December 2014, Vol. 69, No. 12
Features
Tackling Maine’s Largest Solar Power Project
Enterprise Trenchless Technologies Inc. (ETTI) and sister company Enterprise Electric (EE) worked together to complete a key element of the largest solar power project ever built in the state of Maine. It was ETTI’s first “green energy” project.
The 1.2 megawatt project, a partnership of Bowdoin College and SolarCity, is projected to offset approximately 8 percent of the college’s annual electrical usage.
The complex includes approximately 4,500 solar panels mounted on three buildings on the Bowdoin campus and ground-mounted on a three-acre site three-quarters of a mile away on property owned by the college at the former Brunswick Naval Air Station (BNAS).
Directional drilling specialist ETTI installed 3,400 linear feet of 4-inch diameter DR 13.5 HDPE pipe to serve as conduit for medium voltage underground transmission cable running between the campus and former BNAS location. Seven manholes were constructed along the route.
EE installed cable in the conduit and made connections at the solar array and college main power locations.
The 3,400-foot length of conduit was placed in the ground entirely by horizontal directional drilling (HDD) to minimize disturbance of road pavement and traffic disruption and to preserve other surface improvements.
“We were asked to install the line below existing utilities to avoid disrupting these essential services,” said Stephen Bates, ETTI HDD project manager. “Water and sewer lines running parallel to each other were under the road that was a primary route for the pipe.”
Challenges
However, there were challenges. Cost of the pipe installation as specified exceeded the budgeted amount for that part of the project, and drilling at the depth necessary to go under existing utilities prevented connecting pipe through sides of the manholes.
EE and ETTI proposed design changes to reduce material and installation costs without compromising the high quality standards of the project.
“We suggested reducing the size and wall thickness of the pipe,” Bates explained. “And rather than bring pipe through the sides of the manholes, we could enter through the bottom of the crushed stone manhole base which allowed us to dive under the utilities quicker.”
Manholes were precast offsite and installed after the conduit was in place. The conduit was installed at the same elevation as the precast outlet holes and then fitted to the manhole after it was set.
The route of the pipe was along the edge of the college’s soccer field, across a primary road and down the middle of a secondary road to the BNAS site.
Equipment
To install the pipe, ETTI used a 30,000-pound pullback Ditch Witch JT3020 directional drill supported by a truck-mounted FM50 fluid mixing system. Eight bores were completed with a 6-inch pilot hole and 6-inch backreamer. Subsurface soil conditions were sand and clay.
“On average,” Bates said, “we bored between 350 and 500 linear feet per bore. All conduit installed outside of pavement was 4-feet deep and depth under the road was 18 feet. Two bores were made beneath the middle of the road from the center of the secondary street below the utilities and across the primary street. These two bores totaled 850 feet below the utilities. The secondary road was closed for two days to accommodate drilling below the utilities.”
Crews worked normal daylight hours for the entire project. The crews encountered silty sand and clay.
A Ditch Witch FX50 vacuum excavator was used to keep work areas free of drilling fluids and pothole to visibly locate existing utilities.
Pipe installation was completed on Aug. 29. Drilling the pilot holes and product pullback required 12 days with the pulls broken into eight segments, one for each bore hole.
“Using ETTI’s directional drilling technology,” said Bates, “we overcame limited work space, prevented damage to numerous existing utilities and street moratoriums, and other constraints to complete the project with minimal disturbance to landscaping and roads.”
In addition to pullback of 30,000 pounds, the JT3020 drill unit develops 4,000 foot pounds of rotary torque and spindle speeds to 225 rpm.
The Ditch Witch FM50 fluid management system has two 1,000 gallon mixing tanks that can mix as many as 16 bags of additive in 2,000 gallons in less than eight minutes.
The dual-purpose FX50 vacuum excavator is a powerful vacuum and an efficient soft excavation machine that uses high-pressure water for digging small, precisely controlled excavations. In addition to HDD fluid cleanup, it serves as a mobile vacuum system for general cleanup work.
Cable installation
After the conduit was in place, EE used a Sherman & Reilly Duct Dawg to install three cables the entire distance from the college to the solar array. The system, said EE President Jeff Kelly, works by using the onboard compressor to blow a steel cable into the installed conduit. Cable was mounted on a triple-reel trailer to feed into the conduit. A pull calculator analyzed coefficient of friction, weight correction factor, tension and total weight per foot to ensure the cable could be installed without damaging its insulation.
In addition to installing the transmission cable, Kelly said EE made all primary splices and terminations, installed a pad-mounted transformer and all secondary connections, installed MOVE arresters, all required grounding, a new 15KV primary switch and tie into Bowdoin College’s primary distribution system.
“An extensive Job Safety Hazard Analysis was completed prior to starting the work and was followed throughout the project,” he added.
EE completed installation of the cable on Oct. 16.
SolarCity Corp., San Mateo, CA, constructed and operates the Bowdoin solar system with the college purchasing all generated power. SolarCity is a leading provider of solar power for homeowners, businesses, and government agencies in 15 states.
Located in Brunswick, ME, Bowdoin College is a private liberal education institution that was founded in 1794.
“Bowdoin’s solar power initiative is the latest effort to support renewable energy at the Brunswick campus and helps advance the college’s plan to become carbon neutral by 2020,” said S. Catherine Longley, Bowdoin senior vice president of finance and administration and the college’s treasurer.
Based in Lisbon Falls, Maine, ETTI is a directional drilling contract serving a wide range of clients in New England since 1995. Scott Kelly is president. Founded in 1968, EE is a full service electrical contracting company that also installs and maintains communications systems. President Jeff Kelly is Scott’s brother. EE foreman for the Bowdoin project was Jim Kelly, father of Scott and Jeff Kelly.
The Ditch Witch organization designs, manufactures, and distributes underground construction equipment, including horizontal directional drilling systems, drill pipe, downhole tools, and electronic guidance systems; trenchers, digging chain, teeth, and sprockets; vibratory plows, backhoes; mini skid-steer tool carriers, and vacuum excavators.
FOR MORE INFORMATION:
Enterprise Trenchless Technologies Inc., (207) 353-5000, www.hdd-etti.com
Enterprise Electric, (207) 353-2697, www.enterpriseelectric.net
Ditch Witch, (800) 654-6481, www.ditchwitch.com
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