural dynamics. The machines used in Oil and Gas industry are usually supported on a concrete block or a concrete frame and occasionally on steel frames. In majority of the cases the machines are supported on a simple concrete block.Engineering Services – Chennai office January 2011
  4. 4. Types of Machines in Oil and Gas Industry 1.Centrifugal Machines ( Rotating) Fig-1 2. Reciprocating Machines Fig-2Engineering Services – Chennai office January 2011
  5. 5. Centrifugal Machine ( Rotating) Fig-3Engineering Services – Chennai office January 2011
  6. 6. Reciprocating Machine Fig-4Engineering Services – Chennai office January 2011
  7. 7. Classification of rotating machines based onFrequency/speed •Very low speed machines < 100 rpm •Low speed machines 100 rpm to 500 rpm •Medium Speed machines 500 rpm to 1500 rpm •Moderately high speed machines 1500 rpm to 3000 rpm •High speed machines >3000 rpmEngineering Services – Chennai office January 2011
  8. 8. Various Types of Machine Foundations MACHINE CONCRETE CONCRETE/ST BLOCK EEL FRAME CONCRETE BASE RAFT SOIL SOIL 1.Block (Concrete) 2. Frame ( Concrete or Steel) Type Type Fig-5Engineering Services – Chennai office January 2011
  9. 9. MACHINE CONCRETE CONCRETE/ST BLOCK EEL FRAME SOIL SOIL 1.Block (Concrete) 2. Frame ( Concrete or Steel) Supported by piles supported by Piles Fig-6Engineering Services – Chennai office January 2011
  10. 10. 2. INPUTS What inputs are needed for foundation design? The inputs are broadly categorized as  Project Design basis for Machine foundation Soil Parameters for foundation Design Machine vendor inputs The above are elaborated in the subsequent slidesEngineering Services – Chennai office January 2011
  11. 11. What inputs are available in Project Designbasis? The following are the major inputs to be extracted from the project design basis: •Criteria for Dynamic Analysis •Permissible Amplitudes of Displacements and rotation of the foundation in the absence of Vendor data. •Grade of Material ( Concrete/Steel ) to be used for the construction of Machine foundation •Permissible % of bearing pressures for dynamic loading.Engineering Services – Chennai office January 2011
  12. 12. What are the Soil Parameters required? Primary parameters 1. Dynamic Shear Modulus 2. Poisson’s Ratio 3. Damping factor Secondary Parameters 1. Coefficient of Elastic uniform compression Cz 2. Coefficient of Elastic uniform shear Cτ 3. Coefficient of Elastic non uniform compression Cθ 4. Coefficient of Elastic non uniform shear CψEngineering Services – Chennai office January 2011
  13. 13. How to determine the Soil Parameters? Field Tests : •Cross Hole Test ( CHT). •Down hole test (DHT). •Spectral Analysis of Shear wave ( SASW) •Block Vibration Tests Laboratory tests: •Resonant Column test •Cyclical Tri axial Test The above test are conducted usually by a Geotechnical Contractor and appropriate values are recommended by him in the Geotechnical reportEngineering Services – Chennai office January 2011
  14. 14. Cross hole test-schematic Fig-7Engineering Services – Chennai office January 2011
  15. 15. Down hole test- Schematic Fig-8Engineering Services – Chennai office January 2011
  16. 16. Spectral Analysis of Shear Wave - Schematic Fig-9Engineering Services – Chennai office January 2011
  17. 17. Block vibration Test Fig-10Engineering Services – Chennai office January 2011
  18. 18. Damping factor determination- Forced/FreeVibration test Fig-11Engineering Services – Chennai office January 2011
  19. 19. Machine Vendor Input The following input is required from the machine vendor: •Geometric configuration of the Machine •Loads from the Machine: Mass of the stationary as well as rotating parts of the machine and load transfer mechanism from machine to the foundation •Critical Machine performance parameters: Critical speeds of rotors, balance grades and acceptable levels of amplitudes of vibration •Dynamic forces generated by the Machine: forces generated under various operating conditions and their transfer mechanism to the foundation for dynamic response analysis. •Additional forces generated under emergency or faulted conditions, Test condition, Erection condition & Maintenance condition of the machine, forces due to bearing failure ( if applicable) for strength analysis of the foundation.Engineering Services – Chennai office January 2011
  20. 20. Typical Vendor Input ( Machine GAD): Fig-12Engineering Services – Chennai office January 2011
  21. 21. 3.ANALYSIS & DESIGN The Analysis of the Machine foundation is done in two stages: •Dynamic Analysis : Includes determination of the the natural frequencies of the Machine foundation system and calculation of amplitudes of displacements and rotations of the foundation under dynamic loading. •Static Analysis: Includes check for strength of the foundation, stability of the foundation and check for soil bearing capacity.Engineering Services – Chennai office January 2011
  22. 22. The process of evaluating the critical soil properties that influence soil structure interaction is probably the most difficult part of the machine foundation design- Dr. K.G.Bhatia The significant aspects of soil properties which influence soil-structure interactions are: •Energy transfer mechanism- Not quantifiable •Soil mass participation in vibration of the foundation- Not quantifiable •Effect of embedment of foundation- Approximately quantifiable •Applicability of Hooke’s law to soil- To some extent •Dynamic soil parameters-Approximately quantifiableEngineering Services – Chennai office January 2011
  23. 23. Deformation modes of soil Based on above deformable modes of foundation, the following deformable modes can be anticipated for Soil beneath the block foundation: • Uniform Compression •Uniform Shear •Non Uniform compression •Non Uniform shearEngineering Services – Chennai office January 2011
  24. 24. In the context of Machine foundation design, a Machine would necessarily include: •A drive machine •A driven machine •A coupling device Schematically: coupling Driven Drive machine machine Fig-13Engineering Services – Chennai office January 2011
  25. 25. A typical data set required for each of the components shown in the previous schematic is: For dynamic response analysis of foundation: •Total mass of machine ( including rotating parts), Radius of gyration and its over all centroid location. •Mass of rotating parts of the machine, operating speed, height of the centre of the rotor from machine base frame, etc •Foot print of machine base frame, details of holding down bolts •Dynamic forces generated by the machine under operating conditions For strength design of foundation: • Static loads from machine •Equivalent static forces i.e. dynamic forces converted to equivalent static forces •Forces generated under emergency and faulted conditions eg: bearing failure, loss of blade, short circuit etc. •Forces during erection, maintenance and test conditions of the machine.Engineering Services – Chennai office January 2011
  26. 26. Parameters for rotary ( centrifugal) machines: Balancing of rotating machine Centre of Mass Centre of Rotation Balanced condition Fig-14Engineering Services – Chennai office January 2011
  27. 27. Centre of Mass ‘m’ e Centre of Rotation Un- Balanced condition Force Generated due to unbalanced condition F= meω2 The above force is called unbalanced force Fig-15Engineering Services – Chennai office January 2011
  28. 28. Unbalanced forces along the shaft with multiple supports Fig-16Engineering Services – Chennai office January 2011
  29. 29. Picture of RotorEngineering Services – Chennai office Fig-17 January 2011
  30. 30. Picture of RotorEngineering Services – Chennai office January 2011 Fig-18
  31. 31. In every rotating machine there will be certain amount of unbalance( eccentricity) which is inevitable. ISO/ Machine manufacturer has set standards for the allowable eccentricity based on: •Function of the machine •Speed of the machine and •Rotating mass Many rotating machines are balanced to an initial balance quality as per ISO standards. This is called the balance quality grade. Fig-19Engineering Services – Chennai office January 2011
  32. 32. Critical speeds: Correspond to flexural frequencies of the rotor. These are supplied by the vendor. High vibration can occur on account of resonance of foundation with critical speeds Forces due to Emergency conditions: Bearing Failure: Grinding halt of machine due to failure of bearings. Difficult to quantify and can be taken as an static force equivalent to 3 to 5 times the rotor weight. Short circuit force: Furnished by Machine vendor Loss of parts like blade: Furnished by Machine vendor Fig-20Engineering Services – Chennai office January 2011
  33. 33. Parameters for reciprocating machines: Typical arrangement for single cylinder reciprocating system Fig-21Engineering Services – Chennai office January 2011
  34. 34. Dynamic forces transferred at C.G of machine at base frame level Fig-22Engineering Services – Chennai office January 2011
  35. 35. Foundation Parameters Under tuned foundation: The vertical vibration frequency is < operating frequency of the machine Preferred for Medium to High speed Machines Over tuned foundation: The vertical vibration frequency is > Operating frequency of the machine Preferred for very low to low speed machinesEngineering Services – Chennai office January 2011
  36. 36. Vibration Limits in Machine Foundation design Rotary type machines Machine Operating speed Permissible amplitude in in rpm Microns 100 to 500 200 to 80 500 to 1500 80 to 40 1500 to 3000 40 to 20 3000 to 10000 20 to 5 Reciprocating type machines Machine Operating speed Permissible amplitude in in rpm Microns 300 to 1500 1000 to 20 100 to 300 1000 The above are only approximate values. Actual permissible should be given by the Machine Vendor/ManufacturerEngineering Services – Chennai office January 2011
  37. 37. Foundation Sizing ( Block foundation): Foundation should be dimensioned in such a way that the derived eccentricity, in both lateral and longitudinal directions is bare minimum. In no case it should not exceed 5% of the base dimension in the respective direction Foundation should extend by at least 150 mm on all sides of machine base frame The pressure developed in the soil loads due to static loads should not exceed 75% of the allowable safe bearing capacity. Though from strength point of view it may appear adequate to keep foundation mass slightly above the machine mass, a higher mass ratio helps to keep the eccentricity of loading within limits. For rotary machines: foundation mass = 2.5 to 3 times of machine mass For reciprocating machines: foundation mass= 5 to 8 times of machine massEngineering Services – Chennai office January 2011
  38. 38. Foundation Stiffness: Foundation parameters that govern the dynamic response are its mass and its area of contact with the soil. In specific cases projected parts of foundation having finite stiffness also influence the dynamic response. The rigidity of foundation is much higher compared to that of soil supporting it. In the case of block foundation the rigidity is so high that under the influence of static and dynamic forces the deformations of the block are negligible compared to soil. The block foundation is therefore considered as rigid body consisting of mass only. In case the foundation of machine is not a solid block but a frame or some other kind of structure which has stiffness comparable to that of soil, then elements of those structures shall be considered to have both mass and stiffness.Engineering Services – Chennai office January 2011
  39. 39. Strength Design: Since the block foundation behaves like a rigid body supported on a soft media like soil, invariably the block foundations would turn out to be having adequate strength vis-à-vis forces imparted by machine. Strength design is done considering the forces and moments on the foundation due to static loads, dynamic loads, emergency loads and applicable earthquake/wind loads. Anchor bolts: All anchor bolts should be checked for pullout force caused due to Dynamic and Emergency loads Stability Checks: The foundation shall be checked for sliding and overturning . But these checks are not compulsory. Minimum Reinforcement : •25 to 50 kg/m3 •Minimum dia. of the bar 12 mm •Two way reinforcement on all the faces and shrinkage reinforcement ( when thickness of block exceeds 1m)Engineering Services – Chennai office January 2011
  41. 41. Possible Movement for Block foundation: Z Y θ φ X ψ Fig-23Engineering Services – Chennai office January 2011
  42. 42. Single Degree Freedom Mathematical Model Mass of machine and foundation Soil Damping Stiffness of the soil Fig-24Engineering Services – Chennai office January 2011
  43. 43. Lumped Parameter System Z mz   cz z  k z z  P0 sin( t ) z  k n  Cz Kz m Kx Iψ m X ψ Cx Kψ D  c ccr ccr  2 k m Cψ/2Engineering Services – Chennai office Cψ/2 January 2011 Fig-25
  44. 44. Lumped Parameter Values Mode Vertical Horizontal Rocking Torsion 4Gr 8Gr 8Gr 3 16Gr 3Stiffness k 3(1   ) 1  2  3Mass Ratio m(1   ) m(2   ) 3I (1   ) I m mˆ 4 r 3 8r 3 8 r 5 r 5 Damping 0.425 0.288 0.15 0.50 Ratio, D ˆ m1/ 2 ˆ m1/ 2 (1  m)m1/ 2 ˆ ˆ 1  2mˆ Fictitious 0.27m 0.095m 0.24I x 0.24I z Mass mˆ mˆ mˆ mˆ D=c/ccr G=Shear Modulus ν=Poissons Ratio r=Radius ρ=Mass Density Iψ,Iθ=Mass Moment of Inertia
  45. 45. Closed form solution for free and forced vibration of foundation block subjected to dynamic loads: Fig-26Engineering Services – Chennai office January 2011
  46. 46. FE Models of Block foundation Fig-27Engineering Services – Chennai office January 2011
  47. 47. Model with Soil and Block foundation Fig-28Engineering Services – Chennai office January 2011
  48. 48. Foundation Sizing ( Framed foundation): Foundation GA and loading diagram is provided by the Machine vendor/Manufacturer. Typical Steel Frame foundation for Turbo Generator Fig-29Engineering Services – Chennai office January 2011
  49. 49. Eccentricity: For Framed foundation there are two connotations to the term eccentricity: 1. Overall eccentricity: it is defined as the distance between the centre of mass of the over all system ( machine+ foundation) and C.G of the base contact area of the foundation with soil. This should be restricted to 5% 2. Top deck eccentricity: It is defined as the distance between the centre of the Mass Cm ( combined C.G of machine mass, top deck mass and 23% percent of column mass) and centre of stiffness of frames Ck in the transverse and longitudinal directions. It is desirable to restrict this eccentricity to 1% of the respective dimension of the top deck. Top deck sizing: Top deck comprises of transverse and longitudinal beams, slab connecting these beams, projections on all sides of the beams, depressions, cutouts, notches etc. a. The top deck weight > weight of the machine b. For beams: Span/Depth = 3 to 5; Depth/width = 1 to 1.5 c. Extent of cantilever projections in plan should not be more than half the width of the corresponding beam d. Depth of slab should be invariably same as that of the encompassing beams except at areas where the recess or depressions are provided to accommodate machineEngineering Services – Chennai office January 2011
  50. 50. Columns: Total weight of the columns should be close to the weight of the machinery. This is desirable but not essential condition But the following should be kept in mind while assessing the sizes of columns •Centre of stiffness of all the frames should coincide with centre of mass of machine and top deck. •Lateral natural frequencies of each of the column ( along transverse as well as longitudinal directions) considering fixed at both ends should not coincide with Machinery frequency or its harmonics. Base raft: • Raft plan dimensions are selected such that the bearing pressure generated is less than 70% of allowable bearing pressure. • Base raft thickness should be such that it acts like a rigid block and undergoes uniform deformation. •General guide line is weight of the base raft should be about twice the weight of the machineEngineering Services – Chennai office January 2011
  51. 51. Stiffness of frame foundation: Unlike block members of frame foundation have finite stiffness and are subjected to considerable elastic deformations. Framed foundation is considered as elastic body with both mass and stiffness. Strength design: For frame foundation the reinforcement is provided as dictated by the strength design of the structural members i.e. columns, beams and slabs. Minimum Reinforcement: • Reinforcement for top deck and columns to be in the range of 100 to 120 kg/m3 •Reinforcement for base raft shall be in the range of 70 to 80 kg/m3Engineering Services – Chennai office January 2011
  52. 52. Framed foundation under constructionEngineering Services – Chennai office Fig-30 January 2011
  53. 53. Dynamic Analysis ( Framed Foundations) Unlike block foundations, framed foundation has many modes of vibration. Closed form solutions for framed foundation: Before the advent of computers the framed foundations are analyzed manually. The manual procedure aims at analyzing the frames of the foundation independently for free and forced vibrations and algebraically summing up the response for all frames. ( Refer Handbook of Machine foundations by Srinivasulu and Vaidyanathan for Manual Analysis of framed foundation).Engineering Services – Chennai office January 2011
  54. 54. Modeling Methods for Framed foundation: There are two acceptable methods Method 1:the loads are applied to the model of the super structure to determine the foundation response. The computed reactions at the base of the columns are then used as input forces on the model of the mat Method2: in this method both the superstructure and foundation are incorporated into a single model. This procedure will yield results for the entire foundation in one analysis. For both the above methods the beams and columns are modeled using 3 dimensional beam elements, shear walls ( if any) and mat are modeled using plate-bending elements.Engineering Services – Chennai office January 2011
  55. 55. Models for Dynamic Analysis ( some major points to remember): The dynamic model should not be created independently of the static model but should be created from static model by incorporating the following: •Enough nodes should be specified along the length of beams and columns so as to capture frequencies of modes which match with machine frequency. •The nodal masses can be computed either by lumped mass approach or consistent mass approach •It is recommended that 2% of critical damping used for concrete elements and 1% for steel elements. •The dynamic model of foundation should also include the machine. The simplest method to model this is modeling the machine as series of mass points lumped with the foundation.Engineering Services – Chennai office January 2011
  56. 56. Loads for which Static Analysis is recommended: •Dead Load •Live load •Normal torque load •Condenser load •Thermal loading due to machine expansion/contraction •Piping loads Loads for which Pseudo-dynamic Analysis is recommended: •Normal machine unbalanced load for machines mounted on conventional foundations •Seismic loading Loads for which Pseudo-dynamic analysis is acceptable: •Short circuit load •Out of phase synchronization •Loading due to bowed rotor •Load due to missing rotor blade Loads for which dynamic analysis is recommended: •Normal machine unbalance load for machines mounted on low tuned foundation •Seismic loadingEngineering Services – Chennai office January 2011
  57. 57. Typical Free Vibration Response of FE model of Framed foundation Fig-31Engineering Services – Chennai office January 2011
  58. 58. Typical Response of foundation subjected to Dynamic loading Fig-32Engineering Services – Chennai office January 2011
  59. 59. Miscellaneous Topics 1. Soil- Structure Interaction effects: •The presence of supporting soil affects the static and dynamic response of the turbine foundation. These effects are termed as soil structure interaction effects. •Usually soil-structure interaction effects are considered negligible for framed foundation under operating conditions. However, they can be critical under non-periodic forces like earthquake, short circuit etc. •Soil structure interaction effects are more predominant in the case of block foundations.Engineering Services – Chennai office January 2011
  60. 60. 2. Machine foundations on Piles: Piles are specifically required for machine foundations in the following circumstances: • When soil is weak to withstand the loads •When it is required to increase the natural frequency of the machine foundation system. •When dynamic amplitudes are required to be reduced •When it is required to stiffen the support system on account of seismic considerations. Problems associated with Analysis of machine foundations supported on piles •Understanding of dynamic behavior of group of piles is still in its infancy. •As the reliability of dynamic characteristics of group of piles is faced withEngineering Services – Chennai office many questions, so shall be January 2011 of computed dynamic response the status
  61. 61. The following are observations by various researchers with regard to dynamic behavior of piles: •Dynamic stiffness of the pile is generally found to be greater than static stiffness •Both stiffness and damping of pile are found to be frequency dependent •Damping increases with pile length •Embedment of pile cap results in increased stiffness. However, its quantification is not yet established •Damping of group of piles is more frequency dependent •Dynamic group effect considerably differs from static group effect. •Rocking and torsional stiffness of the pile can be safely ignored.Engineering Services – Chennai office January 2011
  62. 62. 4.REFERENCES BOOKS: 1. Barkan D.D.” Dynamics of bases and Foundations”- Mc Grawhill 2. P.Srinivasulu and Vaidyanathan “Hand Book of Machine foundations”- Tata Mc Grawhill 3. “Foundations for Industrial Machines: Hand book for Practicing Engineers”- K.G.Bhatia- DCAD publishers 4. S.Prakash and V.Kpuri “ Foundation for machines- Analysis and Design”- John Wiley 5. Arya, O Neil and Pincus “ Design of Structure and Foundation for Vibrating Machines”- Gulf Publishing 6. Indrajit Chowdhury and P.Dasgupta “ Dynamic of Structure and Foundation” –CRC press 7. “Soil Dynamics and Machine foundations” – Swami Saran- Galgotia 8. “Soil Dynamics “ - Braja M. DasEngineering Services – Chennai office January 2011
  63. 63. CODES: Indian: IS2974: 5 parts of which parts 1,3 & 4 are for Reciprocating and Rotating machines IS 5249: Methods of test for determination of Dynamic soil properties British: CP 2012 German: DIN4024 ( Part 1): For framed type ( Flexible) foundations DIN 4024 ( Part 2): For block type ( Rigid) foundations American: ACI 351 Saudi: SAES-Q-007 ISO: 1S0 10816 ( 7 parts)Engineering Services – Chennai office January 2011
  64. 64. Engineering Services – Chennai office January 2011