The propagation buckling response of Pipe-in-pipe (PIP) system subjected to external pressure is investigated in this paper. Experimental study has been performed on buckling of aluminium PIPs with outer diameter to wall-thickness ratios (Di t) of26.7 and 30, inside a hyperbaric chamber. The experimental protocol is comprised of endsealing concentric PIP with a length of 1.6 m, inserting the PIP inside the 25MPa hyperbaric chamber and increasing the pressure until collapse of the system due to external pressure has occurred. This paper proposes a simple Ring Squash Test (RST) for estimating the propagation buckling pressure of PIPs which is compared against hyperbaric chamber results. The proposed ring squash test is a much expedient test to implement in comparison to hyperbaric chamber test and estimates the propagation pressure of PIPs with reasonable accuracy. Experimental results from the ring squash tests, confined ring squash tests (CRST) and hyperbaric chamber tests are presented, and are compared with empirical equations. The RST gives a lower bound of propagation pressure of PIPs. Previous empirical results agree well with current hyperbaric chamber results.


Pipe-in Pipe (PIP) systems are extensively being used in the design of high pressure and high temperature (HP/HT) flowlines due to their outstanding thermal insulation. A typical PIP system consists of concentric inner and outer pipes, bulk heads and centralizers. The inner pipe (flowline) conveys the production fluids and the outer pipe (carrier pipe) protects the system from external pressure and mechanical damage. The two pipes are isolated by centralizers at joints and connected through bulkheads at the ends of the pipeline. The annulus (space between the tubes) is either empty or filled with non-structural insulation material such as foam or water. A sub-sea pipeline can experience a number of structural instabilities, such as lateral (snaking) buckling, upheaval buckling, span formation and propagation buckling. Among these, propagation buckling is the most critical one, particularly in deep water, and can quickly damage many kilometers of pipeline. Local collapse, irrespective of how it is induced, usually will initiate a propagating buckle which in a short time can catastrophically flatten significant sections of the structure (Palmer and Martin, 1975; Kamalarasa and Calladine, 1988; Xue and fatt, 2001; Albermani et al., 2011).

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