LOS ANGELES (AP) – A freighter anchored in the Pacific is not a fixed point – it is different from parking a car. Even then, with a multi-ton anchor and muscular steel chains resting on the seabed, the huge ships can move in changing winds, ocean currents and tides.

An investigation continues into what caused an offshore pipeline rupture that dumped tens of thousands of gallons of crude oil off Southern California, but an emerging possibility is that a cargo ship – inadvertently or not – has dragged its anchor along the ocean floor, grabbing the steel, an oil pipe covered in concrete and pulling it 100 feet (30 meters) until it was punctured or cracked the way the pressure breaks an eggshell.

Federal transportation investigators said preliminary reports suggest the failure may have been “caused by an anchor that snagged the pipeline, causing a partial tear.”

“A ship at anchor will move around a bit as the tides and winds change direction,” said Steven Browne, professor of shipping at California State University Maritime Academy.

“One explanation would be that they didn’t drop anchor directly on the pipeline,” Browne said. “The vessel shifted and dragged the anchor along the bottom if it was not properly seated. He could have grabbed the pipe and dragged it. “

Many questions remain unanswered.

As the investigation continues, investigators have not yet clarified whether a ship has been ordered by port managers to anchor near the ruptured pipeline. Typically, a ship would receive specific instructions from port officials on where to anchor, and its position would be closely monitored.

Browne said he had never heard of a cargo ship dragging an oil pipeline, but is aware of cases in which telephone cables were lifted from the ocean floor. In such cases, the ship’s staff lower the anchor to free the problem, he said.

“Maybe they didn’t realize they were pulling anchor,” he added. In such a huge ship, “They wouldn’t necessarily know that something was stuck on the bottom.”

Marine traffic in the sister ports of Los Angeles and Long Beach is directed in the same way that air traffic controllers supervise flights entering and leaving airports.

The Marine Exchange of Southern California, in partnership with the United States Coast Guard, manages an area stretching 25 miles (40 kilometers) from the vast coastal complex where goods are unloaded and shipped across the country. It uses a range of technologies to plan the arrivals, anchorages and departures of thousands of ships each year. Computers monitor the speed of ships, and the traffic mirrors a freeway, with lanes for ships moving in different directions.

The forces of nature often come into play.

“Usually, if a ship is pulling anchor, it’s because of a weather event – a strong wind or a strong current,” Browne said. “Because we are talking about thousands of tons of steel and cargo, there is a lot of momentum involved in a ship.”

“A ship at anchor will move a bit as the tides go, the winds change direction,” he said.

Anchors on large ships can weigh 10 tons or more and are attached to hundreds of feet of steel chains. “Whatever the anchor gets fouled on will come with the ship,” Browne said.

Ports have been beset by long backups that have ships scattered on the horizon offshore. Browne said he wouldn’t be surprised if ships were anchored closer to pipelines, internet cables and other dangers “just because there are so many ships in the area.”

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