Instability of mechanically lined pipelines under large deformation

Abstract

Mechanically lined pipe consists of a load bearing carbon steel outer pipe with a thin corrosion resistant alloy (CRA) liner and is a proven and economical solution for oil and gas pipelines. The bond between the two pipes is purely mechanical and under large bending loads plastic deformation of the liner results in non-reversible liner wrinkling. This wrinkling impedes flow and significantly reduces the performance of the pipeline. This study uses non-linear finite element techniques to describe the wrinkling of lined pipe under pure bending and examines the effect of initial material stresses and contact surface friction. This model is then used to test a design failure criterion equation of DNV GL for design of mechanically lined pipelines. In single directional bending to failure the DNV GL failure criterion proves to be conservative in predicting the onset of liner wrinkling for all initial conditions and pipe geometries tested. Under low cycle repeated loading the safety factor in the predicted DNV GL failure curvature is sensitive to the magnitude of initial pre-stress. The results presented herein provide evidence that the magnitude of initial material pre-stress in mechanically lined pipe needs to be considered in designing the CRA lined pipeline even at relatively low curvatures.