The interest in the numerical modeling of a deepwater pipe-in-pipe system (PiP) at a free span location is growing. In a free span, the inner pipe bends substantially due to internal pressure and temperature and possibly contacts the outer pipe. Concerns arise how the interaction between them affects the PiP behavior under thermal expansion and contraction. The study is motivated by the need to understand how a PiP at a free span can be modeled adequately. Different finite element (FE) approaches using Abaqus have been practiced. The pipes were simulated with pipe, elbow, and shell elements. The interaction between the inner and outer pipes was simulated with multi-point constraints (MPC), tube-to-tube contact elements (ITT), and contact surfaces. FE model accuracy and cost effectiveness were compared in a case study. The conclusions in this paper would also be applied for PiP sections with bending, such as for lateral buckling of a PiP laid on the seabed and upheaval buckling of a buried PiP.
A PiP insulates the inner pipe using an outer pipe as shown in Figure 1(a). In a free span, Figure 1(b), the pipes bend and the gap between them allow for relative displacement due to different effective tension and weight of the pipes. A shell FE model of a PiP at a span was developed to investigate the behavior during operating and cool down conditions. In addition, pipe FE models were developed and evaluated against the shell element model. It is shown that while the shell FE model simulates the motion of the inner pipe relative to the outer pipe and the contact between them, it requires a lot of computational effort. Contrary, a pipe FE model provides global deformation and pipe stresses close to those of the shell FE model and is cost effective. Table 1 and Table 2 summarize the PiP properties, span under investigation, and loading conditions for each pipe.