Buckle propagation of pipe-in-pipe systems under external pressure

Pipe-in-pipe systems are widely used in offshore pipeline applications due to favorable thermal insulation capacity. In deep waters, the outer pipe of pipe-in-pipe systems is prone to local collapse and consequent buckle propagation along the length under external hydrostatic pressure. This paper presents the experimental study and numerical simulation of the buckle propagation scenarios for pipe-in-pipe systems in the quasi-static steady-state conditions. The experiments of the buckle propagation for the pipe-in-pipe specimen using 316 grade stainless steel tubes are carried out in a sealed hyperbaric chamber. The comparison between experimental observations and numerical results is made to demonstrate the effectiveness and practicality of numerical simulation technique developed. Based upon the experimental observations and extensive numerical results, as well as the closed-form expressions for the propagation pressures from uniform ring collapse models, a more accurate empirical formula for the propagation pressure PPS of an outer pipe with a solid rod insert of the same diameter as the inner pipe, and a more reasonable empirical formula for the propagation pressure PP2 of pipe-in-pipe systems are proposed, respectively. Furthermore, four collapse propagation modes are identified in the light of geometric characteristics of pipe-in-pipe systems.