Influence of wave modelling on the prediction of fatigue and extreme loads for offshore wind turbines

摘要

Currently it is standard practice to use linear Airy wave theory for the calculation of fatigue loads for offshore wind turbines. However offshore turbines are typically placed in relatively shallow water depths of 10-20m where linear wave theory has limited accuracy. This paper assesses how much wave description accuracy can be improved using 2nd order theory, and what the influence on fatigue loads is. It is found that 2nd order theory works well up to H/d = 0.3, a common case for offshore windfarms. The predicted wave spectrum shape shows agreement with measured spectra. Mudline fatigue bending moment increase by 10-20%. While Airy theory might be sufficiently accurate for fatigue loading, it is clear that loads resulting from extreme wave heights must be calculated with a high-order wave theory. For conventional North Sea structures, Stokes' 5th order wave theory is typically used, while for the shallower waters of offshore wind farms stream function theory has a wider range of applicability. This paper compares extreme wave loads calculated using linear and stream function wave theory. Comparisons of predicted loads are made with loads measured at the Blyth offshore wind farm during storm conditions.