Comparison of Risk Calculation Methods for Offshore O&G Produced Water Discharges

Abstract

Abstract Offshore produced water (PW) risk assessment methods are subject to debate as regards to the type and number of species tested and the use of assessment factor (AF). These include conservative ones with limited species tested and associated with high AF and less conservative ones with lower or no AF. This study was designed to characterize toxicity and calculate risk of PW discharges from platforms in different world areas using various marine species and different calculation methods. Five acute or pseudo chronic toxicity tests (bacteria, microalgae, crustacean, oyster, and sea urchin larvae) were performed on PW samples from twenty-two platforms. Results from the tests were derived into four thresholds for risk calculation using European Union guidelines: one conservative approach with three species and a high AF of 1000, five species and Species Sensitivity Distributions extrapolation (SSDs) with lower AF (10) and five species and acute to chronic ration (ACR) with low AF (10). Risk results calculated using DREAM three-dimensional dispersion model for the four methods were discussed against acceptable risk threshold in use in OSPAR area. The main drivers for risk calculation were first the PW discharge flow rate, then PW toxicity, and sea currents. The results showed that the method with three species (AF of 1000) was the most conservative approach with higher risk figures in 70% of PW samples and the one with 5 species and ACR (AF of 10) in 30% of the samples. Methods using more tests species and statistical extrapolation SSD method (AF of 10) allowed to obtain much lower risk figures. Depending on the risk calculation method used, the number of sites above acceptable risk threshold used in OSPAR area ranged from 17/22 (three species and AF of 10) to 3/22 (five species, SSD and AF of 10). For this study, the European Union guidelines defined for chemical environmental risk assessment, which are among the most conservative ones in the world, were used. Although the SSD method is already quite conservative, a higher AF was used for this study. This study showed that selection of risk calculation method is key for offshore PW discharge risk assessment. It also highlights that the use of limited number of species for risk assessment in offshore environment may also lead to significant risk overestimation.