Numerical Analysis of Pressurized Cold Bend Pipes under Bending to Investigate the Transition from Compression to Tension Side Failures

Abstract

The cold bended parts of pipelines are prone to geotechnical movements due to discontinuous permafrost, slope instabilities and seismic activities. Sen et al carried out an extensive experimental study in 2006 in order to understand the buckling behaviour of cold bend pipes under applied curvature. In one of the experiments in this study, a high pressure X65 pipe specimen failed under applied curvature due to tension side fracture. This fracture was observed after the development of a wrinkle on the compression side. In this paper, the behaviour of this cold bend pipe specimen under applied curvature and bending is investigated using finite element method to understand the conditions leading to pipe body tension side fracture in the post-buckling phase of a cold bend. The finite element analyses - with and without internal pressure and the plastic strain and von Mises stress distributions - showed that the tension side fracture occurs under equal amount of applied curvature only in the case of the pressurized pipes. These results were in accordance with the experimental results obtained by Sen et al. The effect of internal pressure is analyzed further in order to determine a transition point of the mode of failure from compressive to tensile. A parametric study is conducted for a series of internal pressure values ranging from 20% Specified Minimum Yield Strength (SMYS) to 80% SMYS. A failure criterion based on the equivalent plastic strain reaching a critical value was used to assess the models behavior up to failure. It was observed that load cases having an internal pressure less than or equal to 60% SMYS exhibit higher equivalent plastic strains on the compression side throughout the simulations and up to failure. However, for internal pressure values higher than or equal to 67% SMYS, the equivalent plastic strain on the tension side of the cold bend starts increasing dramatically at some point during the loading process, exceeding the equivalent plastic strain values on the compression side up to failure.