LOS ANGELES — As federal investigators examine last month’s deadly natural gas pipeline explosion in San Bruno, engineers already have a strong sense of what went wrong and say the evidence calls into question widely used industry estimates of pipeline safety.
A 28-foot segment of ruptured pipe shows signs that its steel had become brittle over the decades, and the blast occurred at a dip in the landscape that left the underground pipe subject to corrosion from accumulating water and sewage. That bend in the line meant more welds during construction, creating another potential vulnerability.
When the 30-inch-diameter line finally failed, it released an explosive force that ejected tons of asphalt and dirt into surrounding streets. A massive cloud of pressurized gas ignited moments later into a fireball that torched 55 homes and killed eight people.
There are such pipelines under neighborhoods throughout the country. California has 3,600 miles of pipe under densely populated areas.
Nearly half the high-pressure gas transmission lines in the country were installed from about 1950 to 1970. The industry has developed mathematical models to predict which pipelines are at risk and how often they need to be replaced. But a deep divide exists among experts about whether those models are accurate.
The San Bruno disaster on Sept. 9 bolstered the arguments of those who say the industry has put far too much faith in technology and inspection methods.
“The industry worships these models,” said Robert Bea, a professor of civil engineering at the University of California at Berkeley who is a member of the National Academy of Engineering because of his pioneering work in risk analysis. “They are treated as theology.”
The inspections and analysis that Pacific Gas and Electric Co. conducted before the accident did not identify the San Bruno pipeline segment as one of the 100 top safety priorities in the utility’s system. That failure points to an analytical breakdown, Bea said. PG&E officials say they won’t make any conclusions until after investigators determine the cause of the accident.
The failure of the San Bruno line has much in common with an explosion in 2000 that killed a dozen people camping along the Pecos River in rural New Mexico. That disaster led to major changes in federal law and regulation. Until the San Bruno explosion, those changes had swelled confidence in the aging pipeline infrastructure.
Like the New Mexico pipeline, the segment of the San Bruno pipe that ruptured was at a dip in the topography, a vulnerability that almost certainly played a key role. When the pipeline was built in 1956, the dip forced construction crews to weld the line together in small segments. Normally, a pipeline is supposed to flex. But the large number of welds could have created spots that were rigid and others that were weak.
“I would pay attention to the welds, because there are many, many welds,” said Erez Allouche, a professor of civil engineering at Louisiana Tech University and research director for the school’s center for trenchless pipeline technology. “There were so many welds in such a short area, maybe the pipe was rigid.”
The dip would also allow water — among the compounds that can contaminate natural gas — to pool inside the pipe. PG&E only recently began installing collectors and filters to resolve the water problem in portions of the long segments that ran from Milpitas, just north of San Jose, to San Francisco, according to regulatory filings.
The problems relating to the dip continued to multiply when, in recent decades, sewers frequently overflowed in the neighborhood. Sewage can be extremely acid, accelerating corrosion and posing serious problems if the mix saturates the soil around a gas pipe.
If the pipe was brittle with age, it would have been even more vulnerable to internal and external corrosion, which starts as small pits in the metal and has been known in the past to eat away about half the thickness of a pipe wall.
PG&E officials said in a written statement that the pipeline had been repeatedly examined in the last 12 months, undergoing both routine and special inspections that showed it was not among the 100 highest-risk segments of pipe in its system.
There is serious debate over the safety of older pipes.
“Honestly, we have a problem in this country,” said Michael Peevey, president of the California Public Utilities Commission. “It’s much broader than the PG&E problem. We have old and inadequate infrastructure. We also have explosions in places across the United States.”
But Theodore Willke, former chief of the federal Office of Pipeline Safety and now a technical consultant, dismissed such assertions. “Age by itself is not a factor in failure,” he said.
“People who call for replacement of pipe after a certain number of years are misguided. It makes no more sense than replacing the Golden Gate Bridge after a certain number of years.”
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