Vibroacoustic underwater noise from fixed and floating offshore wind turbines

Anthropogenic underwater noise from offshore wind turbines is a growing environmental concern, particularly with the large-scale deployment of bottom-fixed and floating devices. This study presents a reduced-order vibroacoustic framework to analyse continuous structure-borne noise emissions from a 10 MW reference turbine in monopile and semi-submersible floating configurations. The methodology couples aero-hydro-servo-elastic time-domain simulations with a frequency-domain acoustic formulation based on radiation-impedance-corrected dipole sources and the method of images for boundary confinement. The analysis is restricted to continuous operational vibrations from the drivetrain and structural dynamics explicitly excluding transient mooring-related sounds. Results show that floating platforms radiate up to 15 dB higher overall levels below 10 Hz due to rigid-body motions, whereas monopiles emit more efficiently at mid-to-high frequencies associated with drivetrain excitations. Water depth critically modulates propagation: shallow-water conditions act as an efficient waveguide, yielding up to 9 dB higher received levels than in deep water for identical floating sources. Directivity patterns are configuration-dependent, with floating platforms exhibiting complex three-dimensional lobes and depth-dependent variations of 10–15 dB in the 100–1000 Hz band, compared with the nearly axisymmetric monopile response. Comparison against the unweighted NOAA behavioural harassment threshold reveals that the monopile exceeds the limit up to 500 m downstream (specifically below 100 Hz), whereas both floating configurations decay below it before 250 m. Although operational noise remains highly audible for all cases up to 500 m, Low-Frequency (LF) cetacean M-weighted metrics restrict the maximum audibility footprint to 750 m for the deep floating platform compared to 1250 m for the monopile. The proposed framework provides a fast, physics-based screening tool for early-stage design comparisons. It captures first-order propagation effects but is not intended to replace detailed site-specific environmental models. Rather, it supports the identification of configuration-dependent trends and informs more refined assessments when combined with higher-fidelity propagation solvers.