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Engineer details aboveground preference Louis Kozisek of State Pipeline Coordinator’s Office looks at advantages, disadvantages of aboveground vs. HDD line under Nigliq Kristen Nelson Petroleum News
The State of Alaska and federal agencies which are part of the federal-state Joint Pipeline Office have told the U.S. Army Corps of Engineers that its 2010 decision in favor of a horizontal directionally drilled crossing at the Nigliq Channel of the Colville River is incorrect.
The pipeline crossing at the Nigliq Channel should be on a bridge, the corps was told. That was the approach chosen by ConocoPhillips Alaska, which plans to develop its CD-5 drill site, which will be the first oil and gas development in the National Petroleum Reserve-Alaska, across the Nigliq Channel of the Colville River from the company’s Alpine facilities, where oil from CD-5 would be processed.
The engineering considerations behind the opinion of the state and the JPO federal agencies were laid out by Louis Kozisek, P.E., chief engineer in the State Pipeline Coordinator’s Office, in an April technical opinion which evaluates the advantages of the two types of crossings — aboveground or buried under the channel.
Kozisek, who has three engineering degrees and professional engineering licenses in mechanical and civil/structural engineering in Alaska, has 32 years experience with the trans-Alaska oil pipeline and North Slope oilfield facilities, including work for pipeline and oil companies, for an engineering consultant and for federal and state government.
As chief engineer with SPCO his responsibilities include engineering evaluation of most of the long-distance pipelines in Alaska, including the Alpine pipeline and the HDD crossing at the Colville River.
Prevention of spills Kozisek said he placed special emphasis on environmental considerations and on the importance of selecting the least environmentally damaging practical alternative, and gave the greatest weight to prevention of oil spills over the life of the crossing.
The need to design for the long term was also emphasized, he said.
North Pole “operators and engineers are currently struggling with maintaining aging oil production facilities and pipelines. Many design decisions made decades ago have only demonstrated problems with the passage of years and decades. Experience has shown that the difficulties of maintaining integrity over years of service have too often been given insufficient consideration in the original design,” Kozisek said.
Maintenance and repair; surveillance He said the aboveground option has important advantages over an HDD installation for maintenance and repair. “It keeps the pipeline easily accessible, and lowers the costs and reduces the response times.”
The importance of design which allows the easiest and simplest monitoring and repair has been clear over the last 30 years with respect to North Slope pipelines and facilities, Kozisek said.
He said the aboveground option “is greatly preferred for surveillance and monitoring.”
By keeping the pipe high aboveground, crossing the channel on a bridge, nearby traffic can monitor it for leaks and inspection crews can easily and inexpensively reach suspect areas, he said.
While in-line inspection or smart pigging can mitigate some of the disadvantages of surveillance and monitoring of a buried HDD crossing, aboveground lines can be monitored both by in-line inspection, external corrosion direct assessment and other methods and the methods “can be correlated to improve the certainty of condition assessments.”
Kozisek said simple visual inspection is perhaps “the most important surveillance,” and with the pipeline on a bridge “anticipated to be heavily used by the residents of Nuiqsut,” it will be visible to residents crossing to work at Alpine and for subsistence hunting.
This is in addition to sophisticated equipment used by ConocoPhillips.
“None of these methods, which could be used to spot a leak at the Nigliq crossing quickly, can be used for an underground HDD installation,” Kozisek said.
Hydrology, including ice and scour While the HDD option may appear to be the most benign for the hydrology of the channel because it does not interfere with the flow path of the channel, “it does have an Achilles’ heel,” Kozisek said: where the pipeline goes underground.
Because of relatively flat terrain in the area, it is difficult to locate the transition to an area of higher elevation, which would protect it from floodwaters.
He said it would be “paramount to design HDD to withstand a higher flood stage, so that spring breakup would not impinge upon this vulnerable area, potentially rupturing the pipeline.”
Armored revetments are a possibility he said, or locating transitions on higher ground if practical, but “reduction of this risk by engineering countermeasures is not certain prior to more engineering being completed on this option.”
While the HDD option “appears moderately superior for resistance to problems caused by flooding, ice and scour,” this cannot be “fully determined until conceptual or preliminary engineering for an HDD crossing is finished.”
Internal and external corrosion The aboveground bridge option is “greatly superior to the belowground option” for internal corrosion because the HDD line has the potential for increased accumulation of water and sediment and increased corrosion they often cause, Kozisek said.
Many of the recent flowline and pipeline spills on the North Slope have involved corrosion, he said. “Correct engineering to reduce the likelihood of corrosion is paramount. It can be argued that many of the high-profile oil leaks from pipelines on the North Slope could have been prevented had the pipe segments been designed to reduce the accumulation of sediments.”
The aboveground design is superior for internal corrosion because at lower velocities some of the water and some sediments would drop out and remain in the bottom of the pipe and an HDD crossing at the Nigliq Channel could have an elevation drop of as much as 60 to 70 feet, “far greater than the drops in similar pipelines on the North Slope,” and about 10 times the elevation changes in road and caribou crossings, “which are known to be at higher risk for corrosion.”
He said that typical elevation changes in three-phase pipelines are from three to 12 feet.
The aboveground option is also preferred to HDD for external corrosion, Kozisek said, because of the “potential problems of accelerated corrosion due to groundwater” for an installed line “or coating damage during boring or installation.”
External corrosion of a buried line under a river has a greater range of risks, including unknowns in the in situ conditions and possible damage to the coating during installation.
Risks of corrosion at the Colville crossing were mitigated by design features, he said, and while design features have the potential to reduce external corrosion, they have limitations, and such technological fixes are “for increased risks that largely do not exist on the aboveground option.”
Reliability “The reliability (certainty of design) of an aboveground crossing is greater than that of an HDD crossing,” Kozisek said, with more work needed to prove an HDD crossing.
The HDD crossing under the Colville River done for the Alpine pipeline was, he said, to the best of his knowledge, the first HDD installation in a permafrost area — and remains the only one.
The “vast majority” of pipeline crossings on the North Slope are aboveground, “reinforcing the idea that it is the tried and true method.”
ARCO Alaska, which built Alpine and did the Colville HDD crossing, told the press that feasibility studies for the crossing took three years and cost $3 million, Kozisek said.
“The statement that an HDD crossing at the location (the Nigliq Channel) is feasible, without additional engineering analysis, is questionable,” he said.
And ARCO had two contingency plans: an extra winter to complete the HDD crossing as a cushion in the project schedule and an investigation of a pipeline bridge.
He urged caution in applying the experience of the Colville crossing to other sites, especially Nigliq.
“The geotechnical requirements of each site are different, and the two pipelines are fundamentally different,” Kozisek said, with Alpine transporting sales-quality oil processed to trans-Alaska oil pipeline specifications and the pipeline from CD-5 transporting three-phase fluid (oil, gas and water).
“The corrosion rates are typically much higher in three-phase pipelines and flowlines,” he said.
He noted an assumption by the Corps of Engineers that “the Colville River crossing is an analog proving the technical feasibility of the (Nigliq Channel) project.”
“This is an oversimplification and may be incorrect,” he said. “Flow studies, including dynamic modeling, need to be performed to see if an HDD crossing may be constructed and still provide adequate flow assurance. Flow of processed crude oil delivered by pumps is a much different situation than three-phase flow powered by well pressure.”
Overall, “the reliability (certainty of design) of an aboveground crossing is greater than that of an HDD crossing,” he said.
Containment, incident response Containment, the casing of a pipe within another pipe, appears an obvious way to add to the line’s integrity, Kozisek said, but may not be upon closer inspection, with most examples for situations very different than the Nigliq Channel.
Pipeline builders evaluated pipe-in-pipe options for Northstar, Liberty and Oooguruk, but those are not good analogs because they are “relatively shallow trenches in ocean environments.”
A better comparison is cased waterway, road and railroad crossings, but there the move has been from cased to uncased crossings, he said.
“The promises of secondary containment have to be balanced against the practical problems of barriers to inspection and monitoring and ‘indeterminate conditions’ that exist between the inner and outer walls,” Kozisek said. The designers of the Colville Alpine crossing attempted to address the issues but all were partial solutions, he said.
For both HDD and aboveground crossings at Nigliq there are advantages and disadvantages in the area of containment and additional engineering is needed to determine a clear choice, Kozisek said.
Because the aboveground crossing of the Nigliq Channel includes a high-quality vehicle bridge, access is provided for construction equipment and emergency to areas across the channel, making the aboveground crossing “a strongly preferred alternative in this category,” Kozisek said.
Overall, the aboveground crossing “has major advantages for environmental protection in six of seven subject areas,” he said.
From the perspective of a pipeline engineer, Kozisek said, “The decision is clear. For the Nigliq crossing, the aboveground crossing offers the greatest overall benefits for environmental protection because it best accommodates the all-important need to mitigate spill risks at a waterway,” thus providing the least environmentally damaging practical alternative for the site.
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