For the calculation of the ESI SOx sub points the following fuel categories have been defined:
HIGH 0.50 < S % < 3.50 (previously HFO or High sea)
MID 0.10 < S % < 0.50 (previously HFO LS/MDO or ECA)
LOW 0.00 < S % < 0.10 (previously MDO LS or Port)
The MID category is included to allow more ships to benefit from the sub-points that are given for the use of low sulphur fuel. MDO/Gasoil with a sulphur content over 0.5 % is included in the HIGH category.
Fuel oils with a sulphur content of less than 0.1 % have cleaner emission characteristics and their use is mandatory in (Sulphur) Emission Control Areas (S)ECA’s; in certain ports and their approaches comparable requirements are in place. These characteristics are the reason that the MID and LOW category fuels carry more weight in the formula for determining the ESI SOx. Furthermore the calculation method is designed to stimulate the use of the cleanest fuel and this is done by giving bonus points if clean fuels are bunkered in favour of those with higher sulphur contents.
The maximum number of sub-points that may theoretically be obtained is 100 where HIGH fuel with sulphur content between 3.50 and 0.5 % carries a weight of 30 sub-points while MID and LOW fuels weigh in with both 35 sub-points. The formula to calculate the ESI SOx sub points is
x * 30 + y * 35 + z * 35
where x = the relative reduction of the average sulphur content a of HIGH; y = the relative reduction of the average sulphur content b of MID and z = the relative reduction of the average sulphur content c of LOW. The data of the Bunker Delivery Notes (BDNs) for fuels delivered on board of a vessel in the previous two quarters are used for the calculations.
We take a simple example and assume that in a particular two quarter period the vessel bunkered all three types of fuel with the following data: HIGH 100 tonnes @ 2.00 % S (a), MID 100 tonnes @ 0.30 % S (b) and LOW 50 tonnes @ 0.05 % S (c). The next data we need to consider are the baselines for the various fuels and that is where the IMO rules come into the picture with 3.50 % S for HIGH and 0.1 % S for LOW; the ESI Working Group has set the baseline for MID at 0.50 %. The relative reduction may now be calculated as follows:
• HIGH: x = (3.50 – a) / 3.0 = 0.5,
• MID: y = (0.50 – b) / 0.4 = 0.5 and
• LOW: z = (0.10 – c) / 0.1 = 0.5.
We can now calculate the ESI SOx sub points for these two quarters:
ESI SOx = 0.5 x 30 + 0.5 x 35 + 0.5 x 35 = 15.0 + 17.5 + 17.5 = 50.0 (16.13).
In the main formula for calculating the ESI Score the sub-scores are divided by 3.1; the result of this division is the number shown in brackets.
Now how is the use of cleaner fuels rewarded in the calculation: actually the average sulphur content for any fuel type that is more polluting and that is not bunkered is set at the lowest value of its category viz. 0.5, 0.1 and 0.0 % S for HIGH, MID and LOW fuel oils respectively; consequently the full amount of sub-points for any dirtier fuel that is not bunkered is given as a reward.
Continue with the above example and assume that the ship doesn’t bunker any HIGH but only MID and LOW fuels and the above rule would result in ESI SOx = 1.0 x 30 + 0.5 x 35 + 0.5 x 35 = 30 + 17.5 + 17.5 = 65 (20.97). Further cleaning up of the act and only bunkering LOW fuel oil would lead to the even higher value for ESI SOx = 1.0 x 30 + 1.0 x 35 + 0.5 x 35 = 30 + 35 + 17.5 = 82.5 (26.61).
The latter is however only applicable for vessels that have sailed in- and outside (S)ECA’s during the reporting period; where ships sailed in (S)ECA’s only, the 30 sub points for the HIGH range are not added.
The reason for this is that incentives are not provided for mandatory items and in (S)ECA’s vessels may only use LOW fuel oil; however, if the vessel did actually sail outside the (S)ECA while continuing to only use LOW fuel oil, the HIGH bonus will be added. In order to prove this, the number of days outside the (S)ECA during the reporting period has to be entered into a box in a section of the data entry page that will open after an appropriate question has been answered positively.