Design Standards for Offshore Wind Farms

1 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 Submitted to 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, Department of the Interior, Washington, 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.

2 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), Bureau of Ocean Energy Management, Regulation, and Enforcement (BOEMRE), Department of the Interior, Washington, The wind turbine simulation programs used in this project, including FAST, AeroDyn and TurbSim, are developed and maintained by the National Renewable Energy Laboratory (NREL) of the Department of Energy. Technical supports from Carl Stansberg, Casba Pakozdi and Sebastien Fouques at Marintek, Bingbin Yu at the University of Michigan, Jason Jonkman at the National Renewable Energy Laboratory, and John Conlon are gratefully acknowledged. This report is prepared by Qing Yu Kunho Kim and Tzu-Wei Lo at ABS Corporate Offshore Technology, Renewables Group.

3 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 iv List of Figures .. vi Executive Summary ..x 1 Introduction .. 1 2 Design of Offshore wind Turbines Subjected to Hurricanes on the US OCS .. 4 State-of-the-Art Review .. 6 Hurricane 6 Hurricane Generated Waves .. 14 Design Load Case for Hurricane Conditions .. 19 Existing Design Standards and Their Applicability to Hurricane Conditions22 Simulation 25 Technical Approach for Case Studies .. 27 Problem 27 Modeling and Analysis 29 Case Study 41 Site Conditions .. 41 wind Turbine RNA and Tower Specifications .. 45 Support Structure Configurations .. 47 Sensitivity Study to Determine Modeling Parameters .. 60 Comparison of API and IEC wind Models .. 63 API and IEC wind Model Definitions.

4 64 Effect of Using IEC and API wind Models on Turbine Responses .. 72 Summary .. 73 Effect of Fault in Turbine s Yaw and Pitch Control .. 75 Nacelle Yaw 76 Blade Pitch Angle Locking .. 77 Summary .. 79 Case Studies Monopile Support Structure .. 81 Contract M10PC00105: Design Standards for Offshore wind Farms Final Report ii Summary of Model 81 Monopile Case Study Results .. 86 Case Studies Tripod Support Structure .. 91 Summary of Model 91 Tripod Case Study Results .. 96 Case Studies Jacket Support Structure .. 100 Summary of Model 100 Jacket Case Study Results .. 103 Parametric Comparisons of Case Study Results .. 107 Return Period of Environmental Conditions .. 107 Normal and Abnormal Turbine Operating Conditions .. 114 wind and Wave 118 Type of Support Structures .. 125 Conclusions and 129 Hurricane wind Models .. 129 Design Criteria for Offshore wind Turbines in Hurricane-Prone 129 Recommendations.

5 133 3 Breaking Wave Slamming Loads .. 135 State-of-the-Art Review .. 136 Breaking Waves .. 136 Analytical Slamming Load Models .. 140 Numerical 149 Slamming Experiments .. 157 Existing Design 165 Parameter Study .. 168 Wave Related Parameters .. 168 Structure Related Parameters .. 170 Some Important Physical 171 Comparative Study of Analytical Breaking Wave Slamming Load Models .. 173 Analytical Models Used in the Comparative Study .. 173 Comparative Study Results .. 176 Conclusions and 183 Contract M10PC00105: Design Standards for Offshore wind Farms Final Report iii 4 Summary .. 187 References for Section 2 .. 190 References for Section 3 .. 198 Appendix A Abbreviations .. 208 Appendix B Terms and 209 Contract M10PC00105: Design Standards for Offshore wind Farms Final Report iv List of Tables Table Commonly Referenced Design Standards /Regulations for Offshore wind 23 Table General Load Cases Descriptions for the Case 28 Table 50-Year Return Water Depth, Current Speed and wind Speed.

6 42 Table 50-Year Return Wave Condition .. 42 Table 100-Year Return Water Depth, Current Speed and wind Speed .. 43 Table 100-Year Return Wave Conditions .. 43 Table Properties of the RNA of NREL 5-MW Baseline Offshore wind Turbine (Jonkman et al., 2009) .. 46 Table Undistributed Blade Properties (Jonkman et al., 2009) .. 46 Table Turbine Tower Properties (Fabian et al, 2011).. 46 Table Properties of the Grouted TP and Pile .. 49 Table Monopile Support Structure Geometric Properties .. 49 Table Monopile Support Structure Mass Properties (with Marine Growth) .. 49 Table Properties of Equivalent Fixity of Equivalent 50 Table Uniform Cd and Cm of Equivalent Monopile Used in the FAST Analysis .. 50 Table Monopile Natural Frequencies .. 50 Table Properties of the Equivalent Pile Simulating the Leg Sleeve and Pile ..53 Table Tripod Support Structure Geometric Properties.

7 53 Table Tripod Mass Properties (with Marine Growth) .. 53 Table Properties of Equivalent Fixity of the Tripod Equivalent Monopile .. 54 Table Uniform Cd and Cm of the Tripod s Equivalent Monopile Used in the FAST 54 Table Tripod Natural Frequencies .. 54 Table Properties of the Equivalent Pile Simulating the Grouted Leg and 57 Table Jacket Support Structure Geometric Properties .. 58 Table Jacket Mass Properties (with Marine Growth and Sea Water in Flooded Legs) .. 58 Table Uniform Cd and Cm of the Jacket s Equivalent Monopile Used in the FAST 58 Table Properties of Equivalent Fixity of Jacket Equivalent 59 Table Jacket Natural Frequencies .. 59 Table Input Parameters for wind Model Comparison Study .. 64 Contract M10PC00105: Design Standards for Offshore wind Farms Final Report v Table Spectral Parameters for the Kaimal 66 Table Turbulence Properties at Hub Height Based on the API wind Model.

8 69 Table Turbulence Properties at Hub Height Based on the IEC wind Model .. 69 Table Coefficients and Distances for the Coherence Function (API Bulletin 2 INT-MET, 2007).. 70 Table Static wind Forces on the Monopile Support Structure .. 82 Table Mudline Base Shear and Overturning Moment Due to Wave Slamming on the 84 Table Summary of Structural Loads for the Monopile Case Studies .. 85 Table Structural Reponses and Utilization Check Results for the Monopile Support Structure .. 89 Table Monopile Case L12 Member Utilization Ratio Check .. 90 Table Static wind Forces on the Tripod Support 92 Table Mudline Base Shear and Overturning Moment Due to Wave Slamming on the 94 Table Summary of Structural Loads for the Tripod Case Studies .. 95 Table Structural Reponses and Utilization Check Results for the Tripod Support 98 Table Tripod Case L16 Member Utilization Ratio 99 Table Static wind Forces on the Jacket Support Structure.

9 101 Table Summary of Structural Loads for the Jacket Case 102 Table Structural Reponses and Utilization Check Results for the Jacket Support 105 Table Jacket Case L28 Member Utilization Ratio 106 Table Occurrence of Breaking Wave Slamming .. 108 Table Breaking Wave Types .. 139 Table Commonly Used Design Guidelines for Wave Slamming Loads .. 167 Table Load Case Definition for the Wave Slamming Comparison .. 180 Table Maximum Slamming Responses at the Mudline Calculated Using the Wienke Model and the Truncated Campbell-Weynberg Model .. 180 Contract M10PC00105: Design Standards for Offshore wind Farms Final Report vi List of Figures Figure Hurricane (Tropical Cyclone) Generated Wave Field (Young, 2003) .. 16 Figure wind and Primary Wave Field of 1998 Hurricane Bonnie .. 16 Figure Reliability Index for Normal Conditions under wind or Wave Loads (Yu et al.)

10 , 2011).. 24 Figure Reliability Index for Abnormal Conditions under wind or Wave Loads (Yu et al., 2011).. 25 Figure Modeling and Dynamic Analysis Procedure .. 31 Figure Regional wind Conditions in the US Atlantic Coast .. 33 Figure Gulf of Mexico West Central 33 Figure Site Location in the GOM (NOAA NDBC Station 42035) .. 33 Figure Gumbel Curve Fitting for the Buoy wind Data in Hurricane Seasons .. 34 Figure Gumbel Curve Fitting for the Buoy Wave Data in Hurricane Seasons .. 34 Figure Correlation of Tp and Hs Using the Binned Buoy Data in Hurricane 35 Figure Verification of Modal Shapes of Equivalent Monopile for FAST Input .. 38 Figure Verification of Cd and Cm of Equivalent Monopile for FAST 38 Figure Wave Profile of 50-Year Return Waves .. 44 Figure Wave Profile of 100-Year Return Waves .. 44 Figure Undrained Shear Strength of Undisturbed Clay.