Overview of existing Noise Mitigation Systems for reducing Pile-Driving Noise

Underwater noise caused by impulse pile-driving during the installation of offshore foundations is potentially harmful to marine life, especially marine mammals. In Germany the regulation authority (BSH) set following limit values: (i) Sound Exposure Level (SEL) = 160 dB (re 1 μ Pa 2 s) and (ii) Peak Level (LPeak) = 190 dB (re 1 μ Pa) which must be complied with at a distance of 750 m to the construction site. The experience over the last years shows that underwater sound during pile-driving mainly depends on pile diameter (currently up to 6.5 m) and the blow energy used. Measurements show values of up to 180 dB for the SEL and up to 205 dB for the LPeak. Therefore, Noise Mitigation Systems (NMS) are necessary to minimize the underwater noise significantly. Since 2011 NMSs have to be applied during noisy construction work at all OWFs in Germany. The itap measured the underwater noise and evaluated the sound reduction due to the NMS used during nine OWF construction phases and several substations as well as transformer platform installations. Additionally, itap was involved in many funded research projects dealing with the evaluation and enhancement of existing NMS like OFF BW II. Overall, more than 700 piles were installed in the German North and Baltic Sea by using different NMS in different system configurations over the last years. In this paper a general overview of existing and tested noise mitigation systems is given. The main results and general findings regarding the factors that influence noise reduction will be discussed. Finally, it will be discussed if a general State-of-the-Art exists regarding noise reduction and NMS from the acoustical point of view.