Unlocking wind farm life extension, recent trends

Maximizing the performance of wind assets and their life potential can unlock real value for stakeholders. Lifetime extension is one of many options available to a wind farm owner and the results of a lifetime assessment are key to developing effective lifecycle strategies.

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In this blog, I will summarize key insights and trends that DNV has experienced through carrying out life extension projects.

Non-availability of key structural component data
Structural component fatigue accumulation is a key concern for wind turbine life extension. Other factors contribute indirectly, such as operation & maintenance regimes and component degradation due to age and exposure, amongst others. To understand the lifetime of a particular structural component, the knowledge of component strength margins is key. However, this information is typically not available to the wind turbine owners.

Strength margin is the gap between the load level that would risk causing structural failure and the ‘design loading’ level used to dimension the wind turbine components. Whilst turbines are designed to wind ‘type’ classes as defined by the standards which translates to a particular ‘type class’ load level, often additional margin can be found at different structural component locations for a variety of reasons:

• Component design strength may be governed by extreme loading and therefore have significant margin to the design fatigue loading.
• Components may be shared across multiple variant configurations across a turbine platform and be designed to withstand the highest type class associated with the platform.
• Manufacturing constraints may build in margin e.g., component thicknesses being larger than strictly necessary.

Turbine manufacturers (OEMs) typically do not share information on design margins, design loads or aeroelastic model information citing intellectual property protection. This leads to an information gap that only the OEM can fully cover when the question of life extension is being considered.

Third-party approaches governed by standards, such as DNV-ST-262 & 263 and IEC 61400-28 (currently in draft), exist for the assessment of fatigue lifetime through aeroelastic load comparison partnered with inspection. These approaches largely rely on generic models to inform a comparison between site specific loads and type class loads, and therefore do not leverage the component strength margin, leaving conservatism inherent in the lifetime results. This naturally leads to a reliance on practical monitoring to supplement the numerical analysis in an effective risk management strategy to realize a target lifetime objective.

Owners and service providers, including DNV, are exploring and developing methods to reverse engineer aeroelastic and structural component models to provide greater insight into component strength margin. However, this is a time consuming and complex task and hence only applied in a strategic manner when the benefits outweigh the costs.

Removal of conservatism to unlock longer lifetime
DNV has observed trends that OEMs are removing conservatism when calculating site-specific loads in order to increase the lifetime at specific wind farm sites. Fatigue load conservatism is typically reduced through the following approaches that demand more in-depth analysis:

• Per turbine location assessment - Using sophisticated methods to assess each wind turbine location rather than a traditional cluster of locations.
• Full turbulence distribution
• The calculation of fatigue loading using the full turbulence distribution (all percentiles) rather than the traditional industry approach to assume a characteristic (P90) turbulence level. Multidirectional analysis specific to the tower and foundation loading, performing the load calculations assuming a multidirectional inflow and weighting loading based on the wind rose, rather than the traditional industry approach to conservatively assume unidirectional loading.
• Advanced wake models - Modelling wind turbine wakes utilizing more physical engineering models, such as the ‘dynamic wake meandering’ (DWM) model as described in IEC 61400-1 edition 4, rather than the traditional industry approach of the Frandsen effective turbulence method.
• Smart adaptive control - Adoption of intelligent control actions which increase control responsiveness (and hence minimize loading) in response to detection of onerous environmental conditions (e.g., high density, high turbulence) or wind farm control that balances loading across the turbines through wake manipulation.

Longer service agreements
Alongside longer lifetime, there is a trend for OEMs offering longer service agreements, in certain markets as long as 30 years.

Historically for a significant proportion of wind farms globally, the initial term of the service agreement from the OEM was shorter than the design life (e.g., up to ~15 years). Subsequent renewal periods were typically for periods of five years. Now however for some markets we are seeing much longer service agreements from OEMs for new projects (up to 30 years). This indicates that OEMs are getting more comfortable with extending the useful life of wind turbines and see these agreements as a reliable and profitable long-term revenue source. This also lends confidence to developers and investors that the useful life of a wind farm can be more than previously expected.

Adding value, globally
Learn how our independent appraisal and holistic approach to evaluating your assets and their life potential can bring greater certainty and add real value by providing assurance to all stakeholders throughout a wind farm’s lifetime.

11/23/2022 8:00:00 AM