ABSTRACT:

The paper is outlining the main drivers behind the development of various subsea multiphase technologies focusing on multiphase metering and boosting. A complete Twin-Screw Subsea Multiphase Pump module has been designed, fabricated and tested with support by the Norwegian DEMO 2000 program and a number of oil companies. The pump module together with the DUET multiphase meter arranged on a subsea flow base forms a flexible and powerful tool for reservoir management and production enhancement. The final test took place in March 2002 with real crude oil and gas at Statoil's K-lab outside Bergen, Norway. The test was performed with a low viscous mixture of gas and condensate. The main test objective was to demonstrate the performance of the twins screw pump as a wet gas compressor. A low liquid viscosity was therefore utilized. Good results were obtained over the entire flow and pressure range proving the capabilities of such a machine up to 100% gas void fraction. Data obtained for the multiphase pump power consumption indicate, as expected, that the pump efficiency will increase with increased viscosity and lower gas fraction. With the recent development two main multiphase pumping technologies, currently in use topside, are made available for subsea application. The advantages and limitations of the helico-axial pump, a roto-dynamic principle, and the twin screw pump, a displacement pumping principle, are discussed. These two pumping systems have substantial different pumping behavior in a multi-branch production pipe network. Dynamic effects caused by the gas/liquid slugging can be a challenge with respect to flow and pressure stability in such production systems. Full and independent control of each well, without negative dynamic effect on the overall system, is essential for production optimization and enhanced recovery. Such evaluation of the two pumping systems is made by simulation and recommendations given. The paper concludes with an overall cost/benefit analysis giving the basis to state that multiphase subsea technologies are vital tools for designing better future production systems.

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