Thermal Grout, HDD Make 6,200-Foot Electrical Transmission Installation Possible

By Jeff Griffin, Senior Editor | December 2012, Vol. 67 No. 12

A recent project to install more than 6,200 feet of electrical transmission lines in Orange Beach, AL, included multiple challenges, including two intersecting horizontal directional drilling (HDD) pilot holes beneath Wolf Bay inter-coastal waterway channel to complete a single crossing.

Project participants say the project represents the longest installation ever made of continuous cross-linked polyethylene insulated (XLPE) cable rated at 115-121kV and 300MVA and the longest directional drilling installation ever to use specifically-engineered thermal grout.

“This is a very specialized field and I am generally aware of all major projects using thermal grout for underground transmission lines,” says Guy Dickes, president, Constellation Group LLC, thermal grout consultant for the project. “There has been a 3,500-foot installation in Texas, but nothing to compare with Wolf Bay. It’s the longest ever done to my knowledge.”

The project involved installing approximately 6,200 feet of a 36-inch diameter steel casing by intersecting two HDD pilot bores launched from each side of the channel. When the casing was in place, a bundle of five, 10-inch diameter conduits and one eight-inch diameter HDPE conduit was installed to house XLPE cables that would be pulled in later. Thermal grout was installed around the underground conduits to remove heat caused by high-voltage power transmission. If heat was not dissipated, functionality and lifespan of the cables would be reduced.

The project was completed over a six-month period in April 2012.

The project owner is PowerSouth Energy Cooperative. The Wolf Bay crossing was part of a plan developed by PowerSouth to improve transmission reliability in the area and included 4.9 miles of overhead cable and the Wolf Bay underwater crossing.

Atypical underground

The PowerSouth T&D Engineering and Construction Department handled the overhead design and construction work.

“Overhead construction is very typical and something we do every day, but the plan to go underground, beneath Wolf Bay, is completely different from anything we have ever done before,” said Sammy Hogg, T&D Engineering and Construction manager. “A great deal of preliminary work had to be completed before boring the line under the bay could begin, including feasibility studies, environmental reviews, routing, soil testing, land surveys and unique hydrographic surveys of the work site. The Intracoastal Waterway channel is the deepest part of Wolf Bay. The transmission cables would be 90-100 feet below that point.”

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