The ongoing global energy transition has led to significant growth in the offshore wind sector. This growth has resulted in a high demand for offshore installation vessels like the 'Aeolus', and the need to push up their limits to meet the rapidly changing market. In the Saint-Brieuc offshore wind project, the Aeolus had to be installed on a stiff seabed, a situation that was expected to induce high impact forces on the jack-up legs. To mitigate these forces, leg modifications were installed to increase the operational limits for jacking operation. However, Van Oord's operational limits for installation on stiff seabed conditions differed from DNV's Joint Industry Project, leading to the need to reassess the necessity of the leg modifications.

The objective of this research was to identify the main physical phenomena that govern the forces in jack-up legs during installation on a stiff seabed, and to understand how these physical factors and a specific leg modification could influence them. This research followed a systematic approach to address the research objective, comprising a literature study on impact mechanics, model development for axial impact forces, model validation, a sensitivity study, and an analysis of the leg modification. Insights from the literature study guided the development of the model to analyse the governing forces. Validation was performed by using field data from the Aeolus, specifically oil pressure readings from the vertical jacking cylinders and vessel motion measurements.

The research found that what governs the forces in jack-up legs during installation on a stiff seabed depends on the energy contained within the system prior to impact and how this energy is absorbed by the soil and structure. Key factors include wave-induced vessel motions, wave height and period, hull characteristics, vessel's inertia, jacking velocity, and the leg length below the hull. The energy absorbed by the soil and structure depends on their stiffness characteristics and behaviour, which primarily concluded that axial forces dominate over lateral forces during impacts on a stiff seabed. However, lateral forces should be considered in deeper waters or where seabed protrusions are anticipated.

The sensitivity analysis indicated that the vessel mass, initial velocity, and normal soil stiffness significantly influence the impact force, with the initial velocity predominantly influencing the impact force, and the overall system stiffness dictating the impact duration. By introducing the leg modification, the impact force was reduced by approximately 70-75%, along with an increased impact duration of 320-370%. Moreover, it was found that the impact force and duration are interconnected rather than separate occurrences. The leg modification also reduced the influence of soil stiffness variability. Furthermore, for a more realistic representation, incorporating the nonlinear behaviour of soil load-deformation and leg modification characteristics is needed, but this would increase the computational demand. Consequently, this suggests the need for advanced solutions and an engineering assessment to balance desired accuracy against computational complexity.

The amount of floating offshore wind farms under development has drastically increased over the past years as the technology becomes viable for pre-commercial and commercial scale projects. Projects of this size require a streamlined, efficient production of the complete floating offshore wind turbine (FOWT), not only for manufacturing the floater but also for assembling the complete system. Most assembly tasks could, in theory, take place in a port, which is generally preferred, but most ports do not have the sufficient port infrastructure to perform those tasks, while the required infrastructure upgrades are very expensive. This is a significant challenge for smaller-scale, first-mover floating wind farms, which do not have sufficient units to justify those substantial investments for the assembly. To overcome this issue, a jack-up type wind turbine installation vessel could be used nearshore to integrate the wind turbine on the floating unit. This would ensure the realisation of the project without major infrastructure investments. At the same time, it could utilise the sheltered area's advantages and be a kick-off for large-scale projects in the region.

This proposed concept has so far not been investigated in academic research. Therefore, this thesis aims to create a general understanding of the system and its characteristics. Based on the example of Port Talbot in the UK, it should be examined how the concept can be implemented in a location. Furthermore, a response analysis of the system is done in SIMA for the single blade installation to understand which motions characterise the integration task and which environmental conditions limit the operation. Those operational limits are then implemented in a Python model of the complete integration sequence to conduct an operability analysis which should also give estimates for the required installation time and costs of the system when subject to wind and wave loads. Based on the findings of those studies, the technical and economic feasibility of the concept should be investigated.

The study has found that the wave-induced floater motions are mainly governing the systems motions during the single blade installation, which leads to very strict operational limits for the waves during the mating procedure. The technical feasibility of the proposed system is given if it is not subject to large tides and if a sufficient control mechanism is implemented for the installation. The operability analysis has shown that the system can be economically feasible if smaller projects should be implemented. For large projects, it is likely more feasible to invest into the port infrastructure.

Offshore wind energy is poised for significant growth, with three major trends emerging: larger wind turbines, offshore wind farm sites in deeper waters further from the coast, and a shift towards floating offshore wind energy. While current floating wind turbines are installed using a tow-to-site strategy, the increasing size of turbines and foreseen implementation scale raises the question if on-site installation will be more a viable option. This research investigates the operational feasibility of on-site installation of the NREL 15 MW wind turbine blade on a semi-submersible floating wind turbine using a floating monohull crane vessel. This study uses a simplified frequency-domain multi-body model to assess the workability of installing blades on the GustoMSC Tri-floater using a floating monohull crane vessel. The model assumed first order potential flow, rigid body motions for the Tri-floater and the crane vessel, and neglected wind and current loads. The comparison of the multibody and single-body models revealed that positioning the crane vessel next to the Tri-floater resulted in shielding, diffraction, radiation and reflection effects, which were also observed vice versa for the crane vessel but to a lesser degree. Results show that the workability for floating-to-floating operations is 31%, compared to a workability for a fixed-to-fixed operation of 87%. The relative surge was limiting, caused by the crane vessel roll angle. A sensitivity analysis on wave direction showed that the wave direction has significant impact on the relative motion and the workability. This optimal wave orientation resulted in a floating-to-floating workability of 51%. The workability of a floating-to-fixed operation was found comparable to that of a floating-to-floating operation. The preliminary findings indicate that on-site installation for floating-to-floating operations is more challenging than fixed-to-fixed operations. However, workability is comparable to floating-to-fixed operations, suggesting little impact from floating wind turbines. Further research, including wind loads, is recommended to understand the difference between installations. Another area for study is the development of a specialized blade installation tool to aid floating wind turbine installation.

With the highly increased interest in offshore wind turbines and their technologies, the sector has witnessed rapid development in the past decade. For installed offshore wind turbines, there has been a lot of research conducted in the field of aero-, hydro- and structural dynamics, wind turbine-control, operation & maintenance, foundations and moorings. The research field of offshore wind turbine installation is on the other hand relatively new, and the studies regarding this topic are limited. The lifting of heavy objects is one of the most commonly performed offshore installation operations and has become more challenging due to the trend of increasingly larger and heavier payloads. Especially for substantial waves, the pendular motions of the payload may cause operations to be halted.

This thesis performs a study on a positioning control strategy for a complex lifting control scenario, i.e., position-keeping of a complex-shaped 6-DOF payload using a floating vessel equipped with multiple tugger winches. As the system is highly complex and contains non-linear and time-varying dynamic phenomena, it is an impracticable task to formulate a model that meticulously describes the actual system. For this reason, a fully integrated simulation model in Orcaflex has been used to capture the non-linear dynamic behaviour of the system.

The preassembly operation of a Jacket Lifting Tool on a monohull vessel is adopted as a case study to verify the proposed control strategy. Two scenarios are considered -installation and decommissioning- for which an outrigger configuration is used to position the tugger winches. Due to the difference in setpoint (i.e. the desired position) in the two scenarios, the proposed controller is solely implemented in the decommissioning scenario. Damping tuggers, the current state-of-the-art when it comes to motion mitigation, is considered suitable in the installation scenario.

The proposed controller does not consider the state–space equations of the system and only relies on real-time motion and tension measurements of the vessel and suspended payload. In addition, the controller considers the system's velocity tension and power limitations. The controller's impact is evaluated based on the positional error and verified by the peak reduction in the power spectral density spectra of the simulations. Despite its simple form, results show a significant reduction in the positional error, and therefore the possibility to extend the working conditions of the installation vessel. To improve the controller's performance it is recommended to involve derivative control, consider payload motion prediction and to optimise the tugger winch configuration. For further studies, experimental testing is needed to verify the effectiveness of the control scheme as it could appear that the controller does not exhibit similar performance in the real system. However, it is deemed unlikely that the latter would occur as a sensitivity study regarding measurement error indicates a stable response of the controller.

\chapter*{Abstract}
Larger wind turbines are being developed to fill the growing demand for green energy. Wind turbines that need to be installed in the next decade will reach a capacity of 15 MW, with an approximate rotor diameter of 240 meters. The currently used wind turbine installation vessels cannot install these larger wind turbines; hence, investments need to be made. An opportunity arises from the oil and gas industry: investments in this industry are decreasing. Thus, the floating monohull fleet working in this industry will need repurposing in the upcoming decade.

In this thesis, the operational limit of a single blade installation with a floating monohull crane vessel is sought. An installation method is proposed where taglines are used to compensate vessel motions and wind loads on the blade. Motion compensation with taglines is a technology that is already being used in the industry, which makes it attractive to contractors who are hesitant to use new technologies. The proposed installation concept uses three different compensation systems, allowing the blade motions to be decoupled and controlled separately. One of the compensation systems is selected and investigated in more detail in this research. The selected compensation system compensates for the blade motion in y- and yaw-direction with two dynamically controlled taglines horizontally attached from the blade to the crane boom.

The operational limit is determined for the final installation stage, the mating phase between the blade and the hub. The alignment of the blade and the hub is modeled in frequency- and time-domain. From these models, it can be concluded that the control system can limit the y-motions at the blade root below typically accepted motion limits, when the wave direction is limited to an interval of 150-210\deg, for a sea state up to a wave height of $H_s$= 5.6 m and wave peak period $T_p$ of 10 s. However, this is only true when the predisplacement of the blade towards the crane boom is larger than the maximum crane tip displacement.

Based on these preliminary findings, it seems that floating installation of turbine blades could be feasible with proper tagline control. Further investigation, involving dynamic control in all degrees of freedom, is therefore recommended.

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.

One of the world’s biggest concerns is global warming, a solution to this can be wind energy. Offshore wind energy has advantages over onshore wind energy, however, the levelized cost of energy is higher. The maintenance costs are a major cost contributor. To lower these costs, research is performed on faults and its detection. Currently, little is known about fault detectability and vibration propagation in a drive train of an offshore wind turbine. Fault detection and vibration propagation in a drive train of a 10 MW floating offshore wind turbine is therefore investigated to get an insight about the effect of faults on the vibration monitoring data of a drive train. Three different faults with five different degradation levels are applied one by one on the bearings of a 10 MW drive train model. These faults are radial and axial damage in the main shaft front bearing and radial damage in the high speed shaft rear bearing. One traditional, two non-traditional and two novel fault detection methods are used to detect faults and their vibration propagation. One common and one novel fault detection method are deployed in the time domain: the Velocity Root-Mean-Square (RMS) Threshold Method and the Peeters’ Anomaly Detection Method. The Velocity RMS Threshold Method compares the RMS of the vibration velocity of non-rotating parts with a threshold proposed by ISO 10816-21. The latter method makes use of statistical indicators and is tailored for this study. Although changes after fault introduction were observable, the methods can not be used and need to be altered for usage in the wind industry. The non-traditional Angular Velocity Error Energy Method is deployed in the frequency domain. It makes use of the angular velocity measurements from the drive train’s shafts and compares the normalized energy of its spectra with a threshold. This method inspired the development of novel fault detection methods introduced in this study, being the Bearing Velocity Energy Method (making use of bearing velocity measurements and also based on the Velocity Root-Mean-Square Threshold Method) and the Shaft Vibration Energy Method (making use of the velocity and acceleration of shafts). Both methods compare the normalized energy of the spectra with a threshold. Radial damage in the main shaft front bearing could be detected using the Angular Velocity Error Energy Method, the Bearing Velocity Energy Method and the Shaft Vibration Energy Method. Damage was detectable from 15% degradation onwards. Next to a change in vibration in the main shaft and its bearings, a different vibration behaviour was observed at the planet carrier front and rear bearing, intermediate speed shaft front bearing and on the low speed shaft. Axial damage in the main shaft front bearing could only be detected using the Shaft Vibration Energy Method. It was shown that this kind of damage was detectable by monitoring the main shaft’s vibration from 50% degradation and higher. Radial damage in the high speed shaft rear bearing could be detected using the Bearing Velocity Error Method and the Shaft Vibration Energy Method. Damage could only be detected for degradation higher than 70%, by monitoring the high speed shaft and its bearings. Next to the typical measurement locations, it is recommended to place extra sensors measuring velocity on the first stage planet carrier front and rear bearing housings, intermediate speed shaft front bearing housings and on the low speed shaft. The outcome of this study contributes to the understanding of vibration propagation and fault detection in a drive train. The fault detection methods can be implemented in maintenance and monitoring methods for offshore wind turbines. Maintenance engineers can use the detected vibration propagation to check the affected gearbox components and replace them before they fail.

The yearly installed percentage of offshore wind jacket substructures is rising. The most common installation method for a jacket structure nowadays starts by driving the foundation piles in the seabed through a pre-piling template. The jacket is then lowered on top until the legs are resting on the foundation piles through friction-based stopper connections. In order to rigidly fix the connection, grout is pumped into the annulus between the pile and leg. During the curing period of grout, generally taken as 24 hours, environmental loads cause the jacket to oscillate in various directions. As a result, the jacket leg will move relative to the foundation pile. This movement, which is called Early Age Cycling (EAC), can cause crack formation in the cured grout therefore decreasing the shear capacity of the connection. The DNV GL has restricted this relative movement to a conservative 1 mm within the first 24 hours due to a research gap on the subject. The strict regulation forces companies to use expensive EAC mitigation concepts of which the real effects are a debated issue.

The objective of this research is to gain insight into the modelling approach and the magnitude of EAC movements and investigate how they can most efficiently be minimized. This is achieved by investigating three phases: 1) simulating a number of load cases on a global jacket model and extracting interface forces near the seabed, 2) using these interface forces to assess the EAC movement on a detailed pile-leg reference model with a full circular stopper 3) analysing this reference stopper by testing three modified configurations. These configurations are designed with respectively two, three and four brackets yielding the same contact area. A sensitivity study is then performed by increasing the contact area.

The largest EAC movement within all models can be measured at the tip of the jacket leg. The location of this movement on the circumference of the leg varies based on the loading condition and stopper model. In general simulations on the reference model showed EAC movements below 1 mm due to a uniform stress distribution from the stopper to the foundation pile. For larger wave loads, sliding occurred resulting in large EAC movements. The initial modified configurations show significantly larger EAC movement when compared to the reference stopper. In general, the EAC movement decrease when the number of brackets increases. This is the result of a more even stress distribution around the circumference of the pile. A phenomenon visible for the two bracket stopper is rotation around the axis of the wave direction resulting in large EAC movement. This effect, denoted as moment induced rotations, should be limited by all means. A sensitivity analysis on friction showed that sliding could most efficiently be solved by increasing the friction coefficient. This is highly recommended since it greatly improves the performance of the stopper connection for larger waves.

This research could be further extended by performing a large sensitivity study to normalize the current results. This would be needed to verify whether the current conclusions can also be adopted for general use.

The modeling method for the performance of positive displacement pumps of hydraulic pumps is based on collecting operational data of the pump for its operational envelope and using equations with empirical fitting parameters and fit these equations to the data. This way values for the fitting parameters are obtained, with which the equations can be used as model for the operating performance of this specific pump. By implementing this modelling method no information about the contribution of each component present in the pump to the performance losses is obtained. A model which analyzes each coponent and estimates its contribution to the total power losses would be beneficial in the design of pumps. Therefore the answers to the following research questions are sought after in this work:

- What are the components critical to performance within hydraulic pumps?
- To what accuracy can a component-wise, analytical model for pump performance be created?
- How sensitive to certain parameters used in the equations is the model?
- What are the possibilities of implementing this model into pump design?

After the pump system was analyzed and the internal interactions found, the components which effect pump performance the most were identified. These include bearings, rollers, seals, springs, valves and leakage flow which is dependent on gap size, piston movement and viscosity of the hydraulic fluid. For these components analytical equations are presented which are incorporated in the model.

The accuracy of the model is tested by comparing its results to test data of an experimental camring driven radial piston pump. The model predicts leakage flow to an average difference of 3% relative to the data, but outliers up to 49% for high pressures and -35% for low pressures are observed. Input torque is predicted to an average difference of -19% relative to the data. This difference is mainly caused by the inaccuracy of the predictions below 30 bar operating condition, where outliers up to -61% are observed. These results are compared to an empiric modelling method and are found to be more accurate for leakage flow for the entire operational envelope, yet on average less accurate for input torque.
The sensitivity of the model to certain parameters is tested. The parameters investigated are the rolling resistance coefficient of rollers and bearings, bearing lubricant viscosity, hydraulic fluid temperature, piston-cylinder alignment and the corresponding forces resisting piston motion and the assumption of laminar leakage flow.

To show the possibilities of the model in pump design the rollers and appurtenant bearings present in the pump are replaced with hydrostatic bearings. The design is discussed and a hydrostatic bearing with an orifice restrictor is designed of which implementation will approximate the total efficiency of the pump with rollers. The Reynolds number of the flow necessary is found to be too high for capillary restrictors. To approximate the same total efficiency for each operating condition the average bearing flow is found to be 0.0016 L/s, which corresponds to an average fluid film height of 12 μm. The orifice restrictor diameter which allows this flow is found to equal 0.24 mm.

This work shows the possibilities of such a model, yet there is plenty room for improvement of the accuracy of the predictions made. Recommendations to this end are given in the conclusion and recommendations of the report.

New floating wind turbine designs are needed to reduce production costs and to increase mass production feasibility. The TetraSpar floating wind turbine achieves these goals by being constructed using components highly suitable for standardization and industrialization. The design makes use of a suspended submerged counter weight to obtain a low center of gravity for the floating system, while also allowing a low draft during transport and installation. An analytic analysis of the counter weight suspension system is presented that investigates the behavior of the system. The main concern of the novel system is its motion relative to the upper floating platform which it is connected to. It is demonstrated that the suspended system's motion is described by the axial elongation of the suspension lines when positive tension is maintained in all suspension lines. An analytic solution for the equivalent stiffness of the suspension system is given. This suspension system stiffness introduces additional natural frequencies into the system, and the effects of these frequencies are demonstrated.

This novel concept requires a multibody modeling approach to perform a dynamic loads and response analysis, as the stiffness between the floating platform and the counter weight is provided by chains. Additional design criteria are required for the counter weight system dependent on a combination of chain capacity and maintaining positive tension in all of the suspension lines. To satisfy these design criteria a global hydrodynamic load and response analysis of the floating system is performed. In this concept, the counter weight depth contributes significantly to the dynamic properties of the system and therefore a parametric study is conducted. The global response parameters of the rigid-body motion, natural frequencies, nacelle accelerations, counter weight chain tensions, and maximum platform-pitch angles are compared. Following the parametric study, an ultimate limit state analysis is conducted on the original and alternative designs. Design recommendations are made for the counter weight depth and configuration of the suspension system layout.

Resonant motions of a combined wind and wave concept (STC) in waves may induce significant loads in the structural components (such as wind turbine blades, nacelle or the interface between the spar and the torus). One type of resonant motions is related to the so-called Mathieu instability. For the STC under the survival mode where the torus is locked to the spar at the mean water level (MWL mode), the large heave motion in severe waves induces a significant change in the buoyancy center and therefore leads to a periodic change in the restoring term for the pitch motion. When the wave period reaches the half of the natural period, the Mathieu instability (a dynamic instability in pitch for this case) occurs and leads to significant pitch motions.

The purpose of this study is to numerically predict such unstable motions for regular and irregular wave conditions and to compare the results with the experimental observations. First, a literature study on Mathieu instability is carried out to understand the mechanism behind this phenomenon. Then the results of the STC concept from the experiments carried out by Dr. Ling Wan are investigated in order to study the developments of the Mathieu unstable pitch motions and the corresponding wave conditions in which they occur. Finally, a numerical model of Mathieu instability for analysis of the STC pitch motions is established to get insight into the phenomenon.

The numerical model is compared with the experimental results of Dr. Ling Wan. A sensitivity study using the numerical model is then carried out to investigate the conditions where the Mathieu instability may occur.