The calculation of the EEXI follows the calculation of the well-known EEDI. It is based on the 2018 calculation guideline of the EEDI, with some adaptations for existing vessels. The draft guidelines (submission ISWG-GHG 7/2/7), are now replaced by the guidelines adopted at MEPC 76 in June 2021.
In principle, the EEXI describes the CO2 emissions per cargo ton and mile. It determines the standardized CO2 emissions related to installed engine power, transport capacity and ship speed. The EEXI is a design index, not an operational index. No measured values of past years are relevant and no on-board measurements are required; the index only refers to the design of the ship.
The emissions are calculated from installed power of the main engine, the corresponding specific fuel oil consumption of the main engine and of auxiliary engines, and a conversion factor between fuel and the corresponding CO2 mass. The transport work is determined by capacity, which is usually the deadweight of a ship and the ship speed related to the installed power.
The calculation does not consider the maximum engine power, but for most ship types 75% of MCR or 83% of MCRlim (in case of an installed overridable power limitation). Specific fuel oil consumption of the main engine and ship speed are regarded for this specific power.
The EEXI is applied to almost all oceangoing cargo and passenger ships above 400 gross tonnage. For different ship types, proper adjustments of the formula have been introduced to allow a suitable comparison. This is performed by correction factors. Several correction factors are defined to correct the installed power, e.g. for Ice-class ships, as well as to correct the capacity, e.g. to consider structural enhancement. Further correction factors are applicable for cranes on board and for Ice-classed ships having IA Super and IA.
With all these correction factors being applicable only for specific ship types, the initially simple calculation of the EEXI might become quite complex for some ships. DNV's maritime advisory, with eight years and several hundred cases of experience in the calculation of the EEDI, offers reliable, accurate and profound determination of the EEXI attained for each ship, which is subject to EEXI regulations.
Differences between EEDI and EEXI calculations
There are often slight differences between the attained EEDI values of sister vessels. This is partly due to the varying lightweight of ships within a building series, but mainly caused by different ship speeds determined in individual sea trials of each ship.
For EEDI ships the EEXI reference speed is determined from sea trials performed within the EEDI certification, for pre-EEDI vessels the EEXI reference speed is taken from the speed/power curve determined in model tests of the specific design or – under certain circumstances – from sea trials or from CFD calculations.
Additionally, the EEXI calculation guideline offers the option to calculate the reference speed with an approximate formula based on the ship type, deadweight and installed power. With an included margin factor of 5%, this approximated reference speed will be conservative, so that it is recommended to provide model test or sea trial reports whenever possible.
The possible lack of data is the reason for the introduction of the approximated reference speed formula. There might be an equivalent dilemma for the specific fuel oil consumption values of main and auxiliary engines regarding possible lack of shop test values. For this case standard, values for the specific fuel oil consumption of main and auxiliary engines are defined within the EEXI calculation guideline. Again, these standard values are conservative by trend, and the application of measured shop test data will improve the attained EEXI, so it is recommended to provide corresponding reports.