Wake and performance of floating offshore wind turbines under six degrees of freedom conditions

In the marine environment, floating offshore wind turbines (FOWTs) are exposed to perturbations of nonlinear motion response. An in-depth study of the evolution laws of wake vortex and wake deficit under six degrees of freedom conditions in FOWTs can help reduce the blade load and enhance the output for downstream wind turbines. This paper enhances the traditional actuator line model by incorporating velocity corrections that account for platform motion, enabling dynamic simulations of National Renewable Energy Laboratory 5 MW (Megawatt) reference rotor under six degrees of freedom (6-DOF) conditions, including surge, sway, heave, pitch, roll, and yaw. The results indicate except for the surge and pitch, the effect of the motion response of the remaining DOF on the average thrust and power of the FOWT is within 1%. The 6-DOF motion condition drives the evolution of the wake vortex into a vortex ring or long vortex band mode. The wake lengths for surge, sway, and heave are shortened to 0.7, 0.6, and 0.6 times those of the fixed conditions, respectively, while pitch, roll, and yaw slightly increase the wake lengths to 1.1, 1.2, and 1.2 times those of the fixed conditions. All DOF, except for yaw, tend to delay the onset of wake self-similarity, with yaw reducing it by 10% compared to the fixed conditions. The insights garnered from this paper provide guidance for developing engineering wake models and micrositing for floating offshore wind turbines.