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

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.

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

With an increasing shift to renewable energies, solutions are required to overcome that variability and unpredictability of power production that comes with it. Hydrogen is one solution that can overcome this issue since it can be produced using renewable energy through electrolysis, it can be transported to shore efficiently by pipelines and it can be stored, like natural gas. Not only can hydrogen be used as a way to store energy, but it can also be used directly as a basis for steel production and for long-distance transport. Using wind energy, specifically offshore wind farms, as the source for hydrogen is an idea that is gaining attention. Currently, wind turbines are electrically connected, bringing their electricity to shore by high-voltage cables, allowing hydrogen to be produced onshore. Hydrogen could however also be produced at a central location in the offshore wind farm or at each individual offshore wind turbine, removing the need for high-voltage cables. The purpose of this research is to explore different methods of hydrogen production and validate a model that can simulate each method. The goal is to be able to compare the different methods and quantify the losses of producing hydrogen offshore as opposed to onshore. A time-based simulation model has been developed to represent an offshore wind farm, which would either produce electricity or hydrogen, which eventually arrives onshore. High-resolution wind data are used to correctly include short-term fluctuations in wind output. The N.O. Jensen wake model is implemented to capture the wake effects. Since wake is typically modelled steady-state, specific attention has been given to include the moving of the wake through the wind park. The power outputs for each turbine in the model could be accurately determined based on the incoming wind data. It is found that the spatial effect reduces wind power variations at the wind farm level to about 22\% of first turbine in the sting, as expected. The output of the simulation model compares the power losses of producing hydrogen through four scenarios. This model uses a total of eleven wind turbine strings of seven 10 MW wind turbines each. The power output calculated would then be used to determine the amount of hydrogen produced. By applying electrical power loss equations and calculating the pressure drops for hydrogen traveling in a pipeline by assuming steady-state and isothermal, the losses are calculated for each scenario. A sensitivity study is performed to see the effects of changing model parameters on each of the different scenarios. The model developed in this thesis allows for comparison of electricity production and hydrogen production by offshore wind turbines. It is found that producing hydrogen at the wind farm level and then transporting hydrogen to the shore (Scenario 2) has lower overall energy losses (kg of hydrogen available on shore) than using electrical cables. The same goes for individual offshore hydrogen production (Scenario 3). A high-level exploration of the economics indicates that both scenarios 2 and 3 are more economically feasible when only looking at transportation costs. It is recommended that further research is done to be able to determine which method of hydrogen production would be the most effective by taking the transportation losses and economics into account.

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.

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.

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.

As the world economy and population grow, energy consumption grows too at a never before seen pace. Reducing costs and increased environmental awareness resulted in renewable energy sources being the fastest-growing sources in the last decade. Due to its high potential, many offshore wind turbines will be installed in the coming years. These turbines will be installed in deeper waters, usingmainly monopiles as support structures.

In the variety of monopile (MP) installation methods, a distinction exists between installation over the side of a vessel and a novel method where the procedure is repositioned to the vessel’s stern. Experts in the field were convinced that stern installation would be necessary for growing MPs and extended installation timeslots. This thesis aims to create an objective distinction between the installation directions by looking at the following two installation steps.
First, the storage of MPs on the deck of an installation vessel is investigated. For side installation, the MPs are positioned transversely on the deck. This method uses little deck space per MP but includes an overhang which might badly influence the vessel’s behaviour. The latter has been investigated
using the Moment of Inertia (MoI) of the vessel as an indicator of this behaviour. It has been found that transverse storage affects the MoI significantly more than longitudinal storage. However, this longitudinal storage is limited to 4 MPs per transit due to stability, whereas the transverse method can take 6 MPs. The stresses in the MP itself have also been evaluated for these storage methods, as the support locations were different. It has been concluded that there is indeed a difference, but the stress level has been found not governing for this choice.

Second, the upending procedure is investigated, as this is a step in the procedure which is highly influenced by motions and external wave impact. A model is developed that uses tugger line connections from the vessel to the MP to define forces in equipment objectively. It has been found that loads in the tugger lines were significantly lower for stern installation compared to side installation, which leads to a workability comparison. This comparison is based on a specific tugger cable, limited to a 300𝑚𝑡 tugger load. A range of sea states has been analysed and checked on this maximal tugger load. The workability difference for full-year performance is found to go from 64% for side installation to 96% for stern installation. It is realised that these numbers are high compared to the actual installation, but as the assumptions made for this model are equal for side and stern, these percentages are a good comparison between the two methods. The assumptions on which this model is based are checked on
sensitivity, which results in reasonable trend lines and an interesting prospect into the future.

The model presented in this thesis could pose as a hypothetical concept for future installation, and therefore a determination of the natural frequency is added in this thesis. With this natural frequency, the feasibility of a concept can be quickly assessed even though no time-domain simulations have been
executed. The model stays clear from natural periods of the control system and periods of the waves for a large range of upending angles. However, in a nearly vertical position, the control frequency is crossed and later, the regime of wave frequencies is encountered. Adjusting the model slightly in terms of geometry shows that these issues can be solved. However, future research is highly recommended into a time simulation of the model.

Finally, some practical applications of the installation over the side and stern are discussed. Concluding this thesis, the main research question can be answered positively by stating that stern installation can be used to improve the all-year MP installation performance of a floating installation vessel.

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.

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 explores the development of an accurate yet fast methodology to use measured vessel motions for a fatigue damage calculation in the cranes during suspended transport. First, the kinematics, definitions and particulars of the system of interest – the vessel including crane and suspended jacket - are provided. Then, literature research is done on the main two problems with any transport at sea: the inconvenient draft problem, and the forward speed problem. To remove the inaccuracies introduced by both problems, the novel TF A2B Method is introduced. The method can be used to find the required force transmissibility functions at location of the cranes that are most prone to fatigue: the welds at the crane boom pivots. It is mathematically proven that the TF A2B Method can be used and with validation tests set up for unrestrained suspended transport it is shown that the required RAO’s can be obtained from in-house modeling software Liftdyn. Overall, its concluded that the TF A2B Method works in principle, removing the need to consider inconvenient draft and forward speed.

The hypotheses proposed in this research are further validated by the proposed validation tests in combination with a synthetic data model created in software tool Liftdyn. The Synthetic Data Model (SDM) uses a Jonswap Spectrum to obtain the motions at the Motion Reference Unit (MRU, ‘A’) and the Control Point (‘B’) at the modelled SSCV Sleipnir, which is assumed as a rigid body. After successful validation, the SDM model is then used to calculate the motions at the Jacket Sensor (‘B’). Further validation of the found responses is done and it is confirmed that the location ‘B’ can in fact be located on another rigid body as the MRU, which in this research was the location of the jacket sensor. It is also shown
that the transmissibility functions (TFs) between A and B are almost equal for different draft, resulting in that the TFs between A and B are not significantly dependent on the hydrodynamic properties of the modelled system. Therefore, it is concluded that the TF A2B Method can correctly find motions at location B by using the motions of location A in combination with the corresponding TFs between A and B.

The TF A2B Method is then applied to calculate the jacket motions using measured vessel motions. Data obtained from the X suspended transport were processed and applied using the Measured Data Model. The calculated jacket motions are compared to the measured jacket motions during the X suspended transport to the accuracy of the TF A2B Method with real data. An approximate 70% accuracy match with the measured suspended jacket motions, Roll and Pitch during the X Suspended Jacket transport was found. It is noted that not all data was found to be suitable for use. Suggestions for improving this have been made and are expected to further improve accuracy of the match. The validated Liftdyn model of the X transport is then used to model the suspended transport of X. From the Liftdyn model Force RAO’s can be obtained at the selected fatigue location of the cranes. A Fatigue Data Model using the TF A2B Method is proposed to find the stress cycles and thereby the fatigue damage during suspended transport.

Concluding, the TF A2B Method developed in this research shows positive results overall. The method can in principle be used to translate vessel measurements during suspended transport into fatigue life consumption of the crane. Further research is needed to improve the Measured Data Model, for which the use of a Surge and Sway motion sensor at the MRU is recommended. Furthermore, smaller vessel motions and more data samples should be used to further validate the accuracy of the method which is required to validate the Liftdyn model of the transport. Finally, to verify the results of the Fatigue Data Model, it is suggested to install strain gauges at the welds of the crane boom pivots for validation.

In current offshore wind turbine (WTG) tower installations, bolted flange connections are common. These connections are however reaching their limit due to the maximized size of the bolts. Also, bolted connections require the motions between the tower and foundation to be limited during installation, suggesting their workability of installation is low. The Double Slip Joint is a promising and innovative connection between a WTG tower and the support structure, which may remove or reduce these limitations. This research explores the motion and collision limits for WTG tower installation on a monopile (MP) foundation and focuses on the lowering and mating using two types guiding structures, or catchers: the vertical asymmetric catcher and the conical catcher. A first exploration of installation workability is provided as well. A model is developed that simulates the lowering and catcher mating of a WTG tower with a MP. This model includes physical phenomena that affect the motion and collision behavior of the WTG tower. Specific attention is paid to collisions between the catcher and the MP. With the use of finite element analysis, a stiffening non-linearity is observed in the lumped contact element to describe collisions. It is found to be caused by the local deformation of the mating elements. Furthermore, the simulation model calculates 3 degree of freedom in-plane motions numerically of the WTG tower in the time domain using Euler integration. Besides the aforementioned stiffening non-linearity in collisions, other important model features are a time and spatially variable wind load, aerodynamic damping, harmonic horizontal and vertical crane tip motions, crane operator induced crane tip motions and lifting line stiffness and damping characteristics. The model simulates the WTG tower lowering and catcher mating and to determine the installation limits, installation requirements have been defined, aimed at preventing critical events. To be specific, installation requirements aim to prevent slack wires, axial impact, and plastic deformation of the catcher due to contact with the MP. It is also required that the side-lead angle (i.e. the lifting line angle with respect to the vertical) remains below a maximum allowable value. Model simulations are performed for both jack-up and floating installation vessel crane tip excitation characteristics. This research indicates that in both cases, the conical catcher yields higher allowable wind velocities than the vertical asymmetric catcher during installation. This is supported by the findings that the conical catcher has higher allowable contact forces, requires less or no crane operator action during catcher mating and the side-lead angle varies less than with the vertical asymmetric catcher. Another important observation is that the installation limits are often reached through installation requirements related to the motions and collisions of the mating elements. This study shows that applying linear damping to the horizontal WTG tower bottom motions, achievable by (active) tugger winches, the allowable mean wind velocity in installation is increased and this research suggests that the workability of installation increases consequently. The focus in this study is on motion and collision limits in installation as well as on installation requirements. It is recommended for future research to focus on workability and to analyze the entire system consisting of vessel, load, and foundation. As such, wind and waves are considered separately as a source of excitation and vessel-load interaction is incorporated. For 3D analyses, it is recommended to study the control of out-of-plane WTG tower bottom motions and potential inclined collision responses between the catcher and foundation.

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.

Wind farms are increasing in size to keep up with the increased demand for renewable energy. Larger wind farms, leads to more complex wind farm design problems for a designer. This study investigates the wind farm design process and compares computed and manual design approaches in order to find the best solution to the wind farm layout optimization problem.

In this study, the design process of a wind farm is described in detail based on the
experiences of two designers in order to get a clear view on the current state of wind farm designing. Different experiments were performed to compare a manual and a computed design approach for an offshore wind farm. It turned out that the computed designs led to a 1 % increase in energy capture compared to the manual designs. Further research is required to conclude that computed design approaches always lead to higher energy yields, however, it was found likely that the computed approach is the best method to solve the wind farm layout optimization problem. The main problem with the current available optimization software is that it cannot deal with all constraints. Therefore, optimizing a
wind farm design still requires some manual changes to fulfill all requirements for the project.

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.

In the design of offshore wind turbine generator (WTG) support structures, a large number of environmental parameters are used. The most important ones include the wind climate (wind speed and wind direction probability distributions), the wave climate (wave height and wave direction as well as peak frequencies of the waves, and water levels) and the soil conditions. These data in combination with the operational data of the wind turbine, such as the rotor speeds, pitch angle and yaw angle are used in dynamic simulations to determine the loads on the structure. Combining all these data correctly ensures that the support structure design, which is subject to large dynamic loads, is fitting for each specific location of the WTG. In particular, the environmental conditions should not coincide too much/often with the support structure natural frequencies, as that may result in resonance, thereby increasing fatigue life consumption. The dynamic behavior of these offshore support structures is of eminent importance. During the operation of WTGs, the actual dynamical properties of the WTG can be compared to the design. There are several reasons behind the goal of this comparison, for example verifying if the loading assumptions in the design were correct, assessing whether the life time can be extended or even whether the structure can be optimized (e.g. better foundations). Typically, this is done by comparing acceleration measurements of the WTG during idling (e.g. non-operational) conditions and applying Operational Modal Analysis algorithms. Given the high availability of modern wind turbines (e.g. >95% of time in operation) such measurements cannot be done as often as desired for this purpose. Therefore, the question arises to what extent the dynamical properties of WTGs can be estimated using measurement data obtained during operational (operational) conditions. This research explores the applicability of OMA algorithms in estimating the dynamical properties: the natural frequencies, the mode shapes and modal damping for operational conditions of offshore wind turbines on monopile foundations, using the Luchterduinen offshore wind park as a case study. The OMA technique explored in this thesis is Eigensystem Realization Algorithm. Test cases were used to confirm the performance of ERA for the estimation of the dynamic parameters. Stability diagrams were used to identify poles for stable modes (frequencies) in the frequency spectrum of data samples and to estimate a correct size of the Hankel matrices. It is concluded that this preprocessing and pre-analysis of the results is important to confirm the performance of ERA to correctly identify stable modes of support structures of operational offshore WTGs.Overall, it is concluded that ERA can yield useful results in estimating the natural frequencies for both ON and OFF, provided that an automated analysis is combined with a visual analysis. Even then, the accuracy and the consistency of the identified frequencies is moderate. ERA does not perform well for estimating the mode shapes, yielding inaccurate and inconsistent results. Based on the results of this work, ERA is found not to be capable of estimating modal damping ratios. The consistency for the mode shapes is not in all cases good and further study is recommended for both the mode shapes and the modal damping ratio. Another recommendation is to further study the size of Hankel matrices for this application. Furthermore, it is advised to also investigate the use of more advanced Operational Modal Analysis algorithms. The results of this study may be used to explore differences between wind turbines with and without scour protection and also to compare identified dynamic parameters to values used in the design.

During the past decade, the offshore wind energy industry evolved to bigger turbines, going into deeper waters and farther offshore. As bottom-fixed wind turbines are limited to shallow water depths, floating wind structures are the next frontier to unlock the vast potential of wind energy. Despite many techno-economic challenges, several full-scale floating wind structures have been successfully deployed and have shown the potential for floating offshore wind. One project near completion is the 3.6 MW TetraSpar demonstrator developed by Stiesdal Offshore Technologies, Shell and Innogy. With its tetrahedral shaped base and suspended counterweight keel, developed with the focus on ease of fabrication and installation, this spar concept is expected to offer a competitive package for floating wind using future, larger 10 MW+ wind turbines. The goal of this research is to investigate the capability of the TetraSpar platform to accommodate significantly larger wind turbines and to identify challenges in an early stage of development. Since technology upscaling of floating wind substructures has not been done before, this thesis first develops a novel design methodology for upscaling and then is applies it to the TetraSpar as case study. This work builds on academic efforts thus far but focusses on the key design drivers in for upscaling of floating wind, namely the fundamental equilibria in vertical and rotational direction: the structure’s weight is equal to its buoyancy, and the restoring moment equals the maximum overturning moment by wind. Specific emphasis is put on correctly capturing these equilibria, as they generally apply for floating wind substructure technologies, including the TetraSpar. First a design basis is created with functional requirements and design criteria for floating wind structures in general, and specifics to the TetraSpar. Also, key specifications of future 10 MW, 15 MW and 20 MW wind turbine types are explored. Secondly, a model is developed for upscaling based on physical modelling of hydrodynamic stability (water and waves) and aerodynamic thrust (wind). Based on these inputs, the substructure is upscaled using the future turbine type wind thrusts. The model employs an algorithm to find a new equilibrium design point and computes key properties for upscaled substructures. The resulting design concepts are then evaluated for first order wave-structure interactions using a diffraction/radiation solver (WAMIT). Key evaluation aspects are free-floating hydrodynamics, including hydrodynamic coefficients, wave forces and response amplitude operators. Fourth, selected structural elements of the upscaled design concepts are evaluated for structural strength. The fifth and final step assesses the extent to which the now evaluated upscaled design concepts still meet on the functional requirements and design criteria. The thesis concludes that the developed, first-order design methodology is suitable to explore upscaled design concepts of floating offshore wind turbines. By computing an estimation of the physical dimensions and behaviour of the substructure, this allows for the evaluation of the technical and economic feasibility. Key findings of the physical modelling are the linear trends for structure mass over power rating of the wind turbine, sensitivities in design choices for maximum allowable heel angle due to wind, and keel draught for the TetraSpar specifically. Compared to other technologies, it is found that found that the TetraSpar concept offers a relatively lightweight platform for future wind turbines up to 15 – 20 MW. No fundamental technical showstoppers are identified for upscaling, but it is found that as the structure progresses to larger wind turbines, aspects like in-port water depth, physical dimensions of structural elements, and installability of the TetraSpar at sea will become more challenging. It is expected that at some stage in the development towards large-scale floating wind structures, trade-offs will have to be made to arrive at an improved design. For this, the methodology developed for this thesis can be applied, for example by exploring a lighter, wider TetraSpar design with more slender structural elements. Furthermore, it is recommended to further investigate the mooring design, fabrication capacity and deployment procedures of larger floating wind substructures in general, and upscaled TetraSpar designs in particular.

The offshore wind energy industry has been developing rapidly due to the increasing demand for renewable energy. As a result, offshore wind turbines are increasing in mass and height and are installed at larger water depths and further away from shore. A floating vessel is not limited by water depth and soil conditions, but is sensitive to wave-induced motions. Heerema Marine Contractors developed a new installation method using a floating vessel, where the rotor nacelle assembly (RNA) of a wind turbine is assembled on an assembly tower on deck and is installed in one lift. The research objective is to gain a better understanding of the dynamic behaviour of a rotor nacelle assembly, particularly due to the wind, focusing on the horizontal motions of the RNA, when it is hanging in the crane above the tower. To examine the dynamic behaviour of the RNA, separate models are made to assess the effect of the wind and waves individually in OrcaFlex and Liftdyn. This is done to identify which motions are due to the wind, and which motions are due to the waves. Finally, the wind and waves are coupled to gain insight in the coupled response. Furthermore, a case study is performed to prove its applicability for implementing and assessing the effect of using tugger lines on mitigating horizontal motions. In the models the blade pitch angle, crane position and responses for different wind and wave conditions are examined. From the different models is observed that the response due to wind cannot be neglected. Wave-induced motions are governing, due to large crane tip motions. The governing motion is a pendulum motion of the RNA, which is larger for longer wave peak periods. For both short and long wave peak periods, the horizontal motions have to be mitigated. In the case study, tugger lines are implemented to examine the effect on the RNA motions and to assess if the model can be used for developing concepts to reduce the motions. It is shown that providing stiffness in the horizontal plane and in roll help to reduce the sideward pendulum. For long peak periods, the RNA motion has to follow the opposing motion of the crane tip. More research is required to develop a concept that can follow the crane tip for short peak periods and oppose the motion of the crane tip for long wave peak periods.