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Dented defects pose a great threat to the safety of oil and gas pipeline transportation. In this paper, dent behavior of a pipeline caused by a spherical indenter was investigated. Effects of thickness, indenter displacement, indenter radius and pipeline internal pressure on the stress, strain and depth of the pipeline dent are discussed. The results show that internal pressure has a great effect on the pipeline dent. When the internal pressure increases, folds appear on the pipeline wall. With the increasing of wall thickness and indenter displacement, the stress, plastic strain and dent depth increase. But the radius of the indenter has a little effect on the dent's stress and strain. In general, the strain and the depth of the dent achieve an extreme point or maximum near the edge of the dent. [ABSTRACT FROM AUTHOR]

Dent is a common type of defect in oil and gas pipelines and the assessment on dented pipelines is carried out with major international standards and specifications based on dent depth as the evaluation criteria. However, such evaluation criteria based on depth does not account for the impact of various parameters (e.g. parameters of dent, parameters of pipeline and the internal pressure of pipeline) on the evaluation result, as a result, many dented pipelines lose their efficacy, even though they meet the depth-based standards. The influence of parameters changes on the damage degree of II-type dented pipelines is investigated on the basis of Oyane's ductile fracture criterion and by the method adopting finite element numerical calculations. Finally, the non-linear regression analysis is conducted and based on the outputs, a specific expression of dent depth and pipeline damage degree is also acquired within a certain scope of application. [ABSTRACT FROM AUTHOR]

• The ductile damage degree of dented pipes was studied based on the Oyane ductile fracture criterion. • The change of the ductile damage integral value with the change of parameters during each loading process were analyzed. • The relationship between the pipeline damage and the depth of the dent were analyzed. Dent is one of the most common defects in mechanical damage of oil and gas pipelines. Most of the dents generated during pipeline construction are unconstrained dents, and rebound will occur during service of the pipeline. The dent defect can be considered as ductile damage. Taking X52 steel pipe as the research object, the rebound process of unconstrained dents in service is numerically simulated. Based on the Oyane ductile fracture criterion, the ductile damage degree of dented pipes affected by multiple factors was studied. The change of the ductile damage integral value with the change of parameters during each loading process of the pipeline and the relationship between the pipeline damage and the depth of the dent were analyzed. The results show that the analysis results based on the ductile damage model are in accordance with the conclusions of the existing research, and the ductile damage integral value of the pipeline changes with the depth of the dent in a reasonable law, which shows that the ductile fracture criterion can be used for the safety evaluation of dented pipeline. [ABSTRACT FROM AUTHOR]