This study examines the effect of adding helical piles to the foundation mechanism of monopile installation templates. A monopile installation template is a gravity-based structure, which is placed on the seabed. The installation template is utilised for the installation of monopiles, and to ensure its verticality during this installation. At this time, challenges arise for the stability and operability of the installation templates due to the increased dimensions and mass of these structures. The increased demand for green energy leads to the growth of monopiles, in both number and size. The growth in monopile size leads to the increasing required dimensions of the installation templates. The installation templates are, however, often restricted in size and mass by the capabilities of the installation vessel.
The installation templates generally use mud-mats as foundation system to ensure stability and prevent the template from excessive settling. The mud-mats cause significant dynamic effects, especially when taken through the water line. Moreover, the sliding capacity of mud-mats is often insufficient, leading to potential sliding failure of the template. A literature review is conducted using different sources, such as reviews, offshore guidelines, research papers, and expert interviews, to investigate installation templates and different foundation mechanisms. The investigated foundation mechanisms are suction buckets, mud-mats, and (helical) piles. The literature review demonstrates that helical piles show greatest potential of being added to the mud-mat foundation. Therefore, solely helical piles are considered in the current study, and the following research question is answered: How, and to what extent, does adding helical piles affect the footprint of the monopile installation template designed for the benchmark project? The current study uses a project carried out by Heerema Marine Contractors as benchmark. All parameters of this project are used as input for the study. Findings of analytical analyses on the (environmental) loading and mud-mat capacity are validated with the benchmark project results. Subsequently, the helical pile geometry is optimised for maximum uplift capacity, using the methodology shown in the figure below. The capacity is optimised for uplift capacity, as it is assumed that the mud-mats can bear the compressive loading. Additionally, the lateral capacity would significantly increase by utilising piles, and therefore optimising for lateral capacity is not recommended. The obtained geometry, optimised for maximum uplift capacity, is then assumed to be constant and used to determine the uplift, compressive, and lateral capacity. The analysis shows a positive correlation between embedment depth and capacity (uplift, compressive, and lateral). The helical pile capacity is added to the mud-mat capacity. Consequently, effects on design variations are examined, such as reduction in template's mass and size, mud-mat size, and variations in helical pile geometry and soil parameters. The analysis shows a significant improvement in capacity of the foundation, in both compressive and lateral capacity. The uplift capacity of the helical piles ensure no uplift of mud-mats occurs. This leads to a more evenly load distribution among the mud-mats, lowering the maximum factored load on the legs. Consequently, the mud-mat size and the template's mass and dimensions could be reduced. It can be concluded that a reduction in mud-mat size directly results in the potential of reducing the template dimensions. The reduction in both template and mud-mat size would automatically lead to a decrease in mass. However, the reduction in template mass could lead to a decrease in structural performance. This should be investigated in further studies. Variations in soil characteristics show that the performance is even more enhanced in denser sands. Concluding, the helical piles have a positive influence on the template's overall capacity. The findings show that a reduction in the footprint of the template is possible, of maximum 13.6%.

This thesis focuses on the going-on-location (GoL) operation of jack-up vessels in the offshore wind energy industry. The GoL process involves the transition of a jack-up vessel from a free-floating state to a state where it’s elevated above the waterline and pinned to the seabed. The workability of these vessels is significantly affected by the impact loads experienced during the GoL process, particularly when interacting with stiff seabeds and non-negligible sea states. In current methodologies, certain parameters like wave height and wave period set the limitations to when GoL process can proceed. For safety reasons, the GoL process is halted if these conditions are exceeded. Instead of solely depending on external factors like wave height an wave period, the focus transitions towards how the vessel dynamics relate to the impact forces it encounters during the GoL. To address this limitation, a comprehensive framework has been developed that combines hydrodynamic, structural, and soil-spudcan interaction elements to evaluate impact forces during the GoL process. The framework offers a flexible and case-specific configuration. It allows for easy modifications, integration, and
replacement of components and input parameters. This case-specific arrangement is advantageous due to the wide range of jack-up vessels and environmental variations. In model implementations adhering to the framework, the jack-up vessel is represented as a multi-body structure, in contrast to the conventional rigid-body representation often employed. Within the multibody approach the spudcans, the legs, and the vessel are described by separate bodies each with its own properties. The primary focus of this research is on the dynamic soil-spudcan interaction process, which has not been extensively covered in existing standards. The soil-spudcan interaction model is to determine the instantaneous force acting on the spudcan as it contacts the seabed during GoL. By integrating elasto-plastic soil behavior into the
soil-spudcan interaction element, the model encompasses descriptions of soil resistance to spudcan penetration and lateral displacement, taking into account memory and potentially stateful characteristics. A simulation model, adhering to the framework, has been developed, integrating hydromechanical, structural,
and soil-spudcan interaction submodels within the Orcaflex environment. Three distinct simulation scenarios are examined: an undisturbed vessel (free-floating), a disturbed vessel (full GoL), and a pinned vessel (elevated
jack-up). The disturbed vessel scenario, which includes a full GoL process, has exhibited consistency in both undisturbed vessel simulations, where the vessel is the free-floating stage, and in pinned vessel simulations,
where the vessel is in the pinned stage. The impact phase is situated between these two boundary cases, and the framework effectively represents simulation models within its scope. In addition, simulations with varying sea states are performed for regular and irregular sea states. Simulations involving varying regular wave patterns suggest that the maximum downward spudcan velocity (DSV) is a critical parameter influencing the magnitude of impact forces on the spudcans. For irregular waves, the simulations indicate that the maximum impact forces are more closely related to the pinned vessel scenario, as
the maximum impact occurs towards the end of the impact phase.
In conclusion, this thesis has effectively described the behavior of jack-up vessels during the impact phase of the GoL process. For any model utilizing the framework, the GoL process can be simulated, and the results
can be analyzed to assess workability. Furthermore, the study proposes a potential correlation between vessel dynamics and maximal impact forces, a relationship that could potentially guide on-board decision-making
processes. The enhanced understanding of the interaction between the spudcan and the seabed, along with the comprehensive framework, contributes to improving the decision-making process for executing the GoL
operation of jack-up vessels in the offshore wind energy industry.

Suction caisson jacket (SCJ) is one of the most promising substructures, under certain conditions, for offshore wind turbines. Their unique characteristic of large diameter (D) compared to their small wall thickness (t) conducts them an object of study, concerning their transportation to the offshore location, as their structural stability should be ensured. Their large D/t ratio leads the caisson into being a flexible structure, which should be stabilized in an already moving barge (due to wave motions and accelerations). The caisson is partially supported in the base perimeter, in order to leave enough space for both loadout equipment and the barge to coexist. The main objective of this thesis is to apply the most optimal configuration to sufficiently support the caisson on barge. Prerequisite of that is to examine the behavior of the caisson, when interacting with the barge during transportation. A parametric study is performed where the stresses and deformations of the caisson are retrieved and the buckling behavior is examined. The boundary conditions are applied in the bottom perimeter of the caisson and they are designed, so as the stiffness of the barge is taken into consideration. The parametric study considers changing the boundary conditions (supports) in the perimeter of the caisson. In total 6 cases were examined, in which the supports vary with respect to their length, their location in the base of the caisson and finally the value of their stiffness.
A finite element analysis is performed and the results were derived for every case of the parametric study. The equivalent von Mises stress is examined in the full length of the caisson and in the supported perimeter, as well as its most important components. For the supported perimeter, directional deformations are derived in radial, tangential and vertical direction. The results showed that the stresses developed very large peaks above yield limit at the edges of the supports, for each case, and the most influential component causing these peaks turned out to be bending stress in tangential direction. The bending stresses are caused by radial deformations, thus in order to reduce this stress component, radial deformations should be reduced. The comparison of the cases showed that the trend of the stresses remain the same for every case. It is also realized that the peaks in stresses and deformations in radial and tangential direction show the most reduction when the distance between the supported segments is reduced. A non-linear analysis performed indicates that the high stresses in every case (above yielding) are acceptable and this will lead to a redistribution of stresses in the caisson. The buckling behavior of the caisson is examined and in every case buckling failure does not have a high possibility to occur for the applied load combination. As a result of this study it is concluded that the most optimal grillage configuration has to do with reducing the unsupported area between the supported segments.

The demand for offshore wind energy has never been higher, yet the number of ideal areas for wind farm installations is decreasing. As a result, larger wind turbines are being placed in deeper waters, which poses opportunities for floating foundations. However, due to the industry's inability to scale up the production of relatively expensive floaters, companies are exploring alternative foundation methods for offshore wind turbine generators in deep water, including the extension of the use of monopiles, which are now being considered for use in water depths up to 60 meters. However, higher loads in deep water require increased stiffness in the monopile structure, which is typically achieved by using larger thicknesses and diameters, resulting in XXL-monopiles. The steel used for these XXL-monopiles increases exponentially for greater water depths, and the designs cross the production limits. This study investigates the use of a guyed monopile as a more favorable alternative to a conventional monopile for deepwater applications with large wind turbine generators in the range of 60-120 meters of water depth and a rated power of 15 MW. The guyed monopile concept involves adding moorings to a conventional monopile to provide additional stiffness at a certain elevation, reducing the required material within the monopile. A numerical model is elaborated where the structure is represented by a one-direction FEM model having two degrees of freedom. The model is supported by springs that represent the soil and mooring system. The soil is modelled by a series of p-y reaction curves. For the mooring stiffness, a nonlinear stiffness approach is used where the axial stiffness and geometry of the mooring are considered. The method is used to determine the maximum stiffness that a mooring system can bring. After that, the mooring system is included in the FEM model. Static and dynamic analyses are performed to determine the response of the structure. The described methodology also implements requirements based on potential resonance (1P and 3P frequencies), ultimate limit state (yield strength, column buckling, and local buckling), and fatigue limit state (fatigue damage due to wind and waves). The static and dynamic responses are validated using Ansys. It is concluded that the moorings can provide enough stiffness to reduce the impact of the acting loads on the structure. A case study is conducted to compare the use of a conventional monopile and a guyed monopile for a foundation based on realistic wind and wave loads. Data of a reference wind turbine and a reference location is taken to find the advantages of a guyed monopile for a realistic case. The study iterates the process for different water depths and varying numbers of moorings. Greater water depths lead to more issues due to resonance and fatigue, which can only be overcome by increasing the diameter and thickness of conventional monopiles. This can also be overcome for guyed monopiles by increasing the mooring stiffness. The results show that the guyed monopile is favored over the conventional monopile in all cases, with up to a 45% reduction in steel required.
The case study's findings are used to identify key parameters in the design of a guyed monopile. The sensitivity of the mooring system stiffness is compared to other key design parameters, including monopile diameter, thickness, and embedded length. The study finds that the moorings have a favourable effect on the system's natural frequency. Also, bending and shear stress may be reduced when moorings are applied.

Wind energy has seen a transition from onshore to offshore in the last decades. The lack of space on land and up-scaling of turbine sizes led to the need of exploiting further away and harsher environments, where more space is available and there is arguably less of an impact on society. This together with a higher supply of wind resources has led to the development of large numbers of offshore wind farms, of which many are still to be built. Taking Europe as an example, currently the goal is to achieve at least 450[GW] of installed offshore wind power by the year 2050, of which only 25[GW] has been installed up until 2020. This sketches out the enormous challenge that is being tackled by governments and the public market as we speak.
The market is a highly competitive one, where small increases in efficiency of one of the many complex steps within the development of offshore wind farms can make or brake companies and projects. The eventual goal of increasing the efficiency of the overall process, is to decrease the levelized cost of energy of a wind farm. One of the main approaches of increasing this efficiency, has been to up-scale wind turbines, in the last decades the size and power output of wind turbines has already more than doubled from an average of 3MW of installed power per wind turbine 15 years ago to an average of 8.2MW in 2020. This trend does not seem to stop any time soon, as wind turbines of 12-15MW are in active development. The industry has already expressed interest into turbines of 20-25MW and even beyond this. The turbines beyond these sizes will be referred to as Ultra Large Wind Turbines(ULWTs). Due to the size and weight of components of ULWTs traditional top-down installation techniques cannot be used anymore due to relative motion related difficulties during the mating of components. Thus, the following research question will be answered during the defence:

"What would be feasible novel on-site installation methods for a bottom founded Ultra Large Wind Turbines(ULWT) using floating vessels?"

During the research, the base case of, floating installation on a pre-installed jacket type foundation is used as a starting point for the generation of concepts. A total of 15 concepts has been generated and pooled with concepts proposed in the industry and academia. All these concepts were screened to find the most promising ones. These concepts were sequentially tested on their technical feasibility for use with ULWTs, to identify which concepts can possibly be used. The final step was to perform a comparative analysis of the economic feasibility of the technically feasible concepts, to identify the differences in performance and usability of the concepts. From the technically feasible concepts, two main approaches could be identified which are promising for the installation of ULWTs. Further work must be performed to identify the best option from these two and describe their overall economic feasibility.
Self-climbers: Devices which climb the tower without assistance of an installation vessel. Especially interesting due to the fact that they can be used with existing mono-hull lifting vessels. Main downsides are the relatively low flexibility w.r.t. wind turbine size, usability and have a long installation time. They are promising, as they decouple the size of the installation vessel and size of the turbine entirely.
Large semi-submersible vessels: Either using lifting equipment, so that existing vessels can be used. Or specifically designed vessels which provide large flexibility w.r.t. size of turbine and usability. Main downside are the size and costs of these vessels. They are promising, as they provide a platform on which the needed height can be reached effectively.

Although modern technologies such as smartphones with GPS functionality make navigation substantially easier, automobilists still rely on traffic signs to inform them which exit to take or which lane to follow. These traffic signs on highways are of substantial size rendering them, and their support structures, extremely susceptible to aerodynamic load induced fatigue damage.

In this thesis traffic sign support structures in the form of steel portal frames consisting of square hollow sections are considered. Antea Group designs such structures using conservative generalized methods prescribed by the Eurocode and ROK norms. In order to investigate whether or not the structures may be optimized with respect to fatigue, a more elaborate load and dynamic response model is formulated.

Aerodynamic loads are primarily considered as these are the most significant dynamic loads, which are governing for fatigue damage. The two types of aerodynamic loads that are considered are traffic induced pulse loads generated by large vehicles passing close to the sign, and stochastic turbulent wind flow. The former is in the form of a pulse load, while the latter is in the form of a super-position of sinusoidal waves based on the Solari wind turbulence spectrum. These loads are applied to a dynamic finite element method developed in Python specifically for this thesis, consisting of one dimensional Euler-Bernoulli beam elements. The dynamic response is evaluated using a modal analysis. The resulting dynamic cross-sectional signals are transformed into weld-stress signals using a finite element model in \textit{IDEA StatiCa}. Finally the Rainflow Cycle Counting algorithm is used in combination with the Palmgren-Miner rule and SN-curves provided by the Eurocode in order to estimate the fatigue damage of the structure during its lifetime.

This methodology is applied to an existing reference structure, and an optimized design. The optimized design is optimized according to the Eurocode with respect to the ultimate and serviceability limit state, but fails in the fatigue limit state. From the results it is established that the calculation model presented in this thesis yields much lower magnitudes of stress range cycles compared to those found in the design norms, leading to significantly reduced fatigue damage. However this model does not take instantaneous wind gust loads into account, reducing its range of applicability. It is suggested to utilize the simulation model for loads that occur more than 10e5 times in a 50 year life time, but utilize the existing design norm models for loads that occur less often. The traffic induced pulse loads are found to be of significant size under the current assumptions, but due to a lack of research in this field the validity of the magnitude of the pulse loads remains uncertain.

Ultimately it is show that there is potential for optimization with respect to fatigue, but the simulation model should be used on more structures and should be further evaluated in order to create a more simplified and generalized analytical model that may be applied quickly to any design or even structure.

To fulfil the ever-increasing need for wind energy, European offshore wind farm sites are selected in deeper waters with seabed conditions which can consist of hard consolidated sediments or even rock. The deeper sites require the use of floating wind turbine foundations that are moored off to anchor piles in the seabed. For rock seabed sites, the anchor piles must be drilled. As the water depth of these sites increases, commercially available jack-up vessels are no longer able to operate. Therefore, the anchor pile drilling operation must be performed from the deck of a floating vessel. An extensive techno-economic analysis has led to the finding that a topside-operated drilling rig mounted on a large construction support vessel with heave compensation (HC) added is the most cost-effective configuration. The performed research focuses primarily on the determination of which HC method is most effective at changing water depths of 50 m up to 200 m. Leading to the understanding which site requires the use of active HC, limiting the resources required to construct future floating wind farms. The configurations are tested for relevant wave conditions, determined by assessing potential European floating wind farm sites.

Firstly, the research assesses the maximum allowable topside displacements before the drilling column reaches either the operational limits of plastic failure or bottom hole assembly lift-off. Secondly, the operational vessel motions are determined for the relevant environmental conditions. By comparing the results, the need for HC in the drilling configuration is determined. Third and finally, the passive and active HC methods are assessed for a 3-hourly time simulation under the before-mentioned environmental conditions. The assessment is performed using two performance criteria; weight on bit variation and the occurring drill-string stresses.

The performed analyses and simulations show that the vertical upward vessel motion is the limiting factor for the operation’s workability. Also, HC is required in every considered environmental condition. Further, the system operating with passive compensation shows a decreased stiffness with respect to the active system, most noticeable at 50 m water depth. This leads to higher frequency vibrations and stress variations being present in the drill-string of the active system. This effect is no longer noticeable for water depths larger than 50 m.

For locations with a water depth of 50 m, the active system shows favourable workability results. The active system shows a larger sensitivity to wave conditions with larger wave heights, as the stiffness is larger and more stress variations occur as a response. However, the results remain more favourable in comparison to the passive system as the lift-off percentage is significantly smaller. The passive and active systems show similar results when considering short waves in 50 m water depth, this is best witnessed in the weight on bit and lift-off percentages.

For locations with a water depth of 100 m and 200 m, the active and passive HC systems show comparable results for the performance criteria, for all considered wave conditions. The stresses remain within the ultimate limit state, the fatigue damage is negligible in comparison to the time required to perform the drilling operation, and the lift-off percentage for both configurations are in the same order. Therefore, as the workability of the two systems are so comparable for a water depth of 100 m and 200 m the availability, day-rate, and mobilisation complexity of the equipment will determine which HC system is most effective per project.

Changing the way of power production on a Floating, Production, Storage, and Offloading (FPSO)
system by using renewable power can contribute to reducing an FPSO’s carbon footprint while operating. Bluewater Energy Services is interested in the use of a wind turbine placed on top of an
FPSO, to power the FPSO’s internal electricity grid. This suggested configuration of a wind turbine and FPSO results in many different challenges that are new to the offshore industry. Therefore, this thesis considers the following research question: ‘To what extent is placing a 5 MW wind turbine on top of an FPSO feasible?’

To answer this question this research is split into two parts. The first part consists of a literature
research, in which two reference FPSOs, Bleo Holm and Haewene Brim, are introduced, and the decision of considering a 5 MW horizontal axis wind turbine is made. A wind turbine with this capacity can deliver approximately 75% of the FPSO’s required power during its normal operations. This information is used as input for a hazard identification (HAZID), in which a group of experts was challenged to brainstorm on possible hazards concerning the topic. The second part introduces the first design concept and focuses on the assessment of feasibility. Both technical feasibility and non-technical feasibility are evaluated. For technical feasibility, topics such as vessel stability, vessel strength, vibrations, the system’s motions, and wind turbine use during operational events on the FPSO are considered. From the non-technical feasibility, additional focus is put on the health, safety, and environmental (HSE) aspects of the project, and the project’s business case.

From the technical evaluation of the chosen design configuration a motion analysis, which consists of a diffraction analysis in HydroStar and a time domain simulation in OrcaFlex, identified a limitation in the wind turbine’s nacelle acceleration. This motion analysis was limited by the unavailability of a wind turbine model consisting of a yaw control system. In operations, the limit for nacelle acceleration is exceeded, which can be mitigated by performing more frequent wind turbine shutdowns. In extreme conditions for the vessel’s fully loaded condition, this mitigation is not an option. Recommended is an improvement of the design, to reduce the nacelle’s acceleration, with options such as reducing the wind turbine tower height or increasing the tower’s bending stiffness. Also, the found result is linked to the location of the Haewene Brim in the central North Sea, which is generally considered a site with rough environmental conditions. A different location with calmer waters is likely to still facilitate this concept. The considerable HSE-related risks that have been identified are related to the risk of a wind turbine’s dropped objects onto the FPSO, which can occur from different causes such as fire, structural failure, or lightning. This risk comes with a low probability, while its impact can be high. The business case of the project shows the potential of saving 250,000 tonnes of CO2 emissions during a typical wind turbine’s lifetime of 20 years. Also, it is estimated that a positive return on investment can be made in case the capital expenditures are less than €64.3 million. However, this result is sensitive to changes in, among other parameters, the wind turbine’s power production.

Based on the findings described above the conclusion to the question: ‘To what extent is placing a
5 MW wind turbine on an FPSO feasible?’ is that it can be considered feasible in case a design concept is found for which the nacelle acceleration is no longer an issue. However, future research is recommended on comparing this concept to other renewable power alternatives for the FPSO, based on their risks and financials, to see if the safety risks are worth the financial gain. On top of that, the development of a wind turbine yaw-controlled model is suggested to capture the interaction between weathervaning FPSO and yaw-operating wind turbine. In addition, research can be done on the installation method for a wind turbine on an FPSO.

One of the main cost drivers of an offshore wind power plant are the support structures of the wind turbines, it is therefore of primary importance to optimize their design. Among the support structures available, the concept most widely adopted is the monopile-based support structure, whose design is often fatigue-driven. Offshore structures need to withstand the wave loads, that play a major role among the cyclic loads that excite the support structure. The design of the support structure’s geometry is of primary importance, since it determines the structural vibrations.
Therefore, this thesis aims to understand how, varying the diameter of the support structure, affects the wave induced fatigue loads acting on a monopile-based offshore wind turbine. A FE model was developed to represent the structural motion, where the Euler-Bernoulli beam theory was adopted. The linear wave theory was used, and the wave loads were computed according to the Morison equation.
The wave induced fatigue loads were calculated in frequency domain, assuming a narrow-banded response spectrum. A case study was provided by Siemens Gamesa RE, and the wind turbine was assumed in parked mode.

Two assignments were derived, to tackle the research question. First, a sensitivity analysis was
performed, to study the non-local effects on the wave induced loads, due to varying the diameter of the support structure. Then, an analytical optimization was applied to a simplified structure, aiming to find the diameter that minimizes the mass of the support structure, accounting for fatigue damage. The hypotheses of thin wall and deep water regime were assumed.

The results of the sensitivity analysis suggested that the non-local effects do not differ significantly from the local ones, and that to reduce the loads: it is beneficial to reduce the diameter at waterline, to increase it around mudline, while variations along the tower are quite irrelevant to this end. The analytical optimization was run for different load cases. Wave induced fatigue loads alone were first considered,
then a diameter-independent fatigue load was introduced. It was concluded that, accounting for resonant waves only, the smaller the diameter of the support structure, the better.

Offshore structures are often founded on mud-mat foundations which prevent the structure from settling excessively. Pressure differences are generated under mud-mats during removal from the seabed. These differences lead to a force that resists the uplift. A literature study on breakout resulted in a hypothesized distinction of four mechanisms that occur during the uplift of offshore shallow foundations. An experimental program was designed to study these mechanisms. Small-scale tests on clay and sand were executed. The effectiveness of selected mitigation measures was studied, as well as the influence of the measures on the mechanisms. Existing calculation methods that model suction or quantify the breakout force were evaluated. It is believed that the results from the experimental program led to a comprehensive list of parameters that should be included in a design method to model the total resisting force to uplift, taking the mechanisms into account.

This research investigates how the current friction design standard should be improved, specifically for monopile sea fastening with polymers. Literature and results from tests performed previously to this research are investigated in order to obtain a complete overview of the factors that should be considered in the design standard. Additional tests are carried out on factors for which literature and previous test exercises are inconclusive. Friction test methods are compared to determine the best method for testing monopile sea fastening friction. The total test variability is analyzed to determine why safety measures are required. The validity of these measures is studied with a large set of test results. Finally, modelling as an alternative for testing is investigated.

The report summarises the development of a frequency domain Multi-Unit Floating Platform (MUFP) model and a parametric design optimization using the model. The model returns the response statistics of a MUFP due to environmental wind and wave loading using input variables of column separation, column diameter and draft. The model is developed with careful consideration to ensure compatibility with gradient-based optimizers. The low fidelity tool is capable of quickly traversing a design space to land on optimal substructure dimensions. The program uses the input variables to calculate the geometry, buoyancy, mass and stiffness matrices of the MUFP. The design of a mooring system is considered outside the scope of this report. As a result, the stiffnesses in surge, sway and yaw are filled with placeholder stiffnesses. The hydrodynamic coefficients of the three MUFP columns are calculated in HydroD. Smaller drag components such as bracing are assumed to be less influential and are therefore ignored. To maintain computational efficiency during the optimization, the hydrodynamic coefficients are interpolated using surrogate models. The rated power of the MUFP is 10 MW, this is generated using two 5 MW turbines. The aerodynamic loading is accounted for by taking the Power Spectral Density (PSD) of a thrust force time series generated for the NREL 5 MW reference turbine in SIMA. The thrust force time series is taken as the sum of two individual SIMA simulations run in a 280m x 200m wind field. The response statistics of the MUFP are calculated in the frequency domain. This is done by assuming the MUFP motion can be modelled by a Gaussian distribution. The Gaussian distribution is used to derive the zero-crossing periods and the expected number of cycles in a 3-hour timeseries. These values are used to calculate the probability of the maximum wave amplitude. The Most Probable Maxima (MPM) is then found by equating this probability to a Rayleigh distribution. The MUFP is tested against two load cases at rated wind speeds. The first load case includes uni-directional head-on wind and wave loading. The second load case is designed to test the MUFP weathervaning properties. This is done by simulating head-on wind loading, a 90 degree wave heading and a 3 degree yaw misalignment. The optimization is run using the SciPy SLSQP Minimize function. The objective function is the total steel mass and constraints are set on the static heave displacement in addition to the MPM pitch and roll rotations expected in a 3-hour timeseries. The optimization was run three times. Two solutions were found at the global minimum while the third was found at a less optimal local minimum. A comparison of the MUFP optimized dimensions is made against a 10 MW WindFloat design. The comparison revealed the optimization solution was considerably smaller than the WindFloat. Although the physical properties of the MUFP do provide benefits such as a reduction in the aerodynamic pitching moment arm, it is concluded that the reduction in size is a result of the calculation process underestimating the MUFP response statistics.

The goal of this thesis is to investigate the feasibility of optimizing the mooring line characteristics of a semi-taut (chain – polyester – chain) mooring system in combination with drag embedded anchors for a 12 MW wind turbine supported by a semi-submersible in deep water, based on static analysis of the Ultimate Limit State (ULS) condition. The optimization objective is mooring system material cost. The hypothesis is that the ULS condition is design-driving. It is expected that static analysis optimization reduces optimization time and cost significantly in comparison to a full dynamic optimization. Additionally, a framework for a 6 wind turbine shared mooring farm concept is presented and challenges encountered are discussed. A gradient-based sequential quadratic programming optimization algorithm is implemented. Using static analysis results for optimization requires a relation between dynamic and mean mooring line tension. The initially implemented relation was found to be incorrect and showed that the relation is design- and condition-dependent. The optimizer reduces the mooring system costs by reducing the diameter of the mooring line segments, while the lengths of the polyester segment, bottom chain segment and anchor radius are increased to satisfy the constraints. The bottom chain segment is governing in material line material costs. The Minimum Breaking Load (MBL), pretension and anchor uplift constraints are the most critical constraints. The top chain segment reaches the lower limit as it does not significantly affect the properties of the mooring system and other constraints. A total cost reduction of 48% with respect to the initial design and an estimated simulation time reduction of 40% with respect to a dynamic simulation were achieved. Fatigue Limit State (FLS) analysis of the optimized system showed that FLS is design driving, as failure in the chain segments would occur within 1 year. To implement optimization based on static analysis a FLS constraint is required. A full dynamic FLS analysis is time consuming and a single degree of freedom model to estimate fatigue damage in the frequency domain based on surge motion and separation in frequency was implemented. The model was tested on two mooring system designs and compared to the simulated dynamic FLS simulation results. The model underestimates the accumulated damage consistently across the two designs and the segments. It significantly underestimates the damage accumulated in conditions with a long wave periods, which excite the natural frequency in heave, and showed that heave and pitch motion must be included for a more accurate damage accumulation estimate. The shared mooring farm concept design is challenging due to the eccentric placement of the wind turbine. Additionally, the reduction in the number of anchor lines requires increased anchor line strength, causing a vertical force imbalance at the fairleads of the semi-submersible. This force imbalance requires a very robust finite element solution for use in optimization, which was not achieved. Adjustments of the positioning of each turbine, distribution of ballast or dynamic ballasting in the semi-submersible may be required to investigate the feasibility of the designed concept.

Van Oord is active in the subsea cable installation industry and executes most of their cable lay projects with their cable laying vessel: The Nexus. In this thesis the focus lies on the normal lay phase of the cable installation, which comprises the phase when the vessel is pulling out the cable and putting it down on the seabed following the desired cable route. In order to ensure cable integrity during cable installation, a normal cable lay analysis is executed in the dynamic analysis software Orcaflex. The aim of the cable installation analysis is to define the installation limits in terms of the sea state. However, the vessel motions of the Nexus can be measured instantaneously and accurately on board of the vessel. Therefore, an assessment into the use of vessel motions for the expression of the handling limits during cable installation is performed. The assessment is executed for a normal lay configuration with an export cable and a 50 meter water depth. The focus lies on the maximum curvature and maximum tension response of the cable. The cable dynamics result from both the vessel motions and the direct cable loads. First, the effect of these phenomena is assessed independently with the use of Orcaflex simulations. The influence of the vessel motions on the cable dynamics is found to be low for short wave periods, as the vessel hardly reacts to these kinds of waves. As a result, vessel motion limit criteria are less suitable for expressing cable installation limits at less severe sea conditions. However, the handling limits of the cable are not likely to be exceeded during these kinds of sea states, therefore this does not directly prevent the use of vessel motion limit criteria. Next, the most suitable vessel motion, measured at the chute of the vessel, for application of vessel motion limit criteria is determined based on the time lagged cross correlations between the cable response and the vessel motions. By using this method, the vessel motion which is most linearly related to the cable response is selected. In the case study, the heave acceleration and axial acceleration of the vessel are identified as most suitable candidates for application of vessel motion limit criteria. Finally, the performance of the selected vessel motion as limit parameter is compared to the use of the wave elevation, equivalent to sea state limit criteria. The performance of both is assessed by a linear regression analysis of the peaks in the limit parameter time history and the associated peaks in the cable response. This analysis led to the conclusion that in the case study higher certainty can be given to vessel motion limit criteria compared to sea state limit criteria, which eventually can lead to an increase of the workability. Furthermore, a sensitivity analysis is performed to identify if the selected vessel motion for the application of vessel motion limit criteria is sensitive to changes in the normal lay configuration. The selected vessel motion and accompanied magnitude of the correlation were prone to changes in the normal lay configuration. Therefore, the applicability of vessel motion limit criteria for the base case in this thesis cannot straightforwardly be generalised for other normal lay configurations. Due to the nonlinear nature of the cable lay system, all cable installation analysis are executed using time domain simulations in Orcaflex, which are associated with large computational time. In light of reducing the computational time for normal lay analysis, the potential use of a transfer function for estimation of the maximum cable response is evaluated. The transfer function is set up based on the first order frequency response of the cable system to regular waves. Before application of the transfer function approach, the nonlinear behaviour of the system is studied on the basis of the spectral response of the cable towards regular wave simulations in Orcaflex. Especially the contribution of the higher order frequency components and the effect of the nonlinear drag term in the Morison equation are addressed. In order to check the performance of the cable response transfer function, the maximum cable response estimation of the transfer function for a three hour time duration is compared to the statistical three hour maximum resulting from non-linear Orcaflex simulations. The transfer function is found to underestimate both the curvature and the tension response of the cable, leading to the conclusion that this transfer function approach is not suitable for the prediction of the maximum cable response. The underestimation is caused by the high dynamic complexity of the normal lay system.

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.

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.

Offshore wind turbines (OWTs), together with their support structures, are designed for an operational period of 20 years. The first generation of these offshore wind turbines has already reached or is approaching their designed lifetime of 20 years. Depending on the legislation and the governmental subsidies, a decision needs to be made about their future. One option is to keep the OWTs in operation after exceeding the design lifetime while the safety levels are not compromised. Operating after the design life, which is called lifetime extension has become more and more interesting in the current market conditions. To find out whether the safety levels, which are determined by the design standards are not compromised when the lifetime is extended, the OWT support structures should be reassessed when the end of design life is insight. Reassessing the support structure can take out the uncertainties of the parameters that are monitored and can make lifetime extension possible.
The objective of this study is to propose a framework for reassessing existing OWT support structures for lifetime extension since there is not a clear detailed methodology describing the assessment and extension which can be applied for OWT support structures. Because of the complexity of the problem and the limited time, only the governing limit state is studied which is the fatigue limit state. The proposed framework consists of two phases. The first phase is the reassessment phase in which the available documentation and measurements of the (operational) history are taken into account to determine the fatigue damage with more certainty from the installation of the OWT till the point when the reassessment takes place. The second phase is the remaining useful lifetime (RUL) prediction phase, which aims at determining the remaining operational lifetime of an OWT without exceeding the safety limits. For both phases, different methods can be used that can be classified in deterministic methods and probabilistic methods. Finally, the suggested framework is demonstrated in a simplified case study. First, the fatigue lifetime of the simplified structure is calculated with wave conditions of the Gemini wind farm; This calculated lifetime resembles the initial design lifetime and serves as a comparative measure for the following reassessment and RUL prediction phases. Then the simplified structure is reassessed with updated data, using a deterministic method. Subsequently, the RUL is predicted by using probabilistic fatigue calculations whereby different uncertainty distributions are taken into account. From this case study, it can be concluded that the proposed framework is applicable for different amounts and types of measurement data as well as assessment methods. The deterministic reassessment shows different outcomes of fatigue life of the structure even with a small change in the input parameters. The probabilistic fatigue calculations used for the RUL are computational more complicated but very promising since site-specific uncertainty distributions replace the generalized partial safety factors. The suggestion is, therefore, to use probabilistic models to achieve a longer lifetime for the OWT support structure without compromising the safety levels.

The quest for affordable renewable energy causes the need for bigger and more efficient wind turbines. To counter the problem of possible collisions between the growing turbine blades and the tower support structure, downwind turbines might be a solution. For this configuration, blade flexibility no longer forms a problem and besides that passive yaw systems can be applied. However, the fatigue lifetime of the turbine components might significantly decrease due to tower shadow effects. This thesis aims to decrease the uncertainty in the difference between downwind turbines and the conventional upwind turbines. An aeroelastic blade model is constructed using a finite element model of the NREL 5MW turbine blade. This model is subsequently converted to a downwind model including a wake behind the tower structure that temporarily slows down the inflowing wind on the blade when the blade is in the tower shadow area. This wake description follows from a literature review and should be valid for the flow conditions around the turbine. History dependency of the aerodynamic forces on the blades is included in the model using the Küssner function. A comparison in tower wake effects is done using simulations of upwind and downwind turbine blades which result in the blade motions caused by the tower shadow disturbance. From these motions also the root moments are deduced over time as well as the angle of attack variations for the different blade elements. A comparison with other software confirms the order of magnitude of the blade position in equilibrium and a check for dynamic stall issues is done to show the possible influence on the blade lift after the tower shadow disturbance. The root moment variations are used as a fatigue indicator using rainflow analysis. Concluding from this, wind velocities near cut-out speed for downwind turbines cause the biggest wake effects. The static blade deflections and root moments are at the highest for rated wind speed at upwind turbines. Compared to upwind turbines, the downwind turbines are prone to higher wake effects and therefore increased fatigue damage. The overturning root moment and flapwise deflection variations for downwind turbine blade have shown to be up to 88 % and 100 % higher compared to those for upwind turbines respectively.

Recognizing climate change as a result of more than 150 years of industrial activity, and due to the emission reduction protocols set internationally, wind energy has become the mainstream source of energy for many developed nations. Due to the prospect of large scale energy generation, wind energy has moved offshore. Offshore wind farms are relatively more complex to design and costlier to install. Since, substructure design of an offshore wind turbine (OWT) plays an important role and is one of the key drivers for capital cost, optimization of support structure design becomes imperative.
Driven by the universal goal of cost saving in offshore wind industry, the main objective is set to – “reducing computational time required for a typical fatigue design cycle of OWT support structure”. From the perspective of a foundation designer, following practical challenges are identified and investigated:
• Lumping methodology for correlated wind and wave scatter diagrams
• Reduction in number of fatigue design load cases in time domain simulations
• An alternative fatigue design method in frequency domain
This thesis focuses on the fatigue design of jacket type support structure. First, a conceptual 3-legged jacket with a light weight pyramid shaped transition piece is modeled in finite element software. Subsequently, an overview of loads, dynamic properties and fatigue analysis of support structure is presented.
In OWT support structure design, wind and wave loads are equally significant. For fatigue design, the combinations of loads which are likely to occur simultaneously during the design life of the structure are estimated. The correlation established between wind and wave is generally presented in the form of scatter diagrams. To reduce the computational time, common practice is to condense wind and wave scatter diagram and assess fatigue damage for the lumped states. Thus, in this thesis, an investigation is completed on commonly used lumping methodologies and comparison is established on the accuracy of fatigue results. In this study, existing lumping methods are validated for a jacket support structure and it is proven that dynamic characteristic of the structure plays an important role in condensing scatter diagrams.
Traditionally, the complete fatigue load case design of OWT support structure is based on time domain simulations, which is very time consuming. Hence, an important query raised by the Company (KCI) was, whether it is possible to reduce the number of design load cases while retaining a high level of precision in fatigue damage results. A novel method is presented in this thesis, which attempts to reduce the number of design load cases. For jacket support structure, linear-quasi-static assumptions are made to estimate approximate fatigue damages due to wind load alone. Using the estimated damage, critical wind seeds are identified and are sorted for reduction. Results from this case study show that with the reduction of load cases, the accuracy in fatigue damage is also compromised. This experimental method provides good insight into the load case reduction capabilities and is recommended for preliminary design phase.
Since, time domain fatigue design requires very large time records to accurately describe the random loading processes, it proves prohibitive for many finite element analyses. Thus, an alternate fatigue design approach in frequency domain is introduced in this thesis. Contrary to popular use of power spectral density functions, a different method is presented which successfully preserves the phase information of load time series. Frequency domain method offers many advantages like providing clear information regarding the environmental conditions and the dynamic properties of the structure. First, essentials of frequency domain method for a single degree of freedom system is described. Subsequently, this approach is exemplified on OWT support structure with interface wind loads. Based on the assessment of response from time domain and frequency domain analysis, it is proved that frequency domain method is an effective tool which can lead to vast savings in computational times while still producing accurate fatigue results.
To summarize, this report gives an overview of the work done to reduce the computational time and effort required for a typical fatigue design cycle of an OWT support structure. Three measures were presented which helps with the reduction of computational time and the accuracy of end results were checked with full time domain simulations. As a result, one can perform more informed design optimization leading to reduction in costs for the support structure.

The wind energy industry is booming. In 2017 the installed wind capacity has overtaken coal as the second largest form of power generation in Europe. Costs of wind farms are driven down, and even the first offshore wind-farm contract without subsidies is a fact. Installing the Wind Turbine Generators (WTGs) more efficiently increases the profit margins in this highly competitive industry. Nowadays, WTGs are installed using jack-up vessels, but there is little known about the installation of large wind turbines using a floating vessel in offshore sea states. Installation using a floating vessel solves the problem of increasing water depths, critical leg-retrieving forces, the need of more powerful jacking systems, and the time-consuming jacking process. By installing the WTGs in a single lift, the mating operations can be avoided, resulting in shorter offshore installation times.

Though due to the high crane tip, already small vessel motions result in significant WTG motions. The turbine manufacturers only allow minimum accelerations and impact velocities, since the components are sensitive and damaged easily. The objective of this thesis is to identify the dynamical response and coupling of vessel-turbine, to decrease the motions of the WTG and to translate this into a lift and landing system. The focus is on studying influencing parameters, rather than to assess the workability of the proposed concept. The Bokalift 1 is used as a case study, to explore the possibilities of installing WTGs with assets of Boskalis.

An OrcaFlex model is built to simulate the dynamic behavior of the turbine and vessel under regular and irregular wave loads. Both the free-lift and landing phase are studied, with the use of modal analyses and time-domain simulations in OrcaFlex. The hydrodynamic properties of the vessel are calculated in AQWA. A dynamic model is also derived analytically, using the Lagrangian formalism, to validate the modal analysis of OrcaFlex and to study the coupling between the turbine and the vessel.

Resonance is found by the excitation of two lifting configuration modes in the longitudinal plane, namely the pendulum and double pendulum mode. For the transverse plane significant coupling was found between the turbine and the vessel, resulting in a modal shape which is a combination of the pendulum mode and the roll mode of the vessel. A second peak of resonance is caused by the excitation of the double pendulum mode for the transverse plane.

The motion of the WTG, as a consequence of the resonance of the double pendulum mode, is related to the natural period of this mode relative to the RAO of the vessel. Time-domain simulations in OrcaFlex showed that significant motion reduction of the WTG is possible, by lowering this natural period below the wave period excitation range of the vessel. A modal analysis of the analytical model described that the natural period is also a function of the rotational stiffness of the lifting configuration, resulting in an avoidance of resonance when adding enough rotational stiffness. Resonance of the roll mode cannot be avoided without changing the hydrodynamic properties of the vessel. Motion reduction by the usage of tugger line damping is also studied. Tuggers placed under an angle towards the vessel also proved to avoid resonance of the double pendulum mode. An improved setup could increase its potential to damp out the resonance caused by the roll mode as well.

The proposed landing system is modelled in OrcaFlex to assess the performance under irregular wave loads. Time-domain simulations proved to reduce the vertical velocity of the WTG up to 97% and below 0.01m/s. However, the current guiding system still allows for rotations of the WTG. These rotational motions could be significant due to the double pendulum mode and could lead to local vertical impacts of the bottom edge of the tower on the foundation.
Based on the obtained results, it can be concluded that both the horizontal and vertical motions of the WTG can be reduced significantly during the free-lift and landing operation from a floating vessel.