Simplified Limit Load Analysis for Multiple Circumferential Flaws in a Pipe

Abstract

A simplified method is proposed to evaluate limit load at incipient plastic collapse for a pipe made of ductile materials with multiple circumferential flaws. Fitness-for-service codes such as the ASME Boiler and Pressure Vessel Code Section XI specify limit load analysis for a pipe with a single flaw. Multiple flaws caused by stress corrosion cracking have actually frequently been detected in the same or adjacent weld lines. Therefore, some methods of evaluating limit load for a pipe with multiple flaws have been proposed. Bending stress at limit load i.e., collapse bending stress is evaluated regarding actual flaw depths, angles and locations of multiple flaws with these methods. Computational evaluation is required because of the enormous number of repetitive calculations. Thus, simple evaluation that can be applied to multiple flaws is required to incorporate into the code procedure. A simplified limit load analysis for multiple flaws in a pipe was developed in the present study based on a net-section approach. Multiple flaws were characterized as equivalent symmetric through-wall flaws on the basis of the sizes and locations of the original flaws. The collapse bending stress for the equivalent flaws was evaluated from simple equations on the basis of the net-section approach. The collapse bending stress for original flaws was determined from a lower value between the largest single flaw in the original flaws and equivalent flaws. The collapse bending stresses were evaluated based on previous and present studies. The results from a comparison demonstrated that the simplified limit load analysis could accurately be applied to a pipe with multiple flaws.