On the liner wrinkling and collapse of bi-material pipe under bending and axial compression
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Pipelines and flowlines that carry corrosive hydrocarbons are often protected by lining them internally with a thin layer of a corrosion resistant material. In a commonly used fabrication method, the liner is brought in to contact with a carbon steel carrier pipe by mechanical expansion. In applications involving severe plastic loading, such as the reeling pipeline installation method, the liner can detach from the outer pipe and develop large amplitude buckles that compromise the flow. This work examines the mechanics of wrinkling and collapse of such a liner under bending and axial compression. The modeling starts with the simulation of the expansion process through which the two tubes develop interference contact pressure. Bending induced ovalization causes separation of the liner from the outer pipe, which in turn leads to wrinkling of the compressed side and at higher curvature collapse in shell-type mode. The sensitivity of the collapse curvature to the various parameters is studied, and the onset of collapse is shown to be very sensitive to small geometric imperfections in the liner. The models developed are also used to demonstrate that modest amounts of internal pressure can delay liner collapse to curvatures that make it reelable. This framework, suitably extended, is also used to examine the effect of girth welds on liner collapse. It is found that a girth weld locally prevents this detachment creating a local periodic disturbance. With increasing bending, the disturbance grows and eventually yields to a shell-type collapse mode similar to the one that causes collapse away from the weld. The related problem of wrinkling and collapse of lined pipe under axial compression is also studied using a second family of custom models. Following the manufacturing expansion, such a model is compressed with the liner going through axisymmetric wrinkling, followed by localization and collapse via a non-axisymmetric buckling mode. Sensitivity studies show that the collapse strain exhibits a similarly strong sensitivity to small geometric imperfections in the liner. As in bending, modest amounts of internal pressure is demonstrated to delay liner collapse.