A Numerical Study on the Underwater Noise Mitigation in Wind Turbine Pilling

An important engineering challenge of today is developing and harvesting alternative sources of energy and thus offshore wind farms have a great potential as an emerging technology. Unfortunately, the most common technique of wind turbine’s foundation establishment, pile driving, seems to influence marine mammals negatively. The noise generated from driving a foundation pile into the seabed affects the aquatic life several kilometers away, since lower frequencies with larger wavelengths (100-1kHz) do not dissipate in the sea water easily. To mitigate the ecological footprint, several techniques have been introduced, main of which is the placement of an air bubble curtain (ABC) around the pile to increase the compressibility, thereby reducing the velocity and increasing attenuation of acoustic waves. During the last decade, several numerical approaches for the prediction of offshore pile driving noise have been developed and applied for large-scale simulations. The majority of these methods utilize the Finite Element Method (FEM) for near field calculations and eigenfunction expansions for far field predictions. The purpose of this paper is to solve the challenging problem of pile driving, with and without ABC, by employing the Boundary Element Method (FEM). The problem is treated in frequency domain for specific frequencies between 100 Hz and 1 kHz and for noise predictions, with and without ABC, in a distance up to 1.1km from the pile. Also, a practical Iterative Effective Medium Approximation (IEMA) method is proposed for the evaluation of the frequency dependent phase velocity and attenuation that experiences a plane wave propagating in bubbly water.