Multi-dimensional mechanical response of multiple longitudinally aligned dents on pipelines and its effect on pipe integrity

Abstract

Dents have been identified as among the most common deformation defects in oil and gas pipelines. In this study, the multidimensional mechanical responses, springback, and rerounding behaviors of dents as well as the interaction of multiple dents on pipelines and their effect on pipe integrity were investigated through the three-dimensional finite element method and full-scale burst tests. The burst test demonstrates that a plain dent has a minimal effect on the pipeline burst pressure, and the burst failure position is not located in the dented area. Generally, when a dented pipeline that is subjected to monotonically increasing internal pressure bursts, the dent does not fully reround, and a convex protuberance forms in the axial shoulders on both sides of the dent. During pressurization, the maximum equivalent plastic strain in the dented pipe always occurs on the inner surface near the dent center in which no necking is found in the radial direction of the wall thickness. However, a larger equivalent stress is observed around the edge of the dent center, whereas the stress at the center is relatively small, resulting in the formation of a plastic hinge in the dented area. If internal pressure fluctuations occur, this can easily lead to fatigue failure. Finally, the interaction among multiple longitudinally aligned dents was investigated, and the critical spacing affecting the integrity of the pipeline was determined. The investigation reveals that when the dent spacing exceeds the pipe diameter, the interaction between two dents may be ignored, and the dent effect on the pipe integrity may be separately evaluated. However, if the dent spacing is less than 0.5 times the pipe diameter, then the dents must be treated as a combined dent. Further, if the spacing is 0.5–1.0 times the pipe diameter, then dent interaction must be considered.