The transition to a low carbon energy system is focusing attention on the synergies between large scale offshore wind and green hydrogen production. These synergies can have system-wide benefits for the integration of wind farms to the power system, whilst improving their economic performance. Offshore wind investments face multiple uncertainties: commodity prices fluctuate and impact turbine cost, vessel rates required to install the turbines are dynamic and the electricity price fluctuates daily. Additionally, to realize wind-hydrogen synergies (hybrid powerplants) the electrolyzer is an additional source of cost uncertainty. On the contrary, the revenue uncertainty of the wind farm is expected to be mitigated through the use of Hydrogen Purchase Agreements. To take informed investment decisions regarding wind-hydrogen synergies, Vattenfall, a leading European utility, is interested in analyzing the trade-off between increased cost uncertainty and reduced revenue uncertainty.

This thesis investigates how integrating a 400 MW onshore electrolyzer with a 2 GW bottom-fixed wind farm affects the project’s economic uncertainty. To answer the question, a three-step methodology is applied. Firstly, the economics of the offshore wind farm and hybrid powerplant are modelled without the presence of uncertainty, using Vattenfall’s techno-economic model. This model considers investment and operation costs, along with the revenues of the wind farm from electricity sales on the power market. For the revenues of the hybrid powerplant, an optimization algorithm deciding when it is optimal to produce hydrogen or electricity is employed. Secondly, the uncertainties in key commodities (steel, aluminum, copper and shipping fuel oil), vessel day-rates, electricity prices and electrolyzer costs, are defined. Thirdly, the uncertainties are sampled through a Monte Carlo simulation to create 10,000 different realizations of the project, covering the entire range of possible outcomes. By combining the techno-economic model, the dispatch algorithm and the Monte Carlo approach, the uncertainties of the offshore farm and hybrid powerplant can be quantified, and their impact on the project economics can be evaluated.

The methodology allows to compare the effect of hydrogen integration in the business case uncertainty of an offshore wind farm. On a deterministic comparison, the two projects perform similarly. For the considered wind farm, the inclusion of the electrolyzer adds an additional e94 M
in cost uncertainty, while it reduces revenue uncertainty by e18 M. These results hold true for the considered offshore farm, given projected conditions and modelling assumptions. For this case, the offshore wind farm is marginally more certain in terms of economic returns. Given the two projects produce similar investment returns, accepting the increased uncertainty of the hybrid powerplant is non-economical.

The results of the thesis do not favor an investment in hybrid project in terms of uncertainties. However, this result is true for the project investigated and as conditions change and hydrogen technology matures, the analysis can shift in favor of hydrogen. Specifically, hydrogen is a large
scale infrastructure with the first full scale projects currently under development. With more project-experience gained, the uncertainty in costs can be reduced, favoring the hybrid projects. With increased support for large scale hydrogen production, the proposed framework for continuous analysis can be leveraged to keep pace with the external macroeconomic changes. The methodology can be extended and applied to other hybrid solutions, such as hybrid projects with batteries.

The transition to renewable energy has positioned offshore wind power as a cornerstone of sustainable electricity generation. As offshore wind farms expand further from shore, High-Voltage Direct Current (HVDC) transmission systems have become essential for efficient power delivery. These systems consist of multiple cables, preferably installed as bundled units for various reasons, such as cost savings and minimising environmental impact. This thesis investigates the mechanical properties and the static and dynamic behaviour of HVDC submarine power cable bundles during the normal lay operation, a critical installation phase.

The study focuses on a 2 GW 525 kV HVDC transmission system comprising two single-core HVDC cables, a metallic return cable, and a fibre optic cable. Bundled installation is examined due to its potential to reduce installation costs, enhance spatial efficiency, and minimise environmental impact. However, the mechanical complexities of installing bundles with varying cable properties necessitate an in-depth exploration of their behaviour under operational conditions.

A multi-faceted approach was adopted, beginning with a comprehensive characterisation of the mechanical properties of an HVDC submarine power cable bundle. This characterisation included an axial, bending, and torsional stiffness analysis, evaluated under full-stick and full-slip conditions. Analytical and computational models were developed using diverse theories and advanced modelling techniques in OrcaFlex. Particular attention was given to the role of bundling straps in influencing intercable interactions and ensuring overall bundle stability during installation. The analysis revealed that cable bundles predominantly operate in the full-slip regime, where the aggregate stiffness of individual cables primarily defines bundle stiffness.

The installation process was studied from both static and dynamic perspectives, focusing on the behaviour of the bundle during normal lay operations. Three distinct cable (bundle) tensioning strategies
were evaluated, each offering unique insights into the resulting mechanical properties, responses to environmental loads, workabilities, and required strap tensions. The dynamic simulations incorporated
realistic environmental conditions, such as wave-induced motions and seabed interactions, to assess the robustness and efficiency of each strategy. The analysis identified the individual tension control
strategy as the most effective approach. By independently managing the tension in each cable within the bundle, this method offers superior control over compression loads, bending radii, and twist, ensuring the bundle’s structural integrity during installation.

Future research should focus on developing theoretical and computational models to improve the accuracy of predictions regarding the twisting behaviour of cable bundles. These models should comprehensively account for environmental conditions, internal loads, and stick-slip interactions to provide precise estimations and must also be validated and verified. Furthermore, the integration method for the physically sensitive fibre optic cable requires refinement to ensure its structural integrity during installation.

The global concern over climate change has accelerated the shift towards renewable energy sources, in which floating offshore wind energy is expected to play an important role. A floating wind turbine could be installed in a harbour using a quayside crane. Various types of floating foundations are available, where a semi-submersible floater has the advantage of versatility in different water depths due to its small draft. However, the continuous upscaling of wind turbine sizes introduces new challenges for the installation process. Installing these larger turbines requires the use of the world’s largest cranes and large quaysides. The operability of the wind turbine generator (WTG) installation has a significant impact on the planning and feasibility of a floating wind farm project. Therefore, this research is aimed at studying and understanding the factors that influence the operability of the WTG blade installation on a semi-submersible floating wind foundation using a quayside crane.
To investigate how the dynamics of the floater and blade are affected by wind and wind conditions, a numerical model is developed. This is done in frequency domain which allows for fast calculations to gain insight into a variety of parameters. The floater model includes a 3D wave diffraction-radiation analysis, as well as a procedure for including viscous damping. Moreover, a response-based approach is applied to obtain the characteristic displacement and velocity of the turbine hub, which correspond to specific sea states. The blade is modelled as a pendulum with two degrees of freedom in axial and radial directions. Taglines are modelled as spring dampers that reduce the blade motions. A similar response-based method is used to obtain the characteristic motions that correspond to specific wind speeds and directions. During the critical alignment and mating phases, the relative displacement and velocity between the blade root and turbine hub are the limiting factors for installation. These relative motions, that follow from the floater and blade model, are then used to calculate the allowable environmental conditions for the wind turbine blade installation. There after, these allowable conditions are combined with metocean data to obtain the operability.
The model is used to study the sensitivities of parameters for the blade and floater motions. Aligning the blade with the wind is the most effective way to reduce blade motions because the motions in the axial direction are significantly lower compared to the radial direction. The influence of the distance between the quayside and the floater is also studied. For specific sea states, there are beneficial distances. However, when considering a large range of sea states, there is no significant advantage for certain distances. An important parameter affecting the relative motions between the blade and hub is the nacelle and blade heading. Optimising this orientation significantly increases the allowable environmental conditions. For low wind speeds, the optimal relative nacelle heading with respect to the wind and wave directions is dependent on these directions. For high wind speeds, above approximately 6.5 m/s, the blade should be aligned with the wind. Additionally, the lowest motions occur when the nacelle is rotated inward, closer to the floater’s centre of gravity.
Five different blade installation strategies are studied to identify one that yields the highest operability. A method where the nacelle and blade heading is adjusted to the optimal position for each change in environmental conditions yields the highest theoretical operability. However, in practice, a better solution is to select a single nacelle heading that is optimised for high wind speeds from a dominating wind direction. Utilising this strategy, it was found that a yearly operability of approximately 65% and 45% could be achieved for installing the turbine blade in a sheltered and exposed harbour, respectively. During the summer months, this operability increases to approximately 80% and 58%, respectively. The operability is mainly driven by wind-induced motions, so a location where wind speeds are low is favourable for achieving high operability.
In conclusion, this research contributes to the understanding of the critical factors that influence the op- erability of wind turbine blade installation on floating foundations, which is a crucial step in the upscaling of floating offshore wind technology.

The projected rise in monopile installations for offshore wind turbines has spurred interest in innovative installation methods to install larger monopiles using floating vessels while simultaneously enhancing workability and efficiency. One such innovation is the motion-compensated gripper frame (MCPG), which stabilizes the monopile by compensating for environment induced motions affecting both the monopile and the floating vessel. The MCPG is capable of compensating for the wave-frequent motions during lowering of the monopile as well as during the hammering phase of the monopile.
This thesis aims to create a frequency domain model to assess the performance of a motion-compensated pile gripper (MCPG) system for offshore monopile installation using a floating installation vessel. Simulating in the frequency domain offers the advantage of minimum computational time, enabling a large number of scenarios to be simulated in a shorter amount of time compared to time-domain simulations. To assess the installation up until the piling phase, two dynamic models have been created, each based on a particular installation. The first steps of the installation process are divided into three phases: upending, lowering, and pre-piling. Two dynamic models were created, focusing on the second and third phases. The phase 2 model accounts for the lowering of the monopile, while the phase 3 model concerns the situation where the pile has reached its self-penetration depth.
The equations of motion were derived for each system and transformed to a state-space model to implement the controller for the motion-compensated gripper. The state-space models were converted into transfer functions in the frequency domain and subjected to first-order wave forces calculated by the diffraction analysis software WAMIT. The method of obtaining the frequency response was validated using data provided by GustoMSC while the motions of the monopile were validated by comparison to an analytical compound double pendulum model.
This research proposes a response-based approach tune the controller proportional and derivative gains. The gains were determined based on the response characteristics of the phase 3 system. These gains were subsequently applied to the phase 2 system across all drafts. The system’s performance and tuning method were assessed using stability analysis, frequency response, spectral analysis and modal analysis. The models and tuning approach were tested using 6, 12 and 15 meter diameter monopiles, which for the phase 2 system were suspended at drafts of 5, 25 and 45 meters. Simulations were conducted for various sea states, comparing open-loop (without controller) and closed-loop (with controller) scenarios. The systems are analyzed based on the following parameters: the monopile roll angle, crane cable offlead angle, gripper force, gripper stroke and gripper stroke rate.
Results indicated that the MCPG effectively eliminated monopile roll motion in both phases. In the closed-loop phase 3 system, gripper stroke and stroke rate were dominated by vessel sway motion, while phase 2 did not show much reduction in gripper stroke and stroke rate. The operability analysis identified the critical operability limits: in the open-loop phase 3 system, the monopile roll angle was the primary limit, followed by gripper stroke. In the closed-loop system, gripper stroke rate and force became the primary limits. For phase 2, gripper stroke was the main limit in both open and closed-loop systems, with stroke rate becoming more significant in the closed-loop system. The MCPG shifted the limiting criteria from monopile roll angle to gripper stroke rate in phase 3. The combined operability of the phase 3 system including the MCPG was found to be 25%.
Finally, the research recommends the addition of time-domain simulations to capture non-linear effects. It is also recommended to extend the models to three dimensions in order to simulate non-beam or bow waves. The phase 3 model can also substantially benefit from the addition of soil loads.

In recent decades, the global push for sustainable energy has grown, with offshore wind turbines emerging as a successful solution. This has led to the rapid growth of the offshore wind farm industry. As more wind farms are built, the demand for offshore installation vessels has increased significantly. It’s crucial for contractors to complete wind turbine installations safely, reliably, and fast. As turbines continue to increase in size, they require higher lifts of larger blades, complicating the installation process due to component movement.

One of the most critical stages in turbine installation is aligning the rotor blade with the hub. Blades can be attached to the hub from various sides. The most used method involves horizontal blade installation, but also diagonal and vertical installation has been done. To achieve alignment with the hub, the blades are lifted using yokes and their position is controlled using taglines. Taglines connected to the crane boom, slewing platform, or both are used. Taglines can operate in constant tension or speed-dependent tension modes. The question arises, how do different blade installation methodologies perform and compare.

The objective of this research was to model and quantitatively compare the performance of various blade installation configurations. Performance is measured by how well the blade root stays within a 0.2 meter envelope under different wind conditions to allow the final mating of blade and hub. The first step was to study the parameters affecting turbine blade alignment simulation, focusing on wind as the main environmental factor causing blade movement. This led to a detailed examination of the essential parameters to accurately model turbine blades.
A 6 degrees-of-freedom dynamic simulation model of a 15 MW wind turbine blade suspended from an installation crane was developed, focusing on the blade’s dynamic behavior during alignment with the hub. The model simulates the aerodynamic forces on the blade and its motions under different scenarios, considering environmental factors like wind velocity and turbulence intensity, but excluding movement of the jack-up vessel, vessel crane, and hub. The model allows for simulating many combinations of blade position, tagline configuration, and tagline operation mode, comparing the performance of different installation techniques during various wind conditions.

From the results, it is most likely that the way of attacking the hub by the turbine blade (side or bottom) is of high importance. From the results it appears that horizontal installation is most reliable. The model indicates that a tagline system that adjusts the pull force based on the speed of the blade’s movement as the most effective and the tagline configuration was of less importance. The results demonstrated that a higher steepness of the slope (ratio between tagline force and pay- out speed) of the damping tugger improves the damping of the blade and is most favourable for offshore turbine blade installation. The model indicated that the horizontal-horizontal installation method was the most effective. The results showed that it was most feasible to install the blades at wind speeds up to 8 m/s. For higher wind speeds, installations could still proceed if there was lower turbulence intensity.

It is recommended to extend the model to include vessel, crane, and hub motions. Another recommendation is to validate the simplified blade model with detailed (confidential) blade data. Finally, using a more detailed model to explore tools and control strategies to minimize blade root motions is recommended to improve workability.

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

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

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

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

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

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

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

Offshore wind turbines are getting larger and are being installed in ever deeper waters. This requires increasingly stronger yet affordable foundations. The Tri-Suction Pile Caisson (TSPC) is a Wind Turbine Generator (WTG) foundation concept patented by SPT Offshore that requires a smaller amount of structural steel or concrete than a conventional monopile. It can be towed from port to location, and can, thus, be installed by smaller crane vessels. Additionally, it does not require piledriving or noise mitigation measures. Despite its promising potential, unlike its competitors and proven technologies, i.e., the monopile and the Suction Bucket Jacket (SBJ), little is known about the in-place behaviour of the TSPC in sandy or clayey soil types.
This study aims to further existing knowledge regarding the interaction between the three clustered suction piles and the surrounding soil and, subsequently, contribute towards making the TSPC an economically feasible option. To this end, a parametric analysis was conducted using PLAXIS 3D, a Finite Element Modelling (FEM) software, to assess the effect of suction pile centre-to-centre distance, load combination and soil type on the TSPC behaviour under monotonic static loading.
To validate computational results against realistic values, the calculation of the environmental loads acting on the TSPC and the soil constitutive model parameters calibration were made using metocean and geotechnical data from the Aberdeen Offshore Wind Farm (AOWF).
In total, five centre-to-centre distances were considered ranging from 1.2 to 2.0 times the diameter of the suction buckets. Two governing Ultimate Limit State (ULS) load combinations were identified, corresponding to WTG rated wind speed and 50-year storm conditions. Finally, two soil types were addressed, medium-loose sand, modelled by the Hardening Soil Model (HSM) and soft-stiff clay, represented by the NGI-ADP model. The contribution of the above parameters to the mobilization of failure mechanisms, and the evolution of foundation stiffness, was investigated.
Based on this work, it is concluded that the TSPC behaves very similarly to a mono-caisson for the most clustered geometrical configurations, especially in clay, where the formation of an inner soil plug is evident. For larger centre-to-centre distances in sandy soils, the TSPC approaches the behaviour of the SBJ, with the mobilization of axial caisson pair capacity being the main resistance factor against overturning moment loading. With the current knowledge, it is not straightforward to determine which geometrical configuration is the most optimal in terms of structural steel efficiency and installability in sand and clay. Thus, the challenge of the TSPC design optimization remains.
The results of this work, however, can be used to outline the optimization process by providing starting points for normalised centre-to-centre distances in sand and clay, combined with further research on the quantification and normalisation of connection beam costs and rigidity. The investigation of different embedment ratios, accounting for installability, would also contribute to that direction. Finally, the assessment of TSPC behaviour under cyclic and dynamic load is encouraged since it can shed light on a potential strategic advantage against conventional WTG foundations.

All over the world, many offshore structures have been built in the last decades, both by the oil and gas industry and by the renewable industry. When these structures reach their end of life, they have to be removed. The removal is done in decommissioning projects. Many of those offshore structures are bottom founded. Of that amount, a big part has steel space frame substructures called jackets. Jackets are often removed with a heavy lift vessel. The crane hook of such a vessel is connected to the structure to be lifted by means of lift points. These lift points are the connection point of the rigging with the structure. The structures removed in decommissioning projects are often more than 30 years old. Because of that, there are a lot of uncertainties encountered by the industry in decommissioning projects that can lead to issues, such as loss of information or unknown material strength. The result is that all aspects of a decommissioning project have to be reconsidered each time. This makes the preparation phase costly and time-consuming. An aspect of the preparation is the selection of the lift point type, while it is unclear how and whether an optimum solution is chosen. Therefore the question arises, if the decision making process can be optimized by improving the selection of the lift point type in the preparation phase. One way to optimize such a decision-making process is by means of artificial intelligence. AI is able to compare data where humans see no connection, it can analyze large pieces of data in a much shorter time than humans can, it makes unbiased decisions and it can significantly reduce errors and increase accuracy and precision. This thesis will therefore focus on the optimization of the decision-making process of the lift point design of jackets by means of artificial intelligence. To answer the overall research question, first an analysis is made of various decommissioning projects of the past years. These projects were carried out by Boskalis. In this analysis, an attempt is made to visualize which steps are taken in the lift point selection process. By means of these steps it can be investigated whether there is a repetition or trend in the selection process of the lift points. In addition, the analysis aims to find the criteria the decisions are based on. Secondly, the analysis shows that certain steps are taken by Boskalis in the decision-making process of the lift point design in decommissioning projects. A trend was clearly visible in the way the lift points were selected. The relatively simple options are examined first, after which the more difficult options are investigated if the others prove impossible. In addition, the analysis found clear criteria that are used in the selection of the lift points. These criteria were reflected in the selection process of the execution strategy and in the selection process of the lift points. In addition to the total costs, among others ease of installation and the total duration also play a major role in the decision-making process. For example, an option was chosen that was more expensive than its alternative, but had a much shorter duration. This information made it possible to determine whether AI could be used in decommissioning projects in the future, to accelerate the preparation phase. Thirdly, it is investigated whether AI can improve the decision-making process of the lift point design. Therefore, three AI applications are examined and compared with the previous findings. The AI applications are: machine learning, genetic algorithms and single variable optimization. All three turned out not to be a good solution to the problem. For machine learning there is too little data available. Genetic algorithms do not seem to give the desired output with the given inputs. And single variable optimization is too limited in its solution. The final conclusion is therefore that the three investigated AI applications in this way will not improve the decision-making process of the lift point design. If the data set were to be expanded, machine learning could eventually be applied in the future. In addition, it could be investigated in future research whether genetic algorithms can be used if the input is changed. This thesis only briefly discusses what GA entails and whether the method has the desired output with the data found. Also, only a limited number of AI applications have been investigated in this thesis. But there are many today. For example, multi variable optimization could be a starting point for further research. Because there, the optimization function can have two variables, so that, for example, time and cost can be entered in different units.

The offshore wind sector in Europe has grown significantly over the past 20 years. Following this surge in number of installations, the first generation of turbines is reaching its end-of-life (EOL) phase and thought needs to be put into end-of-life strategies. As the market for lifetime extension is still in its infancy and re- powering is economically unappealing, decommissioning is the most obvious solution. The tower, Rotor Nacelle Assembly (RNA) and electrical cables will most likely be removed by reverse-installation methods, as well as the Transition Piece (TP). Removal of the monopile foundation however, is considerably more diffi- cult due to the large embedded length of the pile in the soil. The common way of decommissioning such piles in industry (mostly the Oil & Gas (O&G) industry) is to cut them at or below the seabed and leave the remaining stumps in place. This research investigates more sustainable methods that remove the foundation completely, as this is more in line with the modern circular economy and regulations, i.e., OSPAR, are getting more stringent, focusing on complete removal of all offshore structures in the future. To remove the entire monopile foundation using only a blunt uplift force, tremendous amounts of force are required. It is thus desired to lower this extraction force by some technique to reduce soil resistance. This research investigates the possibility of reducing soil resistance surrounding monopiles by utilizing pitch and heave vessel motions. Such motions can induce an harmonic force on the monopile during the first mo- ments of lifting. The hypothesis is that low frequency oscillations of about 0.1 Hz can induce plastic strain accumulation of the soil, resulting in permanent displacement of the pile. This hypothesis is tested by con- structing two models, one describing vessel motion in MATLAB, and the other describing the pile-soil inter- action under cyclic loading, which is modelled in the open source software OpenSees. An extensive market study is performed to find a representative maximum set of dimensions of the first generation of monopiles installed offshore. Monopiles with a grouted connection to the transition piece are considered as first gener- ation monopiles, and it is these monopiles that are of interest to the study because they need to be removed the first. The vessel considered in this research is the Pioneering Spirit and the crane its Jacket Lift System (JLS), due its large lifting capacity. After delineating to the monopile size and the vessel, assumptions are made regarding the extraction process. A cranemaster is added to the system to avoid slack in the lines and a connection tool between the crane wire cables and the monopile is selected, assuming a rigid connection between the two. The results indicate that applying a low frequency cyclic load to a monopile can result in the accumulation of plastic strain of the soil and, consequently, to larger monopile displacements than in monotonic load cases of the same magnitude. In the simulations performed in this research, this effect, called cyclic degradation, is observed to be more intense in sands than in clays. It is observed that plastic strain is largest when large forces are applied to the model, hence it is desired to maximize the combination of tension and amplitude of the applied cyclic force. The effect of cyclic degradation however, is stronger if the forcing frequency is increased to a value near the resonance frequency of the pile-soil system. This value lies in the region of 4 to 8 Hz, depending on the force input characteristics because of the non-linear behavior of soils. Achieving fre- quency multiplication of the input force has been considered in both active and passive ways. It is concluded that only active frequency multiplication may deliver the needed cyclic degradation, since passive frequency multiplication diminishes the amplitude of the cyclic force, driving up the resonance frequency of the pile- soil system even further. The applicability of the pile-soil model, and thus these conclusions, is only valid during the first moments of lifting, when displacements are relatively small. The results obtained in this thesis are the result of a relatively conservative soil dynamics model and a non- conservative hydrodynamical model. The reliability of the results could be further improved if the two models are combined in a co-simulation, where at every time-step the pile displacement and vessel motion are deter- mined. Additionally, expanding the OpenSees model so that accurate representations of large pile displace- ments are included, will allow for calculation of longer simulations, where the pile is completely removed from the soil. The hydrodynamic model can be improved by using an entire sea-state as input.

Since the introduction of the first offshore wind farm in 1991, the demand for offshore renewable wind energy has experienced exponential growth all around the world. To supply this demand, the power rating and corresponding dimensions of offshore wind turbines have grown significantly. Due to the ever-shrinking availability of easily accessible shallow water sites and the abundance of high quality wind resources in deeper water, the industry is stimulated to come up with innovative, yet cost effective, solutions to tap into these deep water sites. Historically, the use of monopile foundations has been an important facilitator of cost reduction due to its relative ease of manufacturability, transportability and installability. Monopile foundations have, however, thus far only been used in relatively shallow water depths. With jacket-type and floating support structures remaining relatively costly, the question arises if the monopile could yet be scaled up further to be used in water depths beyond the current 40-60mfor future, 10+ MW wind turbines. The goal of this research is to investigate the technical feasibility of monopile foundations in the water depth ’gap’ of 60 to 120 meters, which is currently claimed by jackets, for large wind turbines and determine critical design parameters for up-scaling monopiles to these depths. With an eye on future developments a 15 MW reference turbine is adopted and to make the research widely applicable the Hywind Scotland site in the Northern North Sea is selected, a well-documented reference site with a very severe wind and wave climate. To define monopile designs, a parametric (static) monopile geometry optimization tool is developed in Excel, which transfers the environmental data to forcing components. The monopile geometry is optimized for first natural frequency and ULS resistance (yield and global buckling) by varying the outer diameter and wall thickness along the structure. It was found that the ULS check is governed by the inertial wave forcing during survival case (50 year return period storm conditions). As the acting wave frequency is way lower than the system natural frequency, the monopile can be adequately assessed using a static approach. To assess the effect of critical variables, the tool is used to define monopile geometries for water depths ranging between 60 and 120 m, target first natural frequencies of 0.15, 0.17 and 0.20 Hz and a range of soil types, while complying with known manufacturing limits. The results show that all designs are within manufacturing limits and resistance against ULS decreases for lower target frequencies, making 0.15 Hz monopiles unfeasible. In contrast to the ultimate limit failures, the fatigue damage is largely incurred from normal rather than ultimate wave states, which embrace the first natural frequency of the system, thus warranting a full dynamic analysis. In order to assess the fatigue resistance of 0.17 and 0.20 Hz monopiles, which pass the ULS check, an analytical full dynamic model is developed in Maple based on the fundamental equations of motion, including (added) mass, aerodynamic damping and soil/structural stiffness. The model is verified and validated against numerical modelling using ANSYS Finite Element Analysis software and found to be in very good agreement. The Maple model is then used to test the fatigue performance. It is found that the fatigue damage accumulated over the 25-year design lifetime is governed by the bending stress cycles induced by inertial wave forces. The model is used to generate a transfer function between this wave forcing and the resulting bending stress over the entire range of present wave frequencies. The transfer functions are used to transfer the wave scatter diagram to accumulated fatigue damage against a B2, C1 and D graded S-N curve. The analysis results show increasing fatigue damage for decreasing first natural frequency due to the increasing slenderness and more prominent interference with present wave frequencies, which causes the 0.15 and 0.17 Hz monopiles to fail on fatigue damage. Given the very demanding site conditions assumed in this research, more slender monopiles may be feasible in deep waters with a milder wave climate. Over all, the results show no fundamental technical limitations for a monopile supported 15 MW wind turbine in water depths of up to 120 meters, provided that the stiffness of the structure is sufficiently high. Since steel usage increases for increasing stiffness, technically feasible monopiles will need to be relatively heavy, thus costly. Therefore, a range of strategies to reduce steel weight have been quantitatively assessed. It is found that the amount of steel can be reduced up to 35% by adopting higher-grade steel types and improved weld quality. Based on the fundamental limiting factors for monopiles found, also a novel hybrid floating-fixed bottom concept is proposed aiming at steel weight reduction. Although the concept can be considered promising, it does add complexity to the system and unfortunately does not (yet) result in steel reduction.

This research evaluates and optimises the competitiveness of a combined wind, solar, and electrolyser plant in the Dutch electricity and hydrogen market. For the electricity market, this study used the EPEX spot market, where most electricity is traded. The hydrogen market is far less transparent than the electricity market; hence, a market had to be simulated for this study. It was assumed that the price of hydrogen is set by the production cost of hydrogen through steam methane reforming, because this is by far the largest producer of hydrogen in the current market. For both markets, it was assumed that an unlimited amount of electricity and/or hydrogen could be sold at the market price at any time. The competitiveness of the hybrid plant was determined by the Net present value of the hybrid plant. This NPV was calculated for nine research scenarios placed on two axes, a climate action axis, and a time axis. The climate axis represents the amount of climate action taken by governments and industries. The time axis represents the commercial operating date, varying from 2021 until 2030; this will illustrate how the plant competitiveness evolves.

Wind farm layouts in industry show a range of patterns with an overall trend from regular to irregular patterns over time. Wind farm layout optimisation studies in literature generally result in irregular wind turbine patterns. Review of existing literature shows that the performance of irregular and regular wind farm layouts has not been compared in a consistent way. Although there are indications that irregular layouts outperform regular layouts, it is yet to be determined if this is the case for the overall performance and if so, to what degree. In this research the effect of regular and irregular wind farm layouts on selected performance indicators is quantified. This quantification is performed through means of a comparative case study.
The performance of both regular and irregular wind farm layouts is assessed on the basis of three performance indicator groups: (1) power performance; (2) wake-induced tower fatigue; and (3) inter-array cabling system. The performance indicators in these groups are affected by a change in wind farm layout, feasible, site independent, and technical as concluded from a multi-criteria decision analysis.
The irregular wind farm layout has a higher annual energy production and a higher persistence to wind direction. The net present value of this increase in cash flow over the lifetime of the wind farm is estimated at 10 million Euros. A higher persistence to wind direction means that the power output is less sensitive to fluctuations in wind direction. This characteristic increases the predictability of the wind farm power, which can indirectly lead to a decrease in imbalance cost on the electricity market.
The wake induced tower fatigue is found to be negatively impacted by an irregular wind farm layout. Implementation of the Frandsen model shows that the maximum effective turbulence of the irregular wind farm is 23.8% higher than that of the regular wind farm layout. For fatigue-driven tower design, this leads to an increase in tower wall thickness, which in turn results in an increase in tower material consumption. The increase in tower cost in the wind farm is estimated at 4 million Euros. Application of a minimum inter-turbine spacing to ameliorate the negative effect on effective turbulence is can lead to a decrease of 20 % (as compared to the case study).
The inter-array cable design results show a marginal increase in cable cost of 1.15 % for the irregular wind farm. The analysis reveals that this performance indicator is strongly dependent on site-specific input data. Due to this marginal change and dependency on site specific input data, this performance indicator is omitted from further conclusions.
Comparing the negative effect of the tower cost and the increase in revenue due to the higher AEP, the net present value is computed. With a discount rate of 5 %, the net present value of the AEP reduced by the increase in tower cost results in an increase of 6 million Euros.
For improved performance in future wind farm layouts, the implementation of irregular wind turbine patterns is advisable. That is, with the proviso that the minimum inter-turbine spacing is taken into consideration with respect to wake-added turbulence levels.

With a global growing demand of energy, more offshore wind farms are installed, further away and in deeper waters. Nowadays, offshore wind turbines are mostly installed by jack-up installation vessels. Most existing jack-up vessels have legs that are becoming too short for these water depths while offshore wind turbines are increasing in size year after year. Since 2014, the turbine capacity of newly installed wind turbines has increased by 16% every year. Therefore, larger jack-up crane vessels are needed and installation by floating crane vessels is being considered. Heerema Marine Contractors aims to be one of the leaders in the offshore wind installation with her large crane vessels such as HLV Aegir, SSCV Thialf and SSCV Sleipnir. The challenge of floating installation of offshore wind turbines is that motions of the vessel are transferred to the rotor-nacelle assembly (RNA) while low tolerances apply for the installation of an RNA. Vertical motions can be reduced by means of a heave compensation system. Such systems are available but come with certain drawbacks: they are difficult to retrofit to a vessel, use a large amount of energy and are rather expensive. A possible solution to these problems could be a novel concept called earth-fixed heave compensation. In this concept, the crane wire is connected to the seabed via a transmission on board of the vessel, transforming an upward motion of the vessel into a downward motion of the RNA and vice versa. At present, it is unclear if such a system is technically feasible. In this research, an analytical model of an earth-fixed heave compensation system is developed. The objective of the model is to gain insight in the influence of design parameters such as the transmission and stiffness of the system. The model is set up in three stages: Stage 1 comprising one degree of freedom for the transmission; Stage 2 comprising two additional degrees of freedom for the sheaves that connect the earth-fixed wire from the seabed to the transmission; Stage 3 comprising all other sheaves, crane reeving and payload are added, resulting in a sixteen degrees of freedom model. A first finding is that wire damping has a negligible influence on the results because natural frequencies of the system are found to lie outside the wave frequency range. However, the first natural frequency is close to the wave frequency range, resulting in a larger response amplitude for both transmission and vertical payload motion in the higher frequencies. Furthermore, it is shown that the inertia of the sheaves of the earth-fixed wire can be neglected for a range of transmission inertias and earth-fixed wire stiffnesses. On top of this, the model confirms that the inertia of the other sheaves can be modelled by means of an equivalent inertia block, to simplify the model for time-domain simulations. The losses in wire tension due to wire-sheave interaction were approximately 5% and it is shown that they can be modelled accurately by means of a sigmoid function. Frequency-domain simulations showed that the heading of the vessel, or wave direction, has a significant influence on the vertical RNA motions. On top of that, increasing peak periods generally result in larger vertical RNA motions. A time-domain simulation for typical North Sea environmental conditions, head waves and a fixed crane slew angle without the heave compensator is made to compare results with. These simulations have shown that it is in principle possible to reduce vertical RNA motions. For a given sea state, 80% motion reduction is achieved by tuning the transmission ratio of the system. It is shown that the location of the earth-fixed wire has an impact on the vessel and payload motions. When located on the starboard side of the vessel, rather than reducing the vertical payload motions that are induced by vessel roll, those motions of the vessel itself are actually increased. Further simulations show that the stiffness of the earth-fixed wire and the losses occurring due to wire-sheave interaction determine the performance for a large part. Although the overall performance of the system can be considered promising, additional research is needed to confirm whether earth-fixed heave compensation can be competitive relative to existing passive and active heave compensation systems with a performance of 80% to 95%. Also, it is recommended that the behavior of the system for different sea states is investigated. Sensitivity analyses already show that shorter wave periods result in a significant drop in performance. The vessel heading seems to have a limited influence on the performance of the heave compensator.