The projected rise in monopile installations for offshore wind turbines has spurred interest in innovative installation methods to install larger monopiles using floating vessels while simultaneously enhancing workability and efficiency. One such innovation is the motion-compensated gripper frame (MCPG), which stabilizes the monopile by compensating for environment induced motions affecting both the monopile and the floating vessel. The MCPG is capable of compensating for the wave-frequent motions during lowering of the monopile as well as during the hammering phase of the monopile.
This thesis aims to create a frequency domain model to assess the performance of a motion-compensated pile gripper (MCPG) system for offshore monopile installation using a floating installation vessel. Simulating in the frequency domain offers the advantage of minimum computational time, enabling a large number of scenarios to be simulated in a shorter amount of time compared to time-domain simulations. To assess the installation up until the piling phase, two dynamic models have been created, each based on a particular installation. The first steps of the installation process are divided into three phases: upending, lowering, and pre-piling. Two dynamic models were created, focusing on the second and third phases. The phase 2 model accounts for the lowering of the monopile, while the phase 3 model concerns the situation where the pile has reached its self-penetration depth.
The equations of motion were derived for each system and transformed to a state-space model to implement the controller for the motion-compensated gripper. The state-space models were converted into transfer functions in the frequency domain and subjected to first-order wave forces calculated by the diffraction analysis software WAMIT. The method of obtaining the frequency response was validated using data provided by GustoMSC while the motions of the monopile were validated by comparison to an analytical compound double pendulum model.
This research proposes a response-based approach tune the controller proportional and derivative gains. The gains were determined based on the response characteristics of the phase 3 system. These gains were subsequently applied to the phase 2 system across all drafts. The system’s performance and tuning method were assessed using stability analysis, frequency response, spectral analysis and modal analysis. The models and tuning approach were tested using 6, 12 and 15 meter diameter monopiles, which for the phase 2 system were suspended at drafts of 5, 25 and 45 meters. Simulations were conducted for various sea states, comparing open-loop (without controller) and closed-loop (with controller) scenarios. The systems are analyzed based on the following parameters: the monopile roll angle, crane cable offlead angle, gripper force, gripper stroke and gripper stroke rate.
Results indicated that the MCPG effectively eliminated monopile roll motion in both phases. In the closed-loop phase 3 system, gripper stroke and stroke rate were dominated by vessel sway motion, while phase 2 did not show much reduction in gripper stroke and stroke rate. The operability analysis identified the critical operability limits: in the open-loop phase 3 system, the monopile roll angle was the primary limit, followed by gripper stroke. In the closed-loop system, gripper stroke rate and force became the primary limits. For phase 2, gripper stroke was the main limit in both open and closed-loop systems, with stroke rate becoming more significant in the closed-loop system. The MCPG shifted the limiting criteria from monopile roll angle to gripper stroke rate in phase 3. The combined operability of the phase 3 system including the MCPG was found to be 25%.
Finally, the research recommends the addition of time-domain simulations to capture non-linear effects. It is also recommended to extend the models to three dimensions in order to simulate non-beam or bow waves. The phase 3 model can also substantially benefit from the addition of soil loads.

Offshore wind energy has become a crucial element of the global energy transition, with the North Sea being a major hub for offshore wind farms. As first-generation farms approach the end of their operational life, decommissioning offshore wind export cables has emerged as a significant technical challenge. This thesis focuses on the feasibility and limitations of using the cable pullout method for decommissioning offshore wind export cables, particularly examining the influence of burial depth and soil conditions.

This research is structured in two main phases. The first phase involves a comprehensive literature review to identify critical knowledge gaps and establish an overview and understanding of existing decommissioning practices. The second phase employs simulations using OrcaFlex software to model and analyze various cable pullout scenarios. These simulations focus on determining the limits and constraints imposed by burial depth and soil relative density for sandy soils commonly found in the North Sea.

Key findings from the literature study underscore the complexities of decommissioning, which encompass legal, environmental, economic, and technical considerations. One of the main conclusions is the importance of adopting a 'design for decommissioning' approach during the initial cable installation. This involves designing cables and selecting burial depths that facilitate easier future removal, thus promoting sustainability and cost-effectiveness. Additionally, this approach can help mitigate potential environmental impacts and regulatory challenges associated with cable removal.

Soil modelling plays a crucial role in understanding how burial depth and soil conditions influence resistance forces during cable pullout. Factors such as shear strength and burial depth are analyzed to determine the forces that oppose cable recovery. The model includes scenarios for fully drained, fully undrained, and partially drained uplift resistance, implemented in a Python script to simulate real-time resistance during pullout operations. To address buried cables, an external soil model is integrated into OrcaFlex, allowing dynamic simulations of soil resistance during cable pullout.

The simulation results with a 525 kV HVDC export cable reveal how soil resistance significantly increases with greater burial depth and higher soil density. These findings highlight the critical importance of burial and soil conditions in planning decommissioning operations and suggest that additional deburial techniques may be necessary when cable pullout is not feasible.

This thesis provides valuable insights and recommendations for future research and practical applications, aiming to support the offshore wind industry's evolving needs and enhance the sustainability of decommissioning processes. These findings are crucial as the industry anticipates a substantial increase in decommissioning activities, with offshore wind capacity expected to continue to grow.

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.

Floating wind turbines despite the the potential to harness energy from deep offshore areas where higher average wind speeds face challenges in terms of competitiveness. One approach to raising the competitiveness of a wind farm is to mitigate efficiency losses resulting from the wake effect. This report focuses on the combination of three notable wake effect mitigation strategies: layout optimization, yaw-based wake redirection, and turbine repositioning.

A preliminary analysis of the combined effect of wind turbine repositioning and yaw-based wake redirection on power performance for the case of two turbines only is performed. Above rated wind speeds, upstream turbine yawing reduces downstream turbine movement, with reductions of around 1 to 4 rotor diameters longitudinally and 0.2 to 0.5 times the rotor diameter laterally, keeping the same level of power efficiency.

Nextly, an optimization problem that integrate layout optimization with yaw-based wake steering and turbine repositioning for power maximization across an extended wind farm is formulated. The optimization frame followed a sequential approach. The results on a case study confirmed that the effect of adding yaw-based wake redirection to turbine repositioning remains significant for multiple turbines, with several percent-point efficiency improvements for small movable ranges. For larger ranges, the contribution of yaw control diminishes rapidly to one percent-point or less. Yaw control enables movable range reductions of 10% to 50%, preserving wind farm efficiency. Yet, reductions are more pronounced in smaller, less effective movable ranges. Below rated conditions, the effectiveness of yaw control diminishes swiftly.

Furthermore, the study delves into the implications of integrating position mooring for turbine repositioning and yaw-based wake mitigation strategies on mooring system performance. This examination employs a proposed methodology aimed at minimizing the position error across most points within the movable range Both the tension of the mooring lines and the static stiffness of the floater showed to be sensitive to the position of floater, the direction of the wind load and the yaw of the wind turbine, with percentual changes ranging from 0.5% up to 50%. It results that the orientation of the mooring lines with correspondence of the prevailing wind direction, as well as that restrictive constraints on the tension and stiffness may be taken should be taken into account when designing a mooring system for turbine repositioning.

Overall, combining , yaw-based wake redirection, and turbine repositioning allows for greater wind farm AEP, with gains contingent on turbine movable range and upcoming wind speeds. Designing position mooring systems must factor in the influence of yaw-based wake redirection and turbine repositioning on mooring system tension and stiffness. More advanced analyses, including dynamic assessments, are essential for comprehending the mooring lines system's response to position mooring for turbine repositioning, and yaw-based wake redirection.

In recent years, the interest in deep-water wind energy projects has drastically increased, driven by the considerable wind resource in deep-water locations. In such locations, floating wind turbines are more economically attractive than bottom-fixed wind turbines. Nevertheless, the floating wind industry faces numerous challenges. One major challenge is the high investment costs involved in the manufacturing of the substructure. Additionally, it is important to maintain the wind turbines’ power output within the required standards for grid connection. Therefore, the design and optimization of substructures for floating wind turbines has to account for both capital costs and power quality. To investigate the existing research on the optimization of floating wind turbines, a literature review was conducted. Evidently, there is limited research investigating both cost and power quality optimization. Furthermore, the majority of the optimization techniques presented are accompanied by a computationally expensive substructure analysis. Moreover, in the early stage of the substructure design, a variety of different alternative scenarios are tested, requiring an extensive number of optimizations. Thus, the computational resources required by the optimization become a major concern. One promising approach to address this issue is substituting a computationally expensive analysis with a surrogate model. This solution could significantly increase the computational efficiency of the optimization process.

This thesis investigates the trade-off between substructure capital cost and power quality optimization. To achieve that, a multi-objective optimization workflow is developed for the case of a semi-submersible substructure. In parallel with this task, the optimization’s analysis blocks requiring most of the computational resources are identified and substituted with a surrogate model. Two different surrogate model implementation approaches have been examined. The first approach substitutes the optimization’s substructure analysis tool (NREL’s RAFT) entirely. This approach is called the ”End-to-End” approach. The second approach substitutes a specific analysis block of RAFT that computes the floater’s hydrodynamic properties. This analysis block requires most of RAFT’s computational resources. This approach is defined as the ”PyHAMS” approach.

The thesis outcome shows that improving the quality of power output comes with the drawback of increasing the substructure’s cost. The Pareto front between these two competing criteria is dominated by designs of long pontoons and reduced chain thickness. This is the most economical approach towards reduced cost and improved power quality. These Pareto designs are mainly driven by the boundaries of the design variables. Additionally, the Pareto designs are affected by constraints on the maximum pitch, maximum surge, and the existence of vertical loads on the mooring line’s anchor. Regarding the surrogate model-based optimizations, there is a dramatic reduction in the computational resources required to conduct the optimization. Among the two surrogate model approaches, the ”PyHAMS” approach achieves a better accuracy at mimicking the original optimization workflow than the ”End-to-End”. Instead, the ”End-to-End” approach is the most computationally efficient one to conduct the optimization. Nevertheless, when factoring in the computational resources to produce the training dataset, the ”PyHAMS” approach is the most computationally efficient one. This is the outcome of utilizing a reduced training dataset compared to one utilized by the ”End-to- End” approach.

Recently, there has been an increase in interest in floating wind turbines that are located offshore. These turbines allow for the harvesting of the power of the wind far offshore, where wind speeds are often higher.

Compared to their fixed counterparts, floating wind turbines allow for a certain mobility after the installation. This allows wind farm developers to consider layouts that change throughout the wind farm’s operational phase. The change in layout can increase the energy yield of the wind farm, which may reduce the cost of floating wind energy.

This Master’s Thesis presents a new method for wind farm layout optimization with movable floating offshore wind turbines. The objective function that is maximized is the annual energy production (AEP). The proposed method first finds the optimal installation locations of the turbines, then searches for the optimal wind farm layout for each wind direction while considering the movable range of the turbines. Different movable range sizes are considered in the analysis. These sizes range from small (there is almost no mobility allowed) to large (the turbine is allowed to move anywhere in the wind farm). The results show that the steepest gains are achieved for a movable range size of up to two rotor diameters (i.e., the turbine is allowed to move two rotor diameters in each direction, evaluated from the installation position). Above this range, a large additional movement is required for a minor increase in AEP. Moreover, for this movable range size, repositioning turbines is so effective that their installation positions almost do not affect the AEP.

In addition to the previous method, this Master’s Thesis also presents a novel method to assess the movable range of floating offshore wind turbines. In this method, it is assumed that the mobility is achieved by adjusting the mooring line lengths through a winch system on board of the floater. The proposed method optimizes the line lengths such that an equilibrium is obtained in the relocated position. Various locations are selected for the analysis that cover most of the mooring system footprint on the seabed. The results show that the assumed movable range shape is not the same as the actual movable range shape.
For a 15MW floating offshore reference turbine, the movable range size with the steepest gains in terms of AEP (two rotor diameters) and the actual movable range are compared. The results show that the actual shape covers large parts of the circular shape.

In conclusion, large gains are expected in terms of AEP for movable floating offshore wind turbines. This brings us one step closer to reducing the cost of floating wind energy, which in turn increases its competitiveness with other energy resources.

Simulations of a spar-type floating wind turbine supporting the DTU 10 MW reference turbine under fitted atmospheric conditions are performed. Atmospheric conditions are modelled using the turbulent models recommended by the IEC: the Mann model and the IEC Kaimal and exponential coherence model. Model parameters are fitted to measurements at the FINO-1 platform to generate turbulent boxes that are as close as possible to measurements. The simulations are carried out using OpenFAST for three mean wind speeds: 7.5, 12 and 16 m/s and the power law wind profile is used with fitted shear exponent. The motions, wind, loads spectra are quantified using the WAFO toolbox and the loads are assessed with short-term damage-equivalent loads using MLife. Depending on the wind model, global motions differ by up to 52%. Generally, Kaimal resulted in higher surge and pitch motions, and the opposite was found for the yaw motions. The influence of atmospheric stability on global motions was found to be important as well, unstable conditions giving the largest motions and stable conditions the lowest. It was observed that the wind model influenced the fairlead tension and tower top loads. Mann resulted in up to 35% higher loads on the tower top yaw moment and Kaimal resulted in up to 30% higher fairlead tension loads. Tower base fore-aft bending moment and blade root out-of-plane moment were not very sensitive to the choice of wind model. Atmospheric stability had an influence on all loads by up to 30%. Unstable conditions led to the most damage while stable conditions generally led to the least. Whether the choice of the wind model or the change in atmospheric
stability has more influence on the global motions of the platform and the loads depended on the wind speed, the degree of freedom or the load considered. It was found that wind models should always consider atmospheric conditions, not only neutral. Finally, impact of Mann turbulence model parameter variations on the wind, global motions and loads was investigated. It was found that turbulence intensity is influenced by the three parameters and has a bigger impact than the differences in coherence.