Addressing and explaining EEXI ship power limitations
While compliance with the Energy Efficiency Design Index (EEDI) for newbuild ships was largely achieved by engine derating, larger diameter propellers, lower rpm and hullform optimisation, adherence to the Energy Efficiency Existing Ship Index (EEXI), a technical certification measure for existing ships, will mainly fall to ship power limitations.
At a recent Nautical Institute (NI)/Royal Institution of Naval Architects (RINA) webinar, Edwin Pang, MRINA, Chairman RINA IMO Committee, drew attention to draft guidelines for implementing EEXI, which have been developed by an IMO correspondence group. The guidance, which will be further discussed at the ISWG-GHHG meeting in May and MEPC 76 in June, foresees two main types of ship power limits: shaft power limitation (ShaPoLi) and engine power limitation (EPL).
A limited EEXI calculation
Ship power limitations are proposed as they can facilitate a reduction of power within the existing method for calculating EEDI/EEXI. The key parameters used to calculate EEDI/EEXI, such as the engine power at 75%MCR (Pme), are determined by a vessel engine’s maximum output, known as the maximum continuous rating (MCR). As Pang explains: “Conceptually, the approach is to reduce MCR, because Pme is fixed at 75% MCR, and it is suggested that the way we would want to do this is via a shaft or an engine power limit.”
A ShaPoLi or EPL will restrict the ship’s MCR a percentage below its original output, dependent upon the needs of the individual vessel to meet its required EEXI. The ship’s new Pme,limited, the EEXI calculation point, will be 75% of its limited MCR (MCRlim). Once in place, the ship can operate up to its MCRlim, above which is the ship’s power reserve.
Limiting and unlimiting power
According to the draft IMO guidelines, ShaPoLi will require a control unit for calculating and limiting the power transmitted by the shaft to the ship’s propellers, as well as sensors for measuring torque and rotational speed and a data recording and processing device.
Whereas for EPL, more modern engines have electronic control and the means of limiting such equipment (or accessing the power reserve) is via a fuel index limiter linked to a password, which can electronically lock the fuel index or directly limit power in the engine’s control system. On the other hand, a mechanically controlled engine will be restricted with a sealing device that can physically lock the fuel index, using a mechanical stop screw sealed by wire or an equivalent device with a governor limit setting.
The proposed guidance for EEXI also outlines a range of reporting and general system requirements for ShaPoLi and EPL, as well as an onboard management manual (OMM) used for the purpose of logging, among other functions.
Accessing the reserve power
Although a shaft or engine power limit will make part of a vessel's power range unavailable, a crucial part of the ShaPoLi or EPL actioned for EEXI is that it can be overridden. This allows the ship and its crew to access a vessel's power reserve for the purpose of securing the safety of a ship and saving life at sea. This is different to engine derating, which is permanent and affects engine output characteristics such as SFOC and torque.
The draft guidelines put forward by the IMO correspondence group addresses access to the ship’s reserve power, but Pang notes that a more detailed exploration needs to be undertaken in the event that this power is not used: “The guidelines mostly are along the lines of if you unlimit power and then you use it … For the mechanical EPL, where there is a physical seal, it is less clear cut. If you unlimit or remove the seal as a precautionary measure, just in case, and then you have to put it back even though you’ve never used the power. How that will be treated probably needs some clarity.”
In response to this issue, RINA and NI have recommended an amendment to the proposed guidelines; an insert addressing the precautionary use of power limit override reads as follows:
“Where an EPL/ShaPoLi override is activated but the power reserve is not subsequently used, this event should be recorded in the bridge and engine room logbooks. The engine-room logbook should record power used during the period when the override was activated. The EPL/ShaPoLi should be reset as soon as possible, and details of the reset should also be recorded in the bridge and engine-room logbooks."
NI and RINA have proposed four amendments to the draft guidelines for ShaPoLi and EPL set out by the IMO correspondence group, as well as identifying 11 key areas of concern. These are all covered in a document that NI and RINA have jointly submitted for consideration by the IMO Committee, with a view to inform the discussions at MEPC 76 and allow for further fine tuning of the guidance if required.
See The Naval Architect May 2021 for the full article