The computation of individual EIFs is complex and involves several steps. Here is a grossly simplified description.
Produced water to be discharged into the sea is first subdivided into various pollutants, each of which is characterized by a PNEC (Predicted No Effect Concentration) threshold value.
Produced water to be discharged into the sea is first subdivided into various pollutants, each of which is characterized by a PNEC (Predicted No Effect Concentration) threshold value.
The fate (i.e. the dilution and dispersion) of pollutant groups is then modelled as a basis for predicting environmental concentrations (PECs).
Finally, an EIF index is established by estimating the volume of potentially harmful discharge water in the area under consideration.
The maximum resolution of the dilution model for EIF calculations is 100 m x 100 m x 10 m (where 10 m represents the water depth) – a volume of 100 000 cubic metres.
If this is the total volume containing potentially harmful discharge, the EIF index will be 1.0. If the volume is twice the size – i.e. 200 000 cubic metres - the EIF index will be 2. And so forth.
EIF indices are thus expressed as whole numbers.
They may be used to characterize either a single platform or discharge outlet, a producing oilfield with several platforms and outlets, or an area comprising several fields.
A grand, regional EIF can also be determined; for example, for the entire North Sea or the entire Norwegian sector.
EIF prognoses can also be made for the lifespan of individual fields, including prognoses for each outlet. The higher the EIF index,the greater the potential for environmental damage.
The mathematical procedure also generates a spreadsheet listing the contribution of each produced water group to the EIF, both natural and man-induced.
These data can be manipulated to systematically test the impact of new technologies on the EIF for cleaning discharges and/or substituting or phasing-out certain production chemicals.
In this way, North Sea operators can make informed decisions about which measures should be adopted based on rigorous environmental and cost-benefit ranking.
However, a full environmental risk assessment is needed to properly test those that are finally selected or to help weigh up the pros and cons of re-injection.
So far the technique has been fully implemented on all Statoil-operated fields for discharges from single outlets and is used as a key performance indicator.
It has also been accepted by the Norwegian State Pollution Control Agency, SFT, and is now used by all operators on the Norwegian Continental Shelf.
