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%.

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.

In order to meet the climate goals to minimize the global temperature rise, an accelerated growth of renewable energy sources is necessary. Within the renewables market, the offshore wind sector takes a big part and has experienced a rapid growth over the past years. Around 80% of offshore wind turbines have a monopile foundation, (a large diameter tubular support structure). In present time, monopiles are installed by using either a jack-up or a floating vessel. Due to the trend in the offshore industry that wind turbines increase in size and that wind locations move to deeper waters, an installation with floating vessels may become the most cost-effective option. Jumbo Maritime has designed a new vessel to expand its offshore fleet: HLCV Stella Synergy. This heavy lift crane vessel will be used to install monopiles. Multiple installation methods are under consideration and this thesis focuses on the method with mooring lines to provide station-keeping during the installation. Due to the footprint of the vessel relative to an earth-fixed position, a motion compensated pile gripper is used to maintain upright position of the monopile. During the early driving phase (see Figure 1.1) of the installation, the monopile has limited interaction with the soil and acts as an unstable inverted pendulum. The upright position of the monopile is maintained by the gripper frame. The forces applied by the gripper frame on the monopile are reaction forces on the vessel. These reaction forces on the vessel can cause an increased vessel footprint. A workability assessment is performed for a moored floating monopile installation, and the influence of the gripper frame reaction forces on this workability is analyzed. During the early driving phase (see Figure 1.1) of the installation, the monopile has limited interaction with the soil and acts as an unstable inverted pendulum. The upright position of the monopile is maintained by the gripper frame. The forces applied by the gripper frame on the monopile are reaction forces on the vessel. These reaction forces on the vessel can cause an increased vessel footprint. A workability assessment is performed for a moored floating monopile installation, and the influence of the gripper frame reaction forces on this workability is analyzed. Considering nonlinear phenomena in the installation system, such as a gripper frame control system and viscous drag forces, a time-domain simulation model is made in AnySim XMF for which a hydrodynamic database is used as input. The provided hydrodynamic database is based on the results of a scale model test in a water basin, for one specific loading condition. The provided hydrodynamic database is also compared with an AQWA calculation. The reaction forces from the gripper frame are determined with a MATLAB/Simulink model which generates a gripper force time series based on the behavior of the monopile and the gripper frame due to environmental loading. This gripper force time series is applied to the vessel as an external force during the time-domain simulations. During these 3 hour simulations, the vessel is subjected to co-linear environmental conditions from two different incoming directions, while the vessel is moored to the seabed. The workability is calculated for sea conditions in the North Sea. In a second simulation model, the same simulations are performed but without the gripper frame force time series applied to the vessel to determine its influence on the total workability of the installation. The gripper frame forces have no significant influence on the workability of a monopile installation. In beam waves, the workability is limited by excessive roll motions for longer waves and by insufficient station-keeping from the mooring lines for higher waves. The loading condition addressed in this research proves unfavorable due to its low roll natural frequency. Investigation is needed whether this loading condition is representative of a loading condition which is likely to occur during a monopile installation. In head waves, the system performs satisfactory and is mildly limited by excessive roll and pitch motions.

Using a floating vessel operating on its DP-system and using a motion-compensated pile gripper to install monopiles could be the installation method of the future. Therefore, this thesis focuses on this method. The main objective of this thesis project is to build a model that accurately describes the motions of the Stella Synergy, the monopile, and the motion-compensated gripper, depending on the environmental conditions. This model is built in Anysim, which is a time-domain simulation software program of MARIN based on the RK2 numerical method. The model considers the early pile driving phase because this phase is governing in terms of risk. The monopile acts as an inverted pendulum in this phase, and the motion-compensated pile gripper must guarantee the stability of the monopile. The vessel uses its DP-system for station keeping. The DP-system contains a position reference system, a filter, a control system, and a thruster allocation algorithm. The vessel describes the wind, current and wave forces on the monopile and vessel. The environmental conditions are assumed to be collinear, and wave spreading is added to the model for some simulations. The wave forces on the vessel are determined with diffraction calculations in Ansys AQWA. The diffraction calculation for the vessel is verified with a diffraction calculation of MARIN, and the diffraction calculation for the monopile considers the shielding effect and is verified with a calculation with the Morison equation. A motion-compensated pile gripper with two PD-controllers is built in Python. The gripper considers static and dynamic friction forces and a maximum delta force per numeric timestep to model the pressure build-up time of the hydraulic cylinders. Multiple 3-hour simulations are run to generate results. These simulations, which considers each a different sea condition, are tested by the six limitations of the model. First, the preferable incoming angle of environmental conditions is determined. The workability of the Stella Synergy is calculated operating at the North Sea using this preferable incoming angle of attack. Then, two adaptations to the model are tested to increase the workability. Using fast-rotating thrusters or changing the DP-gains result in the workability of 96.4%. The governing limitation is the pitch motion of the vessel. It is tested if using mooring lines in combination with the DP-system results in a footprint reduction. It is concluded that adding mooring lines could result in a footprint reduction, but it is crucial to gain insight into the optimal axial stiffness of the mooring lines. The monopile's influence on the vessel's motion is also tested. It is concluded that the vessel's surge, sway, roll and yaw motion increases significantly due to the environmental forces on the monopile, which are passed through the gripper to the vessel. Finally, the workability of the vessel during the worst-case single failure is determined. After improving the DP-gains for particular sea conditions, the workability for the worst-case single failure was 96.0%. The failure results thus in a minor difference in the workability.

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.

The Arctic is warming more rapidly than other latitudes, which can result in the release of additional greenhouse gasses, global sea level rise and increase in extreme weather events. Additionally, this causes the rapid decline of sea ice and an ice free Arctic might occur during the summer in the 2040s. The decreasing sea ice cover accelerates the warming of the Arctic, which is known as the albedo feedback system. Solar radiation management (SRM) can be a solution to diminish or possibly stop sea ice decline. Within SRM a proposed technology, known as Arctic Ice Management (AIM), is distributing water on top of existing sea ice to increase the ice thickness enough to survive the summer melt. This raises the question: What water volume should AIM distribute on top of existing sea ice to counteract the annual Arctic sea ice volume loss? Based on data obtained during the period 1979-2020, the September trends for ice extent, ice area and ice volume are -83 400 km2yr-1, -49 200 km2yr-1 and -322 km3yr-1 respectively. The ice volume is considered as target parameter, as it accounts for both absolute areal ice loss and overall decreasing ice thickness. There are two main ice drift patterns in the Arctic: The Beaufort Gyre in the Beaufort Sea and the Transpolar Drift, of which the latter exports ice through Fram Strait into the Greenland Sea. Literature shows the ice remains within the Arctic for about five years when located in the Beaufort Sea and one to two years when located in the Transpolar Drift. For both locations, the ice decay is determined using an analytical approach first. This approach shows resemblance for ice located in the Beaufort Sea, but generally overestimates the ice decay in the Transpolar Drift. For this reason, an empirical approach is developed to determine the survival ice thickness. This results in accurate trends for ice decay of -2.1 to -2.7 cm day-1 in the Beaufort Sea and -0.8 to -1.4 cm day-1 in the Transpolar Drift. Considering 91 melting days results in an average survival thickness of 2.18 and 1 m respectively. AIM can be used to increase the ice thickness beyond this survival thickness and an AIM model is developed to show ice growth including AIM. The model concludes the AIM thickness, initial ice thickness prior to flooding and freezing duration after AIM define the effective ice thickness increase. The model is validated with small scale experiments, which indicate a delay between the flooding phase and continued natural ice growth. This delay can be the effect of the duration required to restore the temperature profile in the ice after flooding as shown by COMSOL Multiphysics simulations. Considering the AIM model, it is discouraged to implement AIM on ice thicknesses below 0.6 m and suggested for ice thicknesses approaching 1 m or higher to optimize the effective increase. The required water volume to compensate the annual sea ice volume loss highly depends on the location, initial ice thickness and target ice thickness and varies between 707 to 1095 km3 in the Beaufort Sea and between 386 to 464 km3 in the Transpolar Drift for the methods discussed in this research. To pump up this water volume, the expected power requirements are 4.5 to 7.0 GW and 2.5 to 3.0 GW respectively.

In the transition from a fossil fuel driven economy to renewable solutions, the activities in offshore wind energy are growing. Operation and maintenance costs of offshore wind turbines are high, therefore alternative methods should be developed to reduce these costs. The replacement of heavy components, like a gearbox or a generator, is a frequently occurring operation that can be improved. Currently the replacement of these components is performed by a jack-up crane vessel which has two downsides: (1) long mobilization time and (2) high operational costs. The development of systems that can replace heavy components can contribute to reduce the maintenance costs of offshore wind turbines. Therefore in the first part of this thesis, developments and techniques were investigated which can do so. It was observed that different types of solutions are already available for onshore maintenance purposes like the crane developed by Liftra and the Gamesa flexifit. Furthermore new concepts are being developed by the wind turbine manufacturer Vestas and Anson, a Chinese crane builder. These developments are addressing the problem of reducing the maintenance costs, but still two important downsides are present: (1) insufficient lifting capacity for gearbox replacement and (2) most systems are designed to be used on specific turbines. The current developments and available techniques were studied to generate concepts that can address these shortcomings. This resulted in a promising solution where a relatively small crane is installed on the wind turbine tower from a floating vessel. A great advantage over currently used method is that no specialised maintenance vessel is required for a gearbox replacement and thereby costs and time can be saved. This crane is attached to the tower structure by use of a clamping mechanism and is kept in position by generating frictional force. To generate this force, hydraulic cylinders are pressing the pads (contact surfaces of the clamp) against the tower structure. For the applicability of this concept it is of major importance that the structural integrity of the tower is maintained, while sufficient frictional force is generated to avoid the crane from slipping down during lifting operations. Therefore, in the second part of this thesis a feasibility study is done on the application of this clamping mechanism on wind turbine towers. By use of finite element modelling a workable contact surface configuration of this clamping mechanism was found. The following four variables were studied: (1) elasticity of the contact layer, (2) number of pads used, (3) width and (4) height of the pads. The latter three relate to the contact surface area and the elasticity of the contact layer influences the distribution of the forces from the clamp to the tower structure. Investigating these variables, it was concluded that; the proposed solution should have a contact layer where the elasticity modulus is higher than 1200 Mpa. By adding more pads, the loading capacity increases. However, it also results in a more complex structure and therefore it is advised to reduce the number of pads and instead widen them. Considering this trade off, a four pad configuration is selected to determine the required width and height of the pad. For this set-up it is advised to use a pad width of eighty degrees and pad height of three meter to meet the requirement of replacing the gearbox, the heaviest component of the wind turbine powertrain. Furthermore, to assure this clamping mechanism works on towers of different dimensions (i.e. diameter and wall thickness combinations), the loading capacity was determined for a variety of tower dimensions. By doing so, insights are gathered on the application of this clamping mechanism on different turbines. To make the data useful for further application, tables are included which indicate the loading capacity for investigated wind turbine towers.

Offshore wind farms are being deployed in ever more challenging conditions. Relatively unexplored are wind farms deployed in hurricane-prone regions. That is exactly the challenge that Mexican government faces as they want to expand their renewable energy resources by developing offshore wind in the Gulf of Mexico. The increased variability in wind resources, due to a combination of a reduced energy-yield design wind speed and increased hurricane structural design wind speed, pushes the overall design challenge of the turbines. Of key importance is the limited knowledge on how hurricane wind affect structures, particularly OWT’s. This study aims to identify how main characteristics of hurricane winds differ from models of regular extreme winds used in engineering simulations, to more accurately quantify hurricane winds loads and response effects on a 10MW turbine and to assess, albeit in a simplified manner, the structural ULS and SLS performance of the turbine under these extreme conditions. The most important distinction found between hurricane winds and regular extreme winds is the turbulence spectrum: Yu [18] found turbulence energy is shifted towards the lower frequencies for hurricanes while Li [16] found that turbulence energy is shifted towards the higher frequencies. Both agreed that, although disagreeing on the turbulence spectra, that these wind parameters are likely storm-dependent and/or location-dependent. In this study, hurricane parameters are incorporated into a wind generation model adopted from Cheynet [2] and altered to incorporate the hurricane spectra. The wind model is limited to the 1D longitudinal case due to limited available information on other wind components for the hurricane winds. To quantify the loads and response effects due to the different spectra, a numerical approach is considered, using a finite-element blade model developed by Pim van der Male [21] applying the DTU’s 10MW reference turbine’s structural and simplified aerodynamic properties. Within the boundaries of the inaccuracies present in the numerical input adn simulations, it was found that both the Yu and Li hurricane spectra show an increased load effect on the turbine blade, the response effect being equally large for both and roughly 20% larger compared to the Kaimal cases. This difference is proven to be predominantly due to the selection of the surface roughness length for hurricane conditions which was found to be larger by both Yu and Li studies [16, 18] for hurricane conditions. The difference due to the spectral change is negligible since the turbulent energy is nearly equal around the natural frequency of the considered 10MW blade thus not giving rise to significant changes in a dynamically amplified response. Selection of accurate hurricane wind parameters such as roughness length are thus equally important as the identified difference in turbulence spectra as they also result in significant changes of about 20% in the final results. Blade orientation has a considerable effect on reducing the response of a single blade if oriented downward. Averaging the thrust forces over all three blades however, effectively negates this advantage. Structural performance was assessed through failure probabilities of the blade given the results of the aforementioned simulations. It was found that the hurricane wind simulations resulted in the largest failure probabilities, showing a non-linear increase in failure probabilities for larger wind speeds. Bending is the governing failure mode of the blade as these failure probabilities are considerably larger compared to the shear failure probabilities for wind speeds exceeding 50 year return period conditions. Verifying the blade model response, it was found that the initially assumed three modeshapes were insufficient to accurately described the blade deformations. The model was therefore also not able to capture the correct internal root shear forces and root bending moments affecting the final results presented.

The ability to predict the fatigue of flood gates due to dynamic wave loading is becoming increasingly important as coasts and waterways worldwide are being reinforced to suit a changing climate. There are few comprehensive ways to do so however, which often leads to conservative estimates. This thesis presents an integral framework with which the fatigue of flood gates due to dynamic waveloading can be described in much more detail, while also being efficient and adaptable to different circumstances. First, the fatigue experienced by a single load event is evaluated based on readily available data. This was done by creating a phase-amplitude model from a wave spectrum based on the site properties and environmental conditions. These wave loads can then be transformed to pressure spectra with linear wave theory and pressure impulse theory, which will dynamically excite the fluid-structure system. The gate is then parametrically modelled in a FEM software package in order to export the in-vacuo response modes. These are combined with a fluid-structure interaction model to derive the response of the system to external pressures. The fatigue was evaluated with the standard Eurocode method to find a fatigue damage factor. Next, the load events imposed on the gate over its lifetime were probabilistically defined. This was done by fitting two independent probability distributions to historical wind- and water level data, the latter of which was also adjusted for climate change. These probability distributions are then split into segments which are integrated to obtain discrete load cases with representative values for the average wind velocity, water level, and probability of occurrence. Two filters were also applied to remove the load cases which cause a negligible amount of fatigue. Computing the fatigue damage caused by the average values of each of these load case bins then gives a set of fatigue damage factors with an associated probability of occurrence, which can be used to perform a Monte Carlo analysis. A case study for a hypothetical parametrically defined gate with an overhang located in the Afsluitdijk was performed, where the response for different gate configurations and environmental conditions was evaluated. The fatigue response was evaluated for fatigue across the gate, at the critical coordinate, and per mode. A ULS check was also done for the chosen design. Results following from the model were also compared to those of methods commonly used in practice.The method gives insight into the relative importance of different modes and load events, can compute fatigue for the entire gate or just a critical coordinate, and does so in a much more time-efficient manner than current numerical models. It gives a more comprehensive view of the fatigue over the lifetime of the gate by modelling its entire lifespan rather than a set of design load events. The modular structure of the integral framework allows for easy adaptation to other use cases in hydraulic engineering where fatigue due to hydrodynamic loading is of interest.

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.

This investigation considers the two floating gates of the Maeslant barrier, which is a storm surge barrier in the Rotterdam area. During the end of each closure operation of the Maeslant barrier, the flow at the barrier may be in seaward direction. Three different effects have been observed in the situation with a flow in seaward direction. The following three effects are considered in this report: 1. A force in the connection between the gate and guide tower 2. A quasi-constant change of the gate orientation 3. A periodic oscillation of the gate orientation In this report, these three effects are described, and data on these effects and relevant hydraulic conditions is collected. These data are obtained from yearly test closures in the period 2007-2021. Based on the collected data, the three effects and hydraulic conditions are quantified. From a statistical analysis based on the data, a relation with the local discharge is found for each of the three effects. To further explain the effects, simple conceptual models are derived for each of the effects. The conceptual models yield the most relevant results for the observed force, which is appropriately modelled based on a momentum balance over the barrier. For the other two effects, the conceptual models presented in this report do not adequately model the gate behaviour, however suggestions are provided for further improved models. Lastly, based on the most relevant statistical and conceptual models, an extrapolation of the three effects is performed to assess the magnitude of the three effects in extreme situations.

In the Dutch coastal waters more and more wind turbines on monopile foundations are built. An important design parameter is the first natural frequency of the wind turbine. The first natural frequency is dependent on environmental conditions and different soil conditions such as scour. This research gives a better insight into the sensitivities of these influences on the first natural frequencies. The first objective was to determine the sensitivity of different influences on the first natural frequencies in a full scale test environment. The first natural frequencies of four wind turbines (two protected against scour and two not protected against scour) in the Eneco Luchterduinen wind farm are constantly monitored and the natural frequencies are identified. It was found that only the water level has an identifiable influence on the identified first natural frequency and that the design natural frequencies and identified natural frequencies differ significantly. After the analysis from the full scale test different influences on the first natural frequency in a numerical computer model are investigated. The computer model predictions confirm that the water level has an identifiable effect and only small effects of backfilling of the scour hole on the first natural frequency are found. The third objective was to compare the sensitivity of different influences on the first natural frequency obtained from the computer model and from the full scale test. The effects of the water level are comparable. And the effect of backfill of the scour hole was found in the model but not in the full scale test. Therefore no conclusion is made on the effect of backfill nor on the presence of backfill. It was found that in order to mitigate the differences in first natural frequency of the test turbines and the model the soil should be modelled twenty times stiffer. The main influence on the first natural frequency of the offshore wind turbines in the Eneco Luchterduinen wind farm is the soil stiffness. The only other identifiable influence was the water level. Also in the design of the wind turbine the soil stiffness is underestimated significantly, implying overconservative designs.

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 extit{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.

This thesis project has set out to investigate the response in fore-aft direction of a monopile wind foundation supporting an NREL 5MW turbine, offshore the coast of Taiwan, which is affected by weak to strong earthquake ground motion in combination with normal sea state conditions. The soil at the location of the monopile is layered, with sand overlying clay, mud-, sandstone and gravel. Liquefaction, although of great concern in seismic design of piled structures, is not investigated. Two soil-structure interaction models’ responses are compared in this study, one denoted as an elastic and one as an inelastic beam on a nonlinear Winkler spring foundation. The goal of using two different soil definitions was threefold: 1. quantify the effect of inelastic soil-structure behavior on the system’s response, by comparing a nonlinear elastic SSI model with a nonlinear inelastic series hysteretic-viscous damping SSI model, 2. analyze the earthquake intensity level at which nonlinear inelastic effects become dominant, 3. verify whether the elastic response assumption made in design standards is valid and leads to a safe design. Incremental dynamic time-series analysis of the response of a Timoshenko beam finite-element model was conducted, where this model is exposed to co-directional steady wind, regular waves and an as-recorded earthquake signal of increasing intensity. This earthquake record is scaled to different magnitudes, including the Extreme Level Earthquake and Abnormal Level Earthquake as defined by ISO19901-2.

This report will give an in depth view on the technical feasibility of placing an offshore platform from which watersports can be practiced in the waters surrounding the island of Bonaire. The feasibility research is done by appointment of two entrepreneurs on the island of Bonaire. The report will give an answer to the research question: Can an offshore floating platform, aimed at fast watersports, be built as sustainable as possible within the context of Bonaire? This research question can be divided into multiple sub-questions, being: 1. What is the context of Bonaire? 2. What is, for this report, the definition of sustainability? 3. What facilities are needed on the platform? 4. How can 100% renewable energy be generated for the platform? 5. What would such a (conceptual) platform look like? To answer these questions, the method as described in Kossiakoff (2011) is used. This method gives structure to the design of a system that has not been used before, but tries to combine older systems in new innovative ways. From this method, three important stages in the design process have been identified: The needs analyses, the concept exploration and the concept definition. This report follows that structure, starting with the chapter: Needs analyses. In this chapter, the report answers the first two sub-questions. The context of Bonaire can be described as: an island with opportunities for every-one, but the local environment suffers from the exponential growth of people and tourists that visit the island. For the second sub-question the definition of sustainability has been placed within this context of Bonaire, leading to a specialized definition of sustainability. This definition combined with the requirements of the clients has lead to a valid need for the platform. The concept exploration, gives options to answer those needs. It does so by researching a broad range of possible facilities for the platform. This broad research eventually leads to a morphological map from which 3 realistic and 1 futuristic concepts are designed. In the concept definition these 3 realistic concepts have been tested by an MCA resulting in one concept that has been worked out for various components, thus answering sub question 5. From this worked out concept, a conclusion is written in where the main conclusion is that an offshore platform aimed at fast water sports van be build sustainable within the context of Bonaire if the clients are able to make some consensus in there plans and the way they will use the platform. Finally, considering the sustainability of the proposed platform, it can be concluded with current technologies it is hard to build a platform without emissions, negative effects or any hidden impact. However, if the schedule and plans of the clients where to change towards a more ’nature dependent’ schedule (so taking peak energy generation into account). The platform could set an example and can even be a global ’first’ when it comes to making the practice of watersports more sustainable.

Offshore wind turbines are being placed all over the world. The increased popularity of these structures comes with a boost to the development of wind turbines in general, resulting in larger and higher constructions. To ensure the support structures of the turbines are able to withstand the extreme loads and environmental conditions, it is subjected to a series of tests and checks, prescribed by codes and standards. This thesis focuses on buckling checks, with global buckling in particular. Several codes and standards define buckling checks, differing in approach. For example, the global buckling check is based on global buckling (compromised stability due to an axial force), but also takes eccentricities into account. How these parameters and safety factors are incorporated is analyzed for the Eurocode and the DNVGL, which are considered the most relevant design standards regarding offshore wind turbines around the Netherlands. Difference between both global buckling checks are analyzed using a monopile based reference wind turbine. Looking to the more mechanical termof global buckling (also called Euler buckling), one finds an expression for the buckling load, based on the flexural stiffness and the buckling length of the structure. However, both parameters vary over the height of the support structure: the monopile has one relevant buckling value, where the flexural stiffness varies with its geometry. Therefore the buckling length cannot be constant for the structure. To tell something about this buckling length, the support structure is modeled with Euler beams as visualized in the figure. Subsequently the buckling force is analyzed, a constant value for the flexural stiffness is derived and the buckling length is calculated. Finally, the influence of the soil conditions on the buckling length of the structure is analyzed. To determine whether a second order analysis is required to calculate the buckling force of the support structure, a comparison ismade. Two Euler-Bernoulli beam models are introduced, one with a second order term, one without. The influence of the term on the bending, rotation, moment and shear force is analyzed, as well as the influence of the Euler buckling force. Subsequently an incremental load factor is introduced to incorporate the second order effect in themore simplified beam model. Finally, the relevance of global buckling is considered. How relevant is the global buckling check for the currently installed wind turbines, and how relevant will it be for the larger support structures that are to be expected? To answer these questions, comparisons are made between the buckling checks and how far from failing the checks are. Subsequently, the local buckling is introduced to compare the global buckling check to the local buckling check regarding relevance.

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.

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.