Long-term durability of offshore wind turbine composite blades based on nonlinear load behavior due to pitch movement

This study proposes a fatigue life prediction method for offshore wind turbines consisting of a fatigue load calculation method that accounts for external environmental factors and system interaction, as well as a fatigue stress spectrum calculation method based on the control strategy. First, we conducted an integrated load analysis considering both external environmental factors and system design characteristics to obtain the ultimate load for flapwise, edgewise, and spanwise directions of the blade. We then applied the ultimate load to the blade finite element model to identify the fatigue critical locations (FCLs) of the blade. Subsequently, we calculated force/moment-local stress relationships considering the applied load magnitude and its change in location according to the control strategy. A 10-min wind speed history was applied to these stress relationships to obtain the unit fatigue stress spectrum at the FCLs. This was then combined with the fatigue resistance of the material to obtain the unit fatigue damage at the FCLs. Finally, we applied the statistical wind speed data at a specific site to the unit fatigue damage to calculate the fatigue damage and life. Based on these results, we evaluated the effect of the loads in each blade direction on the fatigue life of a composite blade.