Operator Walter Oil & Gas reported a loss of well control Tuesday morning at well A-3 on the South Timbalier 220 block about 55 miles off Louisiana, the US Bureau of Safety and Environmental Enforcement (BSEE) said.

The well, near an unmanned platform in 154 feet of water, was not producing and was undergoing completion work for a sidetrack, BSEE said.

"The well is flowing gas and no oil is being released," BSEE said, adding that a quickly dispersing light sheen measuring a half-mile by 50 feet had been reported.

A Coast Guard spokesman said workers had been evacuated from the Hercules 265 jack-up to two life rafts and were later picked up by the Cheramie Marine offshore supply vessel M/V Max Cheramie. Some previous reports had put the count of rescued at 47.

No injuries have been reported and the appropriate authorities have been notified, a Walter spokesman told Upstream. The Coast Guard is also running overflights.

Walter is working with responders to determine the best course to shut in the well, the spokesman said.

"Efforts are ongoing with our client, Walter Oil & Gas, to mobilise the necessary resources to regain control of the well and minimise any potential impact on the environment," Hercules chief executive John Rynd said in a statement.

The Bethlehem JU-250-MC cantilever jack-up can work in up to 250 feet of water and drill up to 25,000 feet, according to Hercules.

The unit was booked for a 30-day contract with Walter at a dayrate of $101,000 to $103,000.

After shipyard time, it was next scheduled to start a 240-day contract with Arena at a rate of $102,000 to $104,000.

In a note to clients Cowen Securities said: "The rig had two 10,000 psi BOP's manufactured by Cameron on board, but the nature of the blowout is as of yet unknown (i.e., a surface or underground blowout)."

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Walter reports blowout, 44 evacuated