In this paper, the local failure criterion using stress modified critical strain method based on annex B of API 579 is applied to evaluate the ductile failure of API X70 pipelines with a volumetric corrosion defect. Ductile failure is quantified in terms of strain, representing the tensile strain capacity (TSC) which is commonly used in strain-based assessment for fitness-for-service of pipelines installed in frozen area where large-scale ground movement can arise due to earthquakes, freezing and thawing.

Based on the local failure criterion suggested for API X70 steel material, the TSCs of the corroded pipelines are evaluated by using the detailed finite element (FE) analyses. The effects of internal pressure and defect size (such as longitudinal length, circumferential width and depth in the direction of thickness) on TSC of pipelines subjected to axial displacement are systematically investigated. In addition, TSCs based on local failure criterion are compared with those based on net-section limit load. The TSCs from the present FE analyses for various defect geometries and internal pressure can be used to predict ductile failure of corroded pipelines and to build the framework for a strain-based assessment for in-service pipelines.

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