How to fix cruise shipping’s CII conundrum

Foreship Co-Founder and Chief Naval Architect Markus Aarnio suggests a way cruise ships could overcome the discrimination caused to them by the IMO’s Carbon Intensity Indicator (CII) regulations

That the current Carbon Intensity Indicator (CII) rules do not work well for cruise ships is now well known. The purpose of the regulation to reduce CO₂ emissions is undercut by its formula (Figure 1), which – for cruise ships – penalizes slow speed operation even if total CO₂ emission decreases.

This means that the first regulation to reduce emissions enacted by the International Maritime Organization can lead to cruise ships increasing their climate warming pollution.

But it does not need to be that way and the solution does not require completely changing the rules.

The answer lies in developing a formula for optimum speed for cruise ships which would better correspond to actual emissions and not encourage vessels to use higher speeds than necessary.

How would this work?

Currently CII ratings are calculated by dividing annual carbon emissions by gross tonnage of a vessel and by its annual distance traveled.

Figure 1: CII formula

This penalizes the time spent in harbor and the periods when the ship is sailing below its optimum speed – that is the speed with the lowest consumption of fuel per nautical mile which for cruise ships – due to their relative high hotel load - is typically around 12 knots.

It is worth focusing on these two issues to understand the clear faults in the current regulation.

First harbor time: Due to cruise ships’ high port load (and thus port emissions) compared to cargo ships, the time spent in port has a significant negative effect on the CII rating. However, emissions in ports also need to be reduced and this can be achieved by using shore power, which is becoming more available, or using a more sustainable fuel in port. Shore power availability increases continuously, and ships are being retrofitted with high power shore connection. New fuels make it easier to generate power in port with minimum CO₂ emissions.

Optimum speed: As an example, if a 100,000 GT ship burning MGO spends 35% of its time in port, and 65% on a constant speed at sea, the vessel would achieve a “mid C” CII rating in 2026 either sailing at a cruise speed of 8.5 knots or 17.5 knots (see Fig.3). But even if the CII rating is the same, there is huge difference in actual fuel consumption: in 8.5 knots operation the vessel would consume 15,000 t/year, but with 17.5 knots consumption 25,000 t/year.  Another example could be that instead of having a peaceful day at sea at 6 knots, current CII regulation could encourage the vessel to zigzag at 12 knots, thus getting a better CII rating but consuming significantly more fuel.

So, one way to overcome the “optimum speed problem” would be to change the CII formula by correcting the “annual distance traveled” for cruise ships to reflect the amount of time a ship is operating at below optimum speed.

This correction could be expressed as: For each operating period with a speed between the minimum speed (to be agreed, for example 6 knots) and optimum speed (could be 12 knots), an “additional distance” will be added to the “annual distance traveled”. The proposed correction is expressed in the formula shown in Figure 2.

Figure 2: Proposed CII correction for cruise ships

Twelve knots could be used as the optimum speed, or a separate ship dependent formula could be developed for this. The “minimum speed” could be, for example, 6 knots, to delete harbor and maneuvering time from the equation.

The graph in Figure 3 shows the 100,000 GT vessel with this approach applied.

This method requires a bit more work to calculate the CII but it would better correspond to cruise ships' actual emissions and not encourage them to use higher speeds than necessary.

Figure 3: CII & Fuel Consumption vs.  Speed