Proposal of Flaw Evaluation Method for Securing Reliability of Circumferential Flawed Pipes

This paper describes the requirements for securing the reliability of pipes with a circumferential surface flaw when they are used continually without repair. In the Rules on Fitness-for-Service (the Rules on FFS) of the Japan Society of Mechanical Engineers (JSME), limitations for flaw angle and flaw depth, and the safety margin for fracture evaluation are prescribed to ensure structural integrity during the evaluation period. In addition, for continued use without repair, three successive inspections are conducted and the prediction of flaw size is reviewed. The influence of these rules on the fracture probability when applying the limit load analysis was evaluated. From these evaluations, the flaw depth limitation and improvement of the safety factor are ineffective in improving the reliability of fracture evaluation, and the most influential factor is the prediction accuracy of flaw size. To improve the prediction accuracy of flaw size, it is important to review the predicted flaw size to envelop its actual size at the end of the evaluation period using the results of successive inspections. Based on this examination, the framework of the rules of the successive inspections and flaw evaluation incorporating the revision of the flaw size at the end of the evaluation period was proposed.