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Enstar to reposition pipeline crossing 5,000-foot horizontal directional drill at Susitna River will exceed the state record set by a previous Beluga pipeline project Alan Bailey Petroleum News Staff Writer
The rapid westward migration of the east channel of the Susitna River is forcing Enstar Natural Gas Co. to reposition more than a mile of the Beluga gas line that carries natural gas from the west side of the Cook Inlet to the Matanuska-Susitna Borough and Anchorage. The pipeline crosses the river near its mouth at the north end of the Cook Inlet. At the crossing point the river consists of three major channels, the west, middle and east channels, in the river’s flood plain.
“You have a five-mile floodplain there with three channels and probably 90 percent of the water goes down this east channel,” John Lau, Enstar’s director of transmission operations, told Petroleum News. “The whole (east) river channel is moving sideways — it’s washing away for about a mile upstream (of the pipeline crossing).”
Enstar is moving the crossing to an alignment about 40 feet north of its current location in a project that involves drilling a 5,000-foot horizontal hole that will accommodate the 20-inch pipeline 60 feet below the river bed. Including trenched sections beyond the ends of the buried pipeline, 5,400 feet of the pipeline will be repositioned.
The project resembles the 2004 repositioning of the middle channel crossing — that project involved a 4,305-foot horizontal directional drill, a state record at the time.
Conam Construction Co. will be the general contractor for the $5 million east channel project, with ARB Inc. doing the directional drilling, Lau said. Enstar has applied to the Alaska Department of Natural Resources for a coastal consistency review for the project and public comments for the review are due on or before Jan. 4. 1993 crossing threatened Enstar constructed the current east channel crossing in 1993 using a 3,400-foot directional drill, as a consequence of erosion of the west bank of the channel since installation of the pipeline in 1984. The company set the entry point for the directional drill 600 feet back from the channel, a distance that seemed more than adequate at the time, Lau said.
But exceptionally large volumes of water coming down the river in recent years have accelerated the river erosion. Last year there was a big runoff in the spring and another heavy flow in August, Lau said.
“We lost about 150 feet in two years,” he said.
The channel has now migrated 400 feet from its 1993 location and, at present rates of erosion, could threaten the current pipeline crossing as soon as the spring of 2006. A loss of as little as 70 feet of additional riverbank would bring the bed of the river dangerously close to the pipeline as it transitions upwards to the surface tie-in at the west end of the crossing. So, with Southcentral Alaska’s dependence on the natural gas that passes through the pipeline, repositioning of the crossing has become urgent.
“This time we’re going back 2,000 feet (from the channel’s west bank),” Lau said.
On the east side of the channel the tie-in to the crossing will be set back about 400 feet from a small river channel off the main channel. Mature technology Fortunately the maturing of directional drilling technology over the past few years has eliminated much of the risk from this type of project, Lau said.
“The drillers didn’t seem too alarmed — they just looked at it and said okay,” Lau said.
In fact, worldwide, there have been horizontal directional drills as long as 6,500 feet to install similar 20-inch pipe, Lau said.
Pulling pipe through this length of hole requires a powerful horizontal drilling rig. The 1993 installation of the east channel crossing used a 330,000-pound rig but ran into problems pulling the pipe. For this new crossing the contractor will be bringing a 750,000-pound rig, a size well in excess of the contract’s minimum requirement of 500,000 pounds.
But drilling the initial pilot hole that’s then reamed out to accommodate the pipeline is actually more challenging than pulling the pipe through the hole, Lau said. It’s critical that the pilot hole follows the precise design path of the crossing and that it surfaces at the correct tie-in point. Unlike directional drilling of an oil well, a shallow horizontal drill uses electromagnetic technology to locate the position of the drill bit and, hence, guide the directional drilling.
“They put a (wire) grid on the surface and it detects where the bit is,” Lau said.
However, the silty sands in the river floodplain should work to the project’s advantage by keeping the hole stable.
“Once the pilot’s across the risk factor is down,” Lau said. Weather challenges The need for cold weather actually presents a bigger challenge than the drilling, Lau said. Cold winter conditions are vital for the construction of ice roads and ice bridges that will enable equipment to reach the work site some 12 miles west of the road system to the west of Goose Bay on the Knik Arm. The ice road route lies along the pipeline right of way.
Although the warm December weather that Southcentral Alaska has been experiencing is cause for concern, a cold November enabled snow machining of a route to the site; good progress in ice road construction should enable project completion this winter.
“Nature did its thing — we have a great ice road out there,” Lau said. “We’re working on the ice bridges and they’re coming along. If we have a reasonably cold January, February there shouldn’t be any risk.”
But how long will this new crossing last, before it, too, is overtaken by the eroding river? Lau said that the 2,000-foot setback should protect the crossing for at least several decades.
“Even if we lose 40 feet a year we’ve got a 50-year pipeline,” he said.
And with most of the river water coming down the east channel, the other two channels shouldn’t present a problem. The 2004 relocation of the middle channel crossing has dealt with problems there and the west channel hasn’t moved at all for many years, Lau said. It’s the east channel that has caused most of the concern.
“We’ve known for years that we’re going to come back out to this one,” Lau said. “It’s not a surprise.”
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