Plastic bifurcation buckling of lined pipe under bending

Linepipe used in offshore and other operations is often protected from corrosive contents by lining internally a carbon steel carrier pipe with a thin layer of corrosion resistant material. In many applications the lined pipeline can experience bending or compression severe enough for the liner to buckle and collapse inside an intact carrier pipe. This paper presents a solution procedure for establishing the onset of the first bifurcation buckling of such a lined pipe under bending. Bending induced differential ovalization of the two shells causes partial separation of the liner from the carrier pipe. The compressed liner intrado at some stage buckles into periodic wrinkles. This bifurcation check is established numerically using the J2-deformation theory of plasticity. It is shown to occur at bending strain levels very similar to those of the liner shell bent alone but smaller than those of lined pipe under uniform compression.The post-buckling behavior of the lined pipe under bending was subsequently studied by introducing to the liner initial imperfections in the form of the wrinkling buckling mode. Under increasing bending the wrinkle amplitude grows eventually yielding to a second diamond-type buckling mode that results in local collapse. Collapse, while shown to be imperfection sensitive, occurs at a much higher bending strain than the critical wrinkling bifurcation values and it should thus be the failure mode governing design.