Offshore oil and HDD technologies merge to build undersea power link
The Strait of Belle Isle is a thin ribbon of ocean that separates the southern part of Labrador from the island of Newfoundland.
The idea of building a fixed link over or under the Strait of Belle Isle to connect the remote Lower North Shore of Quebec to Newfoundland has been around for decades, but has never gone anywhere due to the high costs and the complex engineering such a feat would entail. The only transportation option is currently a 105-minute ferry ride, that takes longer in winter.
In the 1970s the Lower Churchill Development Corporation conducted detailed site investigations looking at a tunnel that would carry electrical wires. A 2005 study concluded that a tunnel link was feasible, but would cost $1.7 billion.
Now the strait is back on the radar as part of a major power transmission project commissioned by Nalcor, the province of Newfoundland and Labrador’s energy company.
Known as the Strait of Belle Isle Marine Cable Crossing, 105 kilometres of power cables will be placed underwater between Forteau Point, Labrador and Shoal Cove, Newfoundland. Three cables will carry power generated from the new Muskrat Falls hydroelectric facility on the Lower Churchill River, cross the Strait of Belle Isle and continue across Newfoundland to the terminus at Soldiers Pond, close to St. John’s.
Construction of the 1,100-kilometre transmission line started in the summer of 2014, with first power from the 824-megawatt Muskrat Falls facility expected at the end of 2017. The rationale for the nearly $7-billion megaproject, including two dams, a power house, transmission links and power infrastructure, is to meet the projected increase in demand for electricity, while reducing the province’s reliance on fossil fuels, particularly the burning of bunker fuel at the Holyrood facility on the island.
Tunnelling vs HDD
As planning got underway around five years ago, one of the chief obstacles to the transmission line was the Strait of Belle Isle, which on average is 20 kilometres across and 120 metres deep in the middle. A taskforce considered two options: a tunnel to carry the set of three cables, or the cable set running along the seabed between six bores directionally-drilled on either side of the strait.
After careful consideration of a tunnel including input from European and Canadian tunnelling experts, it was decided the seabed option was the way to go; the main problem with tunnelling was the highlyfractured rock which would have required an extremely deep tunnel.
“With the head pressure from 120 metres of water plus a saturated zone running 50 metres under the sea floor, it would have been one of the longest, deepest tunnels in the world,” said Greg Fleming, Nalcor’s marine crossings project manager. “None of [the experts] could get comfortable with the solution to execute a tunnel.”
Nalcor therefore decided on horizontal directional drilling as the best solution for installing the cables, hiring Alberta-based Mears to drill a pilot bore 1.5 kilometres underneath the strait as part of a geotechnical derisking program. “It gave us a lot of intelligence around the geology and the drilling methodology, what was going to work,” Fleming said of the pilot bore program. “It allowed us to play with different technologies for survey, bits and fluid mixes.” The upshot, though, was the HDD option presented two fundamental problems for Nalcor and their HDD contractor, Direct Horizontal Drilling: the integrity of the rock; and the lack of a target to survey.
“Effectively granite drilling”
Fleming said the “extremely competent rock” was granite containing quartzite. The hardness of the rock made the prospect of boring through it horizontally a challenge, from both a productivity and a bit-wear perspective.
“It was effectively granite drilling,” he said, noting that typical HDD utility work involves boring through soft soils like clay.
For the choice of bits to use, Nalcor turned to Schlumberger’s Smith Bits, National Oilwell Varco (NOV) and Baker Hughes, whose bit division, Hughes Christianson, offers solutions for tough rock drilling jobs. Hughes recommended a series of PDC (Polycrystalling Diamond Compact) bits and roller cone bits to trial for obtaining the best rate of penetration.
The company also deployed its patented Kymera bit, a hybrid of the traditional roller cone bit and PDC bit.
“We wanted to drill fast but not have the bit wear out too quickly,” said Darren Drake, operations director, drilling services with Baker Hughes, explaining that the Kymera bit offers the advantage of both speed and crushing power. “We employed that technology to make sure [the project] stayed on budget.”
Drilling the six bore holes, one for each cable and three on each side of the strait, took 10 months. From the Labrador side Direct Horizontal Drilling drilled 1,250 metres through solid granite, punching 14 ¾-inch pilot holes. The bores on the Newfoundland side went up to 2,130 metres into the strait. According to Fleming, more metres were drilled, and in harder rock, than the dual-derrick Hibernia offshore drilling platform during the same time frame.
Like hitting a dinner plate
The other challenge for the contractor was the lack of a land-based target from which to plan the bores. Again, Nalcor looked to Baker Hughes for advice. The depth of the holes was touchy – they couldn’t be too deep because the cables, once installed, could overheat, but neither could they be too shallow, due to the icebergs that regularly move in and carve the bottom of the channel. For that reason, the bore holes had to be driven 70 metres underwater. To those challenges, was added the requirement of drilling six holes that lined up perfectly, through which the 10 ¾-inch power cables would eventually be fed.
“When we drilled from the shoreline, we had to pop up on the ocean floor, two kilometres away, in something the size of a dinner plate,” said Drake.
Fortunately Baker Hughes had a steering tool that was up to the task. The company’s AutoTrak X-treme rotary steerable system is designed for specialized oilfield drilling work.
“There’s no wire line, the guidance is done by pumping a binary code down the mud, the tool at the bottom has got hydraulic arms, and the computer will tell when those arms need to come out and steer the bit. It’s very high-tech,” said Lon Briscoe, president of Direct Horizontal Drilling, based in Alberta.
As for the drill rig required to construct the extra-long bore holes at wider diameter than a standard 12 ¼-inch hole used for oil wells, Direct Horizontal Drilling built a customized rig with enough horsepower and pullback to complete what Briscoe described as “massive downhole assemblies.” The company purchases the drill racks from American Augers and then custom-fits each rig at its Edmonton yard. “We shipped 35 truckloads of equipment to Newfoundland and Labrador from Edmonton,” said Briscoe.
The drill had a pullback of 1.1 million pounds and total of over 5,000 hp, including two mud pumps, each rated 1,000 hp. “We were running at 14,000 kpa (2,000 psi) and pumping up to 1,600 gallons a minute and processing it, cleaning and reusing it, Briscoe said.
Drake noted that unlike the process of horizontal directional drilling on land, which starts out as a vertical well, with the undersea bores, they didn’t have “the luxury” of the weight from the drill pipes bearing down on the horizontal section. That meant the drill, mounted to a flatbed trailer, had to be jacked up at different angles, following the trajectory of the sea floor as the pipe extended down.
“Literally we were stabbing into the ground at a 75-degree angle,” said Drake, adding that the straighter the bore, the less force the rig would need to push the pipe around the bends.
HDD activities for the Strait of Belle Isle Marine Crossing were completed in the fall of 2014, on time and on budget, and the next step will be to lay the cables, which are currently being built in Japan.
A cable installation vessel will hook onto wires currently in the holes and winch the cables in using a 7,000-ton turntable. A protective rock berm will be placed over each cable with a remotelyoperated vehicle. Fleming said the berms will take up to 500,000 tonnes of rock.
Once the work in the Strait of Belle Isle is complete and the new Muskrat Falls generating facility and transmission lines are operational, Nalcor will be one step closer to connecting the electricity grid on the island of Newfoundland to Labrador – and North America – for the first time in history.