When a natural-gas pipeline exploded in San Bruno, it split right up the seam of a weld that, according to Pacific Gas & Electric documents, did not exist.
The existence of a weld on the pipeline is significant since in some cases, federal regulations require pipelines with seams to be more stringently monitored.
Investigators from the National Transportation Safety Board released an update Tuesday about their inquiry into the Sept. 9 explosion, in which a 30-inch transmission pipeline exploded, killing eight people, wounding dozens more and destroying or damaging more than 50 homes.
The update does not come to any conclusions about what caused the explosion, but it does rule out some potential triggers, including corrosion, damage from other construction work or an existing leak. The pipeline did have a weakness along the weld, according to the report.
PG&E documents, such as survey sheets and charts, indicated that the pipeline under San Bruno was a seamless steel pipe — which would not have a weld at all.
Instead of a solid pipeline, however, the line that ran under the neighborhood was constructed by curling steel into a tube, and then welding the seam. Most of the seams appear to have been welded from both inside and outside the pipe, but some were only welded from the outside, according to the NTSB findings.
A pipeline welded from just one side of the pipe is likely to be weaker than one welded from both sides, according to Glen Stevick, a pipeline engineering expert from Berkeley.
In the 1950s, if a pipeline was welded at all, the operator would typically allow far less pressure in the line than if it was seamless, explained Stevick. And if it was welded from just one side, that pressure allowed would be even lower.
“If it’s welded just from the outside, you’re just much more likely to have a flaw in the weld,” he said.
Transportation regulations require operators to identify potential manufacturing or construction defects, including seam defects. In those pipelines, if there is ever a pressure spike, the operator is obligated to inspect the pipe with either a full examination, running a device through the pipeline, or hydro-pressure testing, in which pressurized liquid is pumped into a pipeline to see where it bursts.
Those methods have never been employed on the pipeline that burst. Instead, operators employed the less-sensitive direct assessment method, which can only test for corrosion — not weak spots in a weld.
“The first step in good risk management is knowing what you have, because then everything falls into place fairly logically,” Stevick said. “That would lead you to realize that direct inspection would be unlikely to pick up any problems.”
PG&E Vice President Kirk Johnson said the company had not had any pressure spikes in the pipeline. He did not know why the company’s files did not show the pipeline had a seam weld, but said the company is now reviewing all of its documents to find other potential discrepancies.
Congresswoman Jackie Speier, D-Hillsborough, said she hopes to arrange a meeting with representatives from the NTSB, PG&E and their regulator, the California Public Utilities Commission, next week to discuss inspection methods and how the public can be ensured that PG&E knows their own system.
“This pipeline has probably not been examined since 1956, when it was first put in,” she said. “It troubles me greatly.”
There are a few common methods of inspection for natural-gas pipelines:
Direct assessment: An inspector walks along the pipelines with electrified poles that test for corrosion, and small parts of the pipeline are dug up. This method can detect major corrosion, but cannot detect weld weaknesses or many other problems. PG&E uses this method on more than three-fourths of its inspections.
Smart pigging: A device is run through the pipeline, detecting corrosion and some other problems. Some smart pigs can detect weld problems. About 20 percent of PG&E’s transmission pipelines are “piggable.”
Hydro-pressure: The pipeline is taken offline, and pressurized liquid is then pumped into the pipeline at above the maximum pressure allowance. If the pipeline bursts, a weakness has been detected. If not, the pipeline is considered OK for five years. PG&E says this method is typically too inconvenient and expensive.