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.

Deep sea mining refers to the mining of valuable mineral resources from the deep ocean floor. Given the complex and fragile nature of deep-sea ecosystems, adopting an interdisciplinary and holistic approach is crucial to ensure the sustainable and responsible development of deep-sea mining (DSM) operations. This includes work related to the assessment of potential environmental impacts where physical, chemical, and biological characteristics of the target area are studied along with potential short-term and long-term effects on the surrounding ecosystems. These effects will be mining system dependent. Stakeholder engagement is essential. There are however knowledge gaps related to the deep-sea ecosystems and their interconnectedness, biodiversity, ecosystem dynamics and both the potential impacts from a single operation and cumulative impacts of the mining activities, as well as the mining systems themselves and the characteristics of the deposits. Collaboration between marine biologists, oceanographers, geologists, engineers and other relevant disciplines is essential to gain comprehensive insights. Closing these gaps would enable the development and implementation of a robust regulatory framework at both national and international levels to govern potential deep-sea mining operations. Monitoring and enforcement mechanisms must also be put in place to ensure compliance with the not-yet-developed set of standards. Multiscale adaptive management approaches where different temporal- and spatial scales are taken into consideration and where scientific knowledge, stakeholder engagement, robust regulations, and responsible practices are integrated, are the prerequisite for future responsible extraction of mineral resources from the ocean floor. This chapter gives an overview of topics relevant and needed for a proper multiscale marine mineral management. Its focus on the water column is restricted to vertical transportation and the impact of plume resettlement on biogeochemical processes.