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Response team finds source of oil leak Overpressure from expanding ice plugs likely caused 24-inch rupture in pipeline between Lisburne drill pad and production center Alan Bailey Petroleum News
In a situation analogous to the bursting of a frozen water pipe during the winter, it appears that pressure from expanding ice plugs inside the 18-inch pipeline that carries three-phase fluids from Lisburne drill pad L-3 to the Lisburne Production Center caused the oil spill discovered Nov. 29 at one of the pipeline’s vertical support members. The Lisburne field lies within the BP-operated Prudhoe Bay unit on Alaska’s North Slope.
Safety concerns In the initial response to the spill, the response team had been unable to examine the leak site on the pipeline because of concerns that possible high fluid pressures caused by ice plugs in the pipeline might place the responders at risk. However, after making x-ray images of the line to determine the positions of ice plugs and fluids, specialists were able to determine that it was safe to approach the area of the leak, albeit with appropriate precautions for possible toxic fumes.
So, on Dec. 5 responders started entering a previously off-limits 40-foot diameter safety zone around the spill source, to start removing snow that was heavily contaminated with oil and produced water — the responders had previously cleared away more lightly contaminated snow farther out from the spill location. On Dec. 7, when it became possible to closely examine the pipeline, a visual inspection revealed a 24-inch lengthwise rupture in the line, with the rupture displaying characteristics consistent with pressure inside the line having ripped the line open.
It appears that expanding internal ice plugs on either side of the rupture point had built up the fluid pressure in the line, BP spokesman Steve Rinehart told Petroleum News Dec. 8.
“We had ice plugs on either side of this leak. There’s roughly 1,300 feet in between the ice plugs,” Rinehart said. “… Ice expands as it freezes and those ice plugs growing on each side would … have exerted significant pressure on that material between them.”
In fact, the x-ray survey has indicated extensive ice plugging in the line, with one ice plug measuring 1,500 feet in length.
46,000 gallons On Dec. 5 the spill response unified command published an estimated spill volume of about 46,000 gallons, a volume determined using mapping and surveying techniques applied at the spill site. That preliminary estimate will be later superseded by a more accurate estimate based on volumes of material recovered from contaminated snow. Fluid is no longer leaking from the pipeline.
The majority of the spilled material had semi-frozen into a large dome-shaped mass occupying about one-quarter of an acre around the pipeline leak. Another half-acre of snow around the heavily contaminated area had been impacted by light oil misting.
The spilled fluid consists of a mixture of crude oil and produced water. BP does not know the relative proportions of oil and water in the spilled material, but the damaged line typically carried a mix of 75 percent water and 25 percent oil, Rinehart said.
The estimated spill volume can be compared with the approximately 200,000 gallons of crude oil spilled in the largest North Slope spill, the 2006 Prudhoe Bay field spill that resulted from corrosion in a transit line between a Prudhoe Bay gathering center and pump station 1 of the trans-Alaska oil pipeline.
Rinehart declined to speculate on how the ice plugs had formed in the Lisburne line, saying that an investigation into the cause of the problem was already under way. The failed line is paired with a parallel 24-inch line carrying Lisburne three-phase fluids along the same route, with a temperature sensor at the downstream end of each line — BP has assessed its other North Slope pipelines and has determined that the same ice-plug situation is not developing elsewhere, Rinehart said.
“We’ve got other lines that operate in pairs or are looped, but we’ve got the systems in place to monitor them,” he said.
There are means of dealing with ice plugs, an uncommon but not unknown problem in Arctic pipelines, he said.
In parallel with determining how the Lisburne spill happened, response crews have been clearing away contaminated snow from the spill site. Responders have been scooping the material into bins using small mechanical loader vehicles called bobcats, Rinehart said. And according to a response situation report, the material is being moved to an ice pit at the Prudhoe Bay East Dock, for later melting and measurement. Presumably, the contaminants will then be disposed of by recycling them through the field production facilities.
The responders have also placed on site a steamer device with a 4-foot square stainless steel head, to melt the contaminated snow for removal using a vacuum truck. However, mechanical removal using bobcats is proving to be the most successful technique, Rinehart said.
BP is building an ice road from the spill site to enable trucks to remove the bins of excavated material but is meantime shifting the material by rollagon, a special tundra-certified vehicle, Rinehart said.
The damaged 18-inch pipeline was out of operation at the time of the spill and BP has been able to maintain production from the Lisburne field through the parallel 24-inch line.
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