Interpretation and Evaluation of Near Girth Weld, Short Axial Cracks in a Petroleum Pipeline

Abstract

A Canadian crude oil pipeline presented a unique cracking mechanism exhibited by short, branched axial cracks located in the vicinity of girth welds. These attributes, among others, translated into added depth sizing complexity for ultrasonic crack in-line inspection (ILI) tools. The scope presented in this paper encompasses results from three crack ILIs carried out between 2011 and 2013. The assessment and mitigation of such atypical cracks required innovative interpretation and evaluation techniques. First, unique ILI analysis approaches and reporting criteria were implemented and validated beyond established design specifications. The goal was to characterize the very short features at girth welds, while understanding and managing sizing limitations associated with conventional ILI analysis methods. This was attained from a laboratory ILI pull-testing program performed on field cut-outs containing cracks of interest, in addition to detailed non-destructive examinations (NDE) completed in field and laboratory settings. Second, customized, depth-independent, likelihood-based evaluation criteria were developed to identify and mitigate cracking with such distinct attributes. The ensuing model was then validated against a comprehensive field NDE program using different sizing techniques (e.g., Phased Array and Multi-angle Shear Wave). This paper highlights the key findings from the analytical, experimental and field studies and describes the novel methodology followed in the assessment of crack-related features reported by ILI.