Issue34

R.D. Caligiuri, Frattura ed Integrità Strutturale, 34 (2015) 125-132; DOI: 10.3221/IGF-ESIS.34.13 132 C ONCLUSIONS ased on the visual, metallurgical, and fractographic analyses of Pup 1 samples prepared by the NTSB, as well as calculations using established ASME methodologies, it is clear that the September 9, 2010 rupture of PG&E Line 132, Segment 180 in San Bruno, California, was caused by the combination of a missing interior weld, a ductile tear, and fatigue cracking, all of which were present in the Pup 1 longitudinal seam. All three of these factors were necessary for the rupture to have occurred. Metallurgical analysis indicated that short “pups” at the rupture site were not cut from longer line pipe sections. Based on historical record review and the specialized equipment that would have been required to bend the 3/8-inch (0.009 m) plate into a cylinder, PG&E would not have been able to fabricate these pups . The manufacturer of these pups with the missing DSAW remains unknown. Calculations and review of available documents indicate that the ductile tear in Pup 1 was most likely created during a post-installation hydrotest conducted on Segment 180 in 1956. PG&E has no record of such a hydrotest, but a former PG&E employee has testified that he remembers a hydrotest in the vicinity of the later pipe rupture. It is important to note that, but for the missing interior weld, this post-installation hydrotest would not have created the ductile tear. A pressure of approximately 500 psig (3.45 MPa) was required to create the ductile tear, which is well above the 400 psig (2.76 MPa) MAOP for Line 132, Segment 180. The stress concentration created by the missing interior weld and the presence of the ductile tear, combined with normal operating pressure cycles over a long period of time, resulted in the initiation and growth of fatigue cracking. The Segment 180 rupture occurred on September 9, 2010, because the Pup 1 seam weld had been sufficiently weakened by fatigue crack growth to allow rupture at 383 psig (2.64 MPa). Fatigue cracking in Pup 1 developed at a very slow rate, likely requiring the entire 54-year life of the pipe to sharpen the tip of the ductile tear and then propagate to the depth observed at the time of the rupture. The facts and analysis with respect to the fatigue crack growth rate support the conclusion that the ductile tear in Pup 1 was initiated during a 1956 post-installation hydrotest, and that the fatigue cracking developed over time through normal operations until it was large enough to rupture the pipe on September 9, 2010. R EFERENCES [1] National Transportation Safety Board (NTSB) 2011, Pacific Gas and Electric Company Natural Gas Transmission Pipeline Rupture and Fire, San Bruno, California, September 9, 2010, Pipeline Accident Report NTSB/PAR-11/01, Washington, DC, (2011). [2] The American Standards Association – known today as the American National Standards Institute – is an organization that promulgates consensus standards in the pipeline industry, among others. [3] Hydrostatic testing involves the pressurization of water injected into the subject pipeline to establish integrity. [4] National Transportation Safety Board ( NTSB) Report 11-056, Materials Laboratory Factual Report, Table A-1. [5] National Transportation Safety Board ( NTSB) Report 11-057, Materials Laboratory Study Report, table on Page 2. [6] Kiefner, J. F., Evaluating the Stability of Manufacturing and Construction Defects in Natural Gas Pipelines, US DOT, Office of Pipeline Safety, Final Report No. 05-12R, (2007). [7] National Transportation Safety Board ( NTSB) 2011, Pacific Gas and Electric Company Natural Gas Transmission Pipeline Rupture and Fire, San Bruno, California, September 9, 2010, Pipeline Accident Report NTSB/PAR-11/01, Washington, DC, (2011). [8] Barson, J.M., Rolfe, S.T., Fracture and Fatigue Control in Structures, Applications of Fracture Mechanics, 3 rd Edition, ASTM International, Summary of fatigue-crack-growth data for ferrite-pearlite steels, (1999) 202. B