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.

Wind turbines are growing in size and therefore their foundations, become larger as well. Additionally, they are placed in deeper waters. This results in the industry being at the limit of underwater noise levels generated by impact piling during the installation phase of monopiles. The most common installation method for monopiles is impact piling. This installation method comes with high impulse noise emissions which can be harmful for the aquatic environment. Larger piles require more energetic hammer impacts which, in turn, generate more noise. Given the size of the monopiles installed nowadays, noise limits imposed by governmental organisations are exceeded in all cases. The noise due to impact piling can be reduced by applying noise mitigation measures. Several systems have been developed, the most common of which is the Big Bubble Curtain (BBC). Although the working principle of the BBC has been proven in practice, the most effective deployment configuration, i.e. distance from the pile, air-flow volume and pressure, etc., has not been thoroughly investigated. In current projects, the BBC is typically placed at twice the water depth. This study aims to identify the parameters that determine the optimum position of the BBC to achieve maximum noise reduction. First, free-field predictions (without the BBC) using the TU Delft software SILENCE have been carried out for model validation purposes. The accuracy of the noise predictions were found to be within 2 dB (re 1µ Pa) both for the SEL and the Lp,pk. Second, the BBC was implemented by assuming a depth and frequency-dependent transmission loss (TL) factor at the position of the BBC. Noise predictions including the modelling of the BBC were validated against measured data. The key findings in this research are that the location of the bubble curtain is determined by the energy leakage from the soil into the water column. Depending on the damping characteristics of the BBC, this leakage is significant up to a point where the energy does not leak back to the water column anymore. In the examined case, and for blocking the waterborne path, this optimal position is found to be around 70m. The figure below shows the trend of the energy leakage in a schematic way. It shows that if the bubble curtain is placed too close to the pile, noise leaks back into the water column behind the bubble curtain. Thus, depending on the specific geometrical configuration, water depth and soil conditions, it is argued that an optimum position can always be found using the analysis presented in this work.