Offshore wind energy is often considered the Formula One of renewable energies. It is a proven technology that provides millions of people with clean and affordable power. However, the available locations for bottom-fixed turbines are being depleted quickly, while the potential of wind energy in deeper waters is immense. Installing power cables for these large floating electricity generators is more complex than for fixed wind turbines. In November 2025, DEME will install the first dynamic cable for an offshore floating wind farm. During the preparation phase, DEME encountered several challenges related to the attachment of ancillaries and low operability. This thesis focuses on the limitations associated with dynamic cable installation.

This research is structured into three main phases. The first phase consists of an extensive literature study conducted to gain insight into the equipment required for the installation process and the ancillaries that must be attached to the cable to keep it properly in place. The second phase involves developing a new vessel layout to enhance the workability of this installation setup. Once a new configuration is established, the third phase consists of building a model of the setup using the time-domain software OrcaFlex. This model was then used to simulate various environmental scenarios, and the results were analysed to compare different methods based on operability.

Key findings from the literature study highlight the complexities of the cable installation process. These complexities include operational weather windows, onboard logistics, ancillary handling, installation speed, and the limitations of the cable, ancillaries, and equipment. One of the main considerations is the importance of risk-mitigating measures to ensure the safe deployment of the cable and its ancillaries.

Multiple concepts were explored and developed with the goal of improving onboard processes and the overall operability of the system. A multi-criteria analysis resulted in the selection of a stinger frame as an alternative to the current cable installation setup.

Cable modelling was performed to obtain the required insights into cable behaviour. The output from the model identifies the limiting factors during installation, such as curvature, sidewall pressure, and cable tension. The simulations include scenarios with various combinations of environmental conditions, such as wave height, wave direction, and wave period.

The simulations showed that a rigid stinger does not improve operability. Due to increased motions at the stinger tip, tensions rise, which further limits operations. However, the risk of minimum bend radius (MBR) breach for the buoyancy modules (BMUs) in the splash zone was reduced significantly by decreasing the time they spend in high-risk positions. Sidewall pressure (SWP) did not prove to be a critical parameter. These findings suggest that a more advanced stinger design could help increase operability.

This thesis provides valuable insights and recommendations for future research, supporting the offshore wind industry’s transition towards floating solutions to access deeper waters with higher energy yields. This development will further strengthen the global supply of sustainable renewable energy. ... Read more

The need for more sustainable and cost-effective energy generation has increased the interest in offshore wind development in deep-water regions. Floating wind turbines offer a promising solution at water depths where bottom-fixed structures are not feasible. Currently, the implementation of floating wind turbines remains limited due to high costs and complex system dynamics. There is a need for efficient, automated optimisation tools that can efficiently identify cost-effective design spaces early in the development process. While optimisation studies have been conducted for semi-submersible and spar-type platforms, limited optimisation related research exists on tension leg platforms (TLPs). This study addresses that gap by developing an optimisation framework for TLPs supporting 15MW wind turbines, allowing for efficient, accurate and cost-effective design iterations while considering platform specific dynamics,.

To achieve this, a frequency-domain model was developed by extending the open source RAFT software. The extensions incorporate features particularly relevant to TLP modelling, including tower flexibility, a sum-frequency force approximation and an analytical tension leg mooring module. The resulting frequency-domain model achieves an error margin within $\pm16\%$ of non-linear time-domain simulations for TLPs with (near) vertical tendons, while reducing computational time by 98.5\%. This makes the model suitable for dynamic analysis within optimisation studies, while enabling TLP-specific response characteristics to be captured at a fraction of the computational cost.

The developed model was integrated into a multi-step genetic algorithm-based optimisation framework to explore the design space, with the objective to reduce the levelised cost of energy (LCOE). The framework, built around a single-column TLP design with four pontoons with tendons at their ends, took into account six design variables to describe the essential system properties. The resulting six dimensional complex design space considers the main column diameter and draft, pontoon diameter and length, tendon angle and tendon pretension. To enable complete design performance evaluations and potentially provide early-stage insights that support certification preparation, each concept was assessed against an extensive set of load cases according to standards, covering both ultimate and fatigue loads in operational and extreme conditions.

The results of the optimisation study identified draft, tendon pretension, tendon angle and pontoon length as the most influential parameters for dynamic performance due to their strong influence on platform stability and mooring stiffness. The column and pontoon diameter had the most significant influence on platform mass and therefore platform cost, while pretension dominated mooring system cost. Despite the identification of these most influential variables, the highly coupled nature of TLPs requires to take all identified design variables into account.

A detailed cost model was implemented, enabling comparison of design concepts within the study, and comparison with floating wind platform designs in other research. To achieve this, the model combines variable platform and mooring costs with fixed lifecycle costs. Multiple optimisation runs revealed several distinct, cost-efficient design spaces, with convergence toward lower LCOE values with increasing iterations of the optimisation. The most cost-effective designs achieved LCOE values around 65 €/MWh, making them competitive with other floating wind concepts across different studies.

Altogether, this work provides an efficient optimisation framework for TLPs in the context of floating offshore wind. It enables accurate and cost-effective design iterations that account for TLP-specific dynamics while significantly reducing computational time. By considering a wide range of load cases and maintaining a balance between speed, adaptability and physical accuracy within a limited degree of uncertainty, the framework offers a holistic approach. This makes it well suited to support early-stage design decisions and concept selection for future deep-sea wind farms using TLPs. ... Read more

This research investigates the effects of mechanical loads on bottom-founded and floating offshore wind turbines (OWTs), specifically comparing different floater configurations (TLP and semi-submersible) with bottom-founded turbines (monopile). The study identifies three critical intersections for a fair comparison: the base of the blades, the main shaft, and the yaw system. These intersections allow for the prediction of stresses on components when specific material properties are known, which is often not the case in many turbines.

The modeling program Orcaflex is used to describe the motions and loading of TLP and semi-submersible floating offshore wind turbines (FOWTs). Bluewater Energy Services is currently designing a TLP platform for a wind turbine, and this design, along with a semi-submersible FOWT model, is compared with an IEA 15 MW bottom-fixed turbine. External loads such as waves and wind, generated from North Sea data, are considered. Additionally, the effects of design parameters like weight, waterline area, center of mass, and wind turbine generator (WTG) control settings are taken into account.

The study reveals that the semi-submersible platform is more susceptible to environmental loads, leading to some significant translational and rotational motions. Its stability relies on a large water surface area and a catenary mooring system, resulting in low system stiffness. In contrast, the bottom-fixed and TLP turbines exhibit lower motion fluctuations due to their higher system stiffness. The TLP experiences higher nacelle accelerations compared to the semi-submersible, except for heave acceleration, due to resonance with wave frequencies. Mechanical loadings are significantly influenced by wind speed and the turbine's controller. Before reaching the rated wind speed, mechanical loads increase with environmental loads, while post-rated wind speed, the loads stabilize or even decrease due to the controller's intervention.

Furthermore, the study identifies the driving factors for the lifetime of pitch, yaw, and main bearings. The pitch bearing's equivalent load is predominantly influenced by wind-induced moments, while the yaw bearing's load is largely governed by axial loads from the RNA's weight. The main upwind bearing's load is primarily affected by radial loads, with axial loads becoming more significant as wind loads increase.

The overall conclusion indicates that while platform motions influence system dynamics, their direct effect on mechanical loads is less significant compared to other factors such as wind loads and controller actions. The pitch controller plays a crucial role in managing mechanical loads, particularly for pitch bearings. Nevertheless, the relatively large mean angle of the semi-submersible platform impacts bearing lifetimes. The system's angle, combined with the weight of components, especially for the yaw bearing, is a critical factor in determining their lifetime. These findings are supported by existing literature, confirming the complex interplay between environmental conditions, system motions, and mechanical loadings in offshore wind turbines. ... Read more

The growing demand for renewable energy has positioned Floating Offshore Wind Turbines (FOWTs) as a promising solution for harnessing wind energy in deeper waters. Unlike fixed foundation turbines, which are limited to depths of around 50 metres, FOWTs can operate in greater depths. However, this relatively new industry faces numerous challenges, with high costs currently limiting its large-scale deployment. While chains are widely used in mooring systems for the Oil & Gas industry, their required size and supply constraints for floating wind applications have made fibre ropes an attractive alternative. This thesis identifies several factors influencing the use of top chain in a hybrid mooring system, with a primary focus on determining the minimum required length for hook-up operations of semi-submersible FOWTs.

The relationship between the connection height of the top chain to the FOWT and the required top chain length, as well as the relationship between sea state variations and top chain length, were investigated through numerical simulations. Complications related to the depth below the sea surface, fibre elongation behaviour, and the overall performance of hybrid mooring systems were reviewed through existing literature. The dynamic behaviour of the top chain during the hook-up operation was analysed using OrcaFlex, with simulations performed in both frequency and time domains.

Dynamic analyses primarily focused on a dry chain link connection method, revealing that top chain lengths between 61 and 76 metres are required during hook-up operations. These lengths depend on wave conditions, connection height, and the installation vessel. The results showed that the required top chain length increases with wave height and period, while the length itself minimally influences its dynamic behaviour. For the lower segment of the mooring line, at least 10 metres of chain are required to prevent contact between the line and the rudders and thrusters of the installation vessel.

The study concludes that the top chain experiences minimal dynamic tensions relative to the static tension during hook-up, provided it includes sufficient sag and submersion. A bottom connection height offers the most benefits but necessitates a longer top chain during hook-up compared to a middle connection height. While fibre materials show promise as alternatives to chains, further investigation is required for their use in shallow waters. The dry chain link method is considered financially advantageous but it is expected to require the longest top chain length for hook-up operations compared to other methods. An in-depth analysis of applying pre-stretch to a 3-line mooring system is recommended to verify this assumption. These findings contribute to enhancing the economic feasibility of floating wind energy solutions, particularly for large-scale deployment.
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This research addresses the challenges of installing floating wind turbines on semi-submersible platforms, with a focus on managing relative motions during offshore integration. Traditional methods, which rely on large onshore marshaling yards, are constrained by the limited space available at ports to meet the increasing demands of the offshore wind industry.

In response, the study proposes a shift to floating-to-floating installation, where turbine integration takes place on a moored floater directly at the wind farm. A major challenge in this approach is controlling the relative motions between the two floating bodies. To address this, a multibody model is developed in OrcaFlex, simulating the interaction between a heavy lift vessel and the VolturnUS-S platform. This model highlights the complexity of relative displacements, providing the foundation for introducing concepts aimed at minimizing these horizontal motions.

The chosen concept features a donut-shaped external platform equipped with winches for horizontal motion compensation. In the proposed 10-step installation process, the turbine tower is suspended from a crane while a gripper restricts pendulum motions, and an Active Heave Compensation system manages the relative heave motions. The external platform, suspended below the tower as it is attached to the gripper, is first lowered and secured onto the floater. Once in place, the tower is lowered, with the winches providing the necessary stiffness to control relative horizontal displacements during this phase.

The winch wires are modeled as springs with equal stiffness applied in both horizontal directions. Time-domain simulations in OrcaFlex, validated by an analytical model in MATLAB, reveal that a stiffness of 8887 kN/m is required to effectively limit horizontal displacements to 0.2 m for significant wave heights up to 2.5 m. This design choice ensures 92% workability for this installation step, considering a maximum crane angle of 2 deg and a maximum horizontal relative displacement of 0.2 m. Ensuring functionality under the simulated sea states reveals a design requirement of 1823 kN for the compensation system in the horizontal direction for Hs = 2.5 m and a Tp ranging from 6 to 11 s. While this force exceeds the capacity of a simple winch, it remains within a feasible range for a dedicated compensation system.
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This thesis is a about a novel float-over concept for the installation of offshore substations in the Dutch North Sea. This new technology is a topside jacking system integrated into the H-851 float-over barge which should be able to jack the topside to a higher elevation before a float-over installation.

First, a high-level concept design of the novel float-over technology is presented. Thereafter, the stability for the jacking stage is investigated via hand calculations. Analytical models are developed to model the jacking system during select stages of the float-over and jacking of the topside. These models are used to verify the assumed design loads on the jacking system and other components required to install the offshore substation. As of now, the design loads are based on guideline for a different type of float over making the design loads a guestimate.

The float-over barge H-851 proved to be very stable without ballasting during the jacking of the topside. Initial ballast errors did not affect the barges stability significantly, the effect was only a 30% increase in heeling angle for the worst case. The effect for wind loading was more profound as the heeling angle increased with 100%. Due to the large initial stability of the barge, the heeling angle after jacking is still of no concern. For the investigated cases, however, care should be taken to ballasting the barge before jacking as initial errors become larger after jacking.

It was found that the driving systems for the jackings system can be designed in the global horizontal direction using less than 5% of the topside weight, given the sea states in this thesis. A lower stiffness jacking system resulted in lower horizontal loads on the driving system. The jacking system showed a difference in local horizontal loads over the jacks from stern to bow as great as 150%. This is due to the position of the topside with respect to the centre of the barge and in combination with the roll, pitch and heave behaviour of the barge and topside.

The mating analysis had multiple findings. It was found that the impact loads on the LMU’s were significantly lower by using the jacking system compared to the DSF for North Sea waves. However, the effect of the horizontal load (compression force) was less significant which resulted in lower or equal loads on the LMU’s. The horizontal loads at the interface between topside and barge were larger for a jacking system compared to a DSF.

Increasing the stiffness of the jacking system resulted in an increase of loads on the jacks and LMU’s. This was found due to the lower eigen period in the dominant motion for stiffer jacks. An interesting finding is a torsion moment with its centre at the interface between barge and topsides. This point was found to be dependent on the stiffness of the interface.
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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.
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The increasing demand for renewable energy sources has brought about the need for innovative solutions to harness energy from the wind. One such solution is floating offshore wind turbines (FOWT), which offer several advantages over traditional onshore wind turbines and bottom-fixed offshore wind turbines. FOWTs present a challenge concerning overall cost, using fewer mooring lines than seen previously in the offshore industry. Statistically, these mooring line failures are expected to occur annually in large turbine fields and could result in untethered turbines causing extensive financial and reputational damage. It is, therefore, critical to understand whether a single mooring line failure could endanger the entire system, creating a risk that must be reduced to a level that is as low as reasonably practicable (ALARP). The Tetraspar demo FOWT off the coast of Norway is used as a model to investigate the influence of mooring line failure on the mooring system. This thesis investigates the potential risks associated with the three-leg mooring system of a FOWT following mooring line failure. The research employs a simulation-based methodology coupled with insights from previous studies and a fault tree analysis (FTA) to estimate the increase in failure probability of a complete mooring system in case of a single mooring line failure relative to an intact system. Specific assumptions underpin this investigation, including a six-month repair time bridging winter weather till the repair campaign and categorising two mooring line failures in a three-leg mooring system as a complete system failure. This thesis's research is divided into two categories: new failure modes specific to Tetraspar and altered failure modes, which are fatigue-related modes already included in the FTA, adopted from previous studies. Findings highlight the risk of Tetraspar capsizing after a mooring line failure and potential issues with slack line events and fibre sections of the mooring line touching the seafloor. Low-frequency second-order drift significantly increases fatigue in the mooring lines and fairleads, evidenced by an over 1200\% fatigue increase in some instances. A FTA consolidates these findings, showing a total failure probability increase in broken line state of between 32\% to 137\% based on the assumptions made. The study reveals a notable increase in fatigue following a mooring line failure. However, this state will persist for only six months within the turbine's 20-year lifespan, accounting for 1/40th of its design life. With the implementation of a robust safety factor, these fatigue issues can be effectively mitigated. It is advocated that 'design for failure' is incorporated into a three-leg mooring system design to ensure the risks associated with TetraSpar are ALARP. Five recommendations are suggested for the design phase to ensure the TetraSpar and FOWTs achieve ALARP risk levels considering potential mooring line failure, offering solutions that do not necessitate on-site visits, and ideally creating a system that can endure for six months without intervention, allowing for repairs during the summer campaign for more cost-effective and safer operations. ... Read more

This paper investigates the technical, life cycle, and economic feasibility of a 30 MW upscaled downwind turbine, comparing it to a 15 MW X1 Wind PivotBuoy downwind turbine and a benchmark 15 MW IEA Umaine VolturnUS-S upwind turbine in the 450 MW Sud de la Bretagne I wind farm site. The study is significant due to the rising energy demand, the potential for decreasing the levelized cost of energy with increased turbine size, and the optimized use of space. The size limit of current upwind turbine designs could be addressed using a downwind turbine solution.

The research is conducted by modelling the global dynamic response of the structure using OpenFAST and computing the natural frequencies and stresses using a finite element model. A lifecycle analysis is performed to identify potential pitfalls and bottlenecks by analysing the individual lifecycle phases. The economic feasibility is assessed by simulating the annual energy production using TOPFARM and utilizing structural analysis and lifecycle assessment to quantify capital, operational, and abandonment expenditures. Based on the annual energy production and the performance indicators the levelized cost of energy is calculated.

The findings indicate that while the global stability is within boundaries, the stress in members is too high with a simple scale-up of the proposed design. Bottlenecks are found in lifting operations and supply chain readiness. The levelized cost of energy and capital expenditure increased due to substructure self-weight, rendering the proposed 30 MW scale-up currently unfeasible when compared to the other two wind farms.
These findings are important as they demonstrate that the 15 MW X1 Wind PivotBuoy is not scalable without design changes. The levelized cost of energy does not decrease with an increased floater solution. The 15 MW X1 Wind PivotBuoy downwind turbine seems more economically viable, making it a more interesting option for future development. ... Read more