Incorporating wind-wave misalignment and load directionality into the design of offshore wind turbine foundations

The most important foundation design drivers are the Ultimate, Fatigue and Serviceability Limit State requirements. The assumption of aligned wind and waves is considered acceptable for ULS design, however, when the long term performance is assessed, the directionality of wind and wave loading has to be incorporated. In this paper an attempt is made to probabilistically assess the long term performance of offshore wind turbine foundations. Environmental states are represented by a Bayesian network of interdependent variables representing wind and wave parameters, wind direction and wind-wave misalignment. The number of environmental states occurring in the lifetime of the turbine is generated multiple times through simple Monte Carlo methods, and a fast frequency domain approach is used to assess the total lifetime damage probabilistically. The results demonstrate that the lifetime damage taking into account load directionality is significantly reduced as compared to the damage assuming aligned wind and waves.