Midwater oil transfer for deepwater FPSOs

Oct. 1, 2003
ABB Offshore Systems has launched DeeWOS, or Deep Water Offloading System, to provide mid-water oil trans-fer from an FPSO to a tanker in deep-water.

Nick Terdre
Contributing Editor

ABB Offshore Systems has launched DeeWOS, or Deep Water Offloading System, to provide mid-water oil transfer from an FPSO to a tanker in deep-water. DeeWOS is intended for use with spread-moored FPSOs exporting to a remote tanker, usually around one nautical mile – a set-up typical of the West African sector.

DeeWOS has been designed to provide a cost-effective alternative for deepwater tanker loading in such circumstances, using proven technology and components, according to John Namork, DeeWOS marketing manager. The oil transfer system is fully decoupled from the FPSO and offloading buoy motions, minimizing the dynamic loads on the transfer system, Namork says.

The main components of DeeWOS are a subsea pipeline bridge and an offloading terminal. The bridge consists of a pipeline bundle attached at either end to a submerged buoyancy tank. At the FPSO end, the buoyancy tank is suspended from the ship by two heavy chains, while the tank at the offloading end is moored to the seabed by two taut-leg anchor lines.

The pipeline bundle consists of two steel conduit pipes for oil transfer and a carrier pipe that provides buoyancy. The conduit pipes are attached to the carrier pipe using innovative clamps that allow expansion and contraction of the conduits, for example due to thermal expansion, without inducing undesired eccentric loads.

Typical measurements for the pipeline bundle are diameters of 34-36 in. for the carrier pipe and 20-22 in. for the conduits. It sits between 50 and 200 m below the sea surface, sufficiently deep to avoid the effects of waves, upper currents, and marine growth.

Due to the buoyancy provided along its length, the bundle maintains a relatively horizontal configuration. It is significantly shorter than other offloading systems that typically are fitted with buoyancy elements only at their mid-point and therefore take on a W configuration in the water.

In the DeeWOS system, the carrier pipe is attached at either end to the buoyancy tanks, while the conduit pipes are connected by flexible hoses to the FPSO at one end and the offloading buoy at the other. The flexibility of the hoses and the heavy chains at the FPSO end ensure that the motions of the FPSO are absorbed through the buoyancy tank and decoupled from the pipeline bundle. At the offloading end, the motions of the offloading buoy are similarly absorbed through the buoyancy tank. Only insignificant motions are transmitted into the pipeline bundle, and the carrier pipe assimilates these.

This design ensures an excellent fatigue life and reduced maintenance requirements, Namork claims. In addition, system components can be prefabricated locally. DeeWOS is capable of being installed by standard anchor handling tugs, eliminating the need for specialized vessels.

Redundancy is built into most components in the system. In the worst event, operation of the system would cease pending repair, but the chance of catastrophic failure is virtually zero, Namork says.