One US refinery is shutting while management takes control of operations at six others after union workers walked out of the plants in the biggest strike since 1980.

The United Steelworkers union that represents employees at more than 200 refineries, terminals, pipelines and chemical plants stopped work Sunday at nine sites, accounting for 10% of the country’s refining capacity, after contract negotiations fell apart. 

The union rejected five offers made by Royal Dutch Shell on behalf of companies including ExxonMobil and Chevron since talks began Jan. 21. Tesoro Corp. shut half of its 166,000-bpd Martinez plant in California that wasn’t already idled for maintenance.

Union leaders haven’t called a strike nationally since 1980, when a stoppage lasted three months. While only one of the nine plants has curbed production amid the stoppage, a full walkout of USW workers would threaten to disrupt as much as 64% of US fuel output. Shell and union officials began negotiations amid the biggest collapse in oil prices since 2008.

Refiners may cut operating rates ‘a little’ during the strike, said John Auers, executive vice president at Turner Mason & Co. “They won’t push the units as hard. The key will be to run them reliably and safely.”

The USW has been asking employers for “substantial” pay increases, stronger rules to prevent fatigue and measures to keep union workers rather than contract employees on the job, Gary Beevers, the USW international vice president who manages the union’s oil sector, said in an interview in Pittsburgh in October.

The union is also negotiating for better health-care benefits, saying workers are “paying too much” for health-care deductibles and premiums. “For a long time workers have paid 20% of the premiums and the employers have paid 80%,” said Lynne Hancock, a union spokeswoman in Nashville, Tennessee. “Over time this 20% becomes a larger amount of money as health care costs rise.”

United Steelworkers members do everything from operating units to performing maintenance to testing and analyzing samples in labs at US refineries, Hancock said.

The refineries on strike can produce 1.82 million bpd of fuel, data compiled by Bloomberg show. They span the US from Tesoro’s plants in Martinez and Carson, California; and Anacortes, Washington, to Marathon Petroleum’s Catlettsburg complex in Kentucky to three sites in Texas, according to the USW’s statement.

In Texas, Shell’s Deer Park complex, Marathon’s Galveston Bay plant and LyondellBasell’s Houston facility are affected, according to the union. LyondellBasell activated its work-continuation plan, according to spokesman George Smalley on Sunday.

US benchmark West Texas Intermediate oil was up $1.14, or 2.4%, to $49.38/bbl on the New York Mercantile Exchange at 11:58 a.m. New York time, erasing an earlier decline. It jumped 8.3% on Friday, the most since June 2012. Gasoline for March delivery rose 4.4% to $1.5432/gal and the diesel contract for the same month gained 3.1% to $1.7531.

Tesoro is shutting process units at Martinez, said Tina Barbee, a spokeswoman at the company’s headquarters in San Antonio. The facility already had about half its processing capacity offline formaintenance. The company has “successfully transitioned and are operating” the Anacortes and Carson refineries, Barbee said.

More refineries are standing by to join the sites on strike, according to two people familiar with the plan who asked not to be identified because the information isn’t public.

Shell remained “committed to resolving our differences with USW at the negotiating table and hope to resume negotiations as early as possible,” said Ray Fisher, a spokesman for The Hague, Netherlands-based company. Shell activated a “contingency” plan to continue operations at its Deer Park refinery, Fisher said.

Brandon Daniels, a spokesman for Marathon, confirmed work stoppages at its Catlettsburg refinery and Galveston Bay plant and said the company “has plans in place to ensure the continued safe operation.” The walkout also includes Marathon’s Houston Green cogeneration plant in Texas and Shell’s Deer Park chemical plant.

Remaining USW-represented sites are operating under rolling, 24-hour contract extensions, according to the union.

Gasoline in New York was at a premium of $15.55/bbl to crude at Cushing, Oklahoma, the delivery point for WTI contracts. That’s the highest profit from making the motor fuel since September.

“There will be a knee-jerk reaction in gasoline and diesel prices because we don’t know how long this is going to be or how extended it might be,” said Carl Larry, Houston-based director of oil and gas at Frost & Sullivan.

Refiners’ shares on the Standard & Poor’s 500 have more than doubled since the beginning of 2012, when the steelworkers last negotiated an agreement. US fuel producers have been cashing in on the biggest-ever domestic oil boom, which has helped drive oil prices almost 50% lower in 2014.