Offshore renewable energy installations contribute to the continuous underwater sound that has been identified as an environmental concern under the EU Marine Strategy Framework Directive. This study quantified, characterised and compared the continuous underwater sound emitted by steel jacket foundation and monopile Wind turbines during operation at low wind speed (0-12 m/s). The operational sound emitted by a monopile founded and a jacket founded wind farm in the BPNS showed a maximum increase of SPL of about 20 dB re 1 µPa. Spectral analysis showed that this increase occurs at frequencies below 3 kHz. Steel monopile foundations even when equipped with a less powerful generator, emitted significantly more underwater sound than jacket foundations. The addition of underwater sound is increasing with wind speed with a rate dependent of the type of foundation, with monopiles showing a stronger increase with wind speed than jacket foundations. Possible impacts on marine life like fish, marine mammals or invertebrates remain unclear mainly due to the lack of knowledge in disturbance or behavioural response levels for the species that could be found on these sites. Future challenges are to expand the study to higher wind speeds (study ongoing) and to quantify and qualify the additional sound pressure of a larger wind farm or a series of adjacent smaller wind farms (i.e. cumulative effects)
