Mitigationoficeloadingonoff-shorewindturbines:Feasibilitystudyofasemi-activesolution
ArkadiuszMro´za,b,JanHolnicki-Szulcb,*,TuomoKa¨rna¨a
aTechnicalResearchCentreofFinland(VTT),BuildingandTransport,Vuorimiehentie5,P.O.Box2000,FI-02044VTT,FinlandbInstituteforFundamentalTechnologicalResearch(IPPT),SMART-TECHCentre,ul.Swietokrzyska21,00-049Warszwa,PolandAvailableonline6April2007
AbstractThispaperfocusesonmitigationofdynamicloadinginducedonoff-shoretowersbydriftingice.Conicalstructuresattachedtooff-shoretowersatwaterlevelarewidelyusedtoreducestaticanddynamiciceactions.Thelevelofremainingforcesisenoughtocauseseveretowervibrations,thereforeaneedforanadditionaldevicetoreducetheremainingactionsisrecognized.Presentstudyintroducesanew,compliantconnectionbetweentheconeandthetower.Parametersoftheconnectionarecontrolledsemi-activelytooptimizetheeffect.Numericalstudiesaremadetoroughlyestimatetheeffectivenessofthenewsolution.First,contactproblemoficesheetandconeissimulatedusingsimplifiedmaterialmodelforice.Thenanumericalmodelofawindturbinetowerisconstructed.Dynamicinteractionbetweenconeandtowerissimulatedandthenewsolutioneffectivenessisdiscussed.Ó2007ElsevierLtd.Allrightsreserved.Keywords:Impactloadmitigation;Semi-activecontrol;Conicalstructures;Iceloading;Off-shorewindturbine;Structuraldynamics
1.Introduction
Manywindfieldsattractiveforthewindindustry(i.e.overBothnianBay)orlarge,unexploitedoilfieldsarelocatedoff-shore,withinPolarclimatearea,forcingsomeengineeringstructureslikewindturbinetowers,oilplat-formsupportsorlighthousestowithstandveryharshenvi-ronmentalconditions.Strongwinds,seawaveloading,icingaswellasdriftingiceloadingareamongsevereenvi-ronmentalactions,suchstructuresmustberesistantto.Thispaperfocusesonmitigationofdynamicloadingcausedonoff-shoretowersbythedriftingice.Icefieldsoftencoverseveralsquarekilometersofseaandcandriftwithvelocitiesofmorethan1.0m/s.Mechanicalpropertiesoficesuchasbendingstrength,Youngmodulus,iso-orunisotrophydependonseveralparameters,i.e.tem-perature,ageoficesheetormicro-structureoficematerial.Duetoitscomplexityanicesheetinteractingwithoff-shorestructurecanproduceawiderangeofdeformationstates,eachgeneratingdifferentreactionsonthestructure.Dependingonthemechanicalpropertiesofice,itsthick-ness,driftvelocity,aswellasgeometryofthestructure,twomodesoficefailurehavebeenidentified:crushingmodeandbendingmode.Theirphotographs[1]areshownonFig.1,whereasthecorrespondingreactionforces[2]areshownonFig.2.Inpracticeamixtureofthesetwofailuremodesisveryoftenpresent.Timeperiodbetweenconsecu-tivepeaksonFig.2adependsontheflowvelocityandbreakinglengthoftheicesheet.Breakinglengthisdefinedasthedistancebetweenthezoneoficeedge/structurecon-tactandthefirstcircumferentialcrack[3].Ifthebreakinglengthdividedbytheiceflowvelocityequalstothefunda-mentalownfrequencyofthetowerthenanarrowbandexcitationmayoccurinthestructure.Forcylindricalstructuresinteractingwithdriftingice(ofthickness0.2mandmore)themostprobableicefailuremodeiscrushing[1].Suchactionscouldgenerateashighdynamicforcesas5MNandwereofcriticalconcernforthestructuraldesigners.
0045-7949/$-seefrontmatterÓ2007ElsevierLtd.Allrightsreserved.
doi:10.1016/j.compstruc.2007.01.039*Correspondingauthor.E-mailaddresses:amroz@ippt.gov.pl(A.Mro´z),holnicki@ippt.gov.pl(J.Holnicki-Szulc),tuomo.karna@vtt.fi(T.Ka¨rna¨).URLs:http://www.vtt.fi,http://www.ippt.gov.pl(A.Mro´z).
www.elsevier.com/locate/compstrucAvailable online at www.sciencedirect.com
ComputersandStructures86(2008)
217–226Asuccessfulmeasuretoreduceiceinduceddynamicforceswastheintroductionofconicalstructuresfixedtotowersatthewaterlevel(Fig.3).Theiraimistopromotethebendingfailureofanicesheetbymeansofintroducingaverticalcomponenttothetotalinteractionforce[4].Sincethetotalinteractionforceactsalwaysnormaltothesurfaceofstructure,theslopeofaconeintroducestheverticalcomponent.Whenthevalueofverticalforceexceedsacertainlevel,theicebendingstrengthisreachedandthebendingfailureoccursintheicesheet.Thehori-zontalforcecorrespondingtothefailurepoint,i.e.themaximumforceinducingtowervibrationsissubstantiallydecreasedcomparedtothecasewithoutacone,butcanstillreachashighvaluesas800kN.Thereforeafurtherneedtomitigatetheresponseofthestructureand/ortoreducetheiceinducedloadingisrecog-nized.Prospectsofusingeitherpassiveorsemi-activetunedmassdamperswerediscussedbyKarnaandKolari[5].ApproachestouseacompliantconnectioninsteadoffixedconewereinvestigatedbyLindqvistandJuurmaa[6]andWang[7].Inthispaperanumericalstudyofanothersemi-activesolutionismade,aimingatmitigationofdrift-ingiceinducedforces.
2.Ideaofsmartcone
TheideaofSmartConeistofurtherdecreasethehori-zontalcomponentoficeloadingwhichcausestowervibra-tions,whilepreservingtheverticalforcewhichcausesthebendingfailureoficesheet.Thiscouldbeachievedbyintroducinganewcone–towerinterface,asshownonFig.4.Present,fixedconnectionbetweenconeandthetowerisreplacedwithacompliantconnectionbasedonasetofspringsanddampers.Furthermoretranslationiscon-strainedalongtheaxisperpendiculartotheiceflowdirec-tion.Aslongastheeccentricity‘‘a’’isnon-zero,conehastheabilitytorotateaboutthisaxis.Thisresultsinnewbehavioroftheconeduringacone–icecontacteventandconsequentlyleadstodifferentstressdistributionwithintheicesheet.Inturnalsocone–iceinteractionforcesandloadingofthetowerarechanged.Theeffectivenessofpres-entsolutionisdependentonthewaterlevel,aseccentricity‘‘a’’decreaseswiththeloweringofwaterlevel.InmoredetailtheprincipleofSmartConebehaviorcan
beFig.1.Crushing(a)flexural(b)icefailuremodes.
Fig.2.Typicalmeasurediceforcesactingontower.
Fig.3.Conicalstructuresattachedtooilplatform.218A.Mro´zetal./ComputersandStructures86(2008)217–226
