After nearly four months of grueling work 50 ft beneath a state beach in Rhode Island, crews have finished drilling a 2,250-ft tunnel for the conduit that will carry power from the 30-MW Block Island wind farm to a National Grid switching station in Narragansett, R.I.
David Graves, a spoke man for the energy provider to Rhode Island, New York and Massachusetts, says the completion on May 6 will allow the company to begin installation of undersea cable when New Jersey-based subcontractor LS Cable America wraps up installation of Deepwater Wind cabling from five offshore turbines to the island. “We have had lousy weather and are at the mercy of the ocean, but we are prepared to begin laying our 20-mile stretch of cable from the mainland to the island on May 28,” he says.
National Grid’s portion of the transmission work, costing $107 million, includes the submarine cable as well as installation and construction of substations and switching stations on the mainland and Block Island, Graves says.
The wind farm—the nation’s first offshore demonstration project of its type—is now under construction at four port facilities, says Deepwater Wind. Steel jacket foundations are installed, and turbines are set for erection this summer, with grid connection by year’s end.
In addition to mainland work at Scarborough Beach, crews are drilling a 0.8- mile-long tunnel from Crescent Beach on the eastern side of Block Island to the new substation under construction, Graves says. “This is where our 20-mile cable, Deepwater Wind’s 8-mile cable and the Block Island Power Co.’s distribution system will all converge.”
Drilling at Scarborough Beach began on Jan. 8 for the 24-in. high-density polyethylene conduit that will slide through and carry the undersea cable to a mainland parking lot, where it will be spliced to an underground cable that will run 3.5 miles to a switching station being built in Narragansett, Graves says.
Another 0.8-mile stretch of cable will run from the switching station to the existing Wakefield, R.I., substation, tying into National Grid’s distribution system. “It was daunting,” Graves says. “We went through multiple drill bits to get it done.”
Jim Yuille—vice president of horizontal directional drilling for Caldwell Marine International, another New Jersey-based project subcontractor—says, “The work is similar to sea drilling for power-cable installation that [the firm] has performed, but the rock at Scarborough Beach is some of the hardest ever.”
National Grid originally had planned to drill about 3,000 ft, from the Scarborough shore to the parking lot—where the manhole has been installed—but it opted to reduce the length to save on schedule. “The bedrock proved to be such a daunting exercise that we had to reconfigure our permits and cut about 750 feet from that drilling process,” Graves says.
A crew of 12 workers operated the drilling machinery, with another 18 on the barges supporting the operation, Yuilles says. Without support, the driller drills a 9-in. pilot hole before the barge attaches itself to the drill string bit, he explains.
The drillers on shore then attach a reamer that widens the hole to 24 in. The on-shore drill rotates the reamer and supplies drilling mud to the hollow drill string. This process lubricates the hole and reamer while bringing back cuttings, which are separated from the mud in a shaker and pumped back into the drill for reuse. Each joint of the drill string is 30 ft long, “so they drill 30 feet, then add a section and drill another 30 feet and add another section,” Yuille notes. “The drill provides the rotation and the drilling mud. The barge offshore provides the force.”
According to Yuille, crews now are waiting for good weather to float into place the 2,200-ft-long conduit. “Once we complete the hole, we will pull the pipe conduit from offshore. The drill pulls the pipe conduit back through in one continuous pull,” he says. “The pipe is pulled by the drilling rig, and the barge has to work with divers to help guide it into the hole so we don’t overcome the pipe’s bending radius.”