Something weird happened with the trans-Alaska pipeline recently.
Crews in Valdez, at the end of the 800-mile line, on Sept. 8 discovered a random hunk of steel inside what's known as a backpressure control valve.
The disk-shaped object, about 10 inches in diameter, had come down in the oil stream from roughly midway up the line, north of Livengood.
The discovery prompted Alyeska to launch an investigation, and to rally a significant repair to the pipeline at the point where the stray piece originated.
The incident did not result in any oil leak or pipeline shutdown, said Michelle Egan, spokeswoman for Alyeska Pipeline Service Co.
The pipeline has been in service since 1977, moving crude oil from the North Slope south to the tanker terminal at Valdez.
Alyeska was able to trace the steel piece to what it termed a threaded O-ring fitting, near pipeline Milepost 385, that previously had been welded over, according to an internal company note Egan provided to Petroleum News.
Milepost 385 is north of Livengood and south of the Yukon River. The pipeline runs above ground at that point. "The area in question includes a construction-era air vent on top of the mainline. In 2012, this vent was encapsulated to mitigate the risk of an unplanned leak," the Alyeska note said.
Most likely, a pig pushed the stray metal down the pipeline, Egan said. A pig is a device that travels through the pipeline to clean it or to test for corrosion or other problems.
Exactly how the piece broke free and entered the oil stream is unclear. "We just don't know at this point," Egan said. The incident didn't cause any known damage to the pipeline system, Egan said.
Alyeska put the area at Milepost 385 under round-the-clock surveillance, and said there was "no immediate threat of an oil spill."
Crews aimed to make repairs by the end of last week.
The plan was to install a sleeve, or metal covering, over the problem section of pipeline.
The old air vent, where the stray piece originated, had been used for a procedure known as hydrostatic testing in the days before any oil had even entered the pipeline, Egan said.
Workers in Valdez had been doing maintenance on the backpressure control valve, which hadn't been functioning optimally, she said.