Statoil recently developed a tool to improve management of pollution caused by produced water discharges around offshore platforms. In 1998, the Norwegian government focussed atten-tion on the issue of marine pollution caused by discharges from oil and gas installations. The government stipulated that the industry should achieve zero harmful discharges by 2005. This target comes at a time when produced water volumes in the sector are rising substantially.
A joint working group set up by the Norwegian Pollution Control Authority (SFT) and the Norwegian Oil Industry Association (OLF) was charged with developing a management tool to identify pollution caused by produced water and to assess the environmental benefit of possible methods of combating it.
Solving the problem
Coincidentally, such a tool was already under development. This was the environmental impact factor (EIF) which was being developed as part of the Dose-related Risk and Effect Assessment Model project (DREAM) sponsored by Agip, Norsk Hydro, Statoil, and TotalFinaElf.
Concentrations in fields for produced water emissions from Heidrun in 2001 and 2013 (100 x 100 km habitat grid) as calculated using the dilution model developed by Statoil.
The EIF tool has now been fully developed by Statoil in association with the OLF, says Ståle Johnsen, senior research adviser in the environment department at Statoil's R&D center in Trondheim. Johnsen has been closely involved in the work. It has been accepted by the SFT as a management tool for analyzing discharges, and is being actively used in the quest to secure the zero discharge goal.
In 2000 the EIF for all fields was reported to the authorities, providing a picture of the extent of pollution around all producing installations on the Norwegian continental shelf and identifying the harmful substances. Licensees are now due to report back to the SFT on what specific measures must be taken at each installation to achieve zero harmful discharge.
The EIF approach is based on the principle of risk assessment. The approach uses a method developed by the European Union for the handling and regulation of industrial chemicals. The method involves determining the predicted environmental concentration (PEC) and the predicted no effect concentration (PNEC) for related groups of chemicals. A PEC/PNEC ratio (also known as the risk characterization ratio) of less than one indicates that the chemical does not have a harmful effect on the environment.
In an offshore context, the PEC can be measured either through environmental monitoring or calculated by applying a method of environmental dilution/dispersion modeling developed in the Dream project. The complete composition of the produced water from a field has first to be analyzed. This varies from field to field, and can differ significantly between different structures within the same reservoir. To simplify the process, chemicals are divided into 20-25 groups of related substances. The volumes of each group of chemicals discharged are entered in the model, along with the relevant meteorological data, and the model then calculates the changing concentration of each group of chemicals for the area and period under study.
PNEC values are based on toxicity values for each group of chemicals, which are calculated using available toxicity data in the literature. Good chronic toxicity values are known for many of the groups, but only acute toxic data are available for chemicals added by human agents during processing or at other stages, Johnsen says. Allowance for such shortcomings is achieved using a high safety factor.
Taking all this data, the model provides a detailed 3D picture over time of the concentration and dispersal of the various groups of chemicals around a platform. The EIF presents this as a single factor, which provides a quantitative expression of the degree of pollution in a defined volume of water.
By comparing the PEC/PNEC ratio for each group of chemicals, the groups can be ranked in terms of the degree of environmental risk they present. Different means of removing or reducing the risk can be identified and compared by means of cost/benefit analysis.
The model is now well calibrated and can be used predictively, given a sample of reservoir water and knowledge of what process chemicals will be used. An EIF was calculated for Huldra before it came on stream last year, for example. In this way it is possible to work out how harmful discharges can be avoided on new fields before they are brought into production.
The risk assessment approach differs from that adopted by the Ospar Commission, the main European policy-making body with respect to marine pollution in the northeast Atlantic. Ospar is seeking the twin goals of a direct reduction in the proportion of oil-in-water discharges from 40 to 30 ppm, and a 15% reduction in the total volume of oil discharged.
Ospar's approach is based on the principle of hazard assessment, whereby hazardous substances are identified by analyzing the characteristics of the chemicals present in produced water. It does not involve consideration of how the chemicals react with seawater, as does the risk assessment approach, says Johnsen. The issue of risk assessment-based management is now on Ospar's agenda.
The EIF has been accepted by Statoil as a key performance indicator. The company is now carrying out new calculations for all its fields to check whether anti-pollution measures are having the intended beneficial effect. To a lesser extent Norsk Hydro and other oil companies active in the Norwegian sector have also begun to take up the method.