Measurement and characterisation of radiated underwater sound from a 3.6 MW monopile wind turbine

This paper describes underwater sound pressure measurements obtained in close proximity (50 m) to two individual wind turbines, over a 21-day period, capturing the full range of turbine operating conditions. The sound radiated into the water was characterised by a number of tonal components, which are thought to primarily originate from the gearbox for the bandwidth measured. The main signal associated with the turbine operation had a mean-square sound pressure spectral density level which peaked at 126 dB re 1 lPa2 Hz 1 at 162 Hz. Other tonal components were also present, notably at frequencies between about 20 and 330 Hz, albeit at lower amplitudes. The measured sound characteristics, both in terms of frequency and amplitude, were shown to vary with wind speed. The sound pressure level increased with wind speed up to an average value of 128 dB re 1 lPa at a wind speed of about 10 ms 1, and then showed a general decrease. Overall, differences in the mean-square sound pressure spectral density level of over 20 dB were observed across the operational envelope of the turbine.