The offshore wind energy sector requires efficient and environmentally conscious installation of founda-tion piles. Impact-hammering generates significant noise pollution, requires mitigation measures, andcan cause pile fatigue, while vibro-hammers face limitations in achieving target depths in dense soils. This thesis investigates the technical feasibility of applying the GBM Works Vibrojet® – a new method combining vibro-hammering with water jetting near the end of the pile to fluidize internal soil and reduce friction for installing small-diameter piles (0.5-4 meters) used in jacket structures and floating windturbine foundations.

The study employs literature reviews, adapts existing soil resistance (SRD) and bearing capacity (API) models, and introduces a ”Vibrojet® potential reduction ratio” to quantify the resistance of the soil inside the pile, which can potentially be reduced to zero. It analyses various pile dimensions and soil conditions representative of the North Sea. Findings suggest the Vibrojet® can likely be scaled down for small-diameter piles, although submerged installation and the soil plugging effect, for piles with a diameter smaller than 1.5 meters, need further study. The potential reduction in the resistance of the soil in the pile is comparable to monopiles for unplugged small-diameter piles but significantly higher for plugged piles. Consequently, the impact of the Vibrojet® on the axial bearing capacity in comparison to impact-hammering is similar to that of monopiles (20-25% reduction) in the analysed non-uniform soil conditions for unplugged piles, but can be substantially higher for plugged piles, particularly shorter ones.

The study concludes that installation by the Vibrojet® has technical feasibility for small-diameter piles, offering significant soil resistance reduction potential, especially in plugged conditions. While the bearing capacity reduction for unplugged piles is manageable, the impact on plugged piles requires careful design consideration, particularly given the axial loading demands on jacket and mooring piles. Recommendations include using more advanced CPT-based models, conducting physical tests, and further investigating the precise effects of fluidization on soil properties and bearing capacity.

The Nahuel Huapi National Park, in the Lake District of Northern Patagonia, Argentina, is well known for its tourism industry all year round. After COVID-19, the area saw a significant increase in the number of tourists traveling to the area. This means that the lake located in the heart of the district, Lago Nahuel Huapi, is being used more and more to explore the environmental richness of the area by boat. Now, the capacity of mooring spaces is no longer sufficient in the region, resulting in the construction of illegal private docks along the shore. To reduce this impact on the environment the authorities granted in 2024 a concession to develop one of the last not yet commercialized marina’s in the region: the marina in Bahía López.

This report provides a consult for the concessionaire of this development. The process begins with a research phase, consisting of an area study, and the mapping of environmental and hydrodynamic constraints. Subsequently, stakeholders are categorized, as the development of a marina in a national park entails complex regulations from multiple organizations. The outcomes of the research phase are translated into specific functional requirements for the marina. These functional requirements are the basis for the next phase, the design phase. This phase begins with the formulation of a design vision statement, formulating the project response to local conditions. Based on this, three different conceptual designs with various technical solutions are developed. Through a multi-criteria analysis, the concepts are tested on their robustness in order to chose a final concept. This concept is then elaborated into a preliminary design. Presenting an overview of the marina’s facilities, including structural designs, operational needs, and capital costs. Finally, suggestions for future development
are provided, outlining the next steps to advance the marina to a next phase.

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.

This thesis presents the design and evaluation of a reusable elevation system for topsides float-over installations in the offshore industry, aimed at addressing inefficiencies and environmental impacts associated with the current Deck Support Frame (DSF) methodology. Through a structured design approach, the system is developed in four distinct phases: recognition of need, problem definition, concept design, and basic design.

The need for a reusable elevation system arises from the limitations of the DSF, which is a single-use structure fabricated for each project and disposed of afterward. The proposed system offers multiple advantages, including cost savings through the elimination of single-use components, reduced environmental impact by minimizing material waste, adaptability to varying topsides dimensions and weights, and simplified installation processes by integrating functions. Market research into offshore wind trends confirms the relevance and potential demand for a modular, reusable elevation system.

The concept design phase involved generating a wide range of potential solutions and narrowing them down to the most feasible concept. During brainstorm sessions, 126 ideas were generated by 23 participants, resulting in 41 distinct working principles grouped into 21 concept categories. These were refined using the COCD-box framework, which balances practicality and innovation. The skidding-on-a-slope concept emerged as the most viable solution due to its low driving force requirements, direct load transfer, alignment with established offshore methodologies, and streamlined installation sequence. By extending horizontal skidding into a sloped configuration, the concept eliminates the need for the DSF and jacking system while optimizing the overall installation process.

In the basic design phase, the skidding-on-a-slope concept was further refined into a functional system layout. This phase included two sub-phases: modeling the operation and developing the structural design. A shallow slope angle was chosen to minimize driving forces, while a wedge facilitated topsides elevation. Strength analyses revealed insufficient bending capacity under hogging configurations, but recalculations confirmed that bending moments remain within safe limits. Stability evaluations showed that the concept improved barge stability by lowering the center of gravity. The structural design effectively accommodates normal loads but requires further development to manage lateral forces during skidding and mating operations. Modularity was highlighted as a key factor for transport, storage, and reusability across projects.

The thesis concludes that the skidding-on-a-slope system offers clear advantages over the DSF methodology in terms of environmental impact and long-term cost efficiency, provided the system is used more than once. The more the system is reused, the greater the reduction in costs and carbon footprint, with a projected 70% reduction after five installations. This reduction stems from the reusable steel components in the skidding-on-a-slope system compared to the single-use DSF. The adaptability of the system to varying topsides configurations and its streamlined installation sequence further enhance its practicality and relevance. The reduced carbon footprint also strongly aligns with Heerema Marine Contractors’ net-zero goals. Future research and optimization efforts should focus on operational and structural refinements to fully realize the potential of this innovative elevation system.

This report presents a design study focused on a floating offshore barrier for catching sargassum to protect the coastline of the Yucatan region in Mexico. The research was conducted as part of the multidisciplinary course offered by the Civil Engineering faculty. The study involved five students from diverse disciplines: two hydraulic offshore students, one hydraulic engineering student, one structural engineering student, and one chemical engineering student.

An MDP project performed in the scope of the Delta Futures Lab in collaboration with the Dragon Institute. The report delves into the enviromental threats the Mekong Delta is facing and provides a design framework to deal with these problems. Furthermore, a case study is chosen to display how the framework can be adopted in a practical setting, envisioning what a future looks like where the Mekong Delta co-exists with water.

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.