Fiber-reinforced polymer (FRP) composites are used in various engineering applications due to their many advantageous properties, but predicting failure remains challenging due to complex failure behavior. The Computational Mechanics group at TU Delft uses an inter-element cohesive zone method, referred to as the Ortiz model, to analyze microscale crack propagation. Although effective for modeling complex failure processes, the method suffers from severe mesh dependency, restricting large-scale simulations. This research implements and investigates the shifted fracture method (SFM) as an alternative to reduce mesh dependency while maintaining computational complexity. The SFM modifies key aspects of the Ortiz model, including an area correction term, crack propagation algorithm, and shifted weak form equations and cohesive zone conditions using Taylor expansions. Three versions of the SFM were tested: (1) full SFM implementation, (2) reduced SFM with only the area correction and crack propagation algorithm, and (3) an extended Ortiz model with only the area correction term. Numerical simulations show that the reduced SFM is robust, mesh-independent, and efficient in basic fracture tests. The area correction term proved critical in reducing mesh dependency. The extended Ortiz model, on the other hand, showed less accurate results in predicting cracks, despite the additions of the area correction term. Furthermore, the maximum principal tensile stress criterion for determining the crack direction was unsuitable for mixed-mode scenarios, resulting in inaccurate paths. Although the reduced SFM shows promising results, it requires further development to handle complex fracture scenarios like FRP composites. Features such as crack initiation, merging, and termination were implemented and showed potential but require further validation. This research concludes that while the reduced SFM is effective for basic testing, further refinements are needed for broader application to FRP composites.

Offshore floating photovoltaics (OFPVs) show great potential for future renewable energy generation by complementing existing wind energy. To maximize power output, it is essential to accurately predict the response of these structures under wave-induced forces.

Although finite-element fluid-structure interaction (FE-FSI) models can simulate the response of very large floating structures (VLFSs) with high accuracy, they are computationally expensive and time-intensive. To address this challenge, this thesis develops and optimizes three surrogate deep learning models: a Convolutional Neural Network (CNN), a Long Short-Term Memory (LSTM) network, and a hybrid CNN-LSTM model. An iterative trial-and-error approach was employed to fine-tune the hyperparameters and improve model performance.

The CNN model accurately captured spatial features but struggled with generalization across varying sea states. The LSTM model captured temporal patterns well, but had lower accuracy and longer training times. The hybrid CNN-LSTM model combined the strengths of both approaches, achieving a low weighted absolute percentage error (WAPE) of 0.98 %. All models performed well under typical wave conditions but exhibited reduced accuracy in extreme scenarios, particularly for low-amplitude tilts. A sensitivity analysis indicated that these errors were likely caused by insufficient representation of extreme sea states in the training data.

The hybrid CNN-LSTM model was selected as the model that most accurately predicts the response of VLFSs due to its balanced performance, minimal prediction time, and generalization capability across typical operating conditions. While the model shows high reliability for typical sea states, its performance declines for extreme conditions, such as extremely large waves or very small tilts. Despite reduced reliability in extreme conditions, the model provides a promising tool for real-time OFPV response prediction and preliminary studies.

The growing demand for renewable energy has positioned Floating Offshore Wind Turbines (FOWTs) as a promising solution for harnessing wind energy in deeper waters. Unlike fixed foundation turbines, which are limited to depths of around 50 metres, FOWTs can operate in greater depths. However, this relatively new industry faces numerous challenges, with high costs currently limiting its large-scale deployment. While chains are widely used in mooring systems for the Oil & Gas industry, their required size and supply constraints for floating wind applications have made fibre ropes an attractive alternative. This thesis identifies several factors influencing the use of top chain in a hybrid mooring system, with a primary focus on determining the minimum required length for hook-up operations of semi-submersible FOWTs.

The relationship between the connection height of the top chain to the FOWT and the required top chain length, as well as the relationship between sea state variations and top chain length, were investigated through numerical simulations. Complications related to the depth below the sea surface, fibre elongation behaviour, and the overall performance of hybrid mooring systems were reviewed through existing literature. The dynamic behaviour of the top chain during the hook-up operation was analysed using OrcaFlex, with simulations performed in both frequency and time domains.

Dynamic analyses primarily focused on a dry chain link connection method, revealing that top chain lengths between 61 and 76 metres are required during hook-up operations. These lengths depend on wave conditions, connection height, and the installation vessel. The results showed that the required top chain length increases with wave height and period, while the length itself minimally influences its dynamic behaviour. For the lower segment of the mooring line, at least 10 metres of chain are required to prevent contact between the line and the rudders and thrusters of the installation vessel.

The study concludes that the top chain experiences minimal dynamic tensions relative to the static tension during hook-up, provided it includes sufficient sag and submersion. A bottom connection height offers the most benefits but necessitates a longer top chain during hook-up compared to a middle connection height. While fibre materials show promise as alternatives to chains, further investigation is required for their use in shallow waters. The dry chain link method is considered financially advantageous but it is expected to require the longest top chain length for hook-up operations compared to other methods. An in-depth analysis of applying pre-stretch to a 3-line mooring system is recommended to verify this assumption. These findings contribute to enhancing the economic feasibility of floating wind energy solutions, particularly for large-scale deployment.

This research contributes to a novel non-contact installation method for offshore wind turbines, utilizing electromagnetism for positional control. Floating wind installation requires efficient control methods for payload positioning. Current methods use tuggerlines that are physically attached and work through tensile attractive forces. In contrast, electromagnetic control offers a non-contact solution and allows for control through both attractive and repulsive forces. This research explores the applicability of multiple data-driven control algorithms on a magnetically controlled pendulum system. These algorithms are evaluated for both positional control and motion attenuation under pivot-point excitation, using simulations of a numerical model of the electromagnetic pendulum. The model-free control algorithms considered in this study assumed that changes in the system’s state are solely due to the control output. These methods aimed to optimize the control output based on either the state error or estimates of the state gradient with respect to the control output. However, this assumption was found to be valid only for systems dominated by the control response with limited dynamic behaviour over time, which was not the case for the electromagnetic pendulum system.

The study culminates in the development of a controller based on the online estimation of the magnetic interaction force. This controller, named Proportional-Derivative Force estimating neural network controller (PD-FeNN), is a neural-network based state-dependent PD-controller. The neural network is trained during an operational learning phase on estimations of the magnetic interaction force, which are derived using the model of a linear undamped pendulum. By incorporating the neural network, this approach eliminates the requirement of the previous state-of-the-art controller to manual model the magnetic interaction force based on experimental data. The PD-FeNN controller is tested on both numerical and physical models of the electromagnetically controlled pendulum. The results demonstrate efficient control across a wide range of excitation frequencies and amplitudes for both motion attenuation and positional control. As a successor to the modified PD controller, the PD-FeNN controller improves upon its predecessor by enabling positional control without requiring a model of the magnetic interaction. This advancement enhances the applicability of non-contact motion control for payloads in the offshore wind industry.

Offshore wind farms, critical for sustainable energy production, face the challenge of optimization among many parameters influencing the objectives in conflicting ways. In response, this research introduces the novel Integrative Maximized Aggregated Preference Wind Farm Optimization (IMAP-WFO) framework—a comprehensive tool designed to enhance the flexibility, accuracy, and uncertainty quantification of offshore wind farm design and operation.

Existing methods often fall short due to limitations in adaptability and precision, especially when modeling complex multi-physical behaviors under uncertain conditions. IMAP-WFO represents a fundamental change by combining advanced statistical techniques and simulation methods.

At its core are parametric design performance functions, capturing critical aspects of wind farm behavior, including energy production, material usage, and structural fatigue. These functions rely on Kriging meta-models, known for their accurate predictions of unknown values. To address inherent uncertainty, Monte Carlo simulations provide a probabilistic assessment of outcomes. IMAP-WFO’s true innovation lies in translating technical functions into socio-economic objectives. These include sustainability metrics, Annual Energy Production (AEP), Capital Expenditure (CAPEX), Operational Expenditure (OPEX), model uncertainty, and lifetime fatigue. Stakeholders can dynamically weigh these objectives based on their preferences, showcasing IMAP-WFO’s versatility.

A validation process described in this research ensures the accuracy of design performance functions, comparing simulated results with real-world data from operational wind farms. IMAP-WFO’s application is demonstrated through case studies: optimizing the Levelized Cost of Energy (LCOE) and exploring wind farm control strategies. Overall, IMAP-WFO bridges the gap between technical challenges and socio-economic goals, helping offshore wind farm design to increase in efficiency

This thesis is a about a novel float-over concept for the installation of offshore substations in the Dutch North Sea. This new technology is a topside jacking system integrated into the H-851 float-over barge which should be able to jack the topside to a higher elevation before a float-over installation.

First, a high-level concept design of the novel float-over technology is presented. Thereafter, the stability for the jacking stage is investigated via hand calculations. Analytical models are developed to model the jacking system during select stages of the float-over and jacking of the topside. These models are used to verify the assumed design loads on the jacking system and other components required to install the offshore substation. As of now, the design loads are based on guideline for a different type of float over making the design loads a guestimate.

The float-over barge H-851 proved to be very stable without ballasting during the jacking of the topside. Initial ballast errors did not affect the barges stability significantly, the effect was only a 30% increase in heeling angle for the worst case. The effect for wind loading was more profound as the heeling angle increased with 100%. Due to the large initial stability of the barge, the heeling angle after jacking is still of no concern. For the investigated cases, however, care should be taken to ballasting the barge before jacking as initial errors become larger after jacking.

It was found that the driving systems for the jackings system can be designed in the global horizontal direction using less than 5% of the topside weight, given the sea states in this thesis. A lower stiffness jacking system resulted in lower horizontal loads on the driving system. The jacking system showed a difference in local horizontal loads over the jacks from stern to bow as great as 150%. This is due to the position of the topside with respect to the centre of the barge and in combination with the roll, pitch and heave behaviour of the barge and topside.

The mating analysis had multiple findings. It was found that the impact loads on the LMU’s were significantly lower by using the jacking system compared to the DSF for North Sea waves. However, the effect of the horizontal load (compression force) was less significant which resulted in lower or equal loads on the LMU’s. The horizontal loads at the interface between topside and barge were larger for a jacking system compared to a DSF.

Increasing the stiffness of the jacking system resulted in an increase of loads on the jacks and LMU’s. This was found due to the lower eigen period in the dominant motion for stiffer jacks. An interesting finding is a torsion moment with its centre at the interface between barge and topsides. This point was found to be dependent on the stiffness of the interface.

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.

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

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

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

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

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

Hyperloop is a new high-speed transportation system currently in development. Utilising an electromagnetic suspension within a low-pressure tube, this approach eliminates the traditional wheel-track friction and drastically reduces the air resistance, targeting speeds up to 1000 km/h. Compared to traditional trains and aeroplanes, the Hyperloop is a more sustainable alternative as it produces no greenhouse gas emissions and uses ten times less energy than road and aviation transportation. The Hyperloop, therefore, has great potential to contribute to the goal of achieving climate neutrality by 2050.

Ensuring the stability of the Hyperloop is a critical challenge in its realisation. Multiple instability mechanisms are present in the system: (1) electromagnetic instability, (2) wave-induced instability, and (3) aeroelastic effects, such as galloping. Additionally, the imperfections in the guideway, make the Hyperloop system prone to parametric instability. This thesis primarily investigates how aeroelastic forces and parametric resonance interact with the electromagnetic and wave-induced instability mechanisms, impacting the overall stability of the Hyperloop system.

Based on the results, it can be concluded that the aeroelastic force destabilises the system, where its impact increases as the velocity rises. As the aeroelastic force continuously injects energy into the system, the unstable domain expands. Furthermore, parametric resonance, represented by ellipse-shaped indentations in the stability planes, is observed when the excitation frequency is twice the natural frequency of the system. Based on the control parameter range used for maglev trains, it is found that the stable domain is narrow, with only a small region prone to parametric resonance. Therefore, choosing the control parameters precisely is essential to prevent instability and parametric resonance.

When examining the combined effect of aeroelastic force and the irregular guideway profile, the results show that the aeroelastic force shifts the overall position of the stability boundaries but it does not affect the parametric resonance regions. Instead, the amplitude of the guideway’s irregular profile influences the size of the ellipses and the stable domain. A larger amplitude leads to an expansion of the unstable domain and an increase in the size of the ellipse; therefore, it is advised to maintain a smooth guideway.

This thesis further evaluates the capabilities of analytical expressions for the simpler 1.5 degree-of-freedom (DOF) electromagnetically suspended mass system by comparing it to systems that incorporate the beam dynamics. The results show that the analytical expressions from the 1.5 DOF system cannot approximate the position of the ellipses for the more complicated systems across all velocities. However, beyond the velocities of $1.3v_{cr}$, the analytical expressions effectively approximate the ellipse size. Notably, while the position and size of the ellipse are influenced by the guideway’s surface roughness amplitude, only the size of the ellipse is affected by the amplitude of the oscillations in the 1.5 DOF system. For the more complicated systems, the analytical expressions of the ellipse size relative to the amplitude provide accurate results, making the simplified system a valuable approach for estimating the ellipse size of parametric resonance in systems with beam dynamics.

Offshore wind energy has become a important contributor to global renewable energy efforts, particularly as the industry seeks to expand into deeper waters where traditional fixed-bottom turbines are no longer feasible. In these deeper waters, floating offshore wind turbines (FOWTs) offer a promising solution, allowing for the generation of renewable energy in locations with stronger, more consistent winds. However, with the increasing use of FOWTs comes a set of logistical challenges that need to be addressed to ensure their commercial viability and large-scale deployment. One of these challenges is the large-scale production of floating foundations. One significant challenge to the commercial success of FOWTs is the slow fabrication rate of semi-submersible floaters, which require extensive welding and assembly. In contrast to faster monopile fabrication, these floaters are more complex and take longer to produce. Since installation campaigns are typically limited to favorable weather periods, especially in the summer months, many floaters need to be ready in advance, placing strain on fabrication yards and causing space constraints. Wet storage provides a viable solution by allowing temporary storage of floaters in coastal areas, freeing up space in fabrication yards for continuous production. This research explores the existing literature
on wet storage mooring systems and identifies key design criteria, environmental considerations, and components such as mooring lines, anchors, and auxiliary equipment.
To evaluate mooring system designs, a Multi-Criteria Decision Analysis (MCDA) framework was developed, facilitating structured comparisons and trade-offs between various design objectives. Although the MCDA enables informed decision making, dynamic analysis is essential to obtain deeper insights into system performance. In this study, frequency domain (FD) analysis was employed for rapid evaluations of the behavior of the mooring system. FD analysis proved sufficient for preliminary studies and is an ideal tool for exploring design variations. It efficiently captures first-order loading effects, which are typically dominant in expected wet storage conditions. However, for detailed design stages, to account for nonlinear behaviors such as slow drift and mooring line stiffness, time domain (TD) analysis are recommended to validate final configurations and ensure compliance with design codes.
Dynamic analysis of the mooring systems compared catenary and taut configurations under various environmental conditions. Concept-specific constraints such as maximum fairlead tension, uplift allowance, mooring line angles, and minimum anchor distances were incorporated into each mooring concept. Furthermore, diffraction analyzes were conducted with varying water depths and floater drafts to accurately define floater behavior under different conditions, and a mesh validation was performed to ensure that the mesh was fine enough for this study. Further validation of the full model was done by comparing the calculated Load Response Amplitude Operators (RAOs) and natural frequencies with
the reference turbine to ensure model accuracy. Tidal ranges and line diameters significantly influenced mooring performance, impacting tensions loads, and floater displacements. The results indicated that the catenary system consistently met these constraints and demonstrated resilience across the full tidal range, where a clear trade-off between tensions and displacements can be observed. However, the taut system experienced tension overloads, particularly at high tide, where it often exceeded line breaking strength, with only a single taut configuration yielding valid results. Despite being more spatially efficient, the taut system’s reliability was compromised compared to the catenary system. Several recommendations for future research are identified to further the understanding of mooring systems in wet storage applications. These include conducting TD analyzes to capture non-linear behaviors, exploring alternative mooring configurations such as Honeymooring or pile fields, refining design standards, and evaluating the economic and logistical feasibility of wet storage solutions. Additionally, investigating different storage locations is essential, as this study focused on a single location, and variations in environmental conditions could significantly influence mooring performance.

This study focuses on optimizing the use of high-performance computing on public cloud infrastructure, along with information theory, for assessing water systems. These assessments are computationally intensive and can benefit from parallel computing and the evaluation of the collected data with information theory. A case study of a water system analysis for the Vlietpolder was conducted to test various cloud configuration settings using an embarrassingly parallel batch computation. The hydrodynamic simulations involved D-HYDRO 1D2D models with different precipitation events and model resolutions. The modelling results were quantified using normalized Shannon’s Entropy to facilitate the comparison of system configurations, evaluating the batch computation process to determine whether enough simulations have been performed and comparing individual simulations.

The study showed that public cloud infrastructure provides comparable computational performance to local computers and servers, and offers opportunities for vertical and horizontal scaling for parallelization. The study also provides insight into the impact of allocated resources, node size, and node type on cloud infrastructure performance. Furthermore, the quantified information derived from the simulations can be utilized to evaluate the batch
computation output and support cost-benefit analyses for selecting configuration settings and model decisions given modeling scenarios.

The study concludes that combining cloud infrastructure and information theory can enhance hydrodynamic modelling for batch computations in water system analysis. The findings provide insights into the potential benefits of utilizing public cloud infrastructure for large-scale computations of hydrodynamic simulations.

Previous research into floating offshore wind turbines consistently shows some manner of discrepancy between numerical and experimental simulations when it comes to low frequency forces and motions. A part of solving this issue is the generation of accurate data for the calibration and validation of the numerical models, needed because of the coupled analysis introduced due to the relatively large wind force. The goal of this research thus became to generate accurate experimental data focussed on the low natural frequency of the mooring system in surge direction. This data had to be accompanied by an uncertainty assessment and should encompass multiple mooring systems for a good comparison.
In order to achieve these goals experimental model tests were set up using bichromatic wave sets in a long towing tank. The bichromatic wave sets allowed for the creation of beating patterns, targeting the specific low frequencies on and around the surge natural frequency via the difference frequencies. The long towing tank reduced the amount of reflections and clutter in the tank. Together these factors ensured accurate excitation of model at the desired frequencies.
The model used was a 1:96 semi-submersible model with three buoyancy columns supporting a central tower column. The depth in the tank being 1.25 m at model scale ensured a relatively deep testing environment. Two different mooring systems were tested, each with a different surge natural frequency but with the same semi-taut fibre rope-chain lay-out. A comprehensive measurement system consisting of force transducers in the mooring lines and wind force device and a camera tracking system enabled accurate measurement of both the force and motion of the model. Experiments revolved around examining the influence of the difference frequency in the wave excitation on the low frequency behaviour of the mooring system and floater.
From the lead mooring line force time series, it could be observed that during the surge natural frequency test the mooring line had triple the force compared to the wave tests with the monochromatic components, showing a clear effect of the bichromatic beating pattern. From the tests it also became clear that mooring system 1 had rear lines which were slack during wind and wave excitation. These slack lines had a number of effects on the behaviour of the model during the tests, such as increasing the mooring line force and surge response.
The tests thus show that the low frequency response comes from the difference frequency in the bichromatic wave sets, as was the goal. The heightening of the response around the natural frequency also shows that getting close to this frequency amplifies the effect. The comparison between the test results and the results from the OC5 project show that this is likely also the case there. At the same time the surge response does not show this heightened response. Which is likely due to the non-linear stiffness of the mooring system.
Furthermore, it was seen that the slack lines have a very large impact on the force in the mooring lines, especially on the lower frequencies and should thus be avoided at all times. The damping ratio as a parameter for the low frequency response is both physically and experimentally (excluding the slacked lined system) consistent and shows potential for development.

The first part of the thesis explores the process of homogenization, particularly for the permeability properties of rock samples. The Hill-Mandel postulate of energy consistency throughout the transitioning of scales is revisited since traditional homogenization methods rely on applying specific boundary conditions to enforce energy consistency. However, it is shown that the applied boundary conditions influence the effective physical parameter and provide upper or lower bound estimations. Recently, it is shown that these boundary conditions influence a layer near the boundary of the sample and that homogenization applied on the subsample away from this boundary layer is not affected by the boundary conditions.

The research focuses on energy consistency by studying the evolution of the energy within the intrinsic subsample, away from the boundaries. With the help of Finite Element simulations of Stokes-flow through idealized structures, the energy of the fluid is traced without the influences of grain properties on the energy dissipation. By plotting the ratio of the energy dissipation of the macro- and micro-scale, it is shown that the energy consistency is not found within small subsamples. Yet, with a growing subsample, energy consistency is achieved naturally, without the enforcement of boundary conditions. As a result, it is concluded that the energy consistency is found at the Representative Elementary Volume (REV), which is a similar requirement as for traditional homogenization methods. The study of the natural energy consistency in idealized microstructures is extended to real microstructures, which include more natural heterogeneities, such as grain properties. It is shown that energy consistency is also found with the natural heterogeneities included, albeit with a slower convergence.

For the homogenization of the permeability of a sample, the energy ratio is now known to be unitary, which can be used as an accurate indicator to determine the size of the REV.

The second part of the thesis explores the process of determining the size of the REV, which is a common, yet essential practice in Digital Rock Physics. Currently, this is an extensive exercise, involving many and large-size simulations to trace the convergence of the physical property and requires a lot of computational resources and time. Numerical-statistical studies have shown that the convergence of the REV visualizes in a cone-like shape. By plotting the convergence for both the permeability and the energy dissipation ratio for idealized microstructures, this study analyses the shape and evolution of the cone of convergence. From this, the generic evolution law of the convergence is determined.

It is shown that the asymmetrical convergence cone is described with a log-normal distribution, with a stable mean throughout the evolution of the cone and a variance for each sample size. The evolution of the variance is described with the law of large numbers, taking into account a reference value. This makes it possible to determine the size of the REV. Since the statistical method applies, information about the error of the fit, the error of the determined homogenized property, and the error of the size of the REV is provided.

The study is extended to real microstructures to validate whether the determined evolution law applies when natural heterogeneities are included. It is shown that the evolution law still accurately describes the REV's convergence. Therefore, the REV's size of real rock samples can also be determined. Even when the REV is not included within the sample, the evolution law can provide an estimate of the size of the REV or the homogenized property.

By using the cone of convergence, it is not necessary to run simulations on the full sample to find the REV, which is computationally expensive, instead running a number of simulations on small subsamples is sufficient, which saves both time and computational resources. It also unlocks the possibility to find the REV for high-resolution samples, as splitting the sample into subsamples allows for smaller simulations.

With the pressing matter of climate change, the importance of renewable energy sources is increasing day by day. A large portion of this renewable energy is to be generated with the use of wind turbines, which are commonly located onshore. In recent decades, great effort has been put in the development to place these turbines offshore, because of a higher yield in energy production, as a result of higher wind speeds, greater consistency and less interference from human-made objects. These offshore turbines can be placed on floating or bottom-founded structures. Only 20% of all possible wind farm locations are suitable for the application of a bottom-founded wind turbine, due to its depth restriction. Therefore, a great deal of development is focused on the application of floating wind turbines. In essence, there exist three different types of floating structures: semi-submersible, Single Point Anchor Reservoir (SPAR) and Tension Leg Platform (TLP).

In this thesis, the TLP is considered for a floating foundation because to its superior motion characteristics compared to the other structures. The TLP obtains its stability from its mooring system due to excessive buoyancy, which causes the mooring lines to be under tension. This in turn causes the TLP to have compliant and constrained Degrees of Freedom (DOFs). These constrained DOFs cause the eigenfrequencies of the system to be out of reach of first-order wave loading. However, from the oil & gas industry it is known that this type of structure does have a high-frequency response. This is known as springing and ringing, where this thesis is devoted solely to springing. Springing is identified as the resonance of the constrained DOFs of the TLP caused by the subsequent wave loading on the floater.

From initial evaluations of the motion characteristics of the case model TLP, it is seen that there are eigenfrequencies of the system that coincide with both the high frequency tail of the first-order wave load spectrum and the second-order wave load spectrum. Therefore, first- and second-order wave loads are considered. These forces are evaluated with the use of potential theory and calculated with OrcaWave and Hydrostar, which are commercially available diffraction programmes. Springing is subsequently evaluated using time-domain analysis with OrcaFlex, which is a commercially available software capable of performing time-domain analyses with aero-hydro-servo-elastic coupled models.

From initial simulations, it is seen that springing events are indeed present in the system and are mainly affecting the fatigue life of the tendons. Additionally, slack tendon events are present to a larger degree and are correlated to the whipping of the tower and RNA. This mainly causes the exceedance of structural limits. As springing is a resonance problem, only adjustments to the mooring system are considered that influence the location of the eigenfrequency. In this thesis, adjustment of the axial stiffness and inclination of the tendon are considered. Adjusting the axial stiffness did not produce the desired effect, because the range of common values in the mooring industry is rather limited to invoke any considerable change in the motion characteristics of the TLP. Furthermore, the TLP showed a large number of slack tendon events. that are correlated with the whipping of the tower and consequently caused the exceedance of structural limits. Adjusting the axial stiffness is not able to prevent these events. Adjusting the tendon angle did show promising results. For the case study, an optimum angle of 16° is found, which on average halves the tendon tensions. A reason for this is that with the introduction of a tendon angle, the TLP rotates around a certain focal point. At 16°, this focal point is located at the Rotor Nacelle Assembly (RNA). Thus, the TLP tends to rotate around the RNA instead of the other way around. This change of motion behaviour also prevents the occurrence of slack tendon events. Although not preventing the occurrence of springing events, the optimal tendon angle does reduce the effect and rate of occurrence of springing to a satisfactory level.

During ‘float-out’ installation of export cables, the cable is pulled to shore while floating, supported by inflatable floaters. Understanding the hydrodynamic behaviour of these cable-floater-systems (CFS’s) is important, to be able to relate the hydraulic environment to the cable stresses and determine the maximum allowed hydraulic conditions for these procedures. The software package OrcaFlex has appeared to be insufficient and gives untrustworthy results, namely really high peak axial tension and compression forces in the cable. In this research, the CFS is studied with three methods to explain these extreme forces and improve the CFS design.

With the first method, the CFS is modelled as an Euler Bernoulli Beam (EBB) on a continuous elastic foundation. In this analytical model, straight, accessible formulas showed that the CFS is a really stiff dynamic system, dominated by the buoyancy stiffness of the floaters. As a consequence, the natural frequencies are relatively high and in the same range as the wave frequency spectrum. This lead to this research’ hypothesis that the extreme, internal stresses in the cable-floater-system are caused by resonance taking place between the external wave forcing and the CFS itself.

With the second method, the CFS is modelled with the finite-element-method. In the vertical direction, the natural frequencies are again in the range of wave frequencies and are barely increasing for higher modes. The explanation for both of these observations is given by the local oscillations in the modal shapes occurring in between floaters. A parametric study on floater spacing shows that the length of these oscillations is determined by the floater spacing. Since the original floater spacing is small, it causes small wave lengths and high frequencies.

With the third method, dynamic analyses are done in OrcaFlex with cases varying in wave frequency and presence/absence of a current in order to test the hypothesis of resonance. It can be concluded that the CFS is really sensitive to Stokes drift when an other current is absent. This causes a different positioning of the CFS and a mean axial tension in the cable decreasing in wave direction. Consequently, the hydrodynamic behaviour of the CFS is changing throughout its length. Near the cable end most closest from where waves originate, the Stokes drift induced axial tension causes the CFS to be unable to move vertically in waves. Consequently, no interaction between wave and CFS, necessary for resonance, is taking place. Near the other cable end, the mean axial tension is close to zero and the most extreme axial forces occur for resonance conditions with wave frequency equal to natural frequency.

The results do not fully confirm the hypothesis, but do give answer to the first part of the research question. Regarding the second part on the optimisation of a CFS design, one could focus on eliminating the chance on the occurrence of resonance, by increasing the floater spacing or decreasing the floater dimensions, or focus on decreasing the impact of resonance by increasing the floater’s drag area in vertical direction.

Sustainable and renewable energies are essential in achieving climate targets set by the Paris Agreement and Sustainable Development Goals (SDGs). Floating offshore wind turbines (FOWTs) represent an innovative technology with considerable potential to contribute to these goals. However, being a relatively new technology, FOWTs pose challenges that must be addressed to enhance their development. To speed up FOWT installation and optimize their design, accurate numerical simulation tools are essential, particularly mid-fidelity models, which use less computational time but require precise hydrodynamic characteristics to be tuned. These tuning parameters are obtained with experimental tests or with Computational Fluid Dynamics (CFD) simulations.
This thesis aims to analyze the added mass and drag coefficients of the MaRINET2 semi-submersible wind turbine floater when subjected to forced oscillatory motion, utilizing a validated CFD model. To achieve the objective, a nonlinear Navier Stokes numerical model was developed within the open-source CFD software OpenFOAM. The interFoam solver was employed for the simulations. A mesh convergence study identified an optimal mesh configuration, balancing computational efficiency with result accuracy. Two types of simulations were conducted: free decay simulations for model validation against University of Strathclyde (UoS) experimental data and forced oscillatory simulations to compute added mass and drag coefficients under varying oscillation parameters.
It was found that heave added mass is sensitive to both oscillation amplitude and period, while heave drag coefficients displayed minimal influence at shorter periods. For high-amplitude oscillations, surge drag coefficients remained stable across oscillation periods, yet longer periods increased coefficients for
mid and low amplitudes. Pitch added mass coefficients remained unaffected by oscillation amplitude but increased with longer periods. Comparing these hydrodynamic parameters with two reference papers revealed both similarities and discrepancies in trends. It is challenging to attribute these discrepancies
only to floater geometry due to the lack of specific experimental data for this floater.

The advancement of floating wind turbine technology in recent years necessitates accurate predictions of their behaviour using various simulation tools. Utilizing multi-fidelity tools like OpenFAST allows for comprehensive calculations of the entire floating wind turbine structure. However, the precision of these calculations is challenged by the uncertainties associated with different system parameters.

The main goal of this research is to conduct a comprehensive exploration of various parameters, with a particular emphasis on hydrodynamic factors, and their impact on the structural and aerodynamic performance of semi-submersible floating wind turbines. To achieve this, an extensive sensitivity analysis approach is employed, utilizing the multi-fidelity model OpenFAST. The simulations are conducted on a semi-submersible floating wind turbine, specifically the one used in the OC4 experiment, featuring the 5MW NREL wind turbine. The Elementary Effects (EE) method is employed in this study to assess the significance of various factors. These factors include the hydrodynamic drag coefficients, wave height, peak wave period, wind speed, wind direction, current speed, current direction, and turbulence intensity. Their influence on the Damage Equivalent Load (DEL) of different loads acting on the floating offshore wind turbine (FOWT) is thoroughly examined.

The analysis conducted in this study has unveiled several key findings. The most influential parameters identified are the current speed, primarily impacting mooring line tension, and the significant wave height, which exhibits a balanced effect across various outputs, including hydrodynamic loads, tower loads, and rotor dynamics. Additionally, turbulence intensity emerges as a significant factor, particularly concerning rotor thrust and torque.

When employing the Morison equation, the order of parameter significance remains consistent with the hybrid theory (Potential + Drag). However, it is noteworthy that the added mass coefficients carry greater significance compared to the drag coefficient. Furthermore, simulations conducted using probability distributions for different locations confirm that wave height consistently ranks as the most significant parameter. However, the overall significance may vary and decrease in more severe environmental conditions.

The comparison with the OC3 spar platform revealed a similar pattern when examining the influence of current speed and significant wave height. Nonetheless, the simple hydrodynamic drag coefficient used for the whole platform also displayed significance, emphasizing the sensitivity of the spar platform loads to this modelling parameter. This significance is much higher than the corresponding one of the semisubmersible platform.

The research underscores it is imperative to mitigate the uncertainty associated with various parameters. This can be achieved through experimental verification and establishing correlations between modelling parameters and the specific conditions being simulated. Additionally, given that the most influential parameters are related to external conditions, it becomes crucial to simulate conditions that closely resemble the anticipated deployment location for the floating offshore wind turbine (FOWT). By doing so, the accuracy and reliability of the simulations will be enhanced to better inform FOWT design and deployment strategies.

In the last few years, the climate crisis has been accelerating at a dizzying pace and poses an emergency threat to our planet. Rainfalls have transformed into intense downpours, and flash flooding, combined with the sea level rise, leads to a higher risk of inundation of densely populated coastal cities. Moreover, the melting of the permafrost can lead to significant landslides, triggering the generation of mega-tsunami waves. The latter can have a catastrophic impact, not only on the infrastructure but also on human life. Man-induced climate change is responsible for the increase in frequency and intensity of extreme natural events, such as tsunamis, floods, and storm surges. Recent research indicates that almost one-fourth of the world population lives at high-risk locations to at least 0.15m of inundation depths with a return period of 1 in 100 years. Therefore,
the urge of implementing protection measures against unsteady flows is imperative.

Undoubtedly, the involvement of engineers can play a pivotal role in order to analyze these flows in the built environment and provide sufficient coastal and building plans to ensure safety and reduce reconstruction
costs. The behavior of the unsteady flow around a structure is not a well-understood topic and results in a lack of accuracy and reliability. More insights are required into the fluid-structure interactions to come up
with a safe building design.

In the present study, unsteady flows are generated using the dam-break technique in line with previous research. The Thesis aims to model, validate, and implement a simplified approach to analyze the complexity of the hydrodynamic behavior of unsteady flows around impervious buildings, with different orientations
and blockage ratios. To do so, the research introduces a numerical simulation method of a dam-break wave, using the two-dimensional, and non-rotational shallow water equations. The Galerkin finite-element model is applied for the discretization of the solution on a limited domain.

Initially, the flow of the dam-break wave was validated in the absence of the structure, using a dam-break experimental work for comparison. Then, in order to insert a structure in the domain, a second experiment with a structure is used, which generates tsunami-like waves using the vertical release technique. The gate, which represents the dam, is located at x=0 and opens instantaneously at t=0s. Behind the gate, the reservoir maintains an amount of water that flows, after the opening of the gate, into the channel generating the shock
wave. At the channel downstream of the gate, initial water levels are considered and the behavior of a bore propagation is simulated. The building is located on the downstream side and different impervious building configurations were studied in terms of orientation and shape to investigate the impact of the unsteady flow.
To analyze the complex hydrodynamic processes of flooding, four fixed points around the structure are set to measure the action of the bore on different impervious building configurations. The water elevations and the averaged velocity profiles in time were derived at each point. Moreover, the horizontal forces in the x and y directions are calculated by the model, integrating the stresses over the wet surface of the buildings.

The general behavior of the fluid-structure interactions is captured well, especially upstream of the building, and insights are gained regarding the behavior of the fluid around the structure and the parameters of
influence. Results showed that orientation changes completely the impact on the building configurations. The separation of the flow and the blockage ratio are the main parameters that are influenced by the angle of rotation and change the behavior of the loading process at the initial impulsive phase, when the wave arrives at the structure, and at the hydrodynamic phase, where the flow has a quasi-steady behavior. Overall, a good agreement is achieved with the experimental data, although the numerical model overestimates the loads
acting on the different building configurations. Results proved that the orientation of the building with respect
to the flow facilitates the flow around the structure and contributes to lower water levels, and to a better distribution of the horizontal loads on the surface. The best results were achieved for an angle of rotation of 45°, where symmetrical separation of the flow is also playing an important role.

Offshore lifting operations must have reduced payload motion to increase safety and reduce operating time. When payload is retrieved from the splash zone to the deck, besides the crane block, no additional control can be applied on the underactuated system. Existing studies either assume more control over the payload or develop a control system based on a new crane. To reduce payload motion on current crane vessels, a conceptual model needs to be developed.

In this thesis, various state-of-the-art solutions are considered based on four criteria: Time reduction, motion reduction, initial investment required and power required. Eventually, the quantified criteria and an analytic hierarchy process established that the most promising concept is based on an automated side loader of a garbage truck.

The selected concept is developed based on a design process that focuses on optimized material usage. The geometry is determined according to requirements and forms the starting point of the circular design process. A dynamic analysis is conducted to obtain the dynamic response of the payload and eventually the reduced payload motion. The design cycle is complete after a finite element analysis has been conducted to verify the structural integrity of the model. After more than 20 cycles of the design process, the conceptual model is optimized and over 85% of motion is reduced in the X direction. The payload motion in Y- and Z-direction is 20% and 29% respectively.

The simulation results in this study show that the conceptual model is able to reduce the payload motion during offshore lifting operations whilst staying within the limits set by offshore standards. The motion reduction of the payload creates a safer and more efficient environment to execute offshore lifting operations.

Climate change is triggering an ever-growing demand for renewable energy. The U.S. is still far behind Europe when it comes to offshore wind energy. They have made ambitious plans to reach 30 GW in offshore wind energy by 2030 while currently 42 MW is installed. One of the main challenges in the U.S. is the installation method since a legislation called the Jones act prevents the usage of European installation vessels for shuttling (the conventional method). Building a Jones act. compliant installation vessel is a large investment which comes with risks and long lead times. Feedering is an alternative strategy, but barely any research on it is available. Here, a feeder vessel sails back and forth from the storage port to the (non-Jones act. compliant) installation vessel to supply Wind Turbine Generator (WTG) components. These components need to be lifted from the floating feeder in order to be installed. In the literature, this step is deemed to be the riskiest. However, barely any technical research is available with regards to the lift-off.

In the first thesis of this double degree program, a lift/installation sequence called the direct installation method is deemed to be highly interesting with respect to the logistics and costs. However, this research misses a technical study in order to understand if it is technically reachable to directly install these components. In this offshore engineering thesis, a barge is used as a feeder vessel and tower segments of a 20 MW WTG are chosen as the to-be lifted components. This research focuses on the pre-tension phase before the lift-off. This contains the steps where the crane of the installation vessel is already attached to the tower, pre-tension is building up and the release of the sea-fastening. Here, pre-tension is a percentage of the load that is taken in the crane before the lift-off. This research aims to increase the understanding of whether a tower can be released safely on the floating barge and what can be done in order to realise the idea of a direct installation method.

Frequency, as well as time-domain simulations, are used to investigate the problem. The results show that snap loads occur for pre-tensions up to 10\%. From 30\% and higher, the tower will start toppling. Toppling is initiated due to the inertia of the large tower segment when it is released from its sea-fastening. Toppling the tower is not allowed since this could damage the tower itself, the sea-fastening and/or other components on deck of the feeder. Increasing the limiting wave height is a must in order to make the direct installation method more practicable. This can firstly be done by using more tower segments. Therefore, reducing the size of each segment. Another option is to implement a motion compensation tool that decouples the motions of the feeder and the tower. The third option is to design a seafastening system that reduces the moment after the release, a temporary counteracting toppling system. All in all, can be stated that safely releasing a 20 MW tower segment on a floating barge is highly challenging and more research is required to solve the issues that are found in this research. This is necessary to allow the direct feeder method to be used for future offshore wind installation projects in the U.S.