The IMO Carbon Intensity Indicator (CII) gives ship operators wide freedoms on how to reduce their vessel and fleet carbon intensity. However, according to recent analysis carried out by Wärtsilä Marine, 47% of the global merchant fleet will need to upgrade its emissions performance to avoid slipping into the C to E CII bands across their expected lifetime.
Companies can choose to change the fuels they use, implement operational measures such as reducing speed, or install one or more of the 44 energy-saving measures listed in IMO’s fourth Greenhouse Gas Study. The key challenge for owners and operators, then, is not just to familiarise themselves with these measures – a daunting task given the number available – but also to decide when it makes sense to invest in them.
According to Peter Hanstén, director for business development at Wärtsilä Marine: “The question of timing is key because CII compliance requires only a few percentage points in improvement each year. That means, for many vessels, the targets could be met by installing new technologies or employing operational solutions every year or few years, to deliver incremental gains.
“Alternatively, several years’ worth of targets could be banked in a single jump – for example, by switching to clean fuels.”
Which options work best for a company will depend on many factors, says Hanstén, not least the vessel’s current carbon intensity, its remaining lifetime and the operator’s ability to invest. Considerations will also need to include fuel availability and market expectations. It is clear, for example, that reducing reliance on fossil fuels and substituting them with alternative fuels will be the big change needed for vessels to meet the long-term carbon intensity reductions required by CII. But that shift will be expensive and its timing uncertain, as the widespread availability of alternative fuels remains unsettled.
Similarly, reducing vessel speed may be an effective way of conserving energy for some vessels, but will be impractical for the many that rely on speed to fulfil contracts and remain competitive. Hanstén suggests: “On the other hand, stacking marginal energy gains from other measures can keep ships compliant with short- and medium-term targets. These can be planned in advance so that investments are made in line with the required stepped improvements.
“Beyond compliance, these measures cut current fuel costs and give operators an optimised baseline of vessel efficiency that will minimise future fuel costs once vessels do make the leap to cleaner power. This also needs to be factored into calculations of return on investment [ROI].”
The starting point for developing a longer-term CII investment plan needs to involve a rigorous analysis of the existing fleet. “This is the approach adopted by Wärtsilä Decarbonisation Services when supporting shipowners including Princess Cruises, Dubai-based Tristar Eships and Brazilian energy company Raizen,” says Hanstén. “Together, we build a complete picture of the current state of play by gathering data from a variety of sources, including vessel operational profiles, technical characteristics and fuel consumption reports, or from Wärtsilä data collection units installed onboard. Machine-learning techniques are then used to process this data and predict how vessels’ emission performance will degrade over time.” Once processed, the data can be used to build a digital model of each vessel, which is used to simulate the effects of different energy saving measures, or different combinations of technologies and how they interact with each other.
Big efficiency gains can come from some surprising areas, which are sometimes overlooked, Hanstén points out, one example being the propeller. He says: “Propellers are typically designed at newbuild stage to meet a single speed point that may not remain optimised to the vessel’s operating profile in later years. A new propeller design, along with reduced vessel speeds and engine power, can lead to combined propulsive efficiency improvements of up to 15%.” Another high-gain area that Wärtsilä believes is often overlooked is the harnessing of wind power to assist propulsion. Rotor sails, for example, can reduce a vessel’s fuel consumption and associated GHG emissions by up to 30%, based on Wärtsilä’s experience through its license and cooperation agreement with Anemoi Marine Technologies for the latter’s Rotor Sail system.
For the full in-depth story, see the March 2025 issue of The Naval Architect
