Procedures for Using Mooring Where Spill Occurred Are Under Study : Investigation: Depth of the water at the mooring and the ship’s draft are key elements in trying to isolate the cause of the oil spill off Huntington Beach.

Safety requirements for tankers using the offshore mooring where the American Trader spilled 394,000 gallons of crude oil last week are under investigation, U.S. Coast Guard officials said Tuesday.

Coast Guard investigators and documents from the mooring’s owner indicate that the fully laden tanker arrived at the mooring at the maximum allowable water depth when it hit its own anchor and began spilling Alaskan crude oil.

Although the American Trader appears to have been within the safety limit, Coast Guard officials said Tuesday that the mooring’s 51-foot depth and the draft of the ship--the part under water--have raised serious questions about whether large tankers have any room for error at the offshore terminal.

A manual for tankers that use the offshore mooring states that ships with drafts of more than 43 feet cannot use the facility, located 1.3 miles off Huntington Beach. Coast Guard investigators said Tuesday that the draft of the 811-foot American Trader was 43 feet when the tanker bounced on its port anchor.

“Investigators will be looking at the manual and anything the companies use as their standard operating procedures,” said Chief Warrant Officer Dan Dewell at Coast Guard headquarters in Long Beach.

Coast Guard officials said they were particularly concerned about the mooring’s depth. Tuesday they placed restrictions on shipping, to stay in effect at least until the ocean bottom around the pipeline hookup is fully surveyed and recommendations are made.

Investigators want to ensure that navigation charts for the facility are accurate and that no uncharted bottom changes or sand bars are menacing the larger tankers.

Under Capt. James C. Card’s order, tanker barges and ships with drafts of less than 20 feet will be the only vessels allowed to use the mooring.

The mooring is owned by Golden West Refining Co. of Santa Fe Springs, which has issued a manual of detailed procedures for its offshore facility. In 100-plus pages, the manual describes maximum sizes of ships that can safely use the facility. It gives instructions for the mooring master and details oil-spill procedures.

The Coast Guard, which is still investigating the accident, has not determined exactly who is responsible for the spill that has covered miles of beach with oil the consistency of paint, and with a frothy mix of oil and water known to cleanup crews as “mousse.”

Michael J. Murphy, vice president of American Trading Transportation Co., the owner of the tanker, met Tuesday with officials from British Petroleum, the Coast Guard and other agencies aboard the American Trader, now berthed in Long Beach. No fewer than seven theories about the mishap were advanced at the meeting.

Although Murphy would not discuss the possible causes in detail, he said the theories include such possible variables as an unexpected shift in current and an undetected object on the ocean bed.

The mooring master, John E. Keon of San Jacinto, said he is not to blame for the mishap and there was no low tide at the time of the accident, as Coast Guard officials have suggested.

Mooring a tanker for unloading is a two-to-three hour process that involves coaxing the ship into a 700-foot wide, U-shaped cluster of seven buoys.

Tanker crew members told investigators that the American Trader drifted back over its port anchor, and the chain apparently wrapped around the bow. Coast Guard officials believe that as the crew started to raise the anchor to back out of the mooring, the ship came down on the anchor as it rested on the ocean floor.

Charts show that it was low tide at the time, and the ship’s 12-ton anchors are at least 10 feet long. There was a three-foot to four-foot ocean swell and the ship was rolling a few degrees, all of which can reduce the amount of water beneath the hull, according to Coast Guard officials.

“It (the tanker) had the potential for being right on the bottom,” Lt. Cmdr. Alan Carver said.

Times staff writer Eric Lichtblau contributed to this story.