Fatigue load simulation for foundation design of offshore wind turbines due to combined wind and wave loading

Wave- and wind-induced fatigue load plays an important, often crucial role in the design of wind turbine supporting structures. This paper presents a practical procedure of fatigue load simulation for foundation design due to combined wind and wave loading in time domain. The distributions of mean wind speed and wind direction, as well as the wave height and wave period are taken into account. Correlation between wind and wave is considered by introducing the scatter diagram from oceanography. The harmony superposition method (HSM) is applied to simulate fully correlated random time series of wind velocity, from which aerodynamic loads can be obtained. The aerodynamic loads on the wind turbine rotor are determined using the blade element momentum method (BEM), which is computationally efficient but with good precision. The ocean wave model suggested by Longuet-Higgins is adopted, by which an irregular wave can be simulated based on the linear wave theory (Airy theory). Wave loads are finally generated by using the Morison equation. The offshore wind turbine tower is modeled as a discrete multi-degree-of-freedom system, with a lumped mass at the top of tower, representing the nacelle and blades. The bottom of the tower is connected to springs to introduce the effect of soil stiffness. The fatigue load spectrum is extracted by using the rain flow counting method. Through comparison with the capacity spectrum, the situation whether the design requirement is satisfied or not can be verified. Numerical examples are presented in the paper to demonstrate the feasibility of the current procedure. The sensibility of fatigue load due to variation of simulating parameters including water depth and soil stiffness is also numerically studied.