Modeling of mechano-electrochemical interaction at a corrosion defect on a suspended gas pipeline and the failure pressure prediction

Abstract

Suspension effect poses a big threat to the integrity and safety of corroded pipelines. In this work, a finite element (FE) based multi-physics field coupling model was developed to determine the mechano-electrochemical (M-E) interaction at an external corrosion defect on a suspended X100 steel pipe and predict its failure pressure. Theoretical calculations were used for model validation. Parameter effects including the defect location on the pipe, geometry of the defect, suspension length and pipe burial depth were determined. Results demonstared that, generally, the M-E effect at corrosion defect caused an increased stress concentration and anodic current density (i.e., corrosion rate), decreasing the failure pressure of the pipeline. The effect became more apparent when the pipe was in suspension. Both the stress and the anodic current density at the corrosion defect were dependent on the defect geometry, especially the defect depth. When the depth was up to 40% of pipe wall thickness, the von Mises stress exceeded the yield stress of the steel, causing local plastic deformation. A critical defect length of 20Dt (D is the outer diameter of the pipe and t is pipe wall thickness) existed, below which the von Mises stress and the anodic current density at the corrosion defect increased with increased defect length. When the defect length exceeded the critical value, the effect was not obvious. An increased suspension length of the pipe would elevate the local stress and anodic current density at the corrosion defect, reducing the failure pressure of the pipe. A critical pipe burial depth of 2.5 m was identified, exceeding which the failure pressure of the pipe decreased rapidly with the increased burial depth. When the burial depth was smaller than 2.5 m, the effect was marginal. The location of the corrosion defect on the pipe did not affect the local stress, anodic current density and failure pressure at an appreciable level, and could be ignored in defect assessment on suspended pipelines.