Suction Bucket Pile–Soil–Structure Interactions Of O Shore .

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Journal ofMarine Scienceand EngineeringArticleSuction Bucket Pile–Soil–Structure Interactions ofOffshore Wind Turbine Jacket Foundations UsingCoupled Dynamic AnalysisPasin Plodpradit 1 , Osoon Kwon 2 , Van Nguyen Dinh 3, * , Jimmy Murphy 3123*and Ki-Du Kim 1Department of Civil and Environmental Engineering, Konkuk University, Seoul 05029, Korea;P (P.P.); (K.-D.K.)Director of Coastal and Ocean Engineering Division, Korea Institute of Ocean Science & Technology,Busan 49111, Korea; Centre, ERI Beaufort Building and School of Engineering, University College Cork, P43C573 Cork,Ireland; jimmy.murphy@ucc.ieCorrespondence: or nguyendhhh@yahoo.comReceived: 30 April 2020; Accepted: 6 June 2020; Published: 8 June 2020 Abstract: This paper presents a procedure for the coupled dynamic analysis of offshore windturbine–jacket foundation-suction bucket piles and compares the American Petroleum Institute (API)standard method and Jeanjean’s methods used to model the piles. Nonlinear springs were used torepresent soil lateral, axial, and tip resistances through the P–Y, T–Z, and Q–Z curves obtained byeither API’s or Jeanjean’s methods. Rotational springs with a stiffness equated to the tangent or secantmodulus characterized soil resistance to acentric loads. The procedure was implemented in X-SEAprogram. Analyses of a laterally loaded single pile in a soft clay soil performed in both the X-SEA andStructural Analysis Computer System (SACS) programs showed good agreements. The behaviorsof a five MW offshore wind turbine system in South Korea were examined by considering waves,current, wind effects, and marine growth. In a free vibration analysis done with soil stiffness throughthe API method, the piles were found to bend in their first mode and to twist in the second and thirdmodes, whereas the first three modes using Jeanjean’s method were all found to twist. The naturalfrequencies resulting from Jeanjean’s method were higher than those from the API method. In a forcedvibration analysis, the system responses were significantly influenced by soil spring stiffness type.The procedure was found to be computationally expensive due to spring nonlinearities introduced.Keywords: offshore wind turbine; jacket foundation; coupled analysis; soil–pile–structure interaction;suction bucket; finite element model (FEM)1. IntroductionRenewable energy is becoming increasingly necessary in many countries where wind is one ofmost available renewable sources. There are higher potential, steadier, and less-constraining sourcesof wind energy in offshore locations compared to the onshore ones. In order to ensure the feasibility,viability, safety, and serviceability of an offshore wind farm, an engineer needs to select properfoundations for wind turbines and to develop accurate and computationally feasible models at thedesign stage. The selection should be based on water depth, seabed conditions, installation equipment,and supply chains. There are three common types of offshore wind foundations, as shown in Figure 1.The monopile is one of simple and most widely applied foundation types [1], and it is suitable to waterdepth of 20–40 m [2,3]. Tripod foundations, which have an excellent stability and overall stiffnesswhen compared with the monopiles, are utilized in deeper waters [4]. However, the tripod concept isJ. Mar. Sci. Eng. 2020, 8, 416; doi:10.3390/

J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEWJ. Mar. Sci. Eng. 2020, 8, 4162 of 262 of 24and overall stiffness when compared with the monopiles, are utilized in deeper waters [4]. However,the tripodconceptis lessterms ofcomplexityscour, ship ofcollision,complexitylesspreferablein termsofpreferablescour, shipincollision,joints, deflectionat ofthejoints,towerdeflectiontop, andat the towertop,andcomparedoverall weightto the jacket concept [5].overallweightwhento thewhenjacketcomparedconcept [5].A jacketjacket foundationfoundation consistsfour-leggedjacket,whichis supportedbyAconsists gedjacket,whichis supportedmainpiles,is particularlysuitablefor thewindwindindustry[6]. Theof usingjacketbymainpiles,is particularlysuitablefor offshorethe offshoreindustry[6]. benefitsThe benefitsof usingfoundationsare itsarelowerimpactof environmentalloads,highersoiljacketfoundationsits lowerimpactof tiffness, lowerlower eswithpoorsoil,deepwater,orhighwaves.dependency, and suitability for installations at sites with poor soil, deep water, or high waves.Compared toto monopilesmonopiles andand tripods,tripods, jacketjacket foundationsfoundations areare moremore suitablesuitable toto changingchanging waterwater oreHowever, a steel jacket foundation is more complex than other foundations and is therefore aviorofoffshorejacketfoundationsandtheircostly [7]. At the design stage, understanding the behavior of offshore jacket foundations and theirpile supportssupports underunder dynamicwouldassistinpiledynamic loadingloading tudieshaveinvestigatedthein lowering their costs while ensuring safety. Nevertheless, very few studies have investigated turbine–jacketfoundationcoupledsystems[8,9] oupledsystems[8,9] particularlythosethose supportedby suctionsupportedby suctionbucketbucketpiles. piles.(a) Monopile(B) Tripod(C) JacketFigure 1. Three common types of offshore wind turbine foundations.Figure 1. Three common types of offshore wind turbine foundations.A suction bucket is open at the bottom and completely sealed at the top, like an upturned bucket.It is penetratedinto theseabedto abottomcertainanddepthunder itsown atweight,withoutlet valvesonA suction bucketis openat thecompletelysealedthe top,likethean sonescape.SuctionisprocessedbypumpingoutencasedIt is penetrated into the seabed to a certain depth under its own weight, with the outlet valves on thewaterwithtoallallowthe closedvalves,drivingescape.the suctioncaissonto penetratedesignedoutembeddedtop openwateroutletinsidethe caissonSuctionis tallationwater with all the closed outlet valves, driving the suction caisson to penetrate the designedbecausethe dthan thoseof drivenpiles [12].embeddeddepth pile[10,11].An advantageis thatcanthedo fasternot requireheavyequipmentforLatiniand Zania[13] theinvestigatedthefoundations’dynamic responsesof suctioncaissons,and theskirtlengthwasinstallationbecausedriven pileinstallationscan hemodeltestsdriven piles [12]. Latini and Zania [13] investigated the dynamic responses of suction caissons, andandfield installationsof suctionhave proventhatinthedetermininginstallation theirunderbehavior.suction isTheextremelythe skirtlength was foundto becaissonsa sizedsands, primarilyduecaissonsto the reductionin resistancethat resultsunderfromfrom theinmodeltestsand field installationsof suctionhave proventhat the ensuction is extremely effective in fine-or medium-sized sands, primarily due to the reduction intheskirt lengthwas increased.Consequently,a coupleddynamicanalysisof offshorewind turbinesresistancethat resultsfrom seepage.In addition,the dynamicstiffnesscoefficientsof were found to increase when the skirt length was increased. Consequently, a coupled dynamicIn recentstudieswindof offshorewindjacketfoundations,interactionsthe turbine responseswereanalysisof reis necessaryforsimulateda rationalbycoupleddesigndynamicanalysis using Fatigue, Aerodynamic, Structures and Turbulence (FAST), whichstructural[14].was developedby the ynamic,In recent studiesof offshorewind rehydrodynamic,and structuralmodulesandis ableto solveland or offshorefixed-bottomor floatingsimulated by ucturesand Turbulencestructuresquickly[15].In order byto obtainaccurateresults forEnergya sub-structure,the(NREL).loads simulatedby(FAST), whichwasdevelopedthe NationalRenewableLaboratoryFAST c, hydrodynamic, and structural modules and is able to solve land or offshore fixedandvelocityinformationto the quicklyaerodynamicmodulereturnaccuratethe loadsresultsat eachfortimestep. Those loadsbottomor floatingstructures[15]. Inorder andto obtaina sub-structure,the

J. Mar. Sci. Eng. 2020, 8, 4163 of 24pass through the layer of soft, poorly consolidated marine clays and then into stiffer clay or sandstrata [17,18]. The interactions among environmental load conditions, the structure of the pile, andthe soil around the pile constitute complex vibrations in the system. Wei et al. [8] investigatedsoil–structure interaction effects on the responses of an offshore wind turbine with a jacket-typefoundation. Two jacket models using different configurations of braces were used to compare the loadsand responses that resulting from coupled dynamic analyses [19].In the literature, there have been several methods for analyzing pile–soil–structure interactions(PSSIs). One of the most popular methods is the transfer of soil strata properties to spring stiffness.Matlock [20] presented a method for determining the lateral load displacement curve (P–Y) in softclay from static and cyclic loads. The American Petroleum Institute (API) [21] recommends methodsfor determining the pile capacity for lateral and axial end bearing loads in either clay or sandy soilsin which all the information on lateral and axial loads at specific locations with offshore data arefrom laboratory soil sample data tests. Thus they are called the P–Y, axial load displacement (T–Z),and tip load (Q–Z) data. However, because these methods were formulated using results obtainedfrom experiments on piles with small diameters, they have a limited ability to predict the behavior oflarger-scale piles such as suction buckets [22,23].In this paper, the theoretical background of the coupled dynamic analysis of turbine and supportstructures implemented and validated in the previous studies [6,24,25] are improved by including PSSIsand suction bucket pile models. Soil lateral, axial, and tip resistances are included by considering theP–Y, T–Z, and Q–Z curves of soil behavior. Coupled dynamic analyses of a turbine–tower–foundationsystem including PSSIs were performed by using the concept of exchanging of motion and forcecomponents between the X-SEA and FAST programs at the interface nodes. Furthermore, parametricstudies of suction bucket piles supporting an offshore wind turbine jacket foundation at a specific sitein Korea were conducted in which nonlinear translational springs represent soil lateral, axial, and tipresistances, and rotational springs characterize soil resistance to acentric loads in suction bucket piles.2. Coupled Analysis of Turbine and Support StructureX-SEA finite element analysis software was developed for the design and analysis of onshore andoffshore wind turbine platforms. The current version of X-SEA was developed at Konkuk University,Seoul, Korea [24]. The program has an extensive range of uncoupled and coupled analyses betweenthe turbine and sub-structure using the FAST v8 program [15]. In each coupledn . o analysis, the eighteenn . ocomponents of motions represented by the displacement {Ut }, velocity Ut , and acceleration Utvectors are input into the X-SEA program. Simultaneously, X-SEA returns the reaction force vector(Ft (t)) at the interface position every analysis time t, as depicted in Figure 2. This “coupling” conceptimplemented and validated in the study [25] is expressed by:n . on. o [Mt ] Ut [Ct ] Ut [Kt ]{Ut } Ft (t)(1)where Mt ,Ct , and Kt are the mass, damping, and stiffness matrices, respectively, of the coupled systemat the time t.

J. Mar. Sci. Eng. 2020, 8, 416J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW4 of 244 of 26{Ft (t )}Interface position{U } ,{U } ,{U }tttFigureFigure 2.2. TheThe conceptconcept ofof coupledcoupled analysisanalysis throughthrough thethe interface.interface.3. Nonlinear Soil Springs3. Nonlinear Soil SpringsThe API has developed a method for determining the pile capacity for lateral and axial bearingThe API has developed a method for determining the pile capacity for lateral and axial bearingloads in either clay or sandy soils. All the information on pile load tests was derived in the laboratoryloads in either clay or sandy soils. All the information on pile load tests was derived in the laboratoryfrom soil sample data. These datasets describe the lateral load deflection (P–Y), axial load displacementfrom soil sample data. These datasets describe the lateral load deflection (P–Y), axial load(T–Z), and tip load (Q–Z) at specific offshore sites [21]. The pile length is required to achieve tensiondisplacement (T–Z), and tip load (Q–Z) at specific offshore sites [21]. The pile length is required to(pullout) and compression capacities necessary to support the offshore wind turbine foundation.achieve tension (pullout) and compression capacities necessary to support the offshore wind turbineThe soil around the pile must resist lateral and axial bearing loads, which correspond to skin frictionfoundation. The soil around the pile must resist lateral and axial bearing loads, which correspond toand end bearing capacity, respectively. These should not exceed a certain limit under similar conditionsskin friction and end bearing capacity, respectively. These should not exceed a certain limit underwith the same pile diameter and equipment used in the design.similar conditions with the same pile diameter and equipment used in the design.3.1. Pile Lateral Loads3.1. Pile Lateral LoadsPile foundations should be designed to sustain lateral loads, whether static or dynamic. A properPileoffoundationsshouldbe designedto sustain lateraldynamic.A properanalysislateral loadsin cohesiveor cohesionlesssoils loads,shouldwhetheruse P–Ystaticdata,orwhichare ided by geotechnical engineers. P–Y data describe the nonlinear relationship between ilresistance and the depth of the piles [20]. For each layer of the soil along a pile depth, the P–Y dataresistanceand therelationshipdepth of thepiles [20].For eachlayerof the soilandalonga piledepth,the P–Yshowa nonlinearbetweenthe lateralpiledisplacementlateralsoilresistanceper lpiledisplacementandlateralsoilresistancelength. For static lateral loads, the ultimate unit lateral bearing capacity of soft clay has been foundpertounit betweenlength. ityofsoftclayhasbeenvary8cs staticand ofmoredefinitivessfound tothevarybetweenrelationship8c s and 12ciss , where c s is the undrained shear strength. In the absence of morecriteria,followingrecommended by the API for soft clay:definitive criteria, the following relationship is recommended by the API for soft clay:y 13 1P(2)P 0.5( y)Pu 0.5(yc ) 3(2)Puycwhere P is the actual lateral resistance, Pu is the ultimate resistance, y is the actual lateral deflection,andyc isPtheissoilin termsof the pilePudiameter.In a moreresistance,recent study,[26] showedy Jeanjeanthecoefficientactual lateralresistance,is the ultimateis the actuallateralwherethat in conventional methods for assessing the lateral performance of conductors using the P–Y curvesdeflection, and y is the soil coefficient in terms of the pile diameter. In a more recent study,resulting from the cAPI method, the soil springs might be too conservatively soft. That leads to theJeanjean [26]showedthatin displacementsconventional methodsfor stressesassessinglateralofpredictionof largercycliclateraland bendingforthea givenloadperformancerange than theconductorsusing thefromP–Ycentrifugecurves resultingAPI method,springsmight asbe[26]:tooP–Ycurves obtainedtests andfromfinitetheelementanalysis. theThesoillatteris expressedconservatively soft. That leads to the prediction of larger cyclic lateral displacements and bendingstresses for a given load range than the P–Y curves obtained from centrifuge tests and finite elementanalysis. The latter is expressed as [26]:

J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW GPy tanh max ( )0.5 Pu 100.Su D J. Mar. Sci. Eng. 2020, 8, 416whereGmax5 of 26is the shear modulus of the soil,Su(3)5 of 24is the undrained shear strength,yis the lateral"#y 0.5GmaxlateralPThe actualdisplacement, and D is the pile diameter. tanh( ) load and ultimate lateral load relations (3)Pushear strain100.Scan be obtained in terms of cumulative( ξ u) asD[26]:( ξ thed / D ) undrained shear strength, y is the lateralwhere Gmax is the shear modulus of thesoil,P 12 S4ueisDSu(4)udisplacement, and D is the pile diameter. The actual lateral load and ultimate lateral load relations canbewhereobtained in terms of cumulative shear strain (ξ) as [26]:S( ξd/D)DSu u 0 6 Pu 12 S4eu 0for d su D 0.25 0.05d su Dξ for SSu 0 S0.55 0.05 dsuu0D f or dsuu0D 66 0.25 dSu0ξ su D where0.55 f ordsu D(4)(5)(5) 6The above equations were applied to soft clay. The ultimate resistance ( P ) for sand has beenu ) for sand has beenThe above equations were applied to soft clay. The ultimate resistance (Pu) undvarywithwiththethedepthdepth(H)( Hoffoundtotovarythethesoillayer,( ( m H m D )γ H(m1 H m22 D)γH PPu u 1γHmm11DDγH (6) (6)m33 areand m valuevalue ofof thethe ultimateultimate resistance,Equationresistance,wherewheremm11,,mm22 ,, andsoilcoefficientsdeterminedfromfromthe frictionangleandandγ is γtheissoilthedensity.soil density.thesoil coefficientsdeterminedthe frictionangle3.2. Pile Axial Loads and Tip Loads3.2. Pile Axial Loads and Tip LoadsTheaxialresistanceof thesoil soilprovidesaxialaxialadhesionalongalongthe sidethe pile.empiricalTheaxialresistanceof theprovidesadhesiontheofsideof theSeveralpile. Severalrelationsandrelationstheoriesandare availableforavailableproducingfor axialloadfortransfersandtransferspile displacements,empiricaltheories areforcurvesproducingcurvesaxial loadand pileor displacements,T–Z curves. nshipfroma pile loador T–Z curves. Lymon and Michael [27] developed a load deflection relationshipfromtestin arepresentativesoilprofiles basedlaboratoryThe recommendedcurves are displayedpile load test inrepresentativesoilonprofilesbasedsoilon tests.laboratorysoil tests. The recommendedcurves inFigure3 for dendmobilizedbearing resistanceare displayedin Figure3 forcohesivethesoil.Similarly, therelationshipbetweenend bearinga

clay from static and cyclic loads. The American Petroleum Institute (API) [21] recommends methods for determining the pile capacity for lateral and axial end bearing loads in either clay or sandy soils in which all the information on lateral and axial loads at specific locations with o shore data are from laboratory soil sample data tests .

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