Liner wrinkling and collapse of bi-material pipe under axial compression

Lined pipe describes a product where a carbon steel pipe is lined internally with a thin layer of a corrosion resistant material in order to protect it from corrosive constituents in the hydrocarbons it carries. Most commonly the liner is brought into contact with the carrier pipe by mechanical expansion. Full-scale tests on this product have demonstrated that, under mechanical loads that plastically deform the composite structure, the thin liner can buckle and collapse inside an intact outer pipe making the structure unserviceable. This paper investigates the extent to which typical lined pipes can be axially compressed before liner collapse. Demonstration experiments on model lined systems illustrate that the liner, although supported by contact with the outer pipe, first buckles unilaterally into an axisymmetric wrinkling mode at a relatively low strain. The wrinkles grow stably with compression but yield to a non-axisymmetric diamond-type mode that results in the collapse of the liner at a higher strain. This process has been modeled numerically starting with simulation of the mechanical expansion through which the composite structure is manufactured. The sensitivity of the collapse strain to the various parameters of the problem is studied and amongst other findings it is shown to be very sensitive to small geometric imperfections in the liner. It is also demonstrated that even modest amounts of internal pressure can delay liner collapse up to strains at which the outer pipe collapses.