NTSB ISSUES UPDATE ON INVESTIGATION INTO FATAL PIPELINE RUPTURE IN SAN BRUNO, CALIFORNIA
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NTSB ADVISORY
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National Transportation Safety Board
Washington, DC 20594
December 14, 2010
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NTSB ISSUES UPDATE ON INVESTIGATION INTO FATAL PIPELINE
RUPTURE IN SAN BRUNO, CALIFORNIA
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This is an update on the continuing investigation of the
natural gas pipeline rupture and explosion that killed eight
people in San Bruno, Calif., on September 9, 2010.
The investigative group conducting the metallurgical
examination of the pipe is chaired by the National
Transportation Safety Board and includes technical experts
from the Pipeline and Hazardous Materials Safety
Administration (PHMSA), the California Public Utilities
Commission (CPUC), and Pacific Gas & Electric (PG&E). This
group is in the midst of an exhaustive evaluation of the
ruptured pipe pieces. What follows is factual information
that has been developed at this early point in the
investigation.
PG&E survey sheets and charts for the rupture location
indicate that 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 (NDT) 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
currently in the process of 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 that a
pre-existing leak occurred in the ruptured pipe pieces.
The following laboratory work is ongoing:
Chemical compositional analysis and mechanical property
testing of samples taken from the ruptured pipe pieces.
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.
Other areas that investigators are examining include
pipeline control and operations, regulation and oversight,
human performance, survival factors, and pipeline
maintenance and records.
Additional factual updates will be provided and distributed
via media advisory as investigative information is
developed.
PHOTOS:
28-foot-long ruptured section of pipeline at the laboratory
facilities at the NTSB Training Center, Ashburn, Virginia:
http://www.ntsb.gov/Pressrel/2010/images/San_Bruno_28-foot-
long_ruptured_section_of_pipeline.jpg
NTSB investigator examining fracture surfaces:
http://www.ntsb.gov/Pressrel/2010/images/San_Bruno_NTSB_inve
stigator_examining_facture_surfaces.jpg
View of ruptured section of pipeline with investigator
cleaning a fracture surface:
http://www.ntsb.gov/Pressrel/2010/images/San_Bruno_NTSB_inve
stigator_examining_facture_surfaces2.jpg
NTSB investigator using a scanning electron microscope to
examine a pipeline fracture surface:
http://www.ntsb.gov/Pressrel/2010/images/San_Bruno_NTSB_inve
stigator_examining_a_fracture_surface_on_a_scanning_electron
_microscope.JPG
RELATED INFORMATION:
Preliminary Report issued on 10/13/2010:
http://go.usa.gov/1QE
Launch of investigation: http://go.usa.gov/1Qy
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NTSB Media Contact: Peter Knudson
peter.knudson@ntsb.gov
(202) 314-6100
