The RightShip GHG Rating methodology  

RightShip’s GHG Rating compares a vessel’s designed efficiency against peer vessels of similar size and type using a speed-corrected methodology. This approach evaluates all vessels within a peer group at a common speed and focuses on the emissions in the range of a vessel’s market operating speeds, based upon a speed corrected intensity. The vessel’s GHG Rating is presented using an A-E scale, with the most efficient vessels being awarded A and the least efficient awarded E. 

GHG Rating across a ship type 

Ship types for comparison predominantly follow those in the MARPOL, Annex VI (Revised 2021) and regrouped for similar cargo type, including bulk carriers, chemical tankers, container ships, crude and product tankers, passenger and cruise ships, general cargo ships, LNG carriers, LPG tankers, refrigerated cargo ships, ro-ro vessels, and vehicle carriers. 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.  

The GHG Rating size group is based on the size score, which indicates the number of standard deviations by which a vessel varies from the average for similar sized vessels of the same ship type.

If the distribution of the size scores exactly fit a normal distribution, the score ranges would match the fixed percentiles of the data set mentioned in the below chart. The vessel’s position on the A-E scale is determined by the size score and GHG Rating key as follows:

_{(Figure 5: GHG Rating Key)}

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.

Consequently, vessels can have their Ratings upgraded by enhancing design factors which have a measurable impact on efficiency, primarily through the addition of energy saving devices (ESD). The addition of an ESD will require evidence be submitted to RightShip in order to update the vessel characteristics. 

Size score 

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. 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. 

 

The incorporation of EEXI, EVDI, and EEDI   

On 17 June 2021, the IMO adopted amendments to MARPOL Annex VI at MEPC 76, introducing regulations 23 and 25 - the Energy Efficiency Existing Ship Index (EEXI). This amendment requires ships to attain EEXI approval once in a lifetime and must be completed by the first annual, intermediate or renewal survey due, whichever is the earliest from 1st January 2023.

The IMO has defined designed energy efficiency for EEXI as grams of CO2 per tonne nautical mile and the IMO MEPC formulated both EEDI and EEXI as measures of a ship’s CO2 emissions. Both are calculated using characteristics of the ship at build, incorporating parameters that include ship capacity, engine power and fuel consumption.

In 2012, RightShip developed an Existing Vessel Design Index (EVDI) to work alongside the EEDI (introduced by IMO in 2013) as inputs into a Greenhouse Gas (GHG) Rating to enable comparisons of similar vessel types within peer groups for all maritime stakeholders.  

With the implementation on December 6th 2023 of the GHG 2.0 Rating system, RightShip accepts the EEXI as a similar metric overriding EVDI for existing ships. The speed corrected methodology uses EEDI, EVDI ,and EEXI and allows relative comparison of a ship’s CO2 emissions to vessels of a similar size and type. Ship types are largely consistent with those used by IMO MEPC.

Power limitation 

RightShip believes that power limitation should neither give undue advantage nor be discriminatory to vessels, while promoting vessel efficiency to cut down CO2 emissions. Therefore, the GHG Rating methodology addresses such barriers with a speed corrected approach and evaluates all vessel within a peer group (i.e., of similar size and type) at a common speed.  

This common speed is referenced from the Fourth IMO GHG Study 2020, with a reduction of 10% to allow for slowdown of world fleet due to EEXI and power limitation.  

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.  

How a ship’s GHG Rating changes to a 5-point scale 

On December 6th 2023, RightShip’s GHG Rating moved from a 7-point scale (A-G) to a 5-point scale (A-E). Broadly speaking, As and Bs under the old methodology are now As. Fs and Gs are now Es. And all ratings in between have moved one scale up.