Estimation of long-term fatigue damage of fixed substructures using fully-coupled models and non-linear dynamic analysis

Abstract Typical industry models for Offshore Wind Turbines with fixed substructures are based on decoupled models between the wind turbine and the substructure. It is proved that the complete dynamic response of the structure can only be captured with coupled models. In this paper a fully-coupled model for Offshore Wind Turbines with jacket support is presented. The computational model allows to obtain an accurate response of the whole structure and the dynamic interaction between all the elements. It also allows to reproduce particular effects such as the aerodynamic damping without the need for artificial damping ratios which would be needed in decoupled models. Typical environmental offshore conditions are integrated in the model and the response of the structure is obtained by means of a non-linear time integration algorithm in order to include the effect of the continuous rotation of the blades. A cost-efficient approach for the determination of fatigue-damage in the joints of the jackets is proposed. It is based on short-time simulations which allow to accurately estimate the long-term damage in general. The estimation shows a good agreement when compared to the damage values obtained by performing the whole time-interval simulation.