Published: Apr 23, 2010 11:03 PM EDT

 NEW ORLEANS, La. - No oil appeared to be leaking after a drilling rig exploded and sank in the Gulf of Mexico, the Coast Guard said Friday, though officials were trying to contain what spilled from the blast and prevent any threat to the coast's fragile ecosystem.

The search continued for 11 workers missing after the explosion late Tuesday on the Deepwater Horizon, though family members said they had been told they probably did not survive.

The rig burned for nearly two days until it sank Thursday morning. The fire was out, but officials initially feared as much as 336,000 gallons of crude oil a day could be rising from the sea floor nearly 5,000 feet below.

Coast Guard Rear Adm. Mary Landry said Friday morning that no oil appeared to be leaking from a well head at the ocean floor, nor was any leaking at the water's surface. But she said crews were closely monitoring the rig for any more crude that might spill out.

The crew was finishing the well about 50 miles off the Louisiana coast when the rig exploded. Officials have not said what caused the blast, and the oil they are dealing with now is left over from the explosion and sinking.

"If it gets landward, it could be a disaster in the making," said Cynthia Sarthou, executive director for the environmental group Gulf Restoration Network. BP PLC, which leased the rig and took the lead in the cleanup, said Friday it has activated an extensive oil spill response, including using remotely operated vehicles to assess the well and 32 vessels to mop up the spill.

BP Chief Executive Tony Hayward said the company will do "everything in our power to contain this oil spill and resolve the situation as rapidly, safely and effectively as possible." Ed Overton, a Louisiana State University environmental sciences professor, said he expects some of the light crude oil to evaporate while much of it turns into a pasty mess that ultimately breaks apart into small chunks of oily residue that can wash ashore.

"It's going to be a god-awful mess for a while," he said. "I'm not crying doomsday or saying the sky is falling, but that is the potential." Weather forecasts indicate the spill was likely to stay well away from shore at least through the weekend, but if winds change it could come ashore faster, said Doug Helton of the National Oceanic and Atmospheric Administration's office of response and restoration.

The Coast Guard, which was leading the investigation, had not given up the search early Friday for those missing from the rig. Lt. Cmdr. Cheri Ben-Iesau said a Coast Guard cutter would remain on the scene Friday after searching overnight, and a helicopter would take advantage of clear weather to perform three more search flights Friday.

"We use a scientific program to make a best-guess estimate on survivability," Ben-Iesau said, adding, "And then the Coast Guard searches a little longer than that. Because there's always the unknown."

Carolyn Kemp of Monterey, La., said her grandson, Roy Wyatt Kemp, 27, would have been on the drilling platform when it exploded. "They're assuming all those men who were on the platform are dead," Kemp said. "That's the last we've heard."

Most of the crew -- 111 members -- were ashore, including 17 taken to hospitals. Four were in critical condition. Four others made it off safely were still on a boat operating one of several underwater robots being used to assess whether the flow of oil could be shut off at a control valve on the sea floor, said Guy Cantwell, spokesman for rig owner Transocean Ltd.

Landry said crews first saw a 1-mile-by-5-mile rainbow sheen of what appeared to be a crude oil mix on the surface. That had expanded to 10 miles by 10 miles as of Friday morning, Ben-Iesau said. Helton said it is probably good that the sheen is spreading because it will be exposed to more waves and sunlight that will help break it down. But he said a wider area also means a greater chance that seabirds and marine mammals will be affected.

Overton said the sheen's distance from shore means the impact on wildlife is likely not widespread, although some seabirds that dive for food could become coated with oil. The problem, Overton said, would be if thicker globs of oil reach coastal areas such as the Chandeleur islands, home to hatcheries for pelicans and other birds.

At the worst-case figure of 336,000 gallons a day, it would take more than a month for the amount of crude oil spilled to equal the 11 million gallons spilled from the Exxon Valdez in Alaska's Prince William Sound. A turn in winds and currents might send oil toward fragile coastal wetlands - nurseries for fish and shrimp and habitat for birds.

"As you get closer to shore, you get richer and richer marine habitats, and also get the potential for long-term exposure," Helton said. To prevent that, the Marine Spill Response Corp., an energy industry cleanup consortium, brought seven skimmer boats to suck oily water from the surface, four planes that can scatter chemicals to disperse oil, and 500,000 feet - 94.6 miles - of containment boom, a floating barrier with a skirt that drapes down under the water and corrals the oil.

In addition to other environmental concerns, the well is in an area where a pod of sperm whales is known to feed, said Kim Amendola of NOAA. Those who escaped the rig did so mainly by getting on lifeboats that were lowered into the Gulf, said Adrian Rose, vice president of Transocean.

"There are a number of uncorroborated stories, a lot of them really quite heroic stories, of how people looked after each other. There was very little panic," Rose said. Family members of two missing workers filed separate lawsuits Thursday accusing Transocean and BP of negligence. Both companies declined to comment about legal action against them after the first suit was filed.

The U.S. Minerals Management Service, which regulates oil rigs, conducted three routine inspections of the Deepwater Horizon this year -- in February, March and on April 1 -- and found no violations, MMS spokeswoman Eileen Angelico said.

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