Dynamic response and loads analysis of a large offshore wind turbine under low-frequency wind fluctuations

Abstract. We investigate the impact of low-frequency wind fluctuations on the loads and response of a large reference offshore wind turbine. Synthetic wind fields containing low-frequency fluctuations down to 1 hr^-1 are used in aeroelastic simulations with the HAWC2 code. The dynamic response and damage equivalent loads (DEL) for tower and blade moments are evaluated. Both monopile and floating configurations are tested against three wind fields: (i) high-frequency turbulence (3D), (ii) combined low- and high-frequency turbulence (2D+3D), and (iii) high-frequency turbulence scaled to match the measured standard deviation. Low-frequency fluctuations increase DEL for the fore-aft and flapwise moments at the tower base and the blade root, especially at low wind speeds. These are out-of-plane bending moments caused by longitudinal forces. Torsional moments, such as tower-top yaw, exhibit reduced DEL across most wind speeds due to increased coherence. The strongest dynamic response to low-frequency turbulence occurs in the tower fore-aft and blade root flapwise moments at frequencies below 2×10^-3 Hz. For the floating turbine, the platform's surge and pitch motions, and the windward mooring line tension, show pronounced responses. This study underscores the importance of accounting for low-frequency wind fluctuations when simulating the loads and response of large offshore wind turbines.