A hybrid model for the 3D computation of pile driving noise

Piles are the state of the art when mounting offshore constructions to the seafloor, especially for the foundation of wind turbines. During the pile driving process, the pile emits pressure waves directly into the water and the seabed. These waves lead to high underwater noise levels that are potentially harmful to the marine environment. Therefore, several countries have defined limiting values for the noise levels, in order to protect the marine fauna. Noise mitigation systems, such as bubble curtains or mitigation screens, have been developed in order to fulfil these limits. There are several factors influencing the considered noise levels, e.g., the hammer type, the applied hammer energy, the pile geometry, the water depth, the soil type, and the layering of the sediments. All listed factors vary for different piling locations, especially the water depth and sediment characteristics. In order to take all these influences and their interaction into account, numerical models are necessary. An accurate prediction of pile driving noise is often mandatory to check, if noise mitigation systems are needed or to improve the set-up of existing noise reduction measures. Pile driving noise models have been developed by only a few institutions. Most of them are based on a 2D-model for the close vicinity of the pile solved with the finite element method (FEM) and a far-field model for long range predictions. In the current contribution a well validated 2D-FE-model in time domain for the computation of pile driving noise up to distances of 1 km is used. The farfield prediction of noise levels is done by employing the parabolic equations (PE) method, which is also able to take 3D-effects into account. The FE-model for the near-field is split into a precalculation, which computes the interaction of hammer and pile, and the main acoustical model. Within the pre-calculation, all parts of the hammer and the pile are discretized in detail and an excitation signal (e.g. force or velocity) at the pile head is derived. This excitation is used as a boundary condition at the pile head within the main acoustical model, which incorporates pile, water column, and soil. Furthermore, in the model it is possible to consider noise mitigation systems. For the far-field PE computation the split-step Pade technique is used. The necessarý starting field is derived from the results of the FE-model. Within the PE-model, the characteristic acoustical properties of the seabed are incorporated. The PE-model is able to compute 3Deffects, which occur, e.g., due to a varying water depth caused by canyons, shores, or sand dunes. Within this contribution a validation of the hybrid FEM/PE-model for a case with a flat bathymetry is shown. Moreover, the validated case is used to show possible 3D-effects induced by a varying bathymetry. Finally, the influence of a heavily changing bathymetry of a real-life future pile driving scenario is shown.