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.

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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...@en

Offshore wind turbines are complex systems that exhibit intricate interactions among the loads, the environment, and the turbine structure. Therefore, wind turbine design significantly relies on the ability to understand this coupled system and accurately estimate the in-service design loads.

A review of the available literature indicates that most of the current understanding of the structural dynamics of offshore wind turbines stems from research on 5 MW turbines. However, as the wind sector progresses towards taller turbines with larger rotor diameters, the interaction between the rotor and support structure becomes more relevant. In addition, with blades more slender, flexible, and lighter than ever before, the complexity of the turbine dynamics significantly increases. Accentuated geometric nonlinear effects, combined with a more sophisticated blade geometry and anisotropic material behavior, contribute to the challenging structural response of modern blades. Hence, there is a need for further investigation into the influence of modern blades on the turbine response and resulting design loads.

In this context, the present study performs a sensitivity analysis on the IEA 15 MW Reference Wind Turbine to investigate which blade parameters have the greatest influence on fatigue loads during normal turbine operation under turbulence. The sensitivities of different blade parameters are ranked, a reflection on the type of relationship (i.e., either monotonic, quasi-monotonic, or non-monotonic) between each blade parameter and the model response is provided, as well as an indicator of the change caused in the magnitude of the loads. One of the goals of this research is to support blade manufacturers in improving their blade designs by tuning the parameters in new blade models to achieve lower fatigue loads at different positions of the turbine structure. Furthermore, this study aims to highlight parameters that should receive special attention during the manufacturing phase, as slight deviations in their values from the original blade design may considerably increase the fatigue loads.

In this work, aeroelastic simulations are performed with HAWC2, and fatigue loads are quantified using Damage Equivalent Loads, while the first-order Elementary Effects method is employed to assess the parameters’ individual effects on the turbine response. Lastly, two strategies are adopted to ensure that the causes of changes in the dynamic response of the turbine are mainly attributed to variations in blade parameters: turbulence files are generated only once per wind condition and reused in all simulations, and the controller is retuned with HAWCStab2 each time a blade parameter is altered.

The findings reveal a dominant influence of the shear centre on the flapwise moment at the blade root, as well as on the fore-aft moments at the tower-top and tower-bottom across different wind speeds. These moments are critical with respect to fatigue loads since they align with the wind direction and experience a high number of load cycles during turbine operation. This research also unveils that even slight differences in the shear centre position along the chord can have a significant impact on the fatigue loads. For the torsional moment at the blade root - a challenging load component in modern blades due to their increasing length, slenderness, and flexibility - the flapwise bending stiffness, followed by the edgewise bending stiffness, exhibit the highest importance levels at below and near rated wind speeds. In contrast, at above rated wind speeds, the importance of the edgewise swept increases significantly, surpassing the influence of both flapwise and edgewise bending stiffness. Lastly, the torsional stiffness typically plays an important role at near and especially above rated wind speeds in several load components at the tower.

Although the findings may vary based on the size, type, and control of the wind turbine evaluated, the results of this thesis are expected to contribute to the load analysis of other turbines.

This research addresses the challenges of installing floating wind turbines on semi-submersible platforms, with a focus on managing relative motions during offshore integration. Traditional methods, which rely on large onshore marshaling yards, are constrained by the limited space available at ports to meet the increasing demands of the offshore wind industry.

In response, the study proposes a shift to floating-to-floating installation, where turbine integration takes place on a moored floater directly at the wind farm. A major challenge in this approach is controlling the relative motions between the two floating bodies. To address this, a multibody model is developed in OrcaFlex, simulating the interaction between a heavy lift vessel and the VolturnUS-S platform. This model highlights the complexity of relative displacements, providing the foundation for introducing concepts aimed at minimizing these horizontal motions.

The chosen concept features a donut-shaped external platform equipped with winches for horizontal motion compensation. In the proposed 10-step installation process, the turbine tower is suspended from a crane while a gripper restricts pendulum motions, and an Active Heave Compensation system manages the relative heave motions. The external platform, suspended below the tower as it is attached to the gripper, is first lowered and secured onto the floater. Once in place, the tower is lowered, with the winches providing the necessary stiffness to control relative horizontal displacements during this phase.

The winch wires are modeled as springs with equal stiffness applied in both horizontal directions. Time-domain simulations in OrcaFlex, validated by an analytical model in MATLAB, reveal that a stiffness of 8887 kN/m is required to effectively limit horizontal displacements to 0.2 m for significant wave heights up to 2.5 m. This design choice ensures 92% workability for this installation step, considering a maximum crane angle of 2 deg and a maximum horizontal relative displacement of 0.2 m. Ensuring functionality under the simulated sea states reveals a design requirement of 1823 kN for the compensation system in the horizontal direction for Hs = 2.5 m and a Tp ranging from 6 to 11 s. While this force exceeds the capacity of a simple winch, it remains within a feasible range for a dedicated compensation system.

This master thesis proposes a method for assessment of the allowable sea states for the single-lift installation of a Wind Turbine Generator (WTG) with a Quick Connection System (QCS) on a bottom-founded support structure with focus on the landing (mating) operation. The thesis also determines the operability of this installation and examines how it could be optimised.

Due to an increase of WTG size, the current lifting height of state-of-the-art floating Heavy Lift Vessels (HLVs) is insufficient to lift the WTG components to the required installation height. To overcome this limitation, and to further reduce installation time, new installation techniques are being developed. The single-lift installation methodology; where the WTG is lifted at the bottom of the WTG and just above the combined Centre of Gravity, in combination with the C1 Wedge Connection; a newly developed connection with a high ULS and FLS strength and large installation tolerances, forms a promising setup for dual crane HLVs. The Quick Connection System (QCS) of the C1 Wedge Connection can create a temporary connection able to provide sufficient restoring moment, required to keep the WTG upright. In this thesis the operability of a floating single-lift installation of a WTG with a QCS is determined and optimised. The performance of the QCS is compared to alternative connections and design improvements for the operation are proposed.

Two computer simulation models are built and compared, where-after the most promising model is expanded and further developed. This OrcaFlex model includes wave and wind loading in in-plane directions (3 Degree of Freedom directions). Nine critical events and limiting parameters are identified, containing motion limits of the WTG and QCS limits.

The results show that the operability for the base case (no Heave Compensation and strict limiting parameters) is negligible. In all cases assessed, the time required to activate the QCS turned out to be the governing limit. If heave compensation is incorporated and some parameters are altered (e.g. increasing the allowed WTG rotation or the maximum C1 Wedge Connection gap between the flanges) the operability can be increased to become feasible for wave peak periods up to Tp < 9s and significant wave heights Hs < 2m. In this optimised situation the activation of the QCS is the governing installation activity, with respect to the load transfer phase situation. Here, a minimum of 16 QCS Wedges are required for the load transfer phase and the pre-activation load transfer should be around 20% of the WTG static weight. For Hs < 1.4m, the mating operation was found to be feasible without a QCS. Here, it should be noted that optimal wind loading was assumed, and zero DP drift is incorporated.

It is recommended to expand the model to include all 6 DOF directions to verify if these results are still valid for non-optimal wind loading conditions. Furthermore, with these results, a study could be set up to determine the economic feasibility of this installation methodology. Such a study is essential as the economic viability determines the adoption of the technology.

Due to the increasing demand of renewable energy to combat climate change, renewable energy is experiencing a rapid growth. To match this demand for renewable energy, especially offshore wind is popular due to its status as a proven technology. To build offshore wind farms in a space and cost-effective way, wind turbines are increasing in size and the farms are shifting towards deeper water. This also has an effect on the monopiles, the most used wind turbine foundation. They are increasing in size and weight to handle the deeper waters and larger turbines. At this moment, monopile installation is limited to approximately 30 meters water depth because of a scarcity of installation vessels with sufficient capacity. To install these large-size monopiles either the fleet of installation vessels must be expanded or new installation methods must be developed.
This thesis addresses the challenge of installing large expected size monopiles without the use of traditional heavy lift vessels, which are reaching their operational limits due to increasing monopile sizes and water depths. The aim of this study is to develop a craneless upending method for a 130 meters long, 13 meters diameter monopile weighing 3500 tonnes, suitable for deployment in waters 50 meters deep.
Five craneless upending methods are evaluated. Two axes of rotation are considered, namely upending by rotation about a fixed point and upending by rotation about a floating point. Furthermore, to rotate the monopiles during upending, a moment can be applied around the axis of rotation and by ballasting parts of the monopile. After considering the four models that follow from combining the given options, a fifth method was developed that includes upending by rotation about a floating point and rotating the monopile by ballasting and applying a moment both.
A numerical model is developed for each concept to analyse the forces, moments, buoyancy, and draft throughout the upending process. The models were verified against each other and validated through a scaled experiment. The experiments highlighted the importance of friction in the upending process and, after adjusting the numerical model to account for the friction, confirmed its validity.
Of the five methods evaluated, the method that incorporates ballasting and applying a moment is found to be the most optimal. Due to combining the two methods of rotation, the water depth and moment required for the upending process can be minimalized.
In this approach, a floating monopile with end caps arrives at the site and is connected to a gripper frame which allows for axial movement and rotation of the monopile. Once the pile is connected, the ballasting phase starts. During this phase, ballast water is pumped into the monopile through the bottom end cap which causes rotation of the pile through the water. When a certain clearance between the monopile and sea bed is reached, de-ballasting starts while applying a moment around the axis of rotation to continue the upending while maintaining the minimum clearance between sea bed and pile.
The findings demonstrate that the presented hybrid craneless upending method is feasible and offers a practical solution to the installation challenges posed for larger monopiles and installation sites at deeper waters.

Offshore wind energy is a pivotal element in the renewable energy transition and achieving net-zero emissions by 2050. Most offshore wind turbines require the installation of a sturdy foundation called a monopile. The current method of installing monopiles offshore is known as piling, which involves driving large steel cylinders into the seabed with a hydraulic hammer. One of the major problems with this method is that it generates high levels of noise, which travel through the water and are harmful to marine life. Therefore, several European governments have introduced laws that regulate the amount of noise that may be emitted.
Multiple solutions have been developed to reduce the amount of noise that is generated but they are costly or negatively impact operational speed. Therefore, GBM Works, a startup in the offshore wind industry, is developing an innovative method for installing monopiles known as the Vibrojet®. The Vibrojet® combines both a Vibrohammer on top of the monopile with a jet at the bottom. The jet aims to fluidise the sand inside the monopile to reduce the friction on the inner shaft. This not only reduces the amount of noise emitted but also increases the installation speed of the monopile and may reduce the required pile dimensions.
This research aims to optimise the Vibrojet® performance during offshore installation by reducing shaft friction as much as possible while minimising jet flow. This approach maximises the installation speed of the monopile while reducing the capacity requirements for all parts of the Vibrojet® system. When installing a monopile while jetting, a soil skeleton forms (soil plug) in the middle of the pile while all sand particles near the inner shaft gets fluidised. The displacement of the surface of the soil plug has been assessed in this research to estimate the plug shape and optimise the performance of the Vibrojet®.
At Deltares a test setup of a soil container was constructed to imitated a 2D version of the inside of a monopile during Vibrojet® installation. The front of the container was made of see-though Perspex to enable analysis of the processes inside the pile. Inside the container holding 1,500 kg of sand, 0.3\% of the particles were ultraviolet (UV) coated to enable tracking. Visual software was written to track these particles with a camera, allowing for the measurement of flow velocities and visualise the surface of the plug shape.
It was discovered that the equation for plug surface displacement overestimated the displacement observed in the laboratory tests. By analysing the results from various installation settings, discrepancies in the equation were identified. The following improvements were recommended to enhance the accuracy of the equation:
• Addition of a seepage force term to the equation
• Addition of separate term for sedimentation effect
• Factor for the return flow of particles
• Influence of the slope angle on flow erosion
A relationship was discovered indicating that the optimal flow rate for Vibrojet® installations can be determined by the total volume of sand that needs to be fluidised. A model was proposed to optimise the performance of the Vibrojet® by assessing the displacement of the plug and utilising this relationship to calculate the connection between the flow rate and the volume of the fluidised zone.

The offshore wind industry has grown significantly since the installation of the first offshore wind farm in Vindeby, Denmark, in 1991. In recent years, the need to expand offshore wind capacity has led to an increased interest in floating offshore wind turbines (FOWTs), especially for deeper waters where bottom-fixed structures are impractical. This research explores the feasibility and behavior of daisy-chaining FOWTs to reduce mooring costs and anchor points, which currently account for a significant portion of installation expenses and are challenging to install.

The study focuses on spar-type FOWTs, investigating how daisy-chaining impacts the motion, stability, and mooring line forces in different wave and wind conditions. By conducting dynamic simulations with OrcaFlex software, two configurations, triangular and hexagonal, were analyzed to assess the effects of connecting multiple FOWTs. Results indicate that the daisy-chaining doesn’t increase the amplitude of roll and pitch motions compared to a single FOWT, but the system is susceptible to resonance in high sea states, causing excessive yaw motions around the z-axis. These resonances occur due to certain modes in the system being excited by peak wave frequencies, leading to amplified loads.

Despite these challenges, the study demonstrates that daisy-chaining can be feasible if the mooring and connection line design parameters are optimized to avoid resonance issues. The forces experienced in the mooring lines were found to be well within the available safe working load of industry-standard steel wire ropes, but careful design considerations are necessary to mitigate fatigue and ensure stability. This research contributes to the limited literature on shared mooring systems for FOWTs and offers insights into potential cost saving and design strategies for future offshore wind farm projects.

The rise in global energy demand, combined with the imperative to reduce CO2 emissions, has driven significant investments in renewable energy technologies. Offshore wind energy is a critical part of this transition, given its potential to provide large-scale clean energy. However, one of the main chal- lenges facing the offshore wind industry is the high levelised cost of energy (LCOE), driven primarily by the costs of operation, maintenance, and the construction of support stuctures and foundations. Reducing the LCOE is essential to make offshore wind energy more competitive. Innovations in tur- bine design and installation techniques can play a key role in achieving this goal. A good example is the introduction of slipjoint connections, which simplify the installation process and reduce material use. Similarly, new turbine concepts, such as the hydraulic pump generator by Delft Offshore Tur- bine (DOT), aim to reduce the weight and complexity of the rotor nacelle assembly (RNA), potentially cutting both maintenance costs and structural demands.

This thesis investigates the effects of the lighter RNA, due to the innovative hydraulic generator pump, on the support structure. The DOT turbine employs a hydraulic pump directly driven by the rotor, eliminating the need for a gearbox and reducing the RNA weight by up to 50%. Moreover, the hydraulic pump uses significantly less space in the nacelle. This reduction provides an opportunity to enhance the support structure and incorporate dampers in the nacelle to mitigate vibrations caused by environmental loads. In the second half of this research, the potential of a gyroscopic damper to improve the dynamic response of wind turbines subjected to wind and wave loads is explored. The focus lays on the reduction of the undamped side-to-side vibrations.

The research objectives include evaluating the impact of the lighter topside on the design of the support structure and evaluating the effectiveness of gyroscopic dampers in reducing fatigue and wave-induced vibrations. To this end a comparison between a conventional wind turbine support structure and the lighter DOT design is made. The dynamic response of the system is analyzed through ultimate limit state (ULS) and fatigue limit state (FLS) evaluations, using simplified, but accu- rate, dynamic models. In addition, the performance of the gyroscopic damper is modeled and tested to determine its ability to reduce the steady-state amplitude of side-to-side vibrations.

The results demonstrate that the lighter topside of the DOT turbine reduces steel use for the support structure by 13%, compared to the conventional support structure. Furthermore, the implementation of a passive gyroscopic damper without any damping elements demonstrates a frequency skipping tool that can easily be tuned by altering the gyricity of the spinning disk. In addition to that, when a damping element is added to the gyrostabiliser, a positive effect in damping the side-to-side vibrations is observed. Compared to the undamped case, a reduction of more than 90% of the maximum bending stress at the mudline is achieved. Additionally, due to the added weight and the ability of a spinning disk to resist changing its orientation, the natural frequency of the total system lowers. A sensitivity analysis of gyricity and mass variations confirms the potential benefits of such a damper to improve the structural integrity and extend the lifetime of the turbine. The findings suggest that further optimization of the damper configuration could lead to more reliable and cost-effective off- shore wind turbines.

The thesis concludes with recommendations for future research, including the exploration of the effects of lighter topsides on the support structure and the implementation of gyrostabilisers in larger turbines, deeper-water and floating turbine applications.

When drag anchors are no longer suitable for anchoring floating wind turbines to the seabed because of unfavorable soil conditions, it is necessary to use drilled and grouted anchor piles instead. For the installation of such anchor piles it is necessary to drill large diameter holes, with diameters ranging between 1 and 5 meters. The drilling method used to construct such boreholes is the reversed circulation drilling (RCD) method. Floating application of this method has only been done up until now in mild wave conditions. To make this operation more economically feasible, it is necessary to increase the workability of this operation in more challenging environmental conditions. A motion compensation device is a vital aspect in this.
A systematic approach was used to determine which type of motion compensation device is best suited for this operation. Numerical models were set up in the commercial software OrcaFlex, where a wide range of different motion compensation types were tested. These motion compensation systems vary in the amount of degrees of freedom compensated and the actuation method. The actuation methods are passively actuated gas springs and position-based motion control using hydraulic actuators.
First, it was identified what combination of degrees of freedom and actuation method is best suitable. This was determined by performing a regular wave analysis and comparing the performances of the proposed motion compensation systems. The performance was indicated by the ability of the motion compensation system to control the weight on bit, reduce the maximum stress as well as the fatigue damage accumulation in the drill string. From this analysis it was concluded that a passive actuated concept that compensates for the heave, roll, and pitch of the vessel is the most effective scheme.
The optimal type of motion compensation system was then pursued by designing two different systems capable of compensating for the heave, roll, and pitch motion of the vessel. The first, relatively simple, type consists of a vertical gas spring and a universal joint where the drill rig can rotate freely. The other type is a passive variant of the Barge Master platform motion compensation system. This second concept was designed using a multi-objective optimization algorithm, the NSGA-II algorithm. The difference between these two systems is that the platform concept has rotational stiffness, while the simple type is free in this degree of freedom.
It was finally concluded that both concepts are viable solutions, and that both systems represent a motion compensation system that compensates for the heave, roll, and pitch motion of the vessel. The performance regarding the indicators is very similar, therefore choosing one of the concepts would come down to the other advantages and disadvantages of the concepts.

Railway tracks are subjected to constant degradation in terms of geometry, and wear and tear of the track components (rails, sleepers, fasteners, trackbed layers, etc.). Over the years, trains have evolved immensely, but the track infrastructure has not kept pace. With rapid advancements in vehicle technologies, the design of rail infrastructure must cope with the challenges associated with the operation of railway tracks. In addition, railway transition zones (RTZs) degrade even faster (4-8 times) than normal tracks, leading to higher maintenance and operational costs. RTZs are areas where railway tracks cross stiffer structures (e.g., roads, bridges, culverts). The amplified degradation in RTZs is mainly attributed to abrupt changes in vertical stiffness and to differential settlement, resulting in amplified and non-uniformtrack responses. Even though the dynamic behavior of RTZs differs from that of normal tracks, the design of track components in these zones remains similar to normal tracks, with some modifications at the superstructure and/or substructure levels to mitigate the transition effects. There have been several attempts to mitigate the adverse effects of dynamic response amplification in RTZs, but they have proven either marginally effective or counterproductive. Therefore, a comprehensive design methodology for RTZs is needed that addresses the main degradation mechanisms leading to amplified degradation of these zones.

In this paper-based thesis, a novel methodology is proposed to design and evaluate railway transition zones. For this purpose, detailed analysis and design optimization is performed for a bridge-embankment transition using various two-dimensional and three-dimensional finite element models, different vehicle models, and surrogate models (using polynomial chaos expansion). Firstly, the proposed methodology establishes a robust design criterion to design and evaluate RTZs. The criterion relates the magnitude and uniformity (spatial and temporal) of the total strain energy in the trackbed layers to the permanent deformation of RTZs. This novel energy-based criterion is used to evaluate the most commonly used mitigationmeasures at the superstructure and substructure levels and to investigate the key phenomena governing RTZ design. Based on insights obtained from these analyses, a preliminary design of a novel transition structure called SHIELD (Safe Hull-Inspired Energy Limiting Design) is proposed. The second phase of the work is dedicated to identifying the most influential design parameters leading to optimized geometry of SHIELD and the desiredmaterial characteristics. The third phase involves the performance evaluation of optimized SHIELD subjected to critical loading conditions (e.g., critical and supercritical velocities, different directions of movement, hanging sleepers, non-straight rail) and SHIELD is shown to be a robust design solution for all conditions under study. In the end, the use of SHIELD is extended to another type of an embankment-bridge transition (with ballast running over the bridge) where it is shown to be equally (compared to embankment-bridge transition with no ballast layer over the bridge) efficient in mitigating the transition effects, and a laboratory experiment is designed to test the effectiveness of the proposed design criterion and methodology. A robust design methodology for RTZs is proposed in this work, which aims to minimize operation-induced degradation and can be adapted to different transition types and sitespecific conditions. A preliminary optimized design of SHIELD is proposed, which has been shown to be effective in mitigating dynamic amplifications in RTZs under both ideal and non-ideal conditions. The results and conclusions presented in this work demonstrate the promise of SHIELD as an intervention for railway transition zones, outline the next steps toward its practical implementation, and highlight the challenges that need to be addressed in future research.
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Underground tunnels are important infrastructures due to diverse applications in civil engineering. The dynamic behavior of the underground tunnels when exposed to seismic waves or the passage of high-speedmoving trains is of particular interest. Amplifications of displacements and stress concentrations may occur due to wave scattering and wave interference. Engineers concern about the vibration stability of moving trains as so-called anomalous Doppler waves may be generated when trains move at high speeds; the corresponding energy is fed into the vibration of the vehicle. Therefore, the traintrack- soil interaction should be properly considered to predict when the vehicle vibration becomes unstable. This thesis aims to present a semi-analytical solution for the response of a half-space with an embedded tunnel subject to seismic waves and to analyse the vibration stability of high-speed trainsmoving through that underground tunnel. Previous studies indicate that the method of conformal mapping is a promising analytical method to solve the two-dimensional (2D) wave scattering problem due to its computational efficiency and accuracy. Thus, the first objective of this thesis is to extend the method of conformal mapping to three-dimensional (3D) case and systematically evaluate its performance. Results reveal that inaccurate results maybe obtained, particularly at high frequencies. This observation motivates the second objective of this thesis, which focuses on verifying the accuracy of the specific application of the method of conformal mapping in which thewaves scattered fromthe half-space surface are represented by cylindricalwaves that originate from an image source of a priori unknown intensity. To this end, a simpler 2D model is considered, involving a cylindrical cavity embedded in an elastic half-space subject to a harmonic anti-plane shear wave. The performance of the indirect Boundary Element Method (indirect BEM) is evaluated too for this model in view of the choice of the appropriate solution method for the second type of dynamic problem considered in this thesis. For this second type of dynamic problem, due to the identified inaccuracies at high frequencies for the 3D problem, the indirect BEM is utilised to investigate the stability of vibrations of an oscillator moving at high speeds through a tunnel embedded in soft soil, which is the third objective of this thesis… @en

Offshore wind turbines, given their structural properties, are expected to experience severe ice-induced vibrations. However, full-scale events neither have been observed nor published yet and thus predictions of existing numerical models could not yet been validated. Model-scale experiments, aiming to investigate ice-induced vibrations of offshore wind turbines, have been inconclusive as structures with low natural frequencies would exceed the capacity of test facilities (i.e., size and weight limits), while geometrical scaling introduced scaling effects (e.g., buckling) compromising the validity of conducted experiments.

In the absence of full-scale testing capabilities, the main goal of this work was thus to demonstrate how offshore wind turbines behave under dynamic ice loads in smallscale experiments. In total four research questions (RQ) have been formulated and are addressed in this thesis, collectively serving to achieve the primary objective of this thesis.

RQ1: How can ice-induced vibrations of vertically sided offshore structures be scaled?

RQ2: How can offshore structures with low and multiple eigenfrequencies be tested in
ice tank experiments?

RQ3: What types of ice-induced vibrations can an offshore wind turbine experience?

RQ4: What is the effect of wind-ice misalignment on the development of ice-induced
vibrations of offshore wind turbines?@en

The imminence of anthropogenic climate change has motivated a global energy transition towards sustainable power generation. Offshore wind—an important contributor to the energy transition—is expanding, not only in turbine size and number of installations, but also into regions with harsher environmental conditions. One of those conditions in places such as the Baltic Sea is drift ice. Offshore wind turbine support structures, with vertical sides at the waterline, must be designed to survive dynamic ice-structure interaction when ice fails in crushing against the structure. For a safe and efficient design of the support structure, dynamic ice-structure interaction resulting in ice-induced vibrations must be considered. Therefore, both an understanding of the problem and accurate modeling for the prediction of the development of ice-induced vibrations are required.

Significant progress has been made in recent years on the topic of ice-induced vibrations, and a numerical model for prediction of ice-induced vibrations has been developed based on the principles of velocity-dependent deformation and failure behavior of ice, and contact area variation between ice and structure during interaction. However, uncertainty remains regarding physical mechanisms within the ice which govern ice-induced vibrations. The ice mechanics involved in the development of ice-induced vibrations is therefore the main topic of this thesis.

The main objective was to investigate and identify the ice mechanics involved in the development of ice-induced vibrations, especially in the regime of frequency lock-in as historically defined. It was hypothesized that dynamic recrystallization played a relevant role in the ice mechanics involved in ice-induced vibrations. To test the hypothesis, ice mechanics experiments were performed at the ice laboratory specifically developed at Delft University of Technology for this purpose.

To identify grain-scale mechanisms in ice, such as dynamic recrystallization, a method was devised to elucidate ice thin section textures and (quarter) fabrics by means of crossed-polarized transmitted light and interference coloration of ice. An attempt was made to apply the method to the laboratory experiments which applied compressive loading to the edge of a thin freshwater columnar-grained ice plate, laterally confined by glass plates. Crossed-polarized transmitted light was shone through the glass plates to observe the grain structure of the ice during cyclic compression with a haversine velocity waveform. The loading and confinement scenario was intended to reproduce a vertical section of the ice edge during frequency lock-in vibrations. The experimental design demonstrated that the grain-scale mechanics of dynamic recrystallization did not obviously contribute to the peak load-velocity relation associated with frequency lock-in vibrations. As expected, fracture initiated on the grain scale was responsible for load drops. But, more interestingly, stress relaxation during periods of low relative velocity between ice and structure occurred rapidly. Following the stress relaxation, when velocity increased, the peak load was higher than previous brittle peak loads. The results indicated that the mechanisms involved in the stress relaxation were occurring on a scale smaller than the grain size. A loading path dependency was also observed with respect to the peak load-velocity relation.

Ice penetration experiments at the Aalto Ice and Wave Tank in ethanol-doped cold model ice were performed with a rigid structure, controlled oscillation, and a single-degree-of-freedom structure, and comparison of results showed that the peak global ice loads depended on the amount of time spent at low relative velocities where an ice strengthening effect developed. This has implications for the so-called velocity effect and compliance effect in design of structures subject to dynamic ice-structure interaction.

Overall, the load signals from the ice mechanics experiments on freshwater ice resembled the load signals obtained from the controlled-oscillation experiments from the model-scale ice tank tests. The qualitatively similar velocity and resulting load patterns give confidence in the idea that the mechanisms involved in both types of experiments were similar, even for different ice types and loading scenarios.

These similar results demonstrate a link in the ice mechanics across different ice types and loading scenarios, which may be explained with further research on path-dependent constitutive ice behavior, and with scrutiny regarding ice dislocation and grain boundary mechanics. Suggestions for future research are proposed, including the testing of strain rate-varying uniaxial compression of ice and ice penetration experiments with haversine velocity waveforms.@en

The ongoing global energy transition has led to significant growth in the offshore wind sector. This growth has resulted in a high demand for offshore installation vessels like the 'Aeolus', and the need to push up their limits to meet the rapidly changing market. In the Saint-Brieuc offshore wind project, the Aeolus had to be installed on a stiff seabed, a situation that was expected to induce high impact forces on the jack-up legs. To mitigate these forces, leg modifications were installed to increase the operational limits for jacking operation. However, Van Oord's operational limits for installation on stiff seabed conditions differed from DNV's Joint Industry Project, leading to the need to reassess the necessity of the leg modifications.

The objective of this research was to identify the main physical phenomena that govern the forces in jack-up legs during installation on a stiff seabed, and to understand how these physical factors and a specific leg modification could influence them. This research followed a systematic approach to address the research objective, comprising a literature study on impact mechanics, model development for axial impact forces, model validation, a sensitivity study, and an analysis of the leg modification. Insights from the literature study guided the development of the model to analyse the governing forces. Validation was performed by using field data from the Aeolus, specifically oil pressure readings from the vertical jacking cylinders and vessel motion measurements.

The research found that what governs the forces in jack-up legs during installation on a stiff seabed depends on the energy contained within the system prior to impact and how this energy is absorbed by the soil and structure. Key factors include wave-induced vessel motions, wave height and period, hull characteristics, vessel's inertia, jacking velocity, and the leg length below the hull. The energy absorbed by the soil and structure depends on their stiffness characteristics and behaviour, which primarily concluded that axial forces dominate over lateral forces during impacts on a stiff seabed. However, lateral forces should be considered in deeper waters or where seabed protrusions are anticipated.

The sensitivity analysis indicated that the vessel mass, initial velocity, and normal soil stiffness significantly influence the impact force, with the initial velocity predominantly influencing the impact force, and the overall system stiffness dictating the impact duration. By introducing the leg modification, the impact force was reduced by approximately 70-75%, along with an increased impact duration of 320-370%. Moreover, it was found that the impact force and duration are interconnected rather than separate occurrences. The leg modification also reduced the influence of soil stiffness variability. Furthermore, for a more realistic representation, incorporating the nonlinear behaviour of soil load-deformation and leg modification characteristics is needed, but this would increase the computational demand. Consequently, this suggests the need for advanced solutions and an engineering assessment to balance desired accuracy against computational complexity.

An analytical study of an energy-based approach to damping of a clamped timber column in free vibration under axial, lateral and torsional loading conditions was conducted. Backed by experiments it was confirmed that an energy-based approach, as proposed by Sánchez Gómez (2018), could describe the energy dissipation in a laterally vibrating timber column. With the help of an energy balance equation, the energy flow in a timber column was formulated including any energy dissipated during vibration. It was found that the energy in the system could be approximated by an exponential function for which a new dissipation constant E_D (s-1) was introduced. For lateral vibrating columns of Norway spruce, typical values of E_D were 0.4 - 0.6 s-1. Furthermore, a case study on a timber high-rise building demonstrated
that damping has a significant influence on reducing peak accelerations which, in turn, could potentially lead to lowered costs. This largely untapped design lever warrants significant research and design improvement for these timber high-rise structures and, hence, several recommendations for follow-up research are offered.

The offshore monopile decommissioning demand will become definite in the coming years. Our responsibility is to ensure the rights and duties of other legitimate uses by completely removing the ageing monopile from the seabed to continuously redeveloping offshore wind farms within the same location. The growing number of past, present, and future monopile installations opens up the challenges and opportunities to be responsible and lead the decommissioning market. With the goal of complete removal, a novel GDP technique can be the win-win solution for offshore wind operators and contractors to extract the monopiles completely from the seabed using torsional and axial vibration
This thesis seeks to understand the torque and normal force to safely clamp a monopile during a torsional vibration so that the monopile continuously slips over the soil. Gradual soil failure along the pile-soil interface's full depth due to the monopile's torsional motion is a possible theory to explain the failure mechanism. When an upper part of the pile successfully moves relative to the soil, kinetic friction occurs until the soil resistance is larger than the shearing at one point. If more shearing is added by adding more torque, more layers below will be broken while the upper part keeps sliding due to lower friction than static friction. While the linear elastic theory of solid and thin shell bodies is used within a 3D FE modelling in Ansys to couple the soil and pile, the clamping force due to the GDP shaker is decoupled from the analysis. Failure criterion is defined outside the simulation so that the gradual soil failure is done through several simulations assuming discrete soil layers.
The FE model is constructed and verified by analytical calculation through the semi-infinite cavity-pile-soil, wave reflection, and finite cavity-pile-soil-spring-dashpot problems. Several cases of gradual soil failure are simulated and show that the torque amplitudes form a distribution. Firstly, a probabilistic sense is proposed to interpret the torque amplitude and search for the optimum depth of the soil failure. Secondly, a convergence check is made with the help of an analytical shell-spring by considering more soil elements by virtue of good correlation of the shear stress between the analytical and FE model. It eventually suggests that a convergence of the torque amplitude can be achieved, which reinforces the theory of gradual soil failure. The interpretation suggests that the current GDP shaker is one step closer for a monopile extraction test with typical monopile dimensions that correspond to a typical 1 m diameter. A first approximation of the required torque and clamping force is then proposed to benefit the analytical model for larger diameters up to 6 m.

In the last decades, the urgency for sustainability has dawned on humanity. The increasing pressure for more renewable energy sources is accompanied by the success of the offshore wind turbine. As a result, the installation of offshore wind farms has become a major business. The challenges posed during installation become more complicated with larger turbines. Forces exerted on taller and softer support structures and longer blades limit the installation options of the wind turbine components even more.
Nevertheless, the dimensions of wind turbines have grown explosively over the years and this growth is foreseen to continue.
To overcome the growing challenges of blade installation on offshore wind
turbines, it is crucial to better understand the forces and the induced motions. Insufficient understanding of the behaviour of the support structure and blade during installation of the latter could lead to over-conservatism of an already vastly time-constrained process, resulting in loss of workable weather and unnecessary costs.
The objective of the research conducted in this thesis is to develop a dynamic model of a 20 MW offshore wind turbine to quantify motions at the blade-hub interface during blade installation and examine potential mitigations. When the objective is reached, a good comprehension of the blade behaviour is achieved, allowing for exploration of solutions to reduce the observed motions and increase workability.
A 20 MW wind turbine does not exist as of writing, but with a scaling assessment a configuration was composed. The future wind turbine is estimated to have a hub height of 154.7 m and a blade length of 131.5 m with an estimated weight of slightly less than 90 tonnes. Due to offshore placement of the wind turbine, the length of the support structure is estimated to be extended with a water depth of 35 m. The problem has been further delineated by assuming the weather conditions of a typical North Sea offshore site at the boundary of what is currently possible for blade installation. Installation is assumed to be conducted with a floating vessel, posing even more challenges.
Modelling the installation of a blade is subdivided into examining the behaviour of a wind turbine support structure and a blade suspended from a crane separately. The performed simulations are conducted in 3D, with the support structure and blade being multi-body systems excited by unidirectional
waves, wind and current. Calm, normal, and rough weather conditions are assumed as input conditions to examine the environmental impact. A combination of the results of the models simulating the multibody systems provide the relative motions between the support structure and the blade, resulting in potential impact velocities in the horizontal plane. The model is solved in the time domain, because it allows for a better understanding of the input and output variables...

This thesis explores the feasibility of using a helical pile foundation with a tension leg mooring system to support a 15MW floating offshore wind turbine and assesses its financial viability compared to alternative anchor concepts.

The study commenced with an extensive literature review, drawing from a diverse range of sources, including reviews, offshore guidelines, research papers, and expert interviews. The literature review was conducted to enhance the understanding of the use of helical piles in the offshore industry, encompassing their fundamental characteristics, current applications, and potential contributions. For this investigation, a Tension Leg Platform (TLP) featuring a 15 MW wind turbine, engineered by Heerema Engineering Solutions (HES), was employed. Time-domain simulations, accounting for environmental conditions, were carried out using the OrcaFlex software. These simulations led to the determination of the mooring line tensions, with the maximum value identified as the design load case. During the helical pile geometry optimization process, which focused on maximizing uplift capacity, it was established that a single helical pile in dense sand can achieve a maximum uplift capacity of 7.91 MN, while taking into account geotechnical, structural, and installation constraints. As a result, considering the significant uplift capacity demands of TLPs, it is essential to employ helical pile grouping, necessitating a minimum of four helical piles. Consequently, the design of the helical pile group anchors confirms their ability to meet the uplift and lateral capacity requirements of TLPs. Furthermore, the exploration of variations in equipment, steel strength, helical pile geometry, and soil conditions has yielded promising results. It is important to note that achieving the required installation depth for these group anchors involves a significant force and torque during the installation process, presenting potential challenges. Consequently, the development of equipment capable of meeting these installation requirements is vital for ensuring the feasibility of these helical pile group anchors.

The performance of helical pile group anchors was analysed from economic, technical, and environmental perspectives, with a comparison to suction and driven pile anchor concepts. The economic evaluation showed that the estimated total costs for the 4-pile helical group anchor, utilizing a singlepile installation method, are notably higher when factoring in the costs related to developing helical pile equipment, making it less financially attractive compared to the developed suction and driven pile anchor concepts. However, when excluding these equipment costs, the 4-pile helical group anchor ranked as the second-most financially attractive option, regardless of the installation technique, with only the single driven pile anchor being less expensive. Notably, it was discovered that, in a scenario where structural, geotechnical, and installation constraints are disregarded, the single pile helical anchor emerges as the most financially attractive among all anchor types, even surpassing the single pile driven anchor. This suggests that overcoming challenges related to scaling up helical pile dimensions, like enhancing their structural integrity, and reducing installation requirements through innovative designs, could make the use of fewer piles in a helical pile group anchor a feasible choice. In addition to these quantifiable factors, helical piles offer the advantage of a low-noise installation method, which becomes increasingly important due to the growing noise disturbance legislation in certain regions. Furthermore, their adaptability to various ground conditions through flexible installation techniques makes them a convenient choice for sites with limited access or specific inclination requirements...

The offshore wind energy industry is quite literally reaching for the sky to meet the increasing demand for renewable energy in Europe. Offshore wind turbines are sprouting out of the waves off the coast of many European nations and their size is increasing rapidly. As the turbines become larger and the waters in which they are placed become deeper, the foundations that carry these behemoths must grow as well. The most frequently used foundation is the monopile foundation. Over the years, both the length and the diameter of these monopiles have steadily increased. Installing monopile foundations has, as a result of this, become more and more difficult.
An important aspect of the installation of monopile foundations that has become more challenging is the upending of the monopile. When the monopiles are transported to the site of installation, they are sea-fastened to the deck in horizontal orientation. When they arrive, they need to be rotated to a vertical orientation for installation. This can be done with the aid of an upend hinge. This thesis focusses on the modelling of such a hinge for operability studies.
The goal of these operability studies is to determine the weather and wave conditions in which the operation can be safely performed. Conventionally, the stiffness of the upend hinge is approximated by several linear springs between the vessel and the monopile. The aim of this thesis is to apply dynamic substructuring to existing Finite Element models of a hinge to more accurately describe its dynamic behaviour within hydrodynamic simulations. Hereafter, the response of the dynamically substructured model can be compared to the response of the conventional model.
The findings of the research show that it is possible to use dynamic substructuring to reduce the Finite Element models of an upend hinge and implement them into hydrodynamic simulations of an upend operation. When comparing the conventional model to the substructured model, it can be seen that there is a significant difference between the high-frequency response of the models. However, the responses of the models to prevailing ocean waves are very similar. For operability studies, this response is most relevant. Furthermore, the required computation time is significantly higher for the simulations employing the dynamically substructured models than for the conventional models. Therefore, it is not recommended to apply dynamic substructuring to model this upend hinge in operability studies. For situations where the high-frequency response is more relevant, however, dynamic substructuring may prove a valuable tool to more accurately describe the dynamic properties of an upend hinge or other marine equipment.