Codes and standards are often filled with deterministic equations and relationships that are presented without explicit quantification of the uncertainty inherent in their application or derivation. That is not to say that such equations and relationships are not conservative, but to the contrary, that whatever conservatism is dutifully included sometimes goes unquantified. The gamut of Failure Assessment Diagrams (FAD) used for assessing the risk of crack-like flaws provides an excellent example of useful criteria that may benefit from uncertainty quantification. The Level 2 FAD from API 579-1/ASME FFS-1 is used as an exemplar in the probabilistic extension of existing codes and standards. As a first step toward modeling the true probability of failure for assessment points nominally below the FAD envelope, assumptions are made regarding the variance of operating conditions, geometry, and flaw sizes, and these assumptions are utilized to estimate the probability that a nominal assessment point will in fact correspond to conditions exceeding the FAD envelope. This estimation procedure is used to generate contour lines of constant probability within the existing FAD envelope, providing an easily interpretable visualization of the results that may be coupled with risk matrices or other risk-based inspection methodologies.

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