Eversource (NYSE:ES) on July 2 said that a study comparing potential methods to cross Little Bay with a new power transmission line – the Seacoast Reliability Project – shows that the burial of the line in the bottom sediment of the bay would have minimal impact on the environment.
The study also showed that burying the proposed 115-kV line in the bottom sediment of the bay would have the least disruption on area residents and properties, the lowest cost, as well as the shortest schedule, and would be the most appropriate method.
The line would pass through Madbury, Durham, Newington, and Portsmouth along existing utility corridors, including the Little Bay crossing, Eversource said. The project includes proposed substation upgrades and the new line connecting two existing substations in Madbury and Portsmouth, the company said.
Noting that electric demand in the state’s Seacoast region is growing at twice the rate of the rest of New Hampshire, and that ISO New England has determined that additional transmission capacity is needed to support the reliable delivery of electric power to the region, Eversource said that the project is designed to improve the reliability of electric service in the Seacoast area, specifically benefitting the region where the project is located.
The company noted that based on a recommendation by the New Hampshire Department of Environmental Services (NHDES), the state Site Evaluation Committee (SEC) required Eversource to conduct the detailed review – which was filed with the SEC on July 1 – comparing the feasibility of the proposed “jet plow” burial method to that of “Horizontal Directional Drilling” (HDD), which would place the project’s power cable deep beneath the bay by means of a significant drilling operation.
The jet plow method would place the power cables under several feet of the bay sediment by means of pressurized water jets, temporarily suspending some sediment in the water column, the company said. The NHDES in February recommended, with conditions, approval of the cable installation using the jet plow methodology, but also recommended that the SEC require the feasibility study, including the jet plow and HDD methods, Eversource noted.
Full HDD would entail drilling under the entire bay, while shore landing, or partial HDD, would require drilling on both sides of the bay into the channel and connecting the two bore hole cables via jet plowed cable, the company said.
“We understand the importance of selecting the best crossing method, one that’s least impactful to the environment and the community at a reasonable cost to customers,” Eversource New Hampshire President Bill Quinlan said in the statement. “Based on the results of this comprehensive study, we’re confident that the [SEC] members will have a better understanding of why the jet plow method is appropriate for this project and why we recommend its use.”
The company noted that the jet plow method would have a total project cost of $84m, and a construction period of three months; the land rights required are secured. The company also said that the full HDD method would have a total project cost of $216m, and a construction period of 28 months; the land rights required involve more than 10 properties. In addition, the company said that the shore landing, or partial HDD, method would have a total project cost of $184m, and a construction period of 10 months; the land rights required involve more than 10 properties.
As noted in the study, Eversource in April 2016 filed with the SEC for approval to build and operate the line connecting the Madbury and Portsmouth substations. The project would be about 12.9 miles long and include a combination of overhead, underground, and underwater components, the company said in its study, adding that the project route includes an approximate 0.9-mile underwater crossing within a charted cable area of Upper Little Bay, a portion of the Great Bay Estuary system.
Discussing the jet plow method, the company noted, for instance, that the jet plow is laid on the seafloor and towed from a surface barge. The sizes of the barge and tender needed for jet plowing are relatively small and can accommodate the shallow waters and limited access to Little Bay, the company said. The main mechanical components of the jet plow are two skids, which allow the sled to slide across the bottom, and an articulated blade that rotates down into the seafloor, the company noted.
Jet plowing, also referred to as hydro jetting, is a common technique for installing transmission cables requiring burial in unconsolidated sediments, the company said, adding that typical areas are usually shallow waters – <150 feet – where active recreational or commercial boat and fishing activity result in the need for burial for human safety and to protect the cable from damage by anchors or fishing gear.
The benefits of using a jet plow for submarine cable installations include that the jet plow blade’s overall weight is distributed over wide, long skid plates designed to allow the plow body to slide easily over the sea-floor, leaving minimal depressions as it passes along the cable installation route, the company said. Also, jet plow installations typically span a few days versus months or years compared to HDD installation, the company noted.
The challenges associated with jet plow burial are minimal, the company said, adding that the main concerns are such underwater obstacles as boulders or commercial shipping debris like wire ropes, neither of which are anticipated to exist based on the marine route survey data already collected for the work area.