1. Analysis of Structural Failures of Wind Towers AMERICAN WIND ENERGY ASSOCIATION May 2009 Dilip Khatri, PhD, MBA, SE Senior Structural Engineer URS Corporation Los Angeles, CA

2. Analysis of Structural Failures 1. Wind Tower Structures 2. Structural Failures 3. Wind Power Economics 4. Structural Design Considerations 5. Improving Structural Design Practice

3. Wind Farms

4. Wind Tower Structures 1980s – 1990s: Wind Towers < 40m Truss Structures Turbine Sizes 250 kW – 500 kW

5. Wind Towers 1980’s – 2002 Wind Towers 40m – 60m Steel Tubular Towers Spread Footings Pile – Cap Foundations P&H Foundation 1 MW Turbine 1.5 – 2.0 MW Turbine

6. Wind Towers 2002 - present Wind Towers 60m – 80m Steel Tubular Towers Spread Footings Pile – Cap Foundations P&H Foundation 2.0 MW Turbine = 400,000 # - 500,000 # [190-230kN] 2.5 – 3.0 MW Turbine

7. Wind Towers - Future 100m + Tower Heights 3.0 MW – 5.0 MW Turbines 700 kips [300 kN]

8. Why Taller Towers? Wind Energy Basics POWER = dW/dt = Energy (work)/time = Torque x Angular Velocity

9. Swept Rotor Area

10. Structural Failures Structural Tubular Failures: Diameter-thickness ratios are high Buckling failure due to instability of the tower tube

11. Structural Failures Structural Tubular Failures: E-stop load condition Overspeed of the rotor

12. Foundation Failures

13. Foundation Design Issues Overturning moment Soil failure Dynamic stiffness Fatigue causing cracks in concrete Rotational stiffness degradation Soil-structure interaction

14. Structural Failure/Collapse of Wind Towers 3 short videos of wind tower collapse Tower design issues E-stop loading Rotor imbalance Fatigue cracking Buckling/stability failure

15. Tower Failure

16. Wind Power Economics: Utility Grade Projects Typical cost of 1 wind tower is $1,500,000 to $2,000,000/tower 60 – 80m tower height 1.5MW-2.0MW turbine Tower cost = $300,000 –$245,000 for steel materials + labor –$50,000 for exterior painting –$5,000 for engineering design, permitting

17. Foundation Cost = $200,000 –$100,000 for construction, materials, labor, onsite management –$10,000 for design engineering, plans, permit Nacelle + Rotor = $1,100,000 –$900,000 Purchased from the Power Generation company –$200,000 for onsite crane and assembly Tower = $300,000 Total Cost = $1,600,000/tower Wind Power Economics: Utility Grade Projects

18. Wind Power Projects A typical wind power project consisting of 100 towers is 100 X $1.6MM = $160MM project Bank loan (80% debt-equity ratio) = $128,000,000 Risk factor to banks and insurance companies

19. Structural Design Considerations Foundation-Soil-Tower Analysis; combined analysis of the completed structure with soil profile included 3D Finite Element Analysis of taller towers 3D FEA of soil and foundation structure Germanisher Lloyd Guidelines; GL Certificate of Approval Soil-structure interaction analysis of the foundation Consider E-stop loading

20. Structural Design Improvements Finite Element Analysis Perform a detailed FEA of the tower and include the nacelle + rotor into the model Perform a soil-structure interaction model Frequency Response Analysis Dynamic Response Analysis Fatigue analysis on the foundation elements Include soil fatigue

21. Finite Element Analysis 3D FEA Models are necessary to include all vibration modes 3D models capture the torsional behavior and buckling characteristics

22. Tower-Foundation Models capture the full frequency behavior Soil-structure interaction analysis allows for the foundation to be included with the soil strata Finite Element Analysis

23. Tower-Foundation Models capture the full frequency behavior Soil-structure interaction analysis allows for the foundation to be included with the soil strata Finite Element Analysis

24. FLAC3D Soil-Structure Interaction Model FLAC3D for soil- structure interaction analysis to model micropiles with a concrete cap Overturning Moment analysis Uplift capacity Post-Tension Effects

25. Yaw Plate Analysis ANSYS FEA of Yaw Plate Eccentric Loads cause stress concentrations Off-axis wind gusts magnify the moments on the yaw plate

26. Structural Failure of Wind Towers Over-speed condition E-stop loading Fatigue cracking in the tower shell Foundation rotation due to overturning moment, soil creep, soil fatigue, or combination of soil-foundation stiffness degradation

27. Tower buckling Blade separation Eccentric loading due to offset between CG and geometric center of tower nacelle (i.e., built in eccentric loading) Structural Failure of Wind Towers

28. Improving Structural Design Methods 1.Structural Performance Monitoring 2.Comprehensive Research on Structural Failures 3.Evaluation of All Load Conditions 4.Sharing our Design Problems for Discussion 5.Statistical Record Keeping 6.Improving the Design Codes