The rapid advancement of wind turbine technology has enabled the deployment of increasingly powerful wind turbine generators (WTGs). This study focuses on a Siemens Gamesa Renewable Energy (SGRE) WTG with a rotor diameter of 236 m and a hub height of 154 m, installed in a wind farm in the seismically active Taiwan Strait. Seismic ground motions can impose significant loads on WTGs, particularly during the idling state, where the absence of aerodynamic damping can lead to amplified structural responses and potentially challenge project feasibility.

Current deterministic seismic load calculation methods do not account for the probability of such combined scenarios and lack a clear connection with the safety philosophy, where seismic design is associated with a target annual probability of failure that balances the consequences of failure against the cost of the necessary safety measures. Without accounting for these considerations in the simulation, the resulting loads may become either overly conservative or insufficient. This study applies a probabilistic approach to seismic load calculation to address these limitations and provide a clearer understanding of the results

The probabilistic approach models dependencies among four key environmental variables: wind speed, wind direction, wind shear exponent, and ocean wave direction, using a Non-Parametric Bayesian Network (NPBN). In the NPBN, dependencies are captured via a rank correlation matrix describing both conditional and unconditional monotonic relationships. Given wind speed (defining turbine operating conditions), conditional sampling of the remaining variables is performed through Monte Carlo simulation (MCS), preserving these dependencies. Independent variables such as azimuth angle, seismic wave direction, and seismic seeds are also sampled using MCS.

Seismic loads are computed for four seismic return periods (100, 475, 1300, and 4500 years) using SGRE’s aeroelastic simulation tool, BHawC, under both deterministic and probabilistic approaches. The annual probability of failure associated with deterministic seismic load limits is evaluated and compared with optimal load limits derived through iterative optimization to meet the target recommended by the IEC standard and supported by literature. Results show that deterministic approach results in a seismic load limit on the blade that can be up to 26 % higher than what is required, whereas for the tower component, it can be approximately 8 % lower than the optimal level.

The probabilistic framework not only improves the computational efficiency of seismic load simulations by requiring significantly fewer simulation groups but also produces more realistic, site-specific load distributions. Moreover, it enables the seismic load limits to be defined based on a target probability of failure, allowing the wind turbine designs to optimally balance the consequences of failure against economic benefits, such as the material costs of safety measures. This approach is particularly valuable to SGRE, contractor, and client, as even a small percentage reduction in load limit can lead to substantial cost savings for the entire project.

The study explores different system identification techniques that utilize machine learning, including the Restoring Force Surface (RFS) method and Sparse Identification of Nonlinear Dynamics (PySINDy).
These methods have potential to help uncover physical models for soil-pile interaction.
To test these methods , the research first applies them to well-known benchmark systems—simple mechanical models with known nonlinear behaviours. This ensures that the identification techniques work correctly before applying them to real pile-driving experiments.
The experimental data comes from lab-scale vibratory pile-driving tests using strain gauges and accelerometers. The study analyses how forces acting on the pile change over time, focusing on both the tip and shaft resistance. Various mathematical models are tested to see which best captures the nonlinear behaviour.

Gas extraction in the north-eastern Groningen province of the Netherlands has resulted in an increase in induced earthquakes, causing significant damage to homes and affecting residents’ mental health. While reinforcement measures and compensations are being implemented, progress has been slow. The structural analysis for reinforcing these buildings follows various methodologies in accordance with Dutch standard NPR 9998, but these methods are time-consuming and repetitive. This study investigates the use of machine learning techniques to optimize and accelerate the structural analysis of unreinforced masonry (URM) residential buildings. The traditional seismic analysis of URM buildings is computationally expensive, requiring extensive time for model development and assessment. To address this, a classification model and a surrogate model were developed using Deep Neural Network. The data required for both models are obtained from the existing database of Royal HaskoningDHV. Both models provided results indicating whether the building requires reinforcement measures or not. The classification model employs binary classification, using specific building features to categorize structures into those requiring reinforcement
and those that do not. The model generates rapid, implementable outcomes, enabling efficient categorization of buildings. This swift assessment allows for prompt decision-making, particularly in identifying and prioritizing high-risk structures that need more detailed analysis. Surrogate models are efficient approximations of complex analysis, capable of predicting reliable structural responses. Here surrogate for SLaMA analysis is built. The SLaMA surrogate will serve as a proof of concept for developing surrogates of more complex and computationally demanding structural analyses. The results show that, for the classification model, despite sufficient data points, the correlation between input features and the target variable remains weak, primarily due to changes in NPR regulations over the years. This did not allow for a consistent assessment procedure; hence, the same building might be compliant with a recent NPR and not with the older NPRs. For the surrogate model, two different input combinations were experimented with, the analysis provides insights into the impact of input parameters, the need for dimensionality reduction, and the number of data points required for computationally expensive models. The input combinations helped to analyze the curse of dimensionality. The results showed that as more parameters were added, the model required significantly more data to maintain performance. Additionally, the relevance of the selected
reduced input features was observed, to see how well it captures the key aspects of the original analysis with less computational cost. It was observed that the selected features need to remain closely aligned with the original analysis to produce comparable results. In conclusion, while machine learning approaches offer significant potential for improving the efficiency of seismic analysis, further refinement and validation of the models are necessary to address their current limitations and enhance their applicability in real-world scenarios.

To meet the ambitious targets outlined in the Paris Climate Agreement of limiting global temperature rise to 1.5 degrees Celsius, global renewable power generation must triple by 2030, with offshore wind energy expected to increase exponentially to 500 GW, a fourteen-fold rise from 2020. Despite a 48% cost reduction from 2010 to 2020, offshore wind energy remains more costly per megawatt-hour than fossil fuels, highlighting the need for cost-effective innovations in offshore wind farm foundations in the North Sea. Offshore wind farms are increasingly moving into deeper waters and utilizing larger turbines, which arise challenges for traditional monopile foundations. The Hybrid Monopile (HMP) is a promising al- ternative. It integrates features from traditional monopiles and jackets, potentially enhancing structural stability, reducing environmental impact, and optimizing costs in deeper waters. This thesis evaluates the technical and economic feasibility of the HMP compared to traditional foundation types such as the traditional monopile and jacket structure, aiming to accelerate development of offshore wind energy and contribute to global climate objectives. This study uses a comprehensive methodology to assess the HMP’s structural integrity, installation feasibility, and cost-effectiveness relative to traditional monopiles and jackets. The structural analysis begins with a preliminary design phase to establish input parameters for the finite elements analysis in Abaqus. This analysis evaluates natural frequencies and stress levels under varied conditions such as water depth, soil types, and turbine sizes, assessing the HMP’s feasibility across different conditions. The installation procedures are investigated, evaluating various strategies for HMP’s and compare it with the installation strategies of traditional monopiles and jackets. The most efficient way of installation is determined for the installation of the Hybrid Monopile while considering various options. This is compared to industry standards for the installation of the traditional monopile and jacket structure. The economic evaluation involves cost modeling. This analysis provides a cost comparison based on the manufacturing and installation of the structures, highlighting the economic advantages of the HMP in deeper waters and with larger turbine. Structural assessments demonstrate that the HMP is capable of deployment in water depths up to 80 meters and supporting offshore wind turbines of up to 22 megawatts in the North Sea. Design simulations indicate the HMP’s resilience against operational stresses caused by various environmental forces. The installation strategies emphasize efficient methodologies for HMP installation. Due to the fact the hybrid monopile requires an increased number of pin piles compared to jackets, the installation cycle of the hybrid monopile is less superior to the jacket, where the traditional monopile outperforms both of them based on installation time. Economic analyses highlight that while traditional monopiles are cost-effective in waters until 40 meter, the HMP emerges as a competitive solution for deeper waters and larger turbine configurations compared to jacket structures. In conclusion, the HMP represents an advancement over traditional jackets assuming a 20% reduction in interface piece manufacturing costs. Further optimizations in the design of the Hybrid Monopile and installation procedures could enhance its competitiveness in relation with a jacket. The traditional monopile is a more cost-effective solution in environments where possible, however the HMP offers a promising solution where noise and environmental limitation arises

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.

This thesis presents the development and validation of a Semi-Analytical Finite Element (SAFE) model for conical shells. The motivation behind this research is to provide a computationally efficient and accurate numerical model for analysing conical shells. Starting with general equations of motion, kinematic equations, constitutive equations, and boundary conditions, the SAFE method is used to construct a numerical framework for conical shells.

Validation of the SAFE method is performed through comparative analyses with a detailed COMSOL model. The comparison focuses on the natural frequencies, mode shapes, and responses to both uniform and non-uniform harmonic loading. The results demonstrate that the SAFE method achieves accurate predictions in all analyses.

To demonstrate the robustness of the SAFE model, the analysis is extended to include a coupled conical-cylindrical shell system. Similar analyses are performed, and the model continues to provide accurate predictions.

These findings highlight the capability of the SAFE method in delivering both computationally efficient and accurate solutions for the analysis of conical shells.

It is well-established that the effect of the soil significantly influences the overall behaviour of structures, particularly in the presence of vibrations. The majority of the developed approaches can be categorised into two groups: the direct approach and the substructure method. The first one is based on detailed modelling of the soil, foundation and structure using FEM and is not commonly used because it is computationally expensive and thus not feasible for standard engineering practice. The second method uses the dynamic stiffness matrix of the substructure to include it in the analysis of the general structure. Due to the impracticality of the first method, various methodologies have been investigated to better address SSI through the dynamic stiffness matrix, also known as impedance functions. They provide an approximate representation of the SSI that facilitates quicker analysis and sufficiently accurate results.

These approximation techniques typically involve intricate systems composed of lumped masses, dashpots, and springs. The characteristics of these elements vary based on soil and foundation types. For instance, a circular plate laying on homogeneous soil can be modelled as a mass, spring, dashpot single-degree-of-freedom system, where the coefficients of the elements represent the SSI of the disc. Similarly, a pile embedded in soil can be modelled with springs on the side that represent the effect of the friction between the structure and the soil. Some of the existing models for these two cases will be studied in this work.

Four mechanical analogues will be computed and compared for the first SSI problem: Lysmer's, Kausel's, and two of Wolf's models. These models define the mass, stiffness, and damping coefficients differently, but yield similar responses. Additionally, the results will be compared to a finite element (FE) model with the same soil and foundation characteristics to assess their applicability. For the second SSI problem, Novak's solution for piles under vertical vibrations will be studied and compared to the response of an FE model. The relevance of these SSI problems lies in the simplicity of reproducing the analogues to use as a reference. They are suitable benchmark problems to evaluate an alternative computational approach that is proposed in this work.

This work's objective is to determine the feasibility of analysing the response of these systems using a hybrid computational approach. This method involves generating responses using an FE model with a range of soil and foundation parameters as a first step. Secondly, these results are input into a Neural Network (NN). The NN is trained to reproduce the response of any system by knowing only the geometric characteristics of the foundation, and the shear modulus, Poisson's ratio, and density of the soil. The network's output is the frequency response function (FRF) of said system, which is subsequently used in an optimisation process. The output of this second process is the values for the system's mass, stiffness, and damping characteristics. Ultimately, these values can be used to construct the dynamic stiffness matrix necessary for solving the soil-structure interaction of a particular structure.

The central question addressed in this research is: What constitutes an efficient data-driven process for translating large soil-foundation datasets into simplified mechanical analogues that provide a reduced and accurate representation of the system? After assessing its applicability, it can be concluded that a hybrid two-step computational approach is an efficient and generic method for solving SSI problems.

This thesis studies the use of metamaterials for mitigating train-induced ground-borne vibrations, a growing issue with the projected increase in train transportation. Traditional mitigation measures such as trenches often fall short, particularly at low frequencies and small incidence angles. Metamaterials, engineered to alter wave propagation properties, offer a novel solution by creating frequency bands that attenuate vibrations. This study begins with an one-dimensional model of an Euler-Bernoulli beam on a visco-elastic foundation, where metamaterials are modeled as local mass-spring-dash pots and single-degree-of-freedom resonators. This model facilitates an analytical study into wave propagation and band gap formation using Floquet analysis, which identified band gaps arising from both local resonances and the periodicity of resonator arrangements.

The study progresses by examining a finite periodic system using a numerical model. This involves comparing the dispersive properties of the finite system with those observed in its infinite counterpart. The results demonstrate an excellent agreement between the two. A significant focus is on the "metawedge", a metamaterial configuration with unit cells varying in natural frequency, allowing for broadband frequency targeting. This configuration enables elastic rainbow trapping and wave-mode conversion, with detailed three-dimensional analysis performed using FEMIX to explore these phenomena further. The three-dimensional model consists out of a homogeneous elastic half-space with single-degree-of-freedom resonators placed on top of the surface. The results demonstrate that the proposed metamaterial solutions are effective in mitigating low-frequency vibrations caused by trains across all incidence angles. The findings indicate that the classic metawedge configuration traps waves, by slowing them down, while the inverse metawedge accelerates waves, facilitating wave-mode conversion.

Pile run refers to the phenomenon that occurs during the installation of pile and monopile foundations in soils with low bearing capacity, where soil resistance is insufficient for the controlled driving of the monopile. The increasing frequency and associated risks of such occurrences in offshore installations underscore the necessity for effective pile run mitigation strategies.

This thesis investigates the feasibility of utilizing a mitigation structure to reduce pile run during the impact driving of monopile foundations. The primary objective is to model and analyze the dynamics of the hammer-pile-soil system under impact driving and pile run, with a focus on the impact of a mitigation tool in monopile response.

The research adopts a structured methodology, beginning with a qualitative study to identify effective mitigation measures. A 1-D finite element (FE) model is developed to simulate pile run, while a 2-D FE multiphysics model characterizes the fluid-structure interaction (FSI). The study explores the effects of various geometric parameters of the mitigation structure on forces and stresses experienced during pile run.

Key findings indicate that the implementation of a mitigation structure can significantly reduce pile run velocity, with its performance being influenced by the geometry of the structure and the velocity of the pile. A trade-off exists between velocity reduction and maintaining structural integrity. The research also highlights the critical role of FSI in determining the structural response of the mitigation tool and its interaction with the monopile.

Conclusions drawn from the results suggest practical applications for the mitigation measure, emphasizing the need for further investigation into the design and deployment mechanisms of the structure. Recommendations for future research directions are provided, aiming to enhance the understanding and application of pile run mitigation strategies in offshore engineering practices.

Offshore wind energy's rapid expansion underscores the need for accurate and efficient methods to analyze the behavior of monopile foundations supporting wind turbines. While three-dimensional (3D) analyses provide comprehensive insights, their computational demands are significant. As an alternative, one-dimensional (1D) models with spring elements to simulate the interaction between the structure and the surrounding soil, offer efficiency and simplicity. Realistic soil behavior, characterized by elastoplasticity, necessitates proper calibration of the spring models in 1D analysis.
This thesis addresses the challenge of soil-monopile interaction analysis, specifically focusing on the monopile response under lateral static monotonic loading. The research commences by highlighting the development imperatives in monopile-founded offshore wind turbines. The first phase involves calibrating elastic springs through a comprehensive review of existing literature. This calibration accounts for variations in spring stiffness along the monopile's length. Subsequently, the study progresses to elastoplastic soil modelling, adopting a linear elastic perfectly plastic approach and employing only lateral shaft springs. Acknowledging the limitations of linear elastic perfectly plastic p-y response, new material models, namely a bilinear and an exponential model, are examined. A parametric analysis encompassing various monopile geometries and lateral load eccentricities is conducted. An optimization routine refines the bilinear and exponential model parameters to closely match 3D responses. The results demonstrate satisfactory agreement for the analyzed high L/D monopiles, yielding valuable insights and conclusions. However, the low L/D monopiles exhibit a less successful match, primarily attributed to the absence of rotational shaft springs in the analysis.
Furthermore, empirical design processes for applying the bilinear and exponential models are outlined. These processes are founded on the relationships between the model parameters and the length-to-diameter (L/D) ratio as well as the eccentricity-to-diameter (e/D) ratio. The study highlights the applicability of the bilinear model across various soil conditions, monopile geometries and lateral load eccentricities. In contrast, the exponential model's efficacy is constrained by the examined L/D ratios, warranting further analyses for expanded application.
In conclusion, this thesis presents a systematic transition from elastic to elastoplastic modelling for soil-monopile interaction analysis under static monotonic loading. The proposed bilinear and exponential models enhance the accuracy of 1D simulations, facilitating efficient design and analysis of monopile-founded offshore wind turbines. These methodologies contribute to the advancement of sustainable offshore wind energy, catering to diverse soil conditions and design scenarios.

This thesis describes the investigation of the lateral monopile response in weak rock and the effect of a zone of crushed rock that is induced by driven installation. The goal of the investigation is to study the impact of the weaker zone around a monopile on the lateral response. The thesis focuses on the modelling of a monopile in a finite element model to examine the effect. The modelled monopile is compared to field tests that were done in previous research to be able to benchmark the model. To full-fill the research objective several steps were taken. The research can be divided into four parts to get to the final conclusion, being a literature review and three different models that are set up. The goal of the literature review and the two model in 2D are to act as a basis for the setup and inputs of the 3D model. The 3D model can then assess the effect of a crushed zone on the lateral capacity and stiffness response of the pile.

To support offshore wind turbines (OWT), monopiles are currently the most frequently used foundation method. These monopiles are open-ended steel tubes with a diameter of 5 - 12 m, which are most often installed in the seabed by means of hydraulic impact hammer ingusually mounted on a specialised ship. This method is produces high noise levels and releases shock waves into the water, which can damage the sea life’s hearing or even outright kill them. Additional measures can be taken to reduce noise emissions and adhere to strict environmental regulations written for marine biology protection, but they are costly and slow down projects significantly. There are other installation methods which have the potential to be more silent but are not well understood yet from a noise generation, drive ability, and lateral behaviour point of view. These are aspects that will be investigated within the SIMOX Joint Industry Project (JIP). One of these methods is vibratory installation.
A monopile used as a foundation for an OWT will experience a multitude of loads during its service life, the most important of which are the lateral loads. While the behaviour of impact hammered piles under lateral loading has been researched extensively, the behaviour of vibratory driven piles is still relatively unknown. The influence of different parameters used during installation (frequency, penetration speed)and other conditions (wall thickness and soil conditions) on that behaviour must be understood to be able to accurately predict how a vibrated monopile will react to both cyclic and monotonic loading.

The present research explores the behaviour of monopiles under lateral loading and the impact different installation parameters have on it. To achieve this, a laboratory testing campaign was carried out with model piles. The purpose of these tests is to produce qualitative results to be used in following field-testing campaigns within SIMOX. Piles were installed with differing installation parameters in sand beds with different density. These piles were then subjected to initial monotonic loading, followed by cyclic loading, and then monotonic loading again. The data obtained during the experiments was then interpreted and analysed. By comparing the results as well as measurements taken during and after installation, conclusions are made concerning installation parameters and other factors that may play a role on the lateral behaviour of monopiles.

The interpretation focused mainly on pile head displacement during initial lateral loading depending on penetration speed and frequency. The loading tests have shown that impact hammered piles under went lower displacements than vibrated piles after being loaded laterally. When solely considering vibrated piles, piles with a larger wall thickness showed lower displacements in general than thin-walled piles.

Frequency and penetration speed were found to play a role in the lateral behaviour of monopiles. In the experiments considered for this thesis, it seemed in dense sand crane-controlled piles showed lower displacements than free-hanging piles. In medium dense sand, lower penetration speed led to lower displacement during loading, but more research is needed on this topic to be able to formulate clear conclusions on the exact role of each installation parameters. The experiments also show an interesting phenomen on regarding measured soil elevation that might link compaction around the monopile to lower lateral displacements. The difference in elevation before and after installation seemed to correlate with lateral displacements. In general, piles with larger compaction around the pile displaced less during initial loading.

In conclusion, this paper provides a range of observations regarding the impact of installation parameters and other conditions such as wall thickness and sand density on the behaviour of monopiles under lateral loading, as well as offering a comparison with impact hammered piles. Recommendations and suggestions are given for further research and for the field testing experiments, so that the analysis made here may be used to predict the lateral behaviour of vibrated monopiles more accurately.

Wind turbine installation vessels (WTIVs) are ships that are specifically designed to install offshore wind turbines. These WTIVs have four or six large truss-like legs that are lowered towards the seabed by means of jacking systems. These jacking systems are regulated by control systems to ensure the leg is lowered with a constant velocity regardless of external disturbances. The bottom of these legs are outfitted with spudcans, which have a conical shape to allow for penetration into the seabed. When these spudcans have settled in the seabed, the hull is lifted above the waves. Consequently, wave-exciting forces on the hull are prevented and the motions of the hull are near-zero so that the on-board crane can perform the installation operations with minimised disturbances.

The objective of this thesis is to develop and analyse a model that is able to describe and simulate the dynamics of these jacking systems in great detail in response to external loads and the dynamics of the WTIV. The control systems of the jacking systems are included in this model to simulate and evaluate the interaction between the control system and the dynamics of the WTIV. Two conventional control systems are considered: the Volts-per-Hertz (V/Hz) and the direct torque control (DTC) method. In the process of lowering the legs and subsequent platform lifting, a transient phase can be identified during which the spudcans are penetrating the seabed. Due to the periodical motions of the ships, multiple impacts with the seabed are expected. Additionally, the jacking systems and the leg undergo a change of load direction as initially the leg is in tension and the jacking systems are generating power, and afterwards the leg is in compression and the jacking systems are consuming power. This thesis is focused on this seabed penetration phase as this phase introduces complicated dynamics. In literature, no model is available that has the abilities to simulate the WTIVs and its jacking systems with control systems in such level of detail.

This research gap is addressed by developing such a simulation model. This model is written in Python and developed using finite element (FE) techniques and solved using numerical time integration. Seabed characteristics are derived using a detailed coupled Eulerian-Lagrangian (CEL) FE models. Multiple control strategies are simulated and evaluated, each differentiating how the velocity and torque setpoints of the jacking systems are calculated. From the simulation model, it is found that in order to achieve load sharing between jacking systems, torque and velocity require to be independently controlled which only the DTC method has the ability to. Furthermore, each of the jacking systems should be provided with its own power supply. Best performance and stability was achieved when each chord of the leg is given a common torque and velocity setpoint, which is equivalent to a common torque and setpoint per leg in reality. Moreover, load sharing can be improved without a control system by increasing the relative stiffness ratio between the chord and the mechanical contact between rack and pinion.

New offshore wind farms consisting of monopile-founded Offshore Wind Turbines (OWTs) are to be built in earthquake-prone areas. To design the monopile foundation and to accurately define the dynamic response of an OWT, the soil-monopile-superstructure interaction should be modelled properly. Due to the fact that the Three-Dimensional (3D) Finite Element (FE) analyses are complex and computationally expensive, research is focused on One-Dimensional (1D) FE models, in which the soil-monopile interaction is traditionally described via distributed translational springs representing the soil lateral load. Nevertheless, the increase of monopile diameter, followed by the monopile length-to-diameter ratio (L/D) decrease, implies the contribution of additional resistant components such as distributed moment, base shear and moment.

This thesis examines the 3D mechanisms to be accounted for in the 1D FE modelling of the soil-monopile-superstructure seismic response in case of a single-phased, linear visco-elastic soil layer. Both 3D and 1D FE analyses are conducted with the FE software OpenSees. The 1D analyses are simulated in two consecutive steps: first a Site Response Analysis is performed, next the recorded displacements over the soil layer depth are applied to the spring supports and the dynamic interaction of the system is simulated. Three different monopiles are considered with L/D equal to 26, 9 and 5. Two superstructures are examined, which are modelled as Single-Degree-of-Freedom systems. Distributed translational springs are assigned to the slender monopile (L/D=26), while for the stubbier monopiles the contribution of distributed rotational springs is examined as well. Lastly, the effect of considering the base moment and shear is also examined.

The stiffness of the soil reaction curves is calibrated by applying a monotonic lateral load and moment at the pile head, in case of the translational and rotational springs, respectively. The spring stiffness values are assumed uniform along the monopile length. As a next step, the dynamic response of the calibrated 1D models is examined in steady-state conditions, under the action of mono-harmonic excitation, and compared to the 3D results. Ultimately, the seismic response of the 1D models is examined in case of two earthquake excitations with different frequency contents.

In case of the monopiles with L/D=9 and 5, it is concluded that the use of monotonically-calibrated distributed translational and rotational springs provides a good match between 3D and 1D regarding the monopile head and superstructure response under seismic loading. Nevertheless, these 1D FE models cannot predict the base moment, for which a base rotational spring should be employed. In case of the stubbier monopile, with L/D=5, the base shear seems to positively affect the moment profile as well. Lastly, regarding the monopile with L/D=26, the employment of translational springs alone seems sufficient for the accurate prediction of the seismic response; however, the hereby monotonically-calibrated distributed translational springs result in a mismatch between 3D and 1D.

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.

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.

Offshore wind energy is one of the emerging renewable technologies toward a zero-carbon emission society. Historically, offshore wind farms have been developed in the North Sea, but nowadays wind farms are expanded to other regions such as North America and Asia, which are seismic prone.
In the design of Rotor-Nacelle-Assembly (RNA) and support structure for offshore wind farms, the number of design positions is reduced by using representative design positions (i.e. clustering). The number of designs is directly related to the efficiency of the design work, so it is worthwhile to minimize it by wisely selecting the representative position. However, such a clustering methodology has not been systematically developed in seismic design due to the limited number of projects and researches in seismic regions.
As for seismic analysis, there are several challenges to develop an efficient clustering strategy. First, higher (and multiple) vibration modes are excited in the system consisting of the wind turbine itself in interaction with the soil. Second, blade vibration (and blade-tower coupling effect) has a significant role in the output forces. Third, energy intake from the seismic ground motion is dependent on the position due to soil amplification. Considering the problem statement above, this research develops a clustering strategy for seismic analysis to optimize the design process on RNA and support structure for offshore wind farms. To achieve this goal, as a first step, a simplified calculation model is created to efficiently carry out the sensitivity studies. A combination of point masses with Euler-Bernoulli beam is used for RNA model while lumped mass at the tower top is conventionally used in past researches, which allows to properly represent the blade-tower coupling effect contrary to most of past researches. Considering modal participation mass, higher vibration modes are truncated, and principle vibration modes are detected, which improves the computational efficiency. Both the frequency domain method and the response spectrum method are developed and compared with proven software, BhawC, to verify their accuracy. Then, a sensitivity study is carried out by using the developed model. Through the sensitivity study, the system property is approximately quantified as a form of ”scaling factor”. Combining the scaling factor with acceleration response spectrum, the maximum bending moment for an arbitrary position (i.e. arbitrary water depth, soil condition and bedrock depth) can be quickly estimated. Subsequently, the positions in a wind farm are sorted into groups with similar load trends based on the estimated bending moment; this is the basis of the clustering strategy proposed in this thesis. Finally, the strategy is applied to virtual wind farms and its applicability is verified. Although it shows some errors in the estimation of the maximum bending moment itself, the accuracy is considered satisfactory to detect the most loaded position (i.e. design representative position) and categorize the positions which have similar load trend, which are one of the main requirements for a reliable clustering methodology. Also, the strategy is applied to more realistic soil condition, such as inhomogeneous soil and its applicability and limitation are investigated.