1.3 Basic Concepts of ERA Acute
5
1.3.2 ERA Acute Uses Continuous Risk Functions
Where category-based models like MIRA (NOROG 2007) assume a probability
distribution of impacts in categories based on oil amount intervals, ERA Acute applies
a continuous impact function based on the exposure, lethality given exposure and the
VEC fraction present in the cell. The continuous impact and damage functions are
believed to be more suitable for e.g. Net environmental benefit analysis (NEBA)/Spill
Impact Mitigation Assessments (SIMAs) than category-based assessments, since
more subtle differences in exposure will give different impact results which the risk
assessor can then evaluate for decision making.
1.3.3 Two Main Steps—Three Levels of Detail
Questions to be answered by ERA-related studies vary in demand for detail,
depending on e.g. the phase of the project, the maturity of petroleum activity in
the region, or the sensitivity of the environment. The model framework is therefore
designed to be flexible in its uses, and ERA Acute calculates several endpoints.
Chapter 2 is dedicated to the application of ERA Acute results for environmental
risk management purposes.
The modelling is carried out in two main steps. In the first step (A), ERA Acute
methodology uses input from the oil spill trajectory model and the VEC data to
calculate impact in each grid cell for each oil drift simulation (see Fig. 1.6). The
results are summed up or averaged for each VEC in all compartments. In the second
step (B), recovery results are calculated.
1.3.3.1 Impact Calculation (Step A)
Impact modelling in ERA Acute uses the framework of probability of exposure (p exp ),
probability of lethal effect given exposure (p let ) and presence of vulnerable resources
(VEC “unit”) to calculate the mortality in each grid cell for each spill simulation. This
basic principle is the same in all compartments, however the actual calculation of
exposure and lethal effect, as well as the VEC “unit” in each compartment reflects the
differences in mechanism of harmful action. The exposure and lethality parameters
are determined from the compartment-specific oil spill impact parameters, such as
oil coverage above a certain film thickness on the sea surface to induce a mortality
to seabirds, turtles and marine mammals and oil mass on the shoreline to induce
impact to sensitive shoreline types. ERA Acute uses continuous exposure-response
relationships in order to predict mortality, meaning that a change in exposure from
the oil drift will lead to a change in mortality as output. For species at the sea surface
or in the water column compartment, the total injury is calculated for each spill
simulation by summarizing the impact in all grid cells affected by the simulation.
5
1.3.2 ERA Acute Uses Continuous Risk Functions
Where category-based models like MIRA (NOROG 2007) assume a probability
distribution of impacts in categories based on oil amount intervals, ERA Acute applies
a continuous impact function based on the exposure, lethality given exposure and the
VEC fraction present in the cell. The continuous impact and damage functions are
believed to be more suitable for e.g. Net environmental benefit analysis (NEBA)/Spill
Impact Mitigation Assessments (SIMAs) than category-based assessments, since
more subtle differences in exposure will give different impact results which the risk
assessor can then evaluate for decision making.
1.3.3 Two Main Steps—Three Levels of Detail
Questions to be answered by ERA-related studies vary in demand for detail,
depending on e.g. the phase of the project, the maturity of petroleum activity in
the region, or the sensitivity of the environment. The model framework is therefore
designed to be flexible in its uses, and ERA Acute calculates several endpoints.
Chapter 2 is dedicated to the application of ERA Acute results for environmental
risk management purposes.
The modelling is carried out in two main steps. In the first step (A), ERA Acute
methodology uses input from the oil spill trajectory model and the VEC data to
calculate impact in each grid cell for each oil drift simulation (see Fig. 1.6). The
results are summed up or averaged for each VEC in all compartments. In the second
step (B), recovery results are calculated.
1.3.3.1 Impact Calculation (Step A)
Impact modelling in ERA Acute uses the framework of probability of exposure (p exp ),
probability of lethal effect given exposure (p let ) and presence of vulnerable resources
(VEC “unit”) to calculate the mortality in each grid cell for each spill simulation. This
basic principle is the same in all compartments, however the actual calculation of
exposure and lethal effect, as well as the VEC “unit” in each compartment reflects the
differences in mechanism of harmful action. The exposure and lethality parameters
are determined from the compartment-specific oil spill impact parameters, such as
oil coverage above a certain film thickness on the sea surface to induce a mortality
to seabirds, turtles and marine mammals and oil mass on the shoreline to induce
impact to sensitive shoreline types. ERA Acute uses continuous exposure-response
relationships in order to predict mortality, meaning that a change in exposure from
the oil drift will lead to a change in mortality as output. For species at the sea surface
or in the water column compartment, the total injury is calculated for each spill
simulation by summarizing the impact in all grid cells affected by the simulation.