Deepwater Wind to bury transmission line for proposed offshore wind farm

Deepwater Wind has agreed to modify the Block Island Transmission System from an overhead line to a buried line in relation to its proposed 30 MW (nameplate) Block Island Wind Farm off of Block Island, R.I.

According to the company’s May 31 letter to the U.S. Army Corps of Engineers, in response to feedback received from residents in the town of Narragansett, R.I., Deepwater Wind Block Island Transmission has agreed to modify the 0.8-mile segment of the Block Island Transmission System from the Narragansett Town Beach to the proposed Narragansett switchyard from an overhead line to a buried line.

Additionally, based on discussion with Rhode Island Coastal Resources Management Council (CRMC) staff, Deepwater Wind has modified the previously proposed design of the Narragansett switchyard to further minimize land use impacts within buffers to Narrow River Special Area Management Plan wetlands.

According to the company, the demonstration-scale offshore wind farm will be about three miles southeast of Block Island and consist of five wind turbines.  The wind farm is located entirely in Rhode Island state waters.

The company also said that the wind farm will generate more than 125,000 MWh annually, supplying the majority of Block Island’s electricity needs. Excess power will be exported to the mainland via the bi-directional Block Island Transmission System. Deepwater Wind added that it plans to begin construction by 2014.

According to the Block Island Chamber of Commerce, the population rises from 900 to an estimated 12,000 people in the summer.

According to the letter, benefits stemming from the proposed changes include that the existing utility poles along the previously proposed overhead line route no longer require enhancement.

With the letter, the company said it is modifying the contents of the wind farm’s transmission system’s environmental report/construction and operations plan dated September 2012 to reflect the changes.

The transmission system will make landfall at a manhole located within the parking lot of Narragansett Town Beach. As originally proposed, the transmission system would have transitioned to overhead at an existing Narragansett Electric d/b/a National Grid USA riser pole in the Town Beach parking lot and would have primarily followed the existing National Grid overhead distribution system for about 0.8 miles along Beach Street to Narragansett Avenue/Kingstown Road to its interconnection point at the proposed Narragansett switchyard, located next to a Narragansett facility off of Mumford Road.

National Grid is a subsidiary of National Grid plc.

Deepwater Wind has revised the proposed action to a buried cable along that same route from the Town Beach parking lot to the proposed Narragansett switchyard.

The company also said it will build onshore facilities during the fall, winter or spring to avoid impacts to the summer tourist season, which will mitigate construction phase visual and transportation impacts on seasonal residents and tourists.

The buried portions of the project’s terrestrial cables will be installed within a concrete-encased duct bank. In Narragansett, the company added, the installation of the buried portion of the transmission system will require a trench about three feet wide and seven feet deep from the Town Beach to the Narragansett switchyard and about four feet wide and seven feet deep from the switchyard to the interconnection of a National Grid feeder.

In some areas, the excavation will be done by hand and may exceed seven feet deep to avoid disturbing existing utility lines, service connections and stream culverts. Deepwater Wind also said that the trench will be sheeted and stored during construction as required by soil conditions and Occupational Safety and Health Administration safety rules.

Groundwater management during construction of the buried transmission system cable in Narragansett will likely be required.

Deepwater Wind added that regardless of the method used, it will incorporate an appropriate level of erosion and sediment control and be performed in accordance with regulatory erosion and discharge requirements.

Once a portion of the trench is excavated, the conduit will be assembled and lowered into the trench, with the area around the conduit filled with a high-strength thermal concrete. The underground cable will be installed following the installation of the duct bank and manholes. The cable will be installed by stringing it from one manhole to another and will be spliced in the connected manholes after installation.

Deepwater Wind also said that the buried portions of the transmission system on Narragansett will have no regular maintenance needs beyond periodic inspections of the system unless a fault or failure occurs.

The company noted that in response to comments received from CRMC staff in April, Deepwater Wind has modified the switchyard’s design to minimize earth disturbance and permanent fill replacement to the maximum extent possible; prioritize the location of earth disturbances as far as possible from biological wetlands; minimize impacts to the extent possible within the 50-feet perimeter wetland buffer zone(s); and design a maximum grade of 30% for all fill slopes.

Among other things, the company said the proposed transmission system modifications will result in no impacts on marine waters and all transmission system construction activities along the modified buried portion of the cable route are proposed to take place within existing road rights-of-way or previously disturbed lands and will result in no impacts to natural vegetation along the route.

Unlike overhead transmission lines, during operations, the underground portions of the transmission system will not require vegetation clearing, reducing operational impact of the transmission system facilities from what was originally estimated.

The company also said no terrestrial archeological sites have been identified along the portions of the transmission system project area in Narragansett that have been surveyed to date. Furthermore, installation of the modified transmission system facilities in Narragansett will not result in impacts to biological wetlands.

About Corina Rivera-Linares 3286 Articles
Corina Rivera-Linares was TransmissionHub’s chief editor until August 2021, as well as part of the team that established TransmissionHub in 2011. Before joining TransmissionHub, Corina covered renewable energy and environmental issues, as well as transmission, generation, regulation, legislation and ISO/RTO matters at SNL Financial from 2005 to 2011. She has also covered such topics as health, politics, and education for weekly newspapers and national magazines.