Nearly a year after a massive pipeline explosion killed eight and leveled dozens of homes in San Bruno’s Crestmoor neighborhood, some pivotal pieces of the investigative puzzle in the cause of the blast remain a mystery.
A leading Bay Area engineering expert who has closely followed the San Bruno blast expects the National Transportation Safety Board to fill in the blanks on Tuesday when it releases its findings from the most comprehensive investigation yet on origin of the Sept. 9 explosion.
Among the most critical findings experts are interested in is the longterm safety of potentially fatigued steel pipelines in close proximity to densely populated areas throughout the world, UC Berkeley civil engineering professor Bob Bea said.
“One of the most important things not yet understood are the effects of the repeated pressure cycling that the segments (of Line 132)...went though in their lifetime,” Bea said.
"Cycling pressure," the variable stress the pipelines experience as gas pressures fluctuate, could affect their ability to operate safely, Bea said.
“Those pressures aren’t steady, and they have an effect on the strength of the pipelines,” Bea said. It’s just like “flexing a paper clip back and forth, and depending on how strenuously you bend that paper clip back and forth determines what the so-called 'fatigue life' is.”
Of particular interest to experts is the amount of pressure Line 132 experienced leading up to the evening of Sept. 9, Bea said.
“What the (utility) regulators need to understand is how the degrading cycling effects the safety or reliability of the pipeline,” he said.
The Los Angeles Times reported last year that the explosion occurred at “a dip in the landscape” with a bend that required more welding, and in an area where the pipeline was vulnerable to corrosion. Either or both factors may have contributed to a massive explosion whose force "exceeded the thrust of a space shuttle at launch.”
PG&E's acknowledgement that it pressures on Line 132 on two occasions is also expected to figure in the NTSB investigation.
The NTSB in previous reports has already pointed to as a contributing factor. PG&E’s troublesome admission that it by all accounts hasn’t helped anybody get to the bottom of what caused the explosion.
“The NTSB has undoubtedly been pursuing that information to develop the insight concerning these very, very important pressure cycles,” Bea said. “That’s information I haven’t been able to gather and hence, I couldn’t connect those dots.”
Bea expects the NTSB’s report to provide answers.
“The NTSB is one of the finest groups of accident investigators that I’ve come across in my career,” Bea said. “The people of San Bruno can expect to get one of the best jobs of forensic engineering work that we know how to do to properly identify the root causes of this horrible accident.
“Understanding these causes can help put things in place so that we can prevent disasters like this from occurring in the future.”
While the NTSB report is of special interest to San Bruno residents, the agency’s findings will be closely watched by experts around the world, Bea said.
California alone has more than 3,600 miles of natural gas pipeline that snake through densely populated areas, according to the LA Times.
“There are people throughout the world with similar (pipeline) systems,” Bea said. The San Bruno pipeline disaster is “potentially a very valuable learning experience.”