Posters

Abstract

Offshore wind farms are reaching mid-life, and operators face major uncertainty about whether assets can safely and profitably operate beyond design life. This study presents a method that uses already available SCADA data to give a fast, low-cost screening before deeper analyses for certification. The lifetime of an offshore wind turbine is often limited by fatigue on critical components, strongly influenced by mode shape and foundation stiffness, with soil stiffness being a key parameter in load assessment. Using the measured natural frequencies of the as-build structures might reveal discrepancies to the design natural frequency. These differences can be due to various reasons including fabrication tolerances, assumptions in design seabed level and modelling of the scour protection system, and uncertainty in the soil stiffness response. A sensitivity assessment has concluded that the soil stiffness response has the most significant influence on natural frequency. To assess the impact on structural lifetime requires heavy interdisciplinary oversight. The soil stiffness response is calibrated to match the measured natural frequencies, new load envelopes are derived for the updated stiffness properties and new steel utilizations are calculated. Thus, the updated lifetime can be assessed. In this study a sweep has been performed where various increases in natural frequencies have been assessed to see the impact on lifetime. A reference North Sea project in 30-40 m water depth in combination with a 15 MW WTG is considered. The method outlined in this study is appropriate as an initial screening of potential for lifetime extension based on readily available SCADA data. To allow for certification of lifetime extension more extensive analyses are required than simply relying on natural frequency measurements. Although this approach is highly interdisciplinary it is a fairly quick and cheap method than can provide substantial quantification of the potential of lifetime extension.