The National Transportation Safety Board's interim report on the Sept. 9 natural-gas pipeline rupture and explosion that killed eight people in San Bruno, Calif., focuses on defective pipe welds as a cause of the blast.
Owner Pacific Gas & Electric’s records indicate the pipeline was seamless, but NTSB's report, released on Dec. 14, says a portion near the rupture was seam-welded. The report rules out external corrosion and damage caused by a third party as reasons for the explosion.
The investigative group, chaired by the NTSB, includes technical experts from the Pipeline and Hazardous Materials Safety Administration, the California Public Utilities Commission, and PG&E.
PG&E’s survey sheets and charts for the rupture location indicate that the pipeline was constructed of 30-in.-dia seamless steel pipe (API5L Grade X42) with a 0.375-in.-thick wall. Evidence obtained so far, however, reveals that the pipeline in the area of the rupture was constructed, at least in part, with seam-welded pipe, the NTSB report says.
“After the ruptured-pipe surfaces were thoroughly surveyed, all of the asphalt coating was removed and the pipe was cleaned to allow for visual inspection and nondestructive testing … of the surfaces,” according to the report.
The NTSB says it evaluated the welds by visual inspection, X-ray radiography and magnetic-particle inspection to document any defects or irregularities in the material. Pipe-thickness surveys employed ultrasonic testing. Additionally, the pipe pieces were laser-scanned for complete dimensional documentation.
The fracture surfaces of the ruptured pipe pieces were examined using low-magnification optical microscopes. Metallurgists currently are in the process of using this information to determine the direction of crack propagation, the fracture origin and failure mechanisms.
The NTSB says that investigators found that, while the longitudinal seams on some of the pipe segments were fusion-welded from both inside and out, some were fusion-welded only from the outside of the pipe. In order to understand this variance, investigators are researching pipe-welding standards and practices in effect at the time the pipeline was installed in 1956.
“The outer surfaces of the ruptured pipe pieces revealed no evidence of external corrosion,” says the report. “No dents, gouges, or other physical indications consistent with excavation damage were observed. Additionally, no physical evidence suggests that a pre-existing leak occurred in the ruptured pipe pieces.”
Ongoing lab work includes chemical compositional analysis and mechanical property testing of samples taken from the ruptured pipe pieces, along with evaluation of environmental factors at the accident site.
“The investigation is still in an early phase, and there is much factual information to be developed before the Safety Board is positioned to determine the probable cause of the accident,” says the report.
Investigators are examining other areas, including pipeline control and operations, regulation and oversight as well as human performance, survival factors, and pipeline maintenance and records.
The final report on the cause of the blast is due next September.
Meanwhile, PG&E released a statement on the NTSB report that says the “meticulous investigation” is ongoing and “draws no definitive conclusions as to the cause of the accident.”
The NTSB report finds no evidence of external corrosion on the ruptured pipe pieces, no evidence of excavation damage and no physical evidence suggesting a pre-existing leak.
Regarding the seam-welded or seamless pipe issue, PG&E notes that the report “does note a discrepancy in PG&E’s records on the type of pipe installed at the San Bruno location. However, at no time did the pressure in the pipe exceed the Maximum Allowable Operating Pressure, nor did the discrepancy impact the required maintenance and inspection protocols.”