GAPS IN THE CURRENT STRAIN-BASED DENT ASSESSMENT

Dents in pipelines, especially when associated with crack formation, pose a significant pipeline integrity threat. With crack formation during the initial indentation process, the fatigue life and failure pressure can dramatically reduce compared with crack-free dents. Over the past decade, cracks associated with the dent-formation process were typically assessed using strain-based engineering critical assessment (ECA), which compares the dent strain and curvature measured by in-line inspection (ILI) geometry tool, with industry-established damage criteria. However, in recent dent management practice, TC Energy has observed cracks in several shallow dents with strain levels much lower than any established fracture criteria. Cracks were found at some dents with measured depth at only 1.3% outer diameter (OD), and the calculated strain with ASME B31.8 nonmandatory method was similar to 4% [6]. It is believed that the strain-based assessment solely based on post-formation shape measured by ILI geometry tool likely requires improvement to capture additional aspects that might contribute to formation of dent-cracks. Various dent formation processes were reviewed, and the multiple potential contributing factors are discussed in this paper, including loading rate sensitivity, temperature effect, stress triaxiality and geometrical influence. The effects of these factors on material responses, dent-crack formation and fracture damage are outlined. Observed gaps in the current strain-based dent assessment process are also outlined, and a multi-element approach to assess dents and identify dent-induced cracks is proposed, which includes evaluation of combined Caliper, magnetic flux leakage (MFL), electromagnetic acoustic transducer (EMAT) and axial flaw detector (AFD) data, enhanced dynamic testing and ductile failure damage indicator (DFDI) determination, materials modeling and finite element analysis (FEA) techniques. Results of two case studies using multiple real-life dent features are described to illustrate effectiveness of the proposed approach.