Impact pile driving is the predominant foundation technique for offshore wind turbines. The pressure waves generated during pile driving produce high noise levels that can harm marine life. To protect marine mammals, regulations have been established to limit underwater noise. Recent monopile installations already surpass these limits, necessitating noise mitigation measures to proceed with piling operations. As monopiles and hammers grow larger, noise levels are expected to increase further, escalating the need for effective noise mitigation strategies.

The ongoing evolution of offshore technologies makes it essential to understand the behavior and effectiveness of noise mitigation measures and to develop accurate models for predicting their performance. Currently, limited predictive models exist for most noise mitigation techniques, and it remains unclear how their efficiency will be affected by future piling conditions. As monopiles and hammers increase in size, the noise they produce shifts toward lower frequencies, challenging the effectiveness of current mitigation measures.

This study addresses these knowledge gaps by investigating the performance of existing noise mitigation measures under future piling conditions. A numerical model is developed using the Finite Element Method in COMSOL for near-field analysis, coupled with the Boundary Element Method in SILENCE for far-field analysis. Four categories of noise mitigation measures are examined: the bubble-curtain, a resonator-type mitigation screen, an isolation casing, and pulse prolongation. A critical piling scenario -designed to maximize noise propagation- serves as the baseline for analyzing the individual and combined effectiveness of these measures. Additionally, the scenario is re-simulated with softer soil conditions to assess how environmental factors influence mitigation efficiency.

Key findings reveal that most noise mitigation measures are more effective at attenuating higher frequencies. However, as monopiles become larger and stiffer, natural frequencies decrease, while greater hammer strikes and deeper waters enhance the propagation of lower frequencies. These changes result in a general decline in mitigation efficiency for future piling scenarios. Furthermore, the efficiency of each measure is highly dependent on the frequency content and environmental conditions, such as soil stiffness and water depth. For instance, softer soil conditions significantly alter the effectiveness of certain mitigation measures, with some showing improved performance and others becoming less efficient compared to stiffer soil conditions, and the same goes for combinations of measures.

This study highlights the importance of frequency-specific analyses when applying noise mitigation measures. It underscores the need for tailored approaches to ensure effective noise reduction under varying environmental and piling conditions, offering critical insights into the future of noise mitigation in offshore wind development.

Nations worldwide are striving to meet the climate goals set forth by the Paris Agreement and reduce greenhouse gas emissions, with offshore wind farms (OWFs) playing a crucial role. However, OWF construction generates underwater noise from impact piling, which is harmful to marine life. This study examines the effects of PULSE – a novel mitigation measure acting as a spring-damper system between the hydraulic hammer and monopile – on underwater noise and its interaction with air-bubble curtains. Using the semi-analytical SILENCE model, the complete pile-water-soil system is simulated to evaluate noise levels, with the model’s predictions compared to field data from a recent German project. Results show that PULSE effectively reduces both Sound Exposure Level (SEL) and Peak Sound Level (Lpeak) by shifting acoustic energy from higher to lower frequencies. When combined with air-bubble curtain configurations, noise reductions are greater, but the effectiveness of PULSE diminishes. Consequently, the individual performance of PULSE and air-bubble curtains cannot be linearly added to predict their combined effectiveness. In conclusion, while PULSE effectively mitigates underwater noise, its combined use with air-bubble curtains impacts overall mitigation efficiency and requires careful evaluation.

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.

The offshore wind industry is rapidly advancing, expected to play a crucial role in renewable energy production, with a growing emphasis on addressing associated environmental concerns, particularly the impact of underwater noise generated during the installation of wind turbines. In response, bubble curtain technology (BCT) has emerged as a potential solution. While the BCT operational concept has been demonstrated in practice, a detailed investigation related to fluid dynamics, geometric characteristics, and operational parameters has yet to be conducted. It's important to note that this need for investigation is specifically applicable to large BCT systems. This research aims to enhance the understanding of unsteady compressible flow inside BCT hoses and examine the impact of operational parameters on the pneumatic aspects of the bubble curtain system along the hose length.

This thesis systematically defined its fundamental objectives, beginning with the development of a 1D unsteady model for compressible flow in pipeline systems. This model, which served as the basis for subsequent studies, was verified against the existing literature, revealing a 2-4% difference in flow rates and pressure responses. Subsequently, the scope was broadened to include nozzle configurations, which were gradually included in the pipeline model. The model evolved through configurations with one, three, and five nozzles, where larger diameters increased dampening on transient pressure fluctuations. As the research progressed, the final model, incorporating five nozzles, was used as the basis for the scaled Bubble Curtain Technology (BCT) model.

A sensitivity analysis for this study was carried out utilizing parameters from existing research. The sensitivity study specifically emphasized the influence of geometric (hose and nozzle diameters, hose length, nozzle spacing) and operational factors (discharge coefficient, water depth, air flow rate) on the flow dynamic of Bubble Curtain Technology (BCT). The main findings from this research included reducing backflow with smaller diameters and lowering reverse flow with greater discharge coefficients and airflow rates. Changing the hose length and nozzle spacing proved effective for adjusting the required flow rates. The investigation also found that nozzle diameter and discharge coefficient had a considerable impact on nozzle flow rates, with a 2-3% increase over reference values at the maximum value range. Other geometric and operational parameters in the tested ranges had a relatively lower influence on the nozzle flow rates or generated pressure variations.

The scaled BCT unsteady compressible flow dynamics model presented in this thesis is still in its early stages of development, but it can serve as a basis for the development of full-scale pneumatic models that can enhance BCT and lessen the environmental impact of offshore wind farm operations.

Monopiles are by far the most chosen foundation option for most off-shore activities. The installation of the foundation is commonly done by impact pile driving. The biggest issue of the traditional pile driving method is the sound emission, which reaches levels that are harmful to the immediate environment. Partly because of this, an alternative pile driving method is being developed, namely the Gentle Driving of Piles [1], where the hydraulic hammer is replaced by a GDP-shaker, which drives the pile toe into the ground by means of a torsional force. This is an additional incentive for developing a predictive model for the underwater noise radiation due to a non-symmetric force field. The model proposed in this thesis is similar to the existing symmetric semi-analytical model SILENCE [2], where the model consists of two submodules, namely a pile module and a module that describes the acousto-elastic medium. In both submodules the displacement field of the subsystem is described using its modal field. The procedure for both systems are derived and explained in further detail in chapter 3. To combine the submodules two different mode-matching methods, that account for the azimuthal direction, are developed in chapter 4. One of these methods is the so-called Orthogonality method, which is, similar to [3], based on the orthogonality relations of the acousto-elastic medium. The other is referred to as the Point-Collocation method, which is loosely based on the reciprocity theorem. Both methods have proven to be sufficiently stable to achieve good convergence at least for lower frequencies. The model is tested a small sized pile case study. The results are assessed in chapter 5 through the frequency spectra of the acousto-elastic medium as well as the metrics. The time evolution of the response is also looked at, even though these are considered less reliable, due to the limited frequency range.

In the last decades, renewable energies gained an increasing interest due to the environmental awareness of people, especially in the developed countries. Moreover sustainable resources represent a long-term investment full of possibilities of use.
In the Netherlands a technology which is getting more and more popular to produce green energy is represented by offshore wind turbines (OWT). However, during the installation of the structural elements for these systems, the risk of noise pollution and animal harm is an issue that has to be considered.
Previous works developed models to predict the noise emission and propagation during the construction phase, however the uncertainty related to the environmental properties has not been yet fully investigated.
Since the model characteristics are uncertain, so will be the prediction of the noise.
This thesis aims to fill this gap, investigating in the underwater soil property uncertainties and the resulting variation in sound predictions. The main goal of this work is to settle a sounding methodology to model the soil characteristics and interpret the sound levels.
In the first part of this work, the soil uncertainties are treated.
A framework on how to use measurements from cone penetration tests (CPT) and obtain mechanical and dynamic soil features is presented.
By means of statistical approaches, the procedure to define the optimal depth for different homogeneous layers (given the software used for the noise prediction) is described. Another topic dealt with is the definition of proper characteristic distributions and the choice of the optimal one representing the available empirical measurements.
Finally a procedure to generate random samples for the analyses that will follow is shown.
An important feature presented is the use of the correlation between the properties to define copulas. The samples then are not completely random and independent, but instead combinations that are more likely to appear are obtained.
In the second part, the results of several analyses are presented.
The steps to treat the sound levels and obtain the probability density (and cumulative) distributions are discussed. These results will help in estimating the probability of exceeding a particular defined sound level.
With this information, additional measures and precautions, as noise barriers, may be adopted in the installation of the pile to prevent exceeding the threshold.
The correlation between soil properties and obtained sound levels is investigated, in order to highlight if there are soil properties that greatly affect the outcomes.
The insight obtained may help in determining which particular features need a careful estimation, both by more accurate measurements or new techniques. Another benefit related to the detection of parameters not affecting significantly the sound levels, is the reduction of simulations necessary to cover enough combinations. That is because if a property
can be neglected in the generation of samples and be taken as a fixed value, less combinations are needed to be considered.
Finally, a comparison between the obtained estimations and empirical measurements in the North Sea is made, to test the validity of the framework proposed.