San Bruno Fire: NTSB Issues Second Accident Report

Categories: San Bruno Fire
NTSB 01.jpg
No, the investigator hasn't rested his water bottle on the pipeline. He's 'cleaning a fracture surface.'
The National Transportation Safety Board moments ago released an update on "Accident No. DCA10MP008"-- the Sept. 9 San Bruno PG&E pipeline explosion and fire.

Those hoping for a clear explanation of why an entire neighborhood was transformed into a hellish inferno will have to keep waiting. This report mainly concerns itself with highly technical explanations of how various expert bodies inspect welds and seams on incinerated pipes.

Speaking of welds, it seems PG&E wasn't aware that the pipes in question had them. "PG&E survey sheets and charts for the rupture location indicate that the pipeline was constructed of 30-inch-diameter seamless steel pipe (API5L Grade X42) with a 0.375-inch thick wall," reads the NTSB report. "Evidence obtained so far, however, indicates the pipeline in the area of the rupture was constructed, at least in part, with seam-welded pipe."

The ramifications of that revelation are unclear. What's more obvious is that assigning blame for the explosion will be a drawn-out and meticulous affair. Per the report:

The outer surfaces of the ruptured pipe pieces revealed no evidence of external corrosion. No dents, gouges, or other physical indications consistent with excavation damage were observed. Additionally, no physical evidence suggests that a pre-existing leak occurred in in the ruptured pipe pieces.

So much for those apocryphal tales of gas odors pervading the neighborhood prior to the blast.

Investigators also used a variety of methods to inspect the welds and seams on the ruptured pipeline. "Metallurgists are currently in the process of using this information to determine the direction of crack propagation and the fracture origin and failure mechanisms." In other words, they're working on that.

Finally, "Investigators found that while the longitudinal seams on some of the pipe segments were fusion-welded from both inside and outside the pipe, some were fusion-welded only from the outside of the pipe." This may not be unusual, however. The NTSB is currently researching whether this was just how they did things back in 1956, when the pipe in question was installed. 

NTSB 02.jpg
The 28-foot-long ruptured section of pipeline at the NTSB Training Center in Ashburn, Va.
An October 13 preliminary report on the conflagration noted an electrical disruption cut power to the regulating valve for the ill-fated Line 132 -- but the pressure in the pipeline never exceeded the red level of 400 pounds per square inch. Overall, that report did not come up with any, smoking guns, if you can pardon the expression. And neither has this one.

Still under investigation:

  • Chemical compositional analysis and mechanical property testing of samples taken from the ruptured pipe pieces.
  • Evaluation of environmental factors at the accident site.

Per the NTSB: "The investigation is still in an early phase and there is much factual information to be developed before the Safety Board is positioned to determine the probable cause of the accident."

So it would seem.

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