FINAL REPORT Design Standards for Offshore Wind Farms September 2011 Prepared by American Bureau of Shipping Corporate Offshore Technology, Renewables 16855 Northchase Drive Houston, Texas 77060 www.eagle.org Submitted to U.S. Department of the Interior Bureau of Ocean Energy Management, Regulation, and Enforcement 381 Elden Street Herndon, Virginia 20170-4817 Acknowledgement of Sponsorship: This study was funded by the Bureau of Ocean Energy Management, Regulation, and Enforcement, U.S. Department of the Interior, Washington, D.C. under Contract Number M10PC00105. Disclaimer: This report has been reviewed by the Bureau of Ocean Energy Management, Regulation, and Enforcement and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the Service, nor does mention of the trade names or commercial products constitute endorsement or recommendation for use. Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report Acknowledgement This study was funded by the Technology Assessment and Research Program (TA&R Project No. 670),BureauofOceanEnergyManagement,Regulation,andEnforcement(BOEMRE),U.S. Department of the Interior, Washington, D.C. Thewindturbinesimulationprogramsusedinthisproject,includingFAST,AeroDynand TurbSim,aredevelopedandmaintainedbytheNationalRenewableEnergyLaboratory(NREL) of the U.S. Department of Energy. TechnicalsupportsfromCarlStansberg,CasbaPakozdiandSebastienFouquesatMarintek, BingbinYuattheUniversityofMichigan,JasonJonkmanattheNationalRenewableEnergy Laboratory, and John Conlon are gratefully acknowledged. ThisreportispreparedbyQingYu(qyu@eagle.org),KunhoKimandTzu-WeiLoatABS Corporate Offshore Technology, Renewables Group. The project coordinator is Lars Samuelsson at ABSG Consulting. Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report i Table of Contents List of Tables................................................................................................................................. iv List of Figures ............................................................................................................................... vi Executive Summary .......................................................................................................................x 1Introduction ........................................................................................................................... 1 2Design of Offshore Wind Turbines Subjected to Hurricanes on the US OCS................. 4 2.1State-of-the-Art Review................................................................................................ 6 2.1.1Hurricane Wind................................................................................................ 6 2.1.2Hurricane Generated Waves .......................................................................... 14 2.1.3Design Load Case for Hurricane Conditions ................................................. 19 2.1.4Existing Design Standards and Their Applicability to Hurricane Conditions 22 2.1.5Simulation Software....................................................................................... 25 2.2Technical Approach for Case Studies ......................................................................... 27 2.2.1Problem Definition......................................................................................... 27 2.2.2Modeling and Analysis Procedures................................................................ 29 2.3Case Study Models...................................................................................................... 41 2.3.1Site Conditions............................................................................................... 41 2.3.2Wind Turbine RNA and Tower Specifications.............................................. 45 2.3.3Support Structure Configurations .................................................................. 47 2.3.4Sensitivity Study to Determine Modeling Parameters ................................... 60 2.4Comparison of API and IEC Wind Models ................................................................ 63 2.4.1API and IEC Wind Model Definitions........................................................... 64 2.4.2Effect of Using IEC and API Wind Models on Turbine Responses .............. 72 2.4.3Summary ........................................................................................................ 73 2.5Effect of Fault in Turbines Yaw and Pitch Control ................................................... 75 2.5.1Nacelle Yaw Misalignment............................................................................ 76 2.5.2Blade Pitch Angle Locking............................................................................ 77 2.5.3Summary ........................................................................................................ 79 2.6Case Studies Monopile Support Structure ............................................................... 81 Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report ii 2.6.1Summary of Model Parameters...................................................................... 81 2.6.2Monopile Case Study Results ........................................................................ 86 2.7Case Studies Tripod Support Structure.................................................................... 91 2.7.1Summary of Model Parameters...................................................................... 91 2.7.2Tripod Case Study Results............................................................................. 96 2.8Case Studies Jacket Support Structure................................................................... 100 2.8.1Summary of Model Parameters.................................................................... 100 2.8.2Jacket Case Study Results............................................................................ 103 2.9Parametric Comparisons of Case Study Results ....................................................... 107 2.9.1Return Period of Environmental Conditions................................................ 107 2.9.2Normal and Abnormal Turbine Operating Conditions ................................ 114 2.9.3Wind and Wave Misalignment..................................................................... 118 2.9.4Type of Support Structures .......................................................................... 125 2.10Conclusions and Recommendations.......................................................................... 129 2.10.1Hurricane Wind Models............................................................................... 129 2.10.2Design Criteria for Offshore Wind Turbines in Hurricane-Prone Regions.. 129 2.10.3Recommendations........................................................................................ 133 3Breaking Wave Slamming Loads..................................................................................... 135 3.1State-of-the-Art Review............................................................................................ 136 3.1.1Breaking Waves ........................................................................................... 136 3.1.2Analytical Slamming Load Models ............................................................. 140 3.1.3Numerical Methods...................................................................................... 149 3.1.4Slamming Experiments ................................................................................ 157 3.1.5Existing Design Guidelines.......................................................................... 165 3.2Parameter Study ........................................................................................................ 168 3.2.1Wave Related Parameters ............................................................................ 168 3.2.2Structure Related Parameters ....................................................................... 170 3.2.3Some Important Physical Relationships....................................................... 171 3.3Comparative Study of Analytical Breaking Wave Slamming Load Models ............ 173 3.3.1Analytical Models Used in the Comparative Study..................................... 173 3.3.2Comparative Study Results.......................................................................... 176 3.4Conclusions and Recommendations.......................................................................... 183 Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report iii 4Summary ............................................................................................................................ 187 References for Section 2............................................................................................................ 190 References for Section 3............................................................................................................ 198 Appendix A Abbreviations ..................................................................................................... 208 Appendix B Terms and Definitions........................................................................................ 209 Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report iv List of Tables Table 2.1CommonlyReferencedDesignStandards/RegulationsforOffshoreWindTurbines.................................................................................................................................. 23 Table 2.2General Load Cases Descriptions for the Case Studies............................................ 28 Table 2.350-Year Return Water Depth, Current Speed and Wind Speed ............................... 42 Table 2.450-Year Return Wave Condition.............................................................................. 42 Table 2.5100-Year Return Water Depth, Current Speed and Wind Speed ............................. 43 Table 2.6100-Year Return Wave Conditions .......................................................................... 43 Table 2.7Properties of the RNA of NREL 5-MW Baseline OffshoreWind Turbine (Jonkman et al., 2009) ............................................................................................................... 46 Table 2.8Undistributed Blade Properties (Jonkman et al., 2009)............................................ 46 Table 2.9Turbine Tower Properties (Fabian et al, 2011)......................................................... 46 Table 2.10Properties of the Grouted TP and Pile...................................................................... 49 Table 2.11Monopile Support Structure Geometric Properties .................................................. 49 Table 2.12Monopile Support Structure Mass Properties (with Marine Growth) ...................... 49 Table 2.13Properties of Equivalent Fixity of Equivalent Monopile.......................................... 50 Table 2.14Uniform Cd and Cm of Equivalent Monopile Used in the FAST Analysis ............... 50 Table 2.15Monopile Natural Frequencies ................................................................................. 50 Table 2.16Properties of the Equivalent Pile Simulating the Leg Sleeve and Pile ..................... 53 Table 2.17Tripod Support Structure Geometric Properties....................................................... 53 Table 2.18Tripod Mass Properties (with Marine Growth) ........................................................ 53 Table 2.19Properties of Equivalent Fixity of the Tripod Equivalent Monopile........................ 54 Table 2.20Uniform Cd and Cm of the Tripods Equivalent Monopile Used in the FAST Analysis.................................................................................................................................. 54 Table 2.21Tripod Natural Frequencies...................................................................................... 54 Table 2.22Properties of the Equivalent Pile Simulating the Grouted Leg and Pile................... 57 Table 2.23Jacket Support Structure Geometric Properties........................................................ 58 Table 2.24Jacket Mass Properties (with Marine Growth and Sea Water in Flooded Legs) ...... 58 Table 2.25Uniform Cd and Cm of the Jackets Equivalent Monopile Used in the FAST Analysis.................................................................................................................................. 58 Table 2.26Properties of Equivalent Fixity of Jacket Equivalent Monopile............................... 59 Table 2.27Jacket Natural Frequencies....................................................................................... 59 Table 2.28Input Parameters for Wind Model Comparison Study ............................................. 64 Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report v Table 2.29Spectral Parameters for the Kaimal Model............................................................... 66 Table 2.30Turbulence Properties at Hub Height Based on the API Wind Model ..................... 69 Table 2.31Turbulence Properties at Hub Height Based on the IEC Wind Model ..................... 69 Table 2.32CoefficientsandDistancesfortheCoherenceFunction(APIBulletin2INT-MET, 2007)......................................................................................................................... 70 Table 2.33Static Wind Forces on the Monopile Support Structure........................................... 82 Table 2.34MudlineBaseShearandOverturningMomentDuetoWaveSlammingonthe Monopile................................................................................................................... 84 Table 2.35Summary of Structural Loads for the Monopile Case Studies ................................. 85 Table 2.36StructuralReponsesandUtilizationCheckResultsfortheMonopileSupport Structure ................................................................................................................... 89 Table 2.37Monopile Case L12 Member Utilization Ratio Check............................................. 90 Table 2.38Static Wind Forces on the Tripod Support Structure................................................ 92 Table 2.39MudlineBaseShearandOverturningMomentDuetoWaveSlammingonthe Monopile................................................................................................................... 94 Table 2.40Summary of Structural Loads for the Tripod Case Studies...................................... 95 Table 2.41Structural Reponses and Utilization Check Results for the Tripod Support Structure.................................................................................................................................. 98 Table 2.42Tripod Case L16 Member Utilization Ratio Check.................................................. 99 Table 2.43Static Wind Forces on the Jacket Support Structure .............................................. 101 Table 2.44Summary of Structural Loads for the Jacket Case Studies..................................... 102 Table 2.45StructuralReponsesandUtilizationCheckResultsfortheJacketSupportStructure................................................................................................................................ 105 Table 2.46Jacket Case L28 Member Utilization Ratio Check................................................. 106 Table 2.47Occurrence of Breaking Wave Slamming.............................................................. 108 Table 3.1Breaking Wave Types ............................................................................................ 139 Table 3.2Commonly Used Design Guidelines for Wave Slamming Loads .......................... 167 Table 3.3Load Case Definition for the Wave Slamming Comparison.................................. 180 Table 3.4Maximum Slamming Responses at the Mudline Calculated Using the Wienke Modeland the Truncated Campbell-Weynberg Model ..................................................... 180 Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report vi List of Figures Figure 2.1Hurricane (Tropical Cyclone) Generated Wave Field (Young, 2003) ..................... 16 Figure 2.2Wind and Primary Wave Field of 1998 Hurricane Bonnie ...................................... 16 Figure 2.3ReliabilityIndexforNormalConditionsunderWindorWaveLoads(Yuetal., 2011)......................................................................................................................... 24 Figure 2.4ReliabilityIndexforAbnormalConditionsunderWindorWaveLoads(Yuetal., 2011)......................................................................................................................... 25 Figure 2.5Modeling and Dynamic Analysis Procedure............................................................ 31 Figure 2.6Regional Wind Conditions in the US Atlantic Coast ............................................... 33 Figure 2.7Gulf of Mexico West Central Region....................................................................... 33 Figure 2.8Site Location in the GOM (NOAA NDBC Station 42035)...................................... 33 Figure 2.9Gumbel Curve Fitting for the Buoy Wind Data in Hurricane Seasons .................... 34 Figure 2.10Gumbel Curve Fitting for the Buoy Wave Data in Hurricane Seasons .................... 34 Figure 2.11Correlation of Tp and Hs Using the Binned Buoy Data in Hurricane Seasons.......... 35 Figure 2.12Verification of Modal Shapes of Equivalent Monopile for FAST Input .................. 38 Figure 2.13Verification of Cd and Cm of Equivalent Monopile for FAST Input......................... 38 Figure 2.14Wave Profile of 50-Year Return Waves................................................................... 44 Figure 2.15Wave Profile of 100-Year Return Waves................................................................. 44 Figure 2.16Undrained Shear Strength of Undisturbed Clay....................................................... 45 Figure 2.17RNA of NREL 5-MW Baseline Offshore Wind Turbine (Jonkman et al., 2009) .... 45 Figure 2.18Campbell Diagram of NREL 5-MW Baseline Offshore Wind ................................ 47 Figure 2.19Monopile Support Structure and Its Equivalent Monopile....................................... 48 Figure 2.20Modal Shapes of the Monopile Support Structure ................................................... 51 Figure 2.21Tripod Support Structure and Its Equivalent Monopile ........................................... 52 Figure 2.22Modal Shapes of the Tripod Support Structure........................................................ 55 Figure 2.23Jacket Support Structure and Its Equivalent Monopile ............................................ 57 Figure 2.24Modal Shapes of the Jacket Support Structure......................................................... 59 Figure 2.25Mudline Shear Force (Fore-Aft Direction) Averaged over Multiple Realizations... 61 Figure 2.26Mudline Overturning Moment Averaged over Multiple Realizations...................... 61 Figure 2.27Comparison of Mudline Shear Force and Overturning MomentCalculated Using the Two Different Domain Sizes.................................................................................... 62 Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report vii Figure 2.28Sensitivity of Mudline Shear Force and Overturning Moment to the Grid Resolution.................................................................................................................................. 62 Figure 2.29ComparisonofWindShearfortheGOMWestCentral100-yrReturnWind Condition.................................................................................................................. 65 Figure 2.30ComparisonofWindSpectrafortheGOMWestCentral100-yrReturnWind Condition.................................................................................................................. 67 Figure 2.31StandardDeviationandReynoldsStressfortheGOMWestCentral100-yrReturn Wind Condition ........................................................................................................ 69 Figure 2.32TimeSeriesoftheWindSpeedatHubHeightfortheGOMWestCentral100-yr Return Wind Condition ............................................................................................ 69 Figure 2.33Coherence IEC and API Wind Model at f=0.004Hz ................................................ 71 Figure 2.34Coherence IEC and API Wind Model at f=0.01Hz.................................................. 71 Figure 2.35Coherence IEC and API Wind Model at f=0.1Hz.................................................... 71 Figure 2.36Comparison of the Maximum Shear Force and Overturning Moment at the Mudline.................................................................................................................................. 72 Figure 2.37Time Series and Amplitude Spectrum of the Shear force at the Mudline ................ 73 Figure 2.38SensitivityofMudlineShearForcetoNacelleYawAnglefortheGOMWest Central 100-yr Return Wind Condition.................................................................... 77 Figure 2.39SensitivityofMudlineOverturningMomenttoNacelleYawAnglefortheGOM West Central 100-yr Return Wind Condition........................................................... 77 Figure 2.40SensitivityofMudlineShearForcetotheBladePitchAnglefortheGOMWest Central 100-yr Return Wind Condition.................................................................... 78 Figure 2.41Sensitivity of Mudline Overturning Moment to the Blade Pitch Angle forthe GOM West Central 100-yr Return Wind Condition........................................................... 79 Figure 2.42Static Wind Forces on the Monopile Support Structure........................................... 82 Figure 2.43Time History of Breaking Wave Slamming Loads on the Monopile....................... 83 Figure 2.44Mudline Base Shear Forces Due to Wave Slamming on the Monopile ................... 83 Figure 2.45Mudline Overturning Moments Due to Wave Slamming on the Monopile ............. 84 Figure 2.46 Monopile Member Size and Location for Utilization Ratio Calculation................. 88 Figure 2.47Static Wind Forces on the Tripod Support Structure................................................ 92 Figure 2.48Time History of Breaking Wave Slamming Loads on the Tripod............................ 93 Figure 2.49Mudline Base Shear Forces Due to Wave Slamming on the Tripod........................ 93 Figure 2.50Mudline Overturning Moments Due to Wave Slamming on the Tripod.................. 94 Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report viii Figure 2.51 Tripod Member Size and Location for Utilization Ratio Calculation ..................... 97 Figure 2.52Static Wind Forces on the Jacket Support Structure .............................................. 101 Figure 2.53 Jacket Member Size and Location for Utilization Ratio Calculation .................... 104 Figure 2.54 Ratio of 100-yr Return to 50-yr Return Mudline Shear Forces............................. 110 Figure 2.55 Ratio of 100-yr Return to 50-yr Return Mudline Overturning Moments.............. 110 Figure 2.56 Ratio of 100-yr Return to 50-yr Return Maximum Utilization Ratios .................. 110 Figure 2.57 MudlineShearForcesandOverturningMoments(50-yrvs.100-yr)intheATL2 Region..................................................................................................................... 111 Figure 2.58 MudlineShearForcesandOverturningMoments(50-yrvs.100-yr)intheGOM Region..................................................................................................................... 112 Figure 2.59 Utilization Ratios (50-yr vs. 100-yr) at the ATL2 and GOM Regions.................. 113 Figure 2.60 Mudline Shear Forces and Overturning Moments (Normal vs. Abnormal)under the 50-Year Return Conditions..................................................................................... 115 Figure 2.61 Mudline Shear Forces and Overturning Moments (Normal vs. Abnormal)under the 100-Year Return Conditions................................................................................... 116 Figure 2.62 Member Utilization Ratios (Normal vs. Abnormal).............................................. 117 Figure 2.63 Turbine Tower Base Shear and Overturning Moment for the Monopile Case...... 119 Figure 2.64 Turbine Tower Base Shear and Overturning Moment for the Tripod Case .......... 120 Figure 2.65 Turbine Tower Base Shear and Overturning Moment for the Jacket Case ........... 121 Figure 2.66 MudlineShearForcesandOverturningMomentsvs.Wind-WaveMisalignment (under the 50-Year Return Conditions in the GOM West Central Region) ........... 122 Figure 2.67 MudlineShearForcesandOverturningMomentsvs.Wind-WaveMisalignment (under the 100-Year Return Conditions in the GOM West Central Region) ......... 123 Figure 2.68 MemberUtilizationRatiosvs.Wind-WaveMisalignment(GOMWestCentral Region) ................................................................................................................... 124 Figure 2.69 Mudline Shear Forces and Overturning Moments (All Structures in ATL2)........ 127 Figure 2.70 Mudline Shear Forces and Overturning Moments (All Structures in GOM) ........ 127 Figure 2.71 MudlineShearForcesandOverturningMoments(AllStructuresunderthe100-Year Return Conditions in the GOM with Various Wind-Wave Misalignments).. 128 Figure 3.1Definition of Parameters for Calculating Wave Slamming Forces ........................ 142 Figure 3.2Splashing Pattern of Plunging Breaking Wave (IEC61400-3 Annex D)................ 145 Figure 3.3Breaking Wave Slamming Load on a Circular Cylinder (IEC 61400-3, 2009)...... 174 Figure 3.4 Wave Slamming Load of Four Different Models.................................................. 177 Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report ix Figure 3.5 Time History of Mudline Shear Forces ................................................................. 178 Figure 3.6 Time History of Mudline Bending Moments ........................................................ 178 Figure 3.7Undrained Shear Strength for Stiff and Soft Soil ................................................... 180 Figure 3.8Mudline Shear Forces Calculated Using the Wienke Model.................................. 181 Figure 3.9MudlineShearForcesCalculatedUsingtheTruncatedCampbell-WeynbergModel................................................................................................................................ 181 Figure 3.10Mudline Bending Moments Calculated Using the Wienke Model......................... 182 Figure 3.11MudlineBendingMomentsCalculatedUsingtheTruncatedCampbell-Weynberg Model...................................................................................................................... 182 Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report x Executive Summary The main objectives of this project, as outlined in the BOEMRE Contract M10PC00105, are to:Studythegoverningloadcasesandloadeffectsofbottom-foundedoffshorewindturbines subjected to the hurricanes on the US Outer Continental Shelf (OCS) Reviewandevaluatetheexistingmethodsofcalculatingthebreakingwaveslammingload exerted on an offshore wind turbine support structure Providerecommendationstosupportthefutureenhancementtotherelevantdesigncriteria for offshore wind turbinesThisreportpresentstheresultsofthestate-of-the-artreview,thecasestudyresultsforthe characteristicresponsesof bottom-foundedoffshorewind turbinesassumedasbeinginstalledin hurricane-proneregionsontheUSOCS,theresearchfindingsofmodelingbreakingwave slamming loads, and the recommended design methods. The study starts with a review of the existing knowledge applicable to offshore wind turbines to bedeployedontheUSOCS.Casestudies,primarilyinvolvingbottom-foundedwindturbines subjected to tropical hurricane conditions in US waters, are carried out to evaluate governing load cases and load effects. Emphasis is given to both the relevant load cases specified in IEC 61400-3 andthosethatarenotspecifiedintheIECstandardsbutconsideredcriticaltohurricane-prone regions. Three typical configurations of turbine support structure, including monopile, tripod and jacketassumedasbeinginstalledatlocationsintheMid-Atlantic,SouthAtlanticandGulfof MexicocoastalregionsontheUSOCS,areselectedforthecasestudies.Hurricaneconditions withvariouscombinationsofothersiteconditionssuchaswaterdepthandreturnperiodsare considered.Thewell-documented5MWbaselineoffshorewindturbinedevelopedbyNational RenewableEnergyLaboratory(NREL)isemployedinthecasestudies.Theeffectoffault conditionsintheturbinescontrolsystemfornacelleyawandbladepitchisevaluated.The recommendations on the design environmental conditions, design load cases and strength criteria are made based upon the findings from the literature review, comparative studies and case studies. Thestate-of-the-artreviewisalsoperformedtostudytheexistingdesignmethodsforoffshore windturbinesupportstructuressubjectedtobreakingwaveslammingloads.Variousanalytical Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report xi and numerical methods for predicting wave slamming loads are reviewed. An assessment is also performed to identify the relative importance of the parameters that affect the calculation of wave slamming loads. Nonlinear dynamic analyses are carried out to examine the response of a typical offshore monopile support structure under the wave slamming forces, which are estimated using thefourrepresentativeslammingloadmodels.Thecombinedeffectofsoilconditionsand structuraldampingisevaluated.Basedontheresultsoftheliteraturereviewandnumerical analyses,recommendationsaregivenforthedesignofcylindricalstructuralmemberssubjected to the breaking wave slamming (impact) loads. Fourtechnicalareasareconsideredofgreatimportanceandrecommendedforfurtherstudies. Theyincludeturbulenthurricanewindmodeling,foundationsoil-structureinteractionmodels, offshorewindturbineanalysisproceduresandsoftwaretools,andvalidationofslammingload models. Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 1 1Introduction The US offshore oil and gas industry and the Bureau of Ocean Energy Management, Regulation, andEnforcement(BOEMRE)haveextensiveexperiencewithhydrocarbon-relatedoffshore structuresinstalledontheUSOuterContinentalShelf(OCS).AseriesofRecommended PracticeshasbeenpublishedthroughAmericanPetroleumInstitute(API)undertheir SubcommitteeonOffshoreStructures.TheprimarydocumentisAPIRP2A-WSD,which provides a comprehensive basis for the design of offshore structures located on the US OCS and subjectedtowave,wind,currentandearthquakeloads.WhatisnotavailablefromAPIRP2A-WSD,however,isthedefinitionofdesignenvironmentalconditionsanddesignloadconditions that can capture the unique response characteristics of offshore wind turbines. Several design and certification guidelines for offshore wind turbines have been developed in the past ten years. Most of them are based on the experience from offshore wind turbines installed in European coastal areas. Of these guidelines, the International Electrotechnical Commission (IEC) 61400-3(2009)WindturbinesPart3:DesignRequirementsforOffshoreWindTurbines represents industrys state-of-the-art knowledge of offshore wind turbines by the time this IEC standard was published. Although a framework of defining the site-specific Class S wind turbines isincludedinIEC61400-1(2005)WindturbinesPart1:DesignRequirementsandfurther referredtobyIEC61400-3(2009),theapplicabilityofIECstandardstotheoffshoreareas subjected to the risk of hurricane remains unanswered.Thelatestguidelineforbottom-foundedoffshorewindturbinesABSGuideforBuildingand ClassingOffshoreWindTurbineInstallationswaspublishedin2010.TheABSGuide establishedasetofdesigncriteriathattakesintoaccounttheriskoftropicalstormsontheUS OCS.Calibrationshavebeencarriedouttodemonstratethatanoffshorewindturbinesupport structure in US waters designed to ABS criteria is at least as safe as those designed to IEC 61400-3(2009)underEuropeanoffshoreconditions.Thecurrentprojectispartiallybasedonthe experience that the ABS project team has garnered during the development of the ABS Guide.Existingoffshorewindturbinesincommercialoperationaredominatedbybottom-founded designs installedinwaters lessthan30mdeep.Inthisrelativelyshallowwater, theprogressive waveshavehighpossibilityofreachingitsbreakinglimitduetobathymetryeffectsandwill interactwiththewindturbinesupportstructure.Theexpectedloadsexertedbybreakingwaves Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 2 areoftransientnatureandcanbesignificantlyhigherthantheloadsfromnon-breakingwaves. Breakingwavesinshallowwaterscanalsopotentiallyresultinotherissuessuchasfoundation scouring and slamming load induced vibrations of the turbine support structure. Wave slamming is a strongly nonlinear phenomenon that exhibits significant statistical scattering. Hence,thedevelopmentofarobustmodelforpredictingwaveslammingloadsisavery challenging subject. There are a few existing guidelines, such as the one provided in IEC 61400-3 (2007) Annex D, which can be used to determine the breaking wave load for design. This project isaimedtoevaluatetheexistingdesignmethodsandperformcomparativestudiesusingthe published analytical slamming load models. Based on the findings from the literature review and numericalanalyses,recommendationsaremadeforthedesignofoffshorewindturbinesupport structures subjected to breaking wave slamming loads. Thisreportisorganizedaroundthetwomainsubjectsstudiedinthisproject.Thefirstisthe governingloadcasesandloadeffectsofoffshorewindturbinessubjectedtohurricane environmental conditions on the US OCS. The second is the evaluation of the existing methods of calculating the breaking wave slamming loads on offshore wind turbine support structures. Section 2 presents the research findings on the design load cases for offshore wind turbines to be deployed in hurricane-prone regions on the US OCS. The section starts with the discussion of the outcome of the state-of-the-art review of subjects relevant to the hurricane design load cases. The resultsofcomparativestudyoftworepresentativeturbulentwindmodelsandthestudyofthe effectoffailureofturbinesyawandpitchcontrolsystemsarethenreported.Extensivecase studiesusingthreetypical configurationsofoffshore windturbinesupportstructuresarecarried outtoexplorecharacteristicresponsesofoffshorewindturbinessubjectedtohurricane environmental conditions. The correlations of the support structure responses with various design parameters,includingtheenvironmentalconditions,supportstructureconfigurations,site conditions and turbine operating conditions are evaluated in the study and reported in this section. Thissectionconcludeswiththerecommendedstrengthdesigncriteriaforbottom-founded offshore wind turbines to be installed in hurricane-prone regions.Section3isdedicatedtothereviewandevaluationofexistingbreakingwaveslammingload models.Theoutcomeofthestate-of-the-artreview,whichcoversvariousanalyticaland numericalwaveslammingforcecalculationmethodsaswellastheslammingexperiments,are firstreportedinthissection.Anevaluationisperformedtoidentifytherelativeimportanceof Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 3 variousparametersaffectingthecalculationofwaveslammingforces.Thisisfollowedby parametricanalysesoffourrepresentativeanalyticalslammingloadmodelsaswellasthe resultantdynamicresponsesofamonopilesupportstructure.Thecombinedeffectofsoil conditions and structural damping ratios is evaluated. Based on the results of the literature review and numerical analyses, recommendations are made for the design of an offshore wind turbine for which the breaking wave slamming is a matter of concern. Main conclusions and recommendations for future research are provided in Section 4. Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 4 2Design of Offshore Wind Turbines Subjected to Hurricanes on the US OCS Anumberofstudieshavebeenundertakenfromvariousperspectivestoexaminethegaps betweenexistingoffshorewindturbinedesignstandardsandtheestablishedpracticesfor designingoffshoreoilandgasplatformsontheUSOCS.Someofthosestudiesexploredthe means of extending the coverage of IEC 61400-3 (2009), which is primarily developed based on the experience from Europe, to hurricane-prone regions. Efforts have also been made to adapt the designcriteria inAPIRP2A-WSD(2007) forthedesignofoffshorewindturbines subjected to hurricane risks. Thestudyreportedinthissectionexpandedthescopesofpreviousstudies,particularlythose carriedoutduringthedevelopmentoftheABSGuideforBuildingandClassingOffshoreWind Turbine Installations (2010). The following summarizes the scope of work of the present study.State-of-the-artreviewofthemodelingofhurricanewindsandwaves,thedefinitionof designloadcasesforhurricaneconditions,applicabilityofexistingdesignstandardsand regulations, and simulation software. Comparative study of the effect of using the Kaimal wind model referred to in IEC 61400-3 (2009), and the NPD (Frya) wind model recommended by API RP 2A-WSD (2007). Comparative study of the effect of failure of nacelle yaw and blade pitch control systems due to a control system fault or loss of electric power supply. Casestudiesforthethreetypicalsupportstructureconfigurationsviz.monopile,tripodand jacket.Oneconceptualdesignforeachtypeofsupportstructureisdefinedforthecase studies. Regional site conditions in the Gulf of Mexico as well as the Mid-Atlantic and South AtlanticsectionsoftheUSEastCoastareusedinthecasestudies.Siteconditionsatone specific location near the Texas coastline in the Gulf of Mexico are also included. Parametricstudyofthecorrelationsbetweentheglobalandlocalresponsesofturbine supportstructuresandthemaindesignparametersincludingreturnperiodofdesign environmentalconditions,normalandabnormalturbineoperatingmodes,misalignment between wind and wave directions, type of support structures and site variations. Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 5 Conclusions and recommendations drawn based on the findings obtained in this study. Section2.1presentstheresultsofthestate-of-the-artreview.Thetechnicalapproachesand softwareemployedinthecasestudiesaresummarizedinSection2.2.Thecasestudymodels, environmental conditions, foundation properties and load case definitions are described in Section 2.3.Section2.4presentstheresultsofcomparativestudyusingtheIECandAPIwindmodels, whileSection2.5isdedicatedtothecomparativestudyofturbineresponsesundervarious scenarios of nacelle yaw misalignment and blade pitch angle locking. Detailed case study results forthethreeconceptualdesignsarepresentedinSection2.6,2.7and2.8,respectively.Section 2.9summarizesthefindingsfromtheparametricevaluationoftheeffectofvariousdesign parameters on global and local responses of offshore wind turbine support structures. Conclusions and recommendations are discussed in Section 2.10. Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 6 2.1State-of-the-Art ReviewThestate-of-the-artreviewisfocusedonfollowingsubjectsthatareconsideredrelevanttothe objectives of this project. Hurricane conditions and relevant wind and wave models applicable to the US OCS Technical basis employed to define the design load cases and strength criteria in the existing offshore wind turbine design guidelines and its applicability to hurricane-prone regions Available offshore wind turbine analysis tools 2.1.1Hurricane Wind Theextremewindconditionisoneofthemostimportantexternalconditionsthatneedtobe consideredinthedesignofoffshorewindturbines.OntheUSAtlanticCoastandtheGulfof Mexico,thisextremewindconditionismostlycharacterizedbyhurricanes.Asimilartypeof strongweatherconditionisalsoreferredtobynamessuchastropicalcyclonesortyphoonsin other parts of the world. This review is focused on the surface level hurricane wind modeling and datacollection.Informationonmeanwindprofiles,gustfactors,turbulenceintensity,integral scale, and turbulence spectra and coherence is essential for developing a realistic wind model.2.1.1.1Wind DataWind datameasurementprovidesstatisticalwind characteristicsthat canbeusedasinputto the theoreticalwindmodelsforhurricaneforecast.Historicalwinddataarealsocommonlyusedto calibrate wind models. Winddataaremainlymeasuredbysurfacewindmeasuringstations.AutomatedSurface ObservingSystem(ASOS)operatedbyNationalWeatherService(NWS)isoneoftheprimary sourcesforsurfacewindmeasurements.NearlyallASOSstationsarelocatedatairports.At offshoreandcoastalsites,NationalDataBuoyCenter(NDBC)ofNationalOceanicand AtmosphericAdministration(NOAA)managesthedatabuoysandtheCoastalMarine Automated Network (C-MAN) to measure near surface wind conditions. Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 7 Inadditiontothesurfacewindmeasuringstations,thesurfacewindspeedcanalsobeinferred fromotherdatasourcesincludingsatelliteimages,reconnaissanceaircrafts,andradarand radiosonde observations [2.60]. Some of the latest wind field measurement devices include Sonic DetectionandRanging(SODAR),LightDetectionandRanging(LIDAR)andSatellitebased SAR (Synthetic Aperture Radar) [2.51]. There are several hindcast databases that have been developed for the areas subjected to the risk ofhurricanes.TheWaveInformationStudies(WIS)Project[2.72]sponsoredbytheUSArmy Corps of Engineers (USACE) maintains more than 20 years of hourly wind speed, wind direction and wave data along all US coastlines, including the Great Lakes and US island territories.The database is available for the public access through USACEs website.Another useful database is developed by the U.S. National Centers for Environmental Prediction (NCEP)andtheNationalCenterforAtmosphericResearch(NCAR)[2.54].TheNCEP/NCAR databasecoversabout50yearsofwinddataworldwidederivedthroughananalysisofglobal weatherobservationsusinganumericalweatheranalysisandmodelingsystem.Thedatabase provides a useful data source for estimating extreme wind speeds at a given location worldwide. However,asindicatedbyMann,etal.(2002)[2.50],thelowresolutionofthereanalysismodel andthecoarsetemporalresolutionof6hourscouldresultinanunderestimationoftheactual extreme wind speed.OneofthecommerciallyavailablesourcesisfromOceanweather,Inc.[2.56].FortheGulfof Mexico,OceanweathersGOMOS(GulfofMexicoOceanographicStudy)databaseincludes29 yearsofcontinuouswindandwavehindcastdatafortheperiodbetween1980and2008,379 tropicalevents(hurricanes)fortheperiodbetween1900and2008,and68extra-tropicalevents (winterstorms)fortheperiodbetween1950and2008.TheGOMOSdatabasewasusedin supportofthedevelopmentofAPIBulletin2INT-MET(2007)[2.11].Hindcastwindandwave data thatincludethehistoricaltropical andextra-tropicalstormsinthenear-shoreregionsalong part of the US East Coast and West Coast are also available from Oceanweather, Inc. For the western North Pacific region, the best track data are derived by Japanese Meteorological Agency (JMA) and Joint Typhoon Warning Center (JTWC) using the hindcast technique and the available measurement of typhoons [2.60] [2.18]. The data sets contain the center position and the centralpressure(atsealevel)at6hoursintervals.Otherinformationcontainedinthebesttrack data varies at different sites. Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 8 Other than the above hindcast or reanalysis data, measured data of a single hurricane or typhoon (e.g.[2.33],[2.43],[2.80] and[2.85])mayalsobeusedtoverifythephysicalmodels.Thehigh resolution hurricane wind data measured on land near coastline are available through Texas Tech Hurricane Research Team (e.g. [2.33]) and Florida Coastal Monitoring Program (FCMP) [2.73]. Multi-levelmeasurementonamobiletowermakesitpossibletoinvestigatethevariationsof somewindparameterswiththeheightabovesurface.Inaddition,simultaneousmeasurements fromthreeorfourtowersduringthesamehurricanepassagecanbeusedtoanalyzespatial correlations. Theextremewindspeedsforparticularreturnperiodsarespecifiedinnumberofdesign guidelines.RegionalaveragedextremewindspeedsintheGOMcanbefoundinAPIBulletin 2INT-MET (2007) [2.4]. API RP 2A-WSD (2007) [2.5] covers a number of other locations on the USOCS,butonlyprovides100-yearreturnwindspeeds.RecentlypublishedASCE-7(2010) [2.6] includes the updated wind speed contour maps with various return periods for both on-land andcoastalregionsintheUS.BOEMRETA&RProjectNo.672,whichisexpectedtobe completedin2012,ismeanttoprovideacomprehensivemetoceandatasetfortheoffshoreUS Mid-Atlantic Coast. 2.1.1.2Hurricane Wind Field Model Hurricanewindfieldmodelinghasbeenthesubjectofconsiderableresearchovermanyyears [2.76].Dvorak(1975)[2.23]developedahurricanerankingscalebyassigningtheTscale numberstohurricanesrecordedbysatelliteimaging.EachTnumber,whichrangesonascale from1to8,isassociatedwiththelevelofestimatedcentralpressureandmaximumsustained wind. This method is a purely empirical approach because it solely relies on the recognition of the shapesandpatternsofsatelliteimagesofhurricanes.Tosomeextent,italsodependsonthe absolute sizes of the satellite images. Holland(1980)[2.35]proposesatheoreticalhurricanemodelthatusesanexponential formulationtodefinethe relationshipbetweenwindspeed and theradiusfromhurricanecenter. Theparametersofthismodelcanbecalculatedfrombesttrackdata.Thismethodhasbeen validatedbycomparingtothemeasurementofwindfieldofmanytropicalcyclonesand hurricanes, mostly in the central parts of the cyclone and within about 3-4 times the radius of the maximumwindspeed(Rw)(seee.g.[2.81]).Applicationsandvalidityofthismodelarealso Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 9 discussed in Black et al., (1999) [2.12], Ochi (2003) [2.58], Ott (2006) [2.60], and Vickery et al. (2007) [2.76]. AhurricanewindfieldmodeldevelopedbyVickeryetal.(2004)[2.77]hasshownsome advantagesincomparisontoothermodelsdevelopedinpreviousstudies.Ratherthanusingthe spectral model, a fully nonlinear solution to the equations of motion of a translating hurricane is derived.Thismodelappearstobeabletopredictaccuratelytheasymmetriesinfastmoving hurricanesaswellastheeffectoftheseasurfaceroughness.Over90full-scalehurricanewind speedmeasurementsareusedtoevaluatethevalidityofthehurricanewindfieldmodel.The evaluation shows that the model provides a good representation of the hurricane wind field. It is notedthatthebasisofthisapproachistheassumptionthatthelarge-scalestructureofthe hurricane wind fields changes relatively slowly over time. AsindicatedbyVickeryetal.(2007)[2.76],animportantstepintheentirehurricanemodeling processistheabilityofthewindfieldmodelusedinthesimulationproceduretoreproduce measured wind speeds as well as the pressure field. Validation methods include comparison to the observation of past hurricanes [2.60] [2.59] [2.36] and wind tunnel experiment [2.50].2.1.1.3Gust Factor The gust factor is defined as the ratio of the peak wind speed averaged over a short period (e.g., 3 second) to the mean wind speed averaged over a relatively long reference period (e.g., one hour).The gust factor depends on many characteristics of a wind field, such as the roughness length, the transitional flow regimes (change in terrain, distance from upstream terrain, etc.), the anemometer height, the stability of boundary layer and the presence of deep convection.AtechnicalreportpreparedbyHarperetal.(2009)[2.34]andsponsoredbytheWorld MeteorologicalOrganization(WMO)providesaguidelineforconvertingbetweenwindspeeds havingdifferenttimeaveragingperiodsundertropicalcycloneconditions.Thegustfactorsare definedforthedifferentsurfaceconditionsandoriginsofcyclone,i.e.In-Land:Roughlyopen terrain,Off-Land:Offshorewindatacoastline,Off-Sea:Onshorewindatacoastlineand At-Sea:Beyond20kmoffshore.Anextensivereviewisperformedofthetheoreticaland numericalmodelsandtheexistingrecommendedpracticesasdocumentedinthefiveWMO tropicalcycloneregionalassociations.Discussionsarealsodirectedtothetwoprimary Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 10 assumptions made in theoretical models, i.e. the steady mean wind speed and turbulent boundary layer in equilibrium with the underlying surface roughness. In a review paper by Vickery and Skerlj (2005) [2.75], the gust factors obtained using the ESDU model (1982, 1983) [2.25] [2.26] are compared to the observed data and those calculated by the KrayerMarshallmethod,whichassumesthatinagivenhour,thedeviationofashort-duration meanwindspeedfromtheonehourmeanwindspeedfollowstheGaussiandistribution.The studyconcludesthattheESDUmodelprovidesadequateaccuracyfordescribingthehurricane gust factors over both water and land. The study also confirms that gust factor is very sensitive to the local surface roughness. Argyriadis(2003) [2.8]concludesthat thepeakwindspeedsandgust factorsmaybecalculated using the ESDU-spectra and the Charnock equation for roughness length over open sea. It is also stated that this approach is only valid for fully developed seas.Gustfactorscanalsobecalculateddirectlyfromwindspeeddata.Basedonthehigh-resolution measurementdata,PaulsenandSchroeder(2005)[2.61]dividesthetimehistoriesofrecorded wind speed into 10-minute segments, which are then used to calculate the gust factors. The mean gust factor is found to increase with increasing upstream surface roughness. The gust factor also appearstoincludeincreasingamountsofscatterwithdecreasingwindspeed.Somedifferences are observed between data from the tropical storms and the extra-tropical storms.InYu(2007)[2.85],thegustfactorisestimatedusingthemeasurementsofsurfacehurricane windspeedsovertheseasurfaceandopenflatterraininFloridacoastalareas.Thisresearch showsthattheDurstmodelusedinUSdesignstandardsandcodesunderestimatesthegust factorsofhurricanewindsforgustdurationsoflessthan20seconds.Thedependenceofthe estimationofgustfactorsonupstreamsurfaceroughnessconditionsappearsapproximatelyin agreement with the results of other studies (e.g. Paulsen and Schroeder [2.61]). For hurricane wind conditions, Hsu (2008) [2.36] proposes a formula for estimating the 3-second gust from a 1-minute mean wind speed. The formula is developed based on the models developed forstandardneutralboundarylayerflowconditions.Conversionfactorsforthehurricanewinds withdifferentaveragingtimedurationsarealsoreferredinStewart(2010)[2.65],wherethe conversion factor between the 10 minute mean wind and 3 second gust is related to the exponent of the power law wind profile. Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 11 Cautions are raised by Buchan et al. (1999) [2.14] and Black et al. (1999) [2.12] when applying the generic values of gust factors to tropical cyclone wind near the eye wall, where the maximum windspeedoccurs.ThemeasurementrecordedduringthepassageoftropicalcycloneOlivia showsthatthegustfactor(3secondgustto10minutemeanwindspeed)canreachashighas 2.75,whichismuchhigherthanthegustfactorcommonlyreferredintheexistingdesign guidelinesforoffshoreplatformsandoffshorewindturbines.Itshouldbenoted,however,that the observations in these studies are made from the wind speed measurement of a single cyclone. The statistical scattering of gust factors may play a big role in this case such that the conclusion may not have general implications. 2.1.1.4Mean Wind Speed and Profiles Themostcommonlyusedaveragingperiodis10minutes.AccordingtoHarperetal.(2009) [2.34]andBarthandPeinke(2007)[2.10],theintermittentbehaviorofturbulentwindcanbe foundespeciallyonsmallscales,whilethestatisticsonlargerscalesareclosetopureGaussian behavior. A separation of very intermittent and intermittent behavior seems to take place at about 10minutes.Thishasgivensupporttothecommonprocedureofusing10minutesaveragesin order to neglect small-scale turbulence. However, since the fluctuations on smaller time scales are responsiblefortheextremeloadsonwindturbines,thenon-Gaussianbehaviorhastobetaken into account. Thetimeaveragingperiodformeanwindspeedmayvaryduetodifferentdatacollecting methods and conditions. In a WMO report by Harper et al. (2009) [2.34], the conversion factors arerecommendedbetweentheWMOstandard10minuteaveragewindspeedat10mabove waterand1-minute,2-minuteand3-minutesustainedgustwinds,respectively,intropical cycloneconditions.Thereportindicatesthatanat-seaconversionfactorbetweenthe1-minute sustained estimate of peak storm intensity and the 10-minute average wind speed should be 0.93, ratherthanthetraditionalvalueof0.88,whichisshowntobeassociatedmorewithanoff-land exposure.Thevariationofmeanwindspeedwiththeheightabovesurfaceisdescribedbytheso-called wind shear law. The logarithmic law, which is defined as a function of the surface roughness, hasbeenusedinmostcasestodescribetherelationshipbetweenthewindspeedandtheheight abovesurface[2.24][2.85][2.8][2.65].Arelativelysimpleempiricalformatofexponential model has also been widely used in the wind turbine design standards such as IEC 61400-3 [2.40]. Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 12 AsobservedbyArgyriadis(2003)[2.8],thedistributionofmeanwindspeedrecommendedin most of wind turbine related standards follows the Weibull distribution or, with some limitations, the Rayleigh distribution.2.1.1.5Turbulence Intensity The turbulence intensity is the ratio of the wind speed standard deviation to the mean wind speed. Both the wind standard deviation and mean wind speed should be determined from the same set ofmeasureddatasamplesofwindspeedtakenoveraspecifiedperiodoftime(usually10 minutes). The turbulence intensity is dependent on the stability of the atmosphere and the surface roughness length. Dynamicwindloadingcausedbytheatmosphericturbulence isoneofthemostimportantwind turbine design parameters. Simulations of three-dimensional fields of all three components of the windvelocityfluctuationsareparticularlyimportantforthetimedomainsimulationsofgust loading on turbines.AsurveyreportbyArgyriadis(2003)[2.8]providesacomparisonofdifferentmodelsofthe longitudinalturbulenceintensityneartheground.Referencesaremadetothedesignguidelines developed by ESDU, Danish standard DS472, Germanischer Lloyd, ISO, NORSOK and API.In Mann et al. (2000) [2.50], a spectral tensor model of turbulence over flat terrain is discussed in detail,alongwithexperimentalverificationandcomparisonwithothercommonlyreferenced windspectra.Themodelisfurthergeneralizedtocovermoderatelycomplexterrainsthattake into account the influence of roughness changes and gentle hills on the turbulence statistics. The tensormodeldescribedinthisstudyisusedasabasisofthesimulationofturbulentfields implemented in Risos WAsP Engineering software. Yu(2007)[2.85]calculatestheturbulenceintensitybasedonthenormalizedstandarddeviation ofwindspeeddataestimatesforveryshortaveragingperiods.Theresearchshowsthatthe estimatedvaluesof turbulence intensitiesoflongitudinal andverticalwindcomponents increase as the terrain roughness increases. 2.1.1.6Wind Power (Turbulence) Spectra Turbulencespectrarepresentthefrequencydistributionoftheturbulentkineticenergyof fluctuatingwindvelocitycomponents.Sinceturbulentfluctuationsinthesurfacelayerofthe Contract M10PC00105: Design Standards for Offshore Wind Farms Final Report 13 atmospherehaveasignificanteffectonwindloadsaswellasthelossestheyproduceinhigh winds,theresearchofturbulencespectraatthesurfacelayerisofgreatinteresttowindturbine designers.IEC61400-3[2.40]recommendsusingMannsspectrum,butalsomakesareferencetoKaimal spectrum, for application to offshore wind turbine design. Argyriadis (2003) [2.8] states that the informationonthespectralcharacteristicsofoffshorewindisscarcealthoughthereissome evidence that standard spectral models are adequate with some discrepancies at low frequencies, particularlyinstableandunstableatmosphericconditions.GL[2.32]andtheDanish[2.19] standardsrecommendthatthespectrausedfortheonshorewindturbinesmaybeusedfor offshore applications.Ochi and Shin (1988) [2.57] perform a study of the turbulent wind spectrum over a seaway. The measured hurricane wind data at various locations are used to construct the spectral density data set.Thecomparisonofthemeasurementswiththosepredictedbyexistingwindspectra,which aredevelopedbasedonthemeasurementsonland,revealsasignificantdiscrepancyinthelow frequency range. For applications to the design of offshore structures, a new spectral formulation is proposed in the paper to address this discrepancy.Based on the wind turbulence measurements (Frya database) obtained at the exposed sites on the westerncoastofNorway,AndersenaandLvsethlow(2006) [2.7]reachaconclusionsimilarto the one by Ochi and Shin (1988) [2.57] that the low frequency range (