Federal regulators say they are investigating a third Hilcorp pipeline leak in Cook Inlet

Federal pipeline regulators said Friday they are investigating a third recent leak from a subsea pipeline in Cook Inlet — this one involving produced natural gas from a pipeline at the Steelhead platform — although the platform operator said Friday that several helicopter flights spotted no release.

Darius Kirkwood, a spokesman with the U.S. Pipeline and Hazardous Materials Safety Administration, said the agency confirmed on Friday that there was a leak.

And Hilcorp Alaska, the operator, said in a statement that flights were conducted after gas meters reported a "discrepancy" in the volume of gas shipped from the Steelhead platform to a shore-based processing facility on the west side of Cook Inlet. The discrepancy was between a meter on the platform side of the pipe and a meter on the shore side.

"As a precaution, we started emptying the (Steelhead) A pipeline of natural gas on Saturday, and that process was completed early Monday morning," Hilcorp said in a statement released by Lori Nelson, external affairs manager at Hilcorp Alaska. "The line now contains filtered seawater."

"There is no risk of the discharge of oil or gas into the Inlet with the line in its current state," Hilcorp's statement said.

Nelson said she did not know the size of the "discrepancy." The company's announcement did not state whether a leak had occurred.

The announcement comes as the company faces increased scrutiny from watchdog groups and state and federal regulators.

On Feb. 7, Hilcorp discovered a natural gas leak from a pipeline delivering fuel gas to Platform A northwest of Nikiski.

Also, on Saturday, workers on the Anna platform in western Cook Inlet discovered a small release of crude oil following what felt like a violent impact of a natural object to the platform. Several sheens and bubbles were spotted. Hilcorp quickly shut down oil production at the Anna platform and filled that line with filtered seawater to stop oil from leaking.

Inspections of the oil line, including an integrity review from inside the pipe in June, found that line to be in sound condition, Hilcorp said Friday.

"Additionally, the small volume of the release (estimated to be less than 3 gallons) suggests that the pipeline may not be the root cause" of the sheen, the company said. "Whether the cause is found to be the pipeline or otherwise, appropriate action will be taken."

Hilcorp said Friday that the leaking natural gas line to Platform A, in the Middle Ground Shoal field, prompted the company to begin a comprehensive review of all its pipelines in Cook Inlet. The review included the pipeline at the Steelhead platform.

The Steelhead platform was built in 1986. The pipeline delivers natural gas from the Trading Bay Unit to a production facility on the west side of Cook Inlet.

The causes of the leaks are not known, but many of the platforms and related facilities were built decades ago starting in the mid-1960s, prompting critics to suggest that aging facilities in the silty, fast-moving corrosive water is a key factor. Hilcorp began acquiring properties in Cook Inlet in 2011 and is now the dominant producer there. Conservation groups are seeking an inspection by regulators to assess the condition of all facilities in the Inlet.

Hilcorp plans to send divers down as early as Saturday to begin repairing the leaking gas line northwest of Nikiski, if conditions permit. Sea ice has prevented repairs so far this winter by the threat it posed to divers' surface tethers.

Kirkwood, with PHMSA, said he didn't have estimates of the latest leak on Friday.

"I can confirm the pipeline was shut down and that we are investigating," he said.

Hilcorp said it will "leave the line (at Steelhead) filled with seawater until a later time when we are able to further investigate and address the meter discrepancy."