HOUSTON (Reuters) - Shippers and ports on the Gulf of Mexico are prepared for a worst-case oil spill impact, including vessel delays for cleaning, but ships so far had passed through the mess without contamination, officials said.

"We have seen no impacts at all," said an oil trader for a major refiner that receives oil via the Gulf and Mississippi River. "If the oil gets to the mouth of the Mississippi, we think there may be some delays, but nothing substantial."

There could be some delays for ship cleaning from New Orleans to Mobile, Alabama, if the spill spreads farther into shipping lanes, but officials hoped to avoid major slowdowns, said Judith Adams of the Alabama State Port Authority.

"We really need to try and keep commerce moving but also protect the environment," she said.

Onshore preparations accelerated last Friday as oil kept spewing after the blowout and explosion of a BP Plc well in the Gulf off Louisiana April 20, killing 11 workers, sinking a drilling rig and spawning a giant oil spill.

A practice run cleaning a ship Friday went smoothly at Southwest Pass, deepwater entrance to the Mississippi River and New Orleans, officials said.

Work continued installing a boom system with a gate for ships at the entrance to Mobile Bay in Alabama. Sensitive areas were being boomed off without blocking ship channels at Pascagoula and Gulfport, Mississippi.

Louisiana Offshore Oil Port officials said oil had drifted through LOOP's security zone last Thursday but they did not expect to halt operations.

Winds, currents and a rising Mississippi River – due to flooding rains upstream – appeared favorable Friday for keeping the spill away from key Gulf shipping lanes, but officials cautioned conditions could change.

The spill had not seriously threatened the Mississippi-Alabama coast as of Friday, but it was onshore in parts of Louisiana, though it had not seriously marred the Mississippi River entrance as of Friday.

The practice cleaning of a vessel at Southwest Pass last Friday was intended to streamline a system "almost like a carwash," said Captain Jason Bosley of the Associated Branch Pilots.

Sheen contamination can be hosed off in an hour or two, but dirtier ships might have to anchor at Pilottown or Boothville, Louisiana, and face delays up to two days, officials said.

Two cleaning boats were put at Southwest Pass, but more resources could be made available, and cleaning stations were being set up at the other ports. One boat at the pass was dedicated to outbound ships to clean them for their next stop.

"This is not going to be voluntary. If a ship has oil on it, it is going to be cleaned," Bosley said, but he added "every effort is being made" to keep the process speedy.

One ship at Mobile and at least two others at the mouth of the Mississippi reported passing through the spill but were inspected and found not to be contaminated.

They may have benefited from the fact that the oil has been treated with dispersants, which are supposed to make the oil less sticky, Bosley said.

In Alabama, a double boom was being stretched across the Mobile Bay entrance, anchored by pilings and featuring tugboat-driven gates to let ships pass without letting oil into the bay, Adams said.

"What we're trying to do is protect the environment, but we still have a business to run," Mississippi State Port Director Don Allee said.