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R.D. Caligiuri, Frattura ed Integrità Strutturale, 34 (2015) 125-132; DOI: 10.3221/IGF-ESIS.34.13 126 started in Pup 1 and extended some 28 feet (8.54 m). The force of the rupture created a large crater in the ground and ejected a large piece of the pipeline as shown in Fig. 1. The released cloud of natural gas ignited, causing considerable loss of life and property damage in the surrounding subdivision. The National Transportation Safety Board (NTSB), the part of the US Department of Transportation charged with investigating accidents such as this, investigated the incident and prepared a report [1]. Other organizations conducted investigations as well. Some investigations reached the conclusion that the root cause of the incident was the failure of PG&E to hydrotest Segment 180, including the pup section, as required by applicable industry standards when Line 132 was relocated in 1956. However, this root cause conclusion is not correct, because these investigators did not identify the critical crack path that ultimately led to the failure. As discussed in this paper, identifying the correct critical path required careful review of available historical and current documents and reports, detailed microscopic examination of the fracture surfaces, and burst pressure calculations performed in accordance with methods consistent with industry codes and standards. Figure 1 : Photograph taken the day after the incident showing the ruptured segment of Line 132 that was ejected from the ground by the force of the rupture. Source: www.dailyrepublic.com. P IPE MANUFACTURING PRACTICES egment 180 of Line 132 consists of pipe that is nominally 30 inches (0.76 m) in diameter and 3/8-inch (0.009 m) thick. Fabrication of such pipe starts with a flat steel plate, called a skelp, which is then rolled into a cylindrical shape in a pipe rolling machine and welded longitudinally to join the mating edges. The longitudinal seam weld can be made using one of several welding processes. One such process, and the one used to fabricate Segment 180 pipe circa 1948, is called double submerged arc welding (DSAW), which involves welding the seam from both the inside and outside of the pipe, with each weld penetrating more than half way through the thickness of the seam. A metallographic cross section of a proper DSAW seam is shown in Fig. 2. Given the 30-inch (0.76 m) diameter of the pipe in Segment 180, the steel plate used to manufacture the pipe would have been over seven feet (2.13 m) wide. Rolling 3/8-inch (0.009 m) thick steel plate into a pipe requires large machinery that exerts tremendous force. Based on experience reviewing both pipe manufacturing and gas transmission pipeline operations, as well as reviewing records reflecting historic purchase orders and purchasing practices of PG&E and other gas pipeline operators, gas transmission pipeline operators (including PG&E) do not manufacture such pipe; rather, they purchase such pipe from pipe mills. Following receipt of pipe from a mill, a pipeline owner/operator (or contractor hired by them) lays the pipe segments in the ground and welds them together with circumferential welds (which are also referred to as “girth welds”). DSAW pipe is considered by metallurgists in the gas transmission pipeline field today to be one of the highest quality welded pipe. The same was true in 1956 when Segment 180 was constructed and installed. At the time, given the pipe manufacturing techniques available for 30-inch (0.76 m) diameter pipe, DSAW pipe was the highest quality pipe of that size that PG&E could have used for Segment 180. There is no stronger practical way to join together the two edges of a large piece of metal rolled into gas transmission pipe. S