An NTSB official inspects a section of pipe.

Few Clues So Far in San Bruno Pipeline Investigation

No evidence of external corrosion, no physical indications of excavation damage, and no physical evidence suggesting a pre-existing leak has been found so far on the damaged pipe.

The National Transportation Safety Board recently updated its continuing investigation of the natural gas pipeline rupture and explosion that killed eight people in San Bruno, Calif., on Sept. 9, 2010.

The investigative group conducting the metallurgical examination of the pipe is chaired by NTSB and includes technical experts from the Pipeline and Hazardous Materials Safety Administration, the California Public Utilities Commission, and Pacific Gas & Electric. PG&E survey sheets and charts for the rupture location indicate the pipeline was constructed of 30-inch-diameter seamless steel pipe (API5L Grade X42) with a 0.375-inch thick wall. Evidence obtained so far, however, indicates the pipeline in the area of the rupture was constructed, at least in part, with seam-welded pipe.

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. The condition of the welds was evaluated by visual inspection, x-ray radiography, and magnetic particle inspection to document any defects or irregularities in the material. Pipe thickness surveys were conducted using ultrasonic thickness 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. Samples were then cut from the ruptured pipe pieces for detailed metallurgical examination. The fracture surfaces were examined under both high-magnification optical microscopes and scanning electron microscopes to identify and document specific fracture features. Metallurgists are using this information to determine the direction of crack propagation and the fracture origin and failure mechanisms.

Investigators found that while the longitudinal seams on some of the pipe segments were fusion-welded from both inside and outside the pipe, some were fusion-welded only from the outside of the pipe. In order to understand this variance, investigators are in the process of 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. No dents, gouges, or other physical indications consistent with excavation damage were observed. Additionally, no physical evidence suggests a pre-existing leak occurred in the ruptured pipe pieces.

NTSB said much factual information must be developed before the board is ready to determine the probable cause of the accident. Pipeline control and operations, regulation and oversight, human performance, survival factors, and pipeline maintenance and records are being studied.

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