News reports picked up on two new pieces of information about pipe construction:
- While PG&E's specifications on file for the 1956 construction say that the pipe was of seamless manufacture, some of the length had welded seams; and
- While some of those seams were welded inside and out, others were welded only on the outside.
What appears to be a weld bead is visible as a light gray line in the shadow on the lower left. This is in a long segment.
Despite the headlines and commentaries this week it's not yet clear from the NTSB's sparse technical information whether welding methods played a part in this failure. NTSB might have been pointing out that pipeline companies could do a better job of having their filed surveys match the as-builts.
Based on the photographs and two papers released so far, I'm guessing that the fracture started in one of the short segments (called pups), or a junction between them. Pups are short lengths of pipe, typically installed so that the pipeline can make a bend. This location is at the bottom of a hill so that could explain the pups.
The pups mentioned in the report are a little more than three feet long. In this photo they are called Short Segments 1 to 4.
My inference from the photos is that the NTSB is taking a good deal of interest in two of the pups, labeled Short Segments 1 and 2. Take a look at this photo:
The investigator is cleaning fracture faces at the junction of two cracks. One crack runs along a failed girth weld that joined Pups 1 and 2; the second is a longitudinal fracture in Pup 2. This second fracture runs down Pup 2 toward the lower right, joins another girth crack, and trails off a few inches later. It's hard to tell from the photograph whether this second crack follows a seam weld.
All catastrophic fractures have to initiate somewhere and I'm guessing that the starting point is in the foreground of the photograph above, perhaps near the investigator's left hand.
I also feel pretty confident that the segment we see in the photographs is inverted, that is, that we are seeing it upside-down compared to its original position underground.
Note that the NTSB in its update said there is no evidence of external corrosion on the outer surfaces; it didn't dismiss internal corrosion.
Those who want to take a closer look at the photographs and updates can find them on the NTSB's docket page.