A composite modelling approach for the fatigue analysis of offshore wind turbines considering corrosion-induced material degradation

Abstract

Accurate fatigue damage prediction is essential for the safe and cost-effective design of offshore wind turbines. However, the large scale and offshore deployment of modern turbines poses significant challenges for accurately modelling their long-term behaviour. This work proposes a composite modelling framework combining a non-linear multibody dynamic (MBD) model and a high-fidelity finite element (FE) model to improve fatigue life predictions. It represents the first fatigue assessment of the IEA 15-MW reference wind turbine tower and explicitly considers a newly designed bolted ring flange connection at the tower base. The MBD model, with 22 degrees of freedom, accurately captures the system-level response of the turbine under operational wind and wave loading. However, it cannot resolve local stresses at fatigue-critical details. To address this, stress concentration factors (SCFs) derived from a FE model are used to enhance the MBD-derived stress histories. The composite modelling framework is applied to a known fatigue hotspot, specifically the butt weld at the tower base which connects the tower shell to a specifically designed ring flange connection. The inclusion of SCFs arising due to the presence of the bolted ring flange act to reduce the predicted fatigue life by 19.3 %, demonstrating the substantial influence of stresses induced by local geometric features on fatigue performance. The study additionally considers time-varying SCFs resulting from corrosion-induced material loss. Employing time varying SCFs and assuming a five-year effective corrosion protection period, the predicted fatigue life decreases to 13.39 years, failing to achieve the desired 20-year design life. These results demonstrate the critical importance of incorporating the effects of both local geometry and corrosion into fatigue assessments of offshore wind turbines.