Safety factor for flaw evaluation of a pipe having a circumferential surface flaw

Abstract

In order to prevent failure of piping, limitation on flaw depth according to the circumferential flaw angle is prescribed in the Fitness-For-Service Codes (FFS Codes) of the Japan Society of Mechanical Engineers (JSME). First, the applicability of the limit load assessment method was examined when the flaw angle was large. Then, the influence of the crack growth estimation and flaw sizing errors on the reliability of the flaw evaluation was examined. Since the limit load assessment method can predict plastic collapse strength of a flawed pipe appropriately irrespective of a flaw angle, it is applicable to flaw evaluation even if a flaw angle becomes large. However, precision of flaw evaluation much depends on the variation of the material strength (flow stress) and the flaw size measurement error. Probabilistic analyses considering these variations were performed and influence on failure probability of a pipe was evaluated quantitatively. As a result, the simplified method for evaluating the safety factors according to those variations and satisfying target reliability was proposed. The proposed methods give the flaw evaluation result which has equivalent reliability irrespective of a flaw angle.

Keywords

• flaw evaluation
• limit load assessment
• liliability
• safety factor
• probabilistic analysis