Offshore electric transmission cable needs to be reburied, utility saysDavid Wagman | February 14, 2020
Work to rebury an electric power cable that is part of the Block Island Wind Farm in the waters of Long Island Sound is set to begin this fall.
News reports said that the electric cables for the 30 megawatt (MW) offshore wind farm were supposed to have been buried in trenches at least 4 ft below the seabed. At the time of construction, however, workers were unable to dig down as far as they wanted due to the rocky seabed. Over the last four years, waves have exposed portions of the transmission lines that run to and from a beach on the island.
Orsted, the Danish company that owns the wind farm, now plans to rebury one of the two cables starting in the fall. The company reportedly said it should complete the work at Crescent Beach by Memorial Day 2021.
The project means that the wind farm will have to go offline. The island will likely resort to diesel generators for electric power during the outage.
Orsted’s cable runs from the wind farm to the island. A second cable, which connects the island to the mainland electric grid, is owned by the utility National Grid. It plans to rebury its cable, but details on the scope and timing of that work have not yet been released.
Officials linked the problems to a combination of technology that was used to bury the cables and natural forces. The area is subject to both wave action and sediment movement, the combination of which has uncovered the cables.
National Grid installed both cables in 2016 with a device known as a jet plow, which is towed along the seafloor and uses high-pressure water to dig its trench. The technology generally works well in sandy environments but was unable to dig deep enough in the rocky area off Crescent Beach.
Officials recommended that National Grid use horizontal directional drilling equipment to install the cables because of the tough environment. Such equipment was necessary in nearby Narragansett, where a power cable connects the popular resort island to the mainland power system.
Orsted reportedly said it now plans to use a horizontal directional drill to bury its power cable between 10 and 30 ft beneath the seafloor. Work will include replacing a section of cable that stretches from the island to a point about 3,100 ft offshore. The company has to replace the section because the cable is under tension and does not offer enough slack to allow it to drop into a deeper trench. Plans call for the new length of cable to be spliced into the older line.