The emergence in the early 1970s of what about a decade later became the first release of ASME B31G began the sequence of what became the continued development and evolution of criteria to assess the severity of metal-loss defects.  Motivated by the desire to reduce the conservatism built into B31G, the late 1980s saw the release of Modified B31G.  That same report considered the piecewise application of that Modified criterion along the “riverbottom” in a process that was termed RSTRENG.  As modern higher-strength, high-toughness line-pipe steels emerged, by the mid-1990s it was clear that application of those just noted criteria to metal-loss in such pipes could lead to quite conservative predictions.  Again, motivated by the desire to avoid excessive conservatism in applications involving modern steels, several alternative criteria emerged in quick succession.  PCORRC appeared in the spring of 1997, with early versions of DNV RP-F101 and British Gas’ LPC-1 criteria following shortly thereafter.  In contrast to the early criteria that relied on empirical fits to test data for various Grades of steel, the effects of the defect for each of these 1990s schemes were quantified by trending results from parametric FEA.  The defect-free resistance to failure also differed, as these 1990s criteria defined failure in reference to the UTS, in lieu of the previously used flow stress concept.  It has since become evident for isolated smooth-bottomed features that in addition to feature length, L, and depth, d, its width, W, can be a factor, as can its through-thickness profile, and its in-plane ratio of L to W, herein termed aspect ratio. 

Since the mid-90s sometimes subtle variations on these approaches have proliferated, with more than 25 such criteria now in the open literature.  One of the more significant developments in this context came in 2005 in work by Zhu and Leis.  This work showed that the strain-hardening exponent can be a factor in the failure pressure of defect-free line pipe, and as such also could be a factor where metal-loss features develop. 

This paper builds on this wealth of knowledge, presenting and validating a Level 1 failure criteria for isolated metal-loss features.  The defect-free term in this Level 1 criterion is based on the Zhu-Leis criterion for defect-free pipe response.  That defect-free resistance is coupled to a recalibration of the Leis-Stephens defect term in PCORRC, which herein is extended to include the effects of width.  The resulting Level 1 criterion is validated in reference to full-scale tests of pipe with metal-loss that include the mix of real corrosion and flat-bottomed machined features.  These tests involve line-pipe Grades from Gr B to X100, and consider a wide range of diameters and wall thicknesses.  Many of these tests consider the effect of width, in addition to the more commonly studied parameters. 

Finite element results are introduced to illustrate the role of width in the failure behavior of metal-loss features.  Benchmarked against more than 150 full-scale tests it is shown by comparison with other of the often-cited metal-loss criteria that this new approach affects a reduction in conservatism.  At the same time, it provides clear benefits in regard to reduced predictive scatter, as well as a reduction in required maintenance and the scope of features that must be considered in field-digs. 


Continuing Development of Criteria to Quantify Metal-Loss Severity, Including Width

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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