Influence of the Material Plasticity on the Characteristic Behavior of Sandwich Pipes

Sandwich Pipes (SP) can be considered as an enhanced design configuration for Pipe in Pipe (PIP) systems. By improving the structural properties of the core layer and the components’ interface adhesion, SP systems can be an effective design alternative for deepwater applications. However, designing such a hybrid structure demands more knowledge of the response of the system under the governing loading and environmental conditions. A SP system would be a suitable design alternative for offshore pipelines that are subjected to very large hydrostatic pressure in deepwater. Therefore, full understanding of the behavior of such systems under the external hydrostatic pressure is a prerequisite for designing optimum SPs. In this paper a set of parametric models are generated based on practical design configurations. The Finite Element (FE) software package, ABAQUS, is used to create the models and analyze them. The FE models are analyzed through eigenvalue buckling and post-buckling analyses with the assumptions of linear and nonlinear buckling. Appropriate initial imperfections and FE parameters are administered. Moreover, the integrity of FE models is investigated through a mesh convergence study and also by considering various types of element locking mechanism. The results of these three methods of analysis are compared and the discrepancy between the results obtained through the linear analysis in comparison to the nonlinear post-buckling analysis is highlighted. Moreover, the influence of using various material plasticity models on the buckling and post-buckling responses is also investigated. Different models describing the materials stress-strain curves in the form of the elastic perfectly plastic, elastic followed by plastic exponential hardening, as well as the existence of the Lu¨der’s bands are also considered. Furthermore, the effect of core material’s stiffness on the buckling and post-buckling response of the system is also examined. Based on the equivalent plastic strain, it will be shown that in order to ensure system’s effective composite sandwich action, the core must have a certain minimal stiffness. Finally, the influence of the enhancement in the steel grade used to form either the internal or external pipe on the stability response of both PIP and SP systems will be illustrated.