The transition to renewable energy has positioned offshore wind power as a cornerstone of sustainable electricity generation. As offshore wind farms expand further from shore, High-Voltage Direct Current (HVDC) transmission systems have become essential for efficient power delivery. These systems consist of multiple cables, preferably installed as bundled units for various reasons, such as cost savings and minimising environmental impact. This thesis investigates the mechanical properties and the static and dynamic behaviour of HVDC submarine power cable bundles during the normal lay operation, a critical installation phase.

The study focuses on a 2 GW 525 kV HVDC transmission system comprising two single-core HVDC cables, a metallic return cable, and a fibre optic cable. Bundled installation is examined due to its potential to reduce installation costs, enhance spatial efficiency, and minimise environmental impact. However, the mechanical complexities of installing bundles with varying cable properties necessitate an in-depth exploration of their behaviour under operational conditions.

A multi-faceted approach was adopted, beginning with a comprehensive characterisation of the mechanical properties of an HVDC submarine power cable bundle. This characterisation included an axial, bending, and torsional stiffness analysis, evaluated under full-stick and full-slip conditions. Analytical and computational models were developed using diverse theories and advanced modelling techniques in OrcaFlex. Particular attention was given to the role of bundling straps in influencing intercable interactions and ensuring overall bundle stability during installation. The analysis revealed that cable bundles predominantly operate in the full-slip regime, where the aggregate stiffness of individual cables primarily defines bundle stiffness.

The installation process was studied from both static and dynamic perspectives, focusing on the behaviour of the bundle during normal lay operations. Three distinct cable (bundle) tensioning strategies
were evaluated, each offering unique insights into the resulting mechanical properties, responses to environmental loads, workabilities, and required strap tensions. The dynamic simulations incorporated
realistic environmental conditions, such as wave-induced motions and seabed interactions, to assess the robustness and efficiency of each strategy. The analysis identified the individual tension control
strategy as the most effective approach. By independently managing the tension in each cable within the bundle, this method offers superior control over compression loads, bending radii, and twist, ensuring the bundle’s structural integrity during installation.

Future research should focus on developing theoretical and computational models to improve the accuracy of predictions regarding the twisting behaviour of cable bundles. These models should comprehensively account for environmental conditions, internal loads, and stick-slip interactions to provide precise estimations and must also be validated and verified. Furthermore, the integration method for the physically sensitive fibre optic cable requires refinement to ensure its structural integrity during installation.

Offshore Floating Wind Energy (OFWE) presents a promising yet underdeveloped sector due to its higher costs and engineering complexities compared to fixed-bottom systems. Unlike fixed-bottom systems, OFWE isn't restricted by water depth, offering over four times more ocean area for deployment. This flexibility allows for positioning wind farms further offshore where wind speeds are higher and more consistent, maximizing energy generation potential. However, the current viability of OFWE heavily depends on governmental subsidies to remain competitive.
Integrated Systems (IS) have emerged as a promising solution to address the need for affordable, scalable, and transportable renewable energy storage. IS combines offshore wind farms with hydrogen production systems, aiming to enhance the Techno-Economic Performance (TEP) of OFWE and potentially render it economically feasible.
A case study focusing on Japan's Goto City Wind Farm illustrates the challenges and opportunities associated with OFWE and IS integration. Japan, with its ambitious wind energy goals and limited suitable areas for fixed-bottom wind energy, serves as an ideal location for such a study. Additionally, Japan's interest in integrating hydrogen into its energy mix further underscores the relevance of this research.
The initial analysis of the Goto City Wind Farm without IS integration revealed its lack of economic viability over its operational lifetime. However, by integrating additional components for hydrogen production, such as desalination, electrolysis, and hydrogen carrier configuration units, the project's Techno-Economic Performance was significantly enhanced.
One key advantage of the IS approach lies in its flexibility in power allocation between grid supply and hydrogen production. By dynamically adjusting power allocation based on prevailing market conditions and a predetermined switch price, the IS can optimize revenue generation and adapt to fluctuating energy markets.
Further analyses, including examinations of different hydrogen carrier configurations, capacity assessments, scenario analyses, and sensitivity analyses, were conducted to comprehensively evaluate the TEP of the system.
Results indicated that hydrogen production during periods of low power prices substantially improved the TEP of the Goto City Wind Farm, making it economically feasible by the end of its operational lifespan. Moreover, the ability to switch between hydrogen production and grid supply mitigated risks associated with future uncertainties in hydrogen and power prices.
In conclusion, integrating hydrogen production with OFWE through Integrated Systems offers a promising pathway to enhance TEP and improve the economic viability of offshore wind energy projects, contributing to a more sustainable energy future.

Steel is a widely used material in construction because of its availability, high strength-to-weight ratio, and recyclability. However, steel manufacturing consumes a lot of energy, and there is an increasing demand for more environmentally friendly building materials. Consequently, high-strength steel is becoming more popular as it reduces the mass of a structure. Although high-strength steel has been available for many years, its use in offshore topsides is limited due to stability and deflection issues. This research aimed to assess the feasibility of utilizing high-strength structural steel in offshore topsides, and investigated how the use of high-strength steel can be optimized in topside design.

Two different screening tools were constructed to assess the feasibility of high-strength steel within an entire topside. It was concluded that the length of a particular beam can tell an engineer if it is worth further investigating the potential of high-strength steel, while columns showed potential in all cases. The methods were tested with a case study in which an topside was assessed for its feasibility of utilizing high-strength steel beams and columns. Only hot-rolled primary and secondary beams combined with the columns and bracings were considered for this topside. When S460M steel was used for strength-governing beams and seamless tubulars, in combination with S690Q steel for welded tubular columns, the highest benefits were found and a maximum steel weight reduction of 15% was found for the considered components. At the same time, the material costs were reduced by 10%, the welding costs by 13% and the embodied carbon savings equalled 14%. When comparing these results with the total topside steel weight, 5% of the topside steel weight was reduced by using a combination of S460 and S690 steels. It was concluded that high-strength steel is feasible for offshore topsides and is more environmentally friendly and cost-effective, providing a promising alternative to conventional steels within certain components on offshore topsides.

This research presented a screening tool that is simple to use and assessed the feasibility of high-strength steel. Other engineers can easily extend this tool. As more detailed calculations are included in the screening tool, it is expected that additional cost reductions and embodied carbon savings can be found. Furthermore, including additional steel components in the assessment, such as plate girders, may result in finding much higher total weight reductions.

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%.

Methane pyrolysis is a technology that has the potential to greatly reduce the CO2 emissions of hydrogen production on a large scale. The process generates solid carbon instead of gaseous, thereby inhibiting emissions and has a relatively low process energy demand.

A three-step approach was taken to investigate the feasibility of offshore methane pyrolysis for sustainable hydrogen production. Firstly, a model was developed to evaluate the potential for converting methane into hydrogen under various design scenarios. Secondly, the model was utilized as a tool to create an offshore design for the methane pyrolysis reactor, including necessary auxiliary equipment. Finally, a calculation of the levelized cost of hydrogen (LCOH) was conducted to compare it with conventional methane reforming technologies for hydrogen production.

The ultimate results showed a higher LCOH of offshore methane pyrolysis compared to the reforming technologies, however, two main components were identified that have the potential to close this gap. First, the sales of the solid carbon by-product would reduce the LCOH of offshore hydrogen and second the introduction of a CO2 tax would increase the LCOH of reforming technologies where methane pyrolysis is not affected. Thus, it can be inferred that offshore methane pyrolysis can be cost-competitive under the appropriate conditions.

In this thesis, an automated buckling check method has been developed and validated for thin plated stiffened structures used in stressed skin topsides.

Thin plated stiffened structures are widely used in, for example, aircraft, bridges and ships. Recently, it has become competitive to use this type of construction to offshore topsides as well. Originally, topsides were built as a lattice structure, mainly for the oil and gas industry. Over the years, it was found that these lattice structures are sensitive to fluid and air leakage. Due to the later development of offshore wind farms with high voltage equipment, topsides were now also required as transformer houses for these farms. This necessitated topsides less sensitive to leakage, using a stressed skin structure.

Stiffened plated structures in compression are prone to fail by buckling. Therefore, each panel must be checked for every possible load case in the lifetime of the structure. There are three methods in which stiffened panels can be checked for buckling: full-scale experiments, FEA and by design code. Full-scale experiments are not cost-efficient because each topside is exposed to many different load conditions. Therefore, the only economically viable methods are FEA and design codes. Compared to non-linear plastic FEA, design codes are most economical and therefore the most cost-efficient choice for larger structures like topsides. However, the design code method yields more conservative results compared to non-linear plastic FEA.
Due to the use of many panels in a stressed skin topside, a demand has arisen for an automated application of the DNV-RP-C201 buckling standard for stiffened plated structures. This automated method opens up the possibility of stiffener placement optimization. This means that the stiffeners no longer need to be modelled in the design. In other words, during the design phase, non-stiffened panels can be modelled. At a later point in time, the optimization method can determine the number and type of stiffeners and stringers needed in the structure to ensure stability according to the DNV-RP-C201 design code.

Buckling analysis is dependent on a large set of variables and design considerations. The developed method sets clear boundaries for the applicability of the method and justifies the choices made.

Design codes have their limitation concerning assumptions which are not close to real conditions. Panels are subjected to uneven, distributed loads. Design codes provide rules for linear distributed loads over non-stiffened panels, but not for stiffened panels. For stiffened panels, because of the effective width method, an average stress distribution over a plate-stiffener is assumed. It was unclear whether large stress distributions over plate-stiffeners would cause a degrading effect on their ultimate resistance. Therefore, a validation study has been performed for uni-axially loaded stiffened panels. Non-linear plastic FEA has been used to determine the ultimate resistance.

From these “numerical experiments”, it can be concluded that the effective width method can be applied in most cases of stiffened panels. However, in the case of a plate slenderness between 3 and 4.28, large stress distributions can have a degrading effect on the ultimate resistance of the plate stiffener. Therefore, for plate of such slenderness, the maximum of a stress distribution should not exceed twice the minimum.

The stiffener optimization method developed in this thesis allows engineers to design stability-governed structures without modelling each individual stiffener. This enhances design flexibility and majorly simplifies the FE model. Later, the method can quickly generate a stiffener placement optimum, which would not be feasible by hand.

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.

Over the last decade, the demand for offshore power cables has increased significantly due to, in particularly, the emergence of offshore wind. Despite this growth, industry-wide rules and guidelines to analyse fatigue during cable installation operations are yet to be developed, even though fatigue can become an important parameter in the installation design.  In similar offshore operations, e.g. installation of pipelines or umbilicals, fatigue is assessed based on fundamental fatigue theory like rainflow counting, S-N curves and Miner's rule and therefore these principles form the basis of analysis in this research. However, submarine power cables consist of numerous layers of different materials that result in complex cable cross-sections. The fatigue behaviour of the cross-sectional elements due to external loads has been investigated to determine a maximum stand-by time during installation operations. In this regard, a cross-sectional analysis was performed to establish the stress-strain response of the individual cable components to the global cable deformations. For bending behaviour, the analysis was shown to be consistent with existing test data. However, this type of data is scarce and the analysis was based on a limited sample size. Moreover, geometrical cross-sectional data of submarine cables is rarely provided by suppliers and hence several assumptions were made in the analysis. For future work, it is recommended to develop in-house test data of cables such that the cross-sectional model can be accurately verified and improved. The cross-sectional stresses were implemented in a fatigue assessment model, in which the local stresses were calculated based on the output of global modelling software OrcaFlex and subsequently analysed with rainflow counting and Miner's rule. The model was applied to three cable types and it was found hat lead, used for cable sheaths, is the critical cable component in terms of fatigue. When no lead is used in the power cable, the conductors are the critical component. Furthermore, for mild loading scenarios, i.e. waves with significant wave heights H_s≤1.5 m, the model yields components infinite life for all cable components. For higher load cases, i.e. H_s≥2.5m, the model showed that fatigue mitigation measures are required to stay within the fatigue budget. Several mitigation methods were implemented, from which it was found that increasing the layback length of the cable combined with adjusting the vessel heading to favourable conditions most effectively reduces fatigue damage as all load cases with H_s=2.5 m resulted in maximum stand-by times of t_sb≥ 125days. However, for higher loading scenarios, i.e. Hs=4 m, all researched mitigation measures were insufficient to stay within fatigue budget. For future work, it is therefore recommended to improve the mitigation measures such that severe load cases can be survived. Lastly, the wave conditions were simulated as JONSWAP waves. These simulations are time-consuming and therefore not many loading scenarios were modelled. It is recommended to research methods to approximate wave conditions with regular waves, as the duration of OrcaFlex simulations will decrease exponentially. An increase in number of load cases will result in more accurate limit states for the cable components.