The relation between EEXI, EEDI and Shaft Power Limiters

All ocean-going cargo and passenger ships above 400 gross tonnage must have an Energy Efficiency eXisting ship Index (EEXI) measurement before the first survey after January 2023, while cargo, RoPAX and Cruise vessels over 400 gross tonnage will be required to have a Carbon Intensity Indicator (CII) also.

The new regulations have been adopted by the IMO through the Marine Environment Protection Committee (MEPC) following acceptance of the 2021 revised International Convention for the Prevention of Pollution from Ships (MARPOL) Annex VI which includes amendments for mandatory goal-based technical and operational measures to reduce the carbon intensity of international shipping.

What is EEXI

EEXI and CII describe the CO2 emissions per cargo ton and mile of a vessel, with the latter demanding annual updates and the former a once in a lifetime process using the vessel’s design parameters as a basis.

The EEXI determines the standardized CO2 emissions related to installed engine power, transport capacity and ship speed. Ships affected by the EEXI can become compliant by deploying a Shaft Power Limiter (SPL) or Engine Power Limiter (EPL), following guidelines introduced on June 17th 2021 in MEPC76 Annex 9: 2021 Guidelines on the shaft / engine power limitation system required to comply with the EEXI requirements and the use of a power reserve.

‘Use of a power reserve’ refers to any extra power production capability over and above the limit calculated by a ships EEXI figure. An SPL is essentially used to override and log the use of available power over the limit set by EEXI, which can only be accessed for the purposes of saving life at sea or securing the ship’s safety.

Temporary removal of the shaft or engine power limit via the SPL can only be done with approval by the vessel’s Captain or Officer in Charge of Night Watch (OICNW), which is why it is advised that the control interface is installed in the vessels’ bridge. This ensures that those in charge do not have to leave the bridge and that there is no delay in overriding the limiter to access additional power in an emergency.

Use of a power reserve should not have adverse impact on the propeller, shaft and related systems. It is important that the ship master and OICNW are not restricted from exercising judgement to override the SPL when required for safety purposes. The authority for this should be clearly set out in the Onboard Management Manual (OMM) and/or the Safety Management System manual, as appropriate.

Secure system

It is possible, though not mandatory, to configure the SPL to require a password or pin number to access reserve power. It is mandatory that the system be tamper-proof and that any override instances are logged in the OMM and in certain cases, e.g., when the limiter was deactivated but reserve power not used, the engine room and bridge logbooks.

The EEXI is essentially the same as the earlier adopted Energy Efficient Design Index (EEDI), which applies to newbuild vessels only, and while the two have separate guidelines at the moment, it is expected that they will be consolidated into a single set of guidelines on approval by committee in the future.

Shaft Power Limiter technical requirements

The SPL system should consist of the following main arrangements:

  1. sensors for measuring the torque and rotational speed delivered to the propeller(s) of the ship. The system includes the amplifier and the analogue to the digital converter;
  2. a data recording and processing device for tracking and calculation of the data; and
  3. a control unit for calculation and limitation of the power transmitted by the shaft to the propeller(s);
  4. where technically possible and feasible, the SPL system should be controlled from the ships' bridge and not require attendance in the machinery space by ship's personnel.

General requirements

  1. Should a password or pin be required to de-activate the SPL, every effort should be made to ensure the required information is always available to the authorised personnel
  2. For electronically controlled engines, the SPL control unit must provide clear indication when the limiter is currently removed
  3. The SPL should be tamper-proof
  4. The SPL for the electronically controlled engine should indicate shaft rotational speed, shaft torque and shaft power (and total shaft power in case of multiple shaft arrangements) to be recorded constantly in unlimiting mode
  5. The procedure for SPL on the propulsion system should be described in the OMM for SPL in accordance with section 4 of these Guidelines.

Demonstration of compliance of the SPL system

  1. The demonstration of compliance of the SPL system should be verified by an appropriate survey in accordance with regulation 5.4 of MARPOL Annex VI for the verification of the ship's EEXI according to regulation 23. The survey should include the verification and validation of the system by addressing the following items:
    1. the verification of compliance of the system with the OMM for SPL;
    2. the verification of compliance of the system with the specifications set out in section 2 of Annex 9, Resolution MEPC.335(76); and
    3. the verification that the OMM for SPL is in compliance with the specifications set out in section 4 of Annex 9, Resolution MEPC.335(76).
  2. In cases where the SPL system is applied and no changes are made to NOx critical settings and/or components outside what is allowed by the engine technical file as defined in the 2008 NOx Technical Code {NTC 2008), engine re-certification is not needed.
  3. In cases where the SPL system is applied and the NOx critical settings and/or components are altered beyond what is allowed by the engine technical file as defined in NTC 2008, the engine needs to be re-certified. In such a case, for an EEDl-certified ship where the SPL system is applied at a power below that required by regulation 24.5 of MARPOL Annex VI (minimum power requirement), the certified engine power should be at the power satisfying that requirement.

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