HOUSTON (Reuters) - BP Plc on Wednesday became the first major oil producer to say it was evacuating some workers from Gulf of Mexico oil and gas platforms because of a tropical disturbance that could become a named storm this week.

Later on Wednesday, Anadarko Petroleum Corp joined BP saying it has evacuated non-essential workers from three platforms in the Gulf.

Royal Dutch Shell also was preparing to evacuate some workers and expected to decide whether to move forward with that during the day, spokeswoman Kelly op de Weegh said.

Shell was monitoring the system over the northwestern Caribbean Sea and the eastern Gulf. The National Hurricane Center said the weather formation had a 30 percent chance of becoming a cyclone in the next two days.

Chevron Corp, Exxon Mobil Corp, ConocoPhillips and Apache Corp also said they were monitoring the system.

BP, the biggest oil producer in the Gulf with eight company-operated oil and gas platforms, said it was evacuating from five platforms more than 500 "nonessential" workers, or those not directly involved in production, such as cooks.

The platforms involved are Thunder Horse, the world's largest with capacity to produce up to 250,000 barrels of oil per day, and Na Kika, Mad Dog, Holstein and Atlantis.

"Essential personnel remain on board to continue safe operations and prepare the platforms for potential shut-in and full evacuation in the event it becomes necessary," the company said in a statement.

Anadarko said the workers were taken from the Gunnison, Nansen and Boomvang platforms.

"We will continue to monitor the path of the weather and are prepared to evacuate all personnel and shut in production if necessary," Anadarko said in a statement.

Shell is the second-largest producer in the Gulf, operating six oil and gas platforms.

Mexico's state oil monopoly Pemex said it was monitoring the weather but not yet taking any emergency or precautionary steps.

The U.S. National Hurricane Center gives the disturbance, which would be called Lee if it becomes a tropical storm, a 60 percent chance of becoming a tropical cyclone in the next 48 hours, but said it could develop further when it reaches the western Gulf.

U.S.-regulated areas of the Gulf of Mexico roughly account for 30 percent of U.S. oil production and 12 percent of natural gas output, according to the U.S. Bureau of Ocean Energy Management.

About 30 percent of U.S. natural gas processing plant capacity and 40 percent of the nation's refining capacity also line the Gulf Coast, according to the U.S. Energy Information Administration.

Forecasters have said the 2011 hurricane season will spawn six to eight hurricanes, up to six of which half could become major.

(Editing by Dale Hudson, Bob Burgdorfer and Bernard Orr)

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