Nearly four months after an underwater pipeline began leaking almost pure methane into Alaska’s Cook Inlet, Hilcorp Alaska announced on Friday that a temporary repair has stopped the leak.

“The clamp assures a gas tight, liquid tight seal that will reinforce the pipeline,” Hilcorp said in a press release. The next step will be to send divers back down to make a permanent repair.

The company had gradually decreased the amount of gas flowing through the leaking pipeline, but for much of those four months, it was releasing more than 200,000 cubic feet of natural gas into the inlet each day. Not much is known about the impacts of a methane leak on a marine environment, but the leak alarmed regulators, scientists and environmentalists because Cook Inlet is home to endangered beluga whales.

There was no environmental monitoring until mid-March, when Hilcorp reported finding low oxygen and high methane levels at some sites near the leak. Those results were deemed incomplete, however, and the state wrote to Hilcorp that its samples did not appear to have been taken at the “maximum most probable concentrations from the bubble field.”

The divers have been able to determine that the leak was caused by a boulder, said Kristin Ryan, the director of spill prevention and response at the Alaska Department of Environmental Conservation. A three-foot-by-three-foot boulder appears to have rolled over the pipeline, causing it to bend. At the bottom of the bend, there is a small crack, roughly three-sixteenth of an inch long by three-eighth of an inch wide.

Ryan said it wasn’t surprising a boulder cracked the line. “Historically that’s what has happened on that line before,” she said. Cook Inlet is known for violent currents and some of the strongest tides in the world, meaning the water moves rapidly and with great force. As the seabed shifts below a pipeline, the line can be left hanging, leaving it vulnerable to battering. There were two such leaks on this pipeline in 2014, before Hilcorp owned it.

Now that the leak has been stopped, Bob Shavelson of the nonprofit Cook Inletkeeper said he’s concerned about the company’s other operations in the state. “If it takes Hilcorp months and months to shut in a leaky line, we need to re-evaluate whether they can operate in winter,” he said.

Hilcorp’s business model is to buy older oil and gas infrastructure from other companies. It’s a model that has paid off. The company, founded in 1989, is one of the largest privately owned oil and gas companies in the world.

Hilcorp owns much of the oil and gas infrastructure in the inlet. Most of it, including the cracked natural gas line, is more than 50 years old.

Its recent problems in Cook Inlet have raised questions about whether these old pipelines can continue to function safely.

Since identifying the pipeline leak on Feb. 7, the following things have happened:

• The federal Pipeline and Hazardous Materials Safety Administration ordered Hilcorp to repair the pipeline by May 1 and required a comprehensive safety inspection of the line.
• PHMSA later issued an order requiring additional inspections of a nearby oil pipeline. The agency said conditions on the line existed that could “pose a pipeline integrity risk to public safety, property or the environment.”
• After talks with Gov. Bill Walker, Hilcorp shut oil production on the two platforms that are powered by the gas in the pipeline and lowered pressure in the line by more than half.
• On April 1, Hilcorp employees on another oil platform, the Anna Platform, reported feeling an impact and then observed a small oil sheen. The company has said that less than three gallons of oil leaked. Subsequent inspections of the line determined that it was not a pipeline leak but involved the temporary use of oil in the flaring process.
• Less than a week later, on April 7, the company reported a third problem on a different natural gas pipeline after discovering a leak. Hilcorp immediately shut the line and PHMSA is investigating.

Now that the leak has stopped, the agencies can shift from spill response to investigating what happened and why.

Ryan said she expects her agency to review all existing infrastructure within Cook Inlet.