RightShip’s GHG Methodology
Rather than adopting a ‘one size fits all’ approach, the RightShip GHG Rating compares the theoretical CO2 emissions – sometimes known as greenhouse gases (GHG) - of a peer group of vessels across a similar size and type. A vessel’s GHG Rating is presented using an A-G energy efficiency scale, and relative performance is rated, with the most efficient vessels awarded A and the least efficient awarded G.
GHG Rating across a ship type
Ship types for comparison align with the International Maritime Organization’s (IMO) guidelines for calculation of Energy Efficiency Design Index (EEDI) reference lines and include bulk carriers, chemical, crude and product tankers, LNG tankers, LPG tankers, container ships, cruise and passenger ships, general cargo ships, refrigerated cargo ships and ro-ro vessels. Vessels are compared in size to other vessels plus or minus 10% of their DWT.
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.
The GHG Rating is dynamic and will almost always compare a different group of vessels - taken from within a larger sample - for each calculation made. As older vessels are scrapped, new vessels are commissioned, or retrofits/upgrades for existing vessels are verified at sea trials the relative performance of the peer group improves, and vessels are rated against a new average.
As of Q1 2023, the RightShip GHG Rating uses one of three sources when determining an individual vessel’s efficiency: the EEDI, the Existing Vessel Design Index (EVDI), or the Energy Efficiency Existing Ship Index (EEXI). The RightShip GHG Rating is the only environmental rating to encompass all three indices.
Both the EEDI and RightShip’s EVDI measure a ship’s theoretical CO2 emissions per tonne/nautical mile travelled. RightShip designed the EVDI in response to, and to work alongside the IMO’s EEDI. The EEDI is applicable to newbuild ships from 2013, whereas the EVDI focuses on existing vessels.
Where ship specific data is not available, such as specific fuel consumption, the values used in the EVDI calculation are based on the same assumptions used in the Second IMO GHG Study 2009 and/or detailed in IMO Circulars on calculation of the energy efficiency measure.
The IMO owned EEXI is based on the EEDI methodology and calculates the energy efficiency of vessels (g/ CO2/tonne nm). Vessels within scope must attain a score on or below the IMO’s required EEXI reference line. It requires a reduction in vessels’ GHG emissions per transport work, with IMO emissions intensity reduction targets set at 40% by 2030 and 70% by 2050. EEXI approval is required once in a lifetime and must be completed by the first annual, intermediate or renewal IAPP survey due, whichever is the earliest from 1st January 2023.
A vessel’s GHG Rating does not consider operational measures, such as slow steaming or eco speeds as these are included in the Carbon Intensity Indicator (CII), the other new regulation being introduced by the IMO in 2023. By focusing on design, a more meaningful outcome is achievable - enabling a like-for-like comparison and a score which demonstrates how efficiently the vessel can be operated. This benchmark can then be supplemented by operational measures when the ship is in use.
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.
Where GHG Rating calculations before 2023 were based on a vessel’s CO2 emissions at 75% of their Maximum Continuous Rating (MCR) and validated through their EEDI or EVDI certificates and calculations, EEXI compliance may be achieved through Engine Power Limitation (EPL) or Shaft Power Limitation (ShaPoLi). This value is calculated at 83% of the limited MCR (called MCR_lim). Where EPL is not used for EEXi, the value is calculated at 75% of MCR.
Thus, while the CO2 emission number can be significantly reduced through EPL with a relatively small reduction in maximum operational speed leading to a better GHG Rating, the vessel will continue to have the same speed-power-fuel emission profile and therefore may emit the same amount at a given speed.
The 2023 GHG Rating revised methodology addresses issues such as charter policies that discriminate against vessels of older tonnage because they have implemented EPL/ShaPoLi or because they are pre-EEDI, by using a speed corrected approach and evaluating all vessels within a peer group (i.e., of similar size and type) at a common speed. The improved speed-corrected approach to the GHG Rating is a necessary adjustment to an existing system.
Retrofits and upgrades
Retrofits and upgrades such as changes to ship design, propulsion and machinery may help to improve a vessel’s GHG Rating. Any upgrade or retrofit which has been verified by a classification society can be submitted to RightShip, together with new EEXI and certificate enabling the GHG Rating to be recalculated.
Approved enhancement measures will have a plus (+) sign adjoined to their GHG Rating as well as having the types of Energy Saving Devices deployed are featured for each respective vessel on the Platform to enable shipowners to further demonstrate their commitment to decarbonisation.