ANCHORAGE, Alaska (AP) - A federal agency investigating an underwater pipeline leaking natural gas in Alaska’s Cook Inlet is expanding its review to a nearby oil pipeline.

In a proposed safety order issued Friday, the Pipeline and Hazardous Materials Safety Administration said the 8-inch oil pipeline owned by Hilcorp Alaska LLC is subject to the same stresses as Hilcorp’s 8-inch natural gas pipeline and must be quickly inspected.

The natural gas pipeline since mid-December has spewed hundreds of thousands of cubic feet of processed natural gas into the inlet, home to endangered beluga whales.

The PHMSA called for inspection of the oil pipeline, which moves crude oil and water to shore for processing, by side-scan sonar or divers within 21 days of a final safety order. If Hilcorp cannot complete the inspection, the pipeline must be shut down, the agency said in its proposed order.

Hilcorp has 30 days to respond. Spokeswoman Lori Nelson said by email the company will work with state and federal and state agencies to ensure a timely response to concerns.

“Hilcorp continues to focus on addressing the natural gas pipeline leak and ensuring the safety of our responders in the field,” she said.

The leaking line supplies gas to four petroleum platforms. Hilcorp started looking for a leak in January, and on Feb. 7, a helicopter crew spotted gas bubbling to the surface about 4 miles offshore in 80 feet of water.

Hilcorp lowered the pressure in the line Monday and estimates the leak is down to 193,000 to 215,000 cubic feet daily.

Hilcorp purchased the pipeline and other oil and gas facilities from XTO Energy, Inc., in September 2015. Divers repaired two leaks for XTO Energy in the same line in summer 2014. Floating pans of ice make immediate repairs too dangerous, according to Hilcorp.

The PHMSA on March 3 issued an initial proposed safety order requiring the gas line to be repaired by May 1 or shut down.

The safety agency in its new proposed order said the crude oil pipeline is subject to the same threats as the leaking gas line: vibration, excessive bending in pipe that’s not supported by the sea bed, and contact with rocks.

A rupture of the oil pipeline could cause far greater environmental damage than the gas leak, the safety agency said.

The proposed safety order calls for high-resolution side-scan sonar inspection. For areas where the oil pipe is not supported by the ocean floor or more than 10 feet, the safety order calls for inspection by divers or their equivalent.

Hilcorp has said it has observed “no significant impacts to wildlife or the environment” from the natural gas leak.

Environmental groups say processed natural gas will create a low-oxygen dead zone threatening beluga whales, other marine mammals and fish. Two groups have given required 60-days’ notice that they intend to sue Hilcorp over the release.

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