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第21卷第4期2006年8月实验力学JOURNALOFEXPERIMENTALMECHANICSVol.21No.4Aug.2006文章编号:100124888(2006)0420439208ADynamicLoadingDeviceforSuctionFoundationsinCentrifugeModeling3WANGShu2yun1,ZHANGJian2hong2,LUXiao2bing331,JIAOBin2tian1(1.InstituteofMechanics,ChinaAcademyofScience,Beijing100080,China；2.DepartmentofHydraulicEngineering,TsinghuaUniversity,Beijing100084,China)Abstract:Suctionbucketfoundationsarewidelyusedintheoffshoreplatformfortheexploitationoftheoffshorepetroleumandnaturalgasresources.Duringwinterseasons,icesheetsformedinBohaiBaywillimposestrongimpactandresultinstrongvibrationontheplatform.Thispaperdescribesadynamicloadingdevicedevelopedonthegeotechnicalcentrifugeanditsapplicationinmodelingsuctionbucketfoundationundertheequivalentice2inducedvibrationloadings.Someexperimentalresultsarepresented.Itisshownthatwhentheloadingamplitudeisoveracriticalvalue,thesandattheupperpartaroundthebucketsoftensorevenliquefies.Theexcessporepressuredecreasesfromtheupperparttothelowerpartofthesandfoundationinverticaldirectionwhiledecreasesfromneartofarawayfromthebucketssidewallinthehorizontaldirection.Largesettlementsofthebucketandthesandaroundthebucketoccurunderthehorizontaldynamicloading.Thedynamicresponsesofthebucketwithsmallersizeareheavier.Keywords:suctionbucketfoundation；dynamicloadingdevice；ice2inducedvibration；centrifugemodeling0IntroductionInrecentyears,suctionbucketfoundationshavebeenappliedincreasinglyofteninoffshoreengineering(Clukeyetal1,1995；Allersmaetal2,3,1997,2000).Thefirstadvantageofsuctionbucketfoundationsareattractivebecauseoftheconvenientmethodofinstallationandrepeatedlyuse.Foranexample,asuctionbucketfoundationwithadiameterof9mandaheightof10mcanbeinstalledin13hours,byusingonlyapump.Thesecondadvantageisthatitmaymobilizeasignificantamountofpassivesuctionduringuplift.Despiteseveralstudiesontheinstallationandstaticbearingcapacityhavebeencarriedout,thedetailresponsesofsuctionbucketfoundationsunderdynamicloadingshaveremainedunknown(Senpereetal4,1982；Aasetal5,1992；Dymeetal6,1998).Thedynamicloadingconditionissignificantwhensuctionbucketsareusedasthefoundationofaplatform.Waveloadingandice2induceddynamicloadingcausethefoundationtobesubjectedtocyclicloadings(Tjeltaetal7,1990；Byeetal8,1995).Thelackofexperiencesundertheseloading333收稿日期:2005208224；修订日期:2006207210基金项目:ThisprogramissupportedbythefundofChineseOceanOilCo.andChineseAcademyofSciencesKJCX22SW2L03201(40025103)andNationalNaturalScienceFund(No.10202024)通讯作者:鲁晓兵(1968-),男,博士,现在中科院力学所从事岩土力学研究。E2mail:conditionsledtoaproposalforatestprogramintendedtogainadeeperunderstanding.Theconsiderableexpenseandtimeconsumingnatureofprototypetestsmeanthattheinvestigationofthebearingcapacityofrealscaledevicesunderdifferentcircumstancesisoflimitedpracticality.Itismucheasiertochangeparametersinsmallscaletests.Butproblemsariseconcerningthestress2dependentbehaviorofsoilthatthemeasuredloadingsaresolowthatmeasurementsarenotsufficientlyaccuratetovisualizedifferencesindesign.Theserestrictionscanbeovercomeifperformingthetestsinageotechnicalcentrifuge.Inacentrifugethesoilstressesoverasimilardeptharethesameasintheprototypesituation.Foranexample,at100g(100timestheEarthsgravity),asuctionfoundationwithadiameter9mandaheightof9mmaybesimulatedwithafoundationwithadiameterof0.09mandaheightof0.09m.Thescalingfactoroftheprototypetothemodelis1001.Theice2induceddynamicloadingisthecontrollingloadinginBohaiBayinChina.Thistypeofhorizontaldynamicloadingistranslatedtothesoillayerbyplatformandcausestheparameters,e.g.strengthandmodulusofthesoillayer,todegrade.Asaresult,thebearingcapacityofbucketfoundationsdecreases.Therefore,itisimportantandnecessarytoclarifythedynamicbehaviorofbucketfoundationsunderthistypeofloadingsinordertoprovidepracticaldesignmethodandparameters(Dingetal9,2003；Luetal10,2003).Inthispaper,wefirstintroducedasetofdynamicloadingdevicesdesignedforsimulatingthehorizontaldynamicloadings.Thensomeexperimentalresultsoftheice2inducedcyclicbehaviorofsuctionbucketfoundationsarepresented.Theeffectsoftheloadingamplitude,thesizeofthebucketandthestructuralweightaremainlyinvestigated.Fig.1Theelectro2magneticactuator1DynamicloadingdeviceThedynamicloadingdeviceiscomposedprimarilyofanelectro2magneticactuatorasshowninFig.1.Thisactuatorconsistsofapermanentmagnet,acylindricalsilvercoil,twocopperdisksprings,aloadcellandasteelrodconnectedtothecoil.Therearetwomaintechnicalchallengesassociatedwiththedesignoftheactuator.(1)Thecylindricalcoilissuspendedinaprefabricatedspaceinthepermanentmagnet.Thespaceisverytinyinordertoavoidmagneticloss.However,sincethiscylindricalcoilmoveshorizontally,itisorthogonaltothegravitationalforce,thedeformationofthecoilunderthisgravitymaylockitinthistinyspace.Inordertoavoidthissituationalinearbearingandtwodiskspringsareinstalledtomakethecoilmovelinearlywithoutbeinglocked.Thistechniquehasbeendemonstratedtobeveryeffective.(2)Theweightofthecoilshouldbeminimizedtoincreaseitsdeformationinthedirectionofloading.Silverwireisusedtoformthecylindricalcoilsoastoenhancetheperformanceandlimittheweightwithin0.5kg.Thetotalweight044实验力学(2006年)第21卷ofthedeviceis14kg.Consequently,thevibrationofthe0.5kgcoilwouldnotimposesignificantimpactonthestrongboxwhoseweightis150kg.Thecoilmovesbackandforthtoapplycyclicloadingonthemodelbucketthrougharigidrod.Fig.2isthecontrollingsystemfortheloadingdevice.Theloadingfunctionisfedfromacomputersignalgeneratorintoaservoamplifier.Thelatterinturngeneratesthenecessaryelectricalsignalstocontroltheintensityofelectriccurrentintheactuator.Theoutputfromtheactuatorintheformofloadingiscontinuouslymonitoredbytheloadingcellandfedbackthroughachargedamplifiertotheoscillograph.Outputsignalfromporepressuretransducer(PPT)andlinearvariabledisplacementtransformers(LVDT)werepassedacrossthecentrifugeslip2ringandconvertedtodigitaldata.Withthisdevice,differentamplitudesandthefrequenciesofhorizontalcyclicforcesarereproducedatcentrifugalaccelerationof80g.Therefore,thescalingfactoroftheloadfrequencyoftheprototypetothemodelis180,andthescalingfactoroftheloadamplitudeoftheprototypetothemodelis64001.Inthecentrifuge,thepeakcyclicloadingof100Nand64Hzrepresentsaprototypeloadingof640KNand0.8Hz.Table1showsthetechnicalspecificationsofthisactuator.Theforce2electricityrelationat80gisshowninFig.3.Tab.1TechnicalspecificationoftheelectromagneticactuatorDiameter(mm)Length(mm)Weight(kg)Frequencyrange(Hz)Peakcyclicload(N)1802601411201002CentrifugesystemconfigurationFig.4showstheconfigurationofthecentrifugeandthemodel.Theinternalareaofthealuminumstrongboxmeasures600mm350mmandheightis350mm.Thethicknessofthesiltysandis210mm,underlainby20mmcoarsesandwhichistopreventthepipingduringpenetrationofwater.Thereis20mmhighwaterabovethesoilsurface.Theactuatorissupportedbytwosteelbeamsfastenedontheflange.TworodsfixthebackoftheactuatoragainstthesideofthestrongboxasshowninFig.4.AminiatureTVcamerawasattachedtotheflangeofthemodelcontainertomonitorthemotionofthemodelbucket.Themodelbucketisasteelcylinderwhichhasanexternaldiameterof60mm,aninternaldiameterof56mm,andaheightof72mm.Thedeadweightoftheplatformwassimulatedbyaballastmountedonthetopoftheshaft.ThemovementofthebucketwasmonitoredbytwoLVDTs.Whenthecentrifugemodelistestedunder80g,itsbehaviorwouldprimarilycorrespondtothatofaprototypefootingwithadiameterof4.8m.144第4期WANGShu2yunetal:ADynamicLoadingDeviceforSuctionFoundationsinCentrifugeModelingFig.4Thelayoutofthemodel3PreparationoftheexperimentsTheexperimentsareperformedinthe50g2tonscentrifugeinTsinghuaUniversity.Themaximumofthecentrifugesaccelerationis200g.Thepayloadis250kgunder200g.Theexperimentalmaterialisfinesandwhosespecialgravityis2.69andtheaveragegrainsdiameteris0.14mm.Thepermeabilityis510-3mm/s.Threemodelbucketsmadebysteelareusedinexperiments.Theinnerdepths(donotincludethetopcapthickness)are48mm,72mmand90mm,respectively,theinnerdiametersareall60mm,thethicknessesofthewallandthetopcapareboth2mm.Afinepipewithanouterdiameterof10mm,aninnerdiameterof8mmandaheightof100mmisweldedatthecenterofthetopcap.Aslidinggroovepermittingaballmovingupanddowntoadaptthesettlementofthebucketisconnectedwiththepipeclosely.Oneendofasteelpoleisjointedwithaball,theotherendofthepoleisconnectedwiththeelectromagneticvibrationactuator.Thusthepolecannotbedraggedwhenthebucketsettles.OneLVDTislocatedatthetopofthefinepipetomeasuretheverticaldisplacement.OneLVDTislocatedatthefinepipeatthesamelevelasthatofthesteelpoletomeasurethehorizontaldisplacement.TheothertwoLVDTsarelocatedonthesandsurfacetomeasurethesettlementsofthesandsurfaceatdifferentpositions.TenPPTsmadeinDruckCo.(English),areburiedinthesandaroundandinthebucket(Fig.5).ThePPTsarenotfixedbutsuspendedinthesand.Inordertoguaranteetherepeatability,210mmthickmodelsandlayerisdividedintofivelayerstocreateandbecompactedgentlybyhandstriking.Thi