Modelling and development of a resonator-based noise mitigation system for offshore pile driving

Anthropogenic underwater noise generated by pile driving has been an issue of serious concern due to the rapid developments in offshore wind farms. The underwater noise pollution poses a threat to marine mammals. To reduce the noise, many offshore companies have developed various mitigation measures and alternatives to impact piling. One of them is the use of underwater acoustic resonators around the foundation pile. In this paper, a three-dimensional vibroacoustic model is developed in order to find the optimal configuration of the underwater acoustic resonator system and to improve the existing noise reduction potential. The model requires the proper description of the noise source, the resonator and the acoustic domain surrounding the pile. To describe the acoustic performance of the resonators for a more generic use, the frequency response function of an open-ended resonator is analytically derived based on the assumption that the resonator behaves as a linear Single-Degree-of-Freedom (SDoF) system. The derivation of the parameters of the equivalent SDoF system representing each individual resonator is based on appropriate fitting of numerical results obtained in COMSOL for a wide class of parameters. The Boundary Element Method (BEM) is then employed for determining the total pressure field in the acoustic domain in the process of pile driving accounting for the presence of multiple resonators. In this work, noise sources are represented by a distributed array of phased point sources which reproduce adequately the source of the noise field. In addition, a parametric study is presented in order to define the principal factors yielding effective noise mitigation and to obtain the optimal configuration on the predicted sound levels at the low-frequency range.