Optimized design of pipe-in-pipe systems

Deepwater subsea developments must address flow-assurance issues, and these increasingly form a more critical part of the design. Pipe-in-pipe (PIP) systems, one of the options available in the designers' toolbox for overcoming these problems, are recognized as thermally efficient, reliable, and proven technology for insulated, subsea transportation of wellbore fluids. Although extremely low U values are achievable, PIP systems come at a cost, with increased weight as a penalty for use in deepwater developments. By applying an "inside-out" optimization process for the design of PIP systems, the top-tension loading on the surface vessel (installation or production) can be reduced significantly while minimizing procurement expenditure on raw materials. Specifically, the design optimization of each component reduces steel volumes as well as the overall outer diameter (OD) of the system. This paper presents the methodology for optimized design of PIP systems and illustrates the potential cost savings in terms of raw materials and installation through a case study for a typical large west African field. Commercial savings related to surface platform hull costs also are presented for a case in which the development employs PIP in catenary risers.