Six operators on the Norwegian Continental Shelf (NCS) have joined the "DREAM Charter" project to further develop a unified numerical environmental risk-based assessment model for evaluation of environmental risks associated with regular discharges from the petroleum industry in offshore (e.g. produced water and drilling discharges,) and coastal sea areas (land terminals). The operators participating in this project are ENI Norway, Total E&P Norway, BP, ConocoPhillips Norway, Statoil and Det Norske. The environmental risk method applied is following a substance based approach, and is representing one of the two approaches adopted in the OSPAR recommendation for a risk-based approach to the management of produced water discharges from offshore installation (OSPAR Recommendation 2012/5). The environmental risk framework follows principles of environmental risk assessment in use in EU (e.g. ECHA – Technical Guidance documents) and US (US-EPA Guidance on risk assessment).

The numerical model predicts the exposure to substances or stressors present in regular discharges in the receiving environment, expressed as the PEC (Predicted Environmental Concentration). The characterisation of environmental risk of regular discharges is based on the PEC: PNEC ratios for the individual substances. The PNEC (Predicted No Effect Concentration) is the concentration of a substance or environmental stressor which adverse effects on the ecosystem and its organisms will most likely not occur.

To arrive at a single overall risk including multiple substances in the EIF methodology, the individual PEC: PNEC ratios are combined, based on species sensitivity distributions (SSDs), estimating the fraction of species potentially affected (PAF) at a specific exposures concentration. The overall risk can be assessed using multi-substance potentially affected fraction of species (msPAF). The msPAF is a combination of the individual PAF values for all substances and stressors in the discharge. The msPAF < 5% level is used as a cut off criterion, assuming that risk is inacceptable if more that 5% of the species are exposed above their chronic no observed effect concentration. Because this method combines risks of different substances or stressors, the contribution to risk from every single substance (stressor) can also be derived. This allows selection of mitigation measures with the highest risk reduction potential.

The risk-based management model allows estimation of environmental risks both in water column and in sediments from exposure to toxic (e.g. chemical substances) and nontoxic stressors (e.g. suspended particles, sediment burial). The model calculates the "Environmental Impact Factor" (EIF), which characterizes the overall potential impact on the marine environment in terms of recipient water volume and sediment surface area exceeding generic threshold values effects for toxic and nontoxic stressors.

The numeric risk-based model and the EIF concept have been applied by operators on the NCS the last decade to achieve the goal of "zero harmful discharges" to the offshore environment from regular discharges like produced water and drilling discharges. Identification of the potentially most harmful substances in discharges enables the operator to select the most environmental beneficial, optimal and cost-efficient mitigation measure to reduce the risk or potential impact of discharges on the marine environment.

This paper describes briefly the model framework of the environmental risk approach and the EIF concepts applied by operators for different regular discharges on the NCS. Some examples of use of the different EIFs are also illustrated. Furthermore, the current application of the model system and the plan for developments through the "DREAM Charter" joint industry project, are also briefly described.

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