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As towers grow taller, consider structural impacts  

Credit:  By Dilip Khatri | North American Windpower | www.nawindpower.com ~~

Wind towers are proliferating into new sizes and designs. With such developments occurring in the industry, it is worthwhile to take a step back and examine the pitfalls and potential mistakes that can occur when new tower heights are reached.

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 shorter than 40 meters with 500 kW to 660 kW turbines and constructed with a 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, the market has evolved to ever-increasing tower heights. The current trends of the industry are 80-meter, 90-meter and now 100-meter towers. Turbine sizes have increased to 2 MW, 2.5 MW and 3 MW, with increasing sizes exceeding 4.5 MW forthcoming. These dead loads are surpassing 2,600 kN, and the complications of structural dynamics, frequency response and soil-structure interaction are becoming increasingly important.

The tube design is being improved with different versions to accomplish the 90-meter to 100-meter tower heights. Several manufacturing companies are initiating new patented designs that claim higher efficiency, lower cost and stronger tower capacity.

The increased capacity brings more responsibility for developers. Therefore, it is imperative to keep the financial community, investors and insurance underwriters comfortable with the rate of advancement.


Structural failures

To begin, consider the term “structural failure.” In this context, structural failure refers to collapse load – the actual physical destruction of the tower system. Many structural engineers use the term “failure” to refer to exceeding the design-allowable load (or factored capacity load), not that the actual structure is collapsing; it is only “failing in a design calculation.”

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 lengthy periods. The shutdown process leads to lost revenue and increased maintenance costs, which are long-term problems. The big picture is that the wind tower may have this problem repeated on thousands of other designs. Twelve examples of structural failures and maintenance problems are summarized as follows:

Turbine overspeed. The turbine and rotor run into overspeed (>24 rpm) at wind speeds over 50 mph. This results in excessive loads, and the entire system collapses. Turbine/rotor overspeed 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 overspeed from 20 rpm to 25 rpm, calculations show that the energy demand increases by 56%. This additional energy has to be absorbed by the tower structure and foundation. Otherwise, a failure may occur. Energy and structural loads are directly related because the structural system has to absorb these increases. The additional energy input from the rotor has to go into the structure and soil.

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, culminating in large lateral deflections. The magnitude of E-stop loads can be appreciated by examining the fundamental impulse-momentum equations and energy release from the blade rotation. The basis for understanding E-stop loading derives from Impulse Momentum Theory, and the calculated energy release results in an overturning moment of over 200,000 ft-kips, more than five times that of the extreme load. This can cause excessive damage to the tower and foundation.

Frequency-rotational stiffness. Long-term soil fatigue and/or foundation fatigue causes 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 because the issue of soil fatigue is relatively unknown for cycle loads of 20,000,000+. The issue is becoming prevalent in areas of permafrost and weak clays. Anecdotally, there are two cases where soil fatigue caused period lengthening of the global structural system. 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.

Fatigue failure of welds. Steel welds 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 lifecycle 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-rays of the welds.

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.

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 shows a built-in eccentricity effect in the yaw plate. This causes cracks to develop within the structure due to its own eccentric loading. Additional stresses are magnified by off-axis wind loads.

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.

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.

Foundation cracking from fatigue. The base foundation cracks due to fatigue cyclic loads and results in reduction of the foundation rotation stiffness. This impacts the tower performance and may lead to resonance-induced amplitude magnification of the tower structure.

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.

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.

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 an 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 and 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. Short-term loads include E-stop, earthquake, peak wind dynamic pressure and three-second gust load.

Several organizations and associations have standards pertaining to towers, including Germanischer Lloyd (GL), Eurocode, the International Electrotechnical Commission, DNV and the American Wind Energy Association (AWEA).

A tower designer that obtains either GL or DNV certification usually will address financing/investor issues, and the International Building Code certification is obtained through a U.S. 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 overspeed conditions because the turbine is usually expected to be shut down at 50 mph. The problem with this is that there is no factor of safety against an overspeed condition, and unfortunately, these events do occur. The E-stop condition may be the largest load imposed on the tower. Typically, 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 shockwaves 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 but should be. By looking at the big picture, a total design solution can begin to be formulated.

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. Owners and operators typically do not know about these issues until there are cracks developing in the yaw plate or tower shell.


Dynamic and fatigue methods

Towers were designed using static analysis methods up to 50-meter heights. As towers grow beyond 50 meters, the dynamic analysis and fatigue issues become a priority. The following are key issues:

3D vs. 2D dynamic models. Tower designers have traditionally used 2D “stick” models for towers. This does not account for the three-dimensional characteristics of tower shells. The shapes for a 2D model are limited to translation 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 3D model.

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

Tower shell stability. With larger diameter-thickness ratios prevailing, the tower stability is a common cause of collapse. A detailed analysis using an FEA model should examine the shell stability.

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 studied to ensure their interaction with the soil and long-term capabilities. Relaxation of the tendons/bolts/strands will cause the fatigue stresses to be amplified.

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.

Actual performance vs. design. In the final analysis, an understanding of how designs perform in the field is critical. A structural monitoring program should be initiated on projects to assess the actual performance compared with the original design.


Improving standards

Moving forward, tower design practices can be improved by looking at the current inventory of towers and asking the difficult questions.

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 three towers per 100 to be observed with instrumentation for a period of 20 years. For more complex projects (varying soil conditions, different tower-turbine combinations), an increased sample size is warranted.

Structural failure research. 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 more data is needed.

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 the progress and share information among projects.

Evaluate load conditions. There is a lack of information on the scope of design loads and what they should include. Perhaps a revised standard is warranted to form a uniform platform for all manufacturers. Each case of structural issues is somewhat unique.

However, an examination of the big picture finds many commonalities and similarities from which designs can be improved. The general design codes provide minimum standards but do not require uniformity in compliance.

Design problems for discussion. 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 business, it is difficult or impossible to share information between manufacturers/designers. The wind industry needs 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.

Statistical record keeping. The industry should be keeping records of problems and issues to track the performance of various systems. The establishment of a database for tower performance could be made available to design firms to examine and determine the results of their efforts. This can be done formally through a design commission and sharing of published results of tower-foundation issues. Most important, the database could inform which foundations perform better in different soil types.

Improve design codes. All of these efforts should be consolidated into a wind tower structural design committee that can recommend improvements to our design codes. For example, AWEA could take the lead on this effort to establish a design standards committee for taking a global approach to these challenges.

Dilip Khatri is the founder of Pasadena, Calif.-based Khatri International. He can be reached at (626) 475-7660 or dkhatri@aol.com.

Source:  By Dilip Khatri | North American Windpower | www.nawindpower.com

This article is the work of the source indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.

The copyright of this article resides with the author or publisher indicated. As part of its noncommercial effort to present the environmental, social, scientific, and economic issues of large-scale wind power development to a global audience seeking such information, National Wind Watch endeavors to observe “fair use” as provided for in section 107 of U.S. Copyright Law and similar “fair dealing” provisions of the copyright laws of other nations. Send requests to excerpt, general inquiries, and comments via e-mail.

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