By Sharon M. Bueno — Dec 15, 2014
A new, 700-megawatt, gas-fired power generating station is under construction in Woodbridge, N.J., that, when completed, will transmit power generated by this new facility to an existing substation three miles away. Horizontal directional drilling (HDD) was successfully used to install more than 11,000 ft of high-voltage transmission lines across two wetland areas and the Raritan River.
Carson Corp., based in Lafayette Township, N.J., was the HDD contractor for the design-build project. In all, Carson drilled six directional bores in three locations for the 11,000 ft of 30-in. Fusible PVC (fPVC) casing. Each of the three locations necessitated parallel crossings of the casing pipe, which housed three 230 kV electrical cables, each producing enough electricity to power more than 700,000 homes.
Project general contractor was EJ Electric/Ferreira Construction and the owner was Competitive Power Ventures. The power generating station — known as the Woodbridge Energy Center (WEC) — is being constructed at a former chemical plant and will be a state-of-the-art energy center upon completion. According to project information, the WEC will help the state of New Jersey meet its energy demand, while also improving the state’s electric system’s reliability and environmental impact.
According to Carson Corp. vice president Scott Murray, P.E., each 30-in. fPVC casing housed four 8-in. fPVC conduits to carry three 230-kV cables (one of which is a spare conduit for possible future use), two 2-in. HDPE conduits for ground and fiber-optic lines and a 3-in. HDPE grout delivery tube. Casing spacers were installed every 5 ft in the 30-in. fPVC casing pipe. The fPVC casing was supplied by Underground Solutions and Lee Supply supplied the 2-in. and 3-in. HDPE conduits.
The three locations where the six drills took place (parallel bores at each location) were: 1,600 ft, 2,400 ft and 1,500 ft. Crews used three American Augers drills for the project: DD1100 (1.1 million lbs), DD440 (440,000 lbs) and DD140 (140,000 lbs). Drilling took place simultaneously at times, taking advantage of the parallel installation. Crews also used two Tulsa Rig Iron 750 Recyclers, as well as INROCK and SlimDril International steering hands.
Use of traditional installation methods via overhead towers and direct burial were used for most of the installation. “About 1mile of the entire project needed to be installed using the trenchless technology of HDD because the terrain to be traversed was either wetlands or the river,” Murray explains. “When they got to a point where they had wetlands and they couldn’t get through the swamp, they had to bring the wires down from the pole and cross under them to reach the substation.”
The project started in March 2014 and completed in July. However, before drilling could commenced, the Carson crew constructed a mile of timber mats in the wetland/marsh areas in order to use the heavy drilling equipment.
Why Fusible PVC?
When designing the project, Carson Corp. considered a range of casing and conduit pipe materials, including steel and HDPE and fPVC. Murray explains that Carson looked at the design aspects of the project in making its decision to go with fPVC. Pulling strength of the pipe, as well as the internal and external diameters of the pipe, was critical.
“Two drills were over 2,400 ft,” Murray says. “If I was pulling something that long using HDPE, I would need a heavier, thicker wall. When you have a thicker wall, that decreases the size of the inside diameter of the pipe. The whole premise of the project was to get a 7 ½ in. inside diameter space filled with conduit. If I used [a pipe] with a heavier wall thickness, the borehole just got larger and larger.
HDD“The fPVC had a thinner wall thickness that allowed me to create a bigger bundle that had a smaller outside diameter borehole, he adds. “[The fPVC] also had a greater pulling strength, capable of pulling it further.”
Murray further explains that the conduit pipe needed to be large enough inside diameter to allow the proper clearance for the power cables while having a small enough outside diameter to fit within the 30-in. casing. Also, the 8-in. conduit pipe needed sufficient hoop strength to withstand external loading placed on the pipe during the grouting process.
Using the non-metallic fPVC casing pipe also maximized the amount of electrical current a conductor can carry before sustaining progressive deterioration. The fPVC pipe was connected to the open-cut conduit with standard power couplings and fittings.
The ground conditions were of mixed soils, including sand, clay and rock, the latter Carson wasn’t expecting. Murray explains that the final two boreholes proved to be a bit of challenge, given the narrow right of way that the drilling crew had to work in.
Parallel“With the last two drills, we had a very tight tolerance in the right of way,” he says. “With the first four drills, there was a 40-ft right of way but the last two only had a 22-ft right of way. The electrical manufacturer required that the drills be 19 ft apart to prevent the [lines] coming too close together and heating up. But the right of way didn’t allow for any margin of error. We had to drill both drills perfectly along the right of way.”
How did the crews handle these two tricky drills? “Very cautiously and carefully,” Murray says. “When we started to deviate, we made sure we corrected it immediately. There was no room for error.”
Once the casing pipe was in place, the bundle was installed, which was then grouted into place using specially designed thermal grout (Constellation Group), piped in through the 3-in. HDPE grout delivery tubes.
Murray says this is type of project is what Carson does best: handling more than just the drilling.
“We did everything,” he says. “We weren’t just the drillers. We assembled and installed the bundle, grouted the conduits in place, pressure tested it and provided a pull string inside.”
Sharon M. Bueno is managing editor of Trenchless Technology.