Strain response analysis of API 5L X80 pipelines with a constrained dent subjected to internal pressure

Abstract

Constrained dents normally cause serious plastification on the pipe under internal pressure due to the restraint of the indenter. Therefore, in this work, the strain response of the API 5 L X80 (referred to as X80 in the remainder part) steel pipeline containing a constrained dent was investigated. Firstly, the formation process of the constrained dented pipeline was numerically simulated by establishing a 3D finite element (FE) model. The model was then verified against the test results of other researchers. The validated model was employed to investigate the equivalent plastic strain distributions of the constrained dent created during the pipe construction and pipe operation stages. Finally, an extensive parametric study was conducted to examine the effects of the internal pressure, dent depth, diameter-to-thickness ratio, indenter size and shape on the strain response of the constrained dented pipe in detail. The results show that the maximum equivalent strain of shallow dents locates at the pipe inner wall at the dent center. As the dent depth increases, the maximum strain position deviates from the dent center and moves outwards along the pipe circumferential direction. Compared with the constrained dent formed during the pipe construction period, the maximum equivalent strain of the in-service constrained dented pipelines is larger under the identical dent depth. The maximum strain increases significantly as increasing the dent depth and internal pressure. It is noted that the wall thickness has a greater influence on the equivalent plastic strain distributions of the constrained dented pipe than that of the pipe diameter. The equivalent plastic strain caused by a small indenter is more serious than the large one due to the high strain concentration. It is noteworthy that circumferential dent (dent length is less than dent width) can be considered as the most severe form of dent defects on the pipe in comparison to the axial dent (dent length is greater than dent width) and dome dent (dent length is approximately equal to dent width).