What happened

Hilcorp's eight-inch underwater fuel-gas pipeline across Cook Inlet developed a small breach where the pipe rested against a large rock on the seafloor. Operations data later indicated the leak probably began in December 2016, although it was not identified until 7 February 2017. Estimates placed the daily release at roughly 210,000 to 310,000 cubic feet—about 5.9 to 8.8 million litres—of processed dry natural gas containing approximately 98.67 percent methane. Hilcorp said heavy sea ice made diving and repair unsafe and argued that depressurising the line could allow seawater into a system connected to oil-producing platforms. The line therefore continued feeding four platforms while the leak persisted in designated critical habitat for endangered Cook Inlet beluga whales. Environmental groups filed notices alleging violations of federal environmental law. Divers installed a temporary clamp that stopped the leak on 13 April, followed by a permanent repair in May. Reviewed sources did not establish a quantified injury to marine life, so the ecological risk is not presented as confirmed biological damage.

When cited sources give different casualty, damage or spill figures for this event, they’re reported side by side above rather than merged into one number. See how confidence levels work.