Research Article Ultimate Strength of Fixed Offshore ...downloads.hindawi.com/journals/sv/2015/841870.pdf · Research Article Ultimate Strength of Fixed Offshore Platforms Subjected

Research ArticleUltimate Strength of Fixed Offshore Platforms Subjected toNear-Fault Earthquake Ground Vibration

Hesam Sharifian1 Khosro Bargi1 and Mohamad Zarrin2

1School of Civil Engineering College of Engineering University of Tehran Tehran Iran2Department of Civil Engineering K N Toosi University of Technology PO Box 15875-4416 Tehran Iran

Correspondence should be addressed to Mohamad Zarrin mo zarrinsinakntuacir

Received 30 January 2015 Accepted 11 May 2015

Academic Editor Tony Murmu

Copyright copy 2015 Hesam Sharifian et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Thepile foundationnonlinearity and its influence on the ultimate capacity of fixedplatformshave not comprehensively been coveredby previous researchers In this study the seismic behavior and capacity of a newly designed and installed Jacket Type OffshorePlatform (JTOP) located in the PersianGulf is investigated by conducting Incremental Dynamic Analysis (IDA) using a suit of near-fault ground motions Additionally two modified models of the original platform are created by slightly increasing the diameterof the pile foundation and also softening the jacket part for evaluating the importance of the pile foundation and seismic soil-pilestructure interaction on the dynamic characteristics of the JTOPsValuable discussions are provided to explore various aspects of thedynamic behavior of JTOPs by presenting individual andmultirecords IDA curves using effective Engineering Demand Parameters(EDPs) Comparing the results of the three platform collapse fragility curves it is concluded that the pile foundation plays a veryimportant role in the dynamic response of offshore platforms and can drastically alter the ultimate strength of the platform togetherwith its collapse capacity It is observed that the proportional distribution of nonlinear behavior in the pile foundation and jacketpart is the key factor in the enhancement of the ultimate strength of JTOPs On the basis of the results derived from this paper it isrecommended that some basic requirements should be developed in order to ensure that the coupling ductility of pile foundationand jacket part is optimized during the design process Furthermore according to the findings from this study some practicerecommendations are presented to be devised within the design step

1 Introduction

Among all lateral loadings acting on the pile supportedstructures in seismically active areas these structures maybe subjected to strong ground motions where the behaviorof pile foundations under lateral seismic loads becomes animportant factor in efficiently assessing the performanceof such structures Jacket Type Offshore Platform (JTOP)is one of the most important pile supported structures inexploration and production of oil and natural gas Pilefoundations are an essential structural component of JTOPsand the seismic soil-pile-superstructure interaction (SSPSI)is an important issue in the seismic behavior of this kind ofstructure Strong ground motions have been a major causeof past damage in pile foundations and reliably evaluatingthe dynamic response of pile foundations against this type

of lateral loading plays a paramount role in JTOPs designprocedure

The ultimate strength and nonlinear dynamic behavior ofJTOPs subjected to earthquake groundmotions have been thesubject of a number of research papers in recent years How-ever many of the previous works have used static pushoverprocedure to evaluate ultimate capacity seismic reliabilityand seismic assessment of offshore platforms under earth-quake load patterns (Gates et al [1] Kayvani and Barzegar[2] Mortazavi and Bea [3] Asgarian and Agheshlui [4]and Golafshani et al [5]) The ultimate strength hystereticresponse of tubular members and cyclic inelastic behavior ofjacket frames of JTOPs ignoring pile foundation part wereinvestigated in the works of Zayas et al [6] Kayvani andBarzegar [2] Mortazavi and Bea [3] Honarvar et al [7] andGolafshani et al [8] using experimental results of frames

Hindawi Publishing CorporationShock and VibrationVolume 2015 Article ID 841870 19 pageshttpdxdoiorg1011552015841870

2 Shock and Vibration

tested under monotonic increasing static loads or pseudostatic cyclic displacement histories In the works of Bea andYoung [9] Peng et al [10] Bargi et al [11] and El-Din andKim [12] the nonlinear dynamic response and performancecharacteristics of JTOPs were studied by subjecting the plat-form to individual earthquake time histories at a special levelof intensity for example ductility level earthquake Ou et al[13] carried out a comprehensive experimental and numericalstudy in order to examine the vibration control effective-ness of the damping isolation system composed of rubberbearings and viscous dampers for a prototype fixed basejacket platform Similarly Mousavi et al [14] evaluated theoptimum parameters of tuned liquid column-gas damperTLCGD and its efficiency when the platform was subjectedto seismic excitation The jacket structure was considered ashear-flexural cantilever beam fixed at the base in their workLi et al [15] developed a convenient method of applyingwavelet transform to multicomponent earthquake motionsto determine the critical incidence of the seismic wavefrom the viewpoint of the input energy of structures Thesoil-pile system was excluded from their numerical jacketmodel A new robust and practical Beam on NonlinearWinkler Foundation model was presented by Memarpouret al [16] for cyclic lateral behavior of pile foundation ofFixed Platforms which could be utilized for seismic analysisHowever the behavior of the piles in their case studyfixed platform was investigated under wave lateral cyclicand monotonic loadings Recently a practical and accurateapproach named Incremental Dynamic Analysis (IDA) wasproposed by Vamvatsikos and Cornell [17] for the predictionof seismic demands from elastic behavior to yielding andfinally collapse capacity of structures A fundamental stepin IDA is to perform a series of nonlinear dynamic analysesof structures under a suit of multiple earthquake groundmotion records each scaled to several intensity levels sothat it covers the whole range of response To capture thenonlinearity in piles Assareh and Asgarian [18] conductedseveral multirecords IDA analysis on single piles of JTOPSAsgarian and Ajamy [19] investigated the seismic perfor-mance of three newly designed JTOPs using IDA analysisprocedure El-Din and Kim [20] utilized IDA procedurefor obtaining the ultimate capacity of the fixed-based jacketoffshore platforms with various bracing configurations Themain shortcoming of the two later works was that the soil-pile-structure interactions were ignored and the pile founda-tion was replaced by equivalent pile stubs In light of previousstudies the pile foundationrsquos nonlinearity and its systematicinfluences on the seismic ultimate capacity of JTOPs havenot comprehensively been covered by previous researchersThis concluding remark was also emphasized in a state-of-the-art review paper on seismic response and behavior ofFixed Offshore Platforms (Hasan et al [21]) The reviewsuggests that most of the previous analyses are based onhydrodynamic loadings only and that very few investigatorshave accounted for the seismic criteria of offshore structuresThey concluded that more realistic models including soil-structure interaction with multidegrees of freedom need tobe analyzed

Recently El-Din and Kim [12] investigated the effectsof both aleatory and epistemic uncertainty on the seismicresponse parameters of an existing offshore platform Thisstudy revealed that the uncertainty in soil-pile axial frictionhas a notable impact on the response parameters The effectof soil lateral capacity uncertainty on seismic response wasneglected in this research While their study is insightfulto the effects of various parameter uncertainties on thegeneral response of platform they did not study the effectsof using different design strategies or parameters on thenature of platform seismic response rather than the generalmaximum response Furthermore the site response analysisof soil was not conducted and it was assumed that the timehistory of earthquake loading was directly applied to theplatform model This will exclude the influences of soil far-field nonlinearity as well as the kinematic interaction effectson the dynamic response of the platform

This paper investigates the seismic behavior of a newlydesigned and installed Jacket Type Offshore Platform locatedin the Persian Gulf by conducting Incremental DynamicAnalysis (IDA) A suit of 10 pulse-type near-fault groundmotion records is selected to perform dynamic analysisNear-source ground motions especially those containingforward directivity effects impose high seismic demandson the structure and therefore are considered suitable forthe purposes of evaluating ultimate strength and collapsemechanism of a strong structure such as JTOPs Moreoverto date the effects of near-source ground motions on JTOPshave not been investigated in previous works Additionallytwo modified models of the original platform are created byslightly increasing the diameter of the pile foundation andalso softening the jacket part for evaluating the importanceof pile foundation and seismic soil-pile structure interactionin the dynamic characteristics of the JTOPs Some individualand multirecords IDA curves in terms of maximum storey-like drift ratio per unit length of pile and maximum inter-storey drift ratio of jacket bracing storeys are provided andvaluable discussions are presented to explore various aspectsof the dynamic behavior of JTOPs Based on the findings fromthis study some practice recommendations are presentedto be devised within the design step Finally by presentingcollapse fragility curves for all three models the effects ofthe pile foundation and similarly the proportionality ofthe stiffness and strength of the pile foundation and jacketpart of the platform on the ultimate collapse probability ofconsidered platforms are investigated

2 Ground Motion Records Used in This Study

Prior to performing nonlinear dynamic analysis a numberof ground motion records must be selected 10 records outof a set of 40 unscaled three-component ground motions(Baker et al [22]) containing strong velocity pulses ofvarying periods in their strike-normal components havebeen selected Pulse-type near-field groundmotions resultingfrom directivity effects are characterized by a distinct highintensity pulse in the velocity time history of motion ina direction perpendicular to the fault rupture plane Theseground motions are expected to occur at sites near the fault

Shock and Vibration 3

where the fault rupture propagates towards the site and theslip direction (on the fault plane) is aligned with the rupturedirection (Somerville et al [23]) The reason for choosingpulse-type ground motions in this study is that in thesemotions most of the energy from the rupture is concentratedin a single long period pulse of motion at the beginning of theseismogram and this phenomenon transmits into the struc-ture a large amount of energy which should be dissipated ina short time through relatively few plastic cycles causing thestructure to undergo large inelastic displacements Given thisfeature these high seismic demands from forward directivityrecords are suitable for the purposes of evaluating ultimatestrength and collapsemechanismof a strong structure such asJacket Type Offshore Platform which is studied in this workAccording to historic earthquake data near the location of theconsidered case study platform an estimated 68-magnitudeearthquake long decades ago (Ambraseys and Melville [24])as well as a 6-magnitude earthquake in 2005 [25] haveoccurred just a few kilometers away from the current locationof the platform This historic information emphasizes theneed for a detailed study on the dynamic behavior of thecase study platform subjected to near-fault ground motionsFurthermore reviewing the Iranian tectonic setting theregion of interest is located near the NNW trending Zendan-Minab-Palami fault zonewhichmarks thewestern limit of theMakran subduction zone and connects the western Makranto the eastern Zagros deformation domain (Hessami andJamali [26]) The most recently determined focal depths (8ndash14 km) imply that moderate-to-large earthquakes can occurin the uppermost part of the Arabian basement beneath theHormuz Salt Formation (the location of platform) [26 27]It is apparent that the results of the investigations herein areattributed to near-field particular motion type Extending theresults to other types of records would require either payingparticular attention to the differences between the charac-teristics of these motions and non-pulse-type near-fault andfar-field earthquakes as well as their effects on the structuralresponse or verifying the observations documented in thisstudy for other types of earthquakes in a separate study Itwould be valuable to conduct similar dynamic analysis on asuit of far-field records Since the JTOPs are huge structureswith many elements and components the computationalexpenses are high Consequently regarding the aforesaid goalof this study the analyses are only provided for near-fieldrecords However some preliminary analyses by subjectingthe original platform to some representative far-field earth-quake motions revealed consistent failure modes of responsewith near-field analyses results [28]

The original suit of 40 records is a standardized set ofgroundmotions selected as a part of the PEERTransportationSystems Research Program (TSRP) [29] with the goal ofstudying awide variety of structural and geotechnical systemsat a wide range of locations in active seismic regions Themean and variance logarithmic response spectrum of therecord set are depicted in Figure 1 Since the ground motionset is not structure-specific or site-specific the selectedrecords have a variety of spectral shapes over a wide rangeof periods As it is seen from Figure 1 this record suit has alarge and approximately constant variance around the mean

minus15

minus13

minus11

minus09

minus07

minus05

minus03

minus01

01

03

01 1 10Period

10 selected records out of 40 records40 records mean10 selected records mean40 records standard deviation10 selected records standard deviation25 and 975 percentile of 10 records25 and 975

ln(S

a)

Figure 1 The logarithmic response spectrum of individual recordsand their logarithmic mean and variance

spectral values of different periods The later feature guaran-tees that earthquake records with a range of properties areavailable for analysis and ground motion aleatory variabilitycan be captured in the case that the analyst is interestedin evaluating the impact of ground motion variance on thestructural response estimates From another point of viewwhen investigating the behavior of the structure near itsfinal capacity as is the case in this study occurrence ofnonlinearity makes the structure sensitive to excitation at awide range of periods thus utilizing a set of records withvariance in a broad band would be beneficial in highlightingvarious modes of behavior Additionally these 40 groundmotions were chosen to have a variety of pulse periodsAn important feature of pulse-type earthquakes is that thestructural response is a function of the ratio of the pulseperiod to the modal periods of the structure Therefore anyselected suit of records should cover a variety of pulse periods

In the course of this study careful attention has been paidto the selection of 10 records in amanner that they inherit theimportant aforementioned features of the original suit of 40records such as statistical properties and variation in pulseperiod magnitude PGV and duration From Figure 1 it isapparent that the logarithmic mean and standard deviationof these 10 records match fairly well with the original recordset especially in periods larger than the fundamental periodof the structure This ensures that the statistical properties ofthe two record sets remain equivalent Tabulated inTable 1 arethe designation and basic properties of the selected recordsAdditionally depicted in Figure 2 are the histograms of pulseperiods strike-normal peak ground velocities and durationof selected records According to Figure 2(a) the pulse peri-ods of selected records cover a wide bandThree records havetheir ratio of pulse period to fundamental period of JTOP


0

1

2

05

07

12

16 21

25 39 49

15 25 35 45 55 65 75 85 95 105

Num

ber o

f rec

ords

Pulse period (s)

(a)

0

1

2

3

10 30 50 70 90 110 130 150 170

Num

ber o

f rec

ords

Strike-normal PGV (cms)

(b)

0

1

2

3

4

2 6 10 14 18 22 26 30

Num

ber o

f rec

ords

Duration (s)

(c)

Figure 2 Histogram of (a) pulse periods (The ratio of pulse period to fundamental period of JTOP is labeled on the histogram) (b) Strike-normal peak ground velocities (c) Significant duration (25 to 975 AI) of selected records

smaller or equal to one two records have this ratio equal to15 for two other records this ratio is around 2 while for thethree remaining records the aforesaid ratio is larger than 3These motions cover a magnitude range of 653 to 762 and aclosest distance ranging from 395 to 1092 km Strike-normalpeak ground velocities range from 30 to 167 cmsec with amean of 80 cmsec indicating that these ground motions aregenerally very intense (see Figure 2(b)) The selected groundmotions come from earthquakes with a variety of rupturemechanisms Another important parameter that has beenknown to be influential on structural response is recordduration Duration of record is an indirect representation ofearthquake magnitude Careful consideration is given to theselection of records with various durations The durations ofrecord set have been chosen to range from a small value of

707 seconds to a large value of 2907 seconds with a meanvalue equal to 1475 sec

3 Case Study Platform Description

The selected case study platform for seismic investigation isnamed HE2 platform which was planned and designed in2010 and was installed in 2012 The installed location of thisplatform is the PersianGulf near theHengam island and straitof Hormoz The water depth in the central location of theplatform is 746m This platform was designed and analyzedon the basis of the recommendations of API RP2A-WSD [30]

The jacket part of the platform consists of four bracedstoreys and the perimeter legs of the jacket are battered to1 10 in one row and are straight in the other rowsTheutilized


+132

+8

minus19

minus29

minus50

minus71

minus7434

St L 6

St L 5

St L 4

St L 3

St L 2

St L 1

Y X

Figure 3 The schematic configuration of the developed numerical model

Table 1Thedesignation andbasic properties of the selected records

NGA recordsequencenumber

Earthquakename Mw 119877

a

(Km)Vs30(ms)

1 170 ImperialValley-06 653 731 192


10 763 Loma Prieta 693 996 73014 982 Northridge-01 669 543 37317 1045 Northridge-01 669 548 28618 1063 Northridge-01 669 650 28221 1086 Northridge-01 669 530 44124 1161 Kocaeli Turkey 751 1092 79230 1494 Chi-Chi Taiwan 762 530 46138 1530 Chi-Chi Taiwan 762 610 494aClosest distance

lateral resisting system of the jacket are the chevron-type andinverted chevron-type bracing systems which constitute anX-shape (Split X Bracing System) at the connection points of

the second and fourth storey levels (see Figure 3)The reasonwhy the legs are straight in one row is because of the berthingof Jack-up drilling platform besides the fixed platform Theplan of the jacket at working point is square shaped with basedimension of 23m which extends with the aforementionedbatter and becomes 3153 lowast 23m at mud line elevation

The platform is supported by 4 main piles which aredriven to 80m penetrationThe diameter of the piles is 132mand its wall thickness is equal to 55 cm The whole massof the deck is 2810 ton including structural componentsmechanical instruments bridge reaction on deck operatingforces applied by the lifts pipe masses and 20 percent oflive loads This general mass was considered to be lumpedmasses in all nodes of the deck Furthermore these massesare converted to loads and have been applied to decknodes in a separate gravity loading analysis The mass ofthe jacket including structural component masses marinegrowth mass added mass and internal water mass is equalto 4985 ton which is considered to be lumped mass in jacketnodes at corresponding elevations of the jacket

In order to perform dynamic analysis and define fun-damental mode shapes of the structure it is required thatthe dynamic mass matrix of the structure be established Inline with this for defining dynamic masses in longitudinaltransversal and vertical direction the structural elements


Table 2 Design soil parameters for dynamic analysis

Layer Soil Depth [m]1205741015840[KNm3

]Shear strength [KPa]

120576(50) [mdash]From To From To

1 Sand 0 21 652 Clay 21 37 8 55 55 00053 Clay 37 8 9 65 115 00044 Clay 8 264 9 115 176 00045 Clay 264 35 9 164 175 00046 Clay 35 483 95 188 230 00047 Sand 483 492 108 Clay 492 61 95 234 288 00049 Clay 61 631 95 288 288 000410 Clay 631 85 95 288 320 000411 Clay 85 936 95 330 460 000412 Clay 936 100 98 460 460 000413 Clay 100 1105 93 460 530 0004

and other component masses considering all aforementionedmasses are specified at the nodes of both ends of the elements

Based on the in situ drilled soil borings and the geotechni-cal investigations performed on the sampled soil the layeringprofile of the soil at the platform location is characterizedas detailed in Table 2 On the basis of this soil layeringinformation the parameters required for defining p-y t-zand q-z elements which will be described in next sectioncan be determined for a unit length of soil profile A numer-ical model of this platform is developed using the OpenSystem for Earthquake Engineering Simulation (OpenSees)computer program (Mazzoni et al [31]) The schematicconfiguration of the developed numericalmodel inOpenSeesis illustrated in Figure 3 As it can be seen in this figurethe frames of the jacket have both battered and straight legswhich is associated with the phenomenon that the jacket isnot symmetric and has different stiffness along its two majordirections Consequently in order to avoid torsional responsewhen the earthquake loadings are applied dynamic analyseshave been performed along the direction y where the jacketis symmetric

In order to verify the developed analytical model andmake sure that the model is working appropriately thecalculated modal periods of the structure by OpenSees arecompared to the computed periods of Sacs 52 softwareThe fundamental period of the structure obtained usingOpenSees is 216 seconds which is fairly in agreement withits corresponding value of 212 seconds calculated by Sacs52 software Increasing the pile diameter to 142m with awall thickness of 58 cm a modified model of the platformis created in OpenSees program which will be denoted inthe following sections as PSP platform (Pile StrengthenedPlatform) In addition to this another model of platformdenoted hereafter as JS platform (jacket-softened platform)is created by softening the legs of the jacket as a result of thereduction of its diameter from 14542m to 1402m as well asreducing the diameter of the pile inside legs from 13208mto 12708m Furthermore the bracing elements of the jacket

are modified in such a way that the critical compressivebuckling load of brace members in storey levels 1 2 3 and4 are reduced by 34 40 40 and 35 respectively Thefundamental period of the structure in this new model is228 sec It should be emphasized that the pile elements in thismodel remained constant

4 Characteristics of the DevelopedNumerical Model

For the purpose of simulating the response of pile andjacket member elements nonlinear beam-column elementswith distributed plasticity whose analytical formulation ison the basis of flexibility method with or without iterationare used The fiber discretization approach has been utilizedfor modeling the cross section of pile and jacket elementsThe general geometric configuration of a fiber cross sectionis divided into several tiny components with various simpleshapes such as square rectangle and triangle The feasibilityof creating a cross section of member with these subregioncomponents provides appropriate flexibility in definition ofsections composed of different materials The geometricfeatures considered for each of the fibres are the local 119909and 119910 coordinates of fibers and their area The continuityequation of the section is computed on the basis of thestress-strain relationships of the material used The assumedmaterial for the fibers is considered uniaxially and the strainin each fiber is calculated based on the strain at centroidstrain and curvatures at the sections considering Bernoullirsquosassumption that plane sections remain plane and normalto element axis after bending An example of a sectioncomprising different types of materials is the leg element ofthe jacket with the pile element located inside it In somecases the space between the leg and pile is filled with groutTherefore defining the section with fiber approach is themost suitable choice for leg members of offshore platformsThe assigned stress-strain relationship for the steel material


of the pile and jacket members for this research is theMenegotto-Pinto model (Menegotto and Pinto [32]) Oneof the advantages of this type of material is the gradualtransition from linear region to nonlinear range of responseFurthermore the cyclic behavior of this material is fairlyconsistent with experimental tests including Bauschingereffects

Using force-based nonlinear beam-column elementsalong with fiber approach the section stiffness matrix in thesection level is converted to element stiffness matrix at basissystem degrees of freedom at both ends of the elementThenthe element stiffness matrix may be transformed from basicelement to the element local degrees of freedom and finallyto the global degrees of freedom by a matrix transformation(de Souza [33]) The so-called matrix transformation hasbeen done using the well-known and computationally effi-cient corotational formulation The corotational formulationseparates rigid body modes from local deformation usingas reference a single coordinate system that continuouslytranslates and rotates with the element as the deformationproceeds

It should be noted that the library of elements inOpenSees is not able to simulate both global and localbuckling and in light of this fact the only damage that canbe modeled is the strength damage In order to model theglobal buckling in brace members of jacket part a snap-throughmodel is used for brace representation with differentend conditions The utilized procedure defines an additionalnode inmid span of the brace and uses an initial imperfectionequal to 1100 of brace length for this node Utilizing thistechnique which requires two force-based elements per bracemember along with using corotational formulation for coor-dinate transformation buckling and postbuckling behaviorof braces can be modeled accurately The procedure of sim-ulating the cyclic inelastic behavior of steel tubular membersof JTOPs using fiber discretization technique together withnonlinear beam-column elements with distributed plasticitywas investigated in previous research works of Honarvaret al [7] and Asgarian et al [34] and was compared toexperimental results

In this paper in addition to the aforementioned provi-sions for modeling buckling and postbuckling behavior ofbraces the possible failure due to low-cycle fatigue has alsobeen considered in the developed numerical model In amaterial model capable of simulating low-cycle fatigue inOpenSees software (Mazzoni et al [31] and Uriz and Mahin[35]) once the fatigue life of thematerial exhausts and reachesa damage level of unity the load carrying capacity of thematerial becomes zero and this leads to a more realisticestimation of the global collapse of the structureThis fatiguematerial uses a modified rainflow cycle counting algorithmto accumulate damage in a material using Minerrsquos Rule (Urizand Mahin [35]) In the absence of experimental data forlow-cycle fatigue tests of tubular sections and due to theresemblance between tubular sections and box sections of thereported experimental data in Uriz and Mahinrsquos work withthe similar end conditions as is the case here the 120576

0mdashvalue

of strain at which one cycle will cause failuremdashis consideredto be equal to 0095 for brace fatigue material This value

was obtained for a low-cycle fatigue test of a pinned memberwith box section under axial loading It should be noted thataccording to the test results reported in [35] the type ofloading and end connections seems to play amore significantrole than section type in determining the 120576

0value Therefore

it may be interpreted that this value is a reasonable valuefor jacket brace member fatigue material The material alsohas the ability to trigger failure based on a maximum orminimum strain which is not related to the low-cycle fatiguephenomenon For leg and pile members due to the absenceof experimental data to determine fatigue parameters thelower bound conservativemaximumultimate strain values of012 and 01 are considered respectivelyTherefore the fatiguefeatures are not utilized for thesemembers It is advantageousto emphasize that the aim of using fatigue material in thisstudy is to prevent the estimation of the ultimate collapsecapacity of a structure so as not to get values higher thanrealistic values

The Beam on Nonlinear Winkler Foundation (BNWF)method also known as p-y approach is utilized in orderto simulate soil-pile-superstructure interactions In thismethod the pile is modeled as beam elements while thesurrounding soil is modeled using continuously nonlinearsprings and dashpots The soil nonlinearity is modeledusing nonlinear springs whose parameters are determinedanalytically by defining stiffness and strength parametersThe dashpots placed in series or parallel with the nonlinearsprings account for energy loss due to radiation dampingunder dynamic loading conditions The force displacementbehavior of nonlinear springs has been back calculated fromthe results of well instrumented pile lateral load tests indifferent soil conditions Herein soil lateral spring nonlinearbehavior was modeled using the Pysimple1 uniaxial materialincorporated in OpenSees by Boulanger et al [36] Thenonlinear p-y behavior is conceptualized as consisting ofelastic (p-ye) plastic (p-yp) and gap (p-yg) components inseries A radiation damping dashpot is placed in parallel withthe elastic element The gap component itself is composed ofa nonlinear closure spring (pc-yg) in parallel with a nonlineardrag spring (pc-yg) The constitutive behavior of Pysimple1material for clay was based upon Matlockrsquos relations of softclay for static loading condition API [30] recommendedp-y backbone relation for drained sand is approximatedby Pysimple1 material for modeling of cohesionless soilBoulanger et al [37] showed that the resulting p-y curvesbased on the formulation of Pysimple1 material for both clayand sandy soils match API p-y curves within a few percentsover the entire range of y

Instead of using common site response programs thisstudy uses the geotechnical capabilities of OpenSees tocompute free-field response of soil layersThe response of soilprofile is analyzed utilizing a combination of Pressure Inde-pendent Material for clay and Pressure Dependent Materialfor dense sand along with solid-fluid fully coupled quadUpelements of OpenSees The pressure sensitive material stress-strain behavior is based upon one of the most robust consti-tutive models within the general framework of multisurfaceplasticity (Yang et al [38]) which is capable of reliablyanalyzing flow failure type liquefaction and cyclic mobility


scenarios The characteristics of soil in different layers areinput to the model including soil profile height density ofevery soil layer shear wave velocity of soil undrained shearstrength of soil (119862

119906) soil shear modulus reduction curve

(119866119866max) the location of free water and bedrock character-istics and ground motion records at bedrock It should beemphasized that in order to take into account the hardeningdue to strain rate effects in near-fault earthquake groundmotions the upper bound soil characteristics derived fromexperimental data and tabulated in Table 2 are used for theanalyses Then displacement time histories computed fromthe free-filed site response are exerted on the fixed nodes of p-y elements as the input ground motion excitation Thereforefor dynamic analysis of soil-pile-structure system in thecreated numerical model a number of ground motion timehistory records are applied simultaneously to the pile founda-tion at different soil elevations The procedures of computingfree-field soil response using the finite element model on thebasis of geotechnical material and elements of OpenSees andalso the procedure of the soil-pile-superstructure interactionmodeling using this software are verified to experimental testsin a previous work of third author [39] and Assareh andAsgarian [18]

5 Nonlinear Dynamic Analysis Results

51 Definition of Considered IM and EDP From the defini-tion of IDA a 2D IDA curve is a plot of an EngineeringDemand Parameter (EDP) obtained from an IDA analysisversus an intensitymeasure (IM) that characterize the appliedscaled accelerogramTherefore the fundamental step in IDAanalysis is the determination of the IM and EDP The IMis a quantity that represents the intensity of ground motionand is a measure of the strength of the applied earthquakeforce In order to be able to scale the amplitude of the groundmotion accelerogram with a scale factor in such a way thatthe intensity of the scaled accelerogram can be characterizedwith a quantity the IM should be scalable Many scalablequantities have been proposed to be considered to be IMamong which the Peak Ground Acceleration (PGA) and the5 damped pseudo spectral acceleration at the structurefundamental period (119878

119886(1198791 5)) are examples of commonly

used IMs It was found that 119878119886(1198791 5) is more effective due

to its structure-dependent characteristics compared to PGAwhich is totally site-dependent Knowing the shortcomings of119878119886(1198791 5) such as the lack of consideration of higher modes

effects due to its simplicity over other advanced IMs this IMis used as the preferred one in the current study

On the other hand EDP characterizes the response ofthe structure under investigation subjected to a prescribedseismic loading In other words EDP represents the outputof the corresponding nonlinear dynamic analysis It shouldbe specified properly considering the characteristics of thestructural system under investigation In the case of urbanbuildings the possible choices could bemaximumbase shearpeak storey ductility peak roof drift ratio and maximumpeak interstorey drift ratio Considering the fact that foun-dation rotations are negligible in buildings the most suitable

EDP is maximum peak interstorey drift ratio which is satis-factorily related to structural damage For offshore platformsthe EDP should be selected based on the type of platform andthe platform modes of failure Jacket Type Offshore Platformis a multipart multienvironment structure consisting of twomain parts (regardless of the deck equipment above waterlevel) the jacket part above mud line and the piles driveninto the soil Consequently the displacement of the deckis related to the behavior of the jacket and pile parts Inthe well-head jacket platforms the deck equipment is veryexpensive in comparison with other parts such as jacketand pile individually Therefore any proposed EDP shouldmonitor the interstorey drift ratio as well as the displacementof the deck and since this displacement is the consequenceof brace buckling in the jacket and pile nonlinearity thecandidate EDP should take into consideration these two driftratios as well To this end peak drift ratio from mud lineup to deck level can be used in order to account for jacketinterstorey drift ratio as well as pile foundation failure inmost geotechnical conditions This EDP is on the basis ofthe fact that the upper soil layers a few meters beneath themud line are weaker than deeper soil layers which leadsto the more nonlinear behavior of pile in top region thanthe lower elements of pile By scaling up the records andimposing much more severe seismic forces on the platformthe pile gradually fails and plastic hinges are formed Itmay beconcluded that the creation of these hinges causes rigid bodyrotation of the jacket part Since this proposed peak drift ratiocomprise of rigid body rotation of jacket and drift ratios dueto brace buckling in some cases this special drift ratio couldbe considered to be the appropriate EDP for the well-headoffshore platforms However this EDP is only effective whenjust one plastic hinge is formed in the pile and the remainingpart of the platform rotates around this hinge Consequentlythis EDP loses its effectiveness in other failure modes ofwell-head platforms including shear mechanism of responsewhere two plastic hinges are formed in the upper part of thepile foundation Vamvatsikos and Cornell [17] stated that instructures that have several collapse modes such as the onetreated in this paper it is advantageous to detect the structuralresponse with separate EDPs for each individualmode Basedon this recommendationmaximum interstorey drift ratio forthe jacket part and maximum storey-like drift ratio per unitlength of pile for the pile foundation are utilized as consideredEDPs

52 Sample Original Platform Incremental Dynamic AnalysisDepicted in Figure 4(a) is the individual IDA curve of recordnumber 4 for the original platform The vertical axis in thisfigure is the 5 damped spectral displacement at structurersquosfirst mode period Sde(119879

1 5) Sde is analogous to pseudo

spectral acceleration since Sde is equal to pseudo spectralacceleration divided by the square of 119908 where 119908 is the firstmode angular frequency of the structure The horizontalaxis in this figure is maximum storey-like drift ratio perunit length of pile It should be noted that the term ldquodriftratiordquo in the horizontal axis label in this figure and all thefollowing figures is illustrated for brevity as ldquodriftrdquo As it isnoticed from this figure the IDA curve is monotonic and


0 001 002 003 004 005 006 0070

01

02

03

04

05

06

07

Maximum interstorey drift

Sde (

m)

Pile drift (per 1m)

(a) Storey-like pile drift ratio per unit length of pile

0 0005 001 0015 002 0025 003 0035 0040

01

02

03

04

05

06

07 Jacket drifts


Sde (

m)

Story 1 Story 2 Story 3


Story 7

(b) Jacket storey drift ratios

Figure 4 IDA curves of record number 4 for original platform

is gradually degraded towards collapse without any backand forth twisting behavior usually observed in IDA curvesof simple urban buildings Although in other record resultillustrations as will be shown in the following figures alow level of weaving behavior is observed compared tourban structures a more monotonic response is obtained inthis figure The reason for this monotonic behavior is theconcentration of nonlinearity in the upper parts of the pilein almost all records as opposed to building structures wherestiffness and mass along various elevations of structure aredistributed more uniformly and it is likely for all storeys toexperience some levels of nonlinearity Indeed the earliercreation of plastic hinges in the upper part of the pilefoundation causes this region of pile to work as a weakstorey and therefore prevents other parts of the platform tomeet at a nonlinear range of response This leads to a shearmechanism of response in pile foundation where two plastichinges (in each pile) are formed at mud line level and a few(4ndash6m) meters beneath the mud line The outcome is thatthe jacket part remains without rotations This mechanismof response will be discussed in details by presenting IDAcurves of various jacket storeys Illustrated in Figure 4(b) arethe IDA curves of different storeys of the jacket part of theplatform The storey level 1 represents from the mud line upto the first horizontal framing of the jacket at an elevationof 71m and its response behavior is related closely to pilefoundation Actually the braced storeys of the jacket startfrom storey level 2 in this figure As can be observed from thisfigure the EDPs of all braced storeys are constant at variouslevels of intensity measure of input ground motion recordimplying that the jacket remained unrotated in all scalesof input ground motion record Since storeys 6 and 7 areunbraced and only the portal behavior resists lateral loading

0 001 002 003 004 005 006 007minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)

Max drift in platform profile (pile drift per 1m)

Figure 5 The profile of maximum drift ratio in different elevationsof platform from the pile tip up to the deck level

the exhibited EDPs in these storeys are slightly higher thanother braced storeys of jacket

Figure 5 illustrates the profile of the maximum drift ratioat different elevations of the platform from the pile tip up tothe deck level Results show much greater drift ratios at 15meter below mud line in comparison with other levels Thisobservation is due to the weak strength of the soil exactlybeneath the seabed asmentioned in previous discussions andmore importantly due to the weakness of the pile foundationrelative to the jacket part It is apparent in this figure that theEDPs of different storeys of jacket are approximately constant


0 1 2 3 4 5 6 7 8 90

01

02

03

04

05

06

07Overall pile-jacket drift


Sde (

m)

times10minus3

Figure 6 The overall drift ratio of the original platform from thepile tip up to the deck level

at a specified intensity measure being an evidence of straightpositioning of the jacket part during response to earthquakeloading Figure 6 shows the overall drift ratio of the platformfrom pile tip up to deck level

It can be inferred from the consistency between this figureand Figure 4(a) that the failure mechanism of the wholeplatform originates from pile nonlinearity rather than fromthe response of any other parts of the platform It shouldbe noted that in this special record the drift ratios in thelower part of the pile foundation (the lowest 15m of pile)are negligible compared to the upper parts and therefore theoverall pile drift ratio from the pile tip up to the mud line isconsistent with the maximum storey-like drift ratio per unitlength of the pile while in some of the records the driftratios in these parts are considerable which makes overallpile drift ratio inconsistent with maximum storey-like driftratio per unit length of the pile However in all records thejacket response is the same as this record and only the pilefoundation shows nonlinear range of deformation

Depicted in Figures 7(a) and 7(b) are results of IDA curveof record number 17 in terms of maximum storey-like driftratio per unit length of pile and the overall drift ratio ofplatform from the pile tip up to the deck level respectivelyIn Figure 7(a) the EDP increases at a constant rate withincrease in the intensity of earthquake up to Sde equal to06m Subsequently fromSde ranging between 06 and 09mthe IDA curve ldquohardensrdquo having a local slope higher thanthat of the first monotonic part In this range of responsethe structure experiences the deceleration of the rate of EDPaccumulation which leads to a stop in EDP accumulation oreven a reversal Then the IDA curve starts softening againshowing greater range of EDP accumulation as IM increasesFinally the ultimate strength of the pile fiber material isreached and as a consequence the platform reaches itsfinal capacity and experiences global dynamic instabilityComparing Figures 7(a) and 7(b) one can recognize a fairlyconsiderable consistency between IDA curves in terms of

maximum storey-like drift ratio per unit length of pile and theoverall drift ratio of the platform from the pile tip up to thedeck level implying the predominating of pile nonlinearity onplatform mechanism of response

53 Pile Strengthened Platform Incremental DynamicAnalysisFigure 8 illustrates the IDA curves of different storeys ofthe jacket part of the Pile Strengthened Platform (PSP HE2)subjected to record number 4 The storey nomination issimilar to the aforesaid original platform Comparing thisfigure with Figure 4 the first important finding is that theultimate strength of the platform is enhanced to a high degreeThe original platform meets its final capacity at an IM levelequal to 068m as mentioned previously after the formationof plastic hinges at the upper parts of the pile foundation Onthe other hand the PSF HE2 platform reaches its ultimatecapacity at an IM level of 43m where the braces of storeylevel 2 buckles and then the portal behavior of the leg at thiselevation is activated Finally the platform falls down after thecreation of plastic hinges at an elevation of 50mAdditionallyfrom Figure 8 it is observed that a considerable jump occursin the EDP of storey level 2 which is the first braced storey ofthe platformThe reason for this abrupt change in EDP is thatthe ultimate strength of both compression and tension bracematerials at storey level 2 of this IM level is reached and thatthe load carrying capacity of the braces becomes zero Thisabrupt change in IDA slope is a sign of the low redundancyof this Platform model after first member failure In all thesubsequent scale steps of IDA curve only the portal behaviorresists lateral loading until the ultimate strength of the legmaterial is reached and the whole platform collapses

Figure 9 illustrates the profile of the maximum drift ratioat different elevations of the platform from the pile tip up tothe deck levelThis figure illustrates various scale steps of IDAanalysis of record number 4 Similar to the analysis resultsof the original platform the analysis results of PSP platformshow much greater drift ratios at 15 meter below the mudline in comparison with other levels It should be added thateven though the storey braces buckle in this model contraryto original platform model and the collapse of the platformresults from the jacket leg failure the pile foundation stillexhibits high nonlinear response emphasizing the influenceof the pile foundation on platform response As opposed tothe original platform response the drift ratios at elevationof 50m in Figure 9 show abrupt increase in seven final scalesteps of analysis

The proportional distribution of nonlinear behavior is thekey factor in the enhancement of the ultimate strength ofthe platform Considering the results of PSP HE2 platformanalysis it can be inferred that a slight change in the strengthof the pile foundation makes the stiffness and strengthof the pile foundation more proportional to the stiffnessand strength of the jacket part and this leads to uniformdistribution of damage or nonlinear behavior in both jacketand pile foundation Reducing the stiffness of the jacket partin the original platform also makes the strength of thesetwo parts more proportional and leads to similar results Todemonstrate this observation in more details the maximumstress-strain response of the pile foundation and the legs at


0 001 002 003 004 005 006 007 008 0090

02

04

06

08

1

12

14


Sde (

m)

Pile drift (per 1m)


0

02

04

06

08

1

12

14

0 0002 0004 0006 0008 001 0012 0014

Overall pile-jacket drift


Sde (

m)

(b) Overall drift ratio from pile tip up to deck level


0 002 004 006 008 01 012 014 0160

05

1

15

2

25

3

35

4

45 Jacket drift


Sde (

m)



Story 7

Figure 8 IDA curves of different storeys of the jacket part of PSPHE2 Platform

storey level 2 are postprocessed and consequently as depictedin Figure 10 the IDA curve of storey level 2 subjected torecord number 4 is separated into three distinct parts

In part one the platform responds in the samemanner asthe original platform where the braces have not buckled andthe nonlinearity is concentrated in the pile foundation In parttwo the braces buckle and the loads in the platform elementsare redistributed which leads to a sudden drop in the stress-strain demands of the pile foundation and an increase inthe leg of the jacket stress-strain demands Finally in partthree tearing in braces due to high inelastic demands occursand the load carrying capacity of the braces becomes zero

0 002 004 006 008 01 012 014 016 018minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)


Figure 9 The profile of the maximum drift ratio in differentelevations of the platform from the pile tip up to the deck level

The omission of braces forces the legs to absorb the wholeshear loads imposed on the jacket by earthquake groundmotion It is advantageous to note that while in parts 2 and3 the nonlinear demands in the pile foundation decreasescomparatively to part 1 it still experiences high excursionsof inelastic deformations

A special type of IDA response is observed in thisplatform model analysis for some records of ground motionwhich has not been reported previously for urban buildingsIllustrated in Figures 11(a) and 11(b) are the record number17 IDA curves of different storeys of the jacket part of theplatform and the pile foundation drift ratio per unit lengthof the pile respectively As shown in these figures the EDPsincrease monotonically up to some IM levels and then withan increase in IM no EDP accumulation occurs and the IDA


0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3

Figure 10 The IDA curve of storey level 2

curves become approximately vertical As a consequencethe platform does not meet its final capacity at least inthe expected intensity range of probable earthquakes in theengineering field in other words collapse does not occurThe reason behind this phenomenon is that in some scalesof these records the input ground motion is so strong thatit drives the soil to its final capacity and because of soilstrain softening no more load is transferred to the platformstructural part From another stand point in high levels ofseismic intensity the high nonlinearity in the soil profile actsas a source of damping and therefor reduces the seismic riskof the platform

54 Jacket-Softened Platform Incremental Dynamic AnalysisWith the jacket becoming relatively more flexible it isanticipated that the jacket part of the platform contributesmore effectively to the platform nonlinear response charac-teristics and similarly to the input seismic energy dissipationmechanism Therefore it makes the strength and stiffnessdistributed more uniformly in full platform height To clarifythe contribution of this on the dynamic behavior some illus-trations along with their discussion are presented in order tofully disclose the aspects of flexible jacket phenomenon Inaccordance with previous sections the individual IDA curvesof record number 4 for different storeys of the jacket part aredemonstrated in Figure 12 By examining this figure criticallyit is noticed that different storeys of the platform contributeto its nonlinear range of deformation in the whole ranges ofIM This irregularity is even more critical in other records asan inherent aspect of flexible jacket response

The storey level 1 is the first section of the jacket toshow relatively notable inelastic response even in low levelsof earthquake intensity This fact seems to hold for allother records studied in this work As stated previouslythe response of this storey is representative of the pile drift

demands in the upper part of the pile foundation Due tospace limitation they are not shown here At two IM scalesteps between 06m and 09m in this figure the storey levels3 and 4 are the first braced storeys to buckle leading to asudden jump in IDA results This phenomenon is attributedto the fact that the higher modes of vibration contributesignificantly to the seismic interstorey drift response (120579) atleast in the elastic range or low levels of inelastic responseshifting the 120579max drift to higher storeysThis is the case in lowor intermediate ranges of IM for all 10 records studied here

Analogous to PS platform occurring of brace buckling injacket storeys reduces the deformation demands in the pilefoundation (see storey level 1 drift at IM values ranging from06m to 09m in Figure 12) At higher intensity levels theinelastic excursions are gradually transferred to lower storeysof jacket (storey levels 2 and 3) These inelastic demandscommence by brace buckling at intermediate ranges of IMand as the intensity of earthquake increases the contributionof portal resistance in the legs plays a more paramount rolein jacket dynamic resistance Illustrated in Figure 13 is theprofile of maximum drift ration at different elevations ofplatform from the pile tip up to the deck level in variousIDA scale steps As it is seen the lower storeys of jacket showhigh nonlinear behavior in many scale steps Furthermoreexempting some lower IM levels the braces at storey level 5(elevation betweenminus8mup to 9m) buckle in all other IM lev-els participating in the jacket energy dissipation mechanismThis observation can be simply verified by comparing Figures13 and 9

Figure 14 depicts the IDA curves in terms of the overalldrift ratio of the pile from the pile tip up to the mud linelevel overall drift of jacket from the mud line level up tothe deck level and overall general drift of the platform fromthe pile tip in the lowest elevation up to the deck level atthe top most elevation As it is observed there is a mildconsistency between overall pile drift and pile drift per unitlength of pile associated with storey level 1 drift in Figure 12or even inconsistency in higher IM levels Furthermorethere is a close relationship between maximum interstoreydrift over all storeys (see Figure 12) jacket overall drift andwhole pile and jacket overall drift in Figure 14 implyingthe domination of jacket interstorey response on the wholeplatform displacement demands

In order to emphasize he effects of pulse-like records onthe platform response the IDA results of record number 17is presented in Figure 15 which shows a surprising feature ofresponse As opposed to the two previous platform modelsbehavior and results of most of the other records subjected tothis model the nonlinear mechanism of response at higherIM levels is concentrated in upper storeys of the jacket partThis is interpreted as the effect of the ratio of this record pulseperiod to fundamental period of the structure (119879

119901119879119904ratio)

which seems to excite higher modes of response In this caseas the structure behaves nonlinearly at high intensity levelsand its effective modal periods lengthen it is anticipated thatthe lengthened second mode of vibration of the structurecoincides with the record pulse period (119879

1199011198791199042 ratio equal

to unity) which causes larger response compared to theresponse of PS platform to record number 17 According to


0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)

Story 1Story 2Story 3


Story 7

(a) Jacket storeys drift ratio

0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)

(b) Storey-like pile drift ratio per unit length of pile

Figure 11 IDA curves of PSP platform subjected to record number 17

0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7

Figure 12 IDA curves of different stories of jacket part subjected torecord number 4

Figure 15 it is advantageous to add that while the platformexcites with its higher modes the deformation demands inthe pile foundation are significantly reduced This can beproved by simply comparing the pile drifts (storey level 1 drift)in this figure and other record results

55 Sample PlatformModels Pushover Analysis The purposeof pushover analysis is to estimate the expected perfor-mance of a structural system by estimating its strength and

0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)


Figure 13The profile of maximum drift ratio in different elevationsof platform from pile tip up to deck level

deformation demands in future earthquakes by means of anincremental static inelastic analysis which accounts in anapproximate manner for the redistribution of internal forcesoccurring when the structure is subjected to inertia forcesthat can no longer be resisted within elastic range of behaviorIn this part in order to assess the applicability of this simpleprocedure and its comparisonwith final solution time historyanalysis the pushover analysis has been carried out for thethree platform structural models The pushover pattern isselected based on a ldquofirst mode lateral load patternrdquo presentedby Chopra and Goel [40] A modification is applied to thispattern changing the power of the modal vector terms in


0 001 002 003 004 005 006 0070

05

1

15

2

25

3 Overall platform drifts


Sde (

m)

Overall pile driftOverall jacket driftOverall pile-jacket drift

Figure 14 The overall platform drift ratios

minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7

Figure 15 IDA curves of different storeys of jacket part subjected torecord number 17

pattern formulation from a value equal 1 to 04 This modi-fication makes the role of mass distribution more significantrather than the modal shape vectors role Figure 16 depictsthe resulting curves of the conducted pushover analysis ofthe three platform models Results show that the pushoveranalysis can successfully predict the failure mechanism of theoriginal platformmodels where two plastic hinges are formedat mud line level and a few (4ndash6m) meters beneath the mudline and therefore the jacket part remains without rotationsFinally the global instability occurs when the pile materialreaches its ultimate resistance

minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve

Original platformPile-strengthened platformJacket-softened platform

Pile yielding mechanism

Pile tearing

Story L 3 compressionbrace buckling

Compressionbrace tearing

Tearing of legs



Tearing of legs

timesE + 07

Figure 16 Sample well-head platforms pushover analysis curves

In the case of PS platform the jacket maximum loadcarrying capacity is enhanced relative to the original platformmodel and a significant loss in platform lateral stiffnesshappens when the brace of storey level one buckles This isfollowed by the tearing of compression brace and finallyleg failure There is a fairly good agreement between theobserved failure mechanism of pushover analysis and theplatformmechanism of failure within some of the earthquaketime history analysis In the case of flexible jacket model theplatform lateral stiffness and correspondingly the maximumlateral resistance are even lower than the original platformmodel due to flexible jacket concept Herein the softeningbehavior of the jacket is initiated by storey level 1 bracebuckling and followed by tearing of brace and storey level 3brace buckling and finally leg failure leads towhole platformglobal instability Although the pushover analysis could notpredict the whole sequence of failure in this case the generaltrend of failure is in agreement with IDA results

Taking the area under the force-deformation curve ofpushover curve as the potential energy dissipation capacityof the structure according to Figure 16 it is clear that theductility capacity of the structure has been significantlyimproved in the modified models compared to the originalmodel

6 Multirecords IDA andCollapse Fragility Curves

A single-record IDA analysis cannot fully capture the behav-ior that the platform may display in future ground motionsThe IDA study can be highly dependent on the record underinvestigation therefore a sufficient number of records shouldbe selected to cover the full range of responses Herein asstated in previous sections a suit of 10 near-fault groundmotions has been selected for IDA analysis After applyingall these records on the structure multirecords IDA canbe obtained A representative multirecords IDA curve fororiginal platform is illustrated in Figure 17 The EDP in this


0 002 004 006 008 010

05

1

15

2

25 Global dynamic instability (pile drift per length)


Sde (

m)

Figure 17 Multirecords IDA curve for original platform

figure is in terms of storey-like pile drift ratio per unit lengthof pile since the collapse in this platform model is the resultof plastic hinge formation in pile foundation Obviously noapparent difference is seen when the intensity of seismicinput is low but as IM rises differences appear to becomesignificant The variations in values of IM corresponding toflat lines (which represent collapse) indicate that the selectedrecords excite the platform in different manners and this isa remarkable sign of the suitability of the selected records inrepresenting the randomness in future ground motions

Multirecords IDA results of PSPHE2 platform are plottedin Figures 18(a) and 18(b) in terms of storey-like pile drift ratioper unit length of pile and jacket maximum interstorey driftratios respectively Since the collapse does not originate fromthe pile foundation failure the flat lines are not presented inFigure 18(a) As it is observed in this figure in five records ofrecord suit the individual IDA curve becomes approximatelya vertical line since in some scales of ground motion recordthe soil reaches its ultimate shear strength exhibiting aflow-type response These vertical IDA curves can also bedistinguished in IDA curves of jacket interstorey drift ratiosas depicted in Figure 18(b)

From this figure it is apparent that the interstorey driftratios of jacket part show an abrupt increase in EDP forrecords that cause collapse in the structure In three of thecollapsed records the platform experiences global dynamicinstability slightly after this sudden change in platformstiffness implying relatively low redundancy of jacket struc-ture However this is not the case for all records sincefor two remaining records the platform exhibits reservedstrength after the aforesaid change in IDA slope The laterphenomenon is attributed to simultaneous brace bucklingof both storey level 2 and level 3 implying the complicatedmodes of behavior of the platformwhen subjected to differentrecords of various natures

Illustrated in Figures 19(a) and 19(b) aremultirecords IDAresults of JS HE2 platform in terms of storey-like pile driftratio per unit length of pile and jacket maximum interstorey

drift ratios respectively Similarly since the collapse does notoriginate from the pile foundation failure the flat lines arenot presented in Figure 19(a) In this platformmodel in threerecords of the selected record suit the platform does notmeetglobal dynamic instability performance level in the expectedranges of IMThis is interpreted as the effects of soil softeningbehavior both in near field and in far field However in thismodel the platform shows much more nonlinear behaviorat higher IM levels than the PS HE2 platform model andpure vertical lines are not observed in the ID curves of theserecords

According to Figure 19(b) the concept of abrupt increasein the interstorey drift ratios of the jacket part is seenin almost all earthquake records considered here This isattributed to the high flexibility of the jacket part of theplatform In this regard independent of the nature of recordby scaling the ground motion record at an IM level a com-bination of storeys shows high excursions of EDP demandsIt is advantageous to note that these abrupt changes in EDPare less significant comparatively to PSHE2 platform analysisresults Another remark that can be drawn from Figure 19(b)is that the platform exhibits a high level of reserve strengthin the order of 2 to 6 times the intensity level that theaforementioned change in IDA slope took place The maincause of this paramount reserve strength is the contributionof more than one storey to the inelastic demands of the jacketpart within earthquake analysis

The collapse limit state fragility curve is defined as theconditional probability of structural collapse for a given IMlevel Collapse fragility curves relate groundmotion intensityto the probability of collapse of structure The probability ofcollapse at each intensity level is then computed as the frac-tion of groundmotions that causes collapse at the consideredintensity level Naturally the number of groundmotions usedat each intensity level influences the accuracy of this estimateDepicted in Figure 20 are the collapse fragility curves of threeplatform models investigated in this study Comparing thecollapse fragility curves of these platforms it can be inferredthat the probability of collapse is highly reducedwhen the pilefoundation of original platform is strengthened or similarlyby softening the jacket part This reduction in probability ofcollapse in the modified models of the original platform ismostly due to the uniform distribution of nonlinearity alongthe pile foundation and the jacket part and more importantlydue to the effect of soil flow failure An abnormal collapsefragility curve is obtained for these modified models wherefor example for PSP HE2 platform from IM levels higherthan 42m five of the records do not drive the platform to itsfinal capacity and consequently the probability of collapsedoes not reach to unity The pulse periods of the recordsare shown as data labels in Figure 20 As it is obvious theprobability of collapse is directly related to pulse period ofrecord As the pulse period increases the platform tends toexperience collapse at lower levels of IM

7 Design Recommendations

As stated in previous sections the base case platform modelinvestigated in this study is planned and designed based


0 005 01 015 020

1

2

3

4

5

6

7Global dynamic instability (pile drift per length)


Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6

7Global dynamic instability (jacket story drift)


Sde (

m)

(b) Jacket maximum interstorey drift ratio

Figure 18 Multirecords IDA results of PSP HE2 platform

0 002 004 006 008 01 012 0140

1

2

3

4

5

6


Maximum interstorey drift ratio

Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)


Figure 19 Multirecords IDA results of JS HE2 Platform

on the basic requirements of API RP2A-WSD [30] recom-mendation while the analysis in this study revealed a lackof optimum response during earthquake response historyanalysis This does not mean that the platform does notmeet the design environmental load conditions The mainidea behind this paper is to answer the following questionsWhat is the optimum response or what should be taken intoconsideration to achieve a better seismic response mainlynear the ultimate strength performance level The previousquestion leads to a subsequent question What analysis pro-cedures or tools should be utilized during the design processto ensure an optimum response Based on the findings fromthis study the following design recommendations are made

(a) Although the pushover analysis is an evaluationprocess that is relatively simple yet it can predict

the essential features that significantly affect theperformance goal The pushover analysis in generalmanner is able to predict the general trend in platformresponse for example occurrence of a weak link inthe pile foundation in the original platform modelHowever the results can be biased depending on thepushover pattern For example choosing a patternbased on a ldquofirst mode lateral load patternrdquo presentedby Chopra and Goel [40] showed the ldquooverturningfailure mechanismrdquo of response in all three modelsstudied herein where the pile plugged out of the soilConsequently if the pushover analysis is going to beconducted for ductility demand evaluations withinthe design stage the authors recommend that at leasttwo different load patterns with different conceptual


0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e

PSP HE2 platformOriginal HE2 platformJS HE2 platform

1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)

Figure 20 Collapse fragility curve for two platforms model (datalabels are pulse period of records)

natures be chosen for pushover analysis and thedesired response should be checked with these loadpatterns

(b) Two main requirements the strength and ductilitylevel requirements stipulated in API RP2A-WSD [30]recommended practice must be considered in theseismic design of JTOPs Strength level is requiredto make the platform adequately sized for strengthand stiffness to ensure that there is no significantstructural damage during an earthquake vibrationwhich has a reasonable likelihood of not beingexceeded during the life of the structure Usually itis anticipated that the structural part remains elasticduring this phase of design The elastic responsespectrum method is commonly used (both in theplatform studied here and in other studies in literature(El-Din and Kim [12])) in the design of the platformunder the strength level ground vibration Under theductility level ground vibration the platform couldrespond in its nonlinear range of deformation andit is only necessitated that the platform maintains itsstability under the influence of gravity loading Sincethe inelastic characteristic of its behavior requiresan additional detailed knowledge and experience foranalyzing the structure in this range and the designsoftware should also equipped with effective elementsand material constitutive behavior capable of simu-lating nonlinear behavior the response predictionsunder the ductility loading demands could be biasedor even may be neglected by design engineers Inaddition to this some commercial design softwaresuch as Sacs has only the capability to increase theapplied load pattern in a load control manner whichlimit the pushover analysis to the first componentfailure and avoids postnonlinear behavior In light ofthese facts a procedure is presented to be conducted

during the elastic response spectrummethod to avoidpile weak link behavior in rare earthquake eventsIt is recommended to remove the near field soilbearing springs at a distance of 4 to 6 times the pilediameter from the mud line level Then the designresponse spectrum procedure should be repeatedwith the modified model and the pile should besized in a manner that the unity check in the pilesbecomes lower than the unity check in the jacketmembers This procedure has been verified for boththe modified models presented in this paper and fortwo other modifications to the original model whichare not reported here The main reasoning behindthis suggested procedure relies on the fact that whenthe intensity of seismic vibration becomes strongerthe soil in the upper parts of the pile reaches itsultimate shearing capacity and the piles lose theirlateral bearings

(c) The analysis results reported in this paper (both thedynamic and static analysis procedures) reveal thatweak pile strength relative to jacket members cansignificantly reduce the platformrsquos ultimate capacityOn the other hand it is believed that althoughthe consistent strength and stiffness of piles andjacket part are mostly desired to have an optimumbalance between reliable performance and smallersized members the stronger pile foundation relativeto the jacket part shows a better performance thanits counterpart Consequently it is recommendedthat during the design phase the maximum unitychecks in the pile foundation have better be ldquomuchlowerrdquo than the maximum unity checks in the jacketmembers It should be noted that while themaximumunity check in jacket members dominates the pilefoundation unity check in the original HE2 platformthe weak link behaviors in the pile foundation are stillobserved in this case

8 Conclusions

Incremental Dynamic Analysis of a newly designed andinstalled well-head steel offshore platform was performed Inthe same vein a numerical model was developed to assess theperformance of this structure under a suit of 10 pulse-typenear-fault ground motion records OpenSees finite elementsoftware was used to create the numerical model of theplatform considering buckling and postbuckling behaviorof bracing members and material low-cycle fatigue effectsIn order to consider soil-pile-structure interaction effectsthe near-field soil was modeled using Beam on NonlinearWinkler Foundation method Furthermore the geotechnicalcapabilities of OpenSees were utilized to compute free-fieldresponse of soil layers Some individual IDA curves in termsof maximum storey-like drift ratio per unit length of pile andmaximum interstorey drift ratio of jacket bracing storeyswerepresented It was shown that the braces of the jacket storeysdo not buckle and the only source of nonlinearity in thisplatform is the pile foundationThe earlier creation of plastic


hinges in the upper part of the pile foundation caused thisregion of pile to work as a weak storey and prevented otherparts of the platform from meeting at a nonlinear range ofresponse

In order to investigate the importance of pile foundationbehavior in the whole platform response and the probabilityof collapse two modified models of original platform werecreated slightly increasing the diameter of the pile foundationand also softening the jacket part These changes in platformstructural models led to the enhancement of platform seis-mic ultimate strength It was found that the proportionaldistribution of nonlinear behavior in the pile foundationand jacket part is the key factor in enhancement of ultimatestrength of platform This uniform distribution of strengthand stiffness along the platformprofile significantly improvedthe ductility capacity of the structure in the modified modelscompared to the original model which led to higher seismicenergy dissipation capacity of platform In some records theplatform did not reach its final capacity or in other wordscollapse did not occur The reason for this phenomenonis that in some scales of these records the input groundmotion is so strong that it drives the soil to its final capacityand due to strain softening behavior of soil no more loadis transferred to the platform structural part The collapsefragility curve of these modified platform models revealed asignificant reduction in the probability of collapse relative tothe original platformThis reduction in probability of collapseis believed to be the results of (1) a slight increase in thebending strength of the pile in pile strengthened model (2)uniform distribution of damage in the pile foundation andjacket part and (3) passing the soil ultimate shear strengthin higher intensity levels of some records It should be notedthat the pulse period of records was found to be the mostimportant factor influencing the collapse of all platformmodels The higher the pulse period of the record the lowerthe IM at which the collapse occurs There seemed to be amonotonic relationship between the pulse period of recordand the collapse probability This fact is in contrast to thebuilding structures dynamic results for which 119879

119901119879 = 2

(ie a 119879119901that is twice as long as the elastic period of an

oscillator) seems to be the most damaging case because theeffectively elongated period drifts toward the peaks of thepulse period 119879

119901(Alavi and Krawinkler [41]) Comparing

the results of the three platforms dynamic analysis it wasconcluded that the pile foundation plays a paramount rolein dynamic response of platforms and can alter the ultimatestrength of the platform together with its collapse capacityto a higher degree The results of this study reveal thatmuch more investigations should be focused on the couplingbehavior of pile foundation and super structure of JTOPsMoreover current codes of Fixed Offshore Platforms designdo not address direct procedures for evaluating couplinginteraction of pile foundation and jacket part of JTOPs Onthe basis of the findings from this paper it is recommendedthat some basic requirements should be developed in order toensure that the coupling ductility of the various parts of theplatform is optimized during the design process For instancethis study recommended removing the near-field soil bearing

springs at a distance of 4 to 6 times the pile diameter from themud line level during the design phase

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] E Gates W Marshal and S A Mahin ldquoAnalytical methods fordetermining the ultimate earthquake resistance of fixed offshorestructuresrdquo in Proceedings of the Offshore Technology Confer-ence Houston Tex USA 1997

[2] K Kayvani and F Barzegar ldquoHysteretic modelling of tubularmembers and offshore platformsrdquo Engineering Structures vol18 no 2 pp 93ndash101 1996

[3] M Mortazavi and R G Bea ldquoExperimental validation of theultimate limit state limit equilibrium analysis (ULSLEA) withresults from frame testsrdquo in Proceedings of the 7th InternationalOffshore and Polar Engineering Conference pp 321ndash327 Hon-olulu Hawaii USA May 1997

[4] B Asgarian and H Agheshlui ldquoReliability-based earthquakedesign of jacket-type offshore platforms considering pile-soil-structure interactionrdquoAmerican Journal of Applied Sciences vol6 no 4 pp 631ndash637 2009

[5] A A Golafshani M R Tabeshpour and Y Komachi ldquoFEMAapproaches in seismic assessment of jacket platforms (casestudy ressalat jacket of Persian gulf)rdquo Journal of ConstructionalSteel Research vol 65 no 10-11 pp 1979ndash1986 2009

[6] V A Zayas S A Mahin and E P Popov ldquoCyclic inelasticbehavior of steel offshore structuresrdquo Report no UCBEERC-8027 University of California Berkeley Calif USA 1980

[7] M R Honarvar M R Bahaari B Asgarian and P AlanjarildquoCyclic inelastic behavior and analytical modelling of pile-leginteraction in jacket type offshore platformsrdquo Applied OceanResearch vol 29 no 4 pp 167ndash179 2007

[8] A A Golafshani M Kia and P Alanjari ldquoLocal joint flexibilityelement for offshore plateforms structuresrdquo Marine Structuresvol 33 pp 56ndash70 2013

[9] R G Bea and C Young ldquoLoading and capacity effects onplatform performance in extreme condition storm waves andearthquakesrdquo in Proceedings of the 25th Annual Offshore Tech-nology Conference pp 45ndash60 Houston Tex USA May 1993

[10] B Peng B Chang and C Llorente ldquoNonlinear dynamic soil-pile-structure interaction analysis of a deepwater platformfor ductility level earthquakesrdquo in Proceedings of the OffshoreTechnology Conference Houston Tex USA February 2005

[11] K Bargi S R Hosseini M H Tadayon and H Sharifian ldquoSeis-mic response of a typical fixed jacket-type offshore platform(spd1) under sea wavesrdquo Open Journal of Marine Science vol1 no 2 pp 36ndash42 2011

[12] M N El-Din and J Kim ldquoSensitivity analysis of pile-foundedfixed steel jacket platforms subjected to seismic loadsrdquo OceanEngineering vol 85 pp 1ndash11 2014

[13] J Ou X Long Q S Li and Y Q Xiao ldquoVibration control ofsteel jacket offshore platform structures with damping isolationsystemsrdquo Engineering Structures vol 29 no 7 pp 1525ndash15382007

[14] S A Mousavi K Bargi and S M Zahrai ldquoOptimum param-eters of tuned liquid columngas damper for mitigation of


seismic-induced vibrations of offshore jacket platformsrdquo Struc-tural Control and HealthMonitoring vol 20 no 3 pp 422ndash4442013

[15] H-N Li X-Y He and T-H Yi ldquoMulti-component seismicresponse analysis of offshore platform by wavelet energy prin-ciplerdquo Coastal Engineering vol 56 no 8 pp 810ndash830 2009

[16] M M Memarpour M Kimiaei M Shayanfar and M Khan-zadi ldquoCyclic lateral response of pile foundations in offshoreplatformsrdquo Computers and Geotechnics vol 42 pp 180ndash1922012

[17] D Vamvatsikos and C A Cornell ldquoIncremental dynamicanalysisrdquo Earthquake Engineering and Structural Dynamics vol31 no 3 pp 491ndash514 2002

[18] M A Assareh and B Asgarian ldquoNonlinear behavior of sin-gle piles in jacket type offshore platforms using incrementaldynamic analysisrdquo American Journal of Applied Sciences vol 5no 12 pp 1793ndash1803 2008

[19] B Asgarian and A Ajamy ldquoSeismic performance of jackettype offshore platforms through incremental dynamic analysisrdquoJournal of Offshore Mechanics and Arctic Engineering vol 132no 3 Article ID 031301 2010

[20] M N El-Din and J Kim ldquoSeismic performance evaluation andretrofit of fixed jacket offshore platform structuresrdquo Journal ofPerformance of Constructed Facilities Article ID04014099 2014

[21] S D Hasan N Islam and K Moin ldquoA review of fixed offshoreplatforms under earthquake forcesrdquo Structural Engineering andMechanics vol 35 no 4 pp 479ndash491 2010

[22] J W Baker S K Shahi and N Jayaram ldquoNew ground motionselection procedures and selected motions for the peer trans-portation research programrdquo PEER Report Pacific EarthquakeEngineering Research Center University of California Berke-ley Calif USA 2011

[23] P G Somerville N F Smith RW Graves and N A Abraham-son ldquoModification of empirical strong ground motion atten-uation relations to include the amplitude and duration effectsof rupture directivityrdquo Seismological Research Letters vol 68no 1 pp 199ndash222 1997

[24] N N Ambraseys and C P Melville A History of PersianEarthquakes Cambridge University Press Cambridge UK1982

[25] United States Geological Survey USGS Earthquake HazardsProgram httpearthquakeusgsgovearthquakeseqinthenews2005usfyagdetails

[26] K Hessami and F Jamali ldquoExplanatory notes to the map ofmajor active faults of Iranrdquo Journal of Seismology and Earth-quake Engineering vol 8 no 1 pp 1ndash11 2006

[27] M KMohamadi and S Sherkati ldquoThe rupture analysis in SouthPars gas fieldrdquo Oil and Gas Exploration and Production vol1390 no 77 pp 43ndash49 2011 (Persian)

[28] M Abyani Seismic reliability of Jacket Type Offshore Platformwith considering epistemic uncertainty of damping ratio based ongeometric mean [MS thesis] KNToosi University of Technol-ogy Tehran Iran 2013 (Persian)

[29] PEER Transportation Systems Research Program (TSRP)httppeerberkeleyedutransportationprojectsground-motion-studies-for-transportation-systems

[30] American Petroleum Institute Recommended Practice for Plan-ning Designing and Constructing Fixed Offshore Platforms-Working Stress Design API Recommended Practice 2A-WSDRP 2A-WSD 21st edition 2007

[31] S Mazzoni F McKenna M Scott and G Fenves Open Systemfor Earthquake Engineering Simulation (OpenSees)mdashOpenSeesCommandLanguageManual University ofCalifornia BerkeleyCalif USA 2007

[32] M Menegotto and P Pinto ldquoMethod of analysis for cyclicallyloaded reinforced concrete plane frame including changes ingeometry andnon-elastic behavior of elements under combinednormal force and bendingrdquo in Proceedings of the IABSE Sym-posium on Resistance and Ultimate Deformability of StructuresActed on by Well Defined Repeated Loads 1973

[33] R de Souza Forced-based finite element for large displacementinelastic analysis of frames [PhD thesis] University of Califor-nia Berkeley Calif USA 2000

[34] B Asgarian A A Aghakouchak and R G Bea ldquoNonlinearanalysis of jacket-type offshore platforms using fiber elementsrdquoJournal of Offshore Mechanics and Arctic Engineering vol 128no 3 pp 224ndash232 2006

[35] P Uriz and S A Mahin Toward Earthquake-Resistant Designof Concentrically Braced Steel-Frame Structures Pacific Earth-quake Engineering Research Center University of CaliforniaBerkeley Calif USA 2008

[36] R W Boulanger B L Kutter S J Brandenberg P Singh andD Chang ldquoPile foundations in liquefied and aterally spreadingground during earthquakes centrifuge experiments and anal-ysesrdquo Tech Rep UCDCGM-0301 Center for GeotechnicalModeling University of California Davis Calif USA 2003

[37] R W Boulanger C J Curras B L Kutter D WWilson and AAbghari ldquoSeismic soil-pile-structure interaction experimentsand analysesrdquo Journal of Geotechnical and GeoenvironmentalEngineering vol 125 no 9 pp 750ndash759 1999

[38] Z Yang A Elgamal and E Parra ldquoComputational model forcyclic mobility and associated shear deformationrdquo Journal ofGeotechnical and Geoenvironmental Engineering vol 129 no 12pp 1119ndash1127 2003

[39] M Zarrin B Asgarian and R Foulad ldquoA review on factorsaffecting seismic pile response analysis in soft clay and denseliquefying sandrdquo Coupled System Mechanics In press

[40] A K Chopra and R K Goel ldquoModal pushover analysis pro-cedure to estimate seismic demands for buildings theory andpreliminary evaluationrdquo PEER Report 20013 Pacific Earth-quake Engineering Research Center University of CaliforniaBerkeley Calif USA 2001

[41] BAlavi andHKrawinkler ldquoEffects of near-field groundmotionon frame structuresrdquo Tech Rep 138 John A Blume EarthquakeEngineering Center Department of Civil and EnvironmentalEngineering Stanford University Stanford Calif USA 2001

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Active and Passive Electronic Components

Control Scienceand Engineering

Journal of



RotatingMachinery


Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design



Shock and Vibration


Civil EngineeringAdvances in

Acoustics and VibrationAdvances in



Electrical and Computer Engineering

Journal of

Advances inOptoElectronics


Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of


Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014


Chemical EngineeringInternational Journal of Antennas and

Propagation




Navigation and Observation



DistributedSensor Networks



tested under monotonic increasing static loads or pseudostatic cyclic displacement histories In the works of Bea andYoung [9] Peng et al [10] Bargi et al [11] and El-Din andKim [12] the nonlinear dynamic response and performancecharacteristics of JTOPs were studied by subjecting the plat-form to individual earthquake time histories at a special levelof intensity for example ductility level earthquake Ou et al[13] carried out a comprehensive experimental and numericalstudy in order to examine the vibration control effective-ness of the damping isolation system composed of rubberbearings and viscous dampers for a prototype fixed basejacket platform Similarly Mousavi et al [14] evaluated theoptimum parameters of tuned liquid column-gas damperTLCGD and its efficiency when the platform was subjectedto seismic excitation The jacket structure was considered ashear-flexural cantilever beam fixed at the base in their workLi et al [15] developed a convenient method of applyingwavelet transform to multicomponent earthquake motionsto determine the critical incidence of the seismic wavefrom the viewpoint of the input energy of structures Thesoil-pile system was excluded from their numerical jacketmodel A new robust and practical Beam on NonlinearWinkler Foundation model was presented by Memarpouret al [16] for cyclic lateral behavior of pile foundation ofFixed Platforms which could be utilized for seismic analysisHowever the behavior of the piles in their case studyfixed platform was investigated under wave lateral cyclicand monotonic loadings Recently a practical and accurateapproach named Incremental Dynamic Analysis (IDA) wasproposed by Vamvatsikos and Cornell [17] for the predictionof seismic demands from elastic behavior to yielding andfinally collapse capacity of structures A fundamental stepin IDA is to perform a series of nonlinear dynamic analysesof structures under a suit of multiple earthquake groundmotion records each scaled to several intensity levels sothat it covers the whole range of response To capture thenonlinearity in piles Assareh and Asgarian [18] conductedseveral multirecords IDA analysis on single piles of JTOPSAsgarian and Ajamy [19] investigated the seismic perfor-mance of three newly designed JTOPs using IDA analysisprocedure El-Din and Kim [20] utilized IDA procedurefor obtaining the ultimate capacity of the fixed-based jacketoffshore platforms with various bracing configurations Themain shortcoming of the two later works was that the soil-pile-structure interactions were ignored and the pile founda-tion was replaced by equivalent pile stubs In light of previousstudies the pile foundationrsquos nonlinearity and its systematicinfluences on the seismic ultimate capacity of JTOPs havenot comprehensively been covered by previous researchersThis concluding remark was also emphasized in a state-of-the-art review paper on seismic response and behavior ofFixed Offshore Platforms (Hasan et al [21]) The reviewsuggests that most of the previous analyses are based onhydrodynamic loadings only and that very few investigatorshave accounted for the seismic criteria of offshore structuresThey concluded that more realistic models including soil-structure interaction with multidegrees of freedom need tobe analyzed

Recently El-Din and Kim [12] investigated the effectsof both aleatory and epistemic uncertainty on the seismicresponse parameters of an existing offshore platform Thisstudy revealed that the uncertainty in soil-pile axial frictionhas a notable impact on the response parameters The effectof soil lateral capacity uncertainty on seismic response wasneglected in this research While their study is insightfulto the effects of various parameter uncertainties on thegeneral response of platform they did not study the effectsof using different design strategies or parameters on thenature of platform seismic response rather than the generalmaximum response Furthermore the site response analysisof soil was not conducted and it was assumed that the timehistory of earthquake loading was directly applied to theplatform model This will exclude the influences of soil far-field nonlinearity as well as the kinematic interaction effectson the dynamic response of the platform

This paper investigates the seismic behavior of a newlydesigned and installed Jacket Type Offshore Platform locatedin the Persian Gulf by conducting Incremental DynamicAnalysis (IDA) A suit of 10 pulse-type near-fault groundmotion records is selected to perform dynamic analysisNear-source ground motions especially those containingforward directivity effects impose high seismic demandson the structure and therefore are considered suitable forthe purposes of evaluating ultimate strength and collapsemechanism of a strong structure such as JTOPs Moreoverto date the effects of near-source ground motions on JTOPshave not been investigated in previous works Additionallytwo modified models of the original platform are created byslightly increasing the diameter of the pile foundation andalso softening the jacket part for evaluating the importanceof pile foundation and seismic soil-pile structure interactionin the dynamic characteristics of the JTOPs Some individualand multirecords IDA curves in terms of maximum storey-like drift ratio per unit length of pile and maximum inter-storey drift ratio of jacket bracing storeys are provided andvaluable discussions are presented to explore various aspectsof the dynamic behavior of JTOPs Based on the findings fromthis study some practice recommendations are presentedto be devised within the design step Finally by presentingcollapse fragility curves for all three models the effects ofthe pile foundation and similarly the proportionality ofthe stiffness and strength of the pile foundation and jacketpart of the platform on the ultimate collapse probability ofconsidered platforms are investigated

2 Ground Motion Records Used in This Study

Prior to performing nonlinear dynamic analysis a numberof ground motion records must be selected 10 records outof a set of 40 unscaled three-component ground motions(Baker et al [22]) containing strong velocity pulses ofvarying periods in their strike-normal components havebeen selected Pulse-type near-field groundmotions resultingfrom directivity effects are characterized by a distinct highintensity pulse in the velocity time history of motion ina direction perpendicular to the fault rupture plane Theseground motions are expected to occur at sites near the fault




minus15

minus13

minus11

minus09

minus07

minus05

minus03

minus01

01

03

01 1 10Period


ln(S

a)





0

1

2

05

07

12

16 21

25 39 49

15 25 35 45 55 65 75 85 95 105

Num

ber o

f rec

ords

Pulse period (s)

(a)

0

1

2

3

10 30 50 70 90 110 130 150 170

Num

ber o

f rec

ords


(b)

0

1

2

3

4

2 6 10 14 18 22 26 30

Num

ber o

f rec

ords

Duration (s)

(c)








+132

+8

minus19

minus29

minus50

minus71

minus7434

St L 6

St L 5

St L 4

St L 3

St L 2

St L 1

Y X





a

(Km)Vs30(ms)

























0mdashvalue



0value Therefore




















0 001 002 003 004 005 006 0070

01

02

03

04

05

06

07


Sde (

m)

Pile drift (per 1m)


0 0005 001 0015 002 0025 003 0035 0040

01

02

03

04

05

06

07 Jacket drifts


Sde (

m)



Story 7




0 001 002 003 004 005 006 007minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 1 2 3 4 5 6 7 8 90

01

02

03

04

05

06



Sde (

m)

times10minus3










0 001 002 003 004 005 006 007 008 0090

02

04

06

08

1

12

14


Sde (

m)

Pile drift (per 1m)


0

02

04

06

08

1

12

14

0 0002 0004 0006 0008 001 0012 0014



Sde (

m)



0 002 004 006 008 01 012 014 0160

05

1

15

2

25

3

35

4

45 Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014 016 018minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3









119901119879119904ratio)


1199011198791199042 ratio equal



0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation












minus15

minus13

minus11

minus09

minus07

minus05

minus03

minus01

01

03

01 1 10Period


ln(S

a)





0

1

2

05

07

12

16 21

25 39 49

15 25 35 45 55 65 75 85 95 105

Num

ber o

f rec

ords

Pulse period (s)

(a)

0

1

2

3

10 30 50 70 90 110 130 150 170

Num

ber o

f rec

ords


(b)

0

1

2

3

4

2 6 10 14 18 22 26 30

Num

ber o

f rec

ords

Duration (s)

(c)








+132

+8

minus19

minus29

minus50

minus71

minus7434

St L 6

St L 5

St L 4

St L 3

St L 2

St L 1

Y X





a

(Km)Vs30(ms)

























0mdashvalue



0value Therefore




















0 001 002 003 004 005 006 0070

01

02

03

04

05

06

07


Sde (

m)

Pile drift (per 1m)


0 0005 001 0015 002 0025 003 0035 0040

01

02

03

04

05

06

07 Jacket drifts


Sde (

m)



Story 7




0 001 002 003 004 005 006 007minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 1 2 3 4 5 6 7 8 90

01

02

03

04

05

06



Sde (

m)

times10minus3










0 001 002 003 004 005 006 007 008 0090

02

04

06

08

1

12

14


Sde (

m)

Pile drift (per 1m)


0

02

04

06

08

1

12

14

0 0002 0004 0006 0008 001 0012 0014



Sde (

m)



0 002 004 006 008 01 012 014 0160

05

1

15

2

25

3

35

4

45 Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014 016 018minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3









119901119879119904ratio)


1199011198791199042 ratio equal



0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0

1

2

05

07

12

16 21

25 39 49

15 25 35 45 55 65 75 85 95 105

Num

ber o

f rec

ords

Pulse period (s)

(a)

0

1

2

3

10 30 50 70 90 110 130 150 170

Num

ber o

f rec

ords


(b)

0

1

2

3

4

2 6 10 14 18 22 26 30

Num

ber o

f rec

ords

Duration (s)

(c)








+132

+8

minus19

minus29

minus50

minus71

minus7434

St L 6

St L 5

St L 4

St L 3

St L 2

St L 1

Y X





a

(Km)Vs30(ms)

























0mdashvalue



0value Therefore




















0 001 002 003 004 005 006 0070

01

02

03

04

05

06

07


Sde (

m)

Pile drift (per 1m)


0 0005 001 0015 002 0025 003 0035 0040

01

02

03

04

05

06

07 Jacket drifts


Sde (

m)



Story 7




0 001 002 003 004 005 006 007minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 1 2 3 4 5 6 7 8 90

01

02

03

04

05

06



Sde (

m)

times10minus3










0 001 002 003 004 005 006 007 008 0090

02

04

06

08

1

12

14


Sde (

m)

Pile drift (per 1m)


0

02

04

06

08

1

12

14

0 0002 0004 0006 0008 001 0012 0014



Sde (

m)



0 002 004 006 008 01 012 014 0160

05

1

15

2

25

3

35

4

45 Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014 016 018minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3









119901119879119904ratio)


1199011198791199042 ratio equal



0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










+132

+8

minus19

minus29

minus50

minus71

minus7434

St L 6

St L 5

St L 4

St L 3

St L 2

St L 1

Y X





a

(Km)Vs30(ms)

























0mdashvalue



0value Therefore




















0 001 002 003 004 005 006 0070

01

02

03

04

05

06

07


Sde (

m)

Pile drift (per 1m)


0 0005 001 0015 002 0025 003 0035 0040

01

02

03

04

05

06

07 Jacket drifts


Sde (

m)



Story 7




0 001 002 003 004 005 006 007minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 1 2 3 4 5 6 7 8 90

01

02

03

04

05

06



Sde (

m)

times10minus3










0 001 002 003 004 005 006 007 008 0090

02

04

06

08

1

12

14


Sde (

m)

Pile drift (per 1m)


0

02

04

06

08

1

12

14

0 0002 0004 0006 0008 001 0012 0014



Sde (

m)



0 002 004 006 008 01 012 014 0160

05

1

15

2

25

3

35

4

45 Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014 016 018minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3









119901119879119904ratio)


1199011198791199042 ratio equal



0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation


























0mdashvalue



0value Therefore




















0 001 002 003 004 005 006 0070

01

02

03

04

05

06

07


Sde (

m)

Pile drift (per 1m)


0 0005 001 0015 002 0025 003 0035 0040

01

02

03

04

05

06

07 Jacket drifts


Sde (

m)



Story 7




0 001 002 003 004 005 006 007minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 1 2 3 4 5 6 7 8 90

01

02

03

04

05

06



Sde (

m)

times10minus3










0 001 002 003 004 005 006 007 008 0090

02

04

06

08

1

12

14


Sde (

m)

Pile drift (per 1m)


0

02

04

06

08

1

12

14

0 0002 0004 0006 0008 001 0012 0014



Sde (

m)



0 002 004 006 008 01 012 014 0160

05

1

15

2

25

3

35

4

45 Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014 016 018minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3









119901119879119904ratio)


1199011198791199042 ratio equal



0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation














0mdashvalue



0value Therefore




















0 001 002 003 004 005 006 0070

01

02

03

04

05

06

07


Sde (

m)

Pile drift (per 1m)


0 0005 001 0015 002 0025 003 0035 0040

01

02

03

04

05

06

07 Jacket drifts


Sde (

m)



Story 7




0 001 002 003 004 005 006 007minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 1 2 3 4 5 6 7 8 90

01

02

03

04

05

06



Sde (

m)

times10minus3










0 001 002 003 004 005 006 007 008 0090

02

04

06

08

1

12

14


Sde (

m)

Pile drift (per 1m)


0

02

04

06

08

1

12

14

0 0002 0004 0006 0008 001 0012 0014



Sde (

m)



0 002 004 006 008 01 012 014 0160

05

1

15

2

25

3

35

4

45 Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014 016 018minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3









119901119879119904ratio)


1199011198791199042 ratio equal



0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation

























0 001 002 003 004 005 006 0070

01

02

03

04

05

06

07


Sde (

m)

Pile drift (per 1m)


0 0005 001 0015 002 0025 003 0035 0040

01

02

03

04

05

06

07 Jacket drifts


Sde (

m)



Story 7




0 001 002 003 004 005 006 007minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 1 2 3 4 5 6 7 8 90

01

02

03

04

05

06



Sde (

m)

times10minus3










0 001 002 003 004 005 006 007 008 0090

02

04

06

08

1

12

14


Sde (

m)

Pile drift (per 1m)


0

02

04

06

08

1

12

14

0 0002 0004 0006 0008 001 0012 0014



Sde (

m)



0 002 004 006 008 01 012 014 0160

05

1

15

2

25

3

35

4

45 Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014 016 018minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3









119901119879119904ratio)


1199011198791199042 ratio equal



0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0 001 002 003 004 005 006 0070

01

02

03

04

05

06

07


Sde (

m)

Pile drift (per 1m)


0 0005 001 0015 002 0025 003 0035 0040

01

02

03

04

05

06

07 Jacket drifts


Sde (

m)



Story 7




0 001 002 003 004 005 006 007minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 1 2 3 4 5 6 7 8 90

01

02

03

04

05

06



Sde (

m)

times10minus3










0 001 002 003 004 005 006 007 008 0090

02

04

06

08

1

12

14


Sde (

m)

Pile drift (per 1m)


0

02

04

06

08

1

12

14

0 0002 0004 0006 0008 001 0012 0014



Sde (

m)



0 002 004 006 008 01 012 014 0160

05

1

15

2

25

3

35

4

45 Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014 016 018minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3









119901119879119904ratio)


1199011198791199042 ratio equal



0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0 1 2 3 4 5 6 7 8 90

01

02

03

04

05

06



Sde (

m)

times10minus3










0 001 002 003 004 005 006 007 008 0090

02

04

06

08

1

12

14


Sde (

m)

Pile drift (per 1m)


0

02

04

06

08

1

12

14

0 0002 0004 0006 0008 001 0012 0014



Sde (

m)



0 002 004 006 008 01 012 014 0160

05

1

15

2

25

3

35

4

45 Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014 016 018minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3









119901119879119904ratio)


1199011198791199042 ratio equal



0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0 001 002 003 004 005 006 007 008 0090

02

04

06

08

1

12

14


Sde (

m)

Pile drift (per 1m)


0

02

04

06

08

1

12

14

0 0002 0004 0006 0008 001 0012 0014



Sde (

m)



0 002 004 006 008 01 012 014 0160

05

1

15

2

25

3

35

4

45 Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014 016 018minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)






0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3









119901119879119904ratio)


1199011198791199042 ratio equal



0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0 0002 0004 0006 0008 001 0012 0014 0016 00180

05

1

15

2

25

3 Jacket drift


Sde (

m)

Part 1Part 2Part 3









119901119879119904ratio)


1199011198791199042 ratio equal



0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0 001 002 003 004 005 006 007 008 0090

1

2

3

4

5

6 Jacket drift


Sde (

m)



Story 7


0 002 004 006 008 010

1

2

3

4

5

6


Sde (

m)

Pile drift (per 1m)



0 002 004 006 008 01 012 0140

05

1

15

2

25

3Jacket drift


Sde (

m)



Story 7




0 002 004 006 008 01 012 014minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20

40


Hei

ght (

m)





0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0 001 002 003 004 005 006 0070

05

1

15

2

25



Sde (

m)



minus002 0 002 004 006 008 010

05

1

15

2

25

3

35

4 Jacket drift


Sde (

m)



Story 7



minus05

0

05

1

15

2

25

3

35

4

0 05 1 15 2 25 3 35 4 45 5

Base

shea

r (N

)

Displacement (m)

Pushover curve



Pile tearing



Tearing of legs



Tearing of legs

timesE + 07







0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0 002 004 006 008 010

05

1

15

2



Sde (

m)












0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0 005 01 015 020

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 0160

1

2

3

4

5

6



Sde (

m)



0 002 004 006 008 01 012 0140

1

2

3

4

5

6



Sde (

m)


0 002 004 006 008 01 012 014 016 0180

1

2

3

4

5

6



Sde (

m)







0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0010203040506070809

1

0 1 2 3 4

Prob

abili

ty o

f col

laps

e


1049 s824 s

579 s451 s

405 s

1049 s451 s824 s 579 s

405 s352 s

241 s311 s

123 s

18 s

1049 s

824 s

579 s 451 s

405 s

18 s

241 s

Sde (T1 5)






8 Conclusions





119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation












119901119879 = 2








References














































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation








































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation











RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









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