Officials have found a 24-inch jagged rupture in a pipeline that began pouring oil and water Nov. 29, creating one of the biggest North Slope crude oil spills ever.
The on-scene coordinator for the state Department of Environmental Conservation, Tom DeRuyter, said Tuesday that the breach on the bottom of the pipe was the biggest he had ever seen and indicative of the incredible pressure the pipeline was under when it split.
Workers located the source of the leak Monday after cleanup crews hauled away spilled crude and contaminated snow and ice that had been obscuring the area.
Officials say massive ice plugs had formed inside the pipe, which caused BP Exploration (Alaska) Inc. to stop operating it a few weeks ago. Pressure then built up until the pipeline ruptured, according to BP.
"It looks like it was caused by overpressure in the pipe, which we think was linked to ice forming -- the plugs that have formed on either side of the release site," BP spokesman Steve Rinehart said.
Most likely, rapidly forming ice plugs began to grow toward one other, creating a high-pressure area in between, DeRuyter said.
"When a line does that, it rips out with a pretty impressive force and with a very large hole."
As for the specific circumstances that caused the line to fail, those are still under investigation, Rinehart said.
The working estimate of the spill's size is about 46,000 gallons of crude and produced water, the oily water pumped up from the well. The line is no longer leaking. Cleanup operations are well under way, according to state and federal officials.
The 18-inch flow line had carried a mix of oil, water and natural gas from the wells to the Lisburne Production Center, where the materials were separated. About 25 percent of the material was oil, according to BP.
The line had its own temperature sensor at the point it entered the production center, and the manner in which that sensor was monitored will be part of the investigation, Rinehart said.
The line was paired with, and cross-connected to, a bigger, 24-inch line that drew from the same wells and also carried the materials to the processing center, Rinehart said. That line was temporarily shut down. Rinehart said it now is back in operation.
BP engineers do not believe other pipes are similarly threatened, Rinehart said.
"We have looked at the rest of our facilities to see if there are other lines like this one that are vulnerable to this happening. We don't think there are," he said. BP runs most of the North Slope's oil fields on behalf of itself and other oil companies.
The 18-inch ruptured pipeline and its companion had a unique configuration, he said.
"Our other loop lines, we have checked the rest of those, and they are equipped with sensors and alarms that will alert operators if conditions change," Rinehart said. "This pair stood apart in how they functioned and were used together."
Matt Carr, the on-scene coordinator for the federal Environmental Protection Agency, said the pair of lines operated almost like a single pipe, which was unusual.
A FROZEN PIPELINE
The big tear in the pipeline contrasts with the small holes, pinpoint or pinkie size, that occur from corrosion, Carr said. Officials have not yet tested the pipeline itself to see if corrosion was also a factor.
BP began X-raying the failed line on Dec. 3, four days after discovering the spill, to check for ice plugs and assess whether it would be safe for cleanup crews to get close to the pipe. They found it was full of ice, a frozen pipeline. One plug was more than one-quarter of a mile long.
"Analysis of the initial images indicates extensive ice plugs within the 18-inch line; one ice plug measured approximately 1,500 feet in length," officials said in a situation report over the weekend.
The ice plugs are holding in any remaining pressure, officials said, meaning it is safe for cleanup workers to get close to the pipe, something that had been under question. A 40-foot-diameter safety zone centered at the rupture has been lifted, allowing access to the worst-contaminated areas. But heavy equipment still must stay 10 feet away.
Crews have been trying different methods of cleanup. They've used a steamer with a four-foot head to melt contaminated snow, which is then vacuumed up. They've flushed out the spill site with warm water. They've used Bobcats to scoop up the mess and vehicles called Rolligons with huge, low-pressure tires to haul it away. They're building an ice road for trucks, though the weather at times has been too warm for the ice road work.
"They are steadily whittling away at this," Rinehart said.
DEC officials are compiling information about other large spills but, based on past reports, this one appears to be one of the biggest.
The biggest oil spill ever on the North Slope occurred in 2006 when more than 200,000 gallons of crude leaked from a corroded transit line at the Prudhoe Bay field. That led to a criminal misdemeanor conviction for BP, $20 million in fines and restitution, and three years of probation, which the company is still on.
Find Lisa Demer online at adn.com/contact/ldemer or call 257-4390.Photos: Lisburne oil spill
By LISA DEMER