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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
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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
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Wind FarmsWind Farms
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Wind Tower StructuresWind Tower Structures
1980s – 1990s:
Wind Towers < 40m Truss Structures
Turbine Sizes 250 kW – 500 kW
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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
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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
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Wind Towers - FutureWind Towers - Future
100m + Tower Heights3.0 MW – 5.0 MW Turbines700 kips [300 kN]
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Why Taller Towers?Why Taller Towers?
Wind Energy BasicsPOWER = dW/dt = Energy (work)/time = Torque x Angular Velocity
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Swept Rotor AreaSwept Rotor Area
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Structural FailuresStructural Failures
Structural Tubular Failures: Diameter-thickness ratios are high Buckling failure due to instability of the tower
tube
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Structural FailuresStructural Failures
Structural Tubular
Failures:E-stop load conditionOverspeed of the
rotor
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Foundation FailuresFoundation Failures
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Foundation Design IssuesFoundation Design Issues
Overturning momentSoil failureDynamic stiffnessFatigue causing cracks in concreteRotational stiffness degradationSoil-structure interaction
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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
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Tower Failure
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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
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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
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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
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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
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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
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Finite Element AnalysisFinite Element Analysis
3D FEA Models are necessary to include all vibration modes
3D models capture the torsional behavior and buckling characteristics
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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
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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
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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
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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
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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
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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
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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