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Conoco compares HDD with Nigliq bridge Corps of Engineers requested comparison of horizontal directional drilling with company’s proposed vehicle-pipeline bridge Kristen Nelson Petroleum News Editor-in-Chief
ConocoPhillips Alaska is in the process of permitting development of the Alpine West reservoir, the Colville Delta 5 Alpine satellite, CD-5, and has proposed a vehicle and pipeline bridge across the Nigliq Channel of the Colville River. The bridge would provide access between the satellite and the main Alpine facilities for personnel and for three-phase crude oil (oil, gas, water), which would be processed at Alpine. Alpine is not tied into the North Slope road system, but is reached by ice roads in the winter and year-round by air.
The Corps of Engineers requested technical information from the company on an alternative crossing of the channel for the pipeline using horizontal directional drilling, the method used to cross the Colville River to take sales-quality oil from Alpine to Kuparuk.
“This is essentially a discussion about roadless vs. roaded access, as an HDD crossing would only be a consideration in a roadless development,” ConocoPhillips said. The company said it cannot make a direct comparison between a horizontal directional drilling crossing and a pipeline bridge, because while horizontal directional drilling is technically feasible, it is not an alternative to a bridge.
ConocoPhillips told the agency in a Nov. 29 letter that cost estimates are $25 million for a pipeline-only bridge and $28 million for horizontal directional drilling, with an estimated $1 million to support pipelines on an existing vehicle bridge. A vehicle-only bridge is estimated at $39 million compared to an estimated $20 million to support roadless access with an air operation.
“If either a pipeline or vehicle bridge option is required, it is cost effective to install a dual duty bridge to satisfy both access requirements,” the company said. No known three-phase fluid HDD crossings The company said “there are no known three-phase fluid HDD crossings in existence.” Technical concerns include: corrosion; sedimentation; paraffin issues; leak detection; lack of visual inspection; cathodic protection; unknown slugging impacts to existing facilities; and unknown soil conditions.
ConocoPhillips included a white paper on a horizontal directional drilling crossing of the Nigliq Channel prepared by Michael Baker Jr. Inc. Michael Baker concluded that while a horizontal directional drilling crossing is “technically feasible, there are disadvantages to an HDD crossing at the Nigliq Channel. Liquid holdup and slugging can be minimized, but cannot be eliminated and will add increased stresses to the pipe, exacerbate internal corrosion, and require additional slug catching equipment at the Alpine Processing Facility.”
Michael Baker also said that cathodic protection for an HDD pipeline “becomes complicated and expensive.”
Horizontal directional drilling “is sensitive to geotechnical and geothermal conditions,” the company said, and pipeline inspection and leak detection system are not as reliable for buried lines, “especially for the three-phase flow lines.”
Installing a pipeline on a bridge “reduces the risk of an undetectable leak” as well as reducing the cost of the pipeline, Michael Baker said. Issues with multi-phase flow In transporting multi-phase fluids — oil, water and gas — “fluid phase separation becomes increasingly important,” Michael Baker said. Long multi-phase flow lines are “usually accompanied by large pressure drops, and rises and falls in terrain encourage this separation and the formation of slug flow.”
Multi-phase flows, unlike single-phase flows, “are strongly influenced by the geometry of flow,” of which the strongest influence is from pipe inclination, with an inclination of as little as 0.5 degrees having the ability to “dramatically alter the flow patterns, the boundary layer structure, and the fundamental transport mechanisms.” While inclination of a horizontal directional drilling pipe under the Nigliq Channel could be reduced with long approaches, it could not be eliminated, Michael Baker said.
Sediment may also be present in multi-flow, and tends to drop out and accumulate in low spots, such as HDD river crossings, and may increase the risk of corrosion in these areas. In addition, Michael Baker said, sediment may increase erosion of pipe wall, especially at bends in the pipe. That would not be any worse in an HDD installed pipe, but erosion at submerged bends in the pipe could make addressing the issue difficult, “short of complete pipe replacement.”
Michael Baker said slugging would be one of the greatest challenges to an HDD crossing of the Nigliq Channel. That occurs with phase separation. The liquid in a multi-flow accumulates in the pipe and gas pressure builds up behind the liquid, forcing it through the pipe as a slug, which can generate large forces within the pipe “due to changes in momentum as the slug negotiates abrupt changes in direction.” Slug formation is sensitive to pipe inclination and would have to be studied at the Nigliq crossing, Michael Baker said: “Vertical changes in pipe alignment, especially large sags, have the greatest potential to cause liquid holdup and large slugging forces.” An HDD crossing of the Nigliq Channel would require a vertical drop of some 50 feet, Michael Baker said, and while the large slugging forces would require modifications, such as slug catchers, at the Alpine facility, cathodic protection and corrosion issues are of greater concern, and traditional methods of cathodic protection do not work. The slugging forces “increase shear wall stress in the pipe,” increasing the rate of internal corrosion.
If there is a road bridge at the Nigliq Channel, installing a pipe on the bridge would be “the preferred installation,” Michael Baker said.
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