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.

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

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

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.

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.

The potential for developing more efficient offshore wind support structures becomes increasingly critical as the offshore wind sector is faced with challenges such as rising commodity prices and shrinking profit margins. These challenges not only impact the industry’s capacity to meet growing global demand but also hinder efforts to address the imperative challenge of decarbonization. As the offshore wind industry expands, the roles of cost-effectiveness and ongoing research will be pivotal in driving the advancement of offshore wind energy on a global scale.

The support structure is responsible for supporting the turbine, transferring loads to the ground, and allowing access for inspection and maintenance purposes. The environmental loads acting on an offshore wind support structure (OWSS) result from a combination of waves, wind speed, turbulence intensity acting on the shape of the turbine components. These loads can be grouped in a scatter diagrams that depict the probability of occurrence of a given wind wave combined condition which is also known as a "sea state". Different sea states lead to significant vibrations and stresses on the foundation, which can cause fatigue and failure over time.

The perforated monopile consists of a monopile with holes around the splash zone to reduce frontal area, which reduces hydrodynamic loads on the foundation. Assesing the potential of perforated monopiles in deep waters was based on a comparative study of the loads acting on the monopile by developing a model that considers the structural response of the system to different sea states. FEM studies are essential in identifying potential stress concentrations and their effects on the overall integrity and safety of the structure. Thus, analysis focused on assessing the performance of both a reference monopile and a perforated monopile structural models under both parked and power production conditions, including sea states with 50-year and 1-year return periods. The simulations encompassed 35 distinct sea states and computed maximum stresses at critical locations, including the mudline, perforations, and the splash zone.

The study found that the perforated monopile displayed an average reduction of 17% in the maximum stresses found at the mudline compared to the reference monopile. This reduction was attributed to improved flow dynamics facilitated by the perforations. However, the splash zone of the perforated monopile experienced increased stresses around the splash zone of up to a factor of four, attributed to higher overturning moments around the perforated area. Next, the analysis continued with a fatigue life assessment, highlighting sea states that could potentially challenge the structural integrity and longevity of the monopiles over their intended operational lifespan.

The study explored alternative solutions, such as varying thickness parameters at high-stress areas and using different materials at the splash zone. Additionally, the possibility of different perforation geometries was considered, although it could impact natural frequencies and cause resonance with loading frequencies.

To conclude, the research underscores the importance of a comprehensive assessment of monopile designs in offshore renewable energy structures with a discussion and recommendations for further research on the subject. It emphasizes the need for careful consideration of design choices based on specific environmental conditions at the installation site. The findings contribute valuable insights into optimizing monopile designs for long-term performance and structural reliability in varying sea state conditions during power production scenarios.

The Netherlands has agreed, by signing the Paris Agreement, to be climate-neural by 2050. To reach this goal, many large-scale renewable energy projects need to be developed in the coming years using for example wind, geothermal or solar energy. The problem with these projects is that they require large amounts of land and are especially needed in densely populated areas where the energy demand is high. For solar energy, a solution is to move it to bodies of water, creating a floating photovoltaic (FPV) system. Besides not requiring valuable land, FPV offers other benefits; The water has a cooling effect on the panels, increasing the efficiency of the solar panels. There is less water evaporation, which is desired in for example drink water basins. FPV systems have a high potential for system integration with nearshore and offshore wind turbines, optimizing space utilization and making cable pooling possible.

Multiple companies have already developed an FPV system, each designed for specific water categories and having its own (dis-)advantages. This thesis analyses different types of FPV systems and offers a tool for project developers to help decide what kind of system to select in a preliminary phase of project development.

As input to this decision tool, the environmental loads acting on FPV systems are calculated. The forces due to the wind, current and waves are found for three different types of systems. These systems are Floating Solar, Zimfloat and OceanSun. The forces are first found on a small section of the structures and then scaled up to a size of one hectare including sheltering factors. The results show that the membrane-type structure of OceanSun has significantly lower environmental forces than the Floating Solar and Zimfloat systems. These two are both built up of high-density polyethylene floaters and a metal frame holding the PV panels. The wind force is the highest on the Floating solar structure due to the larger tilt angle. The wave forces are found to be the highest for the Zimfloat structure which can be explained by the floaters. These are square blocks instead of circular tubes, making them less aerodynamic and having a larger volume which increases the inertia forces.

The decision tool uses a multi-criteria analysis that takes twelve important aspects of an FPV system
into account including the environmental loads. Examples of these criteria are energy production, costs, energy density and also less quantifiable criteria such as ecological impact, safety and the technical readiness level of the systems. The tool compares six different types of systems and can easily be adjusted to a specific location by changing the weight factors. With these weight factors, the tool provides a clear overview of the most important aspects of an FPV system and helps to decide in an early phase of project development whether a system is suitable for the desired location or not.

The decision tool is used for the case study ’Haringvliet’. Haringvliet is a former estuary in the Netherlands and is chosen as a potential location for an FPV park. The calculations mentioned above are performed using the site conditions found at Haringvliet. The conclusion of the decision tool is that the structure of Floating Solar is the most suitable for the Haringvliet. This is partly because Haringvliet is an ecologically protected area and the floating solar structure is very open, letting sunlight pass through the system into the water.

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.

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.

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.

Continuous development in the wind turbine industry leads to increasing size of wind turbines. Allseas investigates the possibility to enter the offshore wind turbine installation industry with Pioneering Spirit.
For Allseas’ preliminary wind turbine installation design, wind turbines are assembled offshore, requiring wind turbine components to be brought from shore to Pioneering Spirit by means of a cargo barge. This operation requires a proper mooring procedure of which the mooring system is an essential part. The mooring system secures the barge alongside Pioneering Spirit where it has to stay for multiple days.

In this research a pre-defined vessel orientation is analysed where the barge stern is extended 40m in longitudinal direction from Pioneering Spirit stern, to increase the barge area reachable by the unloading crane.
This barge position is challenging due to limited shielding from Pioneering Spirit, leading to excessive environmental loads acting on the barge. Due to the barge extension, also properly connecting the mooring system to Pioneering Spirit is a challenge.
The above leads to the main objective of this thesis: improve the mooring system to secure a barge alongside Pioneering Spirit in offshore conditions, including an evaluation of the dynamic mooring loads.

The main mooring system design requirements are that the system Safe Working Load and motion limits are not exceeded.
Evaluation of the mooring systems starts with a multi-body diffraction analysis with hydrodynamic matrices and other hydrodynamic properties as output. This data is imported in a time domain model which enables capturing non-linearities, e.g. the mooring system and wind and current loads. Validation and verification steps are required to assure realistic outcome and understanding limitations of the numerical models.

Before new mooring systems are introduced, a base case mooring system is defined and modelled in the time domain.
Finding the limiting sea state for which the Safe Working Load limit is reached enables to compute the workability for the reference location defined. Two reference locations are considered: a wind sea area and swell sea area. Wind and current speed is assumed to be constant over time and independent of elevation. Workability is defined as the percentage of time the mooring system is able to operate.

Evaluating the base case mooring system shows performance of 67% workability for wind sea areas and 16% for swell areas. Next to the base case, four mooring improvement concepts are evaluated from which the Cavotec Moormaster® system shows most promising results. This system consists of three main components: ‘fixed structure - hydraulic cylinder - vacuum pad’ (Figure 2) that connects the barge to Pioneering Spirit. This system is modelled as a link with constant stiffness and damping properties within its operational limitations.

The Moormaster system, shows perspective to improve the workability for both wind and swell seas. Besides workability, the Moormaster system improves the entire mooring procedure by quick connection and safe disconnection upon exceeding operational limits.

Why do engineers often design what people don't want? And why do people often want solutions that are not feasible? This is because the current design and decision support optimisation methodologies are one-sided and ignore or fail to capture the dynamic interaction between people's preferences (desirability) and technical assets' performance (feasibility). Furthermore, most methodologies contain fundamental problems and often cannot achieve a single best design solution. Moreover, the offshore & dredging engineering (ODE) industry can significantly benefit from multi-objective design/decision optimisation as projects become larger, more stakeholders are involved in concurrent design/decision-making, and new technologies emerge.

To solve the shortcomings given the current momentum within the ODE industry, a truly integrative Open Systems Design (Odesys) methodology is proposed in this thesis. For this purpose, a new multi-objective optimisation method IMAP is introduced which is operationalised in a Python-based software tool called Preferendus, including a new inter-generational Genetic Algorithm (GA) solver.

The application and added value of the Preferendus/IMAP are validated for two demonstrators within the maritime contractor Boskalis.

The first validation case concerns planning and design optimisation in the development of offshore floating wind farms, focusing on scheduling and mooring design. For this purpose, an integration was made with the wind turbine simulation tool OpenFAST. The new Preferendus/IMAP significantly improved the overall tender performance by: 1) providing initial design approaches within a few hours, where currently tender teams spend days working on design alternatives that, in hindsight, were suboptimal; 2) removing tender team bias from the design process and finding design solutions that were otherwise unfairly disregarded; 3) open glass-box modelling support for concurrent design between the asset owner and the contractor.

The second validation case is a decision support optimisation application of a dredging production in which multiple vessels jointly execute a dredging project. Due to different types of disturbances, these vessels often have to wait for each other, reducing the overall efficiency of the project. Current expert-based optimisation approaches are limited in their ability to adjust best-for-project. The new Preferendus/IMAP shows a clear improvement and finds solutions that significantly reduce waiting time while simultaneously achieving high production levels. Moreover, the Preferendus/IMAP outperforms single-sided optimisation on production alone by achieving similar high production levels while also improving other objectives such as CO2 emissions and vessel efficiency.

Steps for further development include: 1) improving OpenFAST integration and modelling; 2) addition of fatigue loading in the anchor design; 3) improving discrete event simulation scheduling modelling; 4) improving runtime by exploring other algorithms and/or programming improvements.

With the climate goals of the Paris Agreement and the European Green Deal, countries need to reduce their carbon footprint and increase their renewable energy production. For countries with high population density, land-based photovoltaics (solar) takes up valuable space. Therefore, solar application at sea is considered.

One of the proposed designs is by the Joint Industry Project (JIP) Solar@Sea II. The JIP structure is flexible, inflatable, and considerably smaller than Very Large Floating Structures (VLFS). The goal is to mitigate the installation and transportation disadvantages associated with VLFS. Combining multiple of these singular structures in an array allows for the same operational solar footprint as would be the case for a single VLFS.

To keep the structure at the intended location, a mooring system needs to be designed. For the mooring line analysis, the motions of the attachment points of the mooring lines to the floating structure must be deter-mined. These motions are dependent on the interaction between the fluid, structure, and mooring line system. The development of a numerical method that is able to determine the flexible motions of the structure is required. This method should be suitable for the initial design stages of the structure and mooring system.

A numerical method for the deep-water regime in frequency domain was developed. The structural deformations are determined by means of the Finite Element Method (FEM) in ANSYS. The fluid interactions with the structure are determined by means of a lower order Boundary Element Method (BEM). The combined effect of both structural motions and fluid behavior is captured in an equation of motion. The motions of the flexible structure can then be determined by solving the equation of motion. The method is written in Python and the interaction with ANSYS is achieved by means of an ANSYS APDL interface.

The numerical method was successfully verified by comparison of results with analytical solutions. The nu-merical method is validated by comparing the numerical result with experimentally determined responses of a 1:1 scale JIP structure to incident regular deep-water waves as measured by MARIN. Suitable deep-water test cases were determined from the MARIN data. The undisturbed numerical wave was compared with the undisturbed deep wave measured by MARIN. Finally, three test cases were selected for the validation of the interaction of the structure and the wave. These consisted of a wave longer than the structure, a waveslightly shorter than the structure, and a significantly shorter wave than the structure.

The numerical method was able to accurately predict the structure motions for waves longer than the structure. For waves shorter than the structure, the error between the numerical method and the experimental data increased as the wavelength decreased. The errors found can likely be attributed to the nonlinear interaction of the ballast bags, which are not considered in the current numerical method. This cannot be confirmed based on the available experimental data.

The presented work is part of a larger intended method, which is not yet finished. Satisfactory results were found for the components considered in the verification. The validation provided points for improvement for the current numerical method. The response to waves longer than the structure can be determined accurately. The method is less accurate for waves shorter than the structure. The recommendation is therefore to research the effect of the ballast bags on the structure response in shorter waves. This could result in a better approximation for structural responses in shorter waves.

Weirs are constructed inside rivers to manage the water level. On the one hand they need to be designed sufficiently strong to prevent failure or destruction, while on the other hand they need to be economically feasible. If a structure is designed too strong, unnecessary costs are made and the structure becomes too expensive.

One of the potential failure mechanisms for weirs is erosion of the soil downstream of the weir. Bed protections consisting of rock and concrete are placed to prevent this erosion. Numerical modelling can give additional insight in the design of such a bed protection. This thesis aims to improve the design of bed protections downstream of underflow weirs through numerical modelling. This is done through two research objectives, being 1) proposing a numerical modelling strategy to create a design method for riprap bed protections downstream of underflow weirs, and 2) preserving a small computation time compared to full scale 3D models, as this is ideal for a future application in engineering.

A new approach is formulated by using the stability parameter of Steenstra (2014), ψRS, as basis. This parameter is based on multiple flow situations, except the underflow gate. For this reason the underflow gate is investigated in the current research. The output of the created OpenFOAM models can be applied to this parameter, leading to ψ-curves that can be compared with measured damage.

The first research objective is obtained by the application of three different turbulence mixing length approaches, of which two approaches created interesting results. The first approach is the Bakhmetev approach, leading to a conservative design outcome with a gradual decreasing stability pattern. The second is the Shear Stress Relation (SSR), leading to a promising result with better defined instability regions that compare with measured bed damage. From the desire to work conservatively, the results reveal that in the design phase of a bed protection downstream of a weir, the application of the Bakhmetev approach for the mixing length is recommended over the SSR approach.

Extended physical testing containing three aspects is needed to strengthen the promising findings of the SSR. These aspects are 1) a damage stone count including a sieve distribution, 2) a setup with varying gate heights and stone dimensions, and 3) varying water levels and discharges per different gate height and stone size. In addition a preliminary 2D numerical model study is advised, which allows to investigate the areas of interest for PIV flow measurements.

The second research objective is achieved by the application of a 2D RANS model and the simplification of the water level through a rigid lid. This simplification still leads to workable results, with computation time that are in the range of two to eight hours, operating on ten cores in one computer. This makes the approach attractive for engineering applications and for future applications in renovation tasks for weirs in the Meuse.

In contrast to traditional power cables for bottom-founded offshore wind turbines, power cables for floating wind turbines penetrate the water column and are exposed to cyclic loading. Due to the dynamic nature of the cable loading, the cables are prone to fatigue failure. Since the evaluation of the fatigue life of complex structures, like power cables, has to deal with a certain degree of uncertainty, fatigue safety factors are introduced. These factors ensure a desired probability of failure based on the degree of uncertainty present in the modeling of the fatigue life. Limited experience in the evaluation of the fatigue life of dynamic power cables is found in the literature, and no sources have been found attempting to calibrate the prescribed fatigue safety factor. Currently, a fatigue safety factor of 10 is advised. This implies that there is much room for reduction in the uncertainties, meaning that there is a potential for making the cables cheaper and more reliable, which will in turn make floating wind energy more competitive with other (renewable) energy sources.
Due to the lack of available literature, comparisons with umbilicals and flexible risers are made. Different methods for the evaluation of fatigue safety factors for flexible risers are evaluated, and judged on how applicable they are to use on dynamic power cables. It is found that a reliability based approach is most suitable for this purpose.
A case study is carried out for a presently relevant scenario to put the proposed method into practice. It is concluded that for a safety class corresponding to a required maximum probability of failure of 10^-3 in the last operational year, a fatigue safety factor of 3.5 is needed. This safety factor is required for the wave-load induced fatigue, as opposed to the seabed induced fatigue, to which the dynamic power cable is less vulnerable nearby the touch-down zone. 
The main parameter driving the uncertainty in the fatigue analysis is the sensitivity of the local stress analysis, which is in line with what has been found for similar studies for the oil & gas industry. In order to bring down the cost of dynamic power cables, and make them more reliable, more accurate local stress analysis models need to be developed, being validated against test data, in order to reduce the standard deviation of the local stress computation.

With the increase in need for offshore wind exploitation a research is set up with aims to develop a preliminary design tool. The introduction begins with background information on what role floating wind will play in the offshore wind industry and why this is a relevant topic. A general overview of floaters is presented with a comparison of the advantages of each type of floater. As well a chronological review of approaches to preliminary design. After which a numerical load and response model is developed. First theory is discussed regarding load and response modelling as well as the fatigue damage calculations. After the theory is covered, a test case is set up using the IEA 15MW reference turbine. The turbine is subjected to aerodynamic and hydrodynamic loading. This loading is calculated using Morison’s equations and a simplified thrust coefficient. In the test case the IEA15MW reference turbine is put atop a large floating substructure. For the environmental condition a site off the coast of Norway is used with an adjusted depth to allow for the size substructure. The response calculation is then, where the equations of motion are set up in matrix form, and terms in all of the matrices are then derived for the mass, added mass, stiffness and damping matrix. The stiffness matrix considers hydrodynamic stiffness and mooring stiffness. The terms in the damping matrix are derived from aerodynamic and hydrodynamic damping. Hydrodynamic damping only considers viscous damping terms. The model is then tested for a set of regular and irregular wave and wind conditions to analyse the system’s behaviour. For the fatigue calculation the substructure is divided into welded areas that will be investigated for lifetime global fatigue. This response model is validated by comparing natural frequencies of a smaller well documented 5MW floating spar. Optimization theory is covered, where multiple gradient and non-gradient based approaches are discussed. The simulated annealing algorithm is developed and tested on a set of test functions: Ackley & Booth. The algorithm shows quick convergence for areas that have small changes. The optimization problem for this research is formulated. The design space is visualised and compared to the most similar test function. After which, the constraints used by the optimiser are presented. The first constraint set consists of logical design constraints determined by the spar geometry. After which, the constraint set also includes limitations on the extreme response in time domain simulation. In the results chapter, it is found that this slows down the process so much that it is not feasible to include fatigue damage in the constraint function. Furthermore, it is found that global fatigue damage wouldn’t be a governing constraint. The results chapter discusses the importance of mooring stiffness in this optimization. Present the results from constraining the optimiser with the time domain response and compare the fatigue lifetime of the optimal designs. It is found that global fatigue is not a governing design consideration. It is found that both geometrical and response constraints are relevant for this type of optimization. However using global fatigue as a constraint is both irrelevant and computationally infeasible for the optimization of a floating substructure. Finally, a discussion is presented where the work is critiqued, and recommendations are made for further work.

Using a floating vessel operating on its DP-system and using a motion-compensated pile gripper to install monopiles could be the installation method of the future. Therefore, this thesis focuses on this method. The main objective of this thesis project is to build a model that accurately describes the motions of the Stella Synergy, the monopile, and the motion-compensated gripper, depending on the environmental conditions.
This model is built in Anysim, which is a time-domain simulation software program of MARIN based on the RK2 numerical method. The model considers the early pile driving phase because this phase is governing in terms of risk. The monopile acts as an inverted pendulum in this phase, and the motion-compensated pile gripper must guarantee the stability of the monopile. The vessel uses its DP-system for station keeping. The DP-system contains a position reference system, a filter, a control system, and a thruster allocation algorithm.
The vessel describes the wind, current and wave forces on the monopile and vessel. The environmental conditions are assumed to be collinear, and wave spreading is added to the model for some simulations. The wave forces on the vessel are determined with diffraction calculations in Ansys AQWA. The diffraction calculation for the vessel is verified with a diffraction calculation of MARIN, and the diffraction calculation for the monopile considers the shielding effect and is verified with a calculation with the Morison equation.
A motion-compensated pile gripper with two PD-controllers is built in Python. The gripper considers static and dynamic friction forces and a maximum delta force per numeric timestep to model the pressure build-up time of the hydraulic cylinders.
Multiple 3-hour simulations are run to generate results. These simulations, which considers each a different sea condition, are tested by the six limitations of the model. First, the preferable incoming angle of environmental conditions is determined. The workability of the Stella Synergy is calculated operating at the North Sea using this preferable incoming angle of attack. Then, two adaptations to the model are tested to increase the workability. Using fast-rotating thrusters or changing the DP-gains result in the workability of 96.4%. The governing limitation is the pitch motion of the vessel.
It is tested if using mooring lines in combination with the DP-system results in a footprint reduction. It is concluded that adding mooring lines could result in a footprint reduction, but it is crucial to gain insight into the optimal axial stiffness of the mooring lines. The monopile's influence on the vessel's motion is also tested. It is concluded that the vessel's surge, sway, roll and yaw motion increases significantly due to the environmental forces on the monopile, which are passed through the gripper to the vessel. Finally, the workability of the vessel during the worst-case single failure is determined. After improving the DP-gains for particular sea conditions, the workability for the worst-case single failure was 96.0%. The failure results thus in a minor difference in the workability.

At the moment, the world is at the verge of an energy transition. One of the most promising green resources is solar energy, which is a rapidly growing market. However, to fully use its potential of economy of scale, the application of offshore floating solar should be explored. A promising option is the use of a flexible type of Very Large Floating Structures (VLFSs), which are called Very Flexible Floating Structures (VFFSs). They are characterised by their large length to height ratio compared to rigid bodies and depending on their material properties have a hydroelastic response to the incident wave. In the late 1990s, a lot of research has been done on VLFSs by Tsubogo and Okada (1998) who derived an analytical dispersion relation assuming a zero­draught structure. However, only recently, Schreier and Jacobi (2020b) did experimental research on VFFSs in a towing tank at the Delft University of Technology, as little is still known about flexible structures. This report focuses on a numerical alter­ native that covers both VLFSs as well as VFFSs using a Finite Element Method (FEM) Fluid Structure Interaction (FSI) model which has been built based on potential flow to model the fluid and a dynamic Euler­Bernoulli beam that represents the floating structure using the Julia package Gridap. One of the main advantages is that the zero­draught assumption is not necessary and, therefore, structures with larger draughts can also be modelled. Next to this, the numerical model is able to cope with irregular shapes, for which no analytical method yet exists. The model is built such that it can handle 2D as well as 3D domains. A 2D analysis has been made to understand the influence of hydroelastic wave deformation of the incident wave, in terms of wavelength dispersion as well as amplitude dispersion on floating structures. To verify the model, the numerical results were compared to the analytical solution and experimental research in a towing tank, which showed accurate results. Test runs were set up that mimicked the towing tank set­up and a full­scale solar park. Furthermore, a sensitivity study was executed that shows the limits of the flexible domain and to see in which cases significant (>1%) hydroelastic wave deformation would occur using governing mean and extreme ocean waves, as well as a typical lake wave. Finally, the influence of the draught of the structure was examined. This report provides a good overview of when wave deformation should be accounted for in terms of bending rigidity and density. Confirming existing theory, it was found that the stiffness of the VFFS causes wave stretching and the draught of the structure influences the extent of wave shortening. It was also found that significant wave deformation will not occur for ocean waves as the required stiff­ ness is beyond existing materials. For extreme ocean waves, there is even no dispersion at all. As the wave frequency increases, the hydroelastic interaction gets stronger. The typical lake wave showed to be well within the flexible regime and also showed significant dispersion with realistic material parame­ ters. The numerical model is able to cope with large draught scenarios which lead to wave shortening, which in its turn leads to wave focusing. Ultimately, the numerical model showed to be a good alternative to existing methods to investigate the behaviour of VLFSs outside the floating solar domain, where one could think of ice floes, floating islands or floating airports.

A QUAD lift is a new lifting method in which dual crane vessels combine their vessel capability to increase their offshore lift performance. The use of the Jumbo J-Class vessels in a QUAD lift creates the opportunity to increase the offshore lift capacity and to install structures with larger dimensions. When floating vessels are close to each other in an offshore environment, their motion will be different than in the freely-floating situation because of hydrodynamic coupling and wave diffraction forces. The main objective of this thesis is to create a model of the QUAD Lift method which predicts the vessel and load motions and evaluate the workability such a lift. Both potential solvers AQWA and OrcaWave are used to assess the hydrodynamic parameters of the interacting vessels. The gap between the vessels is 40 m and the vessel configuration is such that the cranes are parallel to each other. In between the vessels, transversal wave resonance induces peaks in the frequency dependent radiation forces of the vessels. An additional damping lid in between the vessels effectively reduces the resonance behaviour, which is overestimated by potential solvers. The damping lid has an negligible effect on the final workability of the QUAD lift. A 18-DoF linear Matlab model is created which includes the mechanical connection between the vessels and the load. The cranes and the cables are modelled as linear springs. The natural frequencies and eigenvectors show large coupling between the vessel roll and the load sway motion. Tugger lines between the vessel and load are added to shift the natural frequencies of the system and to decrease the large horizontal responses of the load. A parametric study is done on the effect of the load mass, cable lengths and wave directions on the system motion in the most probable wave condition in the Central North Sea. An increase of the mass of the load leads to larger vessel and load motions. The shorter the cable length, the larger the vessel and load motions. The motions are most severe in beam and quarter waves. Depending on the stiffness of the tugger lines the workability can be improved up to 85, 55 and 24 % in respectively head, quarter and beam waves. The limiting factor for the workability is the off-lead angle of the cranes. Broadening of the off-lead angle limit of the crane shows great potential to further increase the workability.