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R.D. Caligiuri, Frattura ed Integrità Strutturale, 34 (2015) 125-132; DOI: 10.3221/IGF-ESIS.34.13 131 demonstrate, pups 1, 2, and 3 would be expected to survive a 500 psig (3.45 MPa) hydrotest, even with the presence of a ductile tear in Pup 1. Method 1 Method 2 Method 3 psig MPa psig MPa psig MPa Pup 1, no tear 525 3.62 594 4.10 718 4.95 Pup 1, with tear 518 3.57 586 4.04 708 4.88 Pup 2 590 4.07 668 4.61 811 5.59 Pup 3 487 3.36 555 3.83 682 4.70 Table 1 : Calculated burst pressures for pups 1, 2, and 3. D UCTILE TEAR ANALYSIS he stress concentration created by the incomplete seam weld in Pup 1 helped create the ductile tear. Kiefner [6] indicates that ductile tearing can begin at pressures as low as 91% of a pipe’s burst pressure. A lower-bound estimate of the pressure required to create a ductile tear can be obtained by taking 91% of the burst pressure estimates for Pup 1 found in Tab. 1. This gives an estimated range of 478 psig to 653 psig (3.30 to 4.50 MPa) for the pressure to cause a ductile tear in Pup 1. Ductile tearing can occur in a stable manner under a single load application, meaning that even though tearing occurs and results in a larger flaw, the pipe can remain intact and capable of retaining pressure without rupture. Thus, a 500 psig (3.45 MPa) hydrotest could have caused the ductile tear in Pup 1 without causing rupture during the test. A hydrostatic pressure test (hydrotest), in accordance with the 1955 ASA B31.1.8 Standard, would have been performed on Segment 180 to a pressure of 500 psig (3.45 MPa) (1.25 x MAOP). This pressure likely would have been sufficient to create a ductile tear in the one-sided weld of Pup 1 but, as demonstrated in Tab. 1, not burst Segment 180. It is also important to note that no other plausible cause of the ductile tear has been identified. Specifically, PG&E has no record of pressures in Segment 180 ever approaching 500 psig (3.45 MPa) during normal pipeline operation. The NTSB also has ruled out post-installation potential causes such as corrosion, seismic activity, and the 2008 sewer repair [7]. Cold expansion in a pipe mill or a pipe mill hydrotest could not have been the cause of the ductile tear since these activities would have produced stresses far higher than those required to burst the pup. Thus, based on the available information, the ductile tear in Pup 1 was most likely created during a post-installation hydrotest conducted on Segment 180 in 1956. It is important to note that the hydrotest protocol PG&E is currently following would have revealed the missing interior weld in Pup 1. PG&E is generally hydrotesting pipelines like Line 132 at a pressure 1.7 times MAOP, with a spike test to 10% above this test pressure. In the case of Segment 180, PG&E’s current test protocol would subject the pipe to a spike test at 748 psig (5.16 MPa) and a test pressure of 680 psig (4.69 MPa). As indicated by Tab. 1, under this protocol, Pup 1 – or Pup 2 or Pup 3 – would have burst and been replaced. F ATIGUE CRACKING ASSESSMENT ractographic examination of the Pup 1 rupture surface clearly indicated that fatigue cracking extended the length and depth of the ductile tear as shown in Fig. 7. The initiation of fatigue cracking from the “blunted” tip of the ductile tear likely took longer than the time it took to grow the initiated fatigue cracking. This is because of the rounded shape of the tip of a ductile tear, as opposed to the sharper tip of a growing fatigue crack. As the initiated fatigue cracking grew, the strength of the Pup 1 weld gradually decreased. Due to the small size of the pressure fluctuations and the resulting relatively slow growth rates [8], the fatigue cracking would likely have taken decades to reach the size necessary to rupture in September 2010. PG&E’s planned pressure increases on December 11, 2003, and December 9, 2008, were only two among many thousands of pressure cycles contributing to fatigue crack initiation and growth, and were not significantly different from any of the other cycles. Segment 180 operated safely between 1956 and 2010 because the missing interior seam weld and ductile tear were insufficient by themselves to cause the pipe to fail at the relatively low Line 132 operating pressures. The pipe rupture at 386 psig on September 9, 2010, occurred because of initiation and growth of fatigue cracking over a long period of time. T F