Issue34

R.D. Caligiuri, Frattura ed Integrità Strutturale, 34 (2015) 125-132; DOI: 10.3221/IGF-ESIS.34.13 130 B URST PRESSURE ANALYSIS he 1955 American Standards Association (ASA) B31.1.8 Code [2] recommended post-installation hydrostatic testing [3] in the field for all gas transmission pipelines expected to be operating above 30% Specified Minimum Yield Strength (SMYS). The 1955 ASA B31.1.8 code provision regarding hydrostatic testing was not mandatory; rather it provided industry guidance. The 1955 ASA B31.1.8 hydrostatic testing recommendation was the first time any voluntary industry guidelines had recommended hydrostatic testing. The preceding 1952 ASA code gave hydrostatic testing little mention, noting that pipeline operators “may” use hydrostatic testing. In a series of 1954 articles, the Chairman of the ASA Committee charged with drafting the 1955 B31.1.8 provisions noted that “it was quite general practice in the gas industry” not to hydrostatically test pipelines. By 1956, when Segment 180 was being installed, the situation had changed little, as the gas pipeline industry had yet to widely adopt hydrostatic testing. Regulations requiring hydrostatic testing of new pipelines did not go into effect in California until 1961 and under federal law until the 1970s. The ductile tear evident in the weld seam at the Pup 1 rupture origin occurred by two distinctly different fracture mechanisms: tearing and fatigue crack growth. 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. (Conversely, the final rupture of the Pup 1 weld occurred by a brittle cleavage fracture mechanism, which is characterized by relatively small scale plastic deformation, a sharp crack tip, and rapid, unstable propagation.) 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. Fatigue crack initiation and growth is a fracture mechanism that occurs under cyclic application of loads typically well below those needed to cause fracture instability or rupture. It is worth noting here that fatigue crack initiation and growth is typically not a concern for gas transmission pipelines because such pipelines generally operate with fewer and smaller pressure fluctuations compared to liquid transmission pipelines. This is because liquids are virtually incompressible, whereas gas is highly compressible. Available data from PG&E pressure records for Line 132 are consistent with this general observation of relatively small pressure fluctuations. Burst pressure is the internal pressure at which pipe rupture occurs. It is possible to generally reproduce the burst pressures calculated by the NTSB for pups 1, 2, and 3 using the pertinent, accepted ASME Standard, B31G. ASME B31G is used to calculate the failure pressure of pipes with metal loss, which was the situation present in pups 1, 2, and 3 due to the missing internal weld. Burst pressure calculations per B31G were performed using the Level-2 evaluation technique (RSTRENG method), also called the Effective Area Method. This method allows for a longitudinal metal loss profile that varies in depth along the flaw. Two flaw depth profiles were analyzed: (1) a depth profile that only included the incomplete weld (not including the ductile tear), as conducted by the NTSB; and (2) a depth profile that included both the incomplete weld and the 2.4-inch (0.06 m) blunt tipped ductile tear. Average wall thickness values were taken from NTSB Report 11-056 [4]. The yield stresses used in the B31G calculations were taken from NTSB hardness-based estimates for each pup presented in NTSB Report 11-057 [5]. The results of the ASME B31G analyses depend on a material property known as the flow stress, which is defined as the stress required to cause large-scale plastic deformation in a metal. The B31G Standard presents three different methods for estimating the flow stress σ flow : 1.1 flow y     (1)   10 68.9 flow y ksi MPa     (2)   / 2 flow y UTS     (3) In these formulas, σ flow is the flow stress, σ y is the yield stress, and UTS is the ultimate tensile strength. Assuming weld yield stresses from the NTSB and a weld tensile stress of 82 ksi (565 MPa) (based on NTSB-reported weld hardness values for Pup 1), ASME B31G analyses were carried out for pups 1, 2, and 3. Two analyses were done for Pup 1, one analysis for the single-sided weld only and a second analysis for the weld plus the blunt tipped ductile tear. The results of these analyses are tabulated below. The tabulated burst pressures are in psig. Methods 1 through 3 correspond to the three methods of calculating the flow stress shown above. Method 1 was used by the NTSB. As the results in Tab. 1 T