Energy companies braced for the worst storm to threaten the US Gulf of Mexico oilpatch since 2005's devastating hurricanes as Tropical Storm Gustav churned toward the heart of US offshore production.

No company had reported output cuts as they began evacuating staff from offshore oil and natural gas platforms, but Shell Oil Co, which has the largest offshore operations, said it may begin shutting output as early as Thursday while it works to evacuate all of its 1,300 Gulf workers by Saturday.

Gustav is expected to morph into a powerful hurricane as it gains strength from the Gulf's warm waters, and about 85 percent of US offshore oil and gas production could be in its path.

By midday Wednesday, weather forecasters were saying the storm could, as did Katrina and Rita, become a catastrophic category 5 hurricane with winds over 155 mph (249 kph) as it crosses the Gulf, which provides a quarter of US crude oil production and 15 percent of the nation's natural gas output.

"We do believe Gustav is going to emerge into the Gulf as a major hurricane, category 3 or better," said AccuWeather Inc Forecasting Director Ken Reeves. "It has a chance for a brief period to be a category 5."    
"Whether or not it can produce the same amount of damage as Katrina or Rita remains to be seen," Reeves said.

In addition to devastating the Louisiana coast, including the city of New Orleans, Katrina and Rita shut 25 percent of US oil and fuel production. Gulf energy companies needed months to restore operations close to their full capacity.

"I expect somewhere near 85 percent of the Gulf energy infrastructure will be shut-in along with a significant increase in damage potential," said Planalytics forecaster Jim Rouiller in a release.

Computer models see the storm striking land near refining centers along the upper Texas or Louisiana coasts.

ConocoPhillips said its Magnolia platform in the Gulf was shut for a planned overhaul on Monday and 78 workers will be evacuated by Saturday. Transocean Inc said it has pulled 30 workers off drilling rigs.

BP said it was pulling nonessential workers from the Gulf. Other companies were preparing to remove staff as well, according to a helicopter company involved in evacuations.

"Starting today, we're taking out nonessentials for most of the major oil companies," said Jim Shugart, executive vice president of sales for ERA Helicopters. - Reuters