Design of Foundations for Offshore Wind Turbines

SUBHAMOY BHATTACHARYA, PHD, is a Professor and Chair in Geomechanics at the University of Surrey. He is also a visiting fellow at the University of Bristol. His main research interest is the design for offshore wind turbines. He has also worked as a consultant in the in the civil and offshore engineering industry.

• Preface xiAbout the Companion Website xv1 Overview of a Wind Farm and Wind Turbine Structure 11.1 Harvesting Wind Energy 11.2 Current Scenario 21.2.1 Case Study: Fukushima Nuclear Plant and Near-Shore Wind Farms during the 2011 Tohoku Earthquake 51.2.2 Why Did the Wind Farms Survive? 61.3 Components of Wind Turbine Installation 81.3.1 Betz Law: A Note on Cp 111.4 Control Actions of Wind Turbine and Other Details 111.4.1 Power Curves for a Turbine 141.4.2 What Are the Requirements of a Foundation Engineer from the Turbine Specification? 151.4.3 Classification of Turbines 151.5 Foundation Types 161.5.1 Gravity-Based Foundation System 181.5.1.1 Suction Caissons or Suction Buckets 191.5.1.2 Case Study: Use of Bucket Foundation in the Qidong Sea (Jiangsu Province, China) 221.5.1.3 Dogger Bank Met Mast Supported on Suction Caisson 221.5.2 Pile Foundations 221.5.3 Seabed Frame or Jacket Supported on Pile or Caissons 231.5.4 Floating Turbine System 251.6 Foundations in the Future 271.6.1 Scaled Model Tests 331.6.2 Case Study of a Model Tests for Initial TRL Level (3–4) 341.7 On the Choice of Foundations for a Site 351.8 General Arrangement of a Wind Farm 361.8.1 Site Layout, Spacing of Turbines, and Geology of the Site 371.8.2 Economy of Scales for Foundation 401.9 General Consideration for Site Selection 421.10 Development of Wind Farms and the Input Required for Designing Foundations 441.11 Rochdale Envelope Approach to Foundation Design (United Kingdom Approach) 461.12 Offshore Oil and Gas Fixed Platform and Offshore Wind Turbine Structure 481.13 Chapter Summary and Learning Points 502 Loads on the Foundations 512.1 Dynamic Sensitivity of Offshore Wind Turbine Structures 512.2 Target Natural Frequency of a Wind Turbine Structure 532.3 Construction of Wind Spectrum 582.3.1 Kaimal Spectrum 602.4 Construction of Wave Spectrum 612.4.1 Method to Estimate Fetch 632.4.2 Sea Characteristics for Walney Site 632.4.3 Walney 1Wind Farm Example 632.5 Load Transfer from Superstructure to the Foundation 642.6 Estimation of Loads on a Monopile-Supported Wind Turbine Structure 662.6.1 Load Cases for Foundation Design 672.6.2 Wind Load 702.6.2.1 Comparisons with Measured Data 722.6.2.2 Spectral Density of Mudline Bending Moment 762.6.3 Wave Load 762.6.4 1P Loading 792.6.5 Blade Passage Loads (2P/3P) 802.6.6 Vertical (Deadweight) Load 812.7 Order of Magnitude Calculations of Loads 812.7.1 Application of Estimations of 1P Loading 822.7.2 Calculation for 3P Loading 822.7.3 Typical Moment on a Monopile Foundation for Different-Rated Power Turbines 842.8 Target Natural Frequency for Heavier and Higher-Rated Turbines 852.9 Current Loads 862.10 Other Loads 872.11 Earthquake Loads 872.11.1 Seismic Hazard Analysis (SHA) 902.11.2 Criteria for Selection of Earthquake Records 912.11.2.1 Method 1: Direct Use of Strong Motion Record 912.11.2.2 Method 2: Scaling of Strong Motion Record to Expected Peak Bedrock Acceleration 912.11.2.3 Method 3: Intelligent Scaling or Code Specified Spectrum Compatible Motion 912.11.3 Site Response Analysis (SRA) 932.11.4 Liquefaction 942.11.5 Analysis of the Foundation 952.12 Chapter Summary and Learning Points 1013 Considerations for Foundation Design and the Necessary Calculations 1033.1 Introduction 1033.2 Modes of Vibrations of Wind Turbine Structures 1043.2.1 Sway-Bending Modes of Vibration 1053.2.1.1 Example Numerical Application of Modes of Vibration of Jacket Systems 1063.2.1.2 Estimation of Natural Frequency of Monopile-Supported Strctures 1063.2.2 Rocking Modes of Vibration 1093.2.3 Comparison of Modes of Vibration of Monopile/Mono-Caisson and Multiple Modes of Vibration 1153.2.4 Why Rocking Must Be Avoided 1163.3 Effect of Resonance: A Study of an Equivalent Problem 1173.3.1 Observed Resonance in German North Sea Wind Turbines 1193.3.2 Damping of Structural Vibrations of Offshore Wind Turbines 1193.4 Allowable Rotation and Deflection of a Wind Turbine Structure 1203.4.1 Current Limits on the Rotation at Mudline Level 1203.5 Internationals Standards and Codes of Practices 1223.6 Definition of Limit States 1243.6.1 Ultimate Limit State (ULS) 1243.6.2 Serviceability Limit State (SLS) 1253.6.3 Fatigue Limit State (FLS) 1263.6.4 Accidental Limit States (ALS) 1263.7 Other Design Considerations Affecting the Limit States 1263.7.1 Scour 1273.7.2 Corrosion 1293.7.3 Marine Growth 1293.8 Grouted Connection Considerations for Monopile Type Foundations 1293.9 Design Consideration for Jacket-Supported Foundations 1303.10 Design Considerations for Floating Turbines 1313.11 Seismic Design 1323.12 Installation, Decommission, and Robustness 1323.12.1 Installation of Foundations 1323.12.1.1 Pile Drivability Analysis 1333.12.1.2 Predicting the Increase in Soil Resistance at the Time of Driving (SRD) Due to Delays (Contingency Planning) 1343.12.1.3 Buckling Considerations in Pile Design 1343.12.2 Installation of Suction Caissons 1383.12.2.1 First Stage 1383.12.2.2 Second Stage 1383.12.3 Assembly of Blades 1383.12.4 Decommissioning 1393.13 Chapter Summary and Learning Points 1413.13.1 Monopiles 1423.13.2 Jacket on Flexible Piles 1463.13.3 Jackets on Suction Caissons 1464 Geotechnical Site Investigation and Soil Behaviour under Cyclic Loading 1474.1 Introduction 1474.2 Hazards that Needs Identification Through Site Investigation 1484.2.1 Integrated Ground Models 1484.2.2 Site Information Necessary for Foundation Design 1494.2.3 Definition of Optimised Site Characterisation 1514.3 Examples of Offshore Ground Profiles 1514.3.1 Offshore Ground Profile from North Sea 1514.3.2 Ground Profiles from Chinese Development 1524.4 Overview of Ground Investigation 1574.4.1 Geological Study 1574.4.2 Geophysical Survey 1574.4.3 Geotechnical Survey 1584.5 Cone Penetration Test (CPT) 1604.6 Minimum Site Investigation for Foundation Design 1644.7 Laboratory Testing 1644.7.1 Standard/Routine Laboratory Testing 1654.7.2 Advanced Soil Testing for Offshore Wind Turbine Applications 1654.7.2.1 Cyclic Triaxial Test 1664.7.2.2 Cyclic Simple Shear Apparatus 1704.7.2.3 Resonant Column Tests 1724.7.2.4 Test on Intermediate Soils 1744.8 Behaviour of Soils under Cyclic Loads and Advanced Soil Testing 1744.8.1 Classification of Soil Dynamics Problems 1754.8.2 Important Characteristics of Soil Behaviour 1774.9 Typical Soil Properties for Preliminary Design 1794.9.1 Stiffness of Soil from Laboratory Tests 1794.9.2 Practical Guidance for Cyclic Design for Clayey Soil 1814.9.3 Application to Offshore Wind Turbine Foundations 1834.10 Case Study: Extreme Wind and Wave Loading Condition in Chinese Waters 1844.10.1 Typhoon-Related Damage in the Zhejiang Province 1864.10.2 Wave Conditions 1875 Soil–Structure Interaction (SSI) 1915.1 Soil–Structure Interaction (SSI) for Offshore Wind Turbines 1925.1.1 Discussion on Wind–Wave Misalignment and the Importance of Load Directionality 1935.2 Field Observations of SSI and Lessons from Small-Scale Laboratory Tests 1955.2.1 Change in Natural Frequency of the Whole System 1955.2.2 Modes of Vibration with Two Closely Spaced Natural Frequencies 1955.2.3 Variation of Natural Frequency with Wind Speed 1965.2.4 Observed Resonance 1975.3 Ultimate Limit State (ULS) Calculation Methods 1975.3.1 ULS Calculations for Shallow Foundations for Fixed Structures 1975.3.1.1 Converting (V, M, H) Loading into (V, H) Loading Through Effective Area Approach 2005.3.1.2 Yield Surface Approach for Bearing Capacity 2005.3.1.3 Hyper Plasticity Models 2015.3.2 ULS Calculations for Suction Caisson Foundation 2015.3.2.1 Vertical Capacity of Suction Caisson Foundations 2025.3.2.2 Tensile Capacity of Suction Caissons 2035.3.2.3 Horizontal Capacity of Suction Caissons 2035.3.2.4 Moment Capacity of Suction Caissons 2045.3.2.5 Centre of Rotation 2065.3.2.6 Caisson Wall Thickness 2075.3.3 ULS Calculations for Pile Design 2075.3.3.1 Axial Pile Capacity (Geotechnical) 2085.3.3.2 Axial Capacity of the Pile (Structural) 2115.3.3.3 Structural Sections of the Pile 2125.3.3.4 Lateral Pile Capacity 2145.4 Methods of Analysis for SLS, Natural Frequency Estimate, and FLS 2165.4.1 Simplified Method of Analysis 2165.4.2 Methodology for Fatigue Life Estimation 2235.4.3 Closed-Form Solution for Obtaining Foundation Stiffness of Monopiles and Caissons 2235.4.3.1 Closed-Form Solution for Piles (Rigid Piles or Monopiles) 2245.4.3.2 Closed-Form Solutions for Suction Caissons 2275.4.3.3 Vertical Stiffness of Foundations (Kv) 2285.4.4 Standard Method of Analysis (Beam on Nonlinear Winkler Foundation) or p-y Method 2285.4.4.1 Advantage of p-y Method, and Why This Method Works 2305.4.4.2 API Recommended p-y Curves for Standard Soils 2315.4.4.3 p-y Curves for Sand Based on API 2325.4.4.4 p-y Curves for Clay 2325.4.4.5 Cyclic p-y Curves for Soft Clay 2355.4.4.6 Modified Matlock Method 2365.4.4.7 ASIDE: Note on the API Cyclic p-y Curves 2375.4.4.8 Why API p-y Curves Are Not Strictly Applicable 2375.4.4.9 References for p-y Curves for Different Types of Soils 2385.4.4.10 What Are the Requirements of p-y Curves for Offshore Wind Turbines? 2385.4.4.11 Scaling Methods for Construction of p-y Curves 2385.4.4.12 p-y Curves for Partially Liquefied Soils 2405.4.4.13 p-y Curves for Liquefied Soils Based on the Scaling Method 2415.4.5 Advanced Methods of Analysis 2415.4.5.1 Obtaining KL, KR, and KLR from Finite Element Results 2435.5 Long-Term Performance Prediction for Monopile Foundations 2455.5.1 Estimation of Soil Strain around the Foundation 2475.5.2 Numerical Example of Strains in the Soil around the Pile 15 Wind Turbines 2495.6 Estimating the Number of Cycles of Loading over the Lifetime 2535.6.1 Calculation of the Number of Wave Cycles 2565.6.1.1 Sub-step 1. Obtain 50-Year Significant Wave Height 2565.6.1.2 Sub-step 2. Calculate the Corresponding Range of Wave Periods 2575.6.1.3 Sub-step 3. Calculate the Number of Waves in a Three-Hour Period 2575.6.1.4 Sub-step 4. Calculate the Ratio of the Maximum Wave Height to the Significant Wave Height 2575.6.1.5 Sub-step 5. Calculate the Range of Wave Periods Corresponding to the Maximum Wave Height 2575.7 Methodologies for Long-Term Rotation Estimation 2585.7.1 Simple Power Law Expression Proposed by Little and Briaud (1988) 2595.7.2 Degradation Calculation Method Proposed by Long and Vanneste (1994) 2605.7.3 Logarithmic Method Proposed by Lin and Liao (1999) 2605.7.4 Stiffness Degradation Method Proposed by Achmus et al. (2009) 2615.7.5 Accumulated Rotation Method Proposed by Leblanc et al. (2010) 2615.7.6 Load Case Scenarios Conducted by Cuéllar (2011) 2625.8 Theory for Estimating Natural Frequency of the Whole System 2625.8.1 Model of the Rotor-Nacelle Assembly 2635.8.2 Modelling the Tower 2635.8.3 Euler-Bernoulli Beam – Equation of Motion and Boundary Conditions 2645.8.4 Timoshenko Beam Formulation 2645.8.5 Natural Frequency versus Foundation Stiffness Curves 2665.8.6 Understanding Micromechanics of SSI 2686 Simplified Hand Calculations 2736.1 Flow Chart of a Typical Design Process 2736.2 Target Frequency Estimation 2746.3 Stiffness of a Monopile and Its Application 2766.3.1 Comparison with SAP 2000 Analysis 2876.4 Stiffness of a Mono-Suction Caisson 2876.5 Mudline Moment Spectra for Monopile Supported Wind Turbine 2916.6 Example for Monopile Design 299Appendix A Natural Frequency of a Cantilever Beam with Variable Cross Section 333Appendix B Euler-Bernoulli Beam Equation 337Appendix C Tower Idealisation 341Appendix D Guidance on Estimating the Vertical Stiffness of Foundations 345Appendix E Lateral Stiffness KL of Piles 347Appendix F Lateral Stiffness KL of Suction Caissons 349Bibliography 351Index 369