Pipe-in-pipe (PIP) systems are increasingly being used in subsea pipeline applications due to the exceptional level of thermal insulation they provide. Typically, the design of pipe-in-pipe systems consist of an inner pipe, conveying the hydrocarbons, and an outer pipe, withstanding the external hydrostatic pressure and the annulus in between filled with non-structural insulation material complimented by occasional centralizers. These pipes are subjected to high pressures from the water column above. Further, they experience axial loads, hydrostatic pressure, internal pressure, bending and combination of these actions during their installation and operation life. As such, the design of PIP systems is becoming a popular research area. Related studies have demonstrated that for thin cylinders subjected to bending, failure in the form of wrinkling and shell-type buckling which eventually results in a localized collapse (termed bifurcation buckling) may occur. Moreover, it has also been reported that in thick pipelines D/t ≤ 25 failure occurs at moments slightly larger than the full plastic capacity. Altogether, a thorough review of published literature shows that majority of this work was performed on single tubes and the bending response of double cylindrical tubes have been addressed only marginally so far.
Subsea pipelines may collapse under combined actions of bending, axial force and external pressure. Therefore, their behavior under such extreme conditions ought to be meticulously investigated. On this account, Australian researchers form the Griffith School of Engineering and Built Environment at Griffith University: Ali Binazir (PhD candidate), Dr. Hassan Karampour and Dr. Benoit Gilbert, in collaboration with Dr. Adam Sadowski at Imperial College London examined the bending response of PIP systems with diameter-to-thickness ratio (D/t) of 15–40. Specifically, they focused on assessing the behavior of subsea PIP systems under the action of pure bending experimentally and numerically. Their work is currently published in the research journal, Thin-Walled Structures.
In their work, PIPs with certain parameters covering the range of practical offshore applications were selected. The researchers then carried out finite element analyses that were validated against experimental results of single tubes and were further used to perform linear bifurcation analyses and geometrically nonlinear analyses. Lastly, the plastic bending moment and corresponding moment-curvature responses of PIPs were investigated experimentally and using geometrically and materially nonlinear analyses (GMNA) technique.
The authors reported that for the studied PIPs with thick tubes, ovalizations at the ultimate moment were almost negligible. Additionally, finite element GMNA studies of PIPs with centralizers showed that the ultimate moments were not affected by the existence of the centralizers.
In summary, the study presented that in-depth assessment of the flexural response and instability moments in a ‘pipe-in-pipe’ system with 15 < D/t < 40. In general, the classical buckling moment of the PIP system was calculated and the condition of its occurrence in the outer or inner tube defined. Overall, results of the parametric study on ultimate moments of PIPs revealed that, similarly to single-walled pipelines, PIPs with thick outer tubes exhibit ultimate moments slightly larger than the full plastic moment. In a statement to Advances in Engineering, Dr. Hassan Karampour, corresponding author highlighted that it was now clear that the ultimate moments of PIPs with thick tubes are predominantly influenced by the material nonlinearities rather than ovalization of the tubes.
Ali Binazir, Hassan Karampour, Adam J. Sadowski, Benoit P. Gilbert. Pure bending of pipe-in-pipe systems. Thin-Walled Structures, volume 145 (2019) 106381.