* Coast Guard says no timeline for reopening Houston Channel
* Exxon says channel closure has not affected Baytown refinery
* Second barge collision on Houston Channel this month (Updates with statements for Coast Guard commander, Exxon)
By Terry Wade
TEXAS CITY, Texas, March 22 (Reuters) - Four skimming boats surrounded a partially submerged barge on Sunday at the entrance to the Houston Ship Channel, working to clean up fuel spilled from the vessel into Galveston Bay after a Saturday collision with a cargo ship, according to a Reuters eyewitness.
The Houston Ship Channel remained shut on Sunday to contain the environmental damage and prevent additional collisions, said Coast Guard Capt. Brian Penoyer, who commands the maritime safety service's units in Houston and Galveston, Texas.
The Channel will remain shut "until clean water is assured," Penoyer told reporters at a news conference in Texas City.
Eight refineries accounting for nearly 12 percent of U.S. refining capacity are cut off from crude oil tankers while the 53-mile (85-kilometer) waterway linking the busiest U.S. petrochemcial port to the Gulf of Mexico is shut.
The channel closure has not affected production at Exxon Mobil Corp.'s 560,500 barrel per day (bpd) Baytown, Texas refinery, the nation's second largest, a company spokesman said on Sunday afternoon.
Penoyer said an estimated 4,000 barrels or 168,000 gallons (636,000 liters) of fuel oil were spilled due to the collision.
A local official said the channel was expected to be shut for all of Sunday and possibly Monday. The official asked not to identified as the information had not yet been made public.
The Coast Guard said crews completed pumping the remaining fuel oil in the barge's undamaged tanks onto another ship. Next, the Coast Guard will have to move the barge from it's location at the entrance to Galveston Bay.
The Ship Channel is 55-foot (17-meter) deep pathway for barges and deep-draft ships cut into the floor of Galveston Bay, which averages 20 feet (6 meters) in depth.
The closure also affected shipping along the Intracoastal Waterway where it intersects the Ship Channel.
Carnival Corp's Carnival Magic cruise liner was delayed on Sunday from returning to the Port of Galveston due to the spill, according to a Carnival spokesman.
Penoyer said the Coast Guard was making arrangements to allow the Carnival Magic and the Carnival Triumph, due in Galveston on Monday, to dock.
The Carnival Magic has supplies of food and water aboard for the guests, Carnival spokesman Vance Gulliksen said. The Carnival Magic was due to leave Galveston on Sunday afternoon.
The barge is operated by Kirby Inland Marine. A Kirby operated barge carrying fuel oil collided with a ship carrying rice at nearly the same location on March 14. In that accident, the cargo ship was damaged, but no fuel oil was spilled.
This is a far smaller spill than that by the Exxon Valdez tanker, which struck a reef in Prince William Sound, Alaska, in 1989.
A total of 11 million gallons of heavy black crude oil were estimated to have been released by the Exxon Valdez.
In contrast, only one tank on the barge was ripped open by the collision with the cargo ship in the Houston Channel on Saturday, releasing an estimated 168,000 gallons.
Wildlife Response Services, a Texas-based wildlife rehabilitation service, is helping affected birds and marine life. The service has not said how many animals have been received for cleaning.
In addition to four skimming vessels working on the spill, another 20 response vessels were standing by to help with the cleanup on Sunday, the Coast Guard said. About 90,000 feet of boom were staged along the Texas City dike for containment deployment.
Emergency response crews have also laid down floating barriers in hopes of containing the spill. (Reporting by Terry Wade in Texas City, Erwin Seba in Houston, Kevin Murphy in Kansas City, Mo., Alex Dobuzinskis in Los Angeles and Chris Michaud in New York; Editing by Edith Honan, Cynthia Osterman, Jan Paschal and Bernard Orr)