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NTSB reports sections of seam on exploding San Bruno pipeline were welded only on outside

December 14, 2010 | 11:27 am

Some sections of a seam on the Bay Area natural gas pipeline that exploded in a deadly firestorm in a residential area in September were welded only on the outside, according to newly released investigative findings from the National Transportation Safety Board.

Longitudinal seams — meaning along the length of the pipe — were inconsistent, with both external and internal welds found in certain areas and only external welds found elsewhere, federal investigators reported Tuesday in the latest update on their probe of the disaster.

The agency said it is researching pipe-welding standards and practices from 1956, when the large, 30-inch transmission line was constructed, to better understand the variances.

The potential significance of the discovery concerning the welds was not immediately clear.

The NTSB, which is continuing sophisticated forensic testing on the failed pipe, said it would not comment further. A final report on the causes of the blast, which killed eight people and destroyed dozens of homes, is not expected until late next year. A spokesman for Pacific Gas & Electric, which operates the pipeline, said the utility is reviewing the new findings and did not have an immediate comment.

The explosion, one of the deadliest in recent U.S. history, has cost PG&E nearly $250 million in property damage, personal injury compensation, new inspection work and other legal expenses.

The powerful blast occurred after a major transmission line running under a hillside neighborhood began leaking large volumes of gas. When the gas ignited, it blew a 28-foot section of the pipe out of the ground and set fire to more than 50 homes, 37 of which were completely destroyed.

The new report says no evidence of external corrosion, a leading cause of pipeline failures, was found on ruptured sections of the pipe. It does not indicate if internal corrosion was found. But previous reports indicated there did not appear to be a significant deterioration of the pipeline wall thickness.

Also, no dents, gouges or other physical signs of excavation damage to the ruptured pipe have been found, the agency said. Work on city sewer lines running near the pipeline became one focus of media inquiries in the wake of the blast.

Investigators are continuing to examine operations, and control and maintenance of the pipeline before the blast, as well as safety regulation and oversight, which falls primarily to the California Public Utilities Commission.

In an investigative report issued in October, the NTSB reported maintenance work on a pipeline control center triggered electrical problems and a rise in gas pressure just before the explosion.


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-- Rich Connell