The National Transportation Safety Board has found faulty welds, sagging coatings, and large uncoated areas on Pacific Gas & Electric natural-gas pipelines that exploded in September.
PG&E records had described the underground pipe as seamless, and the NTSB is now combing those records.
The explosion in San Bruno, CA, killed eight people and leveled 38 homes on Sept. 9.
The NTSB has issued no cause for the blast, but attention has been focusing on the pipe’s condition.
In a 77-page Metallurgical Group Chairman Factual Report issued Friday (Jan. 21), NTSB described in detail the condition and quality of welds and coatings along three ruptured sections of 30-inch diameter pipe excavated from the blast area. The sections are 12’4”, 27’8” and 15’9” long.
The report did not analyze the results or offer conclusions.
Large areas of pipe were missing coating altogether—one uncoated area started 3’4” from the cut end and continued to the fractured end, the board said. Another section was missing coating “on much of the sides and bottom.”
Coating on the top half of the pipe close to the fracture “exhibited features consistent with drips, sags and charring…. The coating flow patterns were complex with longitudinal and circumferential flow occurring in different regions.”
In one area, a 32-inch piece of coating was detaching from the pipe; another area had an 18-inch-wide strip of coating peeling off less than six feet from one of the fractured areas. “There were also regions on the underside where no coating was observed and the pipe surface was visible,” NTSB reported
It added: “No coating was observed over a cluster of small patches each approximately 2 inches in diameter. … Similar areas of no coating were observed on the undersides of pups 1, 2, and 3.”
Where coating was used, the appearance “was consistent with hot applied asphalt, parts of which had been exposed to elevated temperatures,” NTSB said.
Although the utility company records said the pipe was seamless, NTSB found evidence of numerous girth and seam weld defects, including lack of penetration, incomplete fusion, burn through, slag inclusion, crack, porosity, undercutting, and excess reinforcement.
“All girth welds exhibited incomplete fusion, slag inclusion, and porosity defects on at least one radiograph,” the report said. Lack-of-penetration defects and undercutting defects were exhibited on all but two intact girth welds, the board reported.
For the first time, the report also identified a seam weld as the site of the rupture and said the weld was half as thick as it should have been. The weld would have been considered inferior even by the standards of 1956, when the pipe was installed, the San Francisco Chronicle reported.
Pipes of ‘Unknown Origin’
“PG&E's records—shown by investigators to be in error—indicated that the line was seamless, when in fact the San Bruno site was part of a cobbled assortment of potentially inferior 4-foot-long seamed pipes of apparently unknown origin,” the Chronicle said.
“Records for hundreds of miles of pipelines elsewhere in PG&E's system are now under scrutiny, and regulators might compel the use of high-pressure-water testing to seek out similar weld flaws.”
UC Berkeley engineering Professor Robert Bea told the newspaper that “the seam weld was so weak on the 3/8-inch-thick pipe that a disaster could have occurred at any time.”
"It's not bad welding; it's horrible welding," said Bea, a trained welder. Based on what Bea knew of the welds from reading the report, the newspaper said, the legal maximum pressure on the line should have been "zero."
PG&E President Chris Johns said Friday that it was “premature for PG&E or anyone to speculate on what caused this accident. Nonetheless, today’s report is an important step in the effort to answer that crucial question.”
Johns noted that all of the pipelines in PG&E’s system “are of a similar size and vintage to the line in San Bruno and have not been pressure tested” but are operating “at pressures that have been reduced by 20%,” which “builds a significant additional margin of safety into our current operating conditions.”