Dynamic-Analysis-Method-of-Offshore-Jack-up-Platfo

J.MarineSci.Appl.(2012)11:111-118DOI:10.1007/s11804-012-1112-7DynamicAnalysisMethodofOffshoreJack-upPlatformsinRegularandRandomWavesHaoYu1*,XiaoyuLi1andShuguangYang21.CollegeofShipbuildingEngineering,HarbinEngineeringUniversity,Harbin150001,China2.ResearchCenter,AmericanBureauofShipping,HoustonTX77060,USAAbstract:Ajack-upplatform,withitsparticularstructure,showedobviousdynamiccharacteristicsundercomplexenvironmentalloadsinextremeconditions.Inthispaper,takingasimplified3-Dfiniteelementdynamicmodelinextremestormconditionsasresearchobject,atransientdynamicanalysismethodwasproposed,whichwasunderbothregularandirregularwaveloads.Thestepsofdynamicanalysisunderextremeconditionswereillustratedwithanappliedcase,andthedynamicamplificationfactor(DAF)wascalculatedforeachresponseparameterofbaseshear,overturningmomentandhullsway.Finally,thestructuralresponseresultsofdynamicandstaticwerecomparedandanalyzed.TheresultsindicatedthatthestaticstrengthanalysisoftheJack-upPlatformswasnotenoughunderthedynamicloadsincludingwaveandcurrent,furtherdynamicresponseanalysisconsideringbothcomputationalefficiencyandaccuracywasnecessary.Keywords:jack-upplatforms;waveloads;dynamicanalysis;dynamicamplificationfactorArticleID:1671-9433(2012)01-0111-081Introduction1Jack-upplatformsarewidelyusedinoffshoreoilandgasexploration,thereisgrowingdemandforthistypeofunitsdrillingindeeperwatersandharsherenvironmentalconditions(Lu,2001).Underextremecircumstances,theenvironmentalloading(wind,waves,currents,etc.)andtheirinteractionswithapparentnonlinear,dynamicperformance,randomness,etc.(Xuetal.,2003a)Jack-upplatformiseasytoproducevibrationundertheactionofwavesindeepwaterenvironments,itnotonlyaffectsthestructuralstrength,butalsohasagreaterimpactonthereliabilityandsafetyoftheplatform.Manyscholarsfocusonthedynamicresponseoftheplatformstructureandhavebeencarryingoutseriesofstudies(Zhaoetal.,2007;Cassidyetal.,2002;Xuetal.,2003b;Williamsetal.,1999;Huntetal.,2004;Williamsetal.,1998;Martin,1994).Asaccidentratesduetodynamiceffectsandstructuraldeficienciesremaincomparativelyhigh(Huntetal.,2004),accurateanalysisofjack-upbehavior,avoidingexcessiveconservatism,becomesincreasinglyimportant,thereisanaugmenteddesiretounderstandtheirbehaviorunderdynamicloadingconditions.Thereareseveralareasofuncertainty(Williamsetal.,1998)inthedynamicanalysisofajack-upPlatform.Jack-upsareflexiblestructureswithnaturalperiodsofthesameorderasthepredominantwaveperiodsformanyseas,structuralReceiveddate:2011-07-21.Foundationitem:SupportedbytheNationalNaturalScienceFoundationofChina(GrantNo.51079034)andFundamentalResearchFundsfortheCentralUniversities(GrantNo.HEUCFR1003).*CorrespondingauthorEmail:[e-mail]©HarbinEngineeringUniversityandSpringer-VerlagBerlinHeidelberg2012modelingofthelegs,thedegreeofbaseflexibilityprovidedbythespudcanandthenatureofthewaveloads.Thespudcansprovidethecounterforceoftheplatform’sgravityandslippage,italsobearsthemomentdeliveredbythelegwhichisapttobesimplified．Muchvaluablework(Bienenetal.,2006;Martin,1994;Lietal.,2010)hadbeendoneforthejack-upplatforms’dynamiccharacteristicinconsiderationofthepile-soil-structureinteraction,theWinklerbeammodelisusedtosimulatethedynamicinteractionandstudythedepthofpitchingpile(Luetal.,2005).Thedynamicanalysismethods(Spidsoeetal.,1996)aremainlysingledegreeoffreedom(SDF)method,frequencydomainmethodandtimedomainmethod,SDFmethodcanroughlyestimatedynamicamplificationfactor(DAF)oftheplatform,andtheestimationofthenaturalfrequencyistoolarge.Thefrequencydomainmethoddoesnottakeeachnon-linearfactorintoaccountofloads(Zhenetal.,2004).Theconsiderationwithtimedomainmethodismostcomprehensive,withthedevelopmentofcomputertechnology,thetimedomainmethodistobestudiedfurther.Theauthorsstudiedthecalculationmethodofthedynamicenvironmentalloadsinextremeconditions.TheboundaryconditionsoftheFEmodelweretreatedreasonably,theplatform’sstructuralstrengthandtheDAFwereanalyzedinaccordancewithindustrialstandards,utilizinglinearandnon-linearwavetheoryandthefiniteelementmethod(FEM).Acomparisonwasperformedbetweenthedynamicresultsandthatofthestaticstrengthassessment.Theresultsshowthatfurtherdynamicresponseanalysisisnecessaryandthispaper’stimedomaindynamicanalysismethodisHaoYu,etal.DynamicAnalysisMethodofOffshoreJack-upPlatformsinRegularandRandomWaves112efficientandaccurate.2EnvironmentalloadsEnvironmentalloadsofjack-upplatformsincludewaves,currents,wind,iceloadsandseismicloadsetc.Thispaperonlyconsiderswaveloads(includingtheroleofoceancurrents)andwindloadsunderextremeconditions.2.1Waveloads2.1.1MorrisonforceTypicalk-latticelegisakindofsmall-sizestructures,itsMorrisonforceconsideringcurrentsinfluenceiscalculatedas21(π/4)d2DMFCDUUCDUzρρ⎡⎤=+⎢⎥⎣⎦∫&(1)where,U=componentofthevelocityvector,normaltotheaxisofthemember;U&=componentoftheaccelerationvectorofthestructuralmembernormaltoitsaxis;CM=inertiacoefficient;CD=dragcoefficient;D=projectedwidthofthememberinthedirectionofthecross-flowcomponentofvelocity.ThesevariablesrelatedtotheMorrisonforcearedeterminedafterthechoiceofwaveth