The influence of wind veer on fatigue loading for large floating wind turbines with flexible drivetrains

Abstract. To reduce cost, offshore wind turbines are expected to be designed with significantly increased rotor diameters. Larger turbines become more flexible and span a larger portion of the atmospheric boundary layer. With these changes, the validity of traditional modeling assumptions should be investigated. This work challenges two common assumptions: 1) that the drivetrain can be considered rigid (except in torsion) and does not couple with the rotor and tower and 2) that wind directional change with height (veer) does not greatly influence the fatigue damage in the tower, blades and drivetrain.

Two large floating wind turbines are considered: the International Energy Agency (IEA) 15 MW (Gaertner et al., 2020) reference turbine with the University of Maine VolturnUS-S platform (Allen et al., 2020) and the IEA Wind 22 MW reference turbine (Zahle et al., 2024a). Both are direct-drive generator turbines supported by semisubmersible platforms. Aero-hydro-servo-elastic simulations are performed using OpenFAST, with drivetrain bending flexibility and main bearing response implemented in the coupled analysis. The turbines are subjected to a set of load cases at below-, near- and above-rated mean wind speeds, assembled based on NORA3 hourly wind and wave hindcast data (Haakenstad et al., 2021) for Utsira Nord off the coast of Norway (Cheynet et al., 2023, 2024). Within each load case, conditions with and without veer are simulated to evaluate the influence of veer on damage equivalent loads (DELs) of the turbine tower, blades and main bearings. Further, these load cases are applied to evaluate the influence of drivetrain flexibility on global turbine response.

The results indicate that, depending on the veer gradient, operating regime and turbine size, veer can be very important for DELs of the tower top and blade root and for the fluctuations of the main bearing radial loads. Moreover, drivetrain flexibility is found to influence global DELs, especially for the largest turbine. Comparing flexible and rigid drivetrains, the tower-top fore-aft and torsional damage equivalent moments of the 22 MW turbine are reduced by more than 20 % at near rated wind speeds. For the same turbine in below-rated wind speeds, the blade root flapwise DELs are reduced by up to 6 %.