Finite-Element Modeling of the Mechanoelectrochemical Interaction of Circumferentially Aligned Corrosion Defects on Elbows of Pipelines

Multiple corrosion defects on oil and gas pipeline elbows seriously threaten the safe and smooth operation of pipelines. In this work, a three-dimensional (3D) model of circumferentially aligned corrosion defects on the external surface of the elbow subjected to mechanoelectrochemical (M-E) interaction was developed. Subsequently, the M-E interaction law of circumferentially aligned corrosion defects under internal pressure was investigated. Then the influence of the geometries of circumferentially aligned corrosion defects on the M-E interaction was studied. The critical circumferential spacings between the defects were determined. The results indicate that due to the special characteristics of the elbow structure, the nonuniform distribution of stress under the action of internal pressure leads to different failure pressures at different locations of the elbow. The variation laws of defect width are the same as that of straight pipelines, under a fixed internal pressure or a defect depth, with the increase of the defect width, the maximum stress level and corrosion rate growth slow until both barely change. In addition, this work integrates the mechanical stress field with the electrochemical corrosion field, considering the specificity of the stress distribution in the elbow. A new interaction criterion to judge the critical circumferential spacing of the interaction between adjacent defects is defined. The new interaction criterion indicates that the circumferentially aligned corrosion defects of the elbow separate when neither the mechanical stress field nor the electrochemical corrosion field mutually interact, and can be evaluated as individual corrosion defects. This work complements the integrity evaluation model for elbows containing multiple corrosion defects and provides guidance for oil and gas piping systems integrity management.