Application of Plastic Strain Damage Models to Characterize Dent with Crack

摘要

Currently, the allowable strain limit for a plain dent is 6 percent as per ASME~* B31.8 for gas pipelines. Field experience has shown that the 6% strain limit for plain dents could be overly conservative, which can result in unnecessary excavations and repairs. Recently, efforts to develop an alternative strain limit have been proposed. In this paper, two plastic damage-based models, namely, ductile failure damage and strain limit damage, and one minimum elongation-based criterion are reviewed. Attempts have been made to use these models to characterize rock dents associated with cracks in terms of a plastic damage severity factor and its susceptibility to crack initiation. Field excavations and finite element analysis are utilized to validate these models using two real pipeline dents from two different pipeline operators, operating in USA and Canada. The results have shown that the internal cracks were formed at the time of the initial indentation and can be predicted by the plastic strain damage based ductile failure models. On the basis of this, a newly developed approach that combines in-line inspection (ILI) technologies (caliper and magnetic flux leakage [MFL]) is introduced and utilized to discriminate between dent with corrosion and dent with crack, and identify critical dents in the pipelines.