Since 1991 the North Sea countries (UK, Netherlands, Norway and Denmark) have put a lot of effort in the development of a decision support system for the legislation of the use and discharge of offshore exploration, drilling and production chemicals. The heart of this so-called 'harmonised mandatory control system' is the 'chemical hazard assessment and risk management' (CHARM) model. This model enables the ranking of chemicals on the basis of their intrinsic properties, using a realistic worst-case scenario. To meet the prerequisites of the model (simple and transparent calculation rules), the CHARM model uses a fixed dilution factor, assuming equal and constant dispersion of chemicals around the platform. In reality, however, the chemical follows a three-dimensional dispersion pattern which will change over time. To be able to use the principles of the CHARM model in such a dynamic situation for risk management, a new model has been developed by TNO in cooperation with Dutch Oil (NAM). This model gives a probabilistic estimation of the ecological risk of produced water, based upon a realistic calculation of the fate of components of produced water after discharge from the platform. Spatial and temporal variation in the concentration of chemicals is summarised in frequency distributions. The ecological risk is calculated for aquatic life, benthic life and the food chain. The model aims to support the selection of cost-effective mitigating measures for risk reduction.