Strain-Based Models for Dent Assessment: A Review

The depth-based dent assessment criteria as required by various government regulations and industry codes have served the pipeline industry well by requiring pipeline operators to investigate dents with potentially severe deformational damage. However, the use of depth criterion alone can result in both unnecessary excavations and the mischaracterizing of otherwise severe dents. ASME B31.8-2003 and PHMSA IMP Rules for gas pipelines affecting high consequence areas offer an option for using strain-based criteria and B31.8 provides non-mandatory formulas for strain calculation. Due to complications in dent profile, other formulas in the open literature or those derived by qualified engineers are also allowed by the ASME code. The present paper reviews dent strain calculation methods that are currently available. Comparisons in the calculated strains between methods are made using several case studies. Discrepancies and their effect on the equivalent (or effective) strain are identified. Improvements and alternative strain limit approaches are explored and discussed.