Fracture Arrest Assessment of Low Toughness Pipelines

In support of the development of Fracture Control Plans for older pipelines, a probabilistic approach has been developed for evaluating the susceptibility of these potentially low toughness lines to ductile and/or brittle propagating fractures. In characterizing the fracture arrest capability of these lines, the methodology explicitly accounts for the uncertainties associated with imperfect fracture arrest models, the inherent variability in line pipe material properties, and the added uncertainty associated with estimates based on limited amounts of sample data. A PC-based software implementation of this methodology has been developed that can be used to first prescribe a fracture arrest criterion in terms of a target proportion of arrest joints, and to then estimate the likelihood that a given pipeline segment will meet this target value based on the available line-specific material toughness data. Where the prescribed fracture arrest criterion is not met, a scenario evaluation process can be employed to assess the value of collecting additional material toughness data. The scenario evaluation process involves estimating the likelihood that similar additional sample data will reduce the effects of sample size uncertainty to the point where the chosen fracture arrest criterion is satisfied. The results of this type of propagation susceptibility analysis are useful in assessing the implications of a possible line failure from both a business/financial perspective as well as a life safety perspective. The approach can be used to demonstrate that older existing lines satisfy the intent of current codes with regard to propagation resistance, or to determine the change in operating conditions necessary to achieve a target level of fracture propagation resistance.