Dynamic Assessment of the Ecological Risk of the Discharge of Produced Water From Oil and Gas Producing Platforms
Authors: Karman C.C. , Reerink H.G. 
Published: December 2007

Since 1991 the North Sea countries (United Kingdom, The 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 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 pre-requisites 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 the Dutch Oil Company (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 alter 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.

Introduction

In the offshore production of oil and gas, production chemicals are added to the water pumped into the well to enhance the production, to protect the equipment and to maintain safety on the platform. Part of these chemicals may end up in the marine environment as they are discharged with the produced water, and adverse effects to the biota in the ambient environment may occur. To prevent pollution caused by exploration and production activities, each North Sea country has developed a legislative system regarding the use of chemicals in the offshore exploration and production of oil and gas. By developing the systems individually, the policy became very inconsistent within the North Sea countries. It was therefore that in the North Sea Ministers Conference in 1990 it was decided to harmonise the legislation of the use of offshore exploration and production chemicals. In 1991 the CHARM project was initiated, in which authorities of the North sea countries, the chemical suppliers and operating companies co-operated in the development of a model for evaluating the environmental impact of the discharge of offshore E&P chemicals. The CHARM-model (Chemical Hazard Assessment and Risk Management) calculates the concentration of a chemical in the water at 500m from the platform, based upon worst case platform characteristics, which is compared with the toxicity threshold of biota. The CHARM-model is accepted by PARCOM as a valid model for performing hazard assessment of offshore E&P chemicals, as required by the Harmonised Mandatory Control System.

To calculate the concentration of a chemical at 500 meter from a platform the concentration of the chemical in the discharged produced water is multiplied with a fixed dilution factor. In the Hazard Assessment module (using worst case assumptions) a default dilution of 0.001 is used; in risk assessment experimentally determined figures may be used instead (i.e. rhodamine experiments). But in both cases the spatial and temporal distribution of the chemical is not taken into account, and a constant exposure concentration of the chemical is assumed at a fixed distance from the platform. In reality, however, the produced water will form a plume within the ambient seawater. The concentration will be higher in the center of the plume and decrease with the distance from the center and the distance from the plume (see figure 1). P. 173^