Design methodology for a metamaterial-based cushion for reducing underwater noise from impact-driven offshore monopiles

Abstract

The global energy sector is increasingly shifting towards sustainable solutions, driving the expansion of offshore wind farm installation to meet rising energy demands. The upcoming wind farms will be constituted of larger wind turbines, requiring larger monopile foundations. The monopiles are typically installed using impact hammers that blow several strikes until the monopile reaches a specific penetration depth. The installation process generates high underwater noise levels, posing severe risks to marine life, including hearing loss, behavioral changes, and even death.

Existing noise mitigation systems struggle to attenuate low-frequency noise below1000 Hz due to the mass law limitations of natural materials. This thesis proposes a design methodology for a novel metamaterial-based noise mitigation system–the meta-cushion–designed to brake this limitation. Placed between the monopile and the hammer, the meta-cushion filters out mechanical waves generated by the hammer’s blow that cause high underwater noise levels. The design methodology ensures adaptability to various monopile installation specifications, facilitating the implementation of the meta-cushion in real world pile driving cases.

The thesis begins with an analysis of underwater noise during pile driving and the limitations of existing mitigation solutions, emphasizing the need for novel approaches to reduce low-frequency noise. The analysis shows a strong relation between high underwater noise levels and the low-frequency vibrations of the monopile caused by the hammer’s blow. Then, the concept of metamaterials is introduced, highlighting how their unique properties contribute to reducing low-frequency vibrations. Based on the understanding of the underwater noise-vibration relationship, an initial meta-cushion design exhibiting such filtering features is defined.

The numerical modeling of the meta-cushion is then detailed, with finite element analyses being conducted to investigate the behavior of the meta-cushion when interacting with mechanical propagating waves. The attenuation capabilities are evaluated via analysis of dispersion-curve and transmission loss diagrams. To ensure both mechanical integrity and noise reduction under impact loads, a design optimization strategy is developed, from which genetic algorithm is used to investigate the trade-off behavior between mechanical and attenuation performances. Once verified, the meta-cushion’s functionality is experimentally validated through modal impact analysis and small-scale pile hammering tests, demonstrating effective noise reduction at frequencies below 1000 Hz as also indicated by the numerical results.

Finally, this thesis presents a systematic design methodology–encompassing meta-cushion design selection based on pile specifications, numerical verification, and experimental validation–that can be adapted to full-scale monopile installations. By integrating the meta-cushion into offshore pile driving operations, this work contributes to mitigating underwater noise and its environmental impact.