The section of pipeline that exploded in San Bruno in September had been inspected regularly for years, and no red flags ever arose, Pacific Gas & Electric officials have said.
But experts say the method of inspection PG&E used for that pipeline — and the vast majority of its other lines — is inadequate to detect a whole host of dangers that could have led the pipe to burst.
Federal inspectors have not yet revealed the cause of the explosion, which killed eight people and flattened a neighborhood. However, a preliminary report released by the National Transportation Safety Administration in October indicated that there was not a variation in the thickness of metal in the pipe that burst — a hallmark of general corrosion.
And that’s consistent with the results of the routine inspections that PG&E conducted on the pipeline using direct inspection. That inspection method involves an inspector walking along pipelines with electrified poles and later digging up small sections of the pipeline. This method can detect major corrosion and a handful of other related problems. PG&E has used the direct assessment method on more than three-quarters of its inspections.
But the method cannot detect several other problems, including very small cracks in the pipeline, weak points in a weld, external pressure on a pipeline from seismic shifts, bacteria in the soil that can weaken the steel, and other specific types of corrosion, engineering experts Richard Kuprewicz and Glen Stevick said.
Those engineers provided testimony at a legislative hearing on Tuesday in San Bruno about the explosion. Kuprewicz, a pipeline expert from Washington, said there are three existing methods of inline inspections allowed by the federal government: hydro-pressure testing, in which a length of pipeline is filled with highly pressurized liquid to see whether it bursts; smart pigging, in which a device is run down the length of the pipeline to detect problems; and direct assessment.
The federal government prefers both pressure testing and smart pigging over direct assessment. Kuprewicz said that in his experience with a variety of utilities across the country, few depend so heavily on direct assessment as PG&E.
PG&E representatives said most pipelines in its system are not “piggable,” but the utility is in the process of retrofitting them. Pressure testing is possible in some sections but requires rerouting gas during the inspection for several days and is very expensive and inconvenient to customers.
Assemblyman Jerry Hill, D-San Mateo, who organized Tuesday’s hearing, said regulator California Public Utilities Commission should be insisting that PG&E use other inspection methods.
“We need a stronger public utilities commission and it should be their role to determine the best assessment method to be used,” he said. “We haven’t seen that at all.”