The National Transportation Safety Board has issued seven recommendations—six of them urgent—in its investigation into the pipeline explosion that killed eight people and destroyed 37 homes Sept. 9 in California.
The agency is urging pipeline operators and regulators nationwide to ensure that their system records, surveys and documents accurately reflect the infrastructure as built so that maximum safe operating pressures are calculated accurately.
The recommendations aim to address record-keeping problems that could lead to a pipeline being operated at a higher pressure than it was built to withstand, although it is not yet known if that is what happened in the accident in San Bruno, CA.
Welds Found in ‘Seamless’ Pipe
NTSB announced Dec. 14 that the pipeline section that ruptured had been built of longitudinal seam-welded pipe, although records of pipeline operator Pacific Gas and Electric Company (PG&E) indicate that the section was seamless.
Furthermore, some seams in this section were welded from both inside and outside the pipe, while others were welded only from the outside. To understand this variation, investigators are researching standards and practices that were in effect when the pipeline was installed in 1956.
“The NTSB is concerned that the seam-welded sections may not be as strong as the seamless pipe that was indicated in PG&E's records,” the agency said.
“Because it is critical to consider all of the characteristics of a pipeline in order to establish a safe Maximum Allowable Operating Pressure (MAOP), the NTSB believes that these inaccurate records may lead to potentially unsafe MAOPs.”
The NTSB issued three recommendations, two of them urgent, to PG&E, asking the utility operator to:
• Conduct an intensive records search to identify all gas transmission lines that had not previously undergone a testing regimen designed to validate a safe operating pressure (urgent);
• Determine the maximum operating pressure based on the weakest section of pipeline or component identified in the records search (urgent); and
• If unable to validate a safe operating pressure through the methods described above, determine a safe operating pressure by a specified testing regimen.
Pipeline Records Targeted
The NTSB said it was concerned that other operators may have discrepancies in their records that could compromise the safe operation of pipelines nationwide.
It therefore urged the Pipeline and Hazardous Materials Safety Administration (PHMSA) to “expeditiously inform the pipeline industry” of the circumstances and findings regarding the San Bruno accident so that pipeline operators “can proactively implement any corrective measures” in their systems. It made similar recommendations to the California Public Utilities Commission (CPUC), which regulates intrastate pipeline operations in California.
NTSB also urged CPUC to ensure that PG&E conducts its documents and records search "aggressively and diligently" and to oversee “any testing conducted by PG&E if the document and records search cannot be satisfactorily completed.”
"While it may seem like a small paperwork error, if companies are basing operating pressures on inadequate or erroneous information contained in their records, safety may be compromised," said NTSB Chairman Deborah A.P. Hersman.
"We believe this safety-critical issue needs to be examined carefully to ensure that operators are accurately gauging their risk and that pipelines are being operated at pressures no higher than that for which they were built to withstand."
The investigation has revealed no evidence of external corrosion or obvious physical damage on the ruptured pipe pieces. But those findings, in turn, have raised questions about the adequacy of PG&E’s inspections.
Experts say the utility’s reliance on direct inspection for most of its pipelines, including the San Bruno line, could not detect a variety of problems that could have led to the blast.
With direct inspection, an inspector walks along the line with an electrified pole and later digs up small sections of the pipeline. This can detect major corrosion, but it cannot detect small cracks in the line, weak points in a weld, external pressure from seismic shifts, soil bacteria that can weaken the steel, and other specific types of corrosion, two engineering experts testified in December at a legislative hearing on the disaster.
According to the San Francisco Examiner, pipeline expert Richard Kuprewicz said that the federal government allowed three types of inline inspections: 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.
However, he said, the government prefers pressure testing and smart pigging to direct assessment. He also said that, in his experience, few utilities depended so heavily on direct assessment as PG&E, the Examiner reported.
PG&E said most pipelines in its system are not “piggable,” but they are being retrofitted, according to the Examiner.
The NTSB will hold a fact-finding public hearing March 1-2 in Washington as part of the investigation.
"This accident has exposed issues that merit further attention and have implications for the pipeline infrastructure throughout the country," said Hersman.