Abstract This study presents a numerical investigation of the structural behavior of subsea pipe-in-pipe (PiP) systems designed for transporting cryogenic fluids. The investigation focuses on assessing the effective axial force and the onset of structural instabilities, such as lateral buckling and pipeline walking, under varying operational and geometric conditions. The developed numerical model was validated against reference studies and applied to scenarios with temperatures as low as −253 °C. Results indicate that anchor length is a critical factor in achieving fully restrained effective axial force (EAF). The inclusion of intermediate bulkheads facilitates load redistribution between the inner and outer pipes, while employing Invar material for the inner pipe significantly mitigates thermal effects, potentially allowing residual compression due to the double wall mechanism inherent to pipe-in-pipes, even at low temperatures. Introducing geometric imperfections revealed susceptibility to lateral buckling triggered by the hydrostatic pressure test. Thermal and pressure cycling, simulating operational shutdowns and restarts, demonstrated interaction between lateral buckling and walking, particularly on sloped seabeds, with stabilization occurring after a few cycles. The authors noted that resizing the inner pipe wall thickness could eliminate buckling and walking, confirming that compressive axial force magnitude of the coupled system is the primary trigger for these instabilities. One of the conclusions that arise is that appropriate wall thickness design, combined with materials exhibiting low thermal expansion, is essential to ensure the structural integrity of cryogenic PiP systems. These findings enhance understanding of the underlying mechanisms which are not straightforward.
Torres et al. (2026) studied this question.