Fatigue Analysis

Training Course on Offshore Structures14-24 July 2008at L & T Valdel Office, BangaloreDepartment of Ocean EngineeringIndian Institute of Technology Madras Offshore Structures –Fatigue Analysis FATIGUE ANALYSIS AND DESIGN21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-363Where to Check Fatigue ?Any Connections subjected to cyclic stresses. e.gTubular Connections (Brace-Chord Interface)Tubular transitions both in thickness and diameter (inline fatigue )Plated Connections (Numerous types of connection configurations in offshore structures)Offshore Structures –Fatigue Analysis What Methods are available ? Simple Cumulative Fatigue Damage MethodProven method, for many yearsSimple but not reflecting the actual scenrio Still lot of studies going onFracture Mechanics MethodRecently getting popular but still need lot of researchLack practical application as it involves estimate of initial crack width21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-365Fatigue ConceptFatigue can occur where there is aDiscontinuityChange in stress pathChange in thickness Change in materialApplied load is cyclicOffshore Structures –Fatigue Analysis Typical T Joint21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-367Location of Saddle and Crown Offshore Structures –Fatigue Analysis But Welded Connection21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-369Fabrication MisalignmentOffshore Structures –Fatigue Analysis Tubular Misalignment21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3611FATIGUE ANALYSIS STEPSFatigue analysis methodJacket ModelWave Climate (Scatter Data)Hydrodynamic CoefficientsStructural ResponseCyclic Stress CalculationsDynamic Amplification FactorSCF equationsS-N curveFatigue Damage CalculationsFactor of Safety Offshore Structures –Fatigue Analysis WAVE SCATTER DATAJOINT OCCURANCE DATADIRECTIONAL SCATTER DATA21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3613JOINT OCCURANCE DATA FOR WAVE HEIGHT AND PERIOD H maxOffshore Structures –Fatigue Analysis1521 July 2008Dr. S. NallayarasuDepartment of Ocean EngineeringIndian Institute of Technology Madras-36 Offshore Structures –Fatigue Analysis1721 July 2008Dr. S. NallayarasuDepartment of Ocean EngineeringIndian Institute of Technology Madras-36 Offshore Structures –Fatigue Analysis FATIGUE ANALYSIS METHODSSimplified FatigueAnalysisDetailed FatigueAnalysisDeterministic MethodSpectral Method(Stochastic)21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3619SIMPLIFIED FATIGUE ANALYSISUsed only for preliminary screening of joints for fatigue strength and will be used in concept stageMethod is based punching shear concept for tubular joints using allowable hot spot stresses compared with applied stresses Method is described as below.Static analysis using design wave as per API RP 2A clause 2.3.4-3Compute allowable hot spot stress using API RP 2A Figure C51-1 and 2Unity Check calculated and must be less than 1Offshore Structures –Fatigue Analysis DETERMINISTIC FATIGUE ANALYSISUsed for Detail design of jackets in water depth less than 400 ft and natural period less than 3 secondsMethod is based on Cumulative fatigue Damage concept Method is described as below.Establish sea-state scatter data and define applied cycles (n) for each wave (sea-state is described by discrete waves)Static analysis using design sea-state waves (DAF can be inlcuded)Compute nominal stresses at the brace chord intersectionsCompute SCF using efthymiou equationsCompute Hot Spot Stress Range (HSSR) = nominal stress range x SCFSelect Suitable S-N curve and find out the allowable number of cycles (N) for each wave Cumulative fatigue damage (D) is calculated by adding damage for all waves ∑=)/(N n D21 July 2008Dr. S. NallayarasuDepartment of Ocean Engineering Indian Institute of Technology Madras-3621Wave Scatter diagramDamage scatterdiagramS-N Curves “Centre of Damage”seastateLinearised foundation3-D structure computer model •wave loads•stiffness* -Dynamic analysisLong term cumulative distribution of waveheightsWave heights and periodsE N V I R O N M E N T A L A N DF A T IG U E D A T A & S T R U C T U R A L C O N F I G U R A T I O ND E T E R M I N I S T I C W A V E SContd….Number of occurrences of each deterministic waveOffshore Structures –Fatigue AnalysisNumber ofoccurrences of each stress range valueMiner’s ruleFigure livesS-N CurvesS T R E S S R A N G E S T A T I S T I C SF A T IG U E D A M A G ES T R E S S R A N G E SStructuralanalysisHot spot stress range valuesStressConcentration factorsF o r e a c h w a v e d i r e c t i o n a n dE a c h d e t e r m i n i s t i c w a v eF o r a l l l o c a t i o n s i n s t r u c t u r e21 July 2008Dr. S. NallayarasuDepartment of Ocean Engineering Indian Institute of Technology Madras-3623SPECTRAL FATIGUE ANALYSISUsed for Detail design of jackets in water depth less than400 ft and natural period more than 3 secondsMethod is based on Cumulative fatigue Damage concept Method is described as below.Establish sea-state scatter data and define applied cycles (n) for each wave (sea-state is described spectra, PM, ISSC, JONSWAP)Static / dynamic analysis using design sea-state waves Compute nominal stresses at the brace chord intersections Compute SCF using efthymiou equationsCompute Hot Spot Stress Range (HSSR) = nominal stress range x SCFSelect Suitable S-N curve and find out the allowable number of cycles (N) for each waveCumulative fatigue damage (D) is calculated by adding damage for all waves ∑=)/(N n D Offshore Structures –Fatigue AnalysisWave Scatter diagramDamage scatterdiagramS-N Curves “Centre of Damage”seastateLinearisedfoundation3-D structure computer model •wave loads * mass*•stiffness * damping** -Dynamic analysisWavefrequencies and wave heightsNatural frequencies*E N V I R O N M E N T A L A N DF A T IG U E D A T A & S T R U C T U R E C O N F I G U R A T I O NW A V E S F O R D E T E R M I N I N G T R A N S F E R F U N C T I O N SContd….21 July 2008Dr. S. NallayarasuDepartment of Ocean Engineering Indian Institute of Technology Madras-3625Offshore Structures –Fatigue Analysis Short term statisticsLong term cumulative distribution of hot spot stress rangesWave SpectrumMiner’s ruleFigure livesS-N CurvesS T R E S S R A N G E S T A T I S T I C SF A T IG U E D A M A G EHot spot stress range transfer functionsNominal member stressesStructural analysis(dynamic)*Stressconcentration factorsS T R E S S T R A N S F E R F U N C T I O NF o r a l l l o c a t i o n s i n s t r u c t u r eF o r e a c h w a v e d i r e c t i o nF o r e a c hF o r e a c h T R F f r e q u e n c yS e a s t a t e i n s c a t t e r d i a gOffshore Structures –Fatigue AnalysisPM SPECTRA452)(1exp[)(4)(−−=z z z sh fT fT T H f S ππ=)(f S h Single sided spectral density of waveamplitude )(2ncy unitfreque m Significant wave height (m)Zero up-crossing period (sec)Wave frequency=s H =z T =f Where21 July 2008Dr. S. NallayarasuDepartment of Ocean Engineering Indian Institute of Technology Madras-3627CENTRE OF FATIGUE DAMAGEzii bm sii T P HD =======i zi si i P T H b m D Damage of the ith sea-state Slope of the S-N curveSlope of log-linear wave height versus stress (assumed 1.80)Significant wave height of the ith sea-state Zero up-crossing period of the ith sea-state Probability of occurrence of the ith sea-stateWhereOffshore Structures –Fatigue AnalysisEQUIVALENT WAVE HEIGHT AND PERIODisii zisiiSD T D T D H D H ∑∑==21 July 2008Dr. S. NallayarasuDepartment of Ocean Engineering Indian Institute of Technology Madras-3629Hydrodynamic coefficientsStorm Waves (Large in relation to Size of members)Cd –0.65 (Clean), 1.05 (Rough) Cm –1.6 (Clean), 1.2 (Rough)Fatigue Waves (Small in relation to Size of members)Cd –0.5 (Clean), 0.8 (Rough) Cm –2.0 (Clean), 2.0 (Rough)Cd and Cm depends on the flow field and the structure size and related by Keulegan-Carpenter NumberDT U K m 22=Offshore Structures –Fatigue AnalysisWave DirectionsMinimum 8 Directions for 4 or 8 Leg jacketMinimum 12 Directions for Tripod21 July 2008Dr. S. NallayarasuDepartment of Ocean Engineering Indian Institute of Technology Madras-3631Wave DirectionsOffshore Structures –Fatigue AnalysisGeneration of Cyclic StressesVarious Methods available to generateSimplest will be to calculate static stress amplitude and multiply by a factor 2 (if we know that the stress variation is simple sinusoidal variation)Using wave loads options to generate stresses at crest and trough positions of the passing wave and the difference is the stress range21 July 2008Dr. S. NallayarasuDepartment of Ocean Engineering Indian Institute of Technology Madras-3633Generation of Cyclic StressesBut due to the complexity of the structure like jacket, the methods are not so simpleThe methods available in SACS can be usedWave load with MS and NS options, i.e. Maximum Base shear (MS) andMinimum base Shear (NS), the resulting difference in these two load cases will give the stress rangeOffshore Structures –Fatigue Analysis21 July 2008Dr. S. NallayarasuDepartment of Ocean Engineering Indian Institute of Technology Madras-3635Offshore structures are subjected to dynamic loadsfrom wave, wind and deck machinaryDynamic loads with specific cyclic period close to thenatural period may cause resonanceIn order to assess the vulnerability of the structureagainst dynamic loads, natural period and associated characteristics is requiredFor Simple systems such as cantilevers, simplysupported beams, fixed beams closed form solutions are available to calculate the natural period where as for large structures, FEM methods are usedOffshore Structures –Fatigue AnalysisWhy we need to do Dynamic AnalysisFixed template type platforms have natural periods less than 4 Seconds Compliant structures have natural period in excess of 10 seconds Fatigue wave periods ~ 3 to 12 secondsStorm wave periods ~ 6 to 16 seconds21 July 2008Dr. S. NallayarasuDepartment of Ocean Engineering Indian Institute of Technology Madras-3637Inappropriately configured jacket withnatural period greater than 3 seconds could cause large problem with regards to fatigue loading as DAF will be very highHence as a first step in configuring a jacket structure, we need to asses the natural period quicklyDo remedies by making the jacket baselarger, more foundation piles, less deck mass (if possible)Offshore Structures –Fatigue AnalysisStructural Response –Static Analysis}*{}]{[DAF F X K = If the natural period of the platform is considerably away from fatigue waves, assumption of equivalent static analysis is acceptableSimple calculations for DAF using SDOF model for each of the wave period can be calculated and applied to the wave loads Simple Static Analysis either with Pile Soil Interaction or equivalent linearised foundation can be used.21 July 2008Dr. S. NallayarasuDepartment of Ocean Engineering Indian Institute of Technology Madras-3639DYNAMIC AMPLIFICATION222)2()1(1TT T T DAF n N ς+−=DampingOffshore Structures –Fatigue AnalysisStructural Response –Wave Response analysisIf the natural period of the platform is close to the fatigue waves, assumption ofequivalent static analysis is not acceptableSimple calculations for DAF using SDOF model for will result in very conservative or non-conservative results depending on theassumptions made on average wave periods for the calculation of DAFHence a Dynamic Wave Response analysis needs to be performedDue to iterative calculations in Free Vibration analysis, equivalent linearised Foundation is required21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3641Structural Response –Wave Response analysis 0}"]{[}]{[=+X M X K The dynamic wave response analysis requires the dynamic characteristicsThe results of dynamic analysis will be used in Dynamic Wave Response analysis to generate structure response}{}"]{[}']{[}]{[F X M X C X K =++Free Vibration AnalysisWave Response AnalysisOffshore Structures –Fatigue Analysis STRESS TRANSFER FUNCTIONA transfer function defines the range of cyclic stresses to wave ratio over a range of wave period (or frequency))(*)()(f F f H f R =Including DynamicsForcing Function Final Response21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3643STRESS TRANSFER FUNCTIONA transfer function defines the range of cyclic stresses to wave ratio over a range of wave period (or frequency)WaveHeightStress Cyclic f H / )(=}{}"]{[}']{[}]{[F X M X C X K =++Offshore Structures –Fatigue Analysis STRESS TRANSFER FUNCTIONDetermination of H(f) involves response of the structure for various wave frequency for unit amplitude wavesSelection of frequencies for generation of H(f) is important so that the peaks and valley of the transfer function are included21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3645Selection of wave frequency for transfer function Offshore Structures –Fatigue Analysis WAVE HEIGHT FOR TRANSFER FUNCTIONWave height for each frequency selected shall be definedThis can be calculated by assuming certain wave steepness. Typically the wave steepness of 1/20 to 1/25 can be usedA minimum wave height of 0.3m and the maximum of design wave shall be achieved to cover all ranges21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3647CYCLIC STRESS RANGEOffshore Structures –Fatigue Analysis TYPICAL STRESS TRANSFER FUNCTION21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3649df f S f H f T df f S f H hi RMS z hi iRMS )()()()(22020∞∞∫=∫=σσOffshore Structures –Fatigue Analysis ds RMS s s N s RMS s n D i i )exp()()(2202σσ∞∫=z T mL s n =)(Design LifeFraction ofDesign Life21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3651Stress Concentration Factors (SCF)SCF = Hot Spot StressNominal Stress•SCF can be evaluated using physical models testing•SCF can be computed using FEM model studies•SCF using Lloyds register equation•SCF using Efthymiou Equations•SCF using Kuang et al, or Alpha-KelloggOffshore Structures –Fatigue Analysis FEM Model of T Joint21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3653LOCAL INCREASE IN STRESSES Offshore Structures –Fatigue Analysis TYPES OF JOINTSSIMPLE JOINTSGROUTED TUBULAR JOINTSINTERNAL RING STIFFENED JOINTS OVERLAPPING JOINTSCALCULATION OF SCF FOR THE ABOVE JOINTS DIFFER21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3655GEOMETRIC CLASSIFICATIONTYKX COMBINATION OF THE ABOVEJOINTS ARE TREATED IN ONE PLANE –PLANER JOINTS Offshore Structures –Fatigue Analysis JOINT TYPES21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3657GROUTED PILE / LEG SECTIONPILE JACKET LEGGROUTOffshore Structures –Fatigue AnalysisANNULAR GROUTED TUBULAR JOINTS22Lp eq T T T +=Where p T is the thickness of pile wall and L T is the thickness of jacket leg .21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3659S-N CURVESVARIOUS TYPES OF S-N CURVES AVAILABLE FROMAmerican Welding Society, AWSAmerican Petroleum Institute API RP 2ADet Norske Vritas DNV RP C203American Bureau of Shipping, Guidance Health and Safety Executive, HSE UK Offshore Structures –Fatigue Analysis SCF FOR T JOINT -API RP 2A21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3661S-N CurvesStress-Cycles (S-N) curve is material characteristicsGenerated using experimental methods API RP 2A suggest use of X and X’curves for Tubular Joints (Now its is called WJ and Improved WJ)Health and Safety Executive (HSE) has many Classes of S-N curves for different connections including Tubular JointsOffshore Structures –Fatigue Analysis BASIC S-N CURVE (API RP 2A)21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3663S-N CURVES)(10)1(10)(10S mLog k Log N Log −=)(10log )1(10log 10s m k N −=Offshore Structures –Fatigue AnalysisS-N Curve Improved profile (API RP 2A)21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3665HSE S-N CURVE FOR NODAL JOINTSOffshore Structures –Fatigue Analysis FACTOR OF SAFETY (API RP 2A)∑==alli N n D 1FATIGUE DAMAGE MINERS’RULEn = Number of occurrences in each sea-state N = Allowable number of cyclesOffshore Structures –Fatigue AnalysisWhere to Check ?21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3669LOCAL FATIGUEComputation of Cyclic stresses using global loads can only maximise the global responseLocal member response needs to be looked into especially in deterministic analysisOffshore Structures –Fatigue Analysis RING STIFFENED JOINTS21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3671RING STIFFENED JOINTSRings are effective means of reducing SCF’s at brace / chord interfaceThe SCF’s at saddle get reduced considerably but not at the crownsIf the load is axial or OPB, the rings are effective but for IPB it is not effective in reducing the SCF’s at the Crown locationMay have to use Two rings just below the Crown to be effective in reducing the SCF at crownSCF’s at ring / chord interface shall be checked. This location being cannot inspect, keep the ring bending modulus to ring area high as much as 8 to 15.Offshore Structures –Fatigue Analysis OTHER FATIGUE ISSUESPile Driving fatigueWind induced FatigueFatigue non-tubular connections Fatigue on Topsides due to platform DisplacementFatigue on Riser and Wellhead piping21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3673Fatigue Life ImprovementProfile Grinding the weldWeld Toe grindingHammer peening of the weld toeSmoothening Weld caps in butt weldsPost weld heat treatmentAdding Internal rings Adding external reinforcements Offshore Structures –Fatigue Analysis WELD PROFILE21 July 2008Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-3675TRANSITIONOffshore Structures –Fatigue Analysis BUT WLED。