San Bruno Pipeline Blast: Another NTSB update

On Tuesday the NTSB released a short update and a few more pictures of a 28-foot-long ruptured section of the pipe involved in the San Bruno gas explosion. 

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.

Earlier photographs indicated that some of the pipes had welded seams, like this one:
 
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.