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.

With the increasing water depths of the new offshore wind farms, the challenging soil conditions, the availability of assets, and other factors, jack-up installation vessels may no longer be suitable to complete the installation scope of work for the new wind farms. Therefore, installation methods and techniques using floating vessels must be further developed to allow safe and efficient installation of wind turbines. Due to the response of floating installation vessels, excessive motions can be transmitted to the lifted object, making the installation operation very weather sensitive. To increase workability, Heerema Marine Contractors (HMC) has developed the RNA method for wind turbine installation using a semi-submersible crane vessel. This method uses a temporary support structure on the vessel deck (installation tower) where the RNA is fully assembled with the assistance of the GREP (Guided Root End Positioning) tool to constrain the blade root to the top of the installation tower and consequently the hub. Finally, the RNA is installed in a single lift on the WTG permanent support structure. However, the GREP tool is compatible only with a specific range of wind turbine blade dimensions; therefore, for a different-size wind turbine blade a new GREP tool must be designed, fabricated, and mobilised. This project proposes an improvement to the HMC's RNA method to eliminate the necessity of the GREP tool. That is, a motion-compensated Stewart platform attached to the crane boom, where the blade is fixed to be installed in the hub on top of the installation tower. The project is developed by investigating the initial assumptions; those are crane boom stiffness, blade deflection, installation tower motions and their influence on the vessel's response, and aerodynamic loads acting on the blade during installation. The kinematics (inverse and forward) and dynamics of a Stewart platform are formulated, as well as the mechanical concept of the proposed system and the blade installation process using a Stewart platform attached to the crane boom. Furthermore, to eliminate the requirement of the GREP tool, a control system is developed to compensate for the blade root motions relative to the hub. The motion control system uses sensors to measure the hub's motions and generate the Stewart platform actuators' set points. Different possible sensor set-ups are evaluated, and a filter is designed to reduce the influence of the sensors' noise. The control system is developed on the basis of feedback PI (Proportional and Integral) and adaptive feedforward control to the actuators (hydraulic cylinders). It is concluded that it is technically feasible to use a motion- compensated Stewart platform for blade installation in the RNA method. However, the economic aspects of the proposed solution must be investigated.

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

With the Paris climate accords signed in 2016, most countries have committed themselves to ambitious climate targets during the next decades. One of these targets is a dramatic increase in the overall energy portfolio's market share of renewable energies. This increase in renewable market share will, for a large part, consist of newly built offshore wind farms. In turn, this rise in offshore wind energy projects is expected to be especially dramatic in northern regions, where high and constant wind speeds prevail. However, as offshore wind farm projects move further north, additional challenges need to be faced. One of these is the technical challenge to design offshore wind farms for possible encounters with drifting sea ice. To tackle this challenge, a proper understanding of the mechanics associated with encounters of drifting sea ice with offshore wind turbines is essential. Such encounters are currently primarily understood phenomenologically, and the associated models simulating these encounters – or ice-structure interactions – therefore are phenomenological as well. Moreover, most ice-structure interaction models are fundamentally one-dimensional, whereas ice-structure interactions are generally not one-dimensional. This mismatch holds especially for ice-structure interactions with offshore wind turbines, where wind loads are generally misaligned with ice loads causing highly two-dimensional ice-structure interaction problems. Therefore, the first half of this work sets out to extend one of the industry-leading phenomenological one-dimensional models – the Hendrikse (2017) model – to a two-dimensional environment. The ultimately developed Zero-friction contact Area variation Model By Omnidirectional Numerical Ice (ZAMBONI) attempts to do so by introducing practical extensions rather than introducing new assumptions. Nevertheless, one extension does entail a shift from current one-dimensional ice-structure interaction models. Namely, the assumption that ice experiences neither friction at the ice-structure interface nor internal shear forces. Consequently, much of the correctness of this model hinges on this extension. To assert the correctness of ZAMBONI. A comprehensive verification campaign is performed as well as a simple order-of-magnitude validation campaign. Although both confirm the extensions' correctness, further validation is required, especially concerning the zero-friction principle. Upon developing and discussing this two-dimensional ice-structure interaction model, the second half of this work couples ZAMBONI to an offshore wind turbine model to gain further insight into ice-structure interactions. These dynamically coupled two-dimensional simulations serve two purposes. Firstly, to compare one- and two-dimensionally simulated load cases of aligned ice and wind. Secondly, to perform newly simulable load cases of misaligned ice and wind. Four primary findings are discussed. Firstly, as hypothesized, introducing a disturbing wind load lowers the ice-structure contact area, causing smaller loads and displacements due to ice loads. This effect is especially well observable for misaligned wind loads and low far-field ice velocities. Secondly, a new ice-structure interaction regime is observed where ice and structure synchronize in the structure's first bending mode. This synchronization occurs most dominantly for two-dimensional ice. Thirdly, frequency lock-in occurs solely in the second bending mode and is terminated at lower ice indentation speeds for two-dimensional than for one-dimensional ice. Finally, small ice-wind misalignments, which are most common, appear highly similar to load cases of fully aligned ice and wind.

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.

Power optimization through wake steering and axial induction control is a well investigated topic in wind energy, which is generally proven to work. The influence of control manoeuvres on the fatigue of static components is generally discussed, but drivetrain fatigue due to wake steering and axial induction control is rarely discussed, while it is known that the drivetrain is a highly vulnerable part of the wind turbine and its downtime can result in a significant increase in cost. Having a better understanding of turbine wake interaction and wind farm power optimization and its influence on drivetrain dynamic behaviour serves as a reference for future wind farm cost optimization and predictive maintenance. The main research question answered in the thesis is as follows: To what extent does wind farm power optimization increase profit when wind farm power production and drivetrain bearing fatigue damage is considered? Multiple test cases for wake steering and axial induction control are considered, where different yaw angles, γ, and the blade pitch angles, β, are chosen for the upwind turbine. For each test case, power production and bearing damage is studied. A cost estimation is made and for a range of energy prices, the most profitable test case is found. For verification, a two and four wind turbine case in an uniform wind field is considered. Power production results for this low turbulent case are studied and compared to literature. Turbulent wind field results show that both wake steering and induction control result in a limited power production increase of 0.78% for γ = 7° and 0.17% for β = 1°, shown in the Figure below. The power production increase for the two and four wind turbine case in the uniform wind field for wake steering and induction control are 4.78% for γ = 15°, 16.6% for γ = 20°, 0.19% for β = 1° and 5.04% for β = 3° respectively. Overall absolute bearing damage of WT1 and WT2 increases with increased yaw angles for WT1 and the overall bearing damage of WT1 and WT2 decreases with increased blade pitch angles for WT1. INP-A and PLC-B bearing damage significantly increased for the downwind turbine. In the high turbulent wind field (TI = 0.2), when considering two wind turbines, wake steering can result in an increase of profit ranging from -€3,70 to €4,-, while axial induction control can result in an increase in profit ranging from €3,- to €40,-. In the low turbulent wind field (TI = 0), when considering four wind turbines, the power production increase for wake steering can result in a profit increase ranging from €30,- to €130,-, while axial induction control can result in a profit increase ranging from €15,- to €60,-. Both wake steering and induction control can result in increased profit. The desired control manoeuvre is highly dependent on the ambient wind, wake overlap of the downwind turbine and the wind farm arrangement.

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 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.

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.

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.

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.

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.

Offshore wind energy is one of the solutions to meet the growing demand for renewable energy. The offshore wind turbines producing this energy keep increasing in size and, as a result, the monopile foundations are becoming larger and heavier. The traditional jack-up installation vessels have limited crane capacity and many of these vessels are unable to install the XXL monopiles. Therefore, the offshore industry is currently investigating a new installation method using a motion-compensated gripper frame on floating vessel with a dynamic positioning system. The gripper frame is attached to the vessel and encloses the monopile with a ring to keep it vertical during the installation. In addition, the gripper frame compensates for the vessel motions such that the vessel motions do not influence the monopile motions. The purpose of this thesis is to investigate the feasibility of such a motion-compensated gripper frame and to determine what control settings minimise the monopile inclination and the force exerted on the monopile. The system is composed of three main bodies: the vessel, the gripper frame and the monopile. The monopile and PID controller, which controls the amount of force exerted on the monopile to keep it vertical, have been modelled in the frequency domain to gain insight in the effect of changing the control parameters. To model the dynamics of the coupled system an OrcaFlex model has been set-up. The system has been tested for various values of proportional and derivative gain, kP and kD respectively, in various wave conditions. First the perfect control system is tested, where the force to keep the monopile vertical is applied instantly and the vessel motions are fully compensated. However, as the real world is never perfect, the system tested for sensor lag and imperfect motion compensation as well. The results are judged based on three criteria regarding the maximum monopile inclination, actuator force and actuator stroke. Resonance is observed in case a value of kP is selected such that the natural frequency of the monopile and controller matches the wave forcing frequency. Adding derivative gain kD limits the monopile motions and force exerted in this case. To limit the monopile motion the proportional gain should be selected such that resonance is avoided. Two different control settings are investigated and it has been found that a relatively high value of kP of 10,000 kN/m in combination with a kD of 11,000 kNs/m is a suitable setting based on the three criteria. Furthermore, bow quartering waves is the favourable wave direction compared to head waves for the system considered in this thesis, as the force on the monopile is more evenly distributed over the actuator in x- and y-direction. Introducing a sensor lag into the system results in higher monopile motions and forces on the monopile. If the sensor lag exceeds 0.3 s it leads to instability of the monopile for both settings. The effect of not fully compensating the vessel motions is found to be limited due to the fact that these motions are slowly varying. The results of this work contribute to a better understanding of the dynamics of the system in various wave conditions. Furthermore, it provides insight in the effect of sensor lag and imperfect motions compensation, contributing to the design of a motion-compensated gripper frame for the installation of XXL monopiles.

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.

Climate change, as a result from global warming, requires an energy transition: the reduction of greenhouse gas emissions from fossil fuels and a radical innovation of the global energy system to proceed apace. Offshore wind is an important source of clean, renewable energy, and it plays a key role in the transition. 80% of the worldwide offshore wind is to be produced on locations in deep waters; here floating foundations are required, that to date are far more expensive than their bottom-fixed counterparts. To reduce costs of floating wind energy, reliable, detailed predictions of the system’s loads and motion response are crucial. Floating offshore wind turbine structures are designed using ’aero-hydro-servo-elastic’ software codes that simulate the dynamic response of a floating offshore wind turbine system to the offshore environment. Predictive accuracy can be improved by comparing simulation results from a model of a known system against measurements taken from the real-world system, a so-called model validation. One promising state-of-the-art aero-hydro-servo-elastic software code is BHawC/OrcaFlex, developed by Siemens Gamesa Renewable Energy (SGRE). Due to its novelty, however, validation of the code has only been carried out to a limited extend, giving rise to uncertainty about the interpretation of simulation results. The purpose of this MSc. thesis project is to validate the performance of BHawC/OrcaFlex by comparing its simulated load and motion results to measurements on a real-world floating turbine from the Hywind Scotland floating offshore wind farm (Hywind). Measurement data and a description of the ’as-built’ system were made available by the wind farm owner Equinor ASA. In order to establish an achievable level of modelling accuracy and predictive value of BHawC/OrcaFlex, the code was verified against another aero-hydro-servo-elastic software code: OrcaFlex, by setting up a similar model of the Hywind system in both codes. Limited information is available on the performance of OrcaFlex in floating wind load and motion predictions. Therefore, it was in turn verified against a wide range of industry-standard aero-servo-hydro-elastic software codes, using a modeled system that closely resembled the Hywind turbine and load cases that step-by-step increased in complexity, to further isolate causes of discrepancies between the models. OrcaFlex predictions matched very well across all load cases. The main differences were attributed to differently modeled additional linear hydrodynamic damping, as the official damping prescription resulted in prediction errors. In the BHawC/OrcaFlex verification against OrcaFlex, both models were subjected to multiple load cases that step-by-step increased in complexity, to further isolate causes of discrepancies between the models. Simulation results from running both models appeared to be nearly identical, though some discrepancy was observed from due to the simplified aero-servo-elastic OrcaFlex code. The final validation of BHawC/OrcaFlex to full-scale Hywind measurements is performed at below-rated, rated and above cut-out wind speeds with multi-directional wave and current components. In general, BHawC/OrcaFlex motion frequency domain predictions appeared to correspond well to the actual Hywind measurements. Most phenomena in the low-frequency, wave-frequency and high-frequency region were captured by the simulations. However, large errors were observed in the mean surge, sway and bridle line tensions predictions. Most discrepancies were found originating from errors in the model set-up, e.g. lack of hydrodynamic damping, simplifications in the wave model or errors in the mooring system set-up. Tuning of the mooring system showed improvement of the results, but further improvements could be made. Several sensitivity studies were added on parameters, such as hydrodynamic drag, tower damping and mooring drag. This showed overprediction of the surge/sway and roll/pitch frequency responses can be mitigated by both additional linear and viscous hydrodynamic damping. The main recommendations for further research are to further analyse errors identified in the model set-up. In addition, some yet unexplained phenomena that are not captured by BHawC/OrcaFlex in the current model, are to be addressed. Finally, a the development of a standardized approach to relate model validation studies in the field of floating wind to cost improvements could further quantify the value of future comparison studies.

Floating offshore wind turbines offer a means to access wind resources in waters that are too deep for bottom-fixed offshore wind turbines. To operate a floating wind farm, it must be possible to perform maintenance on the turbines. A critical maintenance aspect is the exchange of a major component such as a blade. It involves a lift of a large component to and from the turbine, which is subject to dynamic excitations from environmental loads. Jack-up vessels are used for major component exchanges on bottom-fixed offshore wind turbines, but due to the increased water depth in floating wind farms these types of vessels cannot extend their legs to the seabed and operate. As an alternative, an onsite blade exchange using a turbine mounted crane is considered in this thesis. The lifting dynamics during the operation are studied, as a knowledge gap is identified in this area. A 15 MW floating wind turbine on a tension-leg platform (TLP) is considered as a case study. This type of floater is considered the most difficult to disconnect from its mooring and tow to a port for maintenance using a shore-based crane. The Offshore Self-Climbing Crane (OSCC) from Huisman Equipment is considered as maintenance equipment, which consists of a base docked on the TLP and a lattice structure coupled to the turbine tower, on which a crane is mounted. Due to the mass of the OSCC, the tension in the tendons of the TLP is reduced, which lowers its natural frequencies in surge, sway and yaw and shifts them closer to wind excitation frequencies. The roll and pitch natural frequencies of the TLP are lowered due to the top mass of the crane and the coupling between the bending modes of the lattice and the tower, shifting them closer to wave excitation frequencies. Snap loads in the tendons occur for sea states with wave peak periods near the roll and pitch natural periods at significant wave heights of 2.5 meters and above. Steps of the blade exchange operation are studied. The operability of the installation of the OSCC is found to be limited by vessel motions, while the TLP remains relatively still. Installation of the OSCC is found to be a bottleneck in the blade exchange due to low operability and a limited number of suitable installation vessels. Free-hanging blade installation is not deemed possible, due to low installation operability and the unconstrained yaw mode. The stiffness in yaw resulting from a line-up tool attached to the blade root or yoke is assessed. The low stiffness of the line-up tool at the yoke results in a large response due to crane tip displacements caused by wind-induced motions of the TLP. Placing a line-up tool at the root of the blade results in the highest operability of the blade lifts. Yoke motions during its attachment to the old blade become limiting instead. The design choices of the top crane mass, lattice stiffness and the type of line-up tool are related to the operability of the blade exchange.