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The14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China1DeterminationofSeismicFortificationLevelofOffshorePlatformsinChinaYuejunLU1,YanjuPENG2,RongyuTANG3andHaijunSHA41Researcher,InstituteofCrustalDynamics,ChinaEarthquakeAdministration,Beijing,ChinaEmail:luyj1@263.
net2AssociateResearcher,InstituteofCrustalDynamics,ChinaEarthquakeAdministration,Beijing,China3Researcher,InstituteofCrustalDynamics,ChinaEarthquakeAdministration,Beijing,China4AssociateResearcher,InstituteofCrustalDynamics,ChinaEarthquakeAdministration,Beijing,ChinaABSTRACT:ThecurrentseismicdesigncodecommonlyusedintheworldisRP2A-WSDcompiledbyAmericanPetroleumInstitute,thereisstillnospecificseismicdesigncodeforoffshoreplatformsinChina.
Inthispaper,abriefintroductionofRP2A-WSDcodeisgivenatfirst,andthecomparisonbetweentheseismicfortificationlevelsofRP2A-WSDandthatofChineseGB17503-1998andGB50011-2001suggeststhatthelevelsofChineseCodesareoverestimated.
Secondly,theseismicdamagesandanti-seismicdesignrequirementsofoffshoreplatformsarediscussedinviewoftheirstructuralcharacteristics.
Onthebasisoftheanalysisabove,summingupthefactors,suchastheseismicactivityinthewatersofChina,structuralcharacteristicsofoffshoreplatform,seismicdesigngoal,andtheseismicfortificationexperiencesofrelatedengineering,werecommendthattheseismicfortificationlevelofstrengthdesigntakeareturnperiodof200years,andthatofdeformationdesigntakeareturnperiodof3000yearsrespectivelyforoffshoreplatformsinChina.
Finally,thecomparisonwithotherChineseCodesofrelatedindustriesshowsthattherecommendedfortificationlevelissafeandrational.
KEYWORDS:OffshorePlatform,SeismicDesign,SeismicFortificationLevel,ReturnPeriod0INTRODUCTIONSincethe80soflastcentury,theexploitationofoceanoilgainedverygreatprogressinChina,Over100offshoreoilplatformshavebeenbuiltinBohai,Huanghai,EastChinaSeaandSouthChinasea,another100willbeerectedinthecoming5years.
Thedamageofoffshoreoilplatformsmaycauseseveresecondarydisasters,andtheseismicityintheseasofChinaisrelativelyhigh,thus,theseismicdesignproblemofoffshoreplatformshasattractedconsiderableattention(LuYuejun,etal,2003).
FixedplatformsaremostlyusedintheexploitationofoceanoilinChina,theirlifespanisratherlong,commonlyismorethan15years.
Duringthelifespantheplatformcannotmove,andmustsupportsomeenvironmentalloads,includingwind,wave,current,ice,earthquakeandsoon.
Thestructuraldamagewillcauseseriouspersonnelcasualty,equipmentloss,oilproductionstop,andenvironmentalpollution.
Inplatformdesigning,earthquakeloadshouldbeimposedontheplatformasaseparateenvironmentalloadingcondition,otherenvironmentalloadsshouldbecombinedinamannerconsistentwiththeprobabilityoftheirsimultaneousThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China2occurrenceduringtheloadingconditionbeingconsidered.
Andearthquakeloadisseriouslydestructive,andcannotbepredicted.
Therefore,theseismicanalysismustbetakenintoaccountinstructuraldesigning.
NospecificseismicdesigncodehasyetbeendraftedforoffshoreplatformsinChinabecauseofinsufficientknowledgeontheseismiccharacteroftheseaareas.
ThecurrentseismicdesigncodecommonlyusedintheworldisRP2A-WSDcompiledbyAmericanPetroleumInstitute(AmericanPetroleumInstitute,2002).
Seismicfortificationistoprovideresistancetoearthquakeforengineeringstructures.
Thefollowingthreefactorsshouldbetakenintoconsiderationinthedeterminationoftheseismicfortificationofengineeringstructures,socialeconomiclevel,seismichazardandimportanceofstructures.
Thekeycontentoftheseismicfortificationisseismiclevelandgrade(XieLili,etal,1996).
Inthispaper,acomparisonbetweentheseismicfortificationlevelsofRP2A-WSDandthatofChineseGB17503-1998andGB50011-2001iscarriedout,anditisregardthatthelevelsofChineseCodesareoverestimated.
Summingupthefactors,suchasseismicactivityintheseasofChina,structuralcharacteristicsofoffshoreplatforms,seismicdesigngoal,andtheseismicfortificationexperiencesofrelatedengineering,werecommendthattheseismicfortificationleveltakesareturnperiodof200yearsforstrengthdesign,and3000yearsfordeformationdesignrespectivelyforoffshoreplatformsinChina.
Finally,thecomparisonwithotherChineseCodesofrelatedindustryshowsthattherecommendedfortificationlevelissafeandrational.
1.
THECURRENTSEISMICDESIGNCODEOFOFFSHOREPLATFORMANDEXISTINGPROBLEMS1.
1.
ThecurrentseismicfortificationcodeThereisnoseismicdesigncodeofoffshoreplatforminChinaatpresent.
TheGB17503-1998suggeststhatthelevelsofthestrengthanddeformationdesigntake10%and0.
5%probabilitiesofexceedancein50years.
The10%and0.
5%probabilitiesofexceedancein50yearscorrespondtoareturnperiodof475and10000yearsrespectively.
Butthiscodedoesnotgiveacorrespondinginterpretationoftheiritems.
BasedontheseismicactivityandseismicriskmapinthecostalwatersoftheUnitedStates,RP2A-WSDiscompiledbyAmericanPetroleumInstitute(AmericanPetroleumInstitute,2002).
Thecodestatesthattwolevelsofgroundmotionintensityduetomoderateandrareintenseearthquakesinthelifespanofoffshoreplatformshouldbeconsideredinstrengthdesignanddeformationdesignrespectively.
Thedesignseismiclevelmeetsthestrengthrequirements,thatis,toprovideresistancetomoderateearthquakeswithoutsignificantstructuraldamage,arecurrenceintervalof200yearsforpermanentstructuresinSouthernCaliforniaisrecommended.
Therareintenseearthquakeistheinputfordeformationdesign,nolifelossandsignificantenvironmentpollutionisallowedatthislevel,structuraldamageislikelytooccur,buttheprovisionsareintendedtopreventcollapseoftheplatform,thereturnperiodmaybeseveralhundredtoafewthousandyears.
Thefirstlevelprovidesthegroundmotioninputfortheelasticdesignofthestructure,andthesecondlevel,ifitisdeemednecessarytoanalyzethestructureforrareintenseearthquakes,providesthegroundmotioninputfortheThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China3analysis.
1.
2.
ExistingproblemsTheprovisionsandmethodofRP2A-WSDCodeareusedinseismicanalysisoftheplatform,namely,strengthdesignanddeformationdesignarecarriedoutrespectively(WangZhongcang,2005).
ButtheseismicfortificationlevelsareaccordingtoGB50011-2001(MinistryofConstructionP.
R.
China,2001)andGB17503-1998.
TheseCodesareinconsistent.
AcomparisonofseismicityismadebetweenSouthernCaliforniaandBohaiSeabyPengYanjuetal(2006),whichshowsthattheearthquakenumber,strainenergyrelease,andPGAinSouthernCaliforniaareallhigherthanthatinBohaianditsadjacency.
TheoilfieldPL19-3isajointventureprojectofChinaandtheUnitedStates,anddesignedinlightofAPIRP2A-WSDcode.
AccordingtoAPIRP2A-WSDandtheChineseCodeGB17503-1998,PengYanjuetal(2007)obtainedtheseismicdesignparametersattheoilfieldsite,thePGAatthestrengthdesignlevelanddeformationdesignlevelis252cms-2and570cms-2respectivelybytheChineseGB17503-1998,andthatis162cms-2and333cms-2respectivelybyAPIRP2A-WSDcode,theratiois1.
55and1.
71times.
Therefore,Chinaisadevelopingcountrywithlimitedfinanceandresources,thereisnoneedtoadoptaseismicfortificationlevelofoffshoreplatformhigherthanthatindevelopedcountries.
2.
STRUCTURALCHATACTERISTICSOFTHEOFFSHOREPLATFORMANDENGINEERINGSEISMICPROBLEMS2.
1.
AbriefintroductionofthestructureofoffshoreplatformThestructuralformsoffixedplatformincludejacket,gravity,tower,andsoon.
Thejacketplatformisthemostpopularstructuralformintheexploitationofoceanoil(gas),andconsistofthefollowing:redundantweldedtubularspaceframeservesasthemainstructuralelementoftheplatform,transmittinglateralandverticalforcestothefoundation,pilespermanentlyanchortheplatformtotheoceanfloor,andcarrybothlateralandverticalloads,asuperstructureprovidingdeckspaceforsupportingoperationalandotherloads.
Theintrinsicperiodofthestructureis1to3sdependingonitssize,weight,pilesdeepness,andwaterdepth.
2.
2EngineeringseismologicalproblemsoftheoffshoreplatformAsformostothertypesoffacilities,itisnotwarrantedandnoteconomicaltodesignoffshoreplatformtoprecludeanydamageforthemostsevereearthquakegroundshakingpossible.
Theactualprincipleisintendedtoprovideresistancetomoderateearthquakes,whichhaveareasonablelikelihoodofnotbeingexceededduringThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China4thelifespanoftheplatform.
Itisneededtoconstituteascientificandreasonableseismicdesigncodeanddetermineseismicfortificationlevel.
Whiledesigningoffshoreplatform,earthquakeforcesaretakenintoaccounttoensurethestructureatlinearelasticityinthelife.
Oncetherearesomestructuralmemberswhicharebendedanddestroyed,thestructureisregardedasreachingtheutmostcarryingcapacityandcannotbeusednormally.
Accordingtotheponderanceofthestructuralmembersinthewholestructure,thedestructivestateofplatformisdividedintotworanksbyWeiWei(2004):(1)Thenon-importancedestructivestate,thisistheutmostbeforethestructuregoesintoplasticstate,butthestructureisstillatlinearelasticity.
Hereon,theplatformcanstillbeused,orcanberepaired.
(2)Thelocalcollapsestate,thisstateisattheutmostofcarryingcapacity,thestructureoritsmembersreachthebiggestallowablecarryingfunction.
Theplatformcannotbeusedanyfurther,somestructuralmembersorthewholestructureundergomortaldeformation,butwithoutcollapsing.
Thestudyresultprovidedatheoreticalbasisforthedeterminationofseismicfortificationgoal.
Theseismicinfluenceontheplatformstructureismostlybroughtbythebasemovement,tofulfillseismicanalysis,someseismicparametersatthebasesitearerequired,includingthepeakvalues,responsespectra,andtimehistoryofthegroundmotion.
Theparticularityofengineeringseismicproblemsoftheoffshoreplatformisthelongperiodgroundmotion,theoffshoreplatformisalongperiodstructure,atthesametime,thereciprocityofplatformandwater(appendedmass)alsomakestheresonanceperiodlonger.
Commonly,themajorityofconstructsandstructuresusethepeakvaluesofseismicaccelerationastheindexofseismicanalysis,andtheplatform,whichislongperiodstructure,shouldusevelocityordisplacement,butthecurrentseismicanalysisoftheplatformstillusesaccelerationbecauseofinsufficiencyofengineeringseismicstudy.
LuYuejunetal(2003)discussedsomeengineeringseismicproblemsofplatformindetail,includingscenarioearthquake,longperiodgroundmotion,andsoon.
3.
DETERMINATIONOFSEISMICFORTIFICATIONLEVELFORLEVELFOROFFSHOREPLATFORM3.
1.
DeterminationofdesignruleBasedontheanalysisofengineeringseismicproblemsoftheoffshoreplatforminabovesection,seismicfortificationforplatformmayhaveananalogywithsomespecialstructureinland,suchasTVtransmissiontower,oversizebridge,greatdamandsoon.
Referringtotheseismicfortificationexperiencesofrelatedengineering,thefollowingfactorsshouldbetakenintoconsiderationinthedeterminationoftheseismicfortificationofplatform.
(1)TomeettheChineseandforeigncurrentcodePresently,inordertofittothedevelopingtrendofinternationalanti-seismictechnology,theseismicdesignThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China5thinkingistransferringfromsafetyfactortoreliabilitydesigninChina.
Sotheseismicfortificationlevelbasedonprobabilityisneededtomeetthedemandofreliabilitydesign.
(2)Considerationoffortificationclassificationandmultiple-stagedesignReliabilitydesignmakesthestructuretomeetthedemandofspecificultimatestatebycontrollingthemajorconditions.
AccordingtoactualengineeringstructuredesigninChina,theultimatestatemaybedividedintothreestates:serviceabilitylimit,ultimatecarryingcapacity,andultimatedeformation.
Theengineeringstructuredesignismadeaccordingtocarryingcapacityforeachstate,andtheseismicfortificationisgivenbasedonthedifferentultimatecarryingcapacity.
Forexample,GB50011-2001adoptsthreeseismicfortificationlevelsandtwo-stagedesign.
(3)KeeptheconsistencywiththestatepolicyofseismicfortificationThestateseismicdesignpolicyrestrainsthefortificationlevel.
Thegroundmotionparameters(e.
g.
peakaccelerationandresponsespectrum)specifiedinthepresentcodessuchasGB50011-2001,arealllowerthanobservationvalues.
Thereforetheseismicfortificationlevelforoffshoreplatformsshouldnotbetooconservative,andthereisnoneedtotakeahighervaluethanthatindevelopedcountries.
3.
2Determinationofseismicfortificationlevelforoffshoreplatform(1)DesignreferenceperiodDesignreferenceperiodliesonthelifespanoffacilities.
Becausethedurationofexploitationofoceanoil(gas)isabout30years,asuitabledesignreferenceperiodofoffshoreplatformis30years.
(2)SeismicfortificationgoalThedestructivestateofplatformisdividedintotworanks:thenon-importantdestructionstateandthelocalcollapsestate.
Hereby,consideringtheexistingdesignrule,theseismicfortificationgoalforoffshoreplatformisdeterminedaccordingtotwolevelsduetofrequentandrareearthquakesduringthelifespan.
Thefrequentearthquakelevelmeetsthestrengthrequirements,thatis,toprovideresistancetofrequentlyencounteredearthquakeswithoutsignificantstructuraldamage,theplatformmaintainserviceability,orcanberepaired.
Therareearthquakeistheinputfordeformationdesign,structuraldamageislikelytooccur,buttheprovisionsareintendedtopreventcollapseofthewholestructure,nolifelossandsignificantenvironmentpollutionisallowedatthislevel.
(3)Thefrequent-earthquakelevelThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China6Thefrequentearthquakescorrespondtoearthquakeforcesunderserviceabilitylimitdesignstate.
Forcommonconstructandstructure(theclassC),thedesignreferenceperiodis50years,thefrequentearthquakeleveltakes63%probabilitiesofexceedancein50years,correspondingtoareturnperiodof50years.
Offshoreplatformisanimportantengineeringstructure,consequentlytheseismicfortificationlevelshouldbehigherthanthatforcommonconstructandstructure.
AccordingtotherecommendedvalueforSouthernCaliforniainRP2A-WSD,thestrengthdesignlevelforoffshoreplatformisareturnperiodof200years,correspondingto14%probabilitiesofexceedancein30years.
Theintensityatfrequent-earthquakelevelforcommonstructuresis1/3ofmediumearthquakes(returnperiod475years).
Accordingtostatisticof128resultsofseismicsafetyassessmentthroughoutthewholecountry,earthquakeforcesatreturnperiodof200yearsis2/3ofreturnperiodof475years,therefore,thefrequent-earthquakelevelforoffshoreplatformtakesareturnperiodof200years,whichcorrespondsto2timesthestrengthatthefrequent-earthquakelevelforclassCstructuresandisequivalenttothatforclassAstructures.
(4)TherareintenseearthquakelevelTherareintenseearthquakescorrespondtoearthquakeforcesinutmostdeformationdesign.
Inthedesignreferenceperiodofplatform,thestructuresuffersfromsuchearthquakeforcesbyaccident,thestructureislikelytoincurthebiggestdeformation,butcankeepoverallstabilitywithoutcollapse.
Thelevelofrareintenseearthquaketakes2~3%probabilityofexceedancein50yearsforclassCstructures,correspondingtoareturnperiodof1500to2500years,thatforclassAstructurestakes2~3%in100years,correspondingtoareturnperiodof3000to5000years.
Usuallythedesignreferenceperiodofoffshoreplatformsis30years,becausethedamageofplatformsmaycausegreateconomiclossandenvironmentpollution,itisappropriatethatthedeformationdesignleveltakes1%in30years,correspondingtoareturnperiodof3000,whichisalreadyhigherthanthatforclassAstructuresandapproximatestotheupperlimitinRP2A-WSD.
Accordingtotheanalysisabove,andtakingRP2A-WSDasareference,itissafeandrationaltotaketheearthquakeswithareturnperiodof200yearsand3000yearsforoffshoreplatformsinChinaasthestrengthanddeformationdesignlevelrespectively.
4.
ACOMPARISONWITHRELATEDCODES4.
1.
Acomparisontothecodesofoil/gasandotherchemicalstructuresOil/gasandotherchemicalstructuresarelargeandcomplexengineeringsystems,includingcomprehensivestation,pipeline,oilstorage,chemicalstructure,andsoon.
Incaseofearthquakedemolishment,thestructuraldamagewillcauseseriouseconomicallossandenvironmentalpollution,sotheseismicproblemsaretakenseriously.
Basedonthestructuralcharacteristicsandimportance,oil/gasandotherchemicalstructuresaredividedintothreeclasses:A,B,andC;theoffshoreplatformisaffirmedasclassAoftheutmostimportance.
Werecommendthattheseismicfortificationlevelofstrengthdesigntakesareturnperiodof200years,andthatThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China7ofdeformationdesigntakesareturnperiodof3000yearsrespectively,thereturnperiodisthelongestinoil/gasandchemicalindustry.
Wemayregardthattheseismicfortificationmeasuresforoffshoreplatformsarestricter,itsseismicriskissmaller.
Table1TheseismicfortificationlevelforoilandgasstructuresClassStructureReferenceperiod(a)Returnperiodforfrequentearthquakes(a)Returnperiodforrareintenseearthquakes(a)AOffshorePlatform302003000BLargecomprehensivestation,importantsectionofoil&gaspipeline,largeoilstructureandimportantchemicalstructure50100300030502500COilgasandotherchemicalstructuresbesidesClassA&B50502500303015004.
2.
ComparisonwithseismicfortificationlevelsofrelevantindustriesBecausetheseismicdesignideavariesindifferentindustrialfields,itisobviouslydifficulttocomprehensivelycomparethedifferencebetweentheirseismiclevels.
Therefore,onlythoseindustrialdepartments,whichhavethesameseismicdesignideaandsimilarseismicfortificationgoal,areselectedtocompare;hereweemphasizeacomparisonofthereturnperiodofrareearthquakebetweendifferentstructures.
Inordertorationallyreflectthedifferenceofseismicfortificationforstructureswithdifferentreferenceperiodandreturnperiod,weconvertreturnperiodtoprobabilityofexceedance(PT)inreferenceperiod,then(PT)canindicatetherelativeseismicriskinthereferenceperiod.
Table2isthedeformationdesignlevelofdifferentstructures.
Thedatashowsthatthereferenceperiodofoffshoreplatformsisrelativelyshort,theprobabilityofexceedanceofrareearthquaketakes0.
01inthereferenceperiod.
Itsseismiclevelisjustlowerthanthatfornuclearpowerstation,andmatchesuptothatfortheThreeGorgeDam.
Thereforeareturnperiodof3000yearsofrareearthquakeforoffshoreplatformissafe.
Table2ThedeformationdesignlevelofdifferentstructuresStructureReferenceperiod(a)RareearthquakeReturnperiod(a)PTOffshoreplatform3030000.
01Nuclearplant40100000.
004ThreeGorgeDam100100000.
01HydraulicstructuresofClassAreservingwater10050000.
02BuildingsandStructuresofClassA1003000~50000.
02~0.
03Large-scaleBridge10033000.
03The14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China8TelevisionTowerofClassA10020000.
05BuildingsandStructuresofClassB5025000.
02BuildingsandStructuresofClassC50~20000.
02~0.
03HydraulicStructureofClassAwithoutwater5010000.
055.
SUMMARYOnthebasisofseismicactivityintheseaareasofChina,structuralcharacteristicsofoffshoreplatform,seismicdesigngoal,andtheseismicfortificationexperiencesofrelatedengineering,werecommendthattheseismicfortificationlevelofstrengthdesigntakesareturnperiodof200years,andthatofdeformationdesigntakesareturnperiodof3000yearsforoffshoreplatformsinChina.
ThereturnperiodsrecommendinthispaperareonlyusedinthecomparisonbetweenChineserelatedCodes,hasnotbeenappliedinstructuralanalysisofoffshoreplatform,alsothereliabilityanalysisofthedesignhasnotbeendonefordifferenceplatformstructures.
Thereisstillalongwaytogobeforetheconstitutionofseismiccodeforoffshoreplatforms,andmanyresearchesshouldbedoneinfuture.
ReferenceLuYue-jun,PengYan-ju,TangRong-yu.
(2003).
Engineeringseismicproblemsoftheoffshoreoilplatforms.
ProgressinGeophysics18:4,662-665.
AmericanPetroleumInstitute,RP2A-WSD.
(2002).
Planning,Designing,andConstructingFixedOffshorePlatforms-WorkingStressDesign.
Houston:AmericanPetroleumInstitute.
ChinaStateBureauofQualityandTechnicalSupervision.
(2005).
ChinaStandardGB17741-2005.
StandardsPressofChina.
ChinaStateBureauofQualityandTechnicalSupervision.
(1998).
GB17503-1998.
StandardsPressofChina.
MinistryofConstructionP.
R.
China.
(2001).
GB50011-2001.
ChinaBuildingIndustryPress.
XieLiliZhangXiaozhiZhouYongnian.
(1996).
OntheDesignEarthquakeLevelforearthquakeResistantWorks.
EarthquakeEngineeringandEngineeringVibrationl6:1,1-18.
WangZhongchang.
(2005).
Thediscussiononsomeproblemsinanti-seismicanalysisforfixedplatform.
ChinaOffshoreOilandGas17:6,421-423PengYanju,LuYuejun,TangRongyu.
(2006).
ResearchontheseismicfortificationlevelforoffshoreplatforminBohaiSeaandadjacentareas,ActaSeismologicaSinica,27:6,647-655WeiWei.
(2004).
Researchonseismicdamagestatesofjacketoffshoreplatformstructures.
AdissertationofOceanUniversityofChinaforDoctoralDegreePengYanju,LuYuejun,TangRongru,etal.
(2007).
ResearchonseismicdesignparametersandearthquakeresistantlevelforanoilfieldplatforminBohaiSea.
Journalofearthquakeengineeringandengineeringvibration.
27:4,8-14
net2AssociateResearcher,InstituteofCrustalDynamics,ChinaEarthquakeAdministration,Beijing,China3Researcher,InstituteofCrustalDynamics,ChinaEarthquakeAdministration,Beijing,China4AssociateResearcher,InstituteofCrustalDynamics,ChinaEarthquakeAdministration,Beijing,ChinaABSTRACT:ThecurrentseismicdesigncodecommonlyusedintheworldisRP2A-WSDcompiledbyAmericanPetroleumInstitute,thereisstillnospecificseismicdesigncodeforoffshoreplatformsinChina.
Inthispaper,abriefintroductionofRP2A-WSDcodeisgivenatfirst,andthecomparisonbetweentheseismicfortificationlevelsofRP2A-WSDandthatofChineseGB17503-1998andGB50011-2001suggeststhatthelevelsofChineseCodesareoverestimated.
Secondly,theseismicdamagesandanti-seismicdesignrequirementsofoffshoreplatformsarediscussedinviewoftheirstructuralcharacteristics.
Onthebasisoftheanalysisabove,summingupthefactors,suchastheseismicactivityinthewatersofChina,structuralcharacteristicsofoffshoreplatform,seismicdesigngoal,andtheseismicfortificationexperiencesofrelatedengineering,werecommendthattheseismicfortificationlevelofstrengthdesigntakeareturnperiodof200years,andthatofdeformationdesigntakeareturnperiodof3000yearsrespectivelyforoffshoreplatformsinChina.
Finally,thecomparisonwithotherChineseCodesofrelatedindustriesshowsthattherecommendedfortificationlevelissafeandrational.
KEYWORDS:OffshorePlatform,SeismicDesign,SeismicFortificationLevel,ReturnPeriod0INTRODUCTIONSincethe80soflastcentury,theexploitationofoceanoilgainedverygreatprogressinChina,Over100offshoreoilplatformshavebeenbuiltinBohai,Huanghai,EastChinaSeaandSouthChinasea,another100willbeerectedinthecoming5years.
Thedamageofoffshoreoilplatformsmaycauseseveresecondarydisasters,andtheseismicityintheseasofChinaisrelativelyhigh,thus,theseismicdesignproblemofoffshoreplatformshasattractedconsiderableattention(LuYuejun,etal,2003).
FixedplatformsaremostlyusedintheexploitationofoceanoilinChina,theirlifespanisratherlong,commonlyismorethan15years.
Duringthelifespantheplatformcannotmove,andmustsupportsomeenvironmentalloads,includingwind,wave,current,ice,earthquakeandsoon.
Thestructuraldamagewillcauseseriouspersonnelcasualty,equipmentloss,oilproductionstop,andenvironmentalpollution.
Inplatformdesigning,earthquakeloadshouldbeimposedontheplatformasaseparateenvironmentalloadingcondition,otherenvironmentalloadsshouldbecombinedinamannerconsistentwiththeprobabilityoftheirsimultaneousThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China2occurrenceduringtheloadingconditionbeingconsidered.
Andearthquakeloadisseriouslydestructive,andcannotbepredicted.
Therefore,theseismicanalysismustbetakenintoaccountinstructuraldesigning.
NospecificseismicdesigncodehasyetbeendraftedforoffshoreplatformsinChinabecauseofinsufficientknowledgeontheseismiccharacteroftheseaareas.
ThecurrentseismicdesigncodecommonlyusedintheworldisRP2A-WSDcompiledbyAmericanPetroleumInstitute(AmericanPetroleumInstitute,2002).
Seismicfortificationistoprovideresistancetoearthquakeforengineeringstructures.
Thefollowingthreefactorsshouldbetakenintoconsiderationinthedeterminationoftheseismicfortificationofengineeringstructures,socialeconomiclevel,seismichazardandimportanceofstructures.
Thekeycontentoftheseismicfortificationisseismiclevelandgrade(XieLili,etal,1996).
Inthispaper,acomparisonbetweentheseismicfortificationlevelsofRP2A-WSDandthatofChineseGB17503-1998andGB50011-2001iscarriedout,anditisregardthatthelevelsofChineseCodesareoverestimated.
Summingupthefactors,suchasseismicactivityintheseasofChina,structuralcharacteristicsofoffshoreplatforms,seismicdesigngoal,andtheseismicfortificationexperiencesofrelatedengineering,werecommendthattheseismicfortificationleveltakesareturnperiodof200yearsforstrengthdesign,and3000yearsfordeformationdesignrespectivelyforoffshoreplatformsinChina.
Finally,thecomparisonwithotherChineseCodesofrelatedindustryshowsthattherecommendedfortificationlevelissafeandrational.
1.
THECURRENTSEISMICDESIGNCODEOFOFFSHOREPLATFORMANDEXISTINGPROBLEMS1.
1.
ThecurrentseismicfortificationcodeThereisnoseismicdesigncodeofoffshoreplatforminChinaatpresent.
TheGB17503-1998suggeststhatthelevelsofthestrengthanddeformationdesigntake10%and0.
5%probabilitiesofexceedancein50years.
The10%and0.
5%probabilitiesofexceedancein50yearscorrespondtoareturnperiodof475and10000yearsrespectively.
Butthiscodedoesnotgiveacorrespondinginterpretationoftheiritems.
BasedontheseismicactivityandseismicriskmapinthecostalwatersoftheUnitedStates,RP2A-WSDiscompiledbyAmericanPetroleumInstitute(AmericanPetroleumInstitute,2002).
Thecodestatesthattwolevelsofgroundmotionintensityduetomoderateandrareintenseearthquakesinthelifespanofoffshoreplatformshouldbeconsideredinstrengthdesignanddeformationdesignrespectively.
Thedesignseismiclevelmeetsthestrengthrequirements,thatis,toprovideresistancetomoderateearthquakeswithoutsignificantstructuraldamage,arecurrenceintervalof200yearsforpermanentstructuresinSouthernCaliforniaisrecommended.
Therareintenseearthquakeistheinputfordeformationdesign,nolifelossandsignificantenvironmentpollutionisallowedatthislevel,structuraldamageislikelytooccur,buttheprovisionsareintendedtopreventcollapseoftheplatform,thereturnperiodmaybeseveralhundredtoafewthousandyears.
Thefirstlevelprovidesthegroundmotioninputfortheelasticdesignofthestructure,andthesecondlevel,ifitisdeemednecessarytoanalyzethestructureforrareintenseearthquakes,providesthegroundmotioninputfortheThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China3analysis.
1.
2.
ExistingproblemsTheprovisionsandmethodofRP2A-WSDCodeareusedinseismicanalysisoftheplatform,namely,strengthdesignanddeformationdesignarecarriedoutrespectively(WangZhongcang,2005).
ButtheseismicfortificationlevelsareaccordingtoGB50011-2001(MinistryofConstructionP.
R.
China,2001)andGB17503-1998.
TheseCodesareinconsistent.
AcomparisonofseismicityismadebetweenSouthernCaliforniaandBohaiSeabyPengYanjuetal(2006),whichshowsthattheearthquakenumber,strainenergyrelease,andPGAinSouthernCaliforniaareallhigherthanthatinBohaianditsadjacency.
TheoilfieldPL19-3isajointventureprojectofChinaandtheUnitedStates,anddesignedinlightofAPIRP2A-WSDcode.
AccordingtoAPIRP2A-WSDandtheChineseCodeGB17503-1998,PengYanjuetal(2007)obtainedtheseismicdesignparametersattheoilfieldsite,thePGAatthestrengthdesignlevelanddeformationdesignlevelis252cms-2and570cms-2respectivelybytheChineseGB17503-1998,andthatis162cms-2and333cms-2respectivelybyAPIRP2A-WSDcode,theratiois1.
55and1.
71times.
Therefore,Chinaisadevelopingcountrywithlimitedfinanceandresources,thereisnoneedtoadoptaseismicfortificationlevelofoffshoreplatformhigherthanthatindevelopedcountries.
2.
STRUCTURALCHATACTERISTICSOFTHEOFFSHOREPLATFORMANDENGINEERINGSEISMICPROBLEMS2.
1.
AbriefintroductionofthestructureofoffshoreplatformThestructuralformsoffixedplatformincludejacket,gravity,tower,andsoon.
Thejacketplatformisthemostpopularstructuralformintheexploitationofoceanoil(gas),andconsistofthefollowing:redundantweldedtubularspaceframeservesasthemainstructuralelementoftheplatform,transmittinglateralandverticalforcestothefoundation,pilespermanentlyanchortheplatformtotheoceanfloor,andcarrybothlateralandverticalloads,asuperstructureprovidingdeckspaceforsupportingoperationalandotherloads.
Theintrinsicperiodofthestructureis1to3sdependingonitssize,weight,pilesdeepness,andwaterdepth.
2.
2EngineeringseismologicalproblemsoftheoffshoreplatformAsformostothertypesoffacilities,itisnotwarrantedandnoteconomicaltodesignoffshoreplatformtoprecludeanydamageforthemostsevereearthquakegroundshakingpossible.
Theactualprincipleisintendedtoprovideresistancetomoderateearthquakes,whichhaveareasonablelikelihoodofnotbeingexceededduringThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China4thelifespanoftheplatform.
Itisneededtoconstituteascientificandreasonableseismicdesigncodeanddetermineseismicfortificationlevel.
Whiledesigningoffshoreplatform,earthquakeforcesaretakenintoaccounttoensurethestructureatlinearelasticityinthelife.
Oncetherearesomestructuralmemberswhicharebendedanddestroyed,thestructureisregardedasreachingtheutmostcarryingcapacityandcannotbeusednormally.
Accordingtotheponderanceofthestructuralmembersinthewholestructure,thedestructivestateofplatformisdividedintotworanksbyWeiWei(2004):(1)Thenon-importancedestructivestate,thisistheutmostbeforethestructuregoesintoplasticstate,butthestructureisstillatlinearelasticity.
Hereon,theplatformcanstillbeused,orcanberepaired.
(2)Thelocalcollapsestate,thisstateisattheutmostofcarryingcapacity,thestructureoritsmembersreachthebiggestallowablecarryingfunction.
Theplatformcannotbeusedanyfurther,somestructuralmembersorthewholestructureundergomortaldeformation,butwithoutcollapsing.
Thestudyresultprovidedatheoreticalbasisforthedeterminationofseismicfortificationgoal.
Theseismicinfluenceontheplatformstructureismostlybroughtbythebasemovement,tofulfillseismicanalysis,someseismicparametersatthebasesitearerequired,includingthepeakvalues,responsespectra,andtimehistoryofthegroundmotion.
Theparticularityofengineeringseismicproblemsoftheoffshoreplatformisthelongperiodgroundmotion,theoffshoreplatformisalongperiodstructure,atthesametime,thereciprocityofplatformandwater(appendedmass)alsomakestheresonanceperiodlonger.
Commonly,themajorityofconstructsandstructuresusethepeakvaluesofseismicaccelerationastheindexofseismicanalysis,andtheplatform,whichislongperiodstructure,shouldusevelocityordisplacement,butthecurrentseismicanalysisoftheplatformstillusesaccelerationbecauseofinsufficiencyofengineeringseismicstudy.
LuYuejunetal(2003)discussedsomeengineeringseismicproblemsofplatformindetail,includingscenarioearthquake,longperiodgroundmotion,andsoon.
3.
DETERMINATIONOFSEISMICFORTIFICATIONLEVELFORLEVELFOROFFSHOREPLATFORM3.
1.
DeterminationofdesignruleBasedontheanalysisofengineeringseismicproblemsoftheoffshoreplatforminabovesection,seismicfortificationforplatformmayhaveananalogywithsomespecialstructureinland,suchasTVtransmissiontower,oversizebridge,greatdamandsoon.
Referringtotheseismicfortificationexperiencesofrelatedengineering,thefollowingfactorsshouldbetakenintoconsiderationinthedeterminationoftheseismicfortificationofplatform.
(1)TomeettheChineseandforeigncurrentcodePresently,inordertofittothedevelopingtrendofinternationalanti-seismictechnology,theseismicdesignThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China5thinkingistransferringfromsafetyfactortoreliabilitydesigninChina.
Sotheseismicfortificationlevelbasedonprobabilityisneededtomeetthedemandofreliabilitydesign.
(2)Considerationoffortificationclassificationandmultiple-stagedesignReliabilitydesignmakesthestructuretomeetthedemandofspecificultimatestatebycontrollingthemajorconditions.
AccordingtoactualengineeringstructuredesigninChina,theultimatestatemaybedividedintothreestates:serviceabilitylimit,ultimatecarryingcapacity,andultimatedeformation.
Theengineeringstructuredesignismadeaccordingtocarryingcapacityforeachstate,andtheseismicfortificationisgivenbasedonthedifferentultimatecarryingcapacity.
Forexample,GB50011-2001adoptsthreeseismicfortificationlevelsandtwo-stagedesign.
(3)KeeptheconsistencywiththestatepolicyofseismicfortificationThestateseismicdesignpolicyrestrainsthefortificationlevel.
Thegroundmotionparameters(e.
g.
peakaccelerationandresponsespectrum)specifiedinthepresentcodessuchasGB50011-2001,arealllowerthanobservationvalues.
Thereforetheseismicfortificationlevelforoffshoreplatformsshouldnotbetooconservative,andthereisnoneedtotakeahighervaluethanthatindevelopedcountries.
3.
2Determinationofseismicfortificationlevelforoffshoreplatform(1)DesignreferenceperiodDesignreferenceperiodliesonthelifespanoffacilities.
Becausethedurationofexploitationofoceanoil(gas)isabout30years,asuitabledesignreferenceperiodofoffshoreplatformis30years.
(2)SeismicfortificationgoalThedestructivestateofplatformisdividedintotworanks:thenon-importantdestructionstateandthelocalcollapsestate.
Hereby,consideringtheexistingdesignrule,theseismicfortificationgoalforoffshoreplatformisdeterminedaccordingtotwolevelsduetofrequentandrareearthquakesduringthelifespan.
Thefrequentearthquakelevelmeetsthestrengthrequirements,thatis,toprovideresistancetofrequentlyencounteredearthquakeswithoutsignificantstructuraldamage,theplatformmaintainserviceability,orcanberepaired.
Therareearthquakeistheinputfordeformationdesign,structuraldamageislikelytooccur,buttheprovisionsareintendedtopreventcollapseofthewholestructure,nolifelossandsignificantenvironmentpollutionisallowedatthislevel.
(3)Thefrequent-earthquakelevelThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China6Thefrequentearthquakescorrespondtoearthquakeforcesunderserviceabilitylimitdesignstate.
Forcommonconstructandstructure(theclassC),thedesignreferenceperiodis50years,thefrequentearthquakeleveltakes63%probabilitiesofexceedancein50years,correspondingtoareturnperiodof50years.
Offshoreplatformisanimportantengineeringstructure,consequentlytheseismicfortificationlevelshouldbehigherthanthatforcommonconstructandstructure.
AccordingtotherecommendedvalueforSouthernCaliforniainRP2A-WSD,thestrengthdesignlevelforoffshoreplatformisareturnperiodof200years,correspondingto14%probabilitiesofexceedancein30years.
Theintensityatfrequent-earthquakelevelforcommonstructuresis1/3ofmediumearthquakes(returnperiod475years).
Accordingtostatisticof128resultsofseismicsafetyassessmentthroughoutthewholecountry,earthquakeforcesatreturnperiodof200yearsis2/3ofreturnperiodof475years,therefore,thefrequent-earthquakelevelforoffshoreplatformtakesareturnperiodof200years,whichcorrespondsto2timesthestrengthatthefrequent-earthquakelevelforclassCstructuresandisequivalenttothatforclassAstructures.
(4)TherareintenseearthquakelevelTherareintenseearthquakescorrespondtoearthquakeforcesinutmostdeformationdesign.
Inthedesignreferenceperiodofplatform,thestructuresuffersfromsuchearthquakeforcesbyaccident,thestructureislikelytoincurthebiggestdeformation,butcankeepoverallstabilitywithoutcollapse.
Thelevelofrareintenseearthquaketakes2~3%probabilityofexceedancein50yearsforclassCstructures,correspondingtoareturnperiodof1500to2500years,thatforclassAstructurestakes2~3%in100years,correspondingtoareturnperiodof3000to5000years.
Usuallythedesignreferenceperiodofoffshoreplatformsis30years,becausethedamageofplatformsmaycausegreateconomiclossandenvironmentpollution,itisappropriatethatthedeformationdesignleveltakes1%in30years,correspondingtoareturnperiodof3000,whichisalreadyhigherthanthatforclassAstructuresandapproximatestotheupperlimitinRP2A-WSD.
Accordingtotheanalysisabove,andtakingRP2A-WSDasareference,itissafeandrationaltotaketheearthquakeswithareturnperiodof200yearsand3000yearsforoffshoreplatformsinChinaasthestrengthanddeformationdesignlevelrespectively.
4.
ACOMPARISONWITHRELATEDCODES4.
1.
Acomparisontothecodesofoil/gasandotherchemicalstructuresOil/gasandotherchemicalstructuresarelargeandcomplexengineeringsystems,includingcomprehensivestation,pipeline,oilstorage,chemicalstructure,andsoon.
Incaseofearthquakedemolishment,thestructuraldamagewillcauseseriouseconomicallossandenvironmentalpollution,sotheseismicproblemsaretakenseriously.
Basedonthestructuralcharacteristicsandimportance,oil/gasandotherchemicalstructuresaredividedintothreeclasses:A,B,andC;theoffshoreplatformisaffirmedasclassAoftheutmostimportance.
Werecommendthattheseismicfortificationlevelofstrengthdesigntakesareturnperiodof200years,andthatThe14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China7ofdeformationdesigntakesareturnperiodof3000yearsrespectively,thereturnperiodisthelongestinoil/gasandchemicalindustry.
Wemayregardthattheseismicfortificationmeasuresforoffshoreplatformsarestricter,itsseismicriskissmaller.
Table1TheseismicfortificationlevelforoilandgasstructuresClassStructureReferenceperiod(a)Returnperiodforfrequentearthquakes(a)Returnperiodforrareintenseearthquakes(a)AOffshorePlatform302003000BLargecomprehensivestation,importantsectionofoil&gaspipeline,largeoilstructureandimportantchemicalstructure50100300030502500COilgasandotherchemicalstructuresbesidesClassA&B50502500303015004.
2.
ComparisonwithseismicfortificationlevelsofrelevantindustriesBecausetheseismicdesignideavariesindifferentindustrialfields,itisobviouslydifficulttocomprehensivelycomparethedifferencebetweentheirseismiclevels.
Therefore,onlythoseindustrialdepartments,whichhavethesameseismicdesignideaandsimilarseismicfortificationgoal,areselectedtocompare;hereweemphasizeacomparisonofthereturnperiodofrareearthquakebetweendifferentstructures.
Inordertorationallyreflectthedifferenceofseismicfortificationforstructureswithdifferentreferenceperiodandreturnperiod,weconvertreturnperiodtoprobabilityofexceedance(PT)inreferenceperiod,then(PT)canindicatetherelativeseismicriskinthereferenceperiod.
Table2isthedeformationdesignlevelofdifferentstructures.
Thedatashowsthatthereferenceperiodofoffshoreplatformsisrelativelyshort,theprobabilityofexceedanceofrareearthquaketakes0.
01inthereferenceperiod.
Itsseismiclevelisjustlowerthanthatfornuclearpowerstation,andmatchesuptothatfortheThreeGorgeDam.
Thereforeareturnperiodof3000yearsofrareearthquakeforoffshoreplatformissafe.
Table2ThedeformationdesignlevelofdifferentstructuresStructureReferenceperiod(a)RareearthquakeReturnperiod(a)PTOffshoreplatform3030000.
01Nuclearplant40100000.
004ThreeGorgeDam100100000.
01HydraulicstructuresofClassAreservingwater10050000.
02BuildingsandStructuresofClassA1003000~50000.
02~0.
03Large-scaleBridge10033000.
03The14thWorldConferenceonEarthquakeEngineeringOctober12-17,2008,Beijing,China8TelevisionTowerofClassA10020000.
05BuildingsandStructuresofClassB5025000.
02BuildingsandStructuresofClassC50~20000.
02~0.
03HydraulicStructureofClassAwithoutwater5010000.
055.
SUMMARYOnthebasisofseismicactivityintheseaareasofChina,structuralcharacteristicsofoffshoreplatform,seismicdesigngoal,andtheseismicfortificationexperiencesofrelatedengineering,werecommendthattheseismicfortificationlevelofstrengthdesigntakesareturnperiodof200years,andthatofdeformationdesigntakesareturnperiodof3000yearsforoffshoreplatformsinChina.
ThereturnperiodsrecommendinthispaperareonlyusedinthecomparisonbetweenChineserelatedCodes,hasnotbeenappliedinstructuralanalysisofoffshoreplatform,alsothereliabilityanalysisofthedesignhasnotbeendonefordifferenceplatformstructures.
Thereisstillalongwaytogobeforetheconstitutionofseismiccodeforoffshoreplatforms,andmanyresearchesshouldbedoneinfuture.
ReferenceLuYue-jun,PengYan-ju,TangRong-yu.
(2003).
Engineeringseismicproblemsoftheoffshoreoilplatforms.
ProgressinGeophysics18:4,662-665.
AmericanPetroleumInstitute,RP2A-WSD.
(2002).
Planning,Designing,andConstructingFixedOffshorePlatforms-WorkingStressDesign.
Houston:AmericanPetroleumInstitute.
ChinaStateBureauofQualityandTechnicalSupervision.
(2005).
ChinaStandardGB17741-2005.
StandardsPressofChina.
ChinaStateBureauofQualityandTechnicalSupervision.
(1998).
GB17503-1998.
StandardsPressofChina.
MinistryofConstructionP.
R.
China.
(2001).
GB50011-2001.
ChinaBuildingIndustryPress.
XieLiliZhangXiaozhiZhouYongnian.
(1996).
OntheDesignEarthquakeLevelforearthquakeResistantWorks.
EarthquakeEngineeringandEngineeringVibrationl6:1,1-18.
WangZhongchang.
(2005).
Thediscussiononsomeproblemsinanti-seismicanalysisforfixedplatform.
ChinaOffshoreOilandGas17:6,421-423PengYanju,LuYuejun,TangRongyu.
(2006).
ResearchontheseismicfortificationlevelforoffshoreplatforminBohaiSeaandadjacentareas,ActaSeismologicaSinica,27:6,647-655WeiWei.
(2004).
Researchonseismicdamagestatesofjacketoffshoreplatformstructures.
AdissertationofOceanUniversityofChinaforDoctoralDegreePengYanju,LuYuejun,TangRongru,etal.
(2007).
ResearchonseismicdesignparametersandearthquakeresistantlevelforanoilfieldplatforminBohaiSea.
Journalofearthquakeengineeringandengineeringvibration.
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