The National Transportation Safety Board (NTSB) issued another preliminary report last Tuesday on the deadly San Bruno, CA, natural gas transmission pipeline rupture last September, eliminating external corrosion, excavation damage or a preexisting leak as possible causes. In the process it raised some doubts about information the pipeline operator provided, stirring concerns among state regulators.

According to the NTSB’s latest report, ruptured pipeline sections indicated “longitudinal seams that were fusion-welded from both inside and outside the pipe, or just the outside of the pipe…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 preexisting leak occurred in the ruptured pipe pieces.”

The Sept. 9 pipeline explosion and fire killed eight people and destroyed 35 homes about 10 miles south of San Francisco (see NGI, Oct. 18).

Regarding the seamed pipe segments and their welds, the NTSB 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.”

In response to the latest NTSB report, California Public Utility Commission (CPUC) CPUC Executive Director Paul Clanon directed the San Francisco-based combination utility to drop the pressure to 20% below each pipe’s maximum allowed levels; assess those pipelines’ integrity with one of four tests; and obtain CPUC authorization to repressure those lines. Since the San Bruno tragedy, the pressure in the three transmission pipelines operating in the area have been kept 20% below their allowable levels.

PG&E officials told reporters that two transmission pipelines in the East Bay area of San Francisco could be impacted by the CPUC directive — one from Oakland to Fremont and the other between Fremont and Milpitas. The officials said immediate steps were being taken to reduce pressures, but when and how inspections would be carried out was not immediately clear.

As part of the investigation, the NTSB said chemical compositional analysis and mechanical property tests of samples taken from the ruptured pipe pieces continues, along with an evaluation of environmental factors at the accident site. There is still more to be done before the board can determine the probable cause of the explosion. 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.

A side issue referenced in the report suggested discrepancies between the utility’s records and what NTSB found in the segments being tested. The issue focuses on why utility records indicated that the ruptured pipe section was seamless when NTSB discovered that it had a welded seam.

PG&E executives said during a conference call last week that the utility is reviewing its records and turning everything it finds over to the NTSB. During the conference call, media expressed concerns about PG&E’s apparent inability to know which sections of Line 132 were seamless and which were not. PG&E utility President Chris Johns and the utility’s gas engineering executive Kirk Johnson were grilled about the pre-blast testing of the ruptured segments and whether it was relevant if the utility didn’t know whether it was a seam-welded piece of pipe.

Johnson stressed that whether the pipe is seamed or seamless made no difference in terms of the 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 pounds per square inch.

The San Francisco Chronicle last week also reported on a nearly two-year-old CPUC safety staff report, which was critical of PG&E’s safety maintenance programs on parts of its distribution system.

In response, PG&E said t it is working closely with the CPUC to make sure everything is being done to keep the utility pipeline systems operating at maximum safety. “The bottom line is that any time we find something that could potentially pose a safety issue, we work to fix it immediately, and we don’t wait for orders from the regulators to do that,” the utility said.

The Interstate Natural Gas Association of America (INGAA) commended the NTSB for its “methodical approach.” INGAA said industry, as well as federal, state and local officials, cannot stand by idly, and emphasized that the industry has to continue to address the issue of third-party damage to pipelines, which it called “the leading cause of fatalities and injuries associated with pipeline accidents.”

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