This article was first published in WindMax, the quarterly technical data publication of Windpower Monthly. Click here to find out more.
Many of the country's developers are considering repowering their ageing wind farms to requalify them for the renewable energy production tax credit (PTC), so repowering is set to be a hot topic within the industry.
There are two approaches to repowering: full and partial. In the former, old turbines (including their foundations) are removed and replaced by new units.
The more popular partial repowering typically involves an upgrade with more advanced and efficient technology to main components, particularly the rotor and gearbox, while other elements, such as the foundation and tower are retained for reuse.
Of the two, partial repowering is the more complex, since the foundation — a technically complicated component that is buried under the ground — can be just as difficult to modify as it is to replace (see Figure 1, below).
Before repowering turbine foundations, it is necessary to conduct a full design review to assess their ability to withstand years of additional service. This review should cover three key aspects.
1 - Original design review
The review of the original turbine foundation design includes a structural and geotechnical evaluation, including the foundation’s fatigue-resistance capabilities, its design loads and the performance criteria provided by the turbine maker.
A qualified structural engineer should carry out a review of the design reports for the foundation with regard to fatigue design, in order to assess the site-specific design lives of critical components.
In addition, the designs should be compared with other foundation designs using similar turbine models but based on up-to-date design codes and standards.
2 - Current foundation review
Next, it is vital to assess the current conditions of the foundation thoroughly. Additional geotechnical and structural investigations and analyses should also be considered for an extended design life, particularly if the foundation being considered is reaching the end of its expected design life.
The investigation should also include a thorough assessment of the loading to which the foundation has been subjected over its operational life. As a minimum, these investigations should include:
- Inspection and evaluation of the visible portions of the foundation.
- Removal of backfill and inspection of the foundation service.
- Evaluation of solid conditions around the foundation, including any ground cracking or deformation.
- Evaluation of 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. It is also important that the geotechnical assessment rules out any soil degradation due to cyclical loading of the foundation. The foundation design must ensure that, under unfactored permanent or normal loads, contact pressure remains compressive under the entire foundation (meaning no ground gap or zero pressures should occur).
The evaluation of the loading the foundation has experienced should include:
- A detailed review of operating records.
- Evaluation of the loading history, using Scada records or direct measurement from load cells or strain gauges mounted within the tower.
The evaluation of the effects of historical loading and accumulated fatigue damage is subject to uncertainty as Scada data tend to be the main sources of such information.
These datasets consist of summaries and statistics of operation during ten-minute periods, which lack many details important for assessing fatigue damage.
They can be useful, however, for an estimation of significantly different operating conditions during the operating period than had been assumed by the original fatigue design.
3 - Full structural assessment
A wind turbine is an operating system and experiences a high number of load cycles during — around a billion during a typical 20-year lifetime.
The key consideration for partial repowering and extending the life of a turbine is the fatigue performance of the foundation.
The term "fatigue" in structural engineering refers to the progressive and localised 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 (see Table 1, right).
The question is: is the projected repowering/life extension possible considering the accumulated fatigue damage to the turbine foundation from both the original and repowered configurations?
Figure 2, below, presents different turbine foundation fatigue scenarios with respect to repowering and life extension.
Repowering could result in either an increase or decrease in fatigue demand; for example, a modern turbine with a more advanced controller might be able to reduce the fatigue demand,
A recent study by DNV GL on the topic provides a framework for the fatigue evaluation of existing foundations for repowering or life extension.
Through case studies (see Figure 3, below), 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 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 central US.
Often, foundations older than ten 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 repowering possible.
The good news for developers is that it is easier to plan for future repowering with new wind projects — for example, by designing the foundations for a 30- to 40-year life, rather than the typical 20 years. Project stakeholders can provide for more options in the future.
Hieu Nguyen is a senior structural engineer, and Matthew Rogers is team leader in the civil engineering department at DNV GL Energy
This article was first published in WindMax, the quarterly technical data publication of Windpower Monthly. Click here to find out more
