Is There a State-of-the-Art to Reduce Pile-Driving Noise?

Abstract

Underwater noise caused by pile-driving during the installation of offshore foundations is potentially harmful to marine life. In Germany, the regulation authority BSH set the following protection values: Sound Exposure Level SEL = 160 dB and Peak Level L_Peak = 190 dB for Harbor Porpoises that must be complied with at a distance of 750 m to the construction site in order to avoid temporal threshold shifts. The experience over the last years shows that underwater sound produced during pile-driving, depending on many parameters and measurements, shows values of up to 180 dB_SEL and up to 205 dB_{L Peak} in a distance of 750 m. Therefore, Noise Mitigation Systems (NMS) are required to significantly minimize the underwater sound. Since 2011, NMS must be applied during all noisy offshore construction work in Germany. The Institute of Technical and Applied Physics GmbH (itap) was involved in many offshore wind farm (OWF) projects with pile-driving activities (>1000 pile installations without and with different NMS). Based on these underwater noise measurements, the tested NMS were evaluated. In this paper, a general overview of existing and tested NMS including tested system variations is provided and the measured data and influencing factors on the resulting noise reduction are discussed. Additionally, combinations of two or more NMS are measured during the construction phase, if monopiles with diameters of up to 8 m are installed. It is demonstrated what level of effect one or more NMS have on the emitted noise. However, it is shown that it is possible to install monopiles with a diameter of >6 m with noise levels below 160 dB_SEL at a distance of 750 m, if combinations of suitable NMS are used. Nevertheless, any kind of noise mitigation will have a significant influence on the ‘disturbed’ or treated area from pile-driving noise for marine mammals. The question of whether a state-of-the-art measure to reduce pile-driving noise exists will be explored, based on measured data and experiences with NMS under real offshore conditions.