Rather than adopting a ‘one size fits all’ approach, the GHG Rating provides comparison of a peer group of vessels, for example comparing a 100,000 DWT bulk carrier to a bulk carrier of a similar size and type.
Ship types for comparison align with the IMO’s documentation and guidance.Read More
Categories include bulk carriers, chemical tankers, container ships, crude & product tankers, cruise passenger and general cargo ships, LNG tankers, LPG tankers, refrigerated cargo ships and ro-ro cargo ships.
In terms of sizes for comparison vessels are compared to other vessels plus or minus 10% of their DWT.
The core measure for comparing the relative efficiency of the world’s fleet is grams of CO2 per tonne nautical mile.Read More
RightShip utilise one of two sources when determining an individual vessel’s efficiency:
As the two methods compare relative efficiency on the same basis, a like-for-like comparison of efficiency is achievable – as outlined in DNV GL’s review of RightShip’s methodology in 2015.
While the GHG Rating is relatively easy to interpret, the methodology used to calculate the GHG Rating is complex. The EVDI formula is as below:
The GHG Rating compares the relative efficiency of a ship using the EVDI (or EEDI as applicable).Read More
The peer group comparison is based on the number of standard deviations a vessel’s EVDI varies from the average for similar sized vessels of the same ship type.
In general, the distribution of the size group fit the following fixed percentiles of the data set.
The bell curve below illustrates the percentage distribution with the corresponding letter displayed in the coloured area under the curve.Read More
The x-axis is expressed as a count of standard deviations which matches the Size Score in the key.
For example, a 176,382 deadweight tonne bulk carrier has a GHG Rating of D, with a size score of -0.345.
It is considered alongside ships of plus or minus 10% of its deadweight, and is therefore compared to vessels within the range of 158,744 to 194,020 deadweight tonne.
In this case, as there are 1,060 ships in the peer group, approximately 735 are more efficient and 325 are less efficient than this particular vessel.
In January 2015 DNV GL commenced an independent methodology review of RightShip’s GHG Rating.Read More
The review was finalised in November 2015 and the key points included:
The executive summary of the outcomes of DNV GL’s methodology review can be found here.