The Perfectly Matched Layer (PML) has become a powerful tool in computational underwater acoustics and elastodynamics. By employing complex coordinate stretching in the wavenumber–frequency domain, PMLs effectively attenuate outgoing waves from the physical domain and thereby provide an efficient means to truncate the computational domain. Although PMLs have been widely adopted in the finite element and finite difference communities, their use in semi-analytical solutions remains limited. A major challenge is that, when modal analysis is applied to the acousto-elastic domain with PML in a semi-analytical framework, the found modes are not orthogonal. This challenge formulates the main motivation of this research. On the other hand, the modes obtained from the discrete solution of the elastic layer with PML, based on the thin-layer method, do preserve orthogonality. Therefore, this thesis aims to understand the differences between the modal solutions of the semi-analytical and thin-layer methods in the elastic domains with PMLs, which may provide insights into the reasons why modes in the semi-analytical solution are not orthogonal to each other.

The main storyline of this thesis is developed through four cases with increasing system complexity. In the first case, the modes of the elastic domain are computed using both the semi-analytical approach and the thin-layer method (TLM), and the comparison demonstrates the equivalence of the two methods in the absence of PMLs. In the second case, the acoustic domain with PML is investigated using the semi-analytical approach, with emphasis on the polynomial order of the complex-stretching function. Mathematical derivations show that a zero-order polynomial induces discontinuities at the interface, leading to uneliminated boundary terms and perturbing modal orthogonality, while numerical results confirm that higher-order polynomials preserve the cross-orthogonality of modes, as well as the continuous slopes of the potential functions at the interface. In the third case, a quadratic complex-stretching function is employed, and the elastic domain with PML is analyzed using both approaches. The comparison reveals differences in eigenvalues and eigenvectors; finer TLM discretization yields increased matches between the two methods, but excessive discretization results in orthogonality violations. Finally, in the fourth case, the semi-analytical modes of the acousto-elastic domain with PML are studied. Propagating, evanescent, and Bérenger modes are identified, with cross-orthogonality preserved given sufficient integration points. Bérenger modes consistently arise in PML formulations and exhibit anomalous dispersion characteristics.

The main contribution of this thesis lies in revealing the influence of the polynomial order of the complex stretching function on the modes of the acoustic domain with PML. When a quadratic complex-stretching functions are employed, the numerical results suggest that the semi-analytical modes of the elastic or acousto-elastic domains with PML are orthogonal. Therefore, it is suggested that a positive value of polynomial order is recommended when computing normal modes of the acousto-elastic domain with PML. However, in the future, a systematic study on the influence of the polynomial order should be conducted for the elastic layer or acousto-elastic domain with PML.
Furthermore, the comparative study of modal solutions highlights the differences between the semi-analytical approach and the thin-layer method. The nature of modal solutions comes from the different formulations of the eigenvalue problem, leading to different eigenvalues and eigenmodes. For TLM, the over-discretization of the PML domain is not suggested due to the violated orthogonality, although the reasons behind that require further investigations.

Overall, this thesis advances the fundamental understanding of the modal basis of acoustic, elastic, and acousto-elastic layers with PML formulations, providing a foundation for future research in two main directions: (i) the study on modes of the acoustic layers coupled with multiple elastic layers with PML, which better represent realistic ocean environments with geological strata; and (ii) the computation of forced responses of structures in acousto-elastic layers with PMLs to model the pile-water-soil interactions using modal matching techniques.

A geotechnical centrifuge is used to conduct tests on small-scale models to obtain data on the response of real-life structures. In the field of earthquake engineering, centrifuges can be used to model soil-structure interaction and to analyse structural response to earthquakes through scaled models. By rotating the centrifuge at high angular velocities, the model inside the centrifuge can experience the stresses corresponding to a full-scale prototype. Through simplified scaling laws, the dimensions of the small-scale model are estimated, and the results from these tests can be extrapolated to study the behaviour of the prototype.

When the small-scale model of a dynamically sensitive structure, such as an offshore wind turbine, is subjected to high angular velocities inside a centrifuge, it is essential that the model remains stable to obtain meaningful results from the test. Instabilities, such as divergence or flutter, may arise in the small-scale model depending on the system’s parameters due to the action of pseudo-forces and coupling between the displacement fields, which may lead to the failure of the model. Moreover, it is unclear whether the simplified scaling laws can still be applied for such slender small-scale models due to the action of these pseudo-forces and the coupling terms. Therefore, understanding the behaviour of these dynamically sensitive small-scale models under centrifuge conditions is important to ensure their stability and to obtain conclusive results about the prototype.

This research represents the small-scale model of a monopile-founded offshore wind turbine inside a centrifuge as a homogeneous Rayleigh cantilever beam rotating about a vertical axis, with each point along the beam consisting of three translational displacements. A mathematical model is formulated using Lagrangian formalism that incorporates all relevant pseudo-forces and coupling of displacement fields that may affect the stability of the small-scale model. Euler-Lagrange equations are then applied to formulate the governing equations of motion, and a dimensionless form of the equations is presented to draw general conclusions. Moreover, the effect of soil-structure interaction is considered using a lumped spring model.

Subsequently, an eigenvalue analysis is performed using analytical and numerical approaches. The numerical approach is more robust and is applied to calculate the eigenproperties of the system, although it requires a good initial guess. The technique of representing the response as a summation of assumed modes is explored in the analytical approach, and a convergence study is conducted. Afterwards, a parametric study is performed to identify the factors influencing stability. The angular velocity at which the onset of instability happens is also determined. Finally, the findings are applied to a case study of a small-scale model of an offshore wind turbine tested inside the centrifuge facility at ETH Zürich as part of the research project, DONISIS, led by TU Delft.

The results of this study demonstrate that, for the small-scale beam model of a monopile-founded offshore wind turbine tested inside a centrifuge, the dominant instability mode is the divergence of chordwise lateral bending. This instability arises from the axial compression induced by the centrifugal forces acting along the beam. The coupling of displacement fields through Coriolis forces does not affect the onset of instability, although its influence on the eigenproperties becomes apparent only at angular velocities far exceeding the practical operating range of a centrifuge. Since the Coriolis terms have no significant impact on system behaviour, their effect on scaling laws can be regarded as negligible. Consequently, the existing scaling laws remain applicable to small-scale models of rotating beams within the practically operating range of angular velocities of the centrifuge.

The large-scale deployment of offshore wind farms is critical to achieving global renewable energy
targets. However, the installation of monopile foundations through impact pile driving generates
significant underwater noise, posing risks to marine ecosystems. While noise mitigation measures,
such as air-bubble curtains and pulse elongation systems, have been developed to reduce peak
sound levels (Lpeak) and sound exposure levels (SEL), less is understood about their effects on
cumulative noise exposure (cumSEL) - a key metric in regulatory frameworks like that of the
United States.
This study investigates the influence of operational parameters, including the target blow count
and impulse elongation, on cumulative underwater noise levels during offshore pile driving. Using
the SILENCE model, a semi-analytical approach is applied to simulate noise propagation in
realistic offshore conditions. The research evaluates the combined effectiveness of mitigation
strategies such as pulse elongation and air-bubble curtains, as well as the effect of frequency
weighting on sound levels for specific species.
The results show that varying the target blow count has negligible influence on cumulative underwater noise, while impulse elongation (PULSE) achieves modest broadband reductions and more
substantial reductions when species-specific1
frequency weighting is applied. The combination of
PULSE with a Double Big Bubble Curtain (DBBC) offers the most effective mitigation across
all marine mammal hearing groups.
This study provides insights into the complex interactions between operational parameters and
noise mitigation strategies, supporting the development of effective solutions to comply with
cumulative noise regulations. The findings aim to contribute to more sustainable offshore wind
farm construction while minimizing impacts on marine ecosystems.

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.

This thesis introduces a novel contactless method for controlling the motion of suspended components during offshore wind turbine installations. Conventional systems rely on mechanical contact and are susceptible to wave-induced disturbances. The proposed technique exploits magnet-to-magnet interactions between mounted permanent magnets and external electromagnetic actuators to achieve non-contact attenuation of both translational and rotational vibrations. The proof-of-concept design of this method is validated through numerical simulations and scaled laboratory experiments. Results demonstrate effective motion mitigation and precise positioning under dynamic conditions. This investigation highlights the potential of magnetic control to enhance safety, accuracy, and efficiency in challenging offshore environments, offering a promising alternative to traditional motion compensation systems.

Wind energy is a crucial component of the transition to a carbon-free energy supply. As offshore wind farms expand into deeper waters, larger monopiles are required, posing technical and environmental challenges. High-blow energy hydraulic hammers drive these piles into the seabed, generating significant underwater noise that propagates through seawater and sediment, potentially impacting marine life. Regulatory bodies enforce noise thresholds and require environmental impact assessments to mitigate these effects.

Current underwater noise models often simplify seabed conditions, overlooking complex pile-water-soil interactions. Semi-analytical models provide accurate near-field predictions but struggle with long-range effects, while empirical models lack adaptability to varying soil conditions and mitigation measures. This thesis addresses these gaps by incorporating detailed sediment descriptions to enhance noise predictions over large distances.

The study also integrates an air bubble curtain into a noise prediction framework, considering pile, water, sediment, and bubbly layers. A noise prediction module estimates non-mitigated pile driving noise, while a noise reduction module quantifies bubble curtain effects using boundary integral equations. This enables efficient noise reduction assessment across different configurations.

Additionally, the study evaluates seabed vibrations and particle motions, crucial for benthic species often neglected in impact assessments. A comprehensive modelling framework is developed, transforming wave fields into Source levels (SL) for both fluid and sediment sources. Sound maps estimate maximum impact distances based on species-specific sensitivity thresholds, offering insights for regulatory compliance and marine conservation.

Offshore wind energy has gained recognition inEurope as a pivotal solution for transitioning to renewable energy sources. Due to limited land space, the European seas offer immense potential for offshore wind energy. The EU Offshore Renewable Energy Strategy advocates for an accelerated expansion of offshore wind, aiming to achieve 60 GW of capacity by 2030 and an ambitious target of 300 GW by 2050. This growth necessitates substantial investments and has been further underscored by the need to replace Russian fossil fuel imports, prompting additional targets for offshore wind capacity. A major environmental concern in offshore wind farm construction is the substantial underwater sound generated during the installation of wind turbine foundations. Most North Sea wind turbines are founded on monopile structures, which involve driving a single hollow cylindrical steel pile into the seabed using impact hammers. This process emits powerful underwater pressure waves that impact marine animals dependent on underwater sound for navigation, communication, and predator-prey relationships. Studies have shown that elevated underwater noise adversely affects marine animals’ physical health and behaviour, with severity dependent on sound pressure levels, frequency bands, and water particle motion. Constructing these wind farms, especially installing monopile foundations, is one of the loudest human-induced underwater sound sources and generates substantial underwater noise that adversely affects marine life. This results in the need for effective noise mitigation strategies, such as quieter driving tools, to protect marine ecosystems while supporting the expansion of renewable energy infrastructure...

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.

Offshore wind energy is expanding rapidly as governments aim for net-zero emissions, with monopiles and jackets being the primary foundation methods for offshore wind turbines (OWTs). Supply chains, heavier turbines and deeper waters influence the efficiency of jacket foundations. This research considers a jacket-founded OWT. The development of OWTs has sparked its expansion to building sites with a proclivity to earthquakes. Such loading involves soil-structure interaction of which more needs to be known to improve accurate computational modelling.

Current practice of Siemens Gamesa Renewable Energy (SGRE) is to perform aeroelastic simulations in their computational tool BHawC. The Foundation Designer delivers a foundation Superelement to maintain secrecy. The equation of motion for the jacket foundation can then be solved linearly with a reduced amount of unknowns. However, prevalence of nonlinearity in soil raises the question to what degree a linear model adequately captures the response.

This study investigates the impact of soil nonlinearity on OWT dynamic behavior under seismic loads by comparing linear and nonlinear soil models. The analysis involves performing seismic simulations using multiple earthquakes. Another distinction is made through soil models with different characteristics. Nonlinearity is introduced to the soil stiffness and energy dissipation mechanism under cyclic loading. The investigated variations are a linear (elastic), geometrically nonlinear (nonlinear elastic) or both geometrically and physically nonlinear soil model (nonlinear plastic).

The numerical model consists of a Rotor Nacelle Assembly (RNA), tower, transition piece, jacket, piles and soil springs. Beam elements are used for the tower, jacket and piles. The transition piece is simulated by stiffening the top jacket braces. The RNA is modelled using a lumped mass with rotational inertia. It is vertically eccentric to the tower top and connected with a rigid link. The earthquake is applied uniform over depth and only horizontal movements are considered.

The findings underscore a difference in results between the linear and nonlinear models. The evaluated results from simulations consist of forces, displacements and dynamic characteristics of the structure. Also noted should be that the computational time of the linear model is significantly lower. The results found in models can differ greatly due to the loading spectrum with highly varying frequency peaks. Another factor is softening of the stiffness. The frequency domain of the elastic model results consists of narrow peaks at the system's natural frequencies. The peaks for the nonlinear elastic model are wider due to softening of the stiffness. For the structure used in this research, softening introduces coupled modes with greater displacements along the height of the structure. This makes it possible for the evaluated results to have higher values, even with less energy put into the system. The plastic models' peaks are of a width in between the elastic and nonlinear elastic model due to the combined use of isotropic hardening and nonlinear stiffness. When the model falls back on its initial stiffness upon unloading, the eigenfrequencies related to that stiffness become more pronounced. To match the occupancy of wider frequency peaks, loading and unloading should both happen nonlinearly. This can be achieved by using kinematic hardening instead of isotropic hardening. Plasticity generally reduces peak displacement and sectional moment values and nonlinear stiffness broadens the response frequency spectrum. Careful consideration of cyclic material behaviour, eigenfrequencies and loading characteristics are essential for a realistic model.

An increased interest in the use of renewable energy, mainly accelerated by the growing awareness of the problem of global warming, has been observed over the last decades. For the installation of the popular monopile and other support structure types, high-energy pile driving is used in around 90% of the cases. The noise emitted by the pile driving forms a serious threat for marine mammals. To mitigate the underwater noise, air-bubble curtain systems are used in the offshore industry. A better understanding is required of the working principle of air-bubble curtain systems in practice to gain more insights into the air transport through the hose-nozzles (perforated pipes).
For this study a physical medium scale pneumatic model of the air-bubble curtain system was designed based on data available from the offshore industry. Tests were performed both in dry and wet conditions which approximate those that are seen in the industrial practice. The role of several geometrical and operational parameters of the pneumatic system, like the inlet pressure, inlet flow rate, internal hose diameter, nozzle diameter, nozzle spacing, static water pressure, on the operational conditions along the hose-nozzle is explored. The influence of the volumetric flow rate along the hose-nozzle on the pressure development along the hose-nozzle is examined. The examined correlation between air flow temperature and air flow rate at the inlet of the compressed air supply and the pressure development over the hose-nozzle forms valuable input for a future numerical model that includes the temperature of the inlet flow rate. The test setup with configurations of reduced lengths in dry conditions is used to experimentally determine the discharge coefficients of a range of different nozzles. Here, the limitations of the test setup are carefully considered and a comparison with numerical results provided by compressible flow theory is followed.

The demand for wind energy has led to the need for larger offshore wind turbines, which require larger monopiles for support. Vibratory pile driving is a common technique for installing these monopiles, but current engineering models are designed for smaller diameter monopiles and may not be applicable to larger ones. Additionally, the vibratory action produced by counter-rotating eccentric masses in the hammer can cause unintended bending motions in the pile if there is a slight misalignment. This study aimed to investigate the effects of gear misalignment on the response of monopiles under vibratory action, including these imperfections.

To achieve this, a hammer model was created and coupled with a pile model and a simplified soil reaction. A parametric study was conducted to evaluate the pile response to the modeled vibratory hammer with imperfections over a range of specified variables. The transfer of force from the hammer to the pile and the range of driving frequencies that activate the isolation springs in the vibratory hammer were examined, as well as the effect of misalignments on the power consumption of the vibratory hammer and the bending displacements caused by these misalignments.

The study found that there was no significant impact on axial vibrations as a result of misalignment of rotating masses.

The force transfer ratio from hammer to pile is 0.5 to 0.7 for smaller diameter piles, and it is important to include force transfer ratios in simple engineering models during the initial design phase of pile drivability prediction models when the decision is made not to model the hammer.

The study also showed that modeling the hammer-pile-soil system with shell elements instead of 1D rod elements yields similar trends in the responses, but the absolute response values of large diameter piles with simple 1D rod elements leads to an underestimation of the response in the lower driving frequency range and an overestimation in the higher driving frequency range.

Even if as much as 50% of the eccentric masses are misaligned, the impact of bending vibrations on the power consumption of the vibratory hammer is insignificant. However, bending displacement can become substantial when 6% of the eccentric masses are misaligned. Smaller diameter piles are more susceptible to vibrations at lower frequency ranges, while larger diameters are impacted in higher frequency ranges.

Only large diameter monopiles are susceptible to axial vibrations near their natural frequencies, while all diameter piles are affected by bending vibrations near their natural frequencies.

Isolator springs are effectively activated in the region of higher driving frequencies (> 28 Hz) for large diameter monopiles. In the case of small diameter piles (< 3 m), the relative motion between suppressor housing and pile head remained above unity for all driving frequencies, meaning poor activation of the isolator springs.

Jack-ups are used for offshore wind turbine installation (OWTI). To assess whether a jack-up can safely operate in a desired geographic location with known seismic activity, an earthquake response analysis is required for structural verification. This analysis is generally carried out according to the codes of the International Organization for Standardization (ISO). The prescribed procedure is called extreme level earthquake (ELE) screening and is performed using response spectrum analysis (RSA). This method is intended to be conservative and should produce higher utilization results than more detailed assessments. With offshore wind farm development moving into regions with greater seismic activity, more detailed assessments are required to demonstrate safe operation in those geographic locations.

The aim of this research was to improve the accuracy of earthquake screening of OWTI jack-ups in order to increase their geographic operability. A new ELE-screening procedure was proposed that uses time-history analysis (THA) and spectrum matched acceleration time history records (THR). For the development and benchmarking of the new method, an earthquake analysis was performed using both the new ELE-screening procedure with THA and the existing ELE-screening procedure with RSA. The analysis was performed for an OWTI jack-up based on the GJ-3750C located in a region offshore Japan. The RSA simulations were performed using Minifem; a new tool was developed in OpenSees for the THA simulations. To assess the effect of various parameters, simulations were run with different soil-structure connections, levels of damping, and design response spectra. The performance is evaluated using the global maximum action effects; limited to the forces and moments at the lower guide and footing. A soil-structure interaction analysis was performed on an equivalent single degree of freedom system to validate the soil-structure interface model, and the use of free field ground motions. A procedure and accompanying tools were developed for acceleration time history record selection and modification. The resulting spectrum matched THR are used for seismic excitation in the THA simulations.

The simulation results showed a reduction in the magnitude of calculated action effects when using the new ELE-screening procedure. A reduction of 10 to 20 percent in the global maximum shear force and moment was observed in the simulations best describing the jack-up and site-specific soil conditions. A small reduction of the global maximum normal force was also observed. The new ELE-screening procedure with THA can be used to demonstrate compliance with earthquake performance requirements when a jack-up does not satisfy the ELE-screening assessment criteria using RSA. Since safe operation can be demonstrated for more areas, the geographic operability of OWTI jack-ups is increased.

The ambitious goals towards the decarbonization of the global energy sector have amplified the demand for renewable energy resources. Amongst the renewables, offshore wind possesses a pivotal role in this endeavour, showcasing remarkable growth in recent years. However, this rapid expansion has been accompanied by a series of technical challenges. Foundation installation comprises one of the most critical phases in the construction of an offshore wind farm and engineering advancements in this topic are vital to accommodate this developmental pace. Bottom-fixed foundations are primarily used to support offshore wind turbines and amongst the available concepts, the monopile is the foremost one. The installation of these substructures is most commonly performed via impact hammering. Notwithstanding the robustness and efficacy of this technique, major environmental concerns have been raised due to the significant levels of underwater noise pollution during driving. In view of this alarming issue, alternative and sustainable pile installation techniques have been progressively drawing attention during the last decade and an increasing number of research projects focus on their investigation and development.

At present, the offshore wind industry is increasingly adopting vibratory pile driving. The previous method has been successfully employed in onshore projects for decades, albeit its wider use in the offshore environment is hindered due to the incompleteness of available field observations. To boost the improvement of vibratory installation methods, a new technology has been recently proposed by the Delft University of Technology, namely the Gentle Driving of Piles (GDP). The preceding method aims to enhance the installation performance of vibratory driving for tubular (mono)piles and to reduce the associated noise emissions, via the simultaneous application of low-frequency/axial and high-frequency/torsional vibrations. Naturally, the shift to these technologies is accompanied by emerging research questions pertaining to pile installation, vibro-acoustic and post-installation performances. In this thesis, the development of an engineering-oriented modelling framework for axial vibratory driving and GDP is the primary objective, thereby focusing on the topic of sustainable monopile installation.

This study presents the structural response of a Submerged Floating Tunnel (SFT) subjected to earthquake loading and other relevant static loading types. The SFT is a competitive solution for crossings of deep seas, canals and fjords, as it is submerged in water and floats at a fixed submersion depth. It is supported by its buoyancy and tethers, connected to the seabed to maintain the structure at an even submersion level. Influence on the environment is low compared to other competitive structures, as the structure does not occupy any surface of the seabed, except for the tether foundations. Moreover, the environment has little influence on the SFT’s operability, as it is insensitive to harsh weather conditions. However, in areas of high seismicity, the SFT’s dynamic characteristics must be carefully tuned to the expected type of earthquake to optimize its performance. Previous research showed that special care should be taken of the transition between the SFT and land tunnels, as this joint has proven itself to be the main challenge.

The purpose of this research is to alter the transition structure between the SFT and adjacent land tunnels, such that an optimal dynamic response of a SFT is found to seismic events (Chapter 1). Initially, its performance is measured by comparing the seismic serviceability limit state (SLS) response to the static ultimate limit state (ULS) response. Subsequently, for more severe earthquakes, it is measured by comparing the SLS seismic stresses with maximum allowed levels of concrete prestress.

To analyse the SFT behaviour, a global model has been created of the SFT+land-tunnel system in Chapter 3, which is built by means of a linear-elastic finite element method in Python. It accounts for dynamic loads through the soil and tethers, as well as static loads by the structure's weight, traffic and buoyancy. The influence of stagnant water is accounted for by the Morison equation. Soil-Structure-Interaction (SSI) is incorporated using a Substructuring method. The properties of the soil and tethers are found in local sub-models, after which the total structural response of the global model is solved in the frequency domain. Later, its response is transformed to the time domain to obtain a time-series of displacements and forces. Its dynamic characteristics are studied by comparing Fourier spectra with SFT natural frequencies, which gives insight in the influence of design choices on the time-domain response.

The global model is validated with a replica model in the finite element software DIANA FEA (Chapter 4). A realistic case study is defined based on a previous TEC project in Chapter 5 and later applied in a parameter study in Chapter 6. Here, the effects of various design choices with respect to the transition structure are monitored using 3 earthquake signals. 5 standard end-joints and 2 special end-joints with seismic base isolation and viscous dampers are tested. The effects of Multi-Support-Excitation is studied by letting earthquakes horizontally approach the SFT with various wave speeds and angles of attack. Finally, the effect of SSI is compared with a non-SSI case, to see its contribution to the research.

In Chapter 7, the results are discussed and in Chapter 8 a conclusion is provided together with recommendations for both TEC and future research.

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.

Climate change is a recurring topic in the news and goes side by side with the energy transition. The energy transition focuses on using clean energy generated by resources like wind and water. Due to the increasing demand for technology that focuses on generating and storing clean energy, the demand for metals grows. These metals are currently extracted from landbased locations but are also found on the bottom of the ocean. Three types of resources are found on the
ocean floor: hydrothermal vents, cobalt crusts, and polymetallic nodules. To extract these resources from the ocean floor, a project called ”Blue Nodules was launched in 2020. This project addresses the challenge of creating a viable and sustainable value chain to retrieve the polymetallic nodules from the ocean floor. Royal IHC is one of the partners on this project and designed the seafloor nodule collector. The total mining system consists of a surface vessel, a vertical transport system, a flexible hose (jumper hose) and a seafloor nodule collector. In combination with a hose developing company, IHC designed the flexible hose. This design was projected on a prior design of the seafloor module collector. Since then, the seafloor module collector design has advanced, but the flexible hose design did not. Therefore the main objective of this master thesis is to design and analyse the flexible hose with its main focus on the dynamic behaviour.
The methodology for this thesis is addressed in the following manner. The old design of the flexible hose was investigated, and all relevant parameters were addressed. A static model was built to verify the parameters that modelled the hose with these parameters. The parameters of interest were: the length of the hose, the distance between crawler and vertical transport system in height and width, number of buoyancy modules, number of buoyancy modules per hose section, starting position of buoyancy nodules. This static model was used to perform a parameter study, resulting in an optimal set of parameters tested against design criteria.
The next stage was implementing the static design in a dynamic software packet called ”OrcaFlex”. Orcaflex is a software program for static and dynamic analysis of offshore systems. The static model is implemented end tested with this software. This resulted in a failure of the set of parameters and adjustment to the static model. After a redo of the parameter study, a new set of parameters was
extracted. This set is tested for an inline motion of the crawler compared to the vertical transport system and for a perpendicular motion. These motions expand in size until the flexible hose reaches its critical values. These tests resulted in a range around the VTS where the crawler could move in.
Results show that the crawler can move in this range for a selected set of parameters. Throughout the research, it is shown that the parameter set needs further development to complete the flexible connection design. It is recommended that some disregarded motions, parameters and environmental forces are further investigated and that the failure criteria that test the hose need to be expanded.

Vibratory installation of piles and casings can be extremely economical, therefore the contractors prefer pile driving. When compared to impact pile driving, vibratory pile driving generates less noise but this installation method is still fraught with uncertainties (Rausche 2003). Environmental effects (such as ground vibrations) have negative impacts which could lead to damage and failure under special circumstances.

The vibrations from the pile driving generate excess pore pressures and cause settles leading to failure in slopes. As part of the optimisation of mooring places for inland shipping and small coasters in Calandkanaal, the port of Rotterdam authority had a number of mooring places modified in the second half of 2016. A point of attention was the lack of analysis and a testing framework as there were uncertainties in excess pore pressure generation, an exposed area in the slope and slope stability. Based on these uncertainties, the work presented in this report tries to bridge this gap by answering
the following research question:

How can the existing knowledge on pile driving by means of vibratory installation be efficiently integrated into a 3D model which can practically estimate the factor of safety of the slope during vibratory installations?

To answer the main research question, a few sub-questions were formed, which are as follows:

• How can the vibratory parameters be integrated to model the slope stability?
• What is the thickness of the highly degraded zone around the pile due to vibratory pile driving?
• What are the limitations of the study?

The proposed work uses the vibratory pile driving data to generate excess pore pressure data using an empirical pore pressure model given by Green & Mitchell (2004) referred here as the GMP pore pressure model. Along with the pore pressures by using an empirical attenuation relation, the extent of the liquefied zone was determined giving an idea about the area of the slope affected by pile driving.

For slope stability, PLAXIS 3D is used along with its flow module. PLAXIS 3D gives an idea of the expected displacements in the geometry (slope) and the factor of safety. In a way to incorporate vibration effects, only the pore pressure are used as load inputs because of the difficulty of inducing
pile driving vibrations in a static calculation.

To add pore pressures into the geometry, a well was used with the infiltration function for the amount of time the pile was driven in the actual project. The volume input for the well infiltration is calculated in the same way as it was done in the actual project shown in the work of de Nijs (2019). PLAXIS 2D and D-geo stability software packages are also used in an attempt to make comparisons with the PLAXIS 3D model and validate the whole analysis.

As the world's population keeps increasing and the rate of urbanizations increases, The rate at which high-rise structures are being built is skyrocketing. In the Netherlands, this is no different. It is expected that the amount of high-rise structures taller than 70 meters will more than double in the coming 20 years. As these structures become taller and more slender, they also become more susceptible to wind-induced excitations. Where once the design of a structure was predominantly governed by the Ultimate Limit State (ULS) design criteria, for tall structures, the Serviceability Limit State (SLS) design criteria becomes as, if not more, important. Therefore, the accurate prediction of the dynamic characteristics are becoming increasingly important. One of these dynamic characteristics is the natural frequency, also called the eigen frequency, of the structure.

The natural frequency is a parameter which is largely influenced by the mass and the stiffness of the structure. One would think that after the completion of structure, that the magnitudes of parameters can be determined with a high level of certainty, and that the natural frequency can be calculated accurately, but this is not the case. When comparing the measured natural frequencies of several high-rise structures in the Netherlands, to their natural frequencies determined in the design phases, an underestimation of between 20\% to 50\% was seen. Although the likelihood that these underestimations will lead to structural failure are small, it does lead to larger design forces and higher peak accelerations, which are used in determining occupant comfort in the structures. The aim of this research is to find the reasons for the discrepancies between the measured and the calculated natural frequencies.

A literature study was performed to determine what the most common methods of determining the natural frequency are during the design phase. There are three main methods which are used throughout different stages of the design phase to approximate the natural frequencies. At the start of the design phase, when structural parameters have not yet been determined, the natural frequency is approximated using empirical formulae. These formulae mostly only depend on 1 or 2 spatial parameters.
As the design progresses and the structural parameters are specified, dynamic beam theory can be used to determine the natural frequency. These calculations take the stiffness of the super- and substructure, and the mass of the structure, into account. As the design nears completion, the structure is modelled in a FE software package. The natural frequency can then be calculated by the software to give a final impression of the natural frequency.

The main parameters influencing the natural frequency of a system are the stiffness and the mass. This is no different for high-rise structures, but how do these parameters affect the natural frequency, and which of these parameters has the greatest effect on the natural frequency? A sensitivity study, looking at 5 existing high-rise structures in the Netherlands, was performed. Each structure was represented by 5 different beam models. One structural parameter was added to each subsequent beam model as to be able to quantify the influence of the added parameter. Lower and upper bounds were determined for each structural parameter. By varying these parameters and calculating the natural frequencies, the effect this variation has on the natural frequency can be determined. It was found that there are 3 parameters which have significant influence on the natural frequencies, namely, the superstructure stiffness, the superstructure density and the rotational stiffness of the foundation.
For all cases with a flexible foundation, the measured natural frequencies could not be reached, even after determining the natural frequencies using the extreme parameter combination, the natural frequencies were still underestimated. The analyses were done for both uniform beam models and multibeam models. The general trend was that the multibeam model produced higher frequencies. This is due to more of the overall stiffness and mass of the structure being situated in the bottom sections of the structure. Using a multibeam can lead to an increase in natural frequency of up to 15\%. Although the natural frequencies were increased, they were still nowhere near the measured natural frequencies.

The underestimation of the natural frequencies using the beams models, led to the question if there are other factors which are not yet taken into account when determining the natural frequencies. In the calculation of the stiffness of the new Erasmus Medical Centre (NEMC), it was assumed that the beams, columns, non-structural elements and the low-rise structure have a negligible influence on the stiffness of the structure. The underestimation in the natural frequencies, led to the conclusion that the stiffness of the structure is underestimated. A complete model of the NEMC was modelled using the SCIA Engineer software. All the structural systems were added to the model. Modal analyses, including different combinations of structural systems and parameter magnitudes, were performed. It was found that for the NEMC the assumption that the beams and columns have a negligible contribution to the natural frequency, was correct. The main contributors to the stiffness of the superstructure were the outer tube and the central cores. The partition walls were added to the model using low stiffness wall elements, by added the walls, the natural frequency was increased by 8.5\%. Assumptions were made to include the influence of the low-rise structure. The determined natural frequency was increased past the measured natural frequency, however, this result might not be realistic.

The final conclusion of the thesis is that the stiffness of the superstructure is underestimated. This leads to the conclusion that there are certain elements which provide the structure with extra stiffness, which is not yet taken into account. At the end of the thesis several recommendations are made as to determine where this extra stiffness comes from.

The increasing global demand of renewable and clean energy has led to the exponential growth, development, and interest in the offshore wind energy industry and expansion towards earthquake-prone areas. Offshore wind turbine structures, typically supported by a tubular monopile foundation, are increasing in size to meet the increasing human demands. In the constant ongoing debate in search of a balance between design accuracy and efficiency, general consensus is yet to be found in search of accurate yet simplified representation of soil-pile interaction. A Winkler foundation principle has shown to be a good compromise regarding this discussion but current knowledge is mainly based on (pseudo-static) small soil-strain inducing wind- and wave load-cases. Adopting this principle, the soil-structure interaction mechanism is represented by local lateral soil reactions (springs with distributed stiffness in mechanical formulation). Strong ground shaking induces shearing and volumetric variation of the soil particles. Through hysteresis the soil material exhibits energy dissipation: hysteretic damping. Accounting for hysteresis is considered computationally more demanding than when the soil continuum is assumed elastic but this is an assumption which only holds when soils undergo very small soil strains.
This thesis explores the amount of energy dissipated as a result of hysteresis during seismic response of an offshore wind turbine and the applicability of such damping in a local linear visco-elastic manner. In order to obtain insight in this nonlinear energy-dissipation mechanism associated with the hysteretic offshore wind turbine model under seismic excitation, a Python code was developed that calculates the energy dissipation of each load-cycle separately. The developed energy dissipation assessment algorithm is effective in application of arbitrary hysteretic response and unloading-reloading rules.
The hysteretic nature of the soil-pile interaction springs in question are calibrated against the widely applied API p-y, force-displacement curves. Unloading-reloading rules are specified to define the load-cycles. Boulanger et al. describes such unloading-reloading rules for pile application under seismic loading. The applicability of these backbone curves and unloading-reloading rules remains questionable in application of rigid monopile foundations. Despite not representing the accuracy of true soil-monopile interaction, obtained results in this research may support the exploration of innovative unloading-reloading rules.
The developed energy dissipation algorithm is proven to be a powerful tool in identifying the amount of energy dissipation over a total timeseries. Reasonable agreement in peak (maximum observed), Ultimate Limit State, deflection and bending moment seismic response at mudline and tower top has been found between a hysteretic supported model and equivalent elastic models using a single (load-dependent and depth-dependent) equivalent damping coefficient in parallel with each soil spring. Representing the hysteretic energy dissipation mechanism using viscous dampers with constant damping coefficients has therefore proven to be an effective modelling strategy to account for the damping mechanism of plastic unloading-reloading rules without accounting for hysteresis. The effectiveness of an equivalent elastic modelling strategy reduces when the response undergoes substantial permanent plastified displacements. A typical property which is unable to be simulated under the application of an elastic modelling strategy.

Pile tip damage due to impact with an object (boulder) during the driving process of monopiles is of main concern in the offshore wind sector. Numerical modelling could accurately describe this failure mechanism and used as a tool in practice but also for the development of appropriate guidelines. In this research project, dynamic Finite Element Analysis is conducted in order to achieve modelling the driving and impact process and properly capture the response of the monopile.