As offshore wind turbines (OWTs) grow in size and are installed in deeper waters, the use of traditional jack-up vessels for installation faces increasing limitations. Semi-submersible crane vessels (SSCVs) offer a promising alternative; however, their floating nature introduces complex dynamic behaviour that can adversely affect installation operability. This thesis investigates the operability of single blade installation using an SSCV.

A numerical model was developed in OrcaFlex to simulate the relative dynamic behaviour between the hub of a bottom-fixed 15 MW OWT and the root of a suspended blade on an SSCV, during the alignment phase of the blade installation. The analysis focuses on the Thialf, an SSCV from the Heerema Marine Contractors fleet.

The relative motions of the OWT hub and blade root were assessed through modal and time domain analyses under environmental conditions representative of a Baltic Sea site. Results show that wave induced motions dominate the system’s dynamic response. Hub motions were strongly amplified when wave peak periods approached the natural frequency of the turbine, while blade root dynamics were governed primarily by vessel motion and the coupling between vessel response and the pendulum behaviour of the suspended blade.

Sensitivity analyses demonstrated that operational parameters, including vessel draught, suspension length, and tugger configuration, significantly affect the system’s dynamic behaviour.

Operability was evaluated using limiting criteria consisting of a maximum displacement threshold and a maximum outcrossing frequency. The analysis revealed that operability is restricted under conditions of hub resonance, vessel resonance, or coupling between the vessel and the pendulum behaviour of the suspended blade, and that the choice of limiting criteria strongly influences the overall operability.

SSCVs can perform comparably to jack-up vessels in moderate sea states, where vessel motions remain limited. In more demanding conditions, however, amplified dynamics and resonant responses may induce excessive motions that render the installation inoperable. The applicability of SSCVs for OWT installation must therefore be evaluated with respect to both site specific environmental conditions and the operational configuration. With careful tuning of the installation setup, SSCVs can provide a viable alternative under favourable conditions.

As offshore wind energy is expanding into seasonally ice-covered regions, accurate estimation of sea ice strength becomes critical for safe and cost-effective design. ISO 19906 provides an empirical method to estimate the global ice force based on the width of the structure, thickness of the ice, and an ice strength coefficient, CR. It suggests predefined reference values for this coefficient depending on the region of interest. However, if the wind farm is located outside one of these predefined refer- ence regions, the ISO standard gives limited guidance on how to adjust the ice strength coefficient to the local ice conditions. This study investigates the method proposed in ISO 19906 to estimate a site-specific ice strength coefficient, CR,s. The method uses the ratio of the ice strength index obtained from strength measurements at a reference site and at a new site.
In this study, two sites were selected to determine this strength index ratio. Hjellbotn, a temperate brackish ice zone near Trondheim, was selected as a potential offshore wind site and falls outside ISO’s predefined CR regions. Svea, in the Svalbard archipelago, was used as a proxy for the Arctic region for which the ice strength coefficient has been determined by ISO. The ice strength index at both locations was estimated using two ISO-recommended approaches: a direct mechanical measurement with the BHJ and an indirect estimate based on brine volume derived from temperature and salinity.
ISO 19906 provides a predefined ice strength index for the Arctic region when using the brine volume method. However, the standard does not offer a similar reference value for measurements taken with the BHJ. BHJ tests from Svea were used as the Arctic BHJ reference. The same test procedure was used at Hjellbotn for comparison. The BHJ strength ratio suggested lowering the ice strength coefficient for Hjellbotn. Brine-based strength ratios used either the ISO Arctic reference or Svea data. The ISO-based approach also indicated a lower strength coefficient for Hjellbotn, as expected for more temperate ice. Using Svea as the proxy for the Arctic gave a higher strength coefficient for Hjellbotn, which was unexpected. It showed that the choice of method and reference value affects the outcome of the strength estimation. This difference indicated a limitation of the brine volume strength method when trying to scale the ice strength coefficient from the Arctic to warm ice conditions. For warm ice, small temperature changes caused large strength variations, which showed the brine-based method’s high sensitivity.
The findings of this work suggest that while the ISO framework provides a basis for estimating an ice strength coefficient for a new area, careful interpretation is required when scaling the ice strength to temperate and marginal ice regimes. The brine volume method is sensitive to measurement uncertainty at high ice temperatures and may not always reflect the full mechanical strength of the ice.

This research addresses key challenges and knowledge gaps in avian safety management within offshore wind farm (OWF) development in the North Sea. As the region undergoes rapid expansion of offshore wind energy, there is a growing need to mitigate ecological impacts on seabirds, particularly in relation to their use of airspace as critical habitat. Concerns around bird collisions and broader environmental changes persist, while existing studies highlight considerable uncertainty in avian fatality estimates and migratory behaviour. In response, this thesis seeks to clarify these uncertainties by identifying data gaps, evaluating current methods, and proposing potential solutions.

Central to this effort is the development of a prototype Ecology-Technology Assessment Model. This foundational tool is designed with the mindset to bridge the gap between ecological insight and engineering practice, by integrating wind turbine characteristics, ecological parameters, and project constraints into a flexible optimisation framework. Built on principles from Multi-Disciplinary Analysis and Optimisation (MDAO), the model estimates bird collisions and assesses the economic consequences of potential mitigation measures. While the explicit incorporation of uncertainty lies beyond the current scope, the model is designed to accommodate such extensions in the future. It ultimately supports collaboration between engineers and ecologists and offers a structured platform for continued development.

Building on this framework, the thesis project applied the model to a practical case study at the IJmuiden Ver Alpha offshore wind farm site in the Dutch North Sea. The case study defined relevant ecological and technical input parameters, including wind farm layout, turbine characteristics, wind resource data, and the presence of key seabird species. Using the stochLAB collision risk model (CRM), design variations in minimum tip height (MTH), rotor diameter, and rated power were evaluated to assess their effectiveness in reducing seabird collisions. In parallel, the WINDOW model was used to calculate the Levelised Cost of Electricity (LCoE) impacts of these design choices, allowing for an integrated assessment of ecological benefit versus economic feasibility.

Simulation results demonstrated that increasing the minimum tip height is generally the most effective and economically feasible strategy for reducing bird collisions, especially for species with higher baseline collision risks. Increasing rated power was found to be more favourable for reducing a small number of collisions, while varying rotor diameter showed minimal impact on collision reduction within the modelled scenario. Ultimately, the findings emphasise that no universal design solution exists, and that the optimal collision mitigation strategy is highly dependent on the project location, site-specific bird characteristics, and wind farm design constraints. The study concludes by highlighting the importance of combining technical modelling with ecological knowledge, and provides recommendations for future research to improve data availability, model accuracy, and integration between ecological and technical disciplines in offshore wind development.

The increase in size of offshore wind turbines has altered the dynamic response of monopile-based support structures, making them more sensitive to wave-induced response-loading during periods of low aerodynamic damping. Current design standards, such as described in IEC-61400, estimate fatigue damage under these conditions, using long-term hindcasts adjusted to a predefined proportion of non-operational periods. While this provides a standardised basis for fatigue estimation, it risks under-representing the impact of fatigue-critical sea states occurring during low-damping conditions.

This thesis investigates the influence of variance in wave-induced response-loading due to operational variances on the fatigue life of monopile-based offshore wind turbines. In order to achieve this, SCADA availability data of an 8 MW wind farm is used to estimate turbine availability patterns and an existing availability model, based on Markov matrices and originally developed to assess component reliability and turbine availability, is adapted to be SCADA-driven. Together with frequency-domain simulations, this adaptation enables the integration of availability variances into fatigue damage calculations. The framework is applied to an 8 MW turbine and a 14 MW turbine, providing insight into fatigue damage sensitivity to operational variances for turbines of different sizes.

By adapting the availability model, several hyperparameters related to the representation of non-operational periods arose, such as the classification of non-operational periods. Multiple model configurations were evaluated. The available SCADA data were split into training and test sets. The training set was used to estimate the transition probabilities between non-operational and operational states, while the test set was used to evaluate the different hyperparameter configurations. The final model was benchmarked against the test set SCADA availability profiles, which were matched with frequency domain simulations. Here, it slightly overpredicted both the mean fatigue damage and its variance for four sectors, including the driving sector, while for two sectors it substantially overpredicted both metrics.

Finally, the availability model was used to predict fatigue damage over the full design life for an 8 MW and 14 MW turbine. The findings show that incorporating availability variances into fatigue damage calculations results in a relatively small variance in fatigue damage, in contrast to the single-value
estimate provided by the deterministic approach recommended by IEC-61400. For the 14 MW turbine, the results indicate that fatigue damage over the design life could be slightly higher than anticipated by the recommended DNV approach, which applies a 10% non-operational ratio as a conservative basis.

Future work should integrate availability modelling with fatigue limit state evaluation together with variance in metocean datasets, so that both operational and environmental uncertainties are captured. This would allow rare but fatigue-critical sea states to be represented more realistically. To ensure fatigue reliability under site-specific wave climates and long-term climate change effects, design methodologies could either adopt more probabilistic approaches or recalibrate partial safety factors in a site-specific manner.

The Netherlands is facing a housing crisis, with a growing shortage due to urbanization and limited available land. An alternative for constructing new buildings is adding levels to existing buildings, but this poses challenges in ensuring structural stability, particularly as existing buildings were designed with older design codes, while new levels must comply with the latest Eurocode. However, an overview of the exact changes in wind loads and what kind of influence this has on adding new levels to an existing building is missing. Additionally, it is unknown what the exact gap is between the building or location specific wind load and the wind load according to the design codes. This research aims to clarify how changes in wind load design codes affect adding levels to existing buildings.
The study analyses the development of design codes from TGB 1955 to the Eurocode, focusing on wind loads. Wind pressure and pressure coefficients are examined through KNMI data and computational fluid dynamics (CFD) simulations. Findings suggest that while overall wind speeds align with design codes, certain coastal locations like Hoek van Holland and Vlissingen show deviations, indicating a need to revise the wind area map. Additionally, pressure coefficients are not constant over building surfaces. Especially the width seems to have an influence and this parameter not considered in the current Eurocode.
Adding levels to an existing building influences the wind load in a twofold manner: extra surface subjected to wind load and a difference in design codes for wind load between the now and past. The structure of the existing building must be used optimally to support the added levels. A case study on the SCYE010 building demonstrates that optimizing the top level's design and geometry can reduce wind pressure and loads, emphasizing the importance of code evolution in such projects. These findings offer critical insights for designing additional levels on existing structures.

Offshore wind energy is considered one of the most promising solutions for sustainably meeting the society's growing energy needs. This is why there are major construction projects and massive expansion plans for new offshore wind farms worldwide. In the North Sea in particular, the technology has been used to generate electricity for several years. Looking at the oldest wind farms still in operation, it will soon be time to decommission them once the turbines have reached their service life of 20 to 25 years. However, especially the removal of the foundations is considered challenging because not many comparable projects have already been realised. The most commonly used type of foundation is the monopile due to its simplicity. During installation, these long steel piles are driven deep into the seabed to provide a secure foundation over the entire service life. One option for decommissioning is to cut the piles from the inside a few metres below the seabed and then pull them out. So far, this procedure has mainly been used for individual small piles. Scaling up to several large monopiles of a wind farm involves uncertainties. On the one hand, it is unclear how larger monopiles will behave during the cutting process, and on the other hand, the operational performance of several piles that are removed one after the other is uncertain.

When cutting monopiles, theoretically no complete cutting progress can be achieved because the pile breaks off beforehand. In this work, a calculation model was developed that predicts the cutting progress at which a monopile fails due to the hydrodynamic forces acting on it. Based on an existing project in which the internal cutting technology was used, a minimum progress was defined that must be reached before failure is allowed to occur. This makes it possible to determine which sea states are permissible in order to achieve the specified cutting progress. With the calculation model, a parameter study was carried out to find out which structural and environmental parameters have the greatest influence on failure. Additionally, two real wind farms were considered as case studies to investigate realistic parameter sets. One of these wind farms has relatively small monopiles as foundations, while the other has much larger ones. An operability analysis was also carried out for these case studies in order to determine the duration of the foundation decommissioning operation and to be able to analyse the weather downtime. In addition to calculating when a monopile fails during the cutting process, the forces required to pull the pile out of the seabed afterwards were calculated. This allows to determine the required crane capacity of the working vessels used for the operation.

The results of the calculations show that larger monopiles fail at an earlier cutting process than smaller monopiles. However, this difference is not large, which is due to the fact that together with the hydrodynamic forces, which increase with the pile size and length, also the wall thicknesses increase. This means that larger forces can generally be withstood. The operability is good for both wind farms from the case studies. Furthermore, it was found that it is important for the operability how the connection between monopile and transition piece was realised and whether ROVs have to be used. The crane capacity of the vessels used can be lower than that of the vessels used during installation. The additional force required to pull the monopiles out of the seabed does not make up for the weight lost by leaving a large part of the foundation in the soil.

Accurate wind turbine modelling is essential for reliable aerodynamic performance predictions. The industry primarily uses the Blade Element Momentum (BEM) method with correction models, but BEM’s assumptions become less valid with larger rotors and in Floating Offshore Wind Turbines (FOWTs), where wave interactions and wake dynamics are more complex. The Lifting Line Free Vortex Wake (LLFVW) method offers higher modelling fidelity but is less computationally efficient.

This study compares a BEM and LLFVW model implemented in the software QBlade. The evaluated parameters include power, torque, thrust, root bending moment, tip deflection, and angle of attack using the floating 15 MW UMaine VolturnUS-S reference turbine under various wind and wave conditions taken from several Design Load Cases (DLCs). The aim is to identify any differences between the methods and the met-ocean conditions under which these are most pronounced.

The results show minimal differences in BEM and LLFVW outputs under varying wave conditions. However, wind conditions have a greater impact, particularly around rated speeds where discrepancies were observed, mainly due to different controller dynamics. Above-rated conditions showed similar power, torque, and thrust predictions, but notable differences in angle of attack. The maximum and standard deviation of the root bending moment and tip deflection were found to be consistently lower for LLFVW compared to BEM.

As offshore wind turbines are moving further from the coast, floating wind turbines are being considered. These structures need to be able to endure high dynamic loads while remaining as still as possible in order to minimize internal stresses and maximize the efficiency of the turbine. To find ways to combat large responses, the effectiveness of tuned mass dampers (TMDs) and tuned liquid dampers (TLDs) has been widely researched. While both systems have been shown to be effective, these damping systems are being used separately.
This thesis investigates the effectiveness of vertical TMDs combined with a TLD by analysing three barge-turbine structures. The first system is the undamped structure, which works as a control. The second system combines vertical TMDs with a TLD. These damping systems are not directly linked. The third system interdependently combines vertical TMDs with a TLD by stacking the damping systems. This makes the TMD displacement determine the shape of the TLD tank, which causes the system to be significantly nonlinear.

The analysis of the three systems begins with the development of a program that creates the frequency response function for each of these systems. The basis for these programs is a linear modal analysis. The nonlinearity of the third system is accounted for by Newton iteration.
The effectiveness of each damping system is determined by the performance index, which is based on the integral of the frequency response function.

The damping parameters of the independent TMD and TLD are optimized. Due to computational difficulties no parameter optimization is performed for the interdependent damping system. In order to judge possible performance improvement for this system, a rough sensitivity study is performed on the damping parameters of this system.

The results of the optimization and sensitivity study show that the independently damped system performs best of all three systems when using these particular parameters. The interdependently damped system has unknown damping potential.

It is not possible to definitively conclude from this research whether the TMDs and TLD damp more effectively when they work either separately or interdependently. Both damped systems are able to perform better than the undamped system with the correct damping parameters.

The developed programs give reasonable frequency response spectra for all three systems. From this it is concluded that the programs of the three systems work as intended, including the nonlinear part.

The increasing popularity of offshore wind as a result of the demand for renewable energy, forces the industry to consider the development of wind farms in sub-arctic areas. One of these areas is the Baltic Sea, where the potential for wind energy is very large, but which is also prone to the occurrence of ice. For the design of offshore foundations it is paramount that ice-structure interaction is appropriately considered as ice-induced vibrations are known to significantly increase the loads. As a result, ice-mitigating measures may need to be included in the design. However, such measures will substantially increase the cost estimates required
for competitive tenders. Hence it is valuable for developers to have early screening tools to assess this. The aim of the research was to provide a ‘feasibility map’, which predicts the necessity for ice-mitigating measures in the entire Baltic region. Feasibility of monopiles was considered both technically and economically. The former by imposing design, installation & fabrication constraints and the latter was measured in terms of weight increase of monopiles.

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.

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.

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.

This report investigates the influence of joint stiffness on the eigenfrequencies of offshore wind turbine supporting jacket structures. The joint is investigated individually, within the context of a jacket structure, and within the context of the entire offshore wind turbine structure. These analyses were specifically conducted for the case of a 10MW wind turbine jacket support structure. This report aims to present an analysis method that can represent the joints of jacket support structures for offshore wind turbines with high fidelity while also ensuring low computational cost during the analyses. The joint stiffness is captured through an implementation of joint submodelling by applying the Craig-Bampton reduction technique. Along with this, the report serves to provide a comparison of innovative wrapped composite joints and traditional welded joints when applied to a 10MW wind turbine. The wrapped composite joint was developed by Dr. Marko Pavlović at the Delft University of Technology, and it was investigated due to its superior fatigue performance compared to welded joint alternatives.
Two sets of analyses were conducted: the jacket analysed by itself, and the jacket analysed along with the tower, turbine, and piles (to be referred to as the ”offshore wind turbine”). Natural frequency analyses were conducted for the jacket and for the offshore wind turbine for the different joint types. It was found that for the jacket and for the entire offshore wind turbine, the models with welded joints had the lowest natural frequencies, the models with wrapped composite joints had the second lowest natural frequencies, and the models with rigid joints had the highest natural frequencies when compared to their respective counterparts.
In conclusion, both the application of the wrapped composite joints and the application of the analysis method of submodelling joints of the jacket are beneficial for solving issues related to the construction of jacket support structures for large wind turbines.

For marine structures subjected to combined wind and wave loads, predicting the long-term extreme response corresponding to a certain return period is an important aspect during the design stage. Generally, a full long-term analysis (FLTA) is recognized as a highly accurate method to determine the long-term extreme responses. FLTA integrates all the short-term extreme responses with their corresponding occurrence probability of environmental conditions. Therefore, this approach is very time-consuming and inefficient, especially for complex marine structures. Therefore, the environmental contour method (ECM) is proposed as a more efficient alternative method for predicting the long-term extreme response of offshore structures. The traditional environmental contour is obtained using the joint distribution of mean wind speed, significant wave height, and spectral peak period. This method has been used extensively on marine structures subjected to wave loads with reasonable numerical accuracy. It decouples the response with the environmental variables and uses the largest most probable short-term response evaluated along with the environmental contour corresponding to a given return period or exceedance probability to represent the extreme response. However, for offshore wind turbines which are subjected to combined wind and wave loads, the ECM performs poorly on wind-induced responses (under wind only or combined wind and wave load conditions). This is due to the non-monotonic behavior of the wind-induced responses resulted from the control system of the wind turbine. The ECM will cause the deviation of the critical environmental conditions between the identified environmental contour and the realistic case. The modified environmental contour method (MECM) is proposed and developed to deal with such a system whose response changes discontinuously due to variations in the operating conditions with additional environmental contours taken into consideration. It is seen that environmental contour plays an important role in the application of both the ECM and the MECM. A set of environmental conditions are selected along the target contour to perform short-term response analysis for the determination of the long-term extreme response. Traditionally, the environmental contour is established based on the inverse first-order reliability method (IFORM) by Rosenblatt transformation with response excluded as a variable. Since this traditional method involves the transformation of the standard normal variables and original physical space which is related to the FORM-approximation closely, a possible over-or underestimation of failure probability may be induced. Another method to obtain environmental contour is the inverse second-order reliability method (ISORM). The ISORM approximates the failure surface by a quadratic function at the design point in standard normal space instead of a linear function in the IFORM. The result of contour based on the ISORM always being conservative, which cannot be ensured by the traditional IFORM method. The highest density region method (HDRM) is to define the environmental contour as the boundary, along which the joint probability density function (PDF) of the environmental variables is a constant. This method solves the contour in the original physical space and can be used in high-dimension conditions. Applying the environmental contour method and modified environmental contour method based on the IFORM, the ISORM, and the HDRM allows to predict the extreme response. The MECM results are much better than the results from the ECM according to safety requirements in the design stage. When applying the environmental contour method, the inverse second-order reliability method to obtain contour is recommended. The ISORM contour is more conservative than the traditional inverse first-order reliability method but the numerical analysis is more stable than the highest density region method in three dimensions, while when applying the modified environmental contour method, the traditional inverse first-order reliability is preferred because of its simplicity and time-efficiency in the calculation.

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.

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.

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

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

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

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

Premature failures in large offshore Wind Turbines are often attributed to bearing failure despite gearboxes being designed and developed using the best bearing design practices. Furthermore, as turbine size and rated power increase, bearings display an enhanced tendency to fail. Unscheduled bearing replacement at sea is a complex, costly, weather-dependent and time-consuming operation that results in high turbine downtimes. Market trends show an increase in turbine rated capacity and a noticeable shift towards deeper waters and far-off remote sites which further delays and complicates unscheduled maintenance activities and aggravates the cost penalties of idle turbines. Detecting an incipient bearing fault (diagnosis task) is therefore a major aspect to evaluate drivetrain and overall wind turbine reliability. Moreover, estimating the remaining useful life of bearings and predicting their operational state in the future (prognosis task) can achieve a breakthrough in optimising maintenance programs, improve wind farm operation and decrease wind turbine downtime which can bring about a significant cost reduction. The purpose of this work is to investigate the health monitoring and prognostics possibilities of drivetrain bearings in a floating spar-buoy offshore wind turbine. The drivetrain concept considered in this work is based on DTU’s 10-MW reference wind turbine. Specifically, this study targets the prognosis of four critical drivetrain bearings located in the main shaft and the high-speed shaft.

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.