Analysis of an Underwater Periodic Helmholtz-type Noise Mitigation System for Offshore Impact Pile Driving

Abstract

Several studies have highlighted the negative impact of underwater impact pile driving on
marine life, underscoring the need for effective noise mitigation systems (NMS). While bubble
curtains are commonly used, recent research also focuses on near-pile NMSs. For instance, with resonating devices such as Helmholtz resonators, which are known for their tunability and effectiveness in specific frequency ranges. However, the potential for broadband noise mitigation using arrays of Helmholtz resonators in offshore impact pile driving remains uncertain. This thesis aims to address this research gap.

Locally resonant acoustic metamaterials, characterised by unusual properties such as negative or zero-valued effective mass, have shown potential for broadband noise mitigation. This thesis defines an acoustic metamaterial composed of Helmholtz-type resonators and investigates its combined effects. The study employs the finite element method (FEM) and the lumpedcomponent method to define the resonator characteristics of a reference Helmholtz-type resonator. Subsequently, the boundary element method (BEM) is used to examine the acoustic behaviour of horizontal arrays of Helmholtz-type resonators, appropriately named Helmholtz-type acoustic metamaterials, in the frequency domain. Multiple configurations of these systems are analysed. Finally, a case study approximating the pressure field radiated by a vibrating monopile excited by an impact hammer is conducted in the time domain using the FEM.

The results indicate that Helmholtz-type acoustic metamaterials amplify sound pressure at
frequencies below the natural frequency of the individual resonators and reduce it at frequencies above. Functional grading, achieved by incrementally decreasing the natural frequencies of the individual resonators in steps of up to 3 Hz along each horizontal array, can reduce the low-frequency amplification while maintaining the high-frequency attenuation. In a vertical system of Helmholtz-type acoustic metamaterials similar behaviour is observed. A vertical system comprising of horizontal arrays of 20 resonators with a horizontal spacing of 0.1 meter and a vertical spacing of 1 meter between each array shows a promising balance between attenuation and unwanted amplification. However, the system’s performance is sensitive to maintaining optimal volume of encapsulated air within the resonators, as the target frequency can significantly increase if a large percentage of the encapsulated air is lost. The transient response behaviour aligns with frequency domain observations, showing low-frequency amplification and high-frequency mitigation. Additionally, it is found that the orientation of the resonators does not significantly affect the transient response. Finally, it is important to note that mechanical coupling effects, which are not included in this study, may introduce additional low-frequency interactions.

This study demonstrates the potential of using Helmholtz-type acoustic metamaterials in near-pile NMSs for offshore impact pile driving, emphasising the system’s sensitivity to the frequency of the applied force and the importance of air volume maintenance. The findings suggest that while Helmholtz-type resonators can effectively mitigate noise in specific frequency ranges, careful configuration is crucial for achieving broadband noise mitigation and for minimising the risk of unwanted amplification of the pressure field.