Research on Fatigue Crack Growth Life of Offshore Wind Turbine Towers with Double Cracks in Extreme Environments

Offshore Wind Turbines (OWT) face fatigue damage from constant exposure to wind, waves, and ocean currents, which can lead to catastrophic tower failure, especially with the presence of double cracks. This study investigates the fatigue crack growth life (FCGL) of a 2.1MW OWT tower under typhoon conditions, considering double cracks. Fluid-structure interaction (FSI) simulations are conducted to reflect real environmental conditions in the Yellow Sea, identifying stress hotspots that serve as crack initiation points. The Forman-Newman-de Koning (FNK) model and linear elastic fracture mechanics are used to calculate crack growth under double crack conditions compared to single cracks. The results indicate that, under typhoon conditions, the location of the maximum equivalent stress on OWT towers shifts upward. During the growth of double cracks, the dominant crack exhibits a similar growth trend to a single crack, while the subordinate crack experiences a gradual decrease in growth rate after penetrating the tower and tends to cease expansion. The presence of double cracks significantly reduces the FCGL of the tower; under wind loading, FCGL decreases from 2.46 years for a single crack to 1.45 years, and under combined wind and wave loading, FCGL decreases from 1.48 years to 0.82 years. This study provides a basis for assessing the damage tolerance of OWT under extreme conditions with double cracks and offers references for maintenance and repair work for engineering professionals.