Analysis of Analysis of Structural Failures Structural Failures of Wind Towersof Wind Towers

AMERICAN WIND ENERGY ASSOCIATION

May 2009

Dilip Khatri, PhD, MBA, SESenior Structural EngineerURS CorporationLos Angeles, CA

Analysis of Structural Analysis of Structural FailuresFailures

1. Wind Tower Structures

2. Structural Failures

3. Wind Power Economics

4. Structural Design Considerations

5. Improving Structural Design Practice

Wind FarmsWind Farms

Wind Tower StructuresWind Tower Structures

1980s – 1990s:

Wind Towers < 40m Truss Structures

Turbine Sizes 250 kW – 500 kW

Wind Towers 1980’s – Wind Towers 1980’s – 20022002

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 - Wind Towers 2002 - presentpresent

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 - FutureWind Towers - Future

100m + Tower Heights3.0 MW – 5.0 MW Turbines700 kips [300 kN]

Why Taller Towers?Why Taller Towers?

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

Swept Rotor AreaSwept Rotor Area

Structural FailuresStructural Failures

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

tube

Structural FailuresStructural Failures

Structural Tubular

Failures:E-stop load conditionOverspeed of the

rotor

Foundation FailuresFoundation Failures

Foundation Design IssuesFoundation Design Issues

Overturning momentSoil failureDynamic stiffnessFatigue causing cracks in concreteRotational stiffness degradationSoil-structure interaction

Structural Failure/Collapse of Structural Failure/Collapse of Wind TowersWind Towers

3 short videos of wind tower collapseTower design issuesE-stop loadingRotor imbalanceFatigue crackingBuckling/stability failure

Tower Failure

Wind Power Economics:Wind Power Economics:Utility Grade ProjectsUtility 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

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 = $300,000Total Cost = $1,600,000/tower

Wind Power Economics:Wind Power Economics:Utility Grade ProjectsUtility Grade Projects

Wind Power ProjectsWind 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 Structural Design ConsiderationsConsiderations

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 Structural Design ImprovementsImprovementsFinite Element AnalysisFinite 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 AnalysisFinite Element Analysis

3D FEA Models are necessary to include all vibration modes

3D models capture the torsional behavior and buckling characteristics

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 AnalysisFinite 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 AnalysisFinite Element Analysis

FLAC3D Soil-Structure Interaction ModelFLAC3D 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

FL AC3D 3.10

Itasca Consulting G roup, Inc .M inneapolis , M N USA

©2006 Itasca Consulting G roup, Inc .

Step 13873 M odel Perspective08:35:57 Wed Apr 01 2009

Center: X: 0.000e+000 Y: 4.500e+000 Z: -9.000e+000

Rotation: X: 360.000 Y: 0.000 Z: 0.000

Dis t: 1.040e+002 M ag.: 1Ang.: 22.500

B lo c k G ro u p Live m ech zones shown

capsoil

S E L G e o m e try M agfac = 0.000e+000

FL AC3D 3.10

Itasca Consulting G roup, Inc .M inneapolis , M N USA

©2006 Itasca Consulting G roup, Inc .

Step 13873 M odel Perspective08:37:28 Wed Apr 01 2009

Center: X: 7.417e+000 Y: 1.749e+000 Z: -5.829e+000

Rotation: X: 40.000 Y: 0.000 Z: 40.000

Dis t: 1.040e+002 M ag.: 1.25Ang.: 22.500

B lo c k G ro u p Live m ech zones shown

capsoil

S E L G e o m e try M agfac = 0.000e+000

FL AC3D 3.10

Itasca Consulting G roup, Inc .M inneapolis , M N USA

©2006 Itasca Consulting G roup, Inc .

Step 52462 M odel Perspective16:44:24 Wed Apr 08 2009

Center: X: 0.000e+000 Y: 7.290e+000 Z: -4.300e+000

Rotation: X: 360.000 Y: 0.000 Z: 0.000

Dis t: 1.040e+002 M ag.: 3.05Ang.: 22.500

C o n to u r o f Z -D isp la c e m e n t M agfac = 0.000e+000 Live m ech 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

S E L G e o m e try M agfac = 0.000e+000

S k e tc h

Yaw Plate AnalysisYaw 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 Structural Failure of Wind TowersTowers

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

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 Structural Failure of Wind TowersTowers

Improving Structural Design Improving Structural Design MethodsMethods

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