Reliability Assessment of Failure Assessment Diagram Based Fitness for Service Procedure Including the Effect of Bias in Modeling

This paper presents the estimation of the reliability levels associated with a cracked pipe found acceptable as per the failure assessment diagram (FAD) based acceptance criteria of ASME Section XI, Appendix H. This acceptance criterion is built on the concepts of fracture mechanics. The parameters which enter the acceptance criteria are piping geometry, applied loading, crack size, and the material properties (tensile and fracture). Most of these parameters are known to exhibit uncertainty in their values. The FAD used also has an associated modeling bias. The code addresses these uncertainties by providing a factor of safety on the applied load. The use of a common factor of safety for a variety of pipe sizes, crack configuration, load combination, and materials may not ensure consistent level of safety associated with the piping component being evaluated. This level of safety can be evaluated by using structural reliability concepts. This paper analyzes the reliability level which is achieved if a cracked pipe passes the acceptance criteria prescribed by the code. The reliability is evaluated for a range of pipe and crack geometry, different load combination, and different materials using Monte Carlo method. The realistic assessment of reliability also requires the assessment of modeling bias associated with the FAD. This bias is also evaluated using the results from the published fracture experiments.