Thanks to their exceptional thermal insulation capability, Pipe-in-Pipe (PIP) systems are well suited for the transportation of hydrocarbons at high pressure and high temperatures (HP/HT), preventing hydrate formation and ensuring high discharge temperatures at the arrival facility. This is the reason why PIP systems are increasingly being used in the design of HP/HT flowlines.
There are two types of PIP systems used in the offshore industry: (i) fully bonded PIP, in which the entire annulus is filled with insulation material like PU, and (ii) unbonded PIP, in which the insulation is achieved by wrapping standard size insulation pads onto the inner pipe.
When a pipe-in-pipe system is installed on an uneven seabed, the inner pipe can experience significant bending due to internal pressure and temperature of the conveyed fluid. This may trigger contact between the inner and outer pipe.
In this paper, different numerical approaches to simulate the structural response of a pipe-in-pipe system are reviewed and compared. The fully bonded PIP can be simulated using an equivalent diameter approach, replacing both pipes by one single pipe with an equivalent mass and bending stiffness. This approach has been pursued to evaluate the mechanical response of a PIP system in a free span. A formulation is presented to reconstruct the stress distributions in the inner and outer pipes based on the strains and bending moments calculated for the equivalent cross section. The results show that the equivalent pipe section can be used for on-bottom roughness analysis and free span assessment of fully bonded Pipe-in-Pipe systems.