Responders to an oil release from a Hilcorp crude oil pipeline on the west side of Upper Cook Inlet stood down at 9 a.m Monday morning, according to an Alaska Department of Environmental Conservation press release, after the two oil platforms connected to the pipeline were shut in and the pipeline purged using a foam plug known as a pig.

On Saturday morning crew members at the Anna Platform, northeast of Trading Bay on Cook Inlet’s west side, felt an impact to the platform. According to a DEC situation report, they looked over the side at about 11:20 a.m and “observed sheen and bubbles coming up near one of the platform legs,” where a 8-inch diameter pipeline carries crude oil to a neighboring platform, Bruce. The pipeline’s capacity is about 461 barrels, and it was full at the time. Following the discovery, workers shut the platform in, dropping the pipeline pressure from 70 pounds per square inch to 5 pounds per square inch.

The release was reported to DEC at 12:05 p.m. A Hilcorp overflight at 12:30 p.m spotted six oil sheens, the furthest 3.5 miles south of Anna Platform, and the largest 10 feet by 12 feet.

Representatives of Hilcorp, DEC, and the U.S Coast Guard formed a Unified Command, based in Nikiski. DEC and Coast Guard personnel were onboard Anna Platform Sunday evening to observe the pigging operation that cleaned out and flooded by the 1.62-mile pipeline by forcing a polyurethane plug through it to squeeze the residual oil to the Bruce Platform, where a separate pipeline carries it to Hilcorp’s Granite Point Tank Farm. 492 barrels of filtered seawater were pumped in behind the plug, filling the pipeline.

After overflights at 7 p.m on Sunday and 7:50 Monday morning found no sheen, the Incident Command stood down at 9 a.m Monday, according to a DEC press release.

In a situation report released Monday afternoon, DEC estimated the spill at less than 10 gallons. Hilcorp put its upper estimate lower.

“Based on standard calculations for the number and size of the initial oil sheens and the amount of oil recovered from the line, Hilcorp estimates the total volume of this spill to be less than three gallons of oil,” Hilcorp Spokesperson Lori Nelson wrote in a statement released Monday.

The two platforms connected by the pipeline were built in 1964 and acquired by Hilcorp in 2011. According to a Hilcorp statement, the pipeline was given an inline inspection in June 2016, with visual inspections of some exterior portions.

Hilcorp spokesperson Nelson wrote in an email that at the time of the leak, the Anna and Bruce platforms were producing about 920 barrels of oil per day. The Bruce Platform remained active throughout the incident, sending oil to the Granite Point Tank Farm, according to a DEC situation report.

“A diving contactor is anticipated to arrive later this week to investigate the line and conduct repairs as necessary,” a Monday DEC report states. “Hilcorp will submit a repair plan to ADEC once the divers complete their initial assessment.”

The divers will go to work “as soon as it is safe to do so,” according to the Monday Hilcorp statement. Ice cover in Cook Inlet makes diving dangerous in Cook Inlet this time of year — a risk that’s also delaying repair of the unrelated methane leak from a Hilcorp natural gas pipeline in the eastern upper Inlet.

Reach Ben Boettger at benjamin.boettger@peninsulaclarion.com.