Floating Offshore Wind Turbine (FOWT) unlock far-offshore wind resources in deep waters that can’t be harvested under economic aspects using bottom-fixed wind turbines. Numerical modelling tools are employed to assess different FOWT designs under various environmental conditions. In order to be competitive and yet guarantee working designs, the numerical tools need to be reliable and computationally efficient. This justifies the need for assumptions simplifying the models.

One such simplification is the rigid floater assumption, where the FOWTs’ substructure is assumed rigid. This reduces the computational effort but at the same time alters the results, like the tower’s first natural frequency and corresponding mode shape.

Multiple approaches are used in literature to match the tower’s first natural frequency of flexible and rigid floaters. This involves adjusting the tower properties, such as length or Young’s modulus, or alternatively, implementing a flexible element between a rigid floater and a flexible tower. Siemens Gamesa currently makes use of the latter method by tuning the flexible elements’ properties to achieve a match in the models’ tower first bending natural frequency.

So far no studies have been conducted on the impact of the flexible element parameters on the tower first mode shape when tuning towards a matching tower first bending frequency. Additionally, the effect of differently correct mode shape variants on the tower dynamics is investigated. This leads to the goal of improving the tower dynamics for a FOWT with a rigid substructure.

The analysis was based on two versions of the U-Maine FOWT model. One fully flexible floater design served as a reference, whilst a fully rigid floater design was used to incorporate the different flexible element designs. Various flexible elements with distinct properties were evaluated to understand the sensitivity of the mode shape to these parameters. Subsequently, selected designs exhibiting varying degrees of accuracy of the mode shape were compared to the flexible floater design in time domain simulation. Furthermore, three separate methods of identifying the tower’s first bending mode are proposed.

In the course of modelling the flexible floater design, a modelling error was made that resulted in double counting of the heave motions. Despite this error, it was concluded that for constant and turbulent wind, all flexible element designs outperform the rigid floater design. Furthermore, a close mode shape match likewise results in an increased match of bending moment and tower top rotation for high and low wind speeds. In the range of rated wind speeds, the shortest flexible element design with the worst mode shape match performs best. Comparing the tower top acceleration also indicates an overall improvement of the results, but less significant. This is expected to result from tuning towards the first tower mode rather than higher-order modes.

Generally, using any flexible element design already results in an improved mode shape match with minor differences. The impact of these discrepancies on the tower dynamics is small. Therefore, it is concluded that any flexible element, even when only tuned to match the tower’s first natural frequency, is an improvement over the rigid floater design.

This thesis investigates the use of Multifidelity and Stochastic Polynomial Chaos Expansion for the assessment of fatigue damage in Floating Offshore Wind Turbine (FOWT) mooring systems. Specifically, the work aims to improve efficiency in providing uncertainty quantification, a probabilistic approach to computer experiments, by balancing prediction accuracy with computational cost. This is crucial for the design and analysis of complex FOWT configurations, where high-fidelity simulations are expensive. The research adopts two approaches: multifidelity (MF) modeling and Generalized Lambda Models (GLaM). While assuming an underlying polynomial structure in the data already provides a speed-up over conventional Monte Carlo estimates. The MF framework builds further on this Polynomial Chaos Expansion (PCE) by employing multiple models of varying resolution, from computationally inexpensive approximations to more accurate versions of the same solvers. Certain optimizations are applied within this method, sparse basis selection, Gaussian process regression, and basis adaptivity, which enable further speed-up and improved accuracy. In addition, when the seed is not fixed (i.e., when the stochastic nature of the problem is preserved), GLaMs are used to approximate the entire output probability distributions. Once trained, these PCE models also allow straightforward sensitivity analysis of the input-output relationships. The primary quantity of interest is mooring line fatigue damage, which results from the cyclic environmental loading experienced by the FOWT. Input uncertainty arises from environmental parameters such as wind speed, wave height, and peak period. Since wind and waves are correlated, their joint distributions are modeled using copulas. Validation is performed using the irregular Ackley function as a benchmark, alongside two realistic mooring case studies that compare quasi-static (MoorPy) and dynamic (MoorDyn) models, and investigate the impact of simulation duration on the fidelity hierarchy. Results for the Ackley benchmark confirm that the MF algorithm captures uncertainty effectively and matches the accuracy of single high-fidelity surrogates, while reducing computational cost, provided the polynomial order and truncation norm are tuned to balance bias and variance. Initial mooring studies show that quasi-static models replicate mean tensions but systematically underpredict fatigue damage. While the MF framework shows promising results for these mooring models, further investigation is needed to address and improve its limitations. The MF approach provides comparable prediction accuracy and slightly reduces the number of required dynamic samples for simpler configurations such as fairlead motion, but its performance degrades in more complex coupled two-dimensional FOWT simulations, where low-fidelity surrogates fail to capture essential coupled dynamics, limiting the effectiveness of MF corrections. Similarly, fidelity hierarchies based solely on varying simulation durations show limited benefit and require further refinement. Overall, with the added cost of the low-fidelity model, there is no clear cost benefit. Results from the GLaM framework also remain subject to further investigation due to limited time.

Mangroves play a vital role in coastal ecosystems by dissipating wave energy, protecting against erosion and supporting biodiversity. Despite their importance, quantification of their attenuation capabilities remains challenging. This study explores the potential of a two-dimensional (2DV) OpenFOAM model to simulate wave attenuation through mangrove forests using a layered approach to capture the vertical variability in mangrove frontal area.

The model is first validated against flume measurements under a range of different hydrodynamic conditions to assess its capability in a controlled, well-measured environment. The goal is that by applying the Reynolds Averaged Navier Stokes equations and a layered approach, a more realistic bulk drag coefficient can be calibrated for different hydrodynamic conditions. One of the objectives is to explore whether this approach can yield a stronger correlation between the bulk drag coefficient (Cd) and the Keulegan-Carpenter (KC) number.

Then, the model is applied to a real-world case study for the Mandai mangrove forest on the north-west coast of Singapore. his application demonstrates the model’s ability to simulate complex coastal environments, including variable bathymetry, forest extent, and wave conditions. A sensitivity analysis is also conducted to evaluate the effect of different parameters on the modelled wave attenuation

Results confirm the model's capability to accurately represent the vertical variations in vegetation structure and their influence on wave attenuation. It showed great agreement with the measured wave attenuation of the flume after calibration. Despite its expected improvement on the Cd-KC relation, it did not succeed in this, but it does reveal the impact of depth-variable drag forces and their effect on the velocity profile and wave attenuation. Furthermore, the case study showed its applicability to real-world coasts. Despite numerical dissipation standing in the way of any quantitative analysis, the runs in combination with the sensitivity analysis provided valuable insights into the model's behaviour and wave attenuation predictions.

This thesis investigates the hydrodynamic behavior of a barge-type Floating Offshore Wind Turbine (FOWT) equipped with a central moonpool, with particular focus on the influence of the damping lid method, a numerical approach used to approximate viscous damping in potential flow simulations. The damping lid factor ε was varied systematically to evaluate its impact on moonpool resonance, platform motions, and mooring system loads under both idealized and realistic sea states.
Hydrodynamic coefficients and motion response amplitude operators RAOs were first analyzed in the frequency domain using DIFFRAC, revealing strong sensitivity of the system to internal resonance modes when no damping is applied. The time domain simulations were then performed in aNySIM to assess the mooring tensions and the motion behavior under regular and irregular wave conditions. A minimum damping lid factor of ε = 0.035 was identified as sufficient to suppress unrealistic resonance amplification.
Finally, a case study for a North Sea scenario was carried out to compare the performance of the system between ε = 0.035 and ε = 0.1 in terms of serviceability and operational availability. Results showed that once the critical damping threshold is reached, the system response becomes relatively insensitive to further increases in ε.
The findings demonstrate that the damping lid is an effective and practical tool for modeling moonpool effects in barge-type FOWTs, and that a carefully selected damping factor for this specific barge type, can eliminate the need for more computationally intensive viscous simulations when evaluating global system behavior.

In this thesis report, we investigate the difference in using physics-compatible elements and standard elements in the context of fluid flow through porous media. For this, we compare the H(div)-conforming Raviart-Thomas elements to Lagrange elements. Our model involves a channel flow over a porous media, resulting in a fluid and a porous subdomain. We distinguish between three approaches, as based on a research paper. The first is the one-domain approach called PE, which sets one equation on the entire domain. The other two approaches are two-domain approaches, which use different equations in the two subdomains, with interface conditions to couple them. The first of these is NSD, which couples Navier-Stokes with the Darcy equation. The second is NSF, which does the same but also incorporates the Forchheimer term, which models inertial effects.

In general, we were not able to make a completely successful model, as the PE case did not match the paper, and the results found when using Raviart-Thomas elements had some unexplained anomalies. We were also unable to carry out an accurate error analysis. We were able to see that Raviart-Thomas produced an exactly divergence-free solution, as expected from the theory, at the cost of more computing power and time needed.

As offshore wind energy expands, the environmental impact of monopile foundations on ocean stratification has become a growing concern. Thermal stratification occurs when layers of seawater with different temperatures form due to solar heating, creating a stable density gradient that limits vertical mixing. However, monopile-induced turbulence can disrupt this layering, a process known as destratification, which may have significant ecological consequences.

This study investigates a novel perforated monopile design as a potential solution to mitigate these effects. Using Computational Fluid Dynamics (CFD) simulations in OpenFOAM, the research compares the hydrodynamic behavior of regular and perforated monopiles, focusing on turbulence intensity, wake formation, and destratification rates.

The results indicate that perforated monopiles reduce turbulence intensity and modify wake dynamics compared to regular monopiles, by allowing partial water flow through the structure and reducing recirculation zones. However, their impact on destratification rates remains minor. While perforations alter hydrodynamic behavior, they do not significantly mitigate stratification breakdown.

These findings suggest that while perforated monopiles influence flow dynamics, their effectiveness in reducing destratification is not significant enough to definitely recommend these structural modifications. Future research should explore different perforation patterns and other alternatives develop more sustainable offshore wind turbine foundations.

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

In the current energy market, where increasing demand for renewable energy is met with larger turbines which require larger installation vessels, understanding operational limits of such vessels becomes invaluable. For the installation of wind turbine generators at great heights, a stable working platform is offered by jack-up (JU) vessels, which use legs to elevate the hull and thus crane of the vessel above the waves. The operation taken into consideration is the preloading process where load is increased on the legs to ensure enough foundation capacity for later operations such as lifting of wind turbine generators. Traditionally, vessels have only been positioned above flat seabeds, but with the increased demand in the wind sector, unfavorable locations with sloped seabeds also need to be considered.
In order to capture global behaviour of a JU on a sloped seabed, two modelling methods are combined to create a framework; soil-structure interaction and structural finite element model. This framework implements a sub structuring approach by cutting and simulating a single leg on a sloped seabed using soil-structure interaction modelling. The results of the first method are foundation stiffness values and reaction loads during preloading of the JU while the second method takes those results as input to calculate internal loads of the JU.
The results show that the proposed framework is able to fulfill the task of combining two models and obtain meaningful results that can inform the JU during preloading phase. Then with this framework, the results show that sloped seabed induces an extra set of moments and lateral loads due to the asymmetric seabed causing an eccentric reaction load. These moments are then redistributed through the JU to other legs and their footings.
These results imply that for a JU on a sloped seabed, a framework combining geotechnical and structural domains can be applied to show the increased load due to the sloped seabed. Additional to the results, a sensitivity study is implemented for a variation of soil type and soil-structure geometry, the latter providing a more realistic seabed slope and larger reaction loads.

Rising global CO2 levels have created a growing need for renewable energy resources. Offshore wind energy is a promising solution, with monopile (MP) foundations preferred for their relatively low cost, provided the site conditions are suitable. Recently, wind turbines have significantly increased in size for greater cost efficiency, leading to the development of 3XL MPs with diameters of 12 meters or more. Depending on the site and marine contractor preferences, a jack-up vessel may be used to install a 3XL MP. Marine contractors typically employ dynamic models to determine suitable weather windows for each installation phase. Lowering the 3XL MP through the splash zone and gripper to just above the seabed is a critical phase. To optimize the dynamic models for this phase, understanding the 3XL MP’s behavior during splash zone lowering is essential. This thesis therefore investigates the dynamic behavior of a 3XL MP during lowering phase through the splash zone until just above the seabed using a jack-up vessel.
A 1-DOF model, considering only the rotational DOF of the MP, is established to examine this behavior. Results from this model show that the most probable maximum (MPM) of the MP rotational motion response, evaluated with a dynamic lowering simulation for irregular spread waves, is 75% lower compared to the conventional steady-state simulations at critical MP drafts. Additionally, the dynamic behavior of the MP is highest for increasing peak periods, as these coincide with the natural periods of the 1-DOF model at larger MP drafts where wave-excitation loads are greatest. Several physical phenomena which are not included in the model are examined to assess their impact. Ambient currents traveling in the same direction as irregular unidirectional waves during a dynamic lowering simulation, reduce the MP motion response by 25%. Vortex shedding frequencies do not coincide with natural periods and are thus neglected. The same goes for the 2nd order difference frequency wave drift forces. Both the dynamic amplification and the mean motion displacement due to these second order wave forces are negligibly small. Lastly, the influence of jack-up motion is negligibly small during peak periods of 8 and 9 seconds but reduces the MP rotational motion during a peak period of 7 seconds.

The exploration of bottom-founded offshore wind turbine development, focusing on monopiles, indicates that while the floating offshore wind sector is emerging to exploit deeper sites, there is a continued rationale for advancing XXL monopiles in the realm of bottom-founded offshore wind. Cost-effectiveness stands out as a pivotal factor among several compelling arguments.
This study aims to improve the economic viability of XXL monopiles by developing a more accurate methodology for calculating wave loads. The introduction of time-varying hydrodynamic coefficients is central to this effort. Building on an extensive literature review, a novel time-varying coefficient method was proposed. This method accounts for subperiodic fluctuations in wave force coefficients by introducing dependencies on the Keulegan-Carpenter and Reynolds numbers, which are determined using zero-crossing periods. This approach marks a significant methodological shift from previous attempts.
The newly developed method was implemented in a MATLAB environment and is compatible with both first and second-order wave theory, as well as the Morison and Rainey load models. Efforts were undertaken to replicate the results of previous experiments. The model successfully reproduced both regular and irregular waves, with particularly accurate results for the least steep regular waves. However, for steeper waves, the second-order wave theory did not fully capture all characteristics, leading to less accurate reproductions.
A simplified fatigue assessment was carried out using MonoPoly, an in-house software package developed by Sea and Land Project Engineering (SLPE). The North Sea Wind Farm, a reference project by SLPE, was used for site characterization and structural configuration.
The fatigue assessment evaluated various test cases to examine the impact of different analysis approaches in the time-varying coefficient method model. Overall, results showed that the time-varying coefficient method decreased the maximum Damage Equivalent Moment (DEM) values by 0.2 to 0.7 percent, depending on the analysis approach. The mean DEM values decreased by 0.2 to 0.5 percent. These findings apply to a more advanced constant coefficient method used in the industry, compared to the one recommended by ISO. A side study suggested that the decrease in DEM values would be even greater when comparing this new method to the ISO-recommended method.
For thin-walled cylinders like monopiles, the relationship between moment and thickness is essentially linear. Therefore, a 0.5 percent reduction in mean DEM allows for a 0.5 percent reduction in monopile thickness while maintaining stress levels. This reduction in material thickness has the potential to save several million euros for wind farms, demonstrating the significant economic benefits of the newly developed time-varying coefficient method.

Plastic has many valuable properties and has become an intrinsic part of our everyday life. An approximated 400 million tonnes is produced every year, consequently, vast quantities of plastic waste find their way into the oceans. The question "Why don't we just clean it up?" led to the founding of The Ocean Cleanup by Dutch entrepreneur Boyan Slat in 2013.

As of the writing of this thesis, The Ocean Cleanup has removed 12,989,690 kilograms of trash from oceans and rivers. System 03, consisting of two towing vessels towing a large net, is utilised to extract plastic waste from the oceans. To catch as much plastic at minimum time, effort and money, it is essential to have a comprehensive understanding of the behaviour of this net in water. A 3D Finite Element model is employed to simulate the system. Drawbacks in this model are the high computational demand and inaccuracies. These inaccuracies are caused by missing knowledge regarding drag coefficients at low angles of attack and the extrapolation of net behaviour to large nets.

The main objective of this research is to develop a simple, yet accurate model to simulate the physical behaviour of a high length-to-depth ratio net towed through water. Achieving accuracy in the model requires a comprehensive understanding of the net's behaviour in water, which has been investigated through a series of experiments. The simplicity of the model ensures low computational demand, enabling its use in offshore operations for swift decision-making on board. The developed model is validated using measurements obtained from campaigns conducted by The Ocean Cleanup. The performance of this simplified model is then compared to that of the 3D Finite Element model. Finally, the simplified model is applied to predict the behaviour of potential future systems.

In the experiment, the loads on two different nets were measured as they were towed through a basin at various velocities and angles of attack. The results show a significant decrease in drag at low angles of attack, attributed to the shielding effect. Experiments with two-meter and six-meter nets revealed that the shielding effect becomes insignificant beyond these lengths. Consequently, the drag coefficients calculated at various angles of attack for the six-meter net are utilised in the model.

To predict loads on the system, the model estimates the system's shape. For an accurate simulation of reality, the model makes an initial guess on how the system could look like. After this guess, an iteration loop starts until convergence is achieved. Convergence indicates that the system has reached equilibrium, accounting for external influences, internal forces, deformations, and the shape of the system. The model uses system properties, vessel positions and environmental conditions as input to generate the predicted shape of the system and the predicted loads acting on the system.

The performance of the simplified model is assessed by examining the error between the predicted and measured loads under various conditions. This performance is then compared to that of the 3D model. Both models demonstrate accuracy, with a MAE of just over 20\%. In the wide-span cases, which occur most frequently in the campaigns, the simplified model performs better than the 3D model. In terms of simulation time, the 2D model significantly outperforms the 3D model. While the 3D model requires approximately one week to complete its simulations, the 2D model finishes in 0.14 seconds, making it a flexible and user-friendly option. Altogether the simplified model is a simple, yet accurate model to simulate the physical behaviour of a high length-to-depth ratio net towed through water.

To demonstrate the model's functionality, it is applied to one of the trips, showing the maximum sailing velocities to ensure the system remains within its limits. The same trip is then simulated using potentially new nets and increased sailing velocities, resulting in a higher plastic catch. To improve the model's accuracy, aspects like wave direction, wind speed and wind direction can be implemented. Additionally, conducting a CFD analysis is recommended to gain a deeper understanding of net behaviour, which can help avoid the expense of additional tank tests.

Thin and large floating structures are increasingly used for a variety of applications, such as floating solar platforms, aquaculture, storage facilities, and even housing. For example, in the Netherlands, where thin plate-shaped floating solar platforms are being deployed in near-shore regions due to their benefits, such as uninterrupted solar irradiance and land conservation. With these deployments, it becomes essential to assess the risk posed to these structures by wave forcing in near-shore environments. Due to the large size of these flexible structures, the potential large impact of long ship-generated waves can be a key scenario of concern.

This thesis explores this scenario using a hybrid model, which couples a weakly nonlinear 2D finite element-based Boussinesq wave-generation model (FEBOUSS) with a 3D linear monolithic fluid-structure interaction (FSI) model. A key contribution of this work is the implementation of a novel, one-way partitioned fluid-fluid coupling algorithm, which efficiently transfers wave-information from the FEBOUSS model to the FSI model. The coupling is achieved by (1) enforcing the normal wave particle velocity from FEBOUSS as a boundary condition at the vertical inlet of the FSI model, and (2) using a damping zone that not only absorbs reflected waves from the membrane but also gradually enforces wave-elevation and normal wave particle velocity at a subsection of the free surface boundary of the FSI model. The accuracy of the coupling algorithm can be adjusted by refining the partition of feeding particles in the overlapping zone while simultaneously controlling the length of the damping zone. This approach ensures both computational efficiency, in terms of total simulation time and data storage requirements, as well as high physical accuracy at the coupling interface. The scenario of ship-induced waves entering a narrow harbour housing a floating structure is demonstrated, focusing on how the velocity of the moving vessel, represented non-dimensionally by the depth-Froude number, influences the hydroelastic response of the membrane. The analysis revealed that for all tested depth-Froude numbers, the membrane initially follows the long ship-induced waves, with subsequent responses that may be either (significantly) amplified or dampened with respect to the incoming waves, depending on the specific case under consideration.

The work presented in this thesis makes a significant contribution to offshore engineering literature by introducing a novel fluid-fluid coupling algorithm that can be extended to even more complex scenarios. Both the FEBOUSS wave-generation model and the FSI model are highly versatile, making them suitable for studying a broad range of fluid-structure interaction problems. Future research could explore the effects of variable bathymetry, irregularly shaped floating structures, and other relevant factors, expanding the scope of this study to include a broader range of physical phenomena.

In this study, we establish a foundational framework for controlling flexible floating structures by integrating state-of-the-art Model Predictive Control (MPC) with a high-fidelity Finite Element model representing these structures. Our specific objective is to enhance the stability of floating structures in offshore engineering applications. We commence with a comprehensive literature review to identify potential challenges, which informs the development of a well-structured research plan that addresses these challenges comprehensively.

From the insights gained during the literature review, we formulate a theoretical framework that serves as the basis for our methodology. We subsequently examine two distinct control scenarios—regular and irregular wave conditions—serving as a Proof of Concept, and we discuss the significant observations derived from these experiments. In conclusion, we summarize our findings and provide recommendations for future research directions.

We utilize a two-way monolithic Finite Element formulation of the flexible floating structure as a representation of the real-world system we aim to control. For our control strategy, we implement Model Predictive Control, which facilitates the incorporation of advanced functionalities, including the application of constraints. To enhance computational efficiency, we employ a Reduced Order Model (ROM) through Dynamic Mode Decomposition with Control (DMDc), trained using open-loop data derived from the Finite Element model. Additionally, we implement a Kalman filter to reconstruct the system's state from sparse and noisy measurements obtained from the floating structure.

We design two Reduced Order Models specifically for controlling the floating structure under the aforementioned conditions. Prior to executing the control strategies in these distinct scenarios, we conduct an in-depth investigation of DMDc, exploring its relationship with Koopman theory. We generate open-loop data free from pollution using the Finite Element model, which is subsequently utilized to derive the Reduced Order Model. A convergence study is performed by analyzing the eigenvalues and amplitudes of the DMDc, following methodologies established in prior research. We validate the DMDc models against validation datasets, allowing us to select the model exhibiting the smallest validation error. Furthermore, we ensure that the training datasets inherently encompass the wet modes by employing Proper Orthogonal Decomposition.

Our initial control scenario involves managing the floating structure under regular wave conditions. This case is pivotal for acquiring fundamental insights into the control mechanisms. We observe that the frequency of the control input aligns precisely with the excitation frequency. Subsequently, we extend our study to encompass the control of the floating structure in irregular wave conditions, characterized by a sea state defined by the JONSWAP spectrum. Consistent with our findings from the regular wave scenario, we discover that the control mechanism exploits the natural frequencies of the floating structure, which closely correspond to the energy-dense region of the sea state's spectrum. This is achieved by amplifying the reflected wave, thereby counteracting the incoming wave and reducing energy input into the system.

In conclusion, we advocate for further research to bolster the proposed method by examining various structural properties and wave environments, thereby providing robust evidence to validate the approach presented in this study. Our findings indicate that the integration of advanced control strategies, such as MPC and DMDc, holds significant promise for optimizing the performance and stability of flexible floating structures in dynamic offshore conditions.

The European offshore wind industry has experienced significant growth in the past decade, mainly focusing on shallow areas in the North Sea to reduce the Levelised Cost of Electricity (LCoE) and compete with fossil fuels. However, as shallow areas become scarcer and the industry seeks greater independence from government subsidies, a shift towards deeper waters is anticipated, and already observed in Europe. In the northern part of the North Sea (60-120 meters deep), jacket foundations are currently favoured, despite drawbacks such as extensive engineering efforts, weld requirements, challenging series production, and high costs. This misalignment with the industry's LCoE reduction goal highlights the need for a technologically viable and economically attractive foundation concept for waters in the 60-120-meter range.

To combat this challenge, perforated monopiles are being developed. The perforated monopile consists of a monopile with perforations, either circular or elliptical, around the splash zone, with the goal of reducing the frontal area, and thus reducing the hydrodynamic loads on the structure. These concepts aim to combine the ease of manufacturing of a monopile, with the reduced area affected by hydrodynamic loads that are common for jacket structures. The research done so far on these perforated monopiles has only looked at the reduction in hydrodynamic loads, which have proven significant. These reductions in hydrodynamic loads should enable the perforated monopiles to be used in deeper waters compared to regular, non-perforated, monopiles. They could provide a tempting alternative for the more expensive jacket structures, but more research is necessary, especially in analyzing other loads that the perforated monopile may be subject to.

This thesis aims to look at one such different load that affects this perforated monopile, namely the installation loads induced by hammering. The first part of this thesis will look at stresses and fatigue damage during the installation of non-perforated monopiles. The second part will analyze the increased stresses, possible losses in hammer energy, and increased fatigue damage, all due to the presence of perforations. Finally, several alternatives, such as different geometries of perforations and different hammer loads will be analyzed with regard to their effect on fatigue damage.

The fatigue damage due to installation is found to increase significantly due to the presence of perforations, increasing from 5% for non-perforated monopiles, to up to 118% and 112% for the two most promising geometries analyzed, thus proving a show-stopper for installation via impact hammer, if no measures are taken.

Changing certain parameters, however, either the geometries of the perforations, or the characteristics of the hammer used, shows that installation is indeed possible. Using different geometries of perforations, that maintain a significant reduction in area, shows installation is possible, whilst limiting the fatigue damage to 53%. A reduction in hammer force by a factor of 2, also decreases the fatigue damage by 34% on average. The use of a so-called vibro-hammer also shows promising, resulting in a halving of the fatigue damage compared to the use of an impact hammer, but more research needs to be done to confirm this final finding.

To conclude, this research shows that installation of a perforated monopile is possible, although most, if not all of the reduction in fatigue damage due to hydrodynamic loading is cancelled out by the increase in fatigue damage due to installation. Geometries and installation methods may exist that improve the fatigue life of the structure, but this research was unable to find them. Future research may be able to find geometries and installation loads that do reduce overall fatigue damage.

Further research is also necessary before perforated monopiles can be taken into service, such as the confirmation of the energy losses in installation due to perforations. Also, several other load cases need to be analyzed, to ensure the perforated monopile survives its designed lifetime.

The demand for clean energy has led to potential cost-effective solutions for the offshore industry. One of these solutions is shared mooring systems, where floating offshore structures for renewable energy are coupled to each other. While this approach saves mooring lines and anchors, it introduces new dynamic loading compared to conventional mooring systems. This study focuses on the feasibility of using a shared mooring system to combine floating offshore wind and floating solar support structures in different configurations in a farm layout.

This research uses the Volturn US-S platform to model the floating wind turbines and recreates the Tractebel Seavolt concept for the modelling of the floating solar arrays. Six configurations are simulated, undergoing irregular waves under various wave headings, load cases, and line materials. The configurations involve two wind turbines with one or more solar arrays in between them. A Quasi-dynamic model is used to identify critical cases. These cases are re-evaluated using OrcaFlex. The assessment of these configurations is done based on two Key Performance Indicators, related to line tensions and the floating support structures' displacements. During the process, polyester lines are chosen due to their favourable characteristics.

The four configurations that met the Key Performance Indicators are compared to a base case of a single turbine regarding its displacements and anchored line tensions. Effective utilisation of the shared lines and limited displacements lead to a preference for the configuration with three solar arrays. Additionally, the anchored line tensions of the turbine in this configuration increase the least compared to other configurations.

In summary, using a shared mooring system to combine both floating offshore wind structures and floating solar structures in a farm layout is feasible. Further research should be done on other configurations and the impact of these mooring systems on the Levelized Cost Of Energy. Additionally, wind and current loading should be incorporated in future studies.

For the installation of suction pile jackets, a new installation method initiated by SPT Offshore could extend the allowable size for suction pile jackets used as foundations for offshore wind turbines. This method involves installing the suction piles separately from the jacket frame. First, the suction piles are installed on the seabed. Then, the jacket frame is placed on top of the suction piles, and the piles and the frame are connected. The installation of the suction piles in the seabed requires a certain level of precision to ensure a proper connection with the jacket frame. This can be achieved by introducing an installation tool called the suction pile installation template (SPIT). The goal of this thesis is to take the first steps towards realizing the SPIT installation method by designing the suction pile installation template and identifying key design challenges for the SPIT.
To bound the scope of the research, assumptions are made regarding the installation vessel, foundation dimensions, jacket-pile connection, and site specifications based on a case study and the resources of DEME. To design the SPIT, the use of the SPIT is analysed, considering a wide range of design options. The most significant inputs to the analysis are the limitations of the installation vessel and operational efficiency. Next, the changes to the suction pile and jacket frame are examined. The selected grout connection between the suction pile and the jacket frame creates a jacket frame similar to a standard pin pile jacket. The suction pile requires a stub on top of the top-plate. An optimization study is conducted to determine the size of the stub. These two analyses provide the general design requirements for the SPIT, which is then checked for structural strength, installation tolerance of the suction piles and lift capacity. The checks are based on industry standard codes.
Analysis shows that the hydrodynamic loading on the suction piles induces the largest loads on the SPIT. However, if the suction piles are incorrectly placed in the seabed, the interaction between the soil and the suction piles could result in even larger loads on the SPIT. The models used in this thesis should provide conservative estimates. In future research, the analysis of hydrodynamic loading, geotechnical analysis, and dynamic response of the SPIT should be verified and justified using more sophisticated models and/or simulation software.
The results from this thesis indicate that the proposed design of the SPIT provides a solution to extent the installation of SPJ for OWT. The research identifies four key design challenges. Each challenge indicates solvable obstacles to the design of the SPIT. Based on the results, the estimated total weight of the SPIT is 240mt. DEME's installation vessel, the Orion, has sufficient lift capacity to perform the installation and deck-space to perform up to 13 installations in one trip.

Global warming represents the most significant threat to humankind, making the need for renewable energy more crucial than ever. However, in densely populated areas near the coast, electricity production faces competition from various sectors such as agriculture, housing, and tourism. To address this challenge, one viable solution is to explore offshore electricity production.

Building upon this context, this research delves into investigating the wave-induced effect on power mismatch losses along a PV string in offshore floating photovoltaic (OFPV) systems. OFPV offers a promising solution for generating electricity in unused marine areas, complementing offshore wind energy. Although OFPV holds great potential, our understanding of its complexities remains limited, particularly regarding the impact of wave-induced power mismatch losses. To bridge this knowledge gap, a comprehensive approach is taken. A floating structure is modeled using the Bernoulli-Euler beam theory, while the fluid domain is analyzed using potential flow/linear wave theory. Structural behavior is examined in the frequency domain through the application of a FEM with the package Gridap in Julia. The wave amplitude spectra are determined using the Jonswap sea spectrum, with consideration given to four distinct sea states based on the Douglas sea scale: slight, moderate, rough and very rough. The optoelectrical modeling is conducted in pvlib in Python.

The results reveal that monthly energy losses due to power mismatch are negligible during summer months for all sea states studied. However, in winter months, monthly energy losses exceed 1%, with daily losses reaching up to 6%. Additionally, the orientation of the PV string is identified as a crucial parameter for minimizing losses. Finally, the findings indicate that using either a thick structure with a stiff and dense or a thin structure with a flexible and lightweight material can help reduce energy losses caused by power mismatch.

As offshore wind turbines are moving further from the coast, floating wind turbines are being considered. These structures need to be able to endure high dynamic loads while remaining as still as possible in order to minimize internal stresses and maximize the efficiency of the turbine. To find ways to combat large responses, the effectiveness of tuned mass dampers (TMDs) and tuned liquid dampers (TLDs) has been widely researched. While both systems have been shown to be effective, these damping systems are being used separately.
This thesis investigates the effectiveness of vertical TMDs combined with a TLD by analysing three barge-turbine structures. The first system is the undamped structure, which works as a control. The second system combines vertical TMDs with a TLD. These damping systems are not directly linked. The third system interdependently combines vertical TMDs with a TLD by stacking the damping systems. This makes the TMD displacement determine the shape of the TLD tank, which causes the system to be significantly nonlinear.

The analysis of the three systems begins with the development of a program that creates the frequency response function for each of these systems. The basis for these programs is a linear modal analysis. The nonlinearity of the third system is accounted for by Newton iteration.
The effectiveness of each damping system is determined by the performance index, which is based on the integral of the frequency response function.

The damping parameters of the independent TMD and TLD are optimized. Due to computational difficulties no parameter optimization is performed for the interdependent damping system. In order to judge possible performance improvement for this system, a rough sensitivity study is performed on the damping parameters of this system.

The results of the optimization and sensitivity study show that the independently damped system performs best of all three systems when using these particular parameters. The interdependently damped system has unknown damping potential.

It is not possible to definitively conclude from this research whether the TMDs and TLD damp more effectively when they work either separately or interdependently. Both damped systems are able to perform better than the undamped system with the correct damping parameters.

The developed programs give reasonable frequency response spectra for all three systems. From this it is concluded that the programs of the three systems work as intended, including the nonlinear part.

To meet global green energy targets, the bottom founded offshore wind industry is looking for ways to economically expand markets to deeper waters. A reduction of the hydrodynamic load is necessary to achieve this. One option is to perforate the monopile around the splash zone. Here the related work of Q. Star is continued through the implementation of a 2D Navier-Stokes based LES CFD model with VMS closure in Gridap.jl. The numerical simulations are gathered to create a dataset on which a machine learning surrogate model is trained. The analysis does not include secondary design aspects such as manufacturing, installation, noise mitigation, scour, corrosion, acidification, or marine growth. Neither does it require secondary fluid phenomena such as 3D simulations, FSI, VIV, and fatigue.
Analysis of 2D perforated cylinders, built up parametrically using the wall thickness, angle of attack, diameter, inflow velocity, number of perforations and porosity as input parameters shows that the first two are of negligible influence, and number three and four can, at least for their mean values, be modelled accurately using Morison equations. A non-scaled diameter does prove important in reducing the random nature of frequency-based effects. The number of perforations and the porosity provide complex interactions both from a design-force mean and variability perspective, as well as for the frequencies and vibrations they generate. A 2D LES-VMS model gives the perfect trade-off between cost and accuracy, predicting a possible drag reduction of more than 50% compared to a closed cylinder with large diameter.
Different machine learning surrogate models are analysed with the goal of massively speeding up the analysis in the future. This is achieved by a factor of 350 thousand using Random Forests, Gaussian Processes and Neural Networks. Although the Gaussian Processes preliminary show the best accuracy, below 6% error for millisecond predictions, further work on Neural Networks could give them the advantage in future analyses.

Global warming has caused an increasing demand for renewable energy. Currently, 50% of the renewable energy is provided by wind and solar combined. However, these renewable energy sources are highly volatile. Wave energy converters can provide a stable renewable energy source. Previous research has contributed to understanding various combined effects of length, aspect ratio, hydro-elasticity and PTO damping on the optimisation of peak power absorption for regular waves. However, research on optimising the combined effects on annual energy production and especially power stability is missing. This thesis aims to better understand these effects by answering the following research question: "How can the combined effects of total length, aspect ratio, raft stiffness and PTO damping influence yearly energy conversion and energy stability of a pitching raftWEC"

To do this, a novel formulation for a monolithic finite element model containing Timoshenko beam theory and Linear potential flow is developed. This FEM consists of a two-dimensional domain, where the water is excited by linear irregular waves. The wave energy converter is modelled as two floating Timoshenko beams connected by a damped joint. The model is verified by proving that the solution converges according to the polynomial order +1 for decreasingmesh size. Additionally, it is proven that energy is conserved which shows that there is no wave reflection resulting from the domain boundaries. Finally, the model is validated by comparing the results of the structure with and without joint to previous research.

Two experiments are developed to understand the combined effects. Initially, an eigenmode analysis of the undamped structure is performed. This gives insight into the influence of individual parameters on the eigenfrequencies and shapes. Moreover, the real eigenmodes are used to decompose the real part of the complex resonance shapes of the relative rotation response amplitude operator (RAO). This provides a coupling between the performance according to the relative rotation RAO and the eigenmodes and frequencies. A limitation of this method is that phase differences cannot be considered.
The second experiment maximises the annual energy conversion and energy stability of thewave energy converter with three sets of variable parameters: (total length, aspect ratio, damping parameter), (total length, symmetrical raft stiffness, damping parameter) and (stiffness fore-aft, stiffness aft-raft, damping parameter). This is done in three steps. First, the RAO for the relative rotation at the joint is constructed. This is converted to a power spectrum. Secondly, the power spectrum is scaled by the JONSWAP energy density spectra. These
spectra are constructed using the annual wave conditions, significant wave heights and peak periods from a wave scatter diagram. Subsequently, the results per spectrum are scaled by the number of occurrences of the wave conditions to obtain the annual energy conversion. Third, The optimum damping parameter per configuration is determined by calculating the annual energy conversion for a set of damping parameters. These results are interpolated to select the damping parameter that maximises annual energy conversion.
Finally, the performance of each configuration with their respective optimum damping parameter is assessed on annual energy conversion and capture width per peak wave period to give insight into the energy conversion and stability. The optimisation results are clarified with the results of the eigenmode analysis.
It was found that the optimal total length, considering raft stiffness, aspect ratio and PTO damping can be determined by matching the dimensionless wavelength corresponding to the third mode, and the annual significant wavelength.

Furthermore, It is proven that an aspect ratio of 0.2 to 0.3 increases the performance in high-frequency waves without compromising on low-frequency waves. The capture width is more evenly distributed over peak wave periods, indicating a higher energy stability. This results in an increase of annual energy conversion of 31% with respect to an aspect ratio of 0.5. Besides the aspect ratio, it was found that an asymmetrical raft stiffness, with a moderately flexible fore raft and a rigid aft-raft increase energy stability without compromising annual energy conversion.