Assessing Environmental Risk of Oil Spills with ERA Acute

Introducing the main concepts of ERA Acute, this chapter describes the overall framework and purpose of the methodology. ERA Acute is a recently developed oil spill risk assessment (OSRA) methodology for quantification of oil spill impacts and risk (Environmental Risk Assessment, ERA). It covers four environmental compartments; sea surface (seabirds, turtles, marine mammals), water column (fish eggs/larvae), shoreline and seafloor (species and habitats) using continuous impact functions and introduces the Resource Damage Factor (RDF). The methodology depends on external oil spill modelling and input data related to the presence and vulnerability of Valued Ecosystem Components (VECs). ERA Acute is developed to provide an improvement over the currently used “MIRA” method on the Norwegian Continental Shelf (NCS) and is better suited for risk management, decision-making and analyses from screening studies to full environmental risk assessments.

ERA Acute supports a variety of analyses, from simple screening studies based on oil spill statistics and potential impact areas to more in-depth impact and recovery calculations on species and habitats. The ERA Acute software tool has been built to enable and provide ease of use of the methodology and results. Visualizations of impact and risk areas can be made at several levels, from simulations and scenarios to whole cases. Results can have a monthly resolution to show variations throughout the year. This enables a wide range of decision-support from risk screening studies, impact assessments, risk quantification, risk management including effect of mitigating measures (NEBA/SIMA) evaluations to properly inform oil spill response planning. The methodology is suitable for global use and will be the recommended approach for oil spill risk assessments for offshore operators on the NCS.

ERA Acute is a model for environmental risk assessment of acute discharges. The calculations follow a common framework for all environmental compartments, whilst maintaining the mechanistic integrity of each compartment and/or VEC group, by using compartment-specific inputs of oil exposure and VEC-specific geographical distribution, vulnerability and recovery-defining parameters/functions. The method allows for using three different levels of detailing in VEC in the exposure and impact calculations. For the highest level of detail, a second step calculates recovery times in three time-factors, as well as the ERA Acute-specific RDF which combines the extent of impact and recovery. The continuous functions of impact and recovery calculations are presented in this chapter, separately for all four compartments. All data are calculated in grid cells, facilitating the use of GIS for viewing inputs and results. The methodology adds up impacts from grid cells to populations, and calculates result statistics from single simulations to scenarios, to multi-scenario DSHAs and cases.

To validate the predictive capability of ERA Acute, a study was carried out using data from two well-studied historic oil spills, the Exxon Valdez Oil Spill (EVOS) and the Deepwater Horizon Oil Spill (DHOS) incidents. Results from the case studies with ERA Acute were compared to the impact estimates and recovery observations that have been reported in the extensive research following the two incidents. Resource data relevant for each of the two incidents were reconstructed within the analysis area. Performance boundaries were set up for evaluating the ERA Acute results, based on the ranges of the impact and recovery estimates reported in the post-spill assessments. Validation of an oil spill ERA model against post-spill assessments of historic spills is a challenging exercise due to scientific limitations of both. ERA Acute performed satisfactorily compared to the performance boundaries and the study gave useful insight into the predictive capabilities of ERA Acute. The results from the study were used to evaluate between two different impact models and to increase the individual vulnerability of cetaceans.

Uncertainty evaluation and sensitivity testing of the functions and parameters used in ERA Acute serve two functions. ERA Acute is a deterministic model which is sensitive to the range of values used for the parameters. Parameters have inherent uncertainties as to what their true values are, and functions may have varying strength of knowledge. The individual functions were tested with respect to their sensitivity towards variation of the parameter values using both deterministic and stochastic testing. Based on the testing, an uncertainty scoring system was used to identify and prioritize the most important parameters for reducing uncertainty. Recommendations for handling the uncertainty and securing comparability in spite of uncertainty were set up as a conclusion of the studies.