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

Analysis of Structural Failures

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Wind Tower Structures Structural Failures Wind Power Economics Structural Design Considerations Improving Structural Design Practice

Wind Farms

Wind Tower Structures

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

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

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

Wind Towers - Future

 100m + Tower Heights  3.0 MW – 5.0 MW Turbines  700 kips [300 kN]

Why Taller Towers?

Wind Energy Basics POWER

= dW/dt = Energy (work)/time = Torque x Angular Velocity

Swept Rotor Area

Structural Failures

Structural Tubular Failures:

 Diameter-thickness ratios are high  Buckling failure due to instability of the tower tube

Structural Failures

Structural Tubular Failures:

 E-stop load condition  Overspeed of the rotor

Foundation Failures

Foundation Design Issues

 Overturning moment  Soil failure  Dynamic stiffness  Fatigue causing cracks in concrete  Rotational stiffness degradation  Soil-structure interaction

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

Tower Failure

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

Wind Power Economics: Utility Grade Projects

    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 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

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

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

Finite Element Analysis

 3D FEA Models are necessary to include all vibration modes  3D models capture the torsional behavior and buckling characteristics

Finite Element Analysis

 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

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

FLAC3D 3.10

©2006 Itasca Consulting Group, Inc. FLAC3D Soil-Structure Interaction Model 08:35:57 Wed Apr 01 2009 Center: X: 0.000e+000 Y: 4.500e+000 Z: -9.000e+000 Dist: 1.040e+002 Rotation: X: 360.000

Y: 0.000

Z: 0.000

Mag.: 1 Ang.: 22.500

    FLAC3D for soil Live mech zones shown structure soil interaction analysis Magfac = 0.000e+000 to model micropiles with a concrete cap ©2006 Itasca Consulting Group, Inc. Step 13873 Model Perspective 08:37:28 Wed Apr 01 2009 Overturning X: 7.417e+000 Y: 1.749e+000 Z: -5.829e+000 X: 40.000

Y: 0.000

Z: 40.000

Moment analysis Ang.: 22.500

Block Group Uplift capacity soil Itasca Consulting Group, Inc.

Magfac = 0.000e+000 Post-Tension Effects

FLAC3D 3.10

©2006 Itasca Consulting Group, Inc. Step 52462 Model Perspective 16:44:24 Wed Apr 08 2009 Center: X: 0.000e+000 Y: 7.290e+000 Z: -4.300e+000 Dist: 1.040e+002 Rotation: X: 360.000

Y: 0.000

Z: 0.000

Mag.: 3.05

Ang.: 22.500

Contour of Z-Displacement Magfac = 0.000e+000 Live mech zones shown -8.6132e-003 to -8.0000e-003 -8.0000e-003 to -7.0000e-003 -7.0000e-003 to -6.0000e-003 -6.0000e-003 to -5.0000e-003 -5.0000e-003 to -4.0000e-003 -4.0000e-003 to -3.0000e-003 -3.0000e-003 to -2.0000e-003 -2.0000e-003 to -1.0000e-003 -1.0000e-003 to 0.0000e+000 0.0000e+000 to 1.0000e-003 1.0000e-003 to 1.2111e-003 Interval = 1.0e-003 SEL Geometry Magfac = 0.000e+000 Sketch Itasca Consulting Group, Inc.

Minneapolis, MN USA Itasca Consulting Group, Inc.

Minneapolis, MN USA

Yaw Plate Analysis

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

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

Structural Failure of Wind Towers    Tower buckling Blade separation 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