A Reconsideration of the Log-Secant Model for Failure Pressure Prediction of Surface-Breaking Axial Cracks in Pipelines

Abstract

In the late 1960s and early 1970s, the researchers of the NG-18 Committee at the Battelle Institute in Columbus Ohio completed a seminal study on the failure pressures of axial flaws in oil and gas pipelines. Key developments included the “ASME B31G” equations for assessment of blunt metal loss flaws, the log-secant model for sharp through-wall cracks, and the log-secant model for sharp surface-breaking cracks. These equations are well-established and feature in various industry standards, recommended practices, and federal regulatory requirements. This work is a reconsideration of the log-secant model for axial surface-breaking cracks. The original equations were derived based on a through-wall crack, for which the crack length is the driving force for crack extension. However, for a surface crack, the crack depth is the correct driving force for crack extension. This work rederives the log-secant model starting with an infinitely long surface crack, and then empirically corrects for a finite length. The result is a new failure pressure model of similar form to the original log-secant model, but with a few key differences. Preliminary validation work using the original NG-18 data shows promising results.