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On Nov. 17, 2013, a blade on a brand-new turbine broke at the Orangeville Wind Farm, located in Orangeville, N.Y. Around the country, there were at least three other incidents that occurred within a close time frame. Most of the incidents had the same root cause: a spar cap manufacturing anomaly. Once the root cause was identified, the manufacturer traced the affected population and implemented corrective actions. In cases like these, corrective actions may include the following: retrofits, blade replacements, load-reducing curtailment, or heightened monitoring and inspection.

Blade failures are often catastrophic and happen without warning. But, is the drama of a sudden failure biasing us to believe there’s a problem where there isn’t one? In this article, we’ll explore the most common causes of blade failures, ways to mitigate the risk of such failures and what the wind industry can do to eliminate unexpected blade failures.

DNV GL has compiled blade failure rates from 10 GW of operating wind projects and found that 1% to 3% of turbines in North America require blade replacements annually in the first 10 years of operation, with the highest failure rates usually occurring in year 1 and year 5.

So, what causes blade failures?

Blades fail when an applied load exceeds the blade strength. A single extreme wind event, such as a severe gust or high shear event, could lead to an applied load that exceeds the blade strength. Blade strength might be reduced in the presence of manufacturing defects, damage incurred during transportation and handling, degradation over time, other minor damage that propagates over time or inadequacies in the original design.

Operational factors can also lead to excessive loads on the blades. Take the following, for example: incorrect pitch set-points, incorrect shutdown sequencing or failure to maintain yaw alignment during high winds can lead to defects. Damage from lightning strikes can also lead to blade failure.

 

Manufacturing defects

In DNV GL’s experience, manufacturing defects are currently the leading cause of blade failure. Defects occur when blades are not manufactured according to the design specifications. Common defects include the following:

Serious manufacturing defects weaken the blade to the point that normal loads cause premature failure.

Most utility-scale wind turbine blades are manufactured in several parts, which are then bonded together in a secondary assembly process. A poor bond may significantly shorten the life of a blade.

The term “wrinkle,” or “wave,” describes the significant misalignment of fibers in one or more layers (lamina) within a composite laminate. A small degree of waviness is expected, due to fabric architecture and construction details, but extensive wrinkles at key locations reduce blade strength. As shown in Figure 1, the severity of the impact of a wrinkle increases with the angle of the wrinkle and with the amount of laminate affected. Wrinkles reduce both the fatigue and ultimate strength of the blade – thus, the blade may fail prematurely due to a gust exceeding the now-reduced ultimate strength or due to fatigue loading over some duration less than a typical 20-year design life.

Manufacturing defects in blades are driven by many things, including the following:

Lightning can also have a big impact on the blade. Lightning strikes are typically intercepted by a blade’s lightning protection system (LPS) and conducted down to ground safely without damaging the turbine. But occasionally, lightning will attach to the blade instead of the LPS and burn or delaminate a section of the blade.

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Sometimes, lightning only leaves pin holes in the shell; sometimes, it results in complete destruction of a blade. However, some lightning damage can propagate to a full blade failure if left unrepaired. Lightning damage is a major problem at some wind farms in North America. Insurance companies report that one-quarter to two-thirds of all wind farm-related claims involve lightning.

Some sites are more prone to lightning damage than others. Factors that influence the lightning damage rate at a site are the LPS design, LPS maintenance and the lightning environment. The lightning environment at a site can be evaluated at a basic level with a lightning flash density map. The National Lightning Detection Network comprises over 100 lightning detection sensors around North America and detects a high percentage of flashes. As shown in Figure 2, a quick look at the lightning flash density map of the U.S. shows that lightning incidence varies widely across the country, implying some projects are at greater risk for lightning damage than others. A flash density map alone is not sufficient to characterize the lightning environment at a specific wind site. A more detailed assessment of the lightning risk requires consideration of terrain effects and the presence of the turbines influencing the lightning environment.

A turbine LPS is typically certified to the IEC 61400-24 (lightning protection) standard for a chosen lightning protection level (LPL). Most modern turbine LPSs are certified to LPL 1, which provides the highest level of protection of the four LPLs. Nonetheless, there are inconsistencies in performance across LPS and blade designs even within the same LPL, so it can be challenging to predict how much lightning damage will be experienced by a new project.

Lightning damage is often covered by insurance, but frequent damage will come at a cost to all parties involved: Insurance may be withdrawn for excessive claims; deductibles transfer some risk and cost to owners; and ancillary costs, such as lost revenue, may be covered in part or in full by insurance. The current practice of writing off lightning damage as force majeure obfuscates responsibility and slows innovation in LPS design. Appropriate allocation of the damage costs will help move LPS design forward.

 

Mitigating blade failure

To reduce the risk of defects, manufacturers should target the drivers of defects listed above through measures including the following:

Whether or not these manufacturing risk mitigation measures are adopted, turbine buyers, owners and operators can always take measures to mitigate the risks of blade failure. Opportunities for risk mitigation exist in all phases of project development and operation.

During the development phase of a project, independent monitoring of blade manufacturing and careful siting can reduce the risks associated with blade failures. Additionally, performing a lightning damage risk assessment reduces the risk of unexpected lightning damage costs.

Practicing careful turbine siting is another way to reduce blade failure risk during the development phase. As turbines designed for low-wind classes are increasingly being sited at higher-wind sites, the need for careful siting, especially through accurate characterization of environmental conditions, is increasingly important.

A lightning damage risk assessment can help illuminate and predict the amount of lightning damage at the project site, given the lightning environment and the turbine design specifics. Although the field performance of the LPS may not meet expectations, the lightning risk assessment can provide owners and investors with a distribution of expected lightning-related costs, given the anticipated performance of the LPS.

Opportunities for risk mitigation also exist in the operations phase, such as through appropriate inspection and repair practices. Customized inspection approaches, including methods and frequency, can be developed by considering the specifics of each site.

 

Eliminating blade failures

A number of areas of research and development should be targeted for the biggest returns:

With careful application of such risk mitigation strategies and focused effort on new pathways, significant occurrences of blade failures could become a thing of the past.

 

Matthew Malkin has 20 years of engineering experience and has led multiple blade failure investigations, blade manufacturing quality reviews and technology reviews. Alex Byrne has 13 years of engineering experience with a focus on wind turbine loads, life assessment and turbine performance. She has participated in and led technology evaluations, site suitability assessments, lightning analysis and failure investigations. Dayton Griffin is an internationally recognized expert on wind turbine blades, with over 20 years of wind energy experience. They can be reached at matthew.malkin@dnvgl.com, alex.byrne@dnvgl.com and dayton.griffin@dnvgl.com, respectively.

Marketplace: Blades

Does The Wind Industry Have A Blade Problem?

By Matthew Malkin, Alex Byrne & Dayton Griffin

After some high-profile incidents in the U.S. last year, some industry watchers are beginning to ponder this question.

 

 

 

 

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