Probabilistic FAD and Ductile Tearing Assessment

The assessment of cracks in pipelines is one of the key elements for the integrity verification and the remaining lifetime evaluation of pipelines. This crack assessment is commonly performed through Failure Assessment Diagram (FAD) methods in which two failure criteria are combined, i.e. brittle fracture and plastic collapse. FAD assessments are, however, highly deterministic and do not take into account the various uncertainties that affect cracking. A probabilistic FAD tool is developed that accounts for these uncertainties and determines the probability of failure as a function of the crack dimensions, steel grade and pipeline characteristics. The failure probabilities are associated with a reliability index β, which is commonly used to describe structural safety. The results allow an objective comparison of different cracks across a wide range of operating characteristics, and can be used to construct a reliability-based inspection plan. Multiple parameter studies are executed in which the influence of specific variables on the reliability level is quantified.