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Burst capacity assessment at a corrosion defect of pipe elbows subjected to internal pressure is essential for oil/gas pipeline integrity management. In this study, a three-dimensional (3D) nonlinear finite element (FE) model validated by burst tests was developed to investigate the effect of corrosion defects on the burst capacity of X80 pipe elbows. Subsequently, the effects of corrosion geometry (i.e. depth, length, and width of the defect) on the pipe burst pressure were determined. Based on the Goodall model of intact pipe elbows, a reasonable mathematical expression for the failure pressure prediction formula for corroded pipelines under external loads was derived by introducing a defect geometry factor function. Then, combined with a series of FE cases, a new prediction model of burst pressure for corroded pipe elbows experiencing internal pressure was developed. Finally, the accuracy and reliability of the proposed model were verified using extensive parametric FE analysis. The results show that the average error of failure pressure between the proposed model and FEM was only 3.63% for 291 cases. Moreover, the prediction accuracy was better than other well-known models. Therefore, the proposed model can be widely used in the safety assessment of corroded elbows of oil/gas pipelines. • A novel model for predicting the ultimate burst capacity of corroded pipe elbows. • A finite element model for burst analysis on corroded elbows. • Determined the failure pressure of pipe elbows containing a corrosion defect. • Dependence of the burst capacity of elbows on corrosion defect geometry. [ABSTRACT FROM AUTHOR]

A dent is a typical defect found in oil and gas pipelines and poses a significant threat to their safety and integrity. In this study, the buckling behavior and limit strain capacity of pipelines with kinked dents under bending moment loads were investigated. First, four-point bending tests of full-scale pipelines with kinked dents were conducted in a laboratory, and the critical buckling load and strain response of the pipelines during buckling failure were tested. The test results revealed that the existence of kinked dents, specifically those with large depths, could weaken the bending resistance of the pipelines. During the buckling process of the pipeline, a considerable compressive strain was generated at the bottom center of the dent, which was considerably higher than the strain value at the corresponding position of the intact pipeline. However, the strain at both axial sides of the dent gradually changed from compressive strain at the initial bending stage to tensile strain after buckling failure. Then, nonlinear finite element (FE) models of buckling failure for pipelines with kinked dents under bending moment loads were developed, and the reliability of the FE models was verified through comparison with the test results. In addition, a parametric sensitivity study was conducted to determine the effect of parameters, such as dent depth, dent angle, diameter-to-thickness ratio of the pipe, and yield ratio of the pipe steel, on the compressive strain capacity of the pipeline. Finally, based on a large number of FE simulation results, a model for predicting the buckling compressive strain capacity of pipelines with kinked dents was developed, which can be used for evaluating buckling instability failure of pipelines with kinked dents subjected to bending moment loads. • A strain-based assessment method for dented pipelines under bending loads. • A finite element model for buckling analysis on dented pipeline. • A bending test of buckling failure of pipelines with kinked dents. • Dependence of the compressive strain capacity on pipe geometry and steel material. • The critical compressive strain of buckling as a function of dent depth, angle. [ABSTRACT FROM AUTHOR]

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. • Developed a finite element model to assess the integrity of pipelines with multiple longitudinally aligned dents. • The edges of the dent center show highly strain sensitivity to the variations of internal pressure. • API 579 suggests the empirical value of rebound coefficient at 1.43 may lead to non-conservative result. • A circle of plastic hinge was observed around the dent which may cause fatigue of pipe material. • The critical spacing distance of interaction between multiple longitudinally aligned dents on pipelines was determined. [ABSTRACT FROM AUTHOR]

The assessment of corrosion defects to determine their effect on pipeline integrity is essential for pipe managers to develop a scientific maintenance strategy. In this work, a nonlinear finite element (FE) model coupled with multi-physical fields was developed to study the mechano-electrochemical (M-E) synergistic effect at an external corrosion defect on X100 pipeline elbow. The results indicated that bending radius has an apparent effect on the burst pressure of the corroded pipe elbow. However, anodic and cathodic reactions was insensitive to bending radius under normal working pressures. Pipes elbows with corrosion defects located in the intrados have a lower burst pressure than those in the extrados and central line crowns. When the deformation or stress generated at the corrosion defect was elastic, the synergistic mechano-electrochemical interaction effect was insignificant. However, when the depth of the defect or the applied internal pressure of the elbow was sufficient to produce a local plastic strain in the corroded region, the corrosion reaction of the local steel at the defect was remarkably enhanced. Under multiaxial stress, the corrosive behaviour of steel in the defective area involved numerous local galvanic batteries, with the anode located in the high-stress zone and the cathode in the lower-stress zone. Anodic polarisation was found to occur in high-stress zone, which accelerated local corrosion. • Developed a finite element model for corrosion defect assessment on pipe elbows. • Determined the failure pressure of pipe elbows containing a corrosion defect. • The location of the defect on an elbow affects the burst pressure of the pipeline. • The defect depth affects the local stress and anodic current density distributions. • The internal pressure affects the stress and anodic current density at the defect. [ABSTRACT FROM AUTHOR]