The National Transportation Safety Board (NTSB) issued another preliminary report Tuesday on the deadly San Bruno, CA, natural gas transmission pipeline rupture last September, eliminating external corrosion, excavation damage or a pre-existing leak as possible causes.

A side issue referenced in the report suggested discrepancies between the utility's records and what NTSB found in the segments being tested. Pacific Gas & Electric (PG&E) executives said during a conference call that the utility is reviewing its records and turning everything it finds over to the NTSB. The issue focuses on why utility records indicated that the ruptured pipe section was seamless when NTSB discovered that it had a welded seam.

NTSB's update -- the second since the Sept. 9 failure of PG&E's 30-inch diameter pipeline -- reported that the federal investigators have found "no physical evidence" suggesting that a preexisting leak occurred in the ruptured pipe pieces from PG&E's Line 132. The resulting explosion and fire killed eight people and destroyed 35 homes in about 10 miles south of San Francisco (see Daily GPI, Oct. 14).

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

Regarding the seamed pipe segments and their welds, the NTSB update said welds varied between those with inside and outside welds and those welds that were only applied on the outside of the pipe. "In order to understand this variance, investigators are in the process of researching pipe welding standards and practices in effect at the time the pipeline was installed in 1956," NTSB said.

As part of the investigation, chemical compositional analysis and mechanical property tests of samples taken from the ruptured pipe pieces continues, along with evaluation of environmental factors at the accident site.

NTSB said there is still a lot more information that needs to be developed before the federal board can determine the probable cause of the tragedy. Investigators are still looking at other related areas, such as pipeline control and operations by PG&E, regulation and oversight, human performance, survival factors and pipeline maintenance and records.

During the PG&E conference call, local news media representatives in the San Francisco Bay Area expressed concerns about PG&E's apparent inability to know which sections of Line 132 were seamless and which were not. Further, they grilled PG&E utility President Chris Johns and the utility's gas engineering executive Kirk Johnson about the pre-blast testing of the ruptured segments and whether it was relevant if the utility didn't know it was a seam-welded piece of pipe.

Johnson stressed that whether the pipe is seamed or seamless makes no difference in terms of the variety of tests the utility subjects its pipelines to over the years.

"In terms of checking the integrity of a pipeline, it does not matter whether the pipe is seamless or has a seam," said Johnson, reiterating that at no time was Line 132 subjected to pressures greater than its designed level of 400 psi.

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