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Structural Failures of Wind Towers and Dynamic Analysis Procedures Dilip Khatri, PhD, SE URS Corporation 915 Wilshire Blvd., STE 700 Los Angeles, CA 90017 Dilip_Khatri@urscorp.com 213-996-2367 This paper is an examination of wind towers and their failure mechanisms. Wind Towers

have now become new rural version of high rise construction. With the growing interest

in renewable energy, the wind tower industry is set to expand to greater heights than ever

before. These towers are now reaching 80, 90, and over 100m heights and support loads

over 2,600 kN (600,000#). Why do these towers fail? How can we prevent them from

failing? These topics are explored and presented in this technical paper.

OUTLINE

1.0 Wind Tower Structures

1.1 Summary of the design practice

1.2 Demands of the Industry

1.3 New Designs

2.0 Structural Failures

2.1 What is structural failure?

2.2 Failure mechanism examples

2.3 Long term Performance vs. Short Duration Loads

3.0 Design Criteria

3.1 Eurocode, GL Criteria, IBC

3.2 Foundation Design

3.3 Static and Dynamic Analysis

4.0 Dynamic and Fatigue Analysis Methods

4.1 3D vs. 2D models

4.2 Foundation and Structure Interaction

4.3 Frequency and Resonance response

4.4 Preventing fatigue failure

4.5 Crack propagation analysis

5.0 Improving Structural Design Practice

5.1 3D Finite Element Analysis

5.2 Soil-structure interaction analysis

5.3 Consideration of E-stop loads

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1.0 WIND TOWER STRUCTURES

Wind Tower structures have evolved from the basic truss/lattice tower design to tall

tubular structures that are carrying higher loads. The original wind towers were less than

40m with 500kW-660kW turbines and constructed with simple tubular steel or lattice

type design. Basic statics and structural engineering practice were the normal rules for

design and these structural systems dominated the market.

In the past 15 years of growth in the wind energy industry, the market has evolved to ever

increasing tower heights. The current trends of the industry are 80m, 90m, with 100m

towers being introduced in Europe. Turbines have increased in size to 2MW, 2.5MW,

3.0MW, and with increasing sizes of 4.5MW+ forthcoming. These dead loads are

surpassing 2600kN (600 kips), and the complications of structural dynamics, frequency

response, and soil-structure interaction are becoming increasingly important. As these

industry demands continue to grow, the budgets are tight and the necessity for cost

effective structural systems is a high priority. This growth trend is propelled further by

the increase in wind energy industry for the next 10 years.

Wind Tower designs are proliferating into new shapes and sizes. The Tube design is

being improved with different versions to accomplish the 90m – 100m tower heights.

Several design-manufacturing companies are initiating new patented designs to introduce

to the marketplace that claim higher efficiency, lower cost, and stronger tower capacity.

With all of these new developments occurring in the industry, it is worthwhile to take a

step back and examine the pitfalls and potential mistakes that can occur when the

profession starts to reach new tower heights. It is imperative to keep our bankers,

financial community, investors, and insurance underwriters comfortable with our

technological growth. As the wind industry continues to grow, the long term prospects

are excellent and it is important to keep track of the basic fundamentals.

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2.0 Structural Failures

Let us first define the term ‘structural failure’ in the context of this paper. The term

‘structural failure’ is referring to ‘collapse load’. Collapse load is the actual physical

destruction of the tower system for any and all reasons. Many structural engineers use

the term ‘failure’ to refer to exceeding the design allowable load (or factored capacity

load), but the actual structure is not collapsing it is only ‘failing in a design calculation’

but nothing is falling apart. We are specifically examining the collapse load situation in

this paper along with other maintenance related issues.

A maintenance related issue is defined as a condition whereby the tower-foundation

system is not performing to its stated specifications and is resulting in turbine

performance degradation. Although this is not technically a ‘failure’ it may certainly lead

to one, and could shut down the turbine for a lengthy period of time. The shut down

process leads to lost revenues and increased maintenance costs which are a long term

problem. The big picture is that the wind tower may have this problem repeated on

thousands of other designs.

Here are twelve examples of structural failures and maintenance problems are

summarized as follows:

2.1 Turbine over-speed: The turbine and rotor run into over speed (>24 rpm) at wind

speeds over 50 mph. This results in excessive loads and the entire system collapses.

Turbine/rotor over speed increases the loads on the structure and foundation system. The

total energy of the rotor is proportionate to the angular velocity squared, by the equation

of kinetic energy. To understand the effects of a slight over speed from 20 rpm to 25

rpm, we can see from the energy equation:

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Kinetic Energy Equation:

E = ½ m v 2

E = ½ m (rω ) 2

If we increase the rotation speed from ω1 = 20 to ω2 =25 rpm,

(ω2/ω1) = (25/20)

= 1.25 { 25% increase in angular velocity}

∴ ∆E = E2/E1 = (ω2/ω1)2

= (25/20)2

= 1.56 ⇒ 56% increase in energy

Energy and structural loads are directly related because the structural system has to

absorb these increases. The additional energy input from the rotor as to go into the

structure and soil.

2.2 E-stop: The short/hard stop creates a shock load spectrum on the structure and

results in a drastic dynamic impulse load to the system, resulting large lateral deflections.

We can appreciate the magnitude of E-stop loads by examining the fundamental impulse-

momentum equations and energy release from the blade rotation.

Impulse Momentum Theorem:

ΣF = ma = m dv/dt

and then integrate with respect to time,

∫∑Fdt = mv2 – mv1

For a wind tower, the equation is based on angular velocity

∫∑Fdt = mrω2 – mrω1

For a 90m wind tower with 90m rotor diameter,

mrotor = 450 kN/g (approximately 100 kips), g = 9.81 m/sec2

r = 45 m (90m rotor diameter)

ω = 20 rpm = 0.67π rad/sec, the stopping time is assumed as ∆t ≈ 1.0 sec

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The impulse force F = mrω2 / ∆t = 450 kN x 45 m x 0.67π /g (1) sec

∴ The impulse force F = 4,344 kN (≈ 978 kips)

This would produce an approximate Rotor Moment at the Hub Height

∴ F x (½ r) = 4,344 kN x 22.5m = 97,740 kN-m (72,200 ft-kip)

This produces a base Overturning Moment (OTM) of

∴ OTM = F x h = 4,344 kN x 90m = 390,960 kN-m ( 288,000 ft-kips)

We may appreciate this result further by examining the energy equation using Kinetic

Energy for a rotating system:

Energy Equation:

E = ½ m v 2

E = ½ m (rω ) 2

∴ E = ½ (450 kN/g) (45m x 0.67π)2 = 205,772 kN-m (152,000 ft-kip)

2.3 Frequency-rotational stiffness: Long term soil fatigue and/or foundation fatigue

cause the tower-structural system to degrade and the rotational stiffness reduces over

time. This causes the structural period to lengthen (frequency shorten) leading eventually

to a resonance problem.

Little research is available on this topic because the issue of soil fatigue is relatively

unchartered for the cycle loads of 20,000,000+. The issue is becoming prevalent in areas

of permafrost and weak clays. This author can specifically site at least two cases where

soil fatigue caused period lengthening of the global structural system, but of course can

not discuss the project location (due to confidentiality restrictions). The soil fatigue issue

was documented through structural testing/monitoring and tower-foundation performance

measurements.

Areas susceptible to this problem should be investigated with sophisticated finite element

analysis to understand the soil-structure interaction characteristics and their impact on the

tower performance.

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2.4 Fatigue Failure of Welds: Steel welds that crack over long term fatigue due to

built up residual stresses. The buckling capacity of the tower is reduced and eventually

the tower shell collapses.

Tower shells have become thinner due to economic pressures. This has pushed the D/t

ratios to over 250. Although these structures are stable under static loads, when a

structural defect (i.e., crack, or new weld) is introduced at the base shell (where the

highest moment stresses exist), the result is possible tower buckling due to instability.

Fatigue stress shortens the life cycle of the tower by causing crack propagation of

microscopic imperfections in the welds. These are difficult to observe visually, and must

be examined using X-ray of the welds.

A sample of such fatigue crack is shown:

Figure 1: Fatigue crack with water leakage

2.5 Blade Failure: Blades are overstressed due to fatigue, wear and tear, excessive

vibration, and collapse from these external loads.

Blades have aeroelastic loads imposed from the wind and have experienced numerous

problems due to structural demands.

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Figure 2: Tower Buckling and Blade Failure

Figure 3: Tower Buckling

2.6 Turbine Eccentricity Loads: The turbine design and yaw plates are not designed

properly and have built in eccentricities that were not accounted for in the original load

calculations. These eccentricities lead to long term fatigue cracking in the yaw plate.

A detailed stress analysis of the yaw plate support structure showed a built in eccentricity

effect in the yaw plate. This caused cracks to develop within the structure due to its own

eccentric loading. Additional stresses are magnified by off-axis wind loads.

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Figure 4: Stress concentrations to due built-in eccentricity

Figure 5: ANSYS FEM showing the stress concentration effect

2.7 Rotor Imbalance: The rotor and blades have built-in imbalance and eccentric

moments are created in the blade rotation. Imbalance due to snow/ice loads will also

cause unusual load spectrums that were not part of the original design spectrum.

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2.8 Residual stresses induced by internal welds: Internal welds were done on the

tower without manufacturer approval and cause built-up stresses. These residual stresses

lead to premature fatigue cracking.

2.9 Foundation Cracking from Fatigue: The base foundation cracks due to fatigue

cyclic loads and result in reduction of the foundation rotation stiffness. This impacts the

tower performance and may lead to resonance induced amplitude magnification of the

tower structure.

2.10 Foundation Softening due to poor drainage: The base foundation structure-soil

interface softens due to concentrated drainage problems. This may lead to weakened soil

parameters (due to the presence of water) and degrading lateral stiffness.

2.11 Corrosion of Foundation Bolts from Aggressive Soils: The soil has aggressive

properties that lead to deterioration of the concrete and penetration into the

bolts/reinforcement. Corrosion of bolts/reinforcement leads to reduced foundation

structural capacity and eventual stiffness degradation.

2.12 Seismic/Earthquake Loads: Tower designers consider wind loads without

evaluating the earthquake/seismic effects and the towers may not be capable of resisting

the local seismic demands.

The most unique aspect of wind tower structural design is the dynamic characteristics and

fatigue related issues. Of all civil engineered structures, the wind tower stands as a

inherently special structure because it is a ‘working machine’ with moving parts, versus a

bridge or high rise building. These inherent dynamic loads create many issues that are

not normally considered in static structures.

Long term loads are the normal operational wind loads that include dead, live, static wind

pressure, and standard operational overturning moments, shears, and axial loads. The

frequency of these long term sustained loads may reach over 20,000,000 cycles in 20

years. Therefore, every aspect of the tower-foundation design must consider these

provisions.

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Short term loads include E-stop, earthquake, peak wind dynamic pressure, and 3-second

gust load.

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3.0 DESIGN CRITERIA

As a summary, the international codes utilized in the wind energy industry are:

3.1 Germanischer Lloyd Rules

3.2 Eurocode

3.3 IEC 61400

3.4 DNV Standards

3.5 International Building Code (IBC) and associated US codes

A tower designer that obtains either GL or DNV certification usually will address their

financing/investor issues and the IBC certification is obtained through a US Engineer.

Most structural issues arise from field conditions that were not properly addressed during

the code review/design process.

For example, there are no loads criteria for turbine over speed condition because we

usually expect the turbine to be shut down at 50 mph. The problem with this is that we

don’t have any factor of safety against an over speed condition, and (unfortunately) these

events do occur.

The E-stop condition may be the largest load imposed on the tower. Usually this load

condition is not part of the structural design and may not be part of the certification

process. E-stops create large dynamic impulse loads that are shock-waves within the

tower. It is a very complex load condition and requires special analysis.

Blade failure and/or rotor imbalance loads are not part of the normal design process. We

just ‘assume’ that these events don’t occur – and if they do, then we claim we had ‘no

knowledge’. This is all technically correct from the Engineer’s standpoint but we should

look at the big picture and start considering these points to formulate a total design

solution.

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As-built imperfections in the nacelle resulting in eccentric loads may be part of the design

specifications but there is no mechanism to cross check the actual structure with the

original design documents until a problem arises. We usually don’t know about these

issues until there are cracks developing in the yaw plate or tower shell.

In summary, we have a list of considerations that need to be revisited from our field

experiences and improve on the design phase of towers and foundation structures. An

improved dialogue between construction/maintenance departments and the

design/engineering team is warranted.

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4.0 DYNAMIC AND FATIGUE ANALYSIS METHODS

Towers were designed using static analysis methods up to 50m height. As we now

progress beyond 50m, the dynamic analysis and fatigue issues become a priority. The

key issues are summarized:

4.1 3D vs. 2D Dynamic Models: Tower designers have traditionally used 2-d ‘stick’

models for towers. This does not account for the 3-d characteristics of tower shells. The

mode shapes for a 2-d model are limited to translational and do not capture the

rotational/twisting, buckling, and eccentric characteristics of real towers. Stress

concentrations in the shell due to the door opening are also not included in the 2d model.

4.2 Soil-Structure Interaction: The soil-structure interaction is a prevailing concern in

wind tower design. This impacts the tower performance. The use of Finite Element

Analyses (FEA) of the soil with the foundation is imperative to capture this behavior.

4.3 Tower Shell Stability: With larger diameter-thickness ratios prevailing, the tower

stability is a common cause of collapse. A detailed analysis using FEA model should

examine the shell stability.

4.4 Pre-stress, Post-Tension, and Reinforced Concrete: Certain foundations depend

on pre-stress or post-tensioned elements for their long term performance. These effects

should be studies to assure their interaction with the soil and long term capabilities are

assured. Relaxation of the tendons/bolts/strands will cause the fatigue stresses to be

amplified.

4.5 Soil Fatigue: There has been minimal consideration of this aspect, yet examples

of soil fatigue do exist. A study of the effects on the soil due to the high cycle demand

should focus on the soil-structure interaction interface. More research, testing, and

structural monitoring is needed in this area.

4.6 Actual performance versus Design: In the final analysis, we need to know how

our designs perform in the field. A structural monitoring program should be initiated on

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projects to assess the actual versus designed characteristics. How are we doing? We

can’t know unless we test, monitor, and check our designs.

Figure 6: Example of Soil-Structure Interaction FEM

Figure 7: 2D Arch Model showing soil reaction loads and Moment Diagram

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Figure 8: 3D tower FEM using plate elements Figure 9: 3D Dynamic Mode Shape

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Figure 10: Pile-Cap Foundation

Figure 11: 3D FLAC model with soil-structure interaction

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FLAC3D 3.10

Itasca Consulting Group, Inc.Minneapolis, MN USA

©2006 Itasca Consulting Group, Inc.

Step 52462 Model 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

Dist: 1.040e+002 Mag.: 3.05Ang.: 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

Figure 12: Soil Stress Profile showing compression effects on the subsurface

Figure 13: 3D view of Foundation and Soil Anchors with Soil Strata

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Figure 14: 3D FEA stress profile using plate element model

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Figure 11: Full Tower and Foundation Soil-Structure Model

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Figure 12: 3D ANSYS FEM of Yaw Plate

Figure 13: Stress Concentrations in Yaw Plate due to eccentric loading

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Figure 14: Stress Concentrations in Yaw Plate

Figure 15:

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5.0 IMPROVING STRUCTURAL DESIGN PRACTICE

As we move forward into the 21st century with wind energy becoming a growing segment

of our power capacity, we can improve our design practice by looking at our current

inventory of towers and asking the difficult questions.

5.1 Structural Performance Monitoring: Monitor a select sample size of towers to

keep a track record of their loads, stresses, and performance.

The scope of the monitoring program should be sufficient to cover the project size with a

reasonable sample size. For example, for a 100 tower project, a 3% monitoring program

would be a minimum. This would involve 3 towers per 100 to be observed with

instrumentation for a period of 20 years (i.e., project life cycle). For more complex

projects (varying soil conditions, different tower-turbine combinations), an increased

sample size is warranted.

5.2 Comprehensive Research on Structural Failures: A research report should be

commissioned to examine the inventory of tower designs and documented failures. Very

little information exists on this topic in the research community, and we need more

information.

This industry desperately needs an independent commission/board to examine structural

issues. An AWEA led committee to establish standards, design rules, and overall

forensic examination of the wind energy industry is a worthwhile venture that will allow

us to ‘self monitor’ our progress and share information between projects.

5.3 Evaluation of All Load Conditions: There is a lack of information on the scope of

design loads and what they should include. Perhaps an revised standard is warranted to

form uniform platform for all manufacturers.

Each case of structural issues is somewhat unique. However, when we examine the ‘big

picture’ we find many commonalities and similarities that we can learn from and improve

our designs. The general design codes provide minimum standards but do not require

uniformity in compliance. It would be useful to share these standards through an AWEA

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led committee and establish a set of tower-foundation guidelines that would lead to a

worldwide set of load conditions to be incorporated in all wind tower designs.

5.4 Sharing our Design Problems for Discussion: Our technical teams should

communicate and share their successes and problems. The best way to improve is to

discuss these in an open forum.

Due to the confidential nature of our business it is difficult or impossible to share

information between manufacturers/designers. As an industry, we need to conquer this

challenge and share technical insight in scientific forums similar to this one. A separate

journal for Wind Towers and Structural issues would be helpful to provide a central

forum for technical discussion.

5.5 Statistical Record Keeping: As an industry, we should be keeping records of

problems and issues so that we can track the performance of various systems.

My recommendation is to establish a database of tower performance that would be

available for design firms to examine and determine the results of their efforts. This can

be done formerly through a design commission and sharing of published results of tower-

foundation issues. Most notably important, which foundations perform better in different

soil types is a common question that remains ‘open’ until we experience it first hand in a

real project. A practical ‘wind tower foundation design handbook’ with case studies is

warranted as move forward with larger/taller towers on more complicated site conditions.

5.6 Improving the Design Codes: All of these efforts should be consolidated into a

wind tower structural design committee that can recommend improvements to our design

codes. AWEA could take the lead on this effort to establish a design standards

committee for taking a global approach to these challenges.

THANK YOU!