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.

This master thesis proposes a method for assessment of the allowable sea states for the single-lift installation of a Wind Turbine Generator (WTG) with a Quick Connection System (QCS) on a bottom-founded support structure with focus on the landing (mating) operation. The thesis also determines the operability of this installation and examines how it could be optimised.

Due to an increase of WTG size, the current lifting height of state-of-the-art floating Heavy Lift Vessels (HLVs) is insufficient to lift the WTG components to the required installation height. To overcome this limitation, and to further reduce installation time, new installation techniques are being developed. The single-lift installation methodology; where the WTG is lifted at the bottom of the WTG and just above the combined Centre of Gravity, in combination with the C1 Wedge Connection; a newly developed connection with a high ULS and FLS strength and large installation tolerances, forms a promising setup for dual crane HLVs. The Quick Connection System (QCS) of the C1 Wedge Connection can create a temporary connection able to provide sufficient restoring moment, required to keep the WTG upright. In this thesis the operability of a floating single-lift installation of a WTG with a QCS is determined and optimised. The performance of the QCS is compared to alternative connections and design improvements for the operation are proposed.

Two computer simulation models are built and compared, where-after the most promising model is expanded and further developed. This OrcaFlex model includes wave and wind loading in in-plane directions (3 Degree of Freedom directions). Nine critical events and limiting parameters are identified, containing motion limits of the WTG and QCS limits.

The results show that the operability for the base case (no Heave Compensation and strict limiting parameters) is negligible. In all cases assessed, the time required to activate the QCS turned out to be the governing limit. If heave compensation is incorporated and some parameters are altered (e.g. increasing the allowed WTG rotation or the maximum C1 Wedge Connection gap between the flanges) the operability can be increased to become feasible for wave peak periods up to Tp < 9s and significant wave heights Hs < 2m. In this optimised situation the activation of the QCS is the governing installation activity, with respect to the load transfer phase situation. Here, a minimum of 16 QCS Wedges are required for the load transfer phase and the pre-activation load transfer should be around 20% of the WTG static weight. For Hs < 1.4m, the mating operation was found to be feasible without a QCS. Here, it should be noted that optimal wind loading was assumed, and zero DP drift is incorporated.

It is recommended to expand the model to include all 6 DOF directions to verify if these results are still valid for non-optimal wind loading conditions. Furthermore, with these results, a study could be set up to determine the economic feasibility of this installation methodology. Such a study is essential as the economic viability determines the adoption of the technology.

Due to the increasing demand of renewable energy to combat climate change, renewable energy is experiencing a rapid growth. To match this demand for renewable energy, especially offshore wind is popular due to its status as a proven technology. To build offshore wind farms in a space and cost-effective way, wind turbines are increasing in size and the farms are shifting towards deeper water. This also has an effect on the monopiles, the most used wind turbine foundation. They are increasing in size and weight to handle the deeper waters and larger turbines. At this moment, monopile installation is limited to approximately 30 meters water depth because of a scarcity of installation vessels with sufficient capacity. To install these large-size monopiles either the fleet of installation vessels must be expanded or new installation methods must be developed.
This thesis addresses the challenge of installing large expected size monopiles without the use of traditional heavy lift vessels, which are reaching their operational limits due to increasing monopile sizes and water depths. The aim of this study is to develop a craneless upending method for a 130 meters long, 13 meters diameter monopile weighing 3500 tonnes, suitable for deployment in waters 50 meters deep.
Five craneless upending methods are evaluated. Two axes of rotation are considered, namely upending by rotation about a fixed point and upending by rotation about a floating point. Furthermore, to rotate the monopiles during upending, a moment can be applied around the axis of rotation and by ballasting parts of the monopile. After considering the four models that follow from combining the given options, a fifth method was developed that includes upending by rotation about a floating point and rotating the monopile by ballasting and applying a moment both.
A numerical model is developed for each concept to analyse the forces, moments, buoyancy, and draft throughout the upending process. The models were verified against each other and validated through a scaled experiment. The experiments highlighted the importance of friction in the upending process and, after adjusting the numerical model to account for the friction, confirmed its validity.
Of the five methods evaluated, the method that incorporates ballasting and applying a moment is found to be the most optimal. Due to combining the two methods of rotation, the water depth and moment required for the upending process can be minimalized.
In this approach, a floating monopile with end caps arrives at the site and is connected to a gripper frame which allows for axial movement and rotation of the monopile. Once the pile is connected, the ballasting phase starts. During this phase, ballast water is pumped into the monopile through the bottom end cap which causes rotation of the pile through the water. When a certain clearance between the monopile and sea bed is reached, de-ballasting starts while applying a moment around the axis of rotation to continue the upending while maintaining the minimum clearance between sea bed and pile.
The findings demonstrate that the presented hybrid craneless upending method is feasible and offers a practical solution to the installation challenges posed for larger monopiles and installation sites at deeper waters.

Offshore wind energy is a pivotal element in the renewable energy transition and achieving net-zero emissions by 2050. Most offshore wind turbines require the installation of a sturdy foundation called a monopile. The current method of installing monopiles offshore is known as piling, which involves driving large steel cylinders into the seabed with a hydraulic hammer. One of the major problems with this method is that it generates high levels of noise, which travel through the water and are harmful to marine life. Therefore, several European governments have introduced laws that regulate the amount of noise that may be emitted.
Multiple solutions have been developed to reduce the amount of noise that is generated but they are costly or negatively impact operational speed. Therefore, GBM Works, a startup in the offshore wind industry, is developing an innovative method for installing monopiles known as the Vibrojet®. The Vibrojet® combines both a Vibrohammer on top of the monopile with a jet at the bottom. The jet aims to fluidise the sand inside the monopile to reduce the friction on the inner shaft. This not only reduces the amount of noise emitted but also increases the installation speed of the monopile and may reduce the required pile dimensions.
This research aims to optimise the Vibrojet® performance during offshore installation by reducing shaft friction as much as possible while minimising jet flow. This approach maximises the installation speed of the monopile while reducing the capacity requirements for all parts of the Vibrojet® system. When installing a monopile while jetting, a soil skeleton forms (soil plug) in the middle of the pile while all sand particles near the inner shaft gets fluidised. The displacement of the surface of the soil plug has been assessed in this research to estimate the plug shape and optimise the performance of the Vibrojet®.
At Deltares a test setup of a soil container was constructed to imitated a 2D version of the inside of a monopile during Vibrojet® installation. The front of the container was made of see-though Perspex to enable analysis of the processes inside the pile. Inside the container holding 1,500 kg of sand, 0.3\% of the particles were ultraviolet (UV) coated to enable tracking. Visual software was written to track these particles with a camera, allowing for the measurement of flow velocities and visualise the surface of the plug shape.
It was discovered that the equation for plug surface displacement overestimated the displacement observed in the laboratory tests. By analysing the results from various installation settings, discrepancies in the equation were identified. The following improvements were recommended to enhance the accuracy of the equation:
• Addition of a seepage force term to the equation
• Addition of separate term for sedimentation effect
• Factor for the return flow of particles
• Influence of the slope angle on flow erosion
A relationship was discovered indicating that the optimal flow rate for Vibrojet® installations can be determined by the total volume of sand that needs to be fluidised. A model was proposed to optimise the performance of the Vibrojet® by assessing the displacement of the plug and utilising this relationship to calculate the connection between the flow rate and the volume of the fluidised zone.

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.

The ongoing global energy transition has led to significant growth in the offshore wind sector. This growth has resulted in a high demand for offshore installation vessels like the 'Aeolus', and the need to push up their limits to meet the rapidly changing market. In the Saint-Brieuc offshore wind project, the Aeolus had to be installed on a stiff seabed, a situation that was expected to induce high impact forces on the jack-up legs. To mitigate these forces, leg modifications were installed to increase the operational limits for jacking operation. However, Van Oord's operational limits for installation on stiff seabed conditions differed from DNV's Joint Industry Project, leading to the need to reassess the necessity of the leg modifications.

The objective of this research was to identify the main physical phenomena that govern the forces in jack-up legs during installation on a stiff seabed, and to understand how these physical factors and a specific leg modification could influence them. This research followed a systematic approach to address the research objective, comprising a literature study on impact mechanics, model development for axial impact forces, model validation, a sensitivity study, and an analysis of the leg modification. Insights from the literature study guided the development of the model to analyse the governing forces. Validation was performed by using field data from the Aeolus, specifically oil pressure readings from the vertical jacking cylinders and vessel motion measurements.

The research found that what governs the forces in jack-up legs during installation on a stiff seabed depends on the energy contained within the system prior to impact and how this energy is absorbed by the soil and structure. Key factors include wave-induced vessel motions, wave height and period, hull characteristics, vessel's inertia, jacking velocity, and the leg length below the hull. The energy absorbed by the soil and structure depends on their stiffness characteristics and behaviour, which primarily concluded that axial forces dominate over lateral forces during impacts on a stiff seabed. However, lateral forces should be considered in deeper waters or where seabed protrusions are anticipated.

The sensitivity analysis indicated that the vessel mass, initial velocity, and normal soil stiffness significantly influence the impact force, with the initial velocity predominantly influencing the impact force, and the overall system stiffness dictating the impact duration. By introducing the leg modification, the impact force was reduced by approximately 70-75%, along with an increased impact duration of 320-370%. Moreover, it was found that the impact force and duration are interconnected rather than separate occurrences. The leg modification also reduced the influence of soil stiffness variability. Furthermore, for a more realistic representation, incorporating the nonlinear behaviour of soil load-deformation and leg modification characteristics is needed, but this would increase the computational demand. Consequently, this suggests the need for advanced solutions and an engineering assessment to balance desired accuracy against computational complexity.

In recent years the global energy demand has rapidly increased and will continue to grow in the next decades. With the growing demand for energy and the international agreement to use more renewable energy, the offshore wind industry is developing rapidly. This causes offshore wind farms to be installed at locations with larger water depths and wind turbines becoming larger and heavier, leading to the use of support jackets. Due to this trend, pre-piling templates are used more often in the industry to decrease the installation time and increase the accuracy of offshore wind parks supported by jackets. Pre-piling templates are made to align and guide foundation piles during pile driving. Huisman built a pre-piling template to install two offshore wind farms off the coast of Taiwan. After installation, it turned out that the secondary steel on the template was severely damaged. In this thesis, research is done to identify the possible reasons for damage to the secondary steel of the pre-piling template during pile driving.

An investigation of currently available literature has identified possible causes of increasing loads on pre-piling templates. The difference between most of the pre-piling templates investigated in the literature and the template of Huisman is the use of friction pads instead of rollers to guide and align the foundation piles during installation. In addition to aligning and guiding the foundation piles, friction pads also clamp the pile. Based on a thorough literature review and the current design of the template, a hypothesis regarding the cause of the damage was formulated. The hypothesis states that the current design of the template causes an increase in normal force and friction force between the pile and the pads during pile driving, damaging the secondary steel of the pre-piling template.

To test the hypothesis, a model is developed to describe and analyze the dynamic interaction between the template and foundation pile during pile driving. The model exists of an external hammer force, a LuGre dynamic friction element to represent soil-pile interaction and a part representing the stick-slip interaction between the pile and the pre-piling template. The model has been implemented using Matlab to generate the system response in the time domain from the corresponding equations of motion. The different components of the model are validated and verified using field data and data from the literature. While the model has been fully verified, the model could not be quantitatively validated due to a lack of data, but the model has been qualitatively validated based on trends from literature.

The use of friction pads leads to energy transfer from the hammer blows into the template due to the existing friction forces between the pile and the pads. Additionally, results show an increase in normal force between the friction pads and the pile during a hammer blow due to the design of the centralizer. The rotation of the upper centralizer causes an increase in normal force between the pile and pad due to its orientation...

Reducing noise pollution during offshore monopile installation is crucial for marine life. Current installation methods create too much noise or do not have the certainty of reaching the required depths. By using a Vibrohammer combined with a Jet-ring this problem could be solved. A Jet-ring is a tool to reduce the sand induced inside shaft friction by fluidizing the inside of the monopile during installation. To make this concept a viable option, the Jet-ring should be retrievable as a reduction in cost but also in materials. The aim of this thesis is to design a retrievable Jet-ring for a variable-diameter monopile without changing the diameter of the Jet-ring. For this goal to succeed the focus will be on the possibility of using inside-out jetting. This design would be an addition to the Vibro-jet® of GBM Works, which is not retrievable yet. Keeping the diameter of the Jet-ring constant keeps the design simple, reducing the chance of failure inside the monopile where the Jet-ring has to withstand harsh conditions under a lot of water and sand.

After creating a complete design of the retrievable Jet-ring, an experiment was designed to prove the concept. The designed concept is a Jet-ring which is small enough to fit through all sections of the monopile. In order to reduce the shaft friction from a distance, the nozzles are pointed outwards. This way the nozzles first have to erode a layer of sand before reaching the monopile wall. For comparison, 4 different water inlet concepts were tested, including the designed Jet-ring. All additional concepts were chosen to confirm and compare certain
aspects of the Jet-ring. The test setup included several cameras, a flow and a pressure meter. The concepts were placed at the bottom of the tank with a sand bed above it during the experiment. A repeatable experiment was created by keeping the relative density equal which was done by keeping the starting bed height constant combined with an equal weight of sand at every experiment. At the top of the sand bed, a bolt was placed which would fall through when the sand was fluidized. After execution, the height of the sand bed was measured
when the bolt fell through. In addition to the bed height, the duration until fluidization was measured. For all concepts, three different flow rates were executed minimally three times per flow rate to reduce outliers.

The processed results can be divided into two categories: visual and numerical results. The visual results contained the flow patterns and other mechanisms helping to fluidize the sand bed. By looking at the flow patterns, explanations were found for the numerical results. The numerical results are the bed height and duration until fluidization. Looking at the results, the importance of having pressure behind the flow was shown. Comparing the base case, with a homogeneous water inlet, to the other concepts jetting with ten nozzles showed that fewer nozzles with a higher pressure resulted in faster fluidization. Furthermore, the importance of prolonging the erosion mechanism is shown. A concept directly jetting into the side of the tank lost a lot of energy in the
flow resulting in almost double the fluidization time. While the energy dissipation against the wall of the tank is substantial, energy dissipation due to erosion seems relatively small. This was concluded by comparing a concept that jetted from the centre of the tank and a concept that jetted from the side. The last conclusion makes the designed tool using inside-out jetting a viable option for the retrievable Jet-ring.

While the results are promising, some remarks should be made on the experiment and the design. While congestion by sand could occur during the experiments the results still give a relevant view on the capability of the concept to reduce the inside shaft friction of the monopile because the same mechanisms such as erosion and fluidization are used to penetrate the sand during the congestion as during monopile installation. Furthermore, because of the smaller diameter of the tank, the complete inside was fluidized. This could be not the case during monopile installation as sand could accumulate at the centre of the monopile. However, this possibly accumulating sand is not sure to influence the shaft friction as the sand against the monopile wall that induces the shaft friction is fluidized or weakened. These additional studies could strengthen and prove the concept of inside-out jetting further in the future.

This thesis explores the feasibility of using a helical pile foundation with a tension leg mooring system to support a 15MW floating offshore wind turbine and assesses its financial viability compared to alternative anchor concepts.

The study commenced with an extensive literature review, drawing from a diverse range of sources, including reviews, offshore guidelines, research papers, and expert interviews. The literature review was conducted to enhance the understanding of the use of helical piles in the offshore industry, encompassing their fundamental characteristics, current applications, and potential contributions. For this investigation, a Tension Leg Platform (TLP) featuring a 15 MW wind turbine, engineered by Heerema Engineering Solutions (HES), was employed. Time-domain simulations, accounting for environmental conditions, were carried out using the OrcaFlex software. These simulations led to the determination of the mooring line tensions, with the maximum value identified as the design load case. During the helical pile geometry optimization process, which focused on maximizing uplift capacity, it was established that a single helical pile in dense sand can achieve a maximum uplift capacity of 7.91 MN, while taking into account geotechnical, structural, and installation constraints. As a result, considering the significant uplift capacity demands of TLPs, it is essential to employ helical pile grouping, necessitating a minimum of four helical piles. Consequently, the design of the helical pile group anchors confirms their ability to meet the uplift and lateral capacity requirements of TLPs. Furthermore, the exploration of variations in equipment, steel strength, helical pile geometry, and soil conditions has yielded promising results. It is important to note that achieving the required installation depth for these group anchors involves a significant force and torque during the installation process, presenting potential challenges. Consequently, the development of equipment capable of meeting these installation requirements is vital for ensuring the feasibility of these helical pile group anchors.

The performance of helical pile group anchors was analysed from economic, technical, and environmental perspectives, with a comparison to suction and driven pile anchor concepts. The economic evaluation showed that the estimated total costs for the 4-pile helical group anchor, utilizing a singlepile installation method, are notably higher when factoring in the costs related to developing helical pile equipment, making it less financially attractive compared to the developed suction and driven pile anchor concepts. However, when excluding these equipment costs, the 4-pile helical group anchor ranked as the second-most financially attractive option, regardless of the installation technique, with only the single driven pile anchor being less expensive. Notably, it was discovered that, in a scenario where structural, geotechnical, and installation constraints are disregarded, the single pile helical anchor emerges as the most financially attractive among all anchor types, even surpassing the single pile driven anchor. This suggests that overcoming challenges related to scaling up helical pile dimensions, like enhancing their structural integrity, and reducing installation requirements through innovative designs, could make the use of fewer piles in a helical pile group anchor a feasible choice. In addition to these quantifiable factors, helical piles offer the advantage of a low-noise installation method, which becomes increasingly important due to the growing noise disturbance legislation in certain regions. Furthermore, their adaptability to various ground conditions through flexible installation techniques makes them a convenient choice for sites with limited access or specific inclination requirements...

Due to the rapidly growing offshore wind market, there is a high demand for capable wind turbine installation vessels. Current wind turbine installation vessels struggle with the installation of large diameter monopiles due to a lack of crane capacity and with the installation of wind turbine components at hub height due to a lack of crane height. Most jack-up vessels currently have a crane capacity of 1600 tonnes or lower. This is adequate for the current average diameter monopile but already too low for the larger diameter monopiles. Due to a lack of crane capacity, these vessels are at risk of no longer being capable of installing wind turbine foundations in the near future. However, if it would be possible to reduce the crane loads during the installation process of monopiles, the commercial viability of these vessels can be extended. Previous research and practical experience have shown that a floating monopile upending process is feasible. A theoretical research with an experiment has been performed on the upending of a monopile where the buoyancy is utilized, but this research does not include the environmental conditions that can impact the monopile during the upending. This research also does not show the maximum amount of buoyancy that can be utilized during the installation process.

This thesis determines the maximum installation capacity that can be reached with a floating upending method, using a 1600 tonnes crane. It also analyzes the impact of the environmental conditions and presents the workability of the upending method. An analytical model has been made to calculate the motions of the monopile while it is partially submerged and hanging in the crane. The analytical model is used to validate the numerical model made in Orcaflex that was developed to consider five different concepts for the upending of the monopile. The best-chosen concept is further developed in the floating upending method.

To make the monopile water and airtight during transportation and upending, both ends of the monopile are plugged. The monopile is transported to the installation site by two tugboats that connect it to a rotating gripper which is placed on the side of the jack-up vessel. Then, the crane upends the monopile while the pile stays connected to the rotating gripper and maintains a certain amount of submergence during the upending which creates the buoyancy force. After the upending, the lower plug is removed by increasing the air pressure inside the monopile whereafter the monopile can be lowered to the seabed. This method is analyzed in the frequency domain for six upend angles whereafter the workability of the method is determined by simulations of the three most critical upend angles in the time domain. The floating upending method increases the installation capacity by 40% while maintaining a workability of 60-70% depending on the monopile diameter. It extends the lifetime of jack-up vessels and is a cost-competitive alternative when compared to mobilizing a floating heavy lift vessel instead.

Heerema Marine Contractors is assigned to execute the installation of 64 monopiles for the He Dreiht Offshore Wind Farm provided by Energie Baden-Württemberg (EnBW). The installation of the monopiles is done by means of a template structure. This template is specifically designed for this project and is installed on the seabed temporarily. The initial design of the template includes large mudmats to ensure the required stability during the full monopile installation sequence. Analysis shows that this initial template design does not perform well with regard to the template installation sequence. Additionally, when lifting the template through the wave zone, the large horizontal surface area of the mudmats yields significant hydrodynamic forces acting on the template in vertical direction. .
Therefore, the main objective of this thesis is to generate and develop alternative foundation concepts to optimize the template installation sequence. The foundation concepts considered in this research are a mudmat, suction bucket, push-in pile and helical pile foundation concept. Using the mudmat as a foundation concept yields a foundation area of 924 m^2 in order to ensure stability on the seabed, independent of the soil type. The suction bucket foundation concepts results in a horizontal foundation area of 390 m^2 for sand, or 117 m^2 for a clay seabed. The push-in pile and helical pile foundation concepts developed for a sand seabed, result in an area of 306 m^2 and 319 m^2. For a clay seabed respectively a foundation area of 75 m^2 and 88 m^2 is required.
The operability of each foundation concept is determined by means of a hydrodynamic analysis of the template when lifting it through the wave zone. The operability is the amount of time the template can be lowered through the wave zone, expressed in percentage based on 24 hours. A simplified method stated by DNV is used to determine the hydrodynamic loads acting on the foundation in heave direction. For the considered homogeneous sand and clay seabed, the mudmat foundation concept proved to have the lowest operability. The suction bucket concept scored second, and the push-in pile and helical pile foundation concept showed the best performance on operability for both types of seabed.
To evaluate which concept yields the most optimal template installation sequence, a multi-criteria analysis (MCA) is performed. This MCA also takes the installation time, construction costs and the risk of damage into account as criteria. Taking all these criteria into account, it is obtained that the mudmat foundation concept does provide the best performance when considering the template installation sequence, based on the assumptions stated throughout this thesis. Even though the mudmat foundation concept has the lowest performance of all foundation concepts when lifting the template through the wave zone, this concept overall performs better than the alternative foundation concept due to a short installation time on the seabed, low construction costs and low damage risks. The suction bucket foundation concept shows an average score on all criteria. This resulted in the second preferred option. The push-in pile and helical pile foundation concepts show a good operability, however, these concepts score low on the construction costs, installation time and damage sensitivity. As a result, these concepts show the lowest performance considering the template installation sequence.

To expand the economic operating range of offshore wind farms, less common but innovative solutions regarding support structures should be considered. To increase the popularity of gravity-based foundations (GBF) within the offshore wind industry, a better understanding of the hydrodynamics on GBFs, as well as an improved insight in the optimal design process for these structures is essential. Therefore, this thesis proposes a universal design methodology for the preliminary design of concrete GBFs for offshore wind turbines. The hydrodynamic GBF load model is based on a linear potential flow solution and was validated using Computational Fluid Dynamics (CFD) data. It provides a sufficiently accurate and highly cost-efficient approximation of the wave loads during the conceptual design phase. For the detailed design phase, CFD is still required since it represents the state-of-the-art in wave load modelling. A simplified structural model was created to perform a natural frequency analysis to ensure structural integrity. Due to the high stiffness of GBFs, it is reasonably easy to design the first natural frequency of the structure to be within the desired soft-stiff region, and thus resonance is only a minor challenge. The main concerns for GBFs rather depend on the weight and cost-efficiency of manufacturing, transportation, and installation. The initial design is based on the environmental conditions, wind turbine data, and design guidelines. The complete model is verified using design checks based on industry standards, which ensures foundation stability for the design limit states. The requirements for foundation stability are often driven by the size and weight of the structure. Therefore, sensitivity analyses were performed with various design parameters to identify the main factors influencing the design. As designs often have multiple objectives, the optimal design depends on the objective considered and thereby numerous optimal designs may exist. For a GBF, these objectives are often related to weight, costs, or even both. Generally, a combination of design objectives with varying priorities should be considered. In addition, implementing optimization algorithms is recommended as manual preliminary design optimization is labor-intensive and has difficulties considering all the various design objectives. For future research, it is recommended to test the universality of this methodology. This can be done by modelling GBFs of varying shapes over a broad range of water depths, wave conditions, and design load conditions.

Structures built in lakes and rivers or at sea are under huge forces induced by ice. These cause friction at the ice-structure interface that may pose a threat to the structural integrity of the construction. For the detailed design and realisation of structures in such regions an understanding is required of this friction process. The friction process at the ice-structure interface is driven by a phenomenon called the stick-slip phenomenon, where during stick mode the structure and the ice move simultaneously with the same velocity, and during slip mode the ice and structure interface slide over one another. This phenomenon has been observed and researched extensively, yet the corresponding static and kinetic friction coefficients reported show a wide range and the relation between the involved parameters and the friction coefficients is inconclusive. The most common theory used in describing general friction processes is Coulomb's Law of Friction. This thesis aims to determine whether and when ice-structure interaction can or cannot be described by Coulomb's Law of Friction in particular cases.

For this graduation project, first an analysis of several test set-ups was carried out to determine which set-up was best suited for experiments with stick-slip behaviour. The chosen test set-up consists of a one-dimensional rotating conveyor belt made from sandpaper on which an ice sample would be placed. This sample would be held in place by four springs attached to the back and front of the sample. This would allow the ice sample to move over the rotating belt, but remain relatively in its place. This would enable to the ice to displace in one direction, the direction of the rotating conveyor belt. This test set-up would be compact and it would be easy to switch the belt or ice sample if they had deteriorated too much. However, the use of a 1D conveyor belt prohibits construction materials such as steel or concrete to be used. Using sandpaper with similar roughness as concrete or steel would serve as a substitute to overcome this problem.

Ultimately, it proved to be impossible to realise the set-up and the focus of this thesis switched towards modelling the two-dimensional stick-slip behaviour based on data from previous research. For this a three degree of freedom (3DOF) model was created. As the forces exerted by the springs on the mass act under an angle that is depending on the position of the mass four different approaches were tested to see how this angle could best be incorporated. Furthermore, the friction force acting on the ice-structure interface was modelled using the friction coefficients found by previous research, and using two different approaches to find out whether the friction force should be modelled using the relative sliding velocity at the interface or not. It proved that the angle between the springs’ original position and it’s instantaneous position should be updated every step during slip mode, and that the friction force should be modelled using the relative sliding velocity. Both are cause for higher computational time. When validating this model against the experimental data it was found that friction coefficients were overestimated substantially for the 3DOF model. Using this the 3DOF model was updated and it was found that it now matches with the experimental data.

After analysing the output of the model, it was found that the kinetic friction coefficients are now in the range of 0.05-0.11 for the used parameters (mass 0.67 kg – 2.03 kg, slab velocity 0.15 m/s – 0.82 m/s, stiffness 20.17 N/m – 68.22 N/m). Also, the friction coefficients show a linear dependency with mass, and an slight inverse linear relationship with the slab velocity. Furthermore, with a low stiffness varying friction coefficients were found when varying the slab velocity, but when increasing the stiffness the friction coefficients converge and this variation decreases. Although further improvements can be made to create a better model, in general it is concluded that under the made assumptions, the 3DOF model describes the two-dimensional behaviour well, and shows how research into friction can be carried out while still using construction materials in an experimental set-up.

A rapid increase in global energy demand and the international objective of increasing the contribution of renewable energy to satisfy this demand have spiked the interest in offshore wind energy. Depletion of suitable locations for bottom founded turbines has led to a new chapter in offshore wind development: floating wind farms. This relatively new business brings many uncertainties and no large-scale projects have been deployed yet. The performance impact of an increase in structural motions, in comparison with bottom founded offshore wind turbines, is not yet fully investigated with respect to power production of the system. This thesis seeks to quantify this impact on performance by performing an integrated system analysis of a 15-MW semi-submersible based offshore wind turbine for operational conditions at a site north of Scotland.
All considered elements in the fully integrated simulation model are broken down individually and assembled in a finite element analysis framework via OrcaWave and OrcaFlex. OrcaWave develops system RAOs and QTFs using linearised potential-flow hydrodynamic radiation and diffraction theory in the frequency domain, whereas OrcaFlex is a finite element analysis software tool with full non-linear capability in the time domain. The non-linear flow separation induced quadratic drag in a real system, which is not included in potential flow theory, is introduced via a damping matrix by addition of Morison elements to the structure. The overall system including tower, RNA, controller and mooring is analysed by OrcaFlex in time domain. This overall integrated model was first validated and verified against a well-documented open-source reference model, provided by NREL and the University of Maine, and then adapted to the semi-submersible substructure selected for this study. A case matrix is developed representing the site environmental loading scenarios under operating conditions to simulate the dynamic interactions between substructure, RNA and controller. The impact of wind-induced yaw misalignment mean offset angles on the mean power production is examined. For wind speeds below rated, an increase in yaw misalignment angle corresponds to a decrease in mean power production. For a functioning active yaw control system, misalignment angles of the rotor will be small and consequently the mean power reductions will be as well. For above-rated wind speeds, sufficient lift can be generated to compensate for the reduced rotor swept area caused by the yaw misalignment and therefore no reduction in mean power production occurs. Load case results show that with increasing sea state severity, the motions of the system increase. The impact of this increase in motions can be observed in the generator power and rotor thrust response through their fluctuations. Performing an FFT on these parameters shows that most energy in the fluctuations is situated at the incident-wave frequencies and correspondingly, the wind and wave unidirectional load cases present the largest spectral density values for generator power and rotor thrust fluctuations. Furthermore, the results present a connection between the severity of power output fluctuations and the mean power production for above-rated wind speeds. For increasing power production fluctuations, the mean power production experiences a reduction up to 0.39\%. A hypothesis is formed that links the mean power production reduction due to its fluctuations to the response time of the controller. In further research this hypothesis is to be tested and it should be further investigated whether a more detailed integrated analysis model produces similar results. The fluctuations in power production impose challenges on the ability to connect to the grid. Connection of a single floating wind turbine to the power grid is not recommended, but when considering a group of multiple turbines it is assumed that the peaks in power production are flattened out over the total power production of the group. Capacitor banks can then be implemented to create a stable power output to the grid. Quantification of this farm effect is recommended for further research. Finally, an indication of the rotor thrust impact on bending stresses at the tower-substructure interface is presented. A more detailed analysis is to be performed to find local stress concentrations and to address potential structural adjustments to fatigue-prone members.

Due to the increasing demand for sustainable wind energy, larger wind farms with more giant turbines are required. Therefore, more is expected of the existing installation vessels for wind turbines. These changes make it increasingly difficult to find capable jack-up vessels to transport and install these new "mega turbines". Another trend in the wind industry is that the demand for wind farms is increasing outside of Europe, where protective laws often apply, such as the Jones Act. The Tetrahedron crane can lift higher without increasing the weight or size of the crane. Installing wind turbines with a Tetrahedron crane whilst supplying the wind turbine components with a feeder vessel would mitigate most problems with the current trends in the offshore wind industry. However, no research is done on performing lifts with a Tetrahedron crane from a moving vessel. This research aims to determine whether it is feasible to perform lifts from a feeder vessel subjected to heave motion with a Tetrahedron crane and analyse this feasibility for large quantities of data points in a relatively short period.
A case study is used to assess the feasibility of executing feeder lifts. The case study is used to provide constraints and parameters for two models that simulate a lifting operation. Both models describe the location of the load from the start of the lifting process till it is safely suspended far above the deck of the feeder vessel. The first model, the 1D model, is based on a free body diagram that provides a simplified representation of the Tetrahedron crane. This model can assess the feasibility of executing feeder lifts for entire data sets. The second model, the 2D model, represents the Tetrahedron crane as a system of elements and nodes. This model is used to validate the single lifting operations of the first model. Both models generate output data that provides insight into the dynamic forces that the crane and components endure during a lifting operation and if the load that is lifted experiences a re-hit or not. From this, it can be concluded whether a lifting operation is successful or not. Successful in this sense means no re-hit takes place, and the dynamic forces remain within limits set by the turbine and crane manufacturer.
While comparing the results of the two models, the 2D model approximates reality better because the dead weight of the crane construction is included where the 1D model does not. By adjusting the 1D model to account for the dead weight, results can be determined for entire data sets. A Tetrahedron crane has a higher lifting speed and a lower crane stiffness than a commonly used luffing boom crane. When both cranes have a comparable re-hit percentage, the results showed that the dynamic forces in the lift operations with the Tetrahedron crane are lower than with a luffing boom crane. In addition, analyses were made regarding the re-hit percentage for varying wave heights, wave periods and masses.
The outlined case study represents a realistic situation, and re-hit percentages within this case study are low. The re-hits that occur can be indicated, and it is also evident in which period lifting operations can be started safely. However, measures must be taken to ensure that the dynamic forces remain within the limits of the crane and turbine components. For the crane, this means that a greater lifting capacity is required. For the vulnerable turbine components, the dynamic forces must be reduced by, for example, a passive heave compensator.

Marine contractors as Heerema Marine Contractors (HMC) have made decommissioning part of their core business. Being the owner of two of the largest semi-submersible crane vessels (SSCVs), the choice for single lift jacket removal is straightforward. However, jackets in the Northern North Sea have a weight and height exceeding the crane capacity of these vessels. Moreover, not all jackets have the structural integrity to be single lifted. Piece small jacket removal can be a solution but is often overlooked by the expectance of being labour-intensive and therefore too expensive. This thesis has the objective to develop a methodology for advising a jacket removal strategy based on a decision support model while comparing piece small versus single lift decommissioning. Comparing the two removal methods is done for a single case study of the Kinsale Bravo jacket, provided by HMC.

The budgets of both removal methods, resulting from the case study, are used as input for the decision support model (DSM). Based on the jacket characteristics, the model uses constrains, rules of thumb and calculations to determine several possible outcomes. These outcomes can then be further optimised based on costs, duration and Co2 emissions. Based on either one of these optimisations a removal scenario can be established by the model.

Due to a high demand for offshore wind energy the industry is expanding to evermore challenging locations such as seismic prone areas. The corresponding seismic shocks and vibrations can be transmitted to jack‐up vessels and their cranes used for the installation of these offshore wind turbines. Since a potential collapse due to seismic activity carries large human and financial risks, the main objective of this thesis is: ”To determine the best solution to mitigate the dynamic response of cranes for offshore wind turbine installation in seismic prone areas.” Therefore, we study existing mitigation measures and find that the dynamic absorber and base isolation systems are promising solutions. However, we only find the dynamic absorber to be feasible for implementation. Subsequently, we investigate the influence of the dynamic absorber on the dynamic response of the crane by performing seismic time history analyses. We find a mitigating influence for the vast majority of simulated cases. More specifically, the dynamic response in the out‐of‐plane direction is reduced by 10 percent on average. Furthermore, the dynamic absorber is highly effective in the mitigation of harmonic vibrations such as those that are induced by Rayleigh waves that occur near the end of earthquakes. This finding suggests that a dynamic absorbers might also be suitable for mitigating wave and wind loads in the crane as these loads prescribe a rather harmonic signal. The conclusions of this thesis give the offshore wind industry a clear indication that a dynamic absorber is an effective measure to mitigate the dynamic response of a crane that is based on a jack‐up.

Areas of railways with considerable variation of track properties encountered near structures such as bridges and tunnels are referred to as transition zones. Degradation rates at these transition zones are higher compared to the remaining railways. These high degradation rates result in high maintenance expenses. In order to come up with solutions that reduce this degradation and thus also the expenses it is needed to have an understanding of the underlying mechanisms in the railway tracks. This is done by formulating a mathematical model of the railway track which can then be analyzed. Researchers have been using different models for this purpose.

The choice is often made to model the railway track an elastically supported beam. This elastic foundation represents the supporting structure of the railway tracks and its stiffness is based on static load cases. This 1D model is often used due to its relative simplicity compared to other more complicated multidimensional models. This model has been thoroughly analyzed and it has turned out to have an important constraint which is its inability to result in a critical velocity that makes sense for railway tracks. The critical velocity of a railway track is that velocity at which waves travel near the surface of the subsoil of the supporting structure. A vehicle that moves with a velocity close to the critical velocity of the railway track causes a strong amplification of the response. Another important constraint of this model is the fact that there is no possibility to directly adjust the stiffness properties of the different components of which the railway track exists individually.

It is opted in this thesis to adjust the previously mentioned 1D model by the addition of a distributed mass and an extra elastic layer. The upper elastic layer represents the pads and the lower elastic layer represents the remaining of the supporting structure. The idea behind the addition of the distributed mass is to include the activated mass of the supporting structure by the moving load. This is done in order to take the dynamical behavior of the supporting structure into account and thus obtain realistic critical velocities. The idea behind the addition of an elastic layer is that it enables to adjust the stiffness of the pads individually. By using this model one can thus modify the stiffness of the pads at transition zones and study whether it is possible to decrease the degradation which is the initial goal of all studies on transition zones.

In this thesis the adjusted model is analyzed thoroughly for different physical phenomena. Such analyses cannot be found in the literature, to the best of the authors knowledge. The system response has been investigated for a uniformly moving load of a constant magnitude. This has been done for homogeneous properties of the elastic layers and for an abrupt jump in the stiffness of the lower layer. Attention has been given to both the displacement fields of the rails and energy propagation in the system. Also a numerical model has been formulated for other load cases and stiffness properties. In order to simulate infinite system behavior non-reflective boundary conditions have been derived and applied to the numerical model. It has been found that indeed a far more realistic critical velocity can be obtained by making use of the adjusted model. It has furthermore turned out that the system response at a transition zone are very similar for both models for the same ratio of load velocity to the critical velocity. The adjusted model has been investigated extensively in this thesis and it can thus be used as a reference work for future researchers that wish to apply the model. The research that should be performed next in the authors view is to investigate the possibility of reducing the degradation at transition zones by adjusting the stiffness of the pads.

A large number of light, modular topsides in the southern North Sea is planned to be removed in the near future. Heerema Marine Contractors (HMC) is currently not the most competitive in this market as smaller contractors can often provide cheaper removal solutions with the use of smaller heavy lift vessels. HMC therefore aims to find a more economically attractive solution for the removal of these topsides. Within HMC, a concept has been proposed to remove these modular topsides using so­called lift beams. The lift beams are large beams that are installed underneath the topsides, and are connected to the topsides’ strong points. The ends of the lift beams provide lift points for the complete topsides, enabling removal in a single lift. Due to their high lifting capacity, and large available deck space, the modular topsides on the lift beams can easily be lifted by a single crane of one of the company’s semisubmersible crane vessels (SSCV). Performing the lift by a single crane allows to put the topsides on deck of the SSCV for transportation. The concept is intended to be reused for a variety of modular topsides in the southern North Sea. In this thesis, the lift beams are designed, and optimised, to withstand the governing load cases, while complying with the practical boundaries that each topsides imposes. Hereafter, it is investigated whether it is structurally feasible to lift the modular topsides using the lift beams. To determine this, structural integrity analyses are performed on a reference topsides using FEM software. Hereby the optimal lift beam setup to lift the topsides is determined. Furthermore, methods are designed for the connection between the lift beams and the topsides, and the installation of the lift beams.The efficiency of the final lift beams concept design is assessed by comparing its performance to conventional removal methods for modular topsides. The lift beams are designed as stiffened box profile beams with a weight of more than 250 mT. The topsides is found to have sufficient capacity to withstand the loads during lifting. The optimal setup to lift the topsides consists of two lift beams per jacket leg row, and cross­beams at each end­on side of the lift beams. The cross­beams add stiffness to the lift beams and thereby redistribute the lift loads in a favourable manner over the connection points. Using a pin­hole connection between the lift beams and the topsides’ legs is found to be most suitable. The lift beams are installed one­by­one using water as the counterweight component. Compared to conventional removal methods, the lift beams concept appears to be profitable after a relatively small number of topsides removals. For future research, it is recommended to investigate the possibilities of performing the lift beams concept with a dual crane lift. It is expected that the practicality of the removal process can be improved in this manner. The increased lifting capacity also allows for removal of wider range of modular topsides. After lifting off its substructure, the topsides could be transported while suspended in the cranes. Furthermore, it is recommended to extend the structural integrity analyses to the other topsides of interest. The structural limitations of the other topsides can be investigated, and the possibility to apply one­sided leg support can be considered. The weight and costs of the lift frame, as well as the installation procedure can be improved this way.