Troll Oil comprises of an extensive subsea production system and two floating production units, Troll B and Troll C. Due to high water production there has been a continuous focus on optimal water treatment, starting with the design of the topside facilities on both platforms, and continuing through operational improvements and modifications. This has resulted in a yearly average oil-in-water concentration of about 10 ppm, well below the current regulation of 40 ppm. The authorities have introduced a requirement of "zero discharge" to sea within 2005. To accomplish this, a project was initiated which consists of a broad variety of activities including offshore testing of technologies capable of removing components dissolved in the water. This paper presents the measures taken to reduce the oil content in the discharged water, through original design, operational experiences and modifications, introduction of new technology and the ongoing project to further reduce harmful discharges to sea towards "zero discharge".
The Troll Oil development in the North Sea comprises more than 100 horizontal wells tied into 27 subsea templates, each with two common production flowlines to a floating production unit, either Troll B or Troll C. After topside treatment oil and gas is transported in pipelines to facilities onshore while produced water is discharged to sea. Figure 1 shows an illustration of the Troll Oil development. The oil column in the reservoir is thin, only 4-27 m1, and the production mechanism is a combination of natural gas cap expansion and water drive. Optimal oil production is obtained by extending the wells horizontally along the oil-water contact. Thus the water production from the field is very large and has predominantly varied between 20.000-40.000 m3/day per platform since start up.
Figure 1. The Troll Oil development. (Available in full paper)
This paper presents the measures taken to reduce the oil content in the discharged water on Troll B and C. Topics to be highlighted are briefly introduced below. When developing the field there was great effort put into the design of the oil separation trains and the produced water treatment systems on both platforms. Experience from start up of Troll B was implemented into the design of Troll C. In the Troll C project a subsea separator was developed to reduce the produce water discharge to sea and to prove that subsea separation of water from the well stream is feasible. Since start up, the oil content in the discharged water has been reduced significantly, and the yearly average is now approx. 10 ppm, well below the present regulation requirement of 40 ppm. This successful approach has been reached by operational improvements and modifications, and the willingness to develop and implement new technology together with suppliers. A tool to quantify the environmental impact of the water discharged to sea, the Environmental Impact Factor (EIF), has been developed by the operators2. The EIF tool shows that both the oil dispersed in the water, which is under current regulation, and components that partly are dissolved in the water phase contributes to the environmental impact.