A study into cable-seabed interaction in offshore cable decommissioning
R.L.J. Helmons (TU Delft - Offshore and Dredging Engineering)
W. Bruinsma (Student TU Delft, Norwegian University of Science and Technology (NTNU), NKT)
E. Bachinsky-Polic (Norwegian University of Science and Technology (NTNU))
O. Kooy (NKT)
Svein Sævik (Norwegian University of Science and Technology (NTNU))
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Abstract
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. There is no industrial standard available (yet) to assess decommissioning of offshore power cables. A thorough understanding of the soil-cable interaction is essential to identify the limitations in the cable pull-out process, enabling cost-effective and safe operations. Factors such as shear strength, burial depth, pull-out velocity and cable stiffness are analyzed to assess the forces that oppose cable recovery. An analytical model of the cable-seabed interaction is developed and implemented in OrcaFlex. The model includes scenarios for fully drained, fully undrained, and partially drained uplift resistance, to simulate real-time resistance during pullout operations, allowing dynamic simulations of soil resistance during cable-pullout. Additionally, experiments have been performed to investigate the influence of flexibility of the cable, pull-out rate and burial depth. These results show that two regimes can be identified, based on the ratio of bending stiffness over burial depth. 1) the cable behaves as a rigid object, and 2) the cable bends within the seabed and exits the seabed with an angle close to being vertical, resulting in a smaller area where sediment is mobilized and a lower pull-out force. These simulations and experiments provide valuable insights, aiming to support the offshore wind industry's evolving needs and enhance the sustainability of decommissioning processes.