The High Pressure and High Temperature (HP–HT) subsea field developments are increasingly using Pipe-In-Pipe (PIP) systems for transportation of production fluids due to superior thermal conductivity performance compared to wet insulated single pipe system. PIP systems can provide the necessary thermal insulation with very low Overall Heat Transfer Coefficient (OHTC) in the order of 0.5 to 1.0 W/m2.K whether installed exposed on the seabed or trenched and buried. An additional inherent feature of PIP systems is that they offer increased protection against third party interaction such as fishing gear and dropped object impact.
PIP system will provide a very long "no touch time" before any intervention (e.g. depresurisation) becomes necessary. This aspect can also be improved by using e.g. hot water circulating and electrical heating in order to overcome difficulties which may arise from prolonged shutdown periods.
The mechanical design of a HP–HT PIP systems is more complex than conventional single pipe system and involves consideration of additional failure modes. This paper gives an overview of the PIP system design, with particular emphasis on in–service buckling and fatigue design. Design considerations for trawl gear impact on lateral buckling and operational aspects of clad and lined pipes for HP–HT and sour service applications are also discussed.