The Tyra East platform in the North Sea is owned by Danish Underground Consortium (DUC) and operated by Maersk Oil. The platform holds separation facilities for well fluids consisting of oil, gas and produced water.
The produced water is discharged to the sea. Prior to discharge the produced water is treated in order to remove residual oil. The main purpose of the treatment is to meet the environmental legislation for discharge to the sea. Despite the environmental demands are being complied with, visible oil sheen is often observed around the Tyra East F (EFA) platform. The visible oil sheen leads to internal reporting to the production technicians as well as flight observations are reported by helicopters and other aviation traffic in the area. Any reporting of this kind is acted upon by the production technicians. However the technicians are facing a technical challenge as the amount of oil dispersed in the produced water is normally discharged within the limits.
The above mentioned leads to the operation analysis carried out in this paper, and suggestions for suitable solutions. A technical solution is needed as the current equipment is not sufficient in order to remove the visible oil sheen.
The produced water treatment on EFA consists of two parallel operated hydrocyclone and degasser units with total treatment capacity of 513 m³/h. In 2011 the total discharge of produced water was approx. 2.3 million m³ with overall DUC average 6.2 m/L dispersed oil in the discharged produced water, which equals to approx 39 kg/day oil discharged from EFA. This amount of oil only equals to some 65000 DKK a year in lost income as the oil is discharged. The gain from improving the produced water quality will be such as positive public relations, acknowledgement in the industry and the ability of spending personnel recourses better compared to the current operation.
Four different types of technical solutions are treated in this paper. All four solutions are expected suitable for improvement of the discharged produced water quality, mainly with the purpose of reducing or removing the visible oil sheen. It is not suggested to replace the existing treatment units. The solutions are suggested to work as polishing units installed downstream the existing treatment.
The first recommendation is a filter operating with molecular cohesion principle. The hydrocarbons in the produced water are attached to the filter media, and will stay within the filter permanently even though the filter media is saturated with hydrocarbons. This makes sure no hydrocarbons are released when the produced water passes the filter. The filter unit has relatively small footprint, which makes it suitable for offshore operations. Each filter unit has a treatment capacity of 795 m³/h which meets the current flow requirements.
The second unit is a Compact Flotation Unit (CFU) using dissolved gas flotation. The produced water is introduced tangentially into a cyclone internal and cleaned produced water containing dissolved gas is added in order to maintain the spinning within the cyclone and the gas bubbles make it easier for oil droplets to rise to the top of the unit, where an oil outlet located.
Both above mentioned units can be tested prior to final decision, as a test unit of each can be temporary installed in order to evaluate the effect. The effect can be measured simultaneously with the current oil in water measurements.