ACCIDENTAL LOAD IMAPACT ON FIXED OFFSHORE STRUCTURES

PUSHOVER AND SHAKEDOWN ANALYSIS OF FIXED OFFSHORE STRUCTURESRALLAPALLI SRIVASTAV (OE13M068)

PROJECT GUIDE Prof. S.K. Bhattacharyya IntroductionStructures possess reserve strength beyond those recognized in traditional designSometimes, demands are placed on existing structures that are outside their original design intent (additional risers or process modules may be added)Additionally, recommendations for the design of new structures may not be satisfied for the existing structuresUltimate strength analysis is needed to account these and demonstrate that a platform has adequate strength to withstand loading criteria with local overstress and damage allowed but without collapseAccurate nonlinear finite element analyses are therefore crucial in order to verify that the design meets the strength requirements

2ObjectiveThe objective of the project is to calculate the potential resistance reserves of the whole structure by performing static pushover and shakedown analysis using Finite Element MethodTo identify the ultimate capacity, static pushover analysis can be performedShakedown analysis is performed in order to investigate if the structure is likely to fail from repetitive loading or remains stableUSFOS is a computer program for nonlinear static and dynamic analysis of space frame structures

3Scope of the studyTo conduct static pushover analysis to calculate ultimate strengthTo discuss the effect of extreme cyclic loading on global integrity and to compare the results with ultimate strength predictions from pushover analysisTo perform shakedown analysis for representative sequence of extreme waves and for earthquake loadingCalculating Reserve Strength Ratio for different bracing patterns of jacket structure

4MethodologyModeling the complete jacket structure using SESAM GeniE Analysis of the structure using USFOS, a module in SESAMPerforming Static Pushover analysis using 100 year design wavePerforming Shakedown analysis for Extreme storm analysis with constant amplitude and Longterm cyclic loadingPerforming shakedown analysis for earthquake

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USFOSUSFOS nonlinear analysis follows the following basic procedure:The load is applied in stepsThe nodal coordinates are updated after each load step The structure stiffness is assembled at each load step. The element stiffness are then calculated from the updated geometry At every load step each element is checked to see whether the forces exceed the plastic capacity of the cross section. If such an event occurs, the load step is scaled to make the forces comply exactly with the yield conditionA plastic hinge is inserted when the element forces have reached the yield surfaceThe load step is reversed (the load is reduced) if global instability is detected

66Incremental and Iterative approachThe approach used is the pure Newton- Raphson method with arc length controlApplies the load gradually, in incrementsAlso performs equilibrium iterations at each load increment to drive the incremental solution to equilibriumSolves the equation [KT]{Du} = {F} - {Fnr} [KT]= tangent stiffness matrix{u}= displacement increment{F} = external load vector{Fnr} = internal force vectorIterations continue until {F} - {Fnr} reaches a minimum value

DisplacementF[KT]1234 equilibrium iterationsFnrDu7Incremental and Iterative approachDisplacementFFnr8General structure and Load input (ufo format)Special USFOS parameters(head.fem)USFOSAnalysis results text fileAnalysis-data-file (containing structure data andanalysis results data)9Portal frame system7 m5 mParametersLegs

BeamDiameter(m) 0.50.5

Thickness(m)0.0200.02Yield strength(Mpa)3003601234Global node4 (displacement in Z direction)Ultimate collapse Load0.96MNTheoritical calculation1.015MNxz41010

11X Brace system10 m10 mParametersLegs

Braces Diameter(m) 0.50.25

Thickness(m)0.02500.0266Yield strength(Mpa)2482481234Compression braceMember 13Global node3 (displacement in x direction)Buckling load4.02 MNUltimate collapse Load4.39 MNEffective Length factor0.65Imperfection applied0.0015L for member 131212

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End Displacement(m)Axial Force(MN)1414NovemberDecemberJanuaryFebruaryMarchModelling the complete jacket structure using Sesam GeniePerforming Static Pushover analysisPerforming shakedown analysis for extreme cyclic wave loadingPerforming shakedown analysis for earthquakeCalculating Reserve Strength Ratio for different bracing patterns of jacket structureTimeline 15ReferencesStewart, G., Moan, T., Amdahl, J., and Eide, O. I. (1993). Nonlinear re-assessment of jacket structures under extreme storm cyclic loading:Part I - philosophy and acceptance criteria. In Proceedings of the 12th International Conference on Offshore Mechanics and Arctic Engineering (OMAE) 1993, Glasgow, ScotlandSkallerud, Amdahl. Nonlinear Analysis of Offshore Structures,Standard Publishers Distribution, 1st Edition (2009).USFOS user manual for USFOS Release 8-5, 2010 16

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