A cargo ship anchored in the Pacific isn’t a fixed point. It’s not like parking a car. The massive vessels can still move in shifting winds and ocean currents, even though they are anchored with a multi-ton anchor.

Investigating the cause of the offshore pipeline rupture that spilled tens to thousands of gallons crude oil off Southern California is ongoing. However, one possibility is that a cargo vessel dragged its anchor along an ocean floor and caught the steel-covered concrete oil pipe. Then it pulled it over 100 feet (30 m) until it was cracked or pierced the same way pressure cracks an egg shell.

Federal transportation investigators stated that preliminary reports indicate the failure could have been caused by an anchor that connected the pipeline to cause a partial tear.

Steven Browne, a professor in marine transportation at California State University Maritime Academy, said that a ship at anchor can move quite a lot as winds and tides change direction.

Browne stated that one explanation is that the anchor was not dropped directly onto the pipeline. If the anchor wasn’t properly fixed, the ship moved and dragged it along the bottom. It could have gotten on the pipe and dragged the anchor along with it.

Many questions remain unanswered.

Investigators are still investigating the matter and have yet to say if port managers directed a ship to anchor near the ruptured pipeline. Port managers would typically give specific instructions to a ship on where to anchor and their position would be closely watched.

Browne stated that he had never heard of any cargo ship towing an oil pipeline. However, he is aware of instances in which phone cables were lifted from the ocean floor. He said that ship personnel will lower the anchor in such cases to release the snag.

He suggested that perhaps they didn’t know they were dragging anchor. They wouldn’t know what was on the bottom of such a large vessel.

The sister ports of Los Angeles, Long Beach and Long Beach are directed in much the same way as air traffic controllers who oversee flights entering and departing airports.

In partnership with the U.S. Coast Guard the Marine Exchange of Southern California manages an area of 25 miles (40 km) from the large coastal complex, where cargo can be unloaded and transported across the country. The Marine Exchange of Southern California uses a variety of technology to schedule the arrivals, anchorings and departures of thousands of vessels every year. Computers monitor ships’ speeds and traffic mirrors a freeway with lanes for ships moving in various directions.

Nature’s forces often play a role.

Browne stated that if a ship drags anchor, it’s usually because of a weather condition – strong current or significant wind. “Because there are thousands of tons steel and cargo in a ship, there is a lot momentum.”

He said that a ship at anchor would move quite a lot as winds and tides change direction.

Large ships have anchors that can support up to 10 tons and are connected to hundreds of feet worth of steel chains. Browne stated that any anchor that is fouled will be taken with it.

Long backups have caused vessels to spread across the horizon, causing the ports to be unable to function. Browne stated that he would not be surprised to see ships being anchored closer than pipelines, internet cables or other hazards because so many ships are in the area.