Modelling and development of a resonator-based noise mitigation system

Abstract

Anthropogenic underwater noise generated by pile driving has been an issue of serious concern
for a long period of time. The underwater noise pollution from pile driving could pose a threat
to marine mammals. To reduce the low-frequency noise, many offshore companies develop
various treatments and alternatives for pile driving. In this study, the focus is placed on a
resonator-based noise mitigation technique.
The first part of the thesis focuses on the investigation of the existing resonator-based noise
mitigation systems. A mathematical expression for the resonance frequency of an individual
open-ended resonator is derived. To validate this expression, a finite element model is built in
COMSOL. To compare the acoustic performance with the HSD, a finite element model is also
built for the HSD mitigation system. To describe the acoustic performance of the resonators
for generic use, the frequency response function of an open-ended resonator is analytically
derived based on the assumption that the resonator behaves as a linear SDoF system. The
derivation of the parameters of the equivalent SDoF system representing each individual
resonator is based on appropriate fitting of numerical results obtained in COMSOL.
The second part of the thesis deals with the development of a new design of a resonator system
named Qiu. To install the resonator system in a more flexible way, the air is encapsulated in
the resonator. A finite element model is also developed in COMSOL for the Qiu resonator.
In the last part of the thesis, a three-dimensional vibroacoustic model is developed in order
to find the optimal properties of the underwater resonator and to improve the existing noise
mitigation techniques. The model requires the proper description of the noise source, the
resonator and the acoustic waveguide. The normal mode method is used to compose the
Green’s function of the waveguide. The boundary element method is then employed in order
to obtain the total pressure field. The frequency response functions derived in the first part
of the thesis are subsequently used to describe the acoustic behaviour of the resonators. A
parametric study is presented in order to define the principal factors for effective noise mitigation.
In addition, the several cases are investigated in order to obtain the optimal properties
of the resonator and the optimum configuration of the array of resonators surrounding the
sound source to maximise noise reduction.