Fatigue life evaluation of offshore wind turbines considering scour and passive structural control

Abstract. Offshore wind turbine (OWT) support structures are exposed to the risk of fatigue damages and scour, and this risk can be effectively mitigated by installing structural control devices such as tuned mass dampers (TMDs). However, time-varying scour altering OWTs’ dynamic characteristics has an impact on the TMD design and fatigue life, which was rarely studied before. In this paper, a simplified modal model was used to investigate the influence of scour and a TMD on the fatigue life evaluation of a 5 MW OWT’s support structure, and the parameters of the TMD were obtained by either a traditional method or a newly developed optimization technique. This optimization technique aims at finding optimal parameters of the TMD which maximizes the fatigue life of a hotspot at the mudline, and effect of time-varying scour can be considered. Results show that scour can decrease the fatigue life by about 26 %, and the TMD can effectively suppress vibration and increase the fatigue life. Further, it is found that the fatigue life can be extended by 7.2 % with the TMD optimized by the proposed optimization technique, compared to that with the traditionally optimized TMD which does not take the change of dynamic characteristics into account.