...The report does...express concern about the "seam weld" in the ruptured section of pipe, which was installed in 1956.
The pipeline that ruptured was constructed - as all pipes were at the time - by fabricators who curled a flat piece of metal around a cylinder and joined the sides together.
A 28-foot length of the line that blew out of the ground had five individual pipes - one bigger section and four smaller pieces, known as pups - welded together to allow the line to change direction with the hilly terrain.
"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," the report says.
It continues: "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 report may put to rest complaints by some residents of the Crestmoor neighborhood of San Bruno that they had heard reports that natural gas was leaking in the days before the explosion. The federal investigators say that "no physical evidence suggests that a pre-existing leak occurred in the ruptured pipe pieces."
The Wall Street Journal puts it this way:
Pieces of a pipeline involved in a September explosion near San Francisco show unusual variations in the welds, the National Transportation Safety Board said Tuesday.
The board also noted discrepancies between utility records concerning the kind of pipe used and what investigators pulled from the ground after the accident.