Bending Capacity of Pipe-in-Pipe Systems Subjected to External Pressure

Abstract

Pipe-in-pipe systems (PIPs) are preferred to conventional single pipelines in deep subsea applications due to their favourable structural performances and superior thermal insulation properties. The PIP systems are comprised of an inner pipe, which conveys the fluid, and an outer pipe, which resists the external hydrostatic pressure. During installation (J-lay, S-lay or Reeling method), offshore pipelines and PIPs may undergo extensive bending moment in presence of substantial external pressure at the touchdown region. On the other hand, in service and due to uncontrolled lateral or upheaval buckling, these pipes may experience large bending moments at the crown segment of the buckled pipe. These bending moments can cause unforeseen ovality of the pipeline’s cross-section, which in turn will reduce their external and internal pressure capacities. Possible interactions between external/internal pressure and bending moment in PIP systems have received limited attention. In this paper, response of the PIP system under combined action of external pressure (on the outer pipe) and bending moment is investigated using finite element analysis (FEA). FE analysis are performed on a PIP system selected within the range of offshore pipeline applications. It is shown that the reduction in bending moment capacity of the PIP systems due to external pressure is more significant than that of the same single outer system without an inner pipe. This shows the susceptibility of the bending moment capacity of the PIPs subjected to the external pressure compare to the conventional single pipes.