Gas Main Risk Assessment through Probabilistic Fracture Mechanics

This paper presents a methodology for quantifying the cumulative probability of failure for a natural gas pipeline in the northeastern United States. The pipeline comprises approximately 21 miles of API 5L X42 seamless line pipe with diameters from 2 to 12 in. and contains roughly 3,600 circumferential welds. Some segments of the pipe date back to the early 1960s, while the newest segments were installed as recently as 2010. Six pipe samples were provided, from which 50 tensile specimens and 510 Charpy V-Notch (CVN) specimens were machined. CVN impact energies were converted to fracture toughness using two-stage CVN-K_1d-K_1c and temperature shift correlations and the master curve approach. Statistical inputs (i.e., mean, standard deviation, minimum, and maximum) for yield strength, flow strength, and fracture toughness were calculated from the mechanical test results. Nearly 700 weld radiographs, taken during installation, were reviewed by a Level III ASNT qualified inspector. The length and width of each discontinuity were used as the data population for the initial flaw size distribution inputs. Rigorous three-dimensional finite element analysis was performed for all diameters with a variety of geometries and considered pressure, thermal, soil bearing, vehicular traffic, and seismic loadings. A probabilistic fracture mechanics evaluation was performed for each pipe diameter considering the worst-case geometry and loading combination from the stress analyses and random samples based on the distributions of the statistical inputs. Cumulative failure probability over a 100-year period was calculated for each pipe diameter, enabling the prioritization of pipe segment replacement based on failure risk.