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.

The Ampelmann system occasionally starts vibrating unexpectedly, especially while the system is placed on a pedestal. These vibrations are believed to be caused by the eigenfrequencies of the system and/or amplification caused by the motion control algorithm. In this research an investigation in this phenomenon was done. This investigation was done via an analysis of the eigenfrequencies of the active controlled hexapod. A finite element method model was created to determine the eigenfrequencies of the system. This model was made using MATLAB and the toolbox StaBIL 2.0, created by the university of Leuven. All the elements of the system are modeled as beams except the hydraulic actuators. The properties of the elements which represent the hydraulic actuators are calculated separately using a modelling study on stiffness characteristics of hydraulic cylinder under multi-factors.
Possible causes for the unexpected vibrations have been investigated via measurements performed on Ampelmann systems. Data which was readily available is analyzed. Based on this data three possible causes have been determined. These are: the influence of the pedestal, residual motions due to limitations of the Ampelmann system and vibrations in the bottom frame due to compensating for gangway motions. To investigate the influence of the pedestal and the vibrations in the bottom frame two experiments have been performed. The residual motions have been investigated via calculations based on data already available. The amplitude of the response in the results from the first experiment performed to investigate the vibrations in the bottom frame due to gangway motions is negligible over the entire test period. The data from the calculations done to investigate the effect of the residual motions show no amplification. The results of the experiment and the calculations lead to the conclusion that these do not cause unwanted behavior. The second experiment to determine the influence of the pedestal shows four peaks in the frequency domain of both the signals. The first is directly caused by vessel motions. The other three have a cause which is not directly related to vessel motions. The data from the sensor on the Ampelmann system, at the top of the pedestal, does not contain a peak which is not present at the ship deck. From this it can be concluded that the eigenfrequencies of the pedestal do not have a relevant influence. For one of these peaks the amplitude of the graph related to the top of the pedestal is higher than the one corresponding to the ship deck. A possible explanation for this phenomenon could be the eigenfrequencies and corresponding eigenmodes of the ship deck. The pedestal functions as a leaver arm amplifying the rotations related to eigenmodes of the ship deck. This may cause the unexpected vibrations.