Offshore piling noise attenuation theory and new applications on monopiles and jacket foundations
Since some years a noise prevention concept for the protection of marine animals exists in Germany. Based on that, the acoustic underwater noise from the pile driving at offshore wind farms is required to be less than 160 dB (SEL) at a distance of 750 m. This value, however, is often exceeded so that the use of a soundproofing system is necessary. Piling noise is radiated from the pile surface directly into the surrounding water and deeper into the soil as travelling waves. These waves can interact with the water and induce additional underwater noise far away from the piling site. Underwater noise in the far field is usually reduced by bubble curtains with diameters of 150 250m. For the special near field conditions and radiation of underwater noise into the surrounding water a very effective noise mitigation method is developed. It uses small elastic elements of different sizes and materials in the surrounding water to produce reflections, scattering effects and material damping effects with resonant vibrations in the water. This leads to controllable effective underwater noise reductions in the near field of radiated piling noise. The effectiveness of the arranged elements were investigated in laboratory tests and in situ under offshore conditions while different offshore installations. In the meantime two offshore wind farms with monopile foundations were installed by realizing these near field damping elements in combination with a far field big bubble curtain. Also the noise mitigation was improved and was determined by an acoustic institute to up to 25 dB (SEL) at a distance of 750m. In current projects the system will be used also during pre piling for jacket foundations and for large the installation of large monopoles with today`s biggest offshore pile drivers. This article describes the theory, flexibility and the implementation of the effective noise mitigation method during previous and current projects. Furthermore a summary will be given about the results to show the effectiveness of the whole noise mitigation system.