Underwater Noise

Abstract

Wind turbines are growing in size and therefore their foundations, become larger as well. Additionally, they are placed in deeper waters. This results in the industry being at the limit of underwater noise levels generated by impact piling during the installation phase of monopiles. The most common installation method for monopiles is impact piling. This installation method comes with high impulse noise emissions which can be harmful for the aquatic environment. Larger piles require more energetic hammer impacts which, in turn, generate more noise. Given the size of the monopiles installed nowadays, noise limits imposed by governmental organisations are exceeded in all cases. The noise due to impact piling can be reduced by applying noise mitigation measures. Several systems have been developed, the most common of which is the Big Bubble Curtain (BBC). Although the working principle of the BBC has been proven in practice, the most effective deployment configuration, i.e. distance from the pile, air-flow volume and pressure, etc., has not been thoroughly investigated. In current projects, the BBC is typically placed at twice the water depth. This study aims to identify the parameters that determine the optimum position of the BBC to achieve maximum noise reduction. First, free-field predictions (without the BBC) using the TU Delft software SILENCE have been carried out for model validation purposes. The accuracy of the noise predictions were found to be within 2 dB (re 1µ Pa) both for the SEL and the Lp,pk. Second, the BBC was implemented by assuming a depth and frequency-dependent transmission loss (TL) factor at the position of the BBC. Noise predictions including the modelling of the BBC were validated against measured data. The key findings in this research are that the location of the bubble curtain is determined by the energy leakage from the soil into the water column. Depending on the damping characteristics of the BBC, this leakage is significant up to a point where the energy does not leak back to the water column anymore. In the examined case, and for blocking the waterborne path, this optimal position is found to be around 70m. The figure below shows the trend of the energy leakage in a schematic way. It shows that if the bubble curtain is placed too close to the pile, noise leaks back into the water column behind the bubble curtain. Thus, depending on the specific geometrical configuration, water depth and soil conditions, it is argued that an optimum position can always be found using the analysis presented in this work.