As maritime transportation has become an important method of international trade, the increase in emissions that harm the nature over time has led to the reduction and control of these emissions, and for this purpose, many global-scale agreements have been made under the leadership of the United Nations. 

Although the role of maritime transport in greenhouse gas emissions is a controversial issue, the International Convention for the Prevention of Pollution from Ships (MARPOL) was adopted by the International Maritime Organization (IMO) in order to ensure a sustainable development in shipping. The Regulation on the Prevention of Air Pollution from Ships is included in MARPOL Annex VI. MARPOL Annex VI applies to all ships and fixed and floating drilling platforms and sets limits for ship-borne SOx and NOx emissions. It also prohibits the intentional release of ozone-depleting substances.

Amendments to Annex VI, which entered into force through a protocol adopted in 1997, were initially adopted under a process of "tacit acceptance", meaning that they entered into force on a specific date unless a certain number of parties objected.  MARPOL Annex VI allows for the declaration of an Emission Control Area (ECA) for more limited SOx and NOx control. In 2008, MARPOL was substantially revised and new rules for exhaust emissions were introduced. The revised MARPOL Annex VI entered into force on July 1, 2010 (Kalendar, 2017).

Emergence of Energy Efficiency Existing Ship Index and Carbon Intensity Index 

Reducing carbon emissions has been an important agenda item in the maritime sector for many years, as in all industrial areas. MPEC (Marine Protection Environment Committee), which meets periodically to revise MARPOL (The International Convention for the Prevention of Pollution from Ships), which includes binding provisions for IMO member states through the United Nations, met in 2021, but the meetings on 10-17 June could not be completed due to lack of time. 

However, at the MPEC76 meeting, the issues of reducing carbon intensity and achieving IMO targets on greenhouse gas emissions were explained with the revised regulations 20 and 21 according to Marpol Annex VI / Part 1. 

These amendments combine technical and operational approaches to improve the energy efficiency of ships in line with the targets set in the 2018 Initial IMO Strategy to Reduce Greenhouse Gas Emissions from Ships and also provide important building blocks for future GHG reduction measures. 

Based on the forecasts made at these meetings, IMO has declared and registered a target to reduce the carbon intensity of international maritime transport by 

What is EEXI?

As a technical approach, the energy efficiency of existing ships is related to the design of the ships during the construction process. Many factors affect the energy efficiency of ships, ranging from the ayrodynamic structure of the ships, the material preferred during the construction process, hull paint, and machinery arrangements. The Energy Efficiency Existing Ship Index (2022) put forward by IMO (2023) is a mandatory procedure for all ships over 400 gross tons as of 2023. At this point, IMO divides the EEXI issue into two parts as the current value and the required value. As a result of the calculations made in the guidelines, the current value called Attained EEXI must be equal to or less than the required value called Required EEXI under all circumstances. 

How are EEXI Calculations Performed?

EEXI calculations can be provided by third-party services or companies can do it in-house. The important thing here is that this is recorded and approved by the authorities.

Attained EEXI ≤ Required EEXI
Required EEXI = (1-Y/100) x EEDI 33Reference line value
- where Y is the reduction factor
- EEDI Reference line value = a x b -c

                                    CO2 Conversion Coefficient x Specific Fuel Consumption (g/kW.h) x Machine Power (kW)
Attained EEXI = ---------------------------------------------------------------------------------
                                    Capacity (tons) x EEXI speed (knots)

* Calculate CO2 emissions from a ship carrying 1 ton of cargo for 1 nautical mile

EEXI Documentation

As stated in the calculations, if Attained EEXI is less than or equal to Required EEXI, the shipowner is required to prepare a technical file, obtain class approval, convert it into a statement and present it at the first survey in 2023. After submitting this, EEXI and IAPP certificates will be renewed and EEXI issue will be closed for this ship until 2026. 

According to the calculations made, if the Attained EEXI is above the Required EEXI value, we need to make some changes in the ship's machinery. 

What to do if the EEXI value is below the required value

If the EEXI value remains below the desired value, more than one application can be made. The main thing here is for the shipowner to decide on the changes to be made by considering the decision to be made in accordance with the commercial operations in the specific case and, if necessary, by consulting with a consultancy service. The first step is to replace the average approximate reference speed and standard fuel consumption values with measured model test and shop test data. At the end of this process, it is envisaged that the model test data will be replaced by actual numerical data (DNV, 2022).

If, after applying individual data based on the documented information provided, there is still a gap between the EEXI value obtained and the required EEXI value, the design does not comply with the EEXI requirements and the design needs to be modified. This could be the installation of energy efficiency technologies or limiting the installed main motor power. At this point, the most suitable in terms of price, performance and benefit is EPL, i.e. Machine Power Limiting. 

Popular energy efficiency changes are energy-saving devices such as ducts or propeller head blades that increase the reference speed for constant installed power. According to EEXI calculation guidelines, the effectiveness of these devices will be determined by model tests (DNV, 2022).

EPL (Machine Power Limitation)

If EPL (Engine Power Limitation) is installed on ships; either necessary arrangements will be made with 75% of the main engine power or necessary arrangements are made with 83% of the reduced power. As a result of this arrangement, the maximum power value of the ship's machinery is reduced and the speed of the ship is also reduced. If this value is at a reasonable value that will not affect the commercial activities of the ship, commercial activities can be continued by preparing a technical file with the new Attained EEXI value obtained and obtaining class approval.

Engine Power Limitation is a system that aims to increase energy efficiency by limiting the ship's engine power to optimum engine settings. As a result, the speed of the ship will be limited. EPL can also be used as an effective measure to increase the energy efficiency of existing ships in terms of EEXI

The EPL consists of a simple device that, under current regulations, can easily limit the maximum engine power by adjusting the fuel index limiter on the engine control system without the need to install a complex system on the engine. The EPL can be installed quickly and easily when the vessel calls at a port without the need to update the EIAPP certificate or NOx technical file.

With the help of a seal to be installed in mechanical machines, the machine is prevented from reaching a power higher than the specified power. In electronic machines, these limitations are realized with the settings to be made on the bridge or from the computer in the machine control room.

These limitations can be deactivated especially in cases such as adverse weather conditions, conflict, pirate attack on electronic machines. Therefore, the limited machine power does not have to meet the minimum power requirement.

When will EEXI come into force?

First of all, ships are required to strengthen the Ship Energy Efficiency Management Plans they are currently using and have them approved by the end of 2022. Thus, these plans, which have existed on the ships without being subject to approval until today, are required to pass class approval in their revised form. Therefore, up-to-date information on EEXI and CII should be included in this management plan. 

As of January 1, 2023, the process will start with the renewal of EEXI Certificates and IAPP (International Air Pollution Prevention) certificates at the first survey (Annual, Interim or Renewal) of the ships.  Thus, as of January 1, 2023, ships will be obliged to calculate and record the EEXI (Energy Efficiency Existing Ship Index) values used to measure energy efficiency. In 2026, it is foreseen that a regulation will be made for the next process to be carried out after the current rules.


In contrast to the one-off situation of EEXI in shipbuilding, the Carbon Insensity Index is an operational process that needs to be continuously improved. For ships of 5000 GT and above (ships subject to IMO - DCS requirements regarding fuel consumption on board ships), the annual operational carbon intensity indicator (CII) is rated to continuously improve the operational carbon intensity of ships at certain levels. 

Currently, within the scope of IMO - DCS, data has been collected for 3 years by recording the annual voyages of the ships, their fuel consumption and the distances traveled. According to the decision taken, a baseline will be determined from the data collected in 2019 (first year).  

The actual annual operational CII level will need to be documented and verified for comparison with the desired annual operational CII level. The rating will be based on a scale established and recorded by IMO. According to this scale, the operational carbon intensity grade can be A, B, C, D and E. These values are respectively: Major superior, minor superior, moderate, minor inferior, or inferior performance level. The performance level is recorded in the Statement of Compliance and more detailed calculations are included in the SEEMP (Ship Energy Efficiency Management Plan).

The performance level will be recorded in the ship's Ship Energy Efficiency Management Plan (SEEMP).

Ships that have received a D or E grade for three consecutive years must have a corrective action plan approved and submitted to the authority within one month showing how to achieve the desired grade (C or above) and demonstrate that they have improved their CII to C or above in the current year. Authorities, port authorities and other appropriate stakeholders are encouraged to offer incentives to ships with an A or B rating.

IMO wants your ship's CII to be 5 percent lower in 2023 than it was in 2019. If you can achieve this, your ship will receive a C value when you achieve a full 5 percent reduction. If you exceed this, your energy efficiency score moves to class A or B, and if it is below these, your ship is categorized in class D or E.

CII Calculation
The CII is basically calculated by multiplying the annual fuel consumption by the carbon emission factor and then dividing this by the capacity times the number of kilometers traveled. The relevant formula is given below.

What the Future Holds for EEXI and CII

As we mentioned at the beginning, greenhouse gas emissions from ships are actually a very controversial issue. Compared to many industrial areas, the maritime sector is far behind in terms of Green House Emissions. However, the main example here is that the maritime sector, which we can think of as an exemplary showcase for the country's economies to change themselves in this direction due to the regulations that connect international trade, should take the lead in this regard. 

As a result, it is obvious that world trade will move forward with a greener perspective, especially with Europe being the dynamo. Therefore, it is expected and normal for the maritime industry to adapt to this before anyone else. The commercially negative impacts of this will in fact be absorbed by the many marginal benefits gained in the long run.


Kalender, S. (2017). Experimental Investigation of the Effects of Ultra-Low Sulfur Diesel Fuel on Particulate Matter and Gas Emissions from Ships. Istanbul Technical University. Unpublished Doctoral Thesis
Rigos, N. (2022). The effect of engine power limitation on the energy efficiency existing ship index (EEXI).

Ivanova, G. (2021, September). Analysis of the Specifics in Calculating the Index of Existing Marine Energy Efficiency EEXI in Force since 2023. In 2021 13th Electrical Engineering Faculty Conference (BulEF) (pp. 1-4). IEEE.

IMO (2022). Rules on ship carbon intensity and rating system enter into force. available at:

DNV. (2022) EEXI - Energy Efficiency Existing Ship Index. access address:

Kpt. Demir (2002). MEPC76 - Everything About EEXI and CII Discussed at KOSDER Academy access address: