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Foundations for Success: Three Steps to Full Turbine Repowering from the Bottom Up

Many U.S. developers are considering repowering for their aging wind farms to requalify them for the renewable energy production tax credit. With nearly 30 percent of U.S. installed capacity over 10 years old this year (25,410 MW out of current 84,407 MW), repowering will continue to be a hot topic in the coming years.

When Full Repowering is Not An Option

Projects can be fully or partially repowered. In full repowering, old turbines (including foundations) are removed and replaced by new turbines. The more popular partial repowering typically involves an upgrade to certain turbine main components, such as the rotor and gearbox, with more advanced and efficient technology while keeping other components, such as the foundation and tower, for reuse. In cases when partial repowering is of interest, it is vital for project owners to understand the technical challenges and to know what to expect.

Of the two, partial repowering is most challenging, since the foundation – technically complicated and buried in the ground — is much more difficult to change or to replace. (See Figure 1).

Figure 1 Cross Section of a typical wind turbine foundation

DNV GL has identified three key steps to successful partial repowering, from the perspective of foundations:

1.Review the Original Design

Conducting a full design review will assess the ability of a foundation to withstand additional years of service utilizing new components. The design review should include:

  • A comprehensive review of the original design
  • Current condition assessment of the foundation
  • A comprehensive structural assessment (fatigue and ultimate)

The review of the original turbine foundation design should include an evaluation of the structural and geotechnical design of the foundation, including its design for fatigue resistance, the foundation design loads and performance criteria provided by the turbine manufacturer.

A qualified structural engineer should review the design reports for the wind turbine foundations, with regard to fatigue design, in order to assess the site-specific design lives of critical components, such as pullout reinforcement, pedestal concrete, top mat reinforcement. In addition, the foundation designs should be compared to other foundation designs using similar turbine models but based on current design codes and standards. Initial fatigue calculations for older foundations may materially underestimate fatigue in accordance with current industry knowledge.

2. Review the Current Foundation Condition

It’s vital to thoroughly assess the current condition of the foundation. Additional geotechnical and structural investigations and analyses should also be considered for an extended design life evaluation, particularly if the foundation being considered is reaching the end of its expected design life.

An investigation of the foundation condition should include a thorough inspection of the foundation and the loading to which it has been subjected over its operational life. These geotechnical and structural investigations and analyses should include the following, at a minimum:

    • Inspection of the visible portions of the foundation and evaluation
    • Removal of backfill and inspection of the foundation surface
    • Evaluation of soil conditions around the foundation including any ground cracking or deformation
    • Evaluation of any potential corrosion to the concrete and steel foundation elements
    • Evaluation of site drainage and erosion conditions
    • Implementation of structural condition monitoring to measure stresses and displacement of the foundation
    • A detailed review of construction records and quality assurance documentation
    • Non-destructive testing of the foundation to detect defects and discontinuities within the concrete

The geotechnical assessment should rule out any soil degradation due to cyclical loading of the foundation. The foundation design must ensure that under unfactored permanent or normal operating loads, contact pressure remains compressive under the entire foundation (i.e., no ground gaps or zero pressures should occur.) The evaluation of the loading the foundation has experienced should include a detailed review of operating records for the subject foundation(s), as well as an evaluation of the loading history of the foundation using SCADA records or direct measurements from load cells or strain gauges mounted within the tower.

The evaluation of the effects of historical loading and accumulated fatigue damage is typically highly uncertain, as SCADA data are usually the main source of such information. SCADA data sets, typically summarized into 10-minute periods, lack important detail to evaluate fatigue damage. However, they can be useful for understanding if the foundation was subject to significantly different operating conditions during the operating period than was assumed in the original foundation fatigue design.

3. Complete a Full Structural Assessment

A wind turbine is an operating system, and during its lifetime experiences a high number of load cycles (1,000,000,000 or 109 cycles during a typical 20-year lifetime). The key element regarding partial repowering and extending the life of a wind turbine is the fatigue performance of the foundation.

The term “fatigue” in structural engineering refers to the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. A fatigue failure can occur at loads and stresses much less than the ultimate strength of the material or structural element.

Is such a repowering/life extension possible considering the accumulated fatigue damage in the turbine foundation from both the original and the repowered configurations?

Figure 2 presents different turbine foundation fatigue scenarios with respect to repowering and life extension. Note that with a modern wind turbine with more advanced controls may actually be able to reduce the fatigue demand.

Figure 2 Repowering/Life extension and wind turbine foundation fatigue scenarios

A recent study by DNV GL has provided a framework for the fatigue evaluation of existing foundations for repowering/life extension purpose. Through case studies, the authors investigated the accumulative fatigue damage at critical structural components such as anchor bolts, grout and concrete at anchor bolt locations, top/bottom reinforcement, and local reinforcement (bursting, pullout) of typical wind turbine foundation designs.

Additionally, typical “weak links” for traditional gravity foundations were identified, including pullout concrete reinforcement fatigue (often referred to as plug failure) and pedestal concrete bearing/bursting fatigue.

As a provider of independent engineering for project financing, DNV GL has reviewed the design and calculations for a variety of repowering projects in the central United States, finding, in certain cases, that turbine foundations can quickly become a hot topic. Often, foundations older than 10 years may not have been designed for fatigue and may not be able to meet both baseline and repowered fatigue demand under current design standards. Fatigue damage is cumulative and foundation retrofit is often difficult and costly. For existing foundations, consideration given to as-built data (concrete strength, mill certificates, etc.) and site-specific loads could make foundation repowering possible.

Fortunately, is it is easier to plan for future repowering with new wind projects by designing the foundations for 30 or 40-year life rather the typical 20-year life. By considering this approach in the design phase, project stakeholders can provide for greater flexibility in repowering options in the future.

About the Authors

Hieu Nguyen has six years of academic/engineering experience in wind energy. As a senior structural engineer in civil engineering department at DNV GL Energy, he supports investors and owners in review of design for wind turbine structural support systems.

Matthew Rogers has fourteen years of engineering experience, with five of those most recent years being in wind energy. As team lead in the civil engineering department at DNV GL Energy, he conducts design and construction review of wind energy projects in support of independent engineering groups.

Cory Gessert has over ten years’ experience in wind and solar project management throughout the development pipeline. She leads North America’s Development and Engineering services team, which strives to manage risk to developer and utility clients through design. Her team provides owner’s engineering support on wind, solar, microgrid, and energy storage projects.

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