Major Maintenance

Following completion of the 172 MW Gunfleet Sands offshore wind farm, constructed by Dong (now Ørsted) between 2008 and 2010, operational surveys revealed an emerging integrity issue around the turbine foundations. Located in very shallow waters of 2–15 metres LAT and subject to a large tidal range, the site experienced more severe seabed scouring around the monopiles than originally expected. Subsequent surveys in the years after completion showed that inter-array cables connected to the turbines were free spanning over distances greater than intended, as scour had excavated larger holes around the monopile bases. To mitigate the risk associated with these scour-induced free spans, Jan De Nul mobilised two of its vessels, the ‘Simon Stevin’ and the ‘Tiger’, for a dedicated rock placement maintenance campaign at Gunfleet Sands. In this operation, the Simon Stevin acted as a supply vessel, enabling the shallower-draught Tiger to place rock around the monopiles even during high tide at the shallowest locations. This coordinated approach allowed more than 50,000 tonnes of rock material to be installed around the monopile foundations in a single campaign carried out in 2014. The intervention represented a major maintenance effort aimed at stabilising the seabed, supporting the cables, and safeguarding the long-term operational reliability of the wind farm’s subsea infrastructure. The rock placement campaign effectively addressed the unintended free spanning, reducing mechanical stress on the cables and limiting future scour development. By reinforcing the immediate vicinity of the monopiles, the works helped preserve both structural and electrical integrity in the challenging shallow-water, high-tidal environment of Gunfleet Sands, supporting continued safe operation of the project.