ANewMethodforReplacingCorrodedBottomPlatesofOilStorageTanksWataruTSUDA*1),AkiraISEDA*2)andKoichiYAMAZAKI*2)Thebottomplatesofoilstoragetankssometimesneedtobereplacedduetocorrosionorrivetted-jointleakage.Thisreplacementworkusuallyinvolvesliftingthetankwithhydraulicjacks,acostlyandtime-consumingexercise.NipponPetroleumRefiningCo.andNiigataConstructionCo.havejointlydevelopedanewnon-jackingmethodthatallowsthetankbottomplatestobereplacedbysupportingthetankwithsimplejigs.Engineeringanalysisandstraingaugemeasurementshaveprovedthatthemethoddoesnotgenerateunacceptablestresslevelsinthetankevenfromearthquakeandwindloadsduringthereplacementwork.Themethodhasbeensuccessfullyappliedtothebottomplatereplacementofmorethan260tanks,andwhencomparedwiththeconventionalJack-upmethod,ithasdemonstratedre-markablecostandtimesavings.1.IntroductionStrictregulationsconcerningtheacceptablebot-tomplatethicknessforlargeoilstoragetanksarenowenforcedthroughoutJapan.Theseregula-tions,whichresultedfromtheoilspillageaccidentataJapaneserefineryin1974,arerequiredbythelocalgovernmentagenciesforregularandsystematicinspectionofalltankbottomplates.TheJack-upMethodisconventionallyusedforbottomplatereplacementwork,involvingjackmountingattachmentstothetankandlocalizedfoundationreinforcementsunderthejacks.Thismethodislabor-intensive,itrequirescostlyhy-draulicjackingequipment,anditistimecon-suming.Asimplermethodreferredtoasthe"SupportPiecemethod"wasintroducedjointlybyNipponPetroleumRefiningCo.andNiigataConstructionCo.in1977 aftertwoyearsofdevelopments.OneoftheoutstandingmeritsoftheSupportPieceMethodisatypical30%reductioninbothrepaircosts andtimeconsumedwhencomparedwiththeconventionedJack-upMethod.Morethan260storagetanksofalltypesandsizes, includingtanksinthe100,000 kiloliterrange,havebeensuccessfullyrepairedbytheSupportPieceMethod.2.On-siteProcedure2.1ReinforcingtheShellAreinforcingringisnormallyinstalledaroundtheinnerortheoutercircumferenceoftheshellplates(Fig.1).ThispreventsanydistortionofFig.1InstallationofReinforcingRingReceivedJune8,1983. 1981TechnicalProgressAwordofJPI.*1) NipponPetroleumRefiningCo.,Ltd.(1-1,Otori-cho,Isogo-ku,Yokohama235)*2) NiigataConstructionCo.,Ltd.(4-1,2-chome,Shiba-koen,Minato-ku,Tokyo105) wJ.JapanPetrol.Inst., Vol.26,No.6,1983419theshellplatesfromresidualstresses,whichmayhaveaccumulatedduringconstructionandservice,andretainsthecorrectcircularprofileduringtherectificationwork.2.2MarkingOffAcuttinglineisnormallymarkedaminimumof1"(25mm)abovethebaseoftheshellplates.Thisdimensionisgovernedbythefollowing:-Workingspace forreplacingtheannularplates.-Weldingandinspectionofnewannularplatebuttjoints.-Removingtheexistingweldmetalfromtheshellandannularplatejoint.-Theextentofcorrosionatthebaseoftheshellplates.-Thepositionofreinforcementforexistingnoz-zles.Thefalsemarkerlinefortheautomaticgasoxygenflamecuttingequipmenttogetherwithatransientlinearemarkedatthesametime(Fig.2).Beforemarking,anypaint,rust,andoilclosetothemarkingareaarethoroughlyremoved.Theautomaticgascuttingmachineistheninstalledagainstoneofthemarkerlines,takingcaretoensurethestraightnessandangleofthecuttingplane.2.3CuttingtheShellPlatesAnopeningcut30 ft(9m)inlengthismadearoundtheshellplates(Fig.3).Allshellcuttingworkneedstobedoneaccuratelyandcarefullybecausethecutsurfacesbecomethenewjointfacesbetweentheshellandannularplates.2.4AnnularPlateReplacementandTempo-rarySupportsThefirstannularsegmentofthetankbottomplatecompletewiththeheeloftheshellplateisthencutoutandwithdrawnthroughthesideplateaperture,takingcarenottodamagetheshellplateorthetankfoundation.Areplacementan-nularplate,preciselycuttosizeintheshop,isnextmaneuveredthroughtheapertureandtackedintoposition.Fig.4showsthetemporarytanksupportmea-sureswhicharethentaken,startingwiththefirstsupportpiecesandshoes.Thesearepositionedatappropriateintervalstosuittheweightofthetank.Thesupportpiecesareweldedtothetankshellplatesandsupportedviashoesonthereplace-mentannularplate.Theseshoesprotectthenewannularplatesfromsubsequentgascuttingopera-tionsanddistributetheloadingstressesfromthesupportpiecesoverthenewannularplate.Guideplatesareinstalledtomaintainthecorrectshapeoftheshellplatesandtoguidethemwhenthetankislaterloweredontothenewannularplates.Theseguideplatesalsopreventanyhori-zontalmovementbywindforcewhenallthebot-tomplateannularsegmentshavebeenreplaced.Asafinalsafetyprecaution,wedgesareinsertedintothegapatregularintervals.Thisprocedureof:-cuttingoutandremovingtheoldbottomplateannularsegment-insertingandtackingthenewbottomplatesegmentintoposition-installingthesupportpieces,supportplates,Fig.2MarkingofCuttingLine,TransientMarkerLineandMarkerLinefortheAutomaticFlameCuttingEquipmentFig.3CutShellandAnnularPlateProgressivelyin3to9MetersIncrementFig.4SupportPiece,otherJigsandInitialWeldingofAnnularPlate wJ.JapanPetrol.Inst., Vol.26,No.6,1983420guideplatesandwedgesiscontinuedaroundthecircumferenceofthetankuntilallthebottomplateannularsegmentshavebeenreplaced.Thetankisthencompetelysup-portedonthenewannularplatesviathesupportpieces.2.5SettingdowntheTankBeforeloweringthetank,theannularplatesarebutt-weldedradiallyoveralengthofabout12"(300mm)inwardsfromtheoutercircumferenceandtheweldsurfaceisgroundflatasshowninFig.4.Thisgivesthefinishedsurfaceonwhichthetankshellplateswillsit.Amagneticparticleorliquidpenetrantexaminationisdoneonthissurface.Allthewedgesarethenremovedandcontrolledloweringofthetankontothenewannularplatesisachievedinsmallstagesbycutting3/8"(10mm)outofthesupportpiecesinaprogressivesequenceuntilthecut-backshellplatesarecompletelysup-portedonthereplacementannularplates.Wheninternal-roof-supportstructureexists,itslengthisalsoadjustedduringthesetting-downoperation.2.6WeldingAfterthetankhasbeenloweredontothenewannularplates,andthejigshavebeeninstalledforanyadjustmenttothecurvatureofthebottomshellplates,theT-jointbetweentheshellandannularplatesiswelded.Thisweldingoperationalsonarrowstheunweldedradialgapbetweenadjacentannularplatesandsometimescancausecrackingtotheexistingweldbeadend.So,beforecompletingthebuttweldbetweenadjacentannularplates,remakingofthegrooveandinspectionoftheexistingweldbeadendareessential.Thefinalweldingoperationisthejointbetweentheannularplatesandthebottomplates.Fig.5showstheorderoftheseindividualweldingoperations.2.7FinishingAftertheweldingworkhasbeencompleted,allthesupportpieces,guideplates,andjigsarefinallyremovedandtheirtemporaryweldmarksarefin-ishedflatbygrinding.2.8TestingThenewbottomistestedinaccordancewiththetestingmethodsspecifiedinAPI.Std.650,5.3.Additionally,amagneticparticleorliquidpenetrantexaminationisconducted.3.EngineeringAnalysisAlltanksarecheckedbyengineeringanalysisbeforestartingthework.Asanexampleofthisanalysis,atankwiththefollowingspecificationswasusedforbothcalculationsandfieldmeasure-ments:1.TankDutya.Content:Crudeb.Type:F.R.T.C.Capacity:96,00kld.Outside diameter:91,135mme.Height:15,846mm2.TankWeightShell690.8 tonAccessories136.2tonTotalW=827.0tonThestressinducedintheshellplateduringthisoperationmustbelowerthanthestressesinthesupportpiecesandmustalsobeatanacceptableleveltocomplywiththelegalsafetystandardsandcodesofpractice.3.1Calculations3.1.1EarthquakeLoad(horizontal)1),2)Ps=kEWPs:earthquakeload=82.7tonk:earthquakefactor=0.1W:tankweight=827ton3.1.2WindLoad(horizontal)3),4)Pw=CEqEAcc(1)A=hEDcc(2)q=1/2EEV02(h1/h0)1/2cc(3)Pw:windload=315ton Fig.5ProcedureofWeldingWork wJ.JapanPetrol.Inst., Vol.26,No.6,1983421C:windfactor=1.0A:projectedwindarea=1,444m2h:tankheight=15.846mD:tankdiameter=91.135mq:airpressure=218kg/m2:airdensity=0.115kgEsec2/m4V0:designwindspeed=60m/sech0:const.height=15mh1:heightfromground=16.346m3.1.3SlidingResistanceRw=WE Rw:slidingresistance=413.5tonW:tankweight=827ton:coefficientoffriction=0.59)WhenRwPsorPw,thetankissafefromhorizontalsliding.(Ifopposite,actionmustbetakentoincreaseRw.)3.1.4OverturningResistanceMr=WED/2cc(1)M0=Rs(Pw)EHcc(2)H=1/2hcc(3)Mr:overturningmomentresistance=37,684tonEmM0:overturningmoment=2,496tonEmH:heightabovegroundofcenterofgravity=7.923mWhenMrM0,thetankissafefromoverturning.(Ifopposite,actionmustbetakentoincreaseMr.)3.1.5SupportPieces(1)LoadPsu=W/N+M0/Zcc(1)Z=NE /Zcc(2)Psu:supportpieceload=3.31ton/pieceN:numberofsupportpieces=284M0:overturningmoment=2,496tonEmZ:modulusofsupportpiecesection=6,471m:tankradius=45.57m(2)Bucklingload5)(1)K=t/12cc(2)Pc/Psu=3.3>1.5ismaintainedPc:max.compressiveload=10.8tonA:supportpiececross-sectionalarea=6.4cm2f:compressivestrength=3,400ton/cm2n:constant(safetyfactor)=1/4a:Rankinefactor=1/7,500l:supportpiecelength=20.0cmK:firstmomentofarea=0.46cmt:supportpiecethickness=1.6cm(3)Filletweldjointstrengthbetweenshellandsupportpiece6).F=Psu/2ECEl02E36L2+l02cc(1)C=S/2cc(2)F:combinedlongitudinal,bendingandshearstress=0.78ton/cm2C:throatoffilletweld=0.56cmS:filletweldsize=0.8cm: weldefficiency=0.857)L:upperwidthofsupportpiece=5.0cml0: lengthoffilletweld=12.0cmp:permissibleshearstress=950kg/cm2(ASTMA570Gr.33)8)WhenF