Critical crack path assessments in failure investigations

Robert D Caligiuri

doi:10.3221/igf-esis.34.13

Abstract

This paper presents a case study in which identification of the controlling crack path was critical to identifying the root cause of the failure. The case involves the rupture of a 30-inch (0.76 m) natural gas pipeline in 2010 that tragically led to the destruction of a number of homes and the loss of life. The segment of the pipeline that ruptured was installed in 1956. The longitudinal seam of the segment that ruptured was supposed to have been fabricated by double submerged arc welding. Unfortunately, portions of the segment only received a single submerged arc weld on the outside, leaving unwelded areas on the inside diameter. Post-failure examination of the segment revealed that the rupture originated at one of these unwelded areas. Examination also revealed three additional crack paths or zones emanating from the unwelded area: a zone of ductile tearing, a zone of fatigue, and a zone of cleavage fracture, in that sequence. Initial investigators ignored the ductile tear, assumed the critical crack path was the fatigue component, and (incorrectly) concluded that the root cause of the incident was the failure of the operator to hydrotest the segment after it was installed in 1956. However, as discussed in this paper, the critical path or mechanism was the ductile tear. Furthermore, it was determined that the ductile tear was created during the hydrotest at installation by a mechanism known as pressure reversal. Thus the correct root cause of the rupture was the hydrotest the operator subjected the segment to at installation, helping to increase the awareness of operators and regulators about the potential problems associated with hydrotesting.

Highlights

The Pacific Gas and Electric Company (PG&E) owns and operates an extensive natural gas pipeline transmission system throughout Northern California
Tearing occurs by a ductile fracture mechanism characterized by relatively wide-scale plastic deformation that typically produces a blunt tip along the leading edge of the tear. The leading edge of the ductile tear in Pup 1 had been transformed from a blunt tear tip to a sharp crack tip by fatigue crack initiation and growth, which occurred over a relatively long period of time due to normal operational pressure fluctuations in the pipeline
Metallurgical, and fractographic analyses of Pup 1 samples prepared by the National Transportation Safety Board (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

Summary

INTRODUCTION

The Pacific Gas and Electric Company (PG&E) owns and operates an extensive natural gas pipeline transmission system throughout Northern California. The maximum allowable operating pressure (MAOP), as set in accordance with applicable regulations, is 400 psig (2.76 MPa). 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. This root cause conclusion is not correct, because these investigators did not identify the critical crack path that 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

PIPE MANUFACTURING PRACTICES

BURST PRESSURE ANALYSIS

DUCTILE TEAR ANALYSIS

FATIGUE CRACKING ASSESSMENT

Findings

CONCLUSIONS