City of San Bruno Disputes Panel’s Report Blaming Pipe Bursting

August 2011, Vol. 66 No. 8

A California blue ribbon panel's report blaming a 2008 sewer pipe bursting project for the natural-gas pipeline explosion that killed eight people in San Bruno, CA, was poorly researched, unsubstantiated and ignored more likely explanations, according to city of San Bruno officials.

San Bruno’s comments were in response to the panel’s report submitted to the California Public Utilities Commission in June. City attorneys filed a response with the state in mid-July claiming the report contained "numerous omissions and erroneous conclusions."

The five-member panel assembled by the California Public Utilities Commission consisted of academics, attorneys and business leaders working with expert consultants, concluded that construction work to replace a sewer pipe was the "most likely" cause of increased stress on a nearby PG&E gas line running beneath a San Bruno neighborhood -- stress that weakened the faulty welds in the pipe that 16 months later exploded, killing eight people and destroying 38 homes. Ground shaking from the work "could have played a key role in transforming a 'stable' threat to an 'unstable' threat, thus triggering the incident."

Determining the final cause of the accident still rests with the National Transportation Safety Board, whose report is not expected until fall; the panel’s report has no formal authority.

Results recanted
Ironically, one of the panel’s expert consultants recanted his earlier conclusions shortly after the report was submitted, citing newly discovered details of the gas line's operating history.

Robert Nickell originally determined that vibrations from the June 2008 sewer pipe bursting project had probably weakened the nearby Pacific Gas and Electric Co. gas transmission pipeline and led to its failure more than two years later. He downplayed the possibility that gas-level fluctuations alone over the years had weakened a poorly constructed seam weld enough so that it finally failed on Sept. 9, 2010.

However, Nickell now says that information contained in hundreds of thousands of documents that PG&E released to the state in July has led him to alter his conclusion. Originally, the gas pressure fluctuations were believed to be only in the 350 – 400 psi range. New information supplied by PG&E now reveal that over the life of the pipeline, gas pressure fluctuations widely varied between 125 and 350 psi. The forces exerted on the pipeline would have been especially damaging if the pressure levels changed often, Nickell observed.