This paper describes Total's patented wash tank oil processing system, now globally licensed to Sulzer 1, which optimizes the water-from-oil separation by relocating part of the process from the FPSO's topsides to specifically designed hull tanks while eliminating the need for electrostatic coalescers and/or desalters. As a result, this technology reduces the topsides oil processing system weight, complexity and CAPEX. Sulzer's patented inlet distributors and static mixers are key building blocks to achieving the performance required.

The technology principles, both from a theoretical and a qualification view, as well as selected operational feedback from the FPSO's which are currently using this system are covered. The proper functioning of the wash tank system relies on the controlled creation of a dense packed zone (DPZ), which consists of a layer of dense droplets located between the water layer and the oil layer which acts as efficient water droplet coalescence promotor to allow gravity to separate these enlarged water droplets from the outlet oil. The behavior of the DPZ and its impact on the water-from-oil separation efficiency is experimentally verified in a wash tank pilot system which operates with actual field oil samples to ensure the full scale production unit will achieve the performance required. The water-from-oil separation efficiency observed at the FPSO's in operation meets typical industry standards for oil export of 0,5 % BS&W (basic sediment & water), while matching with design model predictions and pilot testing results. This is achieved with simplified process topsides and without the use of electrostatic coalescers/desalters and/or excessive heat input to reduce oil viscosity. Because of the large holdup volume and residence time within the wash tanks, the system shows a robust performance and extensive flexibility towards production flow rate variations.

The wash tank oil processing system has demonstrated its separation performance on a total of 5 FPSO's & FPU's currently in operation and can thus be considered a reliable and robust technology. Through the license agreement between Total and Sulzer, this technology is now available for the entire oil processing community for deployment on new-build FPSO's, FPU's or PSO's and converted tankers.

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