The production of heavy oil presents a number of problems to deep water operators. A combination of high fluid density, hence hydrostatic head, and viscosity in the risers tied back to the floating production vessel limits the oil production rate. The Concentric Offset Riser (COR) offers a solution for convenient provision of riser base gas lift all in a single arrangement. The injection of gas at the riser base mixes with and lightens the oil, and hence enhances the production rate from heavy oil reservoirs.
The COR is a free standing and thermally efficient pipe-in-pipe riser system suitable for subsea developments tied back to a floating host vessel. The annular space has many potential uses, one of which is gas lift. The vertical riser is tensioned at the top with a buoyancy tank, and tied back to the production vessel with a flexible jumper. Such systems are being implemented in West Africa.
This paper describes the benefits of implementing the COR design for deepwater heavy oil developments.