Locally Resonant Acoustic Metamaterials for Underwater Pile Driving Noise Mitigation
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During offshore wind farm installation, the pile driving process [1] can result in high sound pressure levels that compromise the safety of marine species [2]. To reduce the intensity of such noise, and to better protect marine species from any form of damage, an alternative solution to the ones commercially available so far, e.g. hydro sound dampers or bubble curtains, is proposed herein. It employs locally resonant acoustic metamaterials (LRAMs) applied directly on the pile. Metamaterials are artificially engineered structures properly designed to exhibit peculiar properties not available in nature. Here, in particular, the local resonance phenomenon is exploited, since it promotes the opening of bandgaps, i.e. frequency ranges where the propagation of elastic-acoustic waves is not allowed, in materials composed of unit cells with subwavelength dimensions. First, a numerical simulation on the pile is carried out, to obtain an insight on the most dangerous pile modes, which are responsible for sound emission in water. An optimization routine based on a genetic algorithm is then performed, to design the LRAM with the bandgap at the desired frequencies, i.e. pile modes identified, as mentioned above. Numerical simulations proved the noise mitigation effect at the real application scale. Finally, an experimental campaign is performed, to validate numerical simulations. Note that the numerical-experimental comparison is done at a smaller scale compared to the real one for the sake of simplicity. Nevertheless, results show a promising proof of concept that can pave the way to the application of the proposed solution at the real scale.
