NTSB finds fusion weld variances in pipe from San Bruno blast

Nick Snow
OGJ Washington Editor

WASHINGTON, DC, Dec. 15 -- US National Transportation Safety Board investigators have initially found longitudinal weld variations in pipe recovered following the Sept. 9 explosion on a San Bruno, Calif., natural gas gathering system, NTSB announced.

NTSB emphasized that its investigation is still in its early stages and more information needs to be developed before it reaches a conclusion about what caused the accident, which killed eight people. It noted that metallurgical examination of the pipe includes technical experts from the US Pipeline and Hazardous Materials Safety Administration, the California Public Utilities Commission, and pipeline owner Pacific Gas & Electric Co.

NTSB said PG&E survey sheets and charts for where the pipe ruptured indicate that the line was constructed of 30-in. seamless steel pipe (AP15L Grade X42) with a 0.375-in. thick wall. Initial evidence also shows that the pipeline in the blast area was constructed, at least in part, with seam-welded pipe, it added.

“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,” NTSB said. “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.”

Outer surfaces of the ruptured pipe’s segments examined by NTSB revealed no evidence of external corrosion, it added. “No dents, gouges, or other physical indications consistent with excavation damages were observed. Additionally, no physical evidence suggests that a preexisting leak occurred in the ruptured pipe pieces,” it said.

Ongoing activity
NTSB said laboratory work is continuing to analyze chemical composition and test mechanical properties of samples taken from the ruptured pipe’s pieces, and evaluation of the accident site’s environment is proceeding. “Other areas that investigators are examining include pipeline control and operations, regulation and oversight, human performance, survival factors, and pipeline maintenance and records,” it indicated.

PG&E said in a response that while NTSB’s update noted a discrepancy in the local natural gas distribution company’s records on the type of pipe which was installed, “at not time did the pressure in the pipe exceed the maximum allowable operating pressure nor did the discrepancy impact the required maintenance and inspection protocols.”

The company nevertheless has launched an exhaustive, system-wide check of its pipeline records to confirm their accuracy and correct any discrepancies, it added.

The Interstate Natural Gas Association of American said in a statement that NTSB’s update demonstrates that the board is taking a methodical and commendable approach in its investigation. “At the same time, we believe that the pipeline industry and federal, state, and local officials cannot sit idly by,” it continued.

“We can better address the issue of damage by third parties such as excavators, the leading cause of fatalities and injuries associated with pipeline accidents,” said INGAA, whose members are primarily interstate gas pipelines. “Local officials need to make better land-use decisions about development near pipelines. And the pipeline industry should keep investing in technology that can reduce risk,” INGAA said.

Contact Nick Snow at nicks@pennwell.com.

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