Analysis of Structural Failures of Wind Towers AMERICAN

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

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

Wind Farms

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

Wind Towers 1980’s – 2002 l l l l Wind Towers 40 m – 60 m Steel Tubular Towers Spread Footings Pile – Cap Foundations P&H Foundation 1 MW Turbine 1. 5 – 2. 0 MW Turbine

Wind Towers 2002 - present l l l l Wind Towers 60 m – 80 m Steel Tubular Towers Spread Footings Pile – Cap Foundations P&H Foundation 2. 0 MW Turbine = 400, 000 # - 500, 000 # [190 -230 k. N] 2. 5 – 3. 0 MW Turbine

Wind Towers - Future l 100 m + Tower Heights l 3. 0 MW – 5. 0 MW Turbines l 700 kips [300 k. N]

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

Swept Rotor Area

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

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

Foundation Failures

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

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

Tower Failure

Wind Power Economics: Utility Grade Projects Typical cost of 1 wind tower is $1, 500, 000 to $2, 000/tower l 60 – 80 m tower height l 1. 5 MW-2. 0 MW turbine l Tower cost = $300, 000 l – $245, 000 for steel materials + labor – $50, 000 for exterior painting – $5, 000 for engineering design, permitting

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

Wind Power Projects l. A typical wind power project consisting of 100 towers is 100 X $1. 6 MM = $160 MM project l Bank loan (80% debt-equity ratio) = $128, 000 l Risk factor to banks and insurance companies

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

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

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

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

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

FLAC 3 D Soil-Structure Interaction Model FLAC 3 D for soilstructure interaction analysis to model micropiles with a concrete cap l Overturning Moment analysis l Uplift capacity l Post-Tension Effects l

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

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

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

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