How ships are compared
Rather than adopting a ‘one size fits all’ approach, the GHG Rating compares the theoretical CO2 emissions of a peer group of vessels, across similar size and type.
Ship types for comparison align with the IMO’s guidelines for calculation of EEDI reference lines. The peer groups for comparison include bulk carriers, chemical tankers, containerships, crude & product tankers, cruise & passenger ships, general cargo ships, LNG tankers, LPG tankers, refrigerated cargo ships and ro-ro cargo ships.
Vessels are compared in size to other vessels plus or minus 10% of their DWT.
RightShip’s GHG Rating utilises one of two sources when determining an individual vessel’s efficiency. We consider the EVDI (Existing Vessel Design Index), or the EEDI (Energy Efficiency Design Index).
The EVDI and EEDI provide an indication of a vessel's design efficiency, representing grams of CO2 per tonne nautical mile, and can be used interchangeably.
GHG Ratings across a ship type
The GHG Rating compares the relative efficiency of a ship using the EVDI (or EEDI as applicable).
This means that a vessel is assigned a rating based on how it’s EVDI compares to the average EVDI score for vessels of a similar type and size.
The rating is designed to place an average vessel in the middle of the D rating band, generally following a normal peer distribution, or bell curve. Vessels which are more efficient than the average are given a higher rating, while vessels which are less efficient are provided with a lower rating.
In general, the ratings for a vessel’s peer group fit the following fixed percentages. When we look at the distribution of ratings across a vessel type, the largest portion of vessels are D rated and only a small number are A or G rated vessels. However, this may vary depending on the makeup of the fleet.
A vessel’s size score represents where it sits within a GHG Rating band. The size score is displayed along the bottom of the bell curve below. You can see that a vessel with a size score of 1.3 is B-rated, where as a size score of -1.3 is a F-rated vessel. The ratings are dynamic and subject to change as the peer group changes, therefore it is common for a vessel’s size score and GHG Rating to slowly change over time.