NTSB -- 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@xxxxxxxx

(202) 314-6100

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