Strain and stress responses of the springback and rerounding processes of dented pipelines

Abstract

Denting, a common geometric defect in oil and gas pipelines, can threaten the structural integrity and safety of pipelines. A pipeline dent is usually caused by the collision or extrusion of hard objects during the pipeline construction and service stages. Some rebound occurs in the dented zone when the external load is removed. In actual engineering, unconstrained dents tend to reround with increasing internal pressure of the pipeline. This study experimentally and numerically investigated the processes of springback and rerounding of a dented pipeline. First, full-scale testing was carried out to simulate the springback and rerounding processes, and the strain variations at the dent were measured. Then, a 3-dimensional numerical model was developed and then validated against the experimental results. According to the experimental and numerical results, the stress and strain responses of the pipeline during the springback and rerounding processes were studied in detail. Furthermore, the factors influencing the springback and rerounding coefficients were discussed. Finally, equations for predicting the springback and rerounding coefficients of dented pipelines were proposed on the basis of a nonlinear regression analysis. The results show that mainly elastic recovery occurs during springback. After a dent is unloaded, the elastic strain and von Mises stress of the pipeline decrease greatly, while the plastic strain remains unchanged. The elastic strain increases with increasing internal pressure. The springback and rerounding coefficients increase with increasing indenter diameter and diameter-to-wall thickness, while the loading depth has a negative effect on these coefficients. The proposed formulas can be used as a reference for estimating the ratios of dent springback and rerounding of dented pipelines.