HOUSTON (Reuters) - Royal Dutch Shell, Anadarko Petroleum and Exxon Mobil announced they were curbing some oil and gas output on Wednesday at facilities in the Gulf of Mexico ahead of a storm expected to hit the Texas coast later this week.

The U.S. National Hurricane Center (NHC) issued a hurricane watch Wednesday for much of the Texas coast, calling for slow-moving Tropical Depression Harvey to intensify as it nears landfall.

Shell said it was evacuating all personnel from the roughly 100,000 barrel-per-day (bpd) Perdido oil and gas production platform as a precaution. Anadarko said it had shut in production and was evacuating workers from its Boomvang, Gunnison, Lucius and Nansen platforms in the Gulf of Mexico.

Exxon was in the process of reducing production at its Hoover facility in the Gulf of Mexico, company spokeswoman Suann Guthrie said.

The company said it was also working on transportation plans for staged evacuation of its personnel from its offshore facilities, expected to be in the path of the storm, to shore.

The U.S. Gulf of Mexico is home to about 17 percent of the nation’s crude oil output and 5 percent of dry natural gas output, according to the U.S. Energy Information Administration. More than 45 percent of the nation’s oil refining capacity is along the U.S. Gulf Coast.

Harvey, which had top sustained winds of about 35 miles (55 km) per hour as it churned over Mexico’s Bay of Campeche late Wednesday, could become a hurricane by Friday, the NHC said in an advisory.

Between 10 inches and 15 inches (25 cm to 38 cm) of rain are expected from the central Texas coast to southwest Louisiana, with some areas receiving up to 20 inches of rain, according to the Miami-based NHC.

“Heavy rainfall is likely to spread across portions of eastern Texas, Louisiana, and the lower Mississippi Valley from Friday through early next week and could cause life-threatening flooding,” the NHC said.

Texas Governor Greg Abbott declared a state of disaster for 30 counties ahead of the storm.

In Corpus Christi, city officials warned residents to expect a storm surge of between four feet and six feet (1.83 m) and began distributing sand bags to local residents.

Other Gulf of Mexico oil and gas operators said they were watching developments closely but operations were unaffected.

Chevron on Wednesday afternoon said it had not evacuated any staff and BP Plc said normal operations were still underway. ConocoPhillips said it was making preparations but had not interrupted any operations.