Offshore multi-rotor wind turbines: Blade interactions under surging conditions

The need to reduce the levelized energy cost of floating offshore wind turbines has spurred interest in scaling up rotor blades. However, this approach poses challenges related to material strength, manufacturing, and logistical hurdles in transportation. An emerging alternative is multi-rotor configurations on a single platform and tower structure. The present study concerns floating multi-rotor wind turbines operating under surge conditions. A comprehensive characterization of rotor and blade loads, power generation dynamics, and a thorough frequency domain analysis are performed within an actuator line model coupled with the OpenFOAM® Navier–Stokes solver. Results show that global loads and power generation associated with a multi-rotor configuration exhibit variability contingent upon blade azimuth and surge displacement, but heightened rotor power and loads compared to single-rotor configurations. Frequency analysis unveils the dominance of the surge frequency alongside the presence of secondary peaks attributed to rotor-to-rotor interactions. The angle of attack is shown to be primarily influenced by surge motion rather than inter-rotor interactions, underscoring the impact of platform motions on load dynamics. The outcomes of this study have significant implications for designing and optimizing multi-rotor wind turbines, particularly in mitigating load-induced fatigue effects, rendering this work a basis for further efforts in this field. • Power and thrust in fixed/floating quad-rotors increase compared to a single-rotor. • Quad-rotor blades show an increase of angle of attack of: 15% mid-board, 7% tip. • Surge frequency is prominent in single rotor power and torque power spectral density. • Surge motion drives azimuthal variation in angle of attack in floating quad-rotors. • Fatigue effects are associated with both surge motions and blade interactions.