HUNTINGTON BEACH, Calif. -- A Southern California underwater oil pipeline was likely struck by an anchor several months to a year before a leak spilled tens of thousands of gallons of crude, the U.S. Coast Guard announced Friday.
A large vessel of some kind may have struck the pipeline, shattering the concrete casing but not necessarily causing the slender crack from which oil spewed last weekend, said Capt. Jason Neubauer, chief of the Coast Guard's office of investigation and analysis.
The longer timeline was partly based on marine growth that was spotted on the pipe in an underwater survey.
The pipe, which was found to be intact last October, may also have been struck several other times by other ships' anchors over the course of the period, he added.
No ships have been identified, however.
"We're going to be looking at every vessel movement over that pipeline, and every close encroachment from the anchor just for the entire course of the year," the captain said.
The pipeline was dragged along the seafloor as much as 105 feet, Neubauer said.
That indicates a large vessel was involved, he said. Cargo ships with multiton anchors routinely move through the area from the ports of Los Angeles and Long Beach.
The leak fouled beaches and killed seabirds.
According to federal records, in some cases an anchor strike is never conclusively proven, such as a 2012 leak from an Exxon Mobil pipeline in Louisiana's shallow Barataria Bay, where a direct strike by a barge or other boat also were considered possibilities.
In others the evidence of an anchor strike was obvious. During 1992's Hurricane Andrew, a 30,000-pound anchor was dragged by a drifting drilling rig over a Texaco pipeline in the Gulf of Mexico, causing a dent that broke open when the line was later restarted.
In 2003, a 7,000 pound anchor was found about 10 feet from a small spill on a Shell Oil pipeline in the Gulf.
A Coast Guard video released Thursday appears to show a trench in the sandy seafloor leading to a bend in the submerged line, but experts offered varied opinions of the significance of the brief, grainy shots. An earlier video showcased a thin, 13-inch long rupture in the line.
Robert Bea, an engineering professor at the University of California, Berkeley and former Shell Oil engineer, said the second video appears to show a furrow in the seabed created by a dragging anchor leading to the damaged pipeline.
Investigators, however, are expected to consider other forces that could have moved and damaged the pipe, including water currents or movement in the seabed.
It will take time.
"The results from the analyses need to be validated -- corroborated. This process can bring even more questions," Bea said. "The shape of the crack indicates that it was caused by internal pressures in the pipeline. But, if that is true, why didn't the pipeline leak" earlier?
Frank Adams, president of Houston-based Interface Consulting International, said in an email that the slight bow in the line displayed in one video "doesn't necessarily look like anchor damage."
When a pipeline is hit by an anchor or other heavy object "that typically results in physical damage that may lead to a fracture," he said.
Reports of a possible spill off Huntington Beach were first coming out late Oct. 1, but the leak wasn't discovered until the next morning. While the size of the spill isn't known, the Coast Guard on Thursday slightly revised the parameters of the estimates to at least about 25,000 gallons and no more than 132,000 gallons.
The Coast Guard said about 5,500 gallons of crude have been recovered from the ocean. The oil has spread southeast along the coast with reports of small amounts coming ashore in San Diego County, some 50 miles from the original site.
Local health officials said Friday that air samples from areas where oil potentially spread are within background levels -- in other words, similar to air quality on a typical day -- and below California health standards for the pollutants that were measured.
As cleanup continued on the shore, some beaches in Laguna Beach reopened Friday, though the public still can't go in the water.
Information for this article was contributed by Brian Melley of The Associated Press.