Failure prediction for Crack-in-Corrosion defects in natural gas transmission pipelines

Abstract

Cracks occurring coincidentally with corrosion (Crack-in-Corrosion or CIC), represent a new hybrid defect in pipelines that are not directly addressed in the current codes or assessment methods. To understand the failure response of these defects, the finite element method using an elastic–plastic fracture mechanics approach was applied to predict the failure pressures of comparable crack, corrosion and CIC defects in 508 mm diameter pipe with 5.7 mm wall thickness. Failure pressure predictions were made based on measured tensile, Charpy impact and J testing data, and validated using experimental rupture tests. Plastic collapse was predicted for corrosion and crack defects using the critical strength based on the material tensile strength, whereas fracture was predicted using the measured J0.2 value. The model predictions were found to be conservative for the CIC defects (17.4% on average), 12.4% conservative for crack-only defects, and 3.2% conservative for corrosion defects compared to the experimental tests, demonstrating the applicability of the material-based failure criteria. For the defects considered in this study, all were predicted to fail by plastic collapse. The finite element method provided less conservative predictions than existing corrosion or crack-based analytical methods.