Mechanical wave filtering through meta-cushions

Abstract

The increasing installation of large offshore monopiles for wind turbine foundations generates harmful underwater noise during pile driving, particularly at low frequencies. Existing noise mitigation systems target noise around 1000 Hz, however, their performance reduces for lower frequency noise expected for the upcoming monopiles. To address this, a metamaterial-based cushion, or meta-cushion, was proposed to filter waves generated by impact loads. Although the meta-cushion principle has been numerically verified, an experimental validation has not been performed yet. Moreover, the meta-cushion was designed for a single frequency range, requiring the development of a meta-cushion with multiple attenuation zones to address various resonant frequencies that could induce noise. In this paper, a graded meta-cushion prototype is manufactured and experimentally tested to validate its vibration attenuation capabilities. The numerical and experimental modal analyses show that the meta-cushion effectively attenuates vibration across single or multiple frequency ranges, particularly when viscoelastic properties are incorporated, from which the results demonstrate the ability of the meta-cushion in attenuating vibration from single and multiple frequency ranges related to small-sized monopiles (around 1000 Hz) and large-sized monopiles (lower than 1000 Hz). This study advances the meta-cushion towards practical application in mitigating undesirable vibrations during full-scale monopile installations.