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GHG Emissions Rating

GHG Rating Methodology

A vessel's GHG Rating is presented using the standard European energy efficiency scale. The relative performance of a vessel is rated from A through to G, the most efficient being A, the least efficient being G.

GHG Rating Clear Background

GHG Emissions A – G Rating Scale

How ships are compared

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. Categories include bulk carriers, chemical tankers, container ships, crude & product tankers, cruise passenger and general cargo ships, LNG tankers, LPG tankers, refridgerated 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.

EVDI

The core measure for comparing the relative efficiency of the world's fleet is grams of CO2 per tonne nautical mile.

RightShip utilise one of two sources when determing an individual vessel's efficiency:

  1. EEDI (Energy Efficiency Design Index) - measures the theoretical CO emission performance of new ships over 400 gross tonnes and is calculated from ship design and engine performance data. This is a regulatory requirement for new ships developed by the IMO (and applied on an ad-hoc basis to existing vessels), or
  2. EVDI (Existing Vessel Design Index) - developed by RightShip, this index also measures a ship's theoretical CO2 emissions per nautical mile travelled. However, the EVDI can be applied to existing vessels as well as new builds (where EEDI is not available/applicable).

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.

EVDI formula

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:

GHg Methodology Formula 3

Source: Based on the IMOs RESOLUTION MEPC.245(66), adopted on 4 April 2014; 2014 GUIDELINES ON THE METHOD OF CALCULATION OF THE ATTAINED ENERGY EFFICIENCY DESIGN INDEX (EEDI) FOR NEW SHIPS.

Rating distribution

The GHG Rating compares the relative efficiency of a ship using the EVDI (or EEDI if applicable). 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.

GHG emissions rating

A

B

C

D

E

F

G

Size Score

> 2.0

> 1.0

> 0.5

> -0.5

> -1.0

> -2.0

<= -2.0

Area Under Curve

2.5%

13.5%

16%

36%

16%

13.5%

2.5%

Normal peer distribution

The bell curve below illustrates the percentage distribution with the corresponding letter displayed in the coloured area under the curve. The x-axis is expressed as a count of standard deviations which matches the size score in the key.

GHG-Emissions -Ratings _2 Clear Background

Example

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 or 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.

 GHG Example

DNV GL methodology review

In January 2015 DNV GL commenced an independent methodology review of RightShip's GHG Rating.

The review was finalised in November 2015 and the key points included:

  • There is a trend for the number of A-rated vessels to increase with younger vessel age. This observation (that there seems to be a correlation between year of build and GHG rating, and not only DWT) further strengthens the case for the robustness for the scheme.
  • By analysing a sample dataset of over 10,000 bulk carriers, it was observed that the median difference of EVDI to EEDI was 5% (thereby EVDI being more conservative), which is enough to give an average improvement of A-G rating by over 1 rate, i.e. more than one A-G letter. 
  • There exists an incentive for interested parties (e.g. ship owners / managers) to update (‘verify’) EVDI values to remove any uncertainty that may exist between ‘verified’ and ‘non-verified’ ratings.

The executive summary of the outcomes of DNV GL’s methodology review can be found here.