Chapter 8 - Deepwater Structural Design.docx

CHAPTER8DeepwaterStructura1.DesignDEEPWATERSTRUCTURA1.STRINGDESIGNIntroductionFunctionDeepwaterstructura1.designformsthefoundationofthewe1.1.boredictatedbyvarious1.oadspressuresandconditionsthatdictatethewe1.1.,sconfiguration.Thestandarddesignofwe1.1.boretubu1.arsusedand1.oadstobeapp1.ied,e.g.zburst,co1.1.apse,axia1.andcombinedoperatingandserviceandwearandtear1.oads,remainsthesame.Structura1.designfordeepwaterwe1.1.smustaccountfora1.1.dri1.1.ingandproduction1.ifecyc1.e1.oadsimposedinstaticandcyc1.icmodes.Operationa1.1.oadsdrivingdri1.1.ingstructura1.casingdesignare1.atera1.(shear)force,axia1.1.oad,andbendingmoments.Productionwe1.1.andfie1.d1.ifecyc1.e1.oadsthenneedtobedeterminedtoconc1.udethestressconcentrationfactorsandspectra1.1.oadsapp1.iedtoeva1.uatefatigue1.ifeandotheraspectsofthewe1.1.'soperatingsystems,e.g.zear,annu1.arpressurevo1.ume,temperatureeffectsasappropriate.DesignMethodDeepwaterstandardsrecommendasimp1.edesignpracticemethodasout1.inedinAPIRP2A-WSD,22ndEd.,Nov2014,forstructura1.pi1.esasusedinoffshorep1.atformdesign.Thisapproachana1.yzesstructura1.stringsbyassumingapointoffixitybe1.owthemud1.inetoca1.cu1.ateabendingmoment,assumingnosoi1.interactionwiththetubu1.arsinsta1.1.ed.Innewmethodsused,thestructura1.stringdesignistreatedasabeamco1.umnwiththesoi1.rep1.acedbyaseriesofnon1.inearspring-typemechanisms.Theaxia1.andtensiona1.stressesaredeterminedbasedontheoperationa1.1.oads,e.g.zshear,moment,andaxia1.imposedatthef1.exjointofthesubseaBOPsystem.Thesystemisgenera1.1.ymode1.edasapi1.ewithID,equiva1.entODzheightsandbuoyantweights,usingthepi1.eana1.ysissoftwareprogram.Thesoi1.pipeinteractionisdeterminedusingspecificorarangeofP-Ydeterminedandeva1.uatedsoi1.dataasout1.inedinthischapter.Deepwaterstructura1.designvariesindifferencefromconventiona1.sha1.1.owoffshorewe1.1.ssincestress1.eve1.saresignificant1.ygreater;evenwiththesamerigtype.A1.1.structura1.designsmustdetermineandeva1.uatea1.1.ca1.cu1.atedcritica1.1.oadsimpartedbythemetocean,operationa1.andsituationa1.effects,tomarineriser,subseaBOEwe1.1.head,ponents.Ivpuuc<Dttghttpsd0iar1.l 016B97S4)X)4 02282*5.000089285»2019 Ekcvicr 1.ed. All rights reserved.Structura1.DesignVerificationTheaxia1.andbendingstressesareca1.cu1.atedincrementa1.1.ya1.ongthe1.engthofthepi1.e.Thestressshou1.dnotexceedtheyie1.dstrengthofthepipeandtheunitcheck,thesumofaxia1.andbendingstresses,shou1.dbeoneor1.essasshowninEq.(8.1)andanexamp1.easshown.Note:Thisismorerigorousthanconsideringbendingstressinastructura1.string.APIANDA1.SCSPECIFICATIONFORTHEDESIGN,FABRICATION,ANDERECTIONOFSTRUCTURA1.STEE1.FORBUI1.DINGSJ+J"、1.0(8.(IFJ(E*Fjwhere： fa=computedaxia1.stress,psi亦=computedbendingstress,psi Fy=yie1.dstressofstee1.zpsi Fs=factorofsafety;recommenda0.8foraxia1.tensionand0.9forbending.Examp1.eca1.cu1.ationofAPIandAISCSpecificationfortheDesign,Fabrication,andErectionofStructura1.Stee1.forBui1.dingsasstatedinEq.(8.1)isi1.1.ustratedbe1.owtorepresentadeepwaterwe1.1.'smaximum1.oadconditions.DataInputfa=computedaxia1.stress,psifb=computedbendingstress,psiFy=yie1.dstressofstee1.,psi-r¼-+-1.0J+&Eb+£Factorofsafety,recommendedFSe1.=O8Fs=0.9 0.8foraxia1.tension(Ei(J) 0.9forbendingmoment(Fs,)M:=5500000-ft-IbfA1.：=600000()IbfM=Maximumbendingmoment,Ibf*ftA1.=Maximumaxia1.1.oadingIbfOd=Externa1.diameteroftubu1.ar,inchesCW；=36ind=33inId=Interna1.diameterOftubu1.ar,inchesMaximum1.oadconditionsexamp1.eStep#1:Ca1.cu1.atethe2ndmomentOfinertiaofthetubu1.ar,I,in4/=24234.197in4Step#2：Ca1.cu1.atethecross-sectiona1.areaofthetubu1.ar,CSa,ft2Csa.=-d2-Id2)Cw=1.129ft2Step#3:Ca1.cu1.ateaxia1.stress/,psi.fA1.1.=36905psiStep#4：Ca1.cu1.atebendingstress/,psi.rMOd1.=i=98043psiStep#5:Ca1.cu1.ate>mbinedstressonthetubu1.ar(Ss),Ss=0.92withsafetyfactorsapp1.iedtoensurethatSiis1.essthan1.0.08,+X800.9+X80DEEPWATERSTRUCTURA1.STRINGDESIGN287Structura1.DesignAna1.ysisInsummary;structura1.stringdesignmustconsiderthepenetrationbe1.owthemud1.ine,thetypeofsoi1.,thepipewa1.1.thickness,diameter,1.ength,materia1.strengthandinsta1.1.ationmethod.Ana1.ysisperformedmustdetermineandeva1.uatethenon1.inearbehaviorofthesoi1.s1.oads,strengths,andresu1.ting1.oaddef1.ectionsofthestructura1.pipe.Structura1.DesignCapacityStructura1.designcapacityisdesignedtomaintainmateria1.stress1.eve1.swithina1.1.owab1.e1.imitsassubjectedbyprimarybendingandaxia1.1.oadsimposedduringwe1.1.1.ifecyc1.eoperationsasi1.1.ustratedinFig.8.1andTab1.e8.1.Note:Fatigue1.ifeandana1.ysisaremorecomp1.exissuestobeunderstoodandappreciated.Theseaspectsandarenotaddressedindetai1.withinthischapterandguide.Axia1.1.oadsFxjoin- anghe)Riserbottom tension1.ateralforce Bending momentsvirtua1.fixity"fopwe1.lheadAxia1.1.oadingofstructura1.pipeistheresu1.tofvertica1.1.oadingatthef1.exjoint,buoyedsubseaBOPweight,buoyedwe1.1.head,andcasingweight.FIG.8.1Bending1.oadsonadeepwaterwe1.1.Source:KingdomDri1.1.ing(2018).TAB1.E8.1KeyFactorsInf1.uencingDeepwaterAxia1.Bendingand1.oading.FactorsInf1.uencingBendingandAxia1.1.oadingBending1.atera1.1.oadsatf1.exjointduetoriser1.oadsWe1.1.headandBOPstickupabovethemud1.ine(inc1.udingsubseatree)Soi1.strengthbe1.owthemud1.ineBOPandwe1.1.headang1.e(wi1.1.effectvertica1.1.oadingandbending)Production1.oads(fatigueandsnag1.oads)Axia1.1.oadingVertica1.1.oadingatf1.exjointduetoriser1.oadsBOPweight(buoyed)We1.1.headandcasingweight(buoyed)Casing(s),Subseatree,CappingstackweightsSource:KingdomDri1.1.ingTraining2006.Bending1.oadsBendingistheresu1.tof1.atera1.1.oadingatthef1.exjointduetoriser1.oads,we1.1.head,andBOPstickupabovethemud1.ine,soi1.strengthbe1.owthemud1.ine,weightoftheBOBandang1.eoftheBOPandwe1.1.head.Note:Bendingstrengthcontributionbytheinnerstringisusua1.1.yre1.ative1.ysma1.1.Bending1.oadsapp1.iedareafunctionoftheconditionsimposedbytheheightOfthe1.owermarineriserpackage(1.MRP)zf1.exjointabovethemud1.ine,risertension,andmaximumang1.eofthef1.exjoint.Thedepthbe1.owthemud1.ineatwhichcombinedstressisatamaximumisusua1.1.y1.ocatedatthemaximumbendingmoment,Ref.Fig.8.1.1.atera1.1.atera1.1.oadingistheresu1.tofbendingatthef1.exjoint.Intherea1.wor1.dascomponentpartsarenottru1.yvertica1.,vertica1.(axia1.)andhorizonta1.(1.atera1.orshear)forcecomponentssha1.1.existatthetopofthesubseaBORNote:Risertensionwithrigoffsetisthemaincauseof1.atera1.orhorizonta1.1.oadingofthestructura1.casings.Contributingfactorsaredeepwater,severeweather,currents,andpoorstationkeepingand/orrisermanagement.1.atera1.1.oadsareobtainedviatheriserana1.ysisandbasedontheoperatingconditionsandspecificsofeachvesse1.MarineRiserBending1.oadsBendingresu1.tsfrom1.oadsimposedbytherigandthemarinerisersystem.Inviewofthis,ariserandstructura1.stringdesignana1.ysissha1.1.considerthefo1.1.owingfactors:Metoceanconditions,vesse1.offsets,BOPstack,andmudeight.Aspectsthataffectstructura1.bending1.oadsinc1.ude:1. Bendingmomentandshearforceincreasebe1.owthemud1.ine,reachingamaximumatadistanceof5to30pipediametersbe1.owthemud1.ine,dependingonsoi1.strengths.Thebendingmomentandshearforcedecreasebe1.owthemaximumpointandreducetozeroatdepth(Fig.8.1).2. Thehigherthewe1.1.headhousingsstickupabovetheseaf1.ooraffectsf1.exjointheighttoresu1.tinhigherbendingmomentsthere.3. We1.1.heads,structura1.casing,conductor;pipebodyandcasingconnectorsmustbedesignedtoaccommodatebendingmomentsandshearforcesthatexist.4. We1.1.headandstructura1.casingdef1.ectionwhenthedri1.1.ingriserisremoved.5. Addedbending1.oadsthatcanresu1.tifacappingstackisinsta1.1.ed.Note:Thismaybethehighestfreestanding1.oadtoconsider.6. Themaximummudweightexpectedinthewe1.1.asthisaffectsrisertension.7. Thetypeofrig-positioningsystem,i.e.zdynamica1.1.ypositionedormooredandwaterdepth,asthesebothaffectthemaximumang1.eofthef1.exjoint.8. Anydeepcurrentsnearthemud1.inethatcanincreasebendingmoment1.oads.9. Soi1.typesandeffectsonwe1.1.headdef1.ectionandbendingmoment1.oads.Notes:Themaximum1.oadscenarioisgenera1.1.yafai1.uretodisconnectthe1.owermarineriserpackage(1.MRP)withthe1.ossofstationkeeping,e.g.,drive-offunderpower,ordrift-offunderenvironmenta1.1.oads.Inviewofthis,ariserfai1.ureana1.ysis(a1.soknownasaweakpointana1.ysis)isconductedbybothoperatoranddri1.1.ingcontractortoassesspotentia1.we1.1.integrityrisks,operatingfai1.ure1.oss,andpotentia1.opportunitiestomitigatemaximum1.oads.We1.1.head,BOPStickUp,andInc1.inationTheheightfromtheseaf1.oortothef1.exjointasstatedimpactsbendingimpartedtothestructura1.conductorandcasing(s).STICKUPThetota1.heightofthehigh-pressurewe1.1.headhousingstick-updesignabovetheseaf1.oorforastanda1.onewe1.1.isas1.oasoperationa1.1.ypracticab1.e.Idea1.1.y±(10-12ft)3-3.7m,±(18in)0.5mtoaccommodateROVandwe1.1.-1.ife-cyc1.eworkrequired.WE1.1.HEADANDSUBSEAINC1.INATIONDespiteAPIRP16Qestab1.ishingadri1.1.ing1.imitof2degreesforthef1.ex-jointsituatedabovethesubseaBe)Hbest-operatingpracticefina1.inc1.inationisdesiredat1.essthan<0.5-1.0degreesoffvertica1.,tominimizeoperationa1.wearand/ordamagerisktowe1.1.head,subseaBc)Handmarinerisercomponentsresu1.tingfromnotab1.ydri1.1.piperotation.Note:A1.1.otherdesign1.oadsimposedonthestructura1.pipeduetoinc1.inationmustbeconsidered.Soi1.StrengthsBe1.owtheSeaf1.oorSoi1.soffer1.essresistanceto1.atera1.def1.ectionthatmayresu1.tinbendingthestructura1.pipe.Undera1.atera1.1.oad,soi1.s,especia1.1.yc1.ays,behaveasap1.asticmateria1.wherethereis1.itt1.ere1.ationshipofpipe1.atera1.deformationandsoi1.resistance.1.atera1.soi1.resistance-def1.ectionp-ycurvesasi1.1.ustratedinfiguresshownareusedinthestructura1.designmode1.topredictthebehaviorofpipeundervarious1.oadconditionsandtypeofsoi1.s.Genericsoi1.,1.ow,mid,andupperrangestrengths,sha1.1.bedeve1.opedforeachdeepwaterregion.MODE1.INGSOI1.RESISTANCETO1.ATERA1.1.OADSANDBENDINGMode1.ingsoi1.andpipeinteractionisimportantindesigningstructura1.casings.Thethreefactorsmostinuencingp-ycurvesaresoi1.properties,pipediameter,andnatureof1.oading.2908.DEEPWATERSTRUCTURA1.DESIGNSevera1.methodshavebeenpub1.ishedintechnica1.1.iteraturebymanyauthorstoana1.yzepi1.es(pipe)1.oadedby1.atera1.forcetoprovetheversati1.ityandapp1.icationtheorythatfinitedifferencemethodscanbeusedtoeva1.uatehigh1.ynon1.inearsoi1.-pi1.einteractions.Asaconsiderab1.evo1.umeofsoi1.isremovedduringjetting,the1.ossofsurroundingsoi1.ifexcessive1.yjettedcanhaveasignificanteffectonthesoi1.response.Best-practice,1.oca1.know1.edge,andexperiencemustapp1.yandp1.aysanimportantpart.Thep-ymethodisapp1.iedthroughproprietarysoftwaremode1.ingbyspecia1.istcompaniestoaccurate1.ymode1.casingbendingandsoi1.interfaceissues.Note:Maxitnum1.oadbendingresu1.tsatapredefineddepthbe1.owtheseaf1.oorasshninFig.8.2.Thephysica1.defectionofsoi1.resistancePjisshowninFig.8.3C.Thisrepresentsaninsta1.1.edpi1.e(thatassumesnobendingsothatsoi1.stressesanddepthXiareuniform1.ydistributed)andathins1.iceofsoi1.atsomedepth%(Fig.8.3A)be1.owthemud1.ine.Ifthepi1.eis1.oaded1.atera1.1.ywithadef1.ectiony1atdepthx1.zthesoi1.stressescou1.dyie1.dthesoi1.resistancewithincreasingdef1.ectiontoapointwherepwou1.dreachanevidentand1.imitingva1.ue(Fig.8.3BandC).Note:Soi1.strengthsvarywithdepth.Inviewofnon1.inearities,numerica1.methodsareuti1.izedtoobtainsuitab1.eresu1.ts.Thepi1.eissubdividedintoaseriesofincrements(Fig.8.4)withequationsformedwithso1.utionstoproceedasi1.1.ustratedinFig.8.5.Fig.8.5Ashowsapi1.esubjectto1.atera1.1.oad.(A)二/1/4fXv1wIWv(B)Fig.8.5Bpresentsafami1.yofp-ycurves.Thedashed1.inei1.1.ustratesthedef1.ectionofthepi1.eeitherassumedorcomputedbasedonestimatedsoi1.strength.FIG.8.2Soi1.pipeinteractioni1.1.ustrationusingarangeofspecificP-Ysoi1.data.Source:KingdomDri1.1.ingmodifiedDeepitfdtercasingdesigntnatu1.,2014.(C)FIG.8.3 Definition of p & y as re1.ated to the response of an installed pipe to lateral loading. Source: Kingdom Drilling nodied Deepiwter casing design manual, 2014.(B)Fig.8.5Cshowstheupperp-ycurveen1.argedwiththepi1.edef1.ectionatthatdepthrepresentedbythevertica1.dashed1.ine.Fig.8.5Dshowstheva1.ueOfESp1.ottedasafunctionofx.Afterdef1.ectioniscomputeddifferentequationsarecomputedtoeva1.uaterotation,bendingmoment,Sheatandsoi1.reactionsasafunctionofx.Insummaryifp-ycurvesareavai1.ab1.e,acomputedso1.utiontoagivendeepwaterstructura1.insta1.1.ationprob1.emisobtainedwith1.itt1.edifficu1.ty.1.oad-BearingCapacityofC1.ayStudiesconductedintheUSGu1.fofMexicobytheNavyderived1.oad-bearingcapacityinstandardc1.ayashavingstrengthsof60-80psf(1.bspersquarefoot)attheseaf1.oor.Supportcapacitythenincreaseseach1.inearfootofpenetrationasperboundedconditionsshowninFig.8.6.FIG.8.4Representationofadef1.ectedpi1.e.Source:KingdomDri1.1.ingmodifiedDeqnwitercasingdesignmanua1.,2014.FIG.8.5I1.1.ustratedrepresentationofso1.vingforresponseofa1.atera1.1.y1.oadedpi1.e.Source:KingdomDri1.1.ingmodifiedDeepwatercasingdesignmanua1.,2014.FIG.8.6 Typica1. Gulf of Mexico soil shear strengths. Source: Kingdom Drilling training 2018.Note:Deeperwaterdepthscanresu1.tinweakerand1.essorstrongerandgreatersupportcapabi1.itiestobeaccountedwithineachindividua1.designeva1.uation.Insta1.1.ationMethods-DesignImpactInsta1.1.ationMethodsAsinsta1.1.ationmethodsimpacttheaxia1.capacityofa1.1.structura1.strings,themostcommondeepwatermethodisjettingaconductorstring,penetratingthroughsoftc1.aysedimentsbe1.owtheseaf1.oortothesha1.1.owdepthsettingsrequired.Ifhardground,g1.acia1.ti1.1.,debris,grave1.beds,ornonc1.aypartic1.egrainsizesarepresent,structura1.stringsareinsta1.1.edinastandardnorm,i.e.zthroughdri1.1.ingawe1.1.bore,runningthestringandgrouting(cementing)itfromsetdepthtotheseaf1.oor.Ofa1.1.methodsuti1.ized,jettingcausesthegreatestdegradationinaxia1.capacity.Ifthestructura1.pipeisnotdesignedorisnotinsta1.1.edproper1.y,thepotentia1.forthestructuretos1.umpandfai1.underaxia1.andbending1.oadsisgreat1.yincreasedasmanyevidentdocumentedcaseshaveproved.Axia1.capacityisconsequent1.yhigh1.ydependentonthesoi1.strength,type,interva1.1.ength,soi1.constituentnature,thedisturbanceofeachsoi1.interva1.,andbestpracticesusedaseachstringisjettedtodepthrequired.Soi1.disturbancedependsontherateofjetting(pumping),thedegreetowhichtheconnector'sODexceedsthecasingOD,theamountofreciprocationsconducted,andthetimea1.1.owedforthesoi1.torecoverfromthejettingoperation.Note:F1.ushconnectorscangreat1.yimproveskinfrictiondeve1.opmentdueto1.essgeometrica1.disturbance(aspipediameterisf1.ushwithnoprotrusionstodisturbthesoi1.).Thejettedpipedesigncriteriaonceatdepthmustinitia1.1.ysupportitsownbuoyantweightandinparttheweightoftheinnerdri1.1.stringbottomho1.eassemb1.yunti1.itisdeemedoperationa1.1.ysafetobere1.eased.Note:Mudmats(hydratedef1.ector)ponentpartoftheaxia1.supportWeightrequiredasi1.1.ustratedintheexamp1.easshowntoserve1.itt1.epurpose.Theevidentpurposeofmudmatsorhydratedef1.ectorsusedinmanycasestobequestionedastowhatex