Recommendations on Standards and Regulatory Frame

IEA Wind TCP - Task 42: Wind Turbine Lifetime Extension

This report aims to assess the fatigue life of tower and support structures based on provisions of standards, as these have been developed during the years. After presentation of turbulence models and a summary of safety factors, based on provisions of the IEC 61400-1 from its initial
introduction in 1994 to its 4th edition of 2019, the design basis with respect to fatigue is established, combining applicable safety factors. Next, a fatigue reliability assessment framework is presented to stand as the basis for different life extension scenarios to be benchmarked.

Outcomes of this work, highlight that fatigue life of components such as towers and support structures is important when considering lifetime extension for wind turbines. The latest version of IEC 61400-1 ed. 4 is calibrated to an annual reliability level after 25 years of 3.3 based on the
assumption that the 90% quantile of the turbulence intensity is used in design using safety factors, whereas the full distribution is used in the reliability assessment. The possibility of using the full Weibull distribution for deterministic design was introduced in IEC 61400-1 ed.4
without explicitly considering the influence it had on the reliability. Accepting a lower annual reliability level for fatigue can be motivated by the fact that the reliability is decreasing over time. Further, for lifetime extension, the economic situation is different when considering lifetime
extension, compared to the situation when the wind turbine is designed.

Going beyond the recommended failure probabilities could be an option, if the expected consequence of a failure is small. It could be a possibility to include directly the risk of structural failure in the economic assessment.

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