Application Of The EEXI Requirements To Steam Turbine LNG Carriers

Starting from 2011, the IMO has introduced in MARPOL Annex VI a series of technical and operational measures to improve the energy efficiency of ships and reduce the amount of CO₂ emissions from international shipping.
In 2011, the IMO has introduced in MARPOL Annex VI the Energy Efficiency Design Index (EEDI) which requires a minimum energy efficiency level for new ships. EEDI is a measure of the ship`s energy efficiency which is expressed in grams of CO₂ per ship`s capacity mile (in case of LNG carriers, tonne mile), in function of the ship type and size. The lower a ship`s EEDI, the more energy efficient it is and the less negative is its impact on the environment.
In 2021, the IMO has introduced in MARPOL Annex VI the Energy Efficiency Existing Ship Index (EEXI) which requires a minimum energy efficiency level for existing ships. EEXI is EEDI for the existing ships.
The EEXI requirements came into force on 1 November 2022. All LNG carriers with a gross tonnage of 400 tonnes and above were asked to calculate their Attained EEXI.
In the case of the steam turbine LNG carriers, the Attained EEXI is calculated by a formula which takes into account the following parameters¹:

- MCR_SteamTurbine is the maximum continuous rated power of steam turbines based on the rated installed power of each steam turbine.
In the case of LNG carriers of 10,000 DWT and above, the value of the Attained EEXI has to be utmost equal to or less than that of the Required EEXI for LNG carriers which sets a minimum energy efficiency level for LNG carriers.
The Required EEXI for LNG carriers is calculated based on a 30% reduction factor applicable to the reference line value set for LNG carriers. If the value of the Attained EEXI exceeds the Required EEXI, the ship does not comply with the EEXI requirements.
In order to comply with the Required EEXI for LNG carriers, the power of the steam turbines (MCR_SteamTurbine) can be reduced by installing a shaft power limitation system.
In the cases where a shaft power limitation system is installed, the installed power limited by the shaft power limitation system is stated as MCR_{SteamTurbine,lim}.
When calculating the acceptable limited power (MCR_{SteamTurbine,lim}), a reduction factor is applied to MCR_SteamTurbine until the Attained EEXI shall be equal to or less than the Required EEXI for LNG carriers.

- P_ME(i) is the power of main engines², i.e. the steam turbines output. It is calculated as 83% of the rated installed power (MCR_SteamTurbine) of each steam turbine (i):

P_ME(i) = 0.83 × MCR_SteamTurbine

In the cases where a shaft power limitation system is installed, P_ME(i) is calculated as 83% of the limited installed power (MCR_{SteamTurbine,lim}) of each steam turbine:

P_{ME_lim} = 0.83 × MCR_{SteamTurbine,lim}

In the cases where shaft generators are installed to steam turbines, the power of the shaft generators (P_PTO(i)) has to be taken into consideration for the calculation of P_ME value. In such case

P_ME(i) = 0.83 × (MCR_SteamTurbine - P_PTO(i))

If a shaft power limitation system is installed, the formula is:

P_ME(i) = 0.83 × (MCR_{SteamTurbine,lim} - P_PTO(i))

The power of shaft generators is calculated as 83% of the rated electrical output power of each shaft generator³.

- P_{ME_revised}

The excessive power from the combustion of the excessive natural boil-off gas in the boilers to avoid the release into the atmosphere can be deducted from P_ME(i). The new power value is stated as P_{ME_revised} and is calculated with the formula:

P_{ME_revised} = 0.83 × (MCR_{SteamTurbine,lim} - P_Excessive)

where:

P_Excessive is the excessive power from the combustion of the excessive natural boil-off gas in the boilers to avoid the release into the atmosphere. It is calculated as the difference between the nominal power generated by consuming all boil-off gas from the cargo tanks (P_BOG) and MCR_{SteamTurbine,lim}⁴.

P_Excessive = P_BOG - MCR_{SteamTurbine,lim}

- Ship`s Capacity – for LNG carriers is the ship`s Deadweight at summer load draft.

- V_ref is the ship`s reference speed at 83% of MCR_SteamTurbine or at 83% of MCR_{SteamTurbine,lim}⁵

- SFC_SteamTurbine is the certified specific fuel consumption of the steam turbines. In case of the power limitation by a shaft power limitation system, the specific fuel consumption is taken at P_{ME_lim}⁶

- C_F is the conversion factor between the fuel consumption and CO₂ emission.
Based on the above-mentioned parameters, the formula for the calculation of the Attained EEXI of the steam turbine LNG carriers is⁷:

Attained EEXI = (P_ME × SFC_SteamTurbine × C_F) / (DWT × V_ref)

In case of the shaft power limitation of P_ME and after the deduction of the power from the combustion of the excessive natural boil-off gas (P_Excessive), P_{ME_revised} shall be used in the formula for the calculation of the Attained EEXI.
In 2023, the LNG carriers of 10,000 DWT and above were subject to a survey by their Administration to verify whether their Attained EEXI were calculated in accordance with the requirements in the Regulation 23 of MARPOL Annex VI and EEXI Calculation Guidelines (Resolution MEPC.350(78)).

by Vlad Cioarec, International Trade Consultant

This article has been published in Commoditylaw`s Gas Trade Review Edition No. 3.

Endnotes:

1. See Resolution MEPC.350(78) – "2022 Guidelines On The Method Of Calculation Of The Attained Energy Efficiency Existing Ship Index (EEXI)"
2. The subscript ME(i) refers to the main engines.
3. See Resolution MEPC.364(79) – "2022 Guidelines On The Method Of Calculation Of The Attained Energy Efficiency Design Index (EEDI) For New Ships".
4. See IACS Recommendation No.172 – "EEXI Implementation Guidelines".
5. For the parameters to be used for the calculation of V_ref see Resolution MEPC.350(78) – "2022 Guidelines On The Method Of Calculation Of The Attained Energy Efficiency Existing Ship Index (EEXI)" and IACS Recommendation No.172 – "EEXI Implementation Guidelines".
6. See Resolution MEPC.351(78) – "2022 Guidelines On Survey And Certification Of The Attained Energy Efficiency Existing Ship Index (EEXI)".
7. See IACS Recommendation No.172 – "EEXI Implementation Guidelines".