Pure bending of pipe-in-pipe systems

Abstract

Subsea pipelines and PIP systems experience large bending moments during installation and operation. However, unlike single-walled pipelines, the behaviour of PIPs under bending has been only marginally addressed. In the current study, the bending response of PIP systems with diameter-to-thickness ratio (D/t) of 15–40 is investigated. Linear bifurcation analyses (LBA) and geometrically nonlinear analyses (GNA) are conducted on PIPs of varying lengths. Analytical expressions are provided to predict the classical and nonlinear limit moments of PIPs, and are compared to existing expressions for single-walled pipelines. Ultimate bending moments of PIPs are obtained from physical four-point bending tests and are compared against geometrically and materially nonlinear analyses (GMNA). The finite element results show that in PIPs with centralizers, the limit moments (GNA) drop slightly, however, the ultimate moments (GMNA) remain unchanged. A parametric study of the effect of geometry and material properties of the inner and outer pipes on the ultimate moment of PIPs is presented. It is understood that the ultimate moments of PIPs with thick tubes are predominantly influenced by the material nonlinearities rather than ovalization of the tubes.