Ultimate Strength Prediction of Steel Liners Using Nonlinear Finite Element Analysis

A new steel liner installed in a prestressed concrete cylinder pipeline failed due to eccentric welds and high pressures in the line. The failure was in the form of tears trough the eccentric welds and the backing bar along the longitudinal joint at the intersection of the longitudinal and the circumferential joints. The failed liners were repaired and critical areas were reinforced using cover plates applied over the longitudinal and circumferential welds. This paper details the nonlinear finite element analyses used to predict the failure of the steel liners before and after weld repair and to determine the effect of additional reinforcement on the safety of the liner in form of local cover plates and butt-and-strap steel repair bands. 2D and 3D finite element analyses accounting for large deflection and plasticity were performed. The failure was governed by the ductile failure criterion, expressed in terms of plastic strain reaching the ultimate strain capacity of steel and the brittle failure criterion expressed in terms of triaxiality criterion. Triaxiality failure criterion is expressed in terms of a relationship between effective stress and maximum principal stress. The resulting stresses and strains were compared to the capacities of the steel in triaxial stress state to determine liner safety against brittle failure and in total plastic strain in ductile failure modes.