A number of major incidents have occurred on pipelines that had recently survived a high-pressure hydrotest, or had recently passed an in-line inspection (ILI).  While uncertainty remains concerning detection and sizing in the context of an ILI, pressure testing is not subject to such concerns.  Thus, the failure of a pipeline that had just passed a proof-pressure test as it was being re-pressurized for its return to service (a so-called pressure reversal) reflects the stable growth of a near-critical anomaly that is still stable as the proof test ends.  In the same way that stable growth of a near-critical anomaly can lead to a pressure reversal, stable growth can occur but remain stable at the pressure first imposed upon the pipeline’s return to service.  Scenarios for which time-dependent degradation mechanisms such as corrosion are not plausible causes of failure have been termed time-delayed failures.  As pressure reversals occurred during the early years of hydrotesting pipelines understanding emerged regarding a defect’s response to pressure, and criteria for a pressure reduction were established to avoid leaks becoming ruptures that in turn could pose issues for company personnel and/or first responders.  The advent of tough steels opens to the possibility that such line pipe can sustain deep cracks at high pressures, which in turn opens to the potential for crack initiation and stable tearing of that cracking at service pressures under circumstances that differ from that for the early line pipe steels. 

Whether cracking remains stable or continues to grow slowly at sustained pressure is a concern regarding long-term integrity management, as well as for company personnel and first-responders called in to deal with a leak from a gas or HVP-liquid pipeline.  The 2004 incident at Ghislenghien involving a modern high-toughness X70 pipeline raised the need to better understand how to manage leaks that could become ruptures in such steels. 

This paper develops a model to quantify the extent of stable tearing and possible instability at axial part-through-wall (PTW) defects as a function of the steel, the length and depth of the defect, and the operating pressure.  First, the theoretical basis for the model is developed in terms of nonlinear fracture (NLFM) mechanics analysis for such defects.  Thereafter this model is recast for time-marching analysis that is coupled with isochronous stress-strain curves for the X70 steel of interest.  As the NLFM solution generalizes handbook results, a finite-element analysis (FEA) of the specific pipe and flaw geometry are used to benchmark that solution.  The model then is used to predict cracking and instability in a step-load and hold testing first for a compact-tension (CT) specimen, and then for a pipe containing PTW slits.  It is shown that the time-marching scheme leads to accurate blind predictions of the stable tearing in the CT specimen, and for the failure response of the full-scale pipe tests.  The predictive results are shown to be consistent with the failure response of full-scale tests under constant pressure leading to delayed failures.  Companion papers present the other aspects of this multiyear project.


Modelling Stress-Activated Creep at Axial Cracks in Pipelines

Division / Sponsors: 
Pipeline Systems Division

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

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