Plastic Collapse Evaluation for Multiple Circumferential Flaws in a Pipe

Multiple stress corrosion cracks initiate in a weld joint of primary loop recirculation system (PLR) piping in many cases. To prepare a stability assessment method for a pipe with these flaws is one of the major interests for Fitness-for-Service (FFS) Codes of Japan Society of Mechanical Engineers (JSME). This paper presents plastic collapse assessment method of a pipe with multiple circumferential flaws and proposals to revise FFS Codes. Plastic collapse strength of a pipe with multiple circumferential flaws is evaluated with respect to the weakest axis where the minimum plastic collapse moment is obtained, and a program assessing the weakest axis is presented in this study. A ratio of collapse strength corresponding to the weakest axis to that putting all cracks together is defined as strength correction factor, FMC. The strength correction factors when the number of flaws is two and three are summarized in the assessment diagrams, and proposals to revise FFS Codes are reported based on these results.