With a worldwide growing consensus on fossil fuel combustion have a negative impact on climate, society, and biodiversity, the urge for renewable energies is growing rapidly. One the most widely used renewable energy source is wind energy and for this purpose both onshore and offshore wind farms (OWFs) are being developed. In recent years the development of OWFs is getting more attractive due to fewer constraints on turbine size, more stable wind resources, and generally less environmental impact.

Offshore wind turbine generators (OWTGs) have various possible foundation concepts for shallow waters of which the monopile (MP) is the most widely used foundation concept. The dominant installation method for MPs is percussive pile driving where energy is transferred from a hydraulic impact hammer to the MP driving the latter into the seabed. Due to this energy transfer, a structural response is induced in the MP which in turn inflicts an acoustic response in the water column and surrounding soil.

The underwater noise emissions generated during offshore pile driving have a negative effect on the marine fauna. This negative effect can be subdivided in three categories, that is instant death or injury from a single noise pulse, auditory damage due to accumulative noise, and behavioural disturbance. For this reason, governmental bodies have imposed rules and legislation concerning underwater noise thresholds during offshore pile driving. To comply with these noise limits, noise mitigation systems are often deployed during offshore construction works of which the big bubble curtain (BBC) is the most frequently applied. A BBC is formed by air being injected into the water through nozzles in air supply hoses laying enclosing the entire MP.

Currently, the BBC configuration is often based on rules of thumb and experience from similar past projects. However, a BBC configuration can be optimized beforehand, depending local technical-constructive and site-specific parameters. This thesis focuses on the link between the influence of different soil configurations on the characteristics of the underwater noise emissions and the intrinsic mitigation performance of a BBC and what this means for the design of a BBC configuration. One objective of this thesis is to provide a framework for optimizing a BBC configuration which consists of a quantitative and qualitative analysis.

For both analyses, use is made of the semi-analytical model SILENCE BUBBLES, which captures the acoustic interaction between MP, fluid, soil, and air bubble curtain. This air bubble curtain model makes use of a one-dimensional coupling approach to capture the interaction between the bubbly medium and the travelling sound wave. Another objective of this thesis is to investigate an alternative approach for modelling waves travelling through the bubbly medium taking into account its two-dimensional properties, thus include the angle dependency of the incident sound wave.

Therefore, the focus of this thesis is twofold with the overarching theme; "Noise mitigation by a BBC". On the one hand, a framework for optimizing a BBC configuration is proposed based on three different soil configurations. This framework consists of a quantitative and qualitative analysis. The qualitative analysis focuses on the effect of different soil configurations on the mitigation effectiveness of a BBC. The quantitative analysis provides an optimal combination of BBC configuration parameters for complying with a set noise limit. On the other hand, an alternative two-dimensional (2D) coupling approach is examined for coupling an air bubble curtain model to a non-mitigated field.

Optimizing mitigation by a BBC
A sensitivity analysis was performed to identify the model sensitivity to different BBC parameters. Here, it was shown that the model is sensitive to the nozzle diameter and the gas velocity of the nozzle. Furthermore, the radial distance of the BBC is of significant importance for the SEL at r = 750 m.
SILENCE BUBBLES can be used for a quantitative robust approach for determining an optimal configuration of a BBC for different soil configurations. Each soil configuration has its own distinct optimization process.
Together with a qualitative analysis regarding energy distribution over the radial distance and the total energy being irradiated into the fluid domain, the framework presented can lead to a better estimation of whether noise limits will be met for future noise prognoses.
An alternative coupling approach for integrating an air bubble curtain model
This thesis presents a mode-coupling approach for coupling a similar air bubble curtain model as the one used in SILENCE BUBBLES to a non-mitigated field. The results showed that for higher frequencies (f > 300 Hz), the 1D approach is conservative compared to the 2D approach. For lower frequencies, the 2D approach is more conservative where the 1D approach shows significantly higher transmission loss for certain frequencies below 300 Hz. It could not be identified where these large differences come from.

In the underwater environment, it is all but silent. In most parts of the oceans, sunlight is barely available and thus marine animals have evolved to rely on sound for navigation, foraging and communication. Marine animals are not the only sources of underwater sound. Other natural sources such as earthquakes, waves and rain cause ambient noise, but also loud impulses. Furthermore, human activities consisting of dredging, surveying, construction and shipping cause loud noise in a wide frequency range. Underwater radiated noise (URN) from shipping has severe negative consequences on marine animals. The negative effects include auditory masking, stress, and behavioural and acoustic responses, possibly leading to collision with ships. It is evident that the mitigation of the underwater radiated noise of a ship is advantageous and worth researching.

In the past, plenty of research has been done into the modelling of the acoustic characteristics of vessels. A research gap was identified in the modelling of mitigation methods of the underwater radiated noise from ships with a focus on marine mammals. Up until now, most research has focused on the attenuation of vibrations on board a ship or radiated noise due to the propellers. At all times, the focus was either on human comfort or the radiated noise in general, however, for marine mammals certain frequency bands are of greater importance. It is valuable to assess the URN of a ship in the design phase, such that adjustments can be made to decrease the URN without excessive costs. The research goal of this thesis is:
Predict and mitigate the structure-borne underwater radiated noise of a ship in the design phase caused by onboard machinery.

The research goal and corresponding research questions are answered by first setting up the framework for the models. This framework sets the frequency analysis range to 20 - 200 Hz, formulates a reference ship case for validation of the data and gives the inputs and boundary conditions for the models. The acoustic metric of interest is set to the source level (SL) in dB re 1 μPa2m2.

Secondly, a simplified model is setup in Ansys 2021R2. This model is a 3D solid element model shaped like a beam. The equivalent beam (EB) model has similar global properties as the reference ship case. Around the EB model an acoustic domain is located that is modelled to represent an infinite domain. No physical boundary effects are included as the source level is per definition not dependent on this.
Subsequently, mitigation methods for machinery URN are researched and the resilient mount was found to be the most promising. A resilient mount is applied to the EB model in Ansys 2021R2 and source level spectra for different mount parameters are investigated.
Lastly, a two degree of freedom (2-DOF) schematisation is made that incorporates the Ansys model using a dynamic stiffness. The 2-DOF schematisation allows for faster computations of the complete model and thus a more extensive parameter study of the resilient mount is possible.

Over the frequency analysis range of 20 - 200 Hz, the results show that machinery structure-borne URN can be reduced by 45 - 65 dB re 1 μPa2m2. The reduction oscillates over the frequencies at lower frequencies. A linear SL reduction was observed from 60 Hz and above, which gradually lessened for higher frequencies. The SL reduction increased from 45 dB re 1 μPa2m2 to 60 dB re 1 μPa2m2 when the resilient mount damping ratio was changed from 0.18 to 0.02. In addition, the normalised resilient mount parameter study showed the system's parameter sensitivities and responses. It became clear that the resilient mount does not respond to a change in resilient mount damping as expected. The accuracy of the absolute results is subject to assumptions and limitations, which introduce uncertainties.

The absolute decrease in URN with the resilient mount was computed using acceleration input rather than force input. The acceleration input was found to have overestimated the 'no-mount' case, which was used to compare cases with the resilient mount. The total URN reduction with the resilient mount could thus have been overestimated. Furthermore, an effect of the model boundaries and the model domain size on the results was present in the models. The magnitudes of the results were influenced by this effect, which could not be eliminated due to the computational limitations reached. Finally, there was a scarcity of model input data and reference data. The magnitudes of the results were obtained and compared to limited data in order to determine the accuracy of the results.
Taking these limitations into consideration, the findings of this study should be interpreted with caution. The findings support literature claims that a resilient mount can reduce structure-borne machinery URN by 20 - 40 dB re 1 μPa2m2, with more reduction at higher frequencies.

The effect of the application of the resilient mount on marine mammals was hard to quantify. The structure-borne machinery URN is a part of the total URN of a ship. Due to the logarithmic relation of the noise, the reduction of one part could have very limited effects. Furthermore, the total soundscape in the ocean is formed by the combined noise of many ships. Moreover, the relation between the perceived nuisance of marine mammals and the URN levels is hard to indicate. The effect is undoubtedly positive but could be negligible in the bigger picture. At low speeds and close distances, the machinery URN is governing and the influence of URN from other ships is reduced. In those cases, the reduction of structure-borne machinery URN with resilient mounts could be expected to be the most positive for marine mammals.