Scour influence on the fatigue life of operational monopile-supported offshore wind turbines

Offshore wind turbines supported on monopiles are an important source for renewable energy. Their fatigue life is governed by the environmental loads and in the dynamic behavior, depending on the support stiffness and thus soil-structure interaction. The effects of scour on the short-term and long-term responses of the NREL 5-MW wind turbine under operational conditions have been analyzed by using a finite element beam model with Winkler springs to model soil-structure interaction. It was found that due to scour, the modal properties of the wind turbine do not change significantly. However, the maximum bending moment in the monopile increases, leading to a significant reduction in fatigue life. Backfilling the scour hole can recover the fatigue life, depending mostly on the depth after backfilling. An approximate fatigue analysis method is proposed, based on the full time-domain analysis for 1 scour depth, predicting with good accuracy the fatigue life for different scour depths from the quasi-static changes in the bending moment.