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R.D. Caligiuri, Frattura ed Integrità Strutturale, 34 (2015) 125-132; DOI: 10.3221/IGF-ESIS.34.13 127 Figure 2 : Metallographic cross section of a proper DSAW weld from NTSB Final Report, Fig. 19b. Absent any corrosion damage, well-manufactured DSAW pipe from the late 1940s or early 1950s would not have needed replacement merely due to its age in 2010 under any industry practice or standard. Between installation of Segment 180 in 1956 and the September 2010 accident, there would be no reason to believe that any soundly manufactured piece of DSAW pipe would be at risk of rupturing. Certainly, the increase in pressure experienced by Segment 180 on September 9, 2010, would not have caused properly made DSAW pipe to fail. As noted earlier, Segment 180 was constructed with 30-inch (0.76 m) diameter, 0.375-inch (0.009 m) wall thickness DSAW pipe. However, an approximately 23-foot (7.01 m) long portion of Segment 180 – the section of pipe that ruptured – contained several short pieces of pipe commonly referred to as “pups.” Specifically, the approximately 23-foot (7.01 m) section of pipe was made up of six pups, from south to north. Those pups are referred to as Pup 1 through Pup 6. These pups ranged in length from 3.5 to 4.7 feet (1.07 to 1.43 m) in length. The use of pups was a common industry practice for pipelines constructed in the late 1940s and early 1950s. R OOT CAUSE ANALYSIS : SEGMENT 180 RUPTURE ased upon visual and metallurgical analysis of the subject pipe, it is clear that portions of Segment 180 – namely pups 1, 2, and 3 – were missing the interior seam weld found in properly fabricated DSAW pipe as shown with respect to Pup 1 in Fig. 3. Tensile testing conducted by the NTSB indicated that the base metal in Pup 1 through Pup 6 did not meet the 52,000 psig (358 MPa) strength requirement specified by PG&E in purchases of 30-inch (0.76 m) pipe in the late 1940s and 1950s. Further, the directionality of manganese-sulfide stringers determined during metallographic examination indicated the rolling direction of Pup 1 through Pup 3 was in the circumferential direction rather than the longitudinal direction associated with full lengths of pipe manufactured at a mill. This indicates the pups were not cut from longer pipe sections but were individually fabricated from smaller pieces of plate oriented 90 degrees from full length pipe rolling operations. Given that the rupture initiated through weld metal in Pup 1, the lower-than- specified base metal strengths exhibited in Pup 1 through Pup 6 and transverse rolling direction orientation in pups 1, 2, and 3 did not contribute to the rupture. The length of the pups (less than 5 feet (1.52 m)) also had no relationship to the root cause of the accident. The failure of Segment 180 was in part due to the missing interior seam weld in Pup 1. However, the missing weld in and of itself was not sufficient to cause the rupture in Pup 1 on September 9, 2010. Two additional factors were also necessary to cause the pipeline failure in San Bruno: (1) ductile tearing at the root of the exterior longitudinal seam weld on Pup 1; and (2) fatigue cracking that initiated and grew from the ductile tear slowly over time by the action of normal operational pressure fluctuations in the pipeline. This fatigue cracking grew to a point that the relatively small increase in pressure on September 9, 2010, caused the Pup 1 longitudinal seam to rupture. Thus, the root cause of the failure was the unique combination of three factors; the elimination of any one of which, and the rupture would not have occurred. B