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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10012488820060420439208ADynamicLoadingDeviceforSuctionFoundationsinCentrifugeModeling3WANGShu2yun1,ZHANGJian2hong2,LUXiao2bing331,JIAOBin2tian11.InstituteofMechanics,ChinaAcademyofScience,Beijing100080,China2.DepartmentofHydraulicEngineering,TsinghuaUniversity,Beijing100084,ChinaAbstractSuctionbucketfoundationsarewidelyusedintheoffshoreplatformfortheexploitationoftheoffshorepetroleumandnaturalgasresources.Duringwinterseasons,icesheetsformedinBohaiBaywillimposestrongimpactandresultinstrongvibrationontheplatform.Thispaperdescribesadynamicloadingdevicedevelopedonthegeotechnicalcentrifugeanditsapplicationinmodelingsuctionbucketfoundationundertheequivalentice2inducedvibrationloadings.Someexperimentalresultsarepresented.Itisshownthatwhentheloadingamplitudeisoveracriticalvalue,thesandattheupperpartaroundthebucketsoftensorevenliquefies.Theexcessporepressuredecreasesfromtheupperparttothelowerpartofthesandfoundationinverticaldirectionwhiledecreasesfromneartofarawayfromthebucket′ssidewallinthehorizontaldirection.Largesettlementsofthebucketandthesandaroundthebucketoccurunderthehorizontaldynamicloading.Thedynamicresponsesofthebucketwithsmallersizeareheavier.Keywordssuctionbucketfoundationdynamicloadingdeviceice2inducedvibrationcentrifugemodeling0IntroductionInrecentyears,suctionbucketfoundationshavebeenappliedincreasinglyofteninoffshoreengineeringClukeyetal1,1995Allersmaetal2,3,1997,2000.Thefirstadvantageofsuctionbucketfoundationsareattractivebecauseoftheconvenientmethodofinstallationandrepeatedlyuse.Foranexample,asuctionbucketfoundationwithadiameterof9mandaheightof10mcanbeinstalledin1~3hours,byusingonlyapump.Thesecondadvantageisthatitmaymobilizeasignificantamountofpassivesuctionduringuplift.Despiteseveralstudiesontheinstallationandstaticbearingcapacityhavebeencarriedout,thedetailresponsesofsuctionbucketfoundationsunderdynamicloadingshaveremainedunknownSenpereetal4,1982Aasetal5,1992Dymeetal6,1998.Thedynamicloadingconditionissignificantwhensuctionbucketsareusedasthefoundationofaplatform.Waveloadingandice2induceddynamicloadingcausethefoundationtobesubjectedtocyclicloadingsTjeltaetal7,1990Byeetal8,1995.Thelackofexperiencesundertheseloading333收稿日期2005208224修订日期2006207210基金项目ThisprogramissupportedbythefundofChineseOceanOilCo.andChineseAcademyofSciencesKJCX22SW2L0320140025103andNationalNaturalScienceFundNo.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。E2mailxbluimech.ac.cnconditionsledtoaproposalforatestprogramintendedtogainadeeperunderstanding.Theconsiderableexpenseandtimeconsumingnatureofprototypetestsmeanthattheinvestigationofthebearingcapacityofrealscaledevicesunderdifferentcircumstancesisoflimitedpracticality.Itismucheasiertochangeparametersinsmallscaletests.Butproblemsariseconcerningthestress2dependentbehaviorofsoilthatthemeasuredloadingsaresolowthatmeasurementsarenotsufficientlyaccuratetovisualizedifferencesindesign.Theserestrictionscanbeovercomeifperformingthetestsinageotechnicalcentrifuge.Inacentrifugethesoilstressesoverasimilardeptharethesameasintheprototypesituation.Foranexample,at100g100timestheEarth′sgravity,asuctionfoundationwithadiameter9mandaheightof9mmaybesimulatedwithafoundationwithadiameterof0.09mandaheightof0.09m.Thescalingfactoroftheprototypetothemodelis100∶1.Theice2induceddynamicloadingisthecontrollingloadinginBohaiBayinChina.Thistypeofhorizontaldynamicloadingistranslatedtothesoillayerbyplatformandcausestheparameters,e.g.strengthandmodulusofthesoillayer,todegrade.Asaresult,thebearingcapacityofbucketfoundationsdecreases.Therefore,itisimportantandnecessarytoclarifythedynamicbehaviorofbucketfoundationsunderthistypeofloadingsinordertoprovidepracticaldesignmethodandparametersDingetal9,2003Luetal10,2003.Inthispaper,wefirstintroducedasetofdynamicloadingdevicesdesignedforsimulatingthehorizontaldynamicloadings.Thensomeexperimentalresultsoftheice2inducedcyclicbehaviorofsuctionbucketfoundationsarepresented.Theeffectsoftheloadingamplitude,thesizeofthebucketandthestructuralweightaremainlyinvestigated.Fig.1Theelectro2magneticactuator1DynamicloadingdeviceThedynamicloadingdeviceiscomposedprimarilyofanelectro2magneticactuatorasshowninFig.1.Thisactuatorconsistsofapermanentmagnet,acylindricalsilvercoil,twocopperdisksprings,aloadcellandasteelrodconnectedtothecoil.Therearetwomaintechnicalchallengesassociatedwiththedesignoftheactuator.1Thecylindricalcoilissuspendedinaprefabricatedspaceinthepermanentmagnet.Thespaceisverytinyinordertoavoidmagneticloss.However,sincethiscylindricalcoilmoveshorizontally,itisorthogonaltothegravitationalforce,thedeformationofthecoilunderthisgravitymaylockitinthistinyspace.Inordertoavoidthissituationalinearbearingandtwodiskspringsareinstalledtomakethecoilmovelinearlywithoutbeinglocked.Thistechniquehasbeendemonstratedtobeveryeffective.2Theweightofthecoilshouldbeminimizedtoincreaseitsdeformationinthedirectionofloading.Silverwireisusedtoformthecylindricalcoilsoastoenhancetheperformanceandlimittheweightwithin0.5kg.Thetotalweight044实验力学2006年第21卷ofthedeviceis14kg.Consequently,thevibrationofthe0.5kgcoilwouldnotimposesignificantimpactonthestrongboxwhoseweightis150kg.Thecoilmovesbackandforthtoapplycyclicloadingonthemodelbucketthrougharigidrod.Fig.2isthecontrollingsystemfortheloadingdevice.Theloadingfunctionisfedfromacomputersignalgeneratorintoaservoamplifier.Thelatterinturngeneratesthenecessaryelectricalsignalstocontroltheintensityofelectriccurrentintheactuator.Theoutputfromtheactuatorintheformofloadingiscontinuouslymonitoredbytheloadingcellandfedbackthroughachargedamplifiertotheoscillograph.OutputsignalfromporepressuretransducerPPTandlinearvariabledisplacementtransformersLVDTwerepassedacrossthecentrifugeslip2ringandconvertedtodigitaldata.Withthisdevice,differentamplitudesandthefrequenciesofhorizontalcyclicforcesarereproducedatcentrifugalaccelerationof80g.Therefore,thescalingfactoroftheloadfrequencyoftheprototypetothemodelis1∶80,andthescalingfactoroftheloadamplitudeoftheprototypetothemodelis6400∶1.Inthecentrifuge,thepeakcyclicloadingof100Nand64Hzrepresentsaprototypeloadingof640KNand0.8Hz.Table1showsthetechnicalspecificationsofthisactuator.Theforce2electricityrelationat80gisshowninFig.3.Tab.1TechnicalspecificationoftheelectromagneticactuatorDiametermmLengthmmWeightkgFrequencyrangeHzPeakcyclicloadN180260141~1201002CentrifugesystemconfigurationFig.4showstheconfigurationofthecentrifugeandthemodel.Theinternalareaofthealuminumstrongboxmeasures600mm350mmandheightis350mm.Thethicknessofthesiltysandis210mm,underlainby20mmcoarsesandwhichistopreventthepipingduringpenetrationofwater.Thereis20mmhighwaterabovethesoilsurface.Theactuatorissupportedbytwosteelbeamsfastenedontheflange.TworodsfixthebackoftheactuatoragainstthesideofthestrongboxasshowninFig.4.AminiatureTVcamerawasattachedtotheflangeofthemodelcontainertomonitorthemotionofthemodelbucket.Themodelbucketisasteelcylinderwhichhasanexternaldiameterof60mm,aninternaldiameterof56mm,andaheightof72mm.Thedeadweightoftheplatformwassimulatedbyaballastmountedonthetopoftheshaft.ThemovementofthebucketwasmonitoredbytwoLVDTs.Whenthecentrifugemodelistestedunder80g,itsbehaviorwouldprimarilycorrespondtothatofaprototypefootingwithadiameterof4.8m.144第4期WANGShu2yunetalADynamicLoadingDeviceforSuctionFoundationsinCentrifugeModelingFig.4Thelayoutofthemodel3PreparationoftheexperimentsTheexperimentsareperformedinthe50g2tonscentrifugeinTsinghuaUniversity.Themaximumofthecentrifuge′saccelerationis200g.Thepayloadis250kgunder200g.Theexperimentalmaterialisfinesandwhosespecialgravityis2.69andtheaveragegrainsdiameteris0.14mm.Thepermeabilityis5103mm/s.Threemodelbucketsmadebysteelareusedinexperiments.Theinnerdepthsdonotincludethetopcapthicknessare48mm,72mmand90mm,respectively,theinnerdiametersareall60mm,thethicknessesofthewallandthetopcapareboth2mm.Afinepipewithanouterdiameterof10mm,aninnerdiameterof8mmandaheightof100mmisweldedatthecenterofthetopcap.Aslidinggroovepermittingaballmovingupanddowntoadaptthesettlementofthebucketisconnectedwiththepipeclosely.Oneendofasteelpoleisjointedwithaball,theotherendofthepoleisconnectedwiththeelectromagneticvibrationactuator.Thusthepolecannotbedraggedwhenthebucketsettles.OneLVDTislocatedatthetopofthefinepipetomeasuretheverticaldisplacement.OneLVDTislocatedatthefinepipeatthesamelevelasthatofthesteelpoletomeasurethehorizontaldisplacement.TheothertwoLVDTsarelocatedonthesandsurfacetomeasurethesettlementsofthesandsurfaceatdifferentpositions.TenPPTsmadeinDruckCo.English,areburiedinthesandaroundandinthebucketFig.5.ThePPTsarenotfixedbutsuspendedinthesand.Inordertoguaranteetherepeatability,210mmthickmodelsandlayerisdividedintofivelayerstocreateandbecompactedgentlybyhandstriking.Thiscourseiscontrolledbydrydensity.Thewaterdepthoverthesandsurfaceis1cm.Afterthedrysandsamplehasbeenprepared,thesandlayerissaturatedbypenetratingwaterintofromthebottomofthetankthroughavulva.A20mmthickcoarsesandlayerisdepositedatthebottomofthetanktoguaranteethewaterriseuniformlyandpreventingthepiping.Whenthewaterlevelisoverthesandsurface,avacuumpumpisusedtopumpthegasinthesandfor38hourstoincreasethesaturation.TwotypesofPPTlayoutareadoptedinthetests.ThefirsttypeonerowPPTsaredisposedinthesandalongdepthattheactuatorsideand2mmawayfromthebucket′ssidewall.Theupperoneis20mmbelowthesandsurface.Theothershavea20mmdistanceonebyonefromtheuppertothelowest.Tworowsaredisposedinthesandattheoppositeside.Onerowis2mmawayfromthebucket′swallandtheotherrowis52mmawayfromthebucket′swall.Thesecondtypetworowsareboth2mmawayfromthebucket′ssidewallontheoppositesideintheloadingdirection.ArowofPPTsaredisposedinthehorizontaldirection20mmbelowthesandsurface.ExceptforthePPTs244实验力学2006年第21卷
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