America’s pioneering Block Island offshore wind farm will be down for a period in 2021 to repair and rebury part of two 34.5kV grid-linked power transmission cables that became exposed in the approach to a popular swimming beach on the island off the state of Rhode Island.

“At this time we cannot predict exactly how long the wind farm will be offline, but we are working to keep it limited to a few weeks – not months,” Lauren Burm, spokeswoman at Orsted North America – which took over the five-turbine, 30MW array when it bought the project’s developer, Deepwater Wind, in 2018 for a reported $510m – told Recharge.

Construction related activities will be ongoing throughout the fall and winter, with an anticipated completion date before 31 May 2021, according to Orsted.

Power off-taker National Grid, which owns the 34km (21 mile) sea2shore export cable from Block Island to the mainland electric grid at Narragansett, anticipates its share of the repair bill at $30m with recovery of that cost from ratepayers.

Orsted, which owns the much shorter export cable from the array to the Block Island itself, is not disclosing repair costs. Its power contract does not provide for passing them on to ratepayers, the US offshore wind industry pacesetter told ecori.org.

The demonstration project in state waters, brought online in 2016 by Deepwater Wind, employs GE 6MW Haliade-150 turbines located about 6km off Block Island’s southeastern coast.

The array gave birth to an industry that now has commitments from east coast states to procure more than 29GW of power capacity through 2035.

The Block Island remedial work will require sections of both export cables to be spliced and lengthened up to 0.8km, and then re-laid at greater depth nearshore. That effort will include replacement of cable sections totaling 518 metres by Orsted and 975 metres by National Grid.

The drilling phase of the construction work at Town Beach is scheduled to begin in this fall, with the ocean-based cable installation scheduled to begin in early spring.

For the build phase, Rhode Island’s Coastal Resources Management Council (CRMC) approved use of high-pressure jet-plow technology by National Grid and Orsted for cable burial, a cheaper alternative to horizontal directional drilling (HDD) recommended by its staff.

The jet plow functioned well in burying the cables beneath the sandy sea bottom in the open ocean, but problems arose in the approach to Crescent Beach when the jet plow went up and over a concentration of boulders and pebbles.

This led to the cables being buried at 0.6-0.9 metres (2-3 feet) depth versus 1.2-2.4 metres specified by state officials during the permitting process, according to former CRMC executive director Grover Fugate.

The upcoming horizontal directional drilling will bury the cables at a depth of between 7.6- and 15.2-metres below the seafloor. This is deep enough to withstand changing ocean floor conditions.

“Throughout the planning process leading up to the repairs, Orsted considered multiple solutions and ultimately went with the most comprehensive option by choosing HDD,” said Burm.

This option will allow the cable to be buried at the greatest depth, with the least amount of surface level disturbance and “best limit the time between needing to take the wind farm offline and bringing it back online,” she added.

National Grid and Orsted have not provided a timeline for the repair work. According to ecori.org, drilling is expected to begin in October and conclude over the winter.

When the wind farm is offline, power supply to Block Island will continue via National Grid’s sea2shore cable and the Block Island Power Company’s fleet of diesel generators.

“Both National Grid and Ørsted have been working rigorously with the town and state and federal partners on an enduring plan to address burial depth, and even amidst the pandemic, we are on track to having it completed in time for next summer,” said Terry Sobolewski, president of National Grid Rhode Island.

Updates with details of planned repairs and timetime, and comment by National Grid

Wind

Orsted faces Block Island switch-off to fix US offshore wind array export line