The aim of this report is to complete a desk-based study to inform an evidence gap in relation to the feasibility of, and options for, removal of scour prevention and cable protection upon decommissioning of offshore wind farms and to identify potential impacts to marine habitats.
The report identifies scour and cable protection types commonly used in the marine environment including rock dump, rock bags, grout bags and concrete, fronded, bitumen and poly mat mattresses. The report assesses removal options, degradation resistance, current ease of removal and availability of future technologies to provide an overall ranking for each type, which were in descending order rock bags, concrete mattresses, grout bags, fronded mattresses, bitumen mattresses, and lastly rock dump.
For each scour and cable protection type the various decommissioning methods were considered including leave in situ, removal by divers, ROV dredgers, rock removal tool, trailing suction hopper dredge, backhoe dredge, crane lift, subsea grapples and lifting baskets, speed loaders, wet store systems, and mass flow excavators. The positives, risks, limitations, and environmental implications for each decommissioning option were assessed to provide an overall (very good to very poor) grade for each method.
Quantitative and qualitative evidence was gathered from the renewables and oil and gas sectors on the current ability to decommission scour prevention and cable protection. Input was collated from suppliers and contractors relating to current best practice, possible future improvements and recommendations that could lead to improving outcomes.