Buckle propagation of sandwich pipes under external pressure

Sandwich pipe (SP), consisting of two comparatively thin-walled metal tubes and a thick polymer or cement-based core, is believed to be a feasible conception to transport hydrocarbons in deep sea. In this paper, the buckle propagation experiments were carried out on small-scale SP test specimens composed of two aluminium tubes and the polypropylene (PP) core. In association with experimental efforts, dedicated finite element models of SP test specimens under external pressure were established to reproduce the local collapse and consequent buckle propagating scenarios using the software ABAQUS, and good agreements were observed between the experimental and numerical results. Then, influencing mechanisms of inter-layer adhesion behaviour on the stress state, cross-section deformation and buckle propagation pressure of SPs were systematically studied, and broad parameter correlation analyses were performed to explore geometric and material properties of two steel tubes and polymer core on the buckle propagation pressure of SPs. The results show that good interface bonding conditions can markedly enhance the buckle propagation pressure of SPs and improve the energy dissipation and the deformation capacity of the structures. The core layer thickness and the ratio of the wall thickness between inner and outer tubes have extremely noticeable effect on the buckle propagation pressure of SPs. Additionally, based upon extensive numerical results, an empirical design expression was developed to conservatively estimate the buckle propagation pressure of SP systems for the no inter-layer adhesion case.