The IMO Carbon Intensity Indicator (CII) is going to limit the real CO2 emission for sea-going vessels greater 5,000 GT from 2023 onwards. This paper intends to gives an overview of the CII basics and present some use cases for typical container vessels and bulk carriers. The respective operation was analysed and potential measures to improve the CII situation were evaluated.
The results demonstrate that different solutions for CII improvements have to be applied for each specific ship and application. Further on, the improvements need to be tackled continuously further on, to keep the vessel in an acceptable ranking.
According the regulatory framework set by the IMO, starting from 2023, the Carbon Intensity Indicator (CII)  rating will apply on all sea-going vessels greater 5,000 GT. Those vessels will have to report their yearly CO2 emissions according to their actual operation while based on the reported values the vessels will be ranked in A to E class categories.
After 2023, the mean baseline for the class definition will be reduced annually, leading to a continuous ranking deterioration if no countermeasures for emissions reduction are applied. If a vessel is ranked in class D or worse, the owner must implement improvement measures to reach at least class C within three years, or the vessel will lose its emissions certificate. The first review of the reduction factors is scheduled for 2026
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This paper analyses the typical operation of container vessels and bulk carriers. Based on the results, potential CO2 reduction measures for the propulsion function, are evaluated e.g. engine power limitation (EPL) or the use of alternative fuels, while the respective effects and potential risks are taken into account too. These insights aim to deliver decision making support to owners, operators and other stakeholders.
Currently, the IMO uses a tank-to-propeller (TTP) approach to CII calculation, meaning that the CO2 emissions considered are those generated by the fuel in a vessel’s bunker storage. With the source of the fuel not yet considered, the use of ‘green fuels’ does not help the owner to improve the CII ranking.
In 2025, the EU is set to introduce the FuelEU Maritime regulation, allowing for a well-to- propeller (WTP) approach. Here, emissions impact is considered from the fuel production and transport stage, supporting the use of green fuels. The IMO is discussing a similar approach, but there is no defined date for its introduction as yet. A review of the regulations is expected in 2026.
The table below displays the CII class ranking with regard to the speed for a number of CPO Containerschiffreederei’s 14k TEU-Class container vessels.. According to this evaluation, current operations at high speeds consistently yield D and E ratings. If this were to continue with no improvement measures implemented, the vessel’s emissions certificate would end in 2028. In 2026, the vessel would have to limit its speed to 15 knots to maintain a category C rating, and to 14 knots in 2030.
One typical way to to improve the CII rating would be by exchanging the propeller for one optimized to lower speeds. However, this can affect powertrain behaviour, with altered torsional vibrations potentially increasing ageing and wear of the powertrain components.
Bulk carrier evaluation was performed on an exemplary bulk carrier built in high production numbers, worldwide.
Based on the ship characteristics and the operation data from the 2021 evaluation period (annual distance travelled: approx. 55,000 nm, annual fuel consumption: approx. 4,300 mt LSFO & 900 mt MDO) lead to the following CII results.
As displayed above, without changes, the vessel would reach a D rating in 2026. To maintain a C rating, its fuel consumption would have to be reduced by 13% in 2028, or by 18% for a B rating. This could be achieved by limiting the main engine output.
The impact of the reduction and limitation of the main engine output in order to achieve specific CII rating reductions. can be seen in the figure on the left.
Limiting the engine power by 13% would maintain the C rating until 2030. At -18%, the C rating could last two more years. If the shipowner were to select this approach, the operational impact would require further investigation. With lower ship speeds leading to more days at sea, the vessel’s availability might be reduced.
Potential impacts on the powertrain is part of the discussion too. For this the actual vessel operation have to be brought into relation to the required engine output. The typical operation profile of an exemplary bulk carrier from 2021 can be seen in the figure on the left.
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From the left diagram, of the figure on the side, which displays the ship resistance curve as function of ship speed, a typical engine power of 60 % MCR can be retrieved. This value is projected in an engine speed/load-diagram along with the reduced output values. The diagram shows clearly that limiting the output will bring the engine operation close to the barred speed range (grey marked area). This will give the crew a very small operation window of the main engine.
The challenges of these restrictions can be overcome by installing a new torsional vibration damper. But the new damper would have an approx. 30% increased weight compared to the original damper.
Beside the effort to exchange the torsional vibration damper the following questions needs to be answered beforehand:
•Will the maximum load of the engine support bearing be exceeded?
•Would there be the need for an additional, external support bearing?
•Is there enough space availability in front of the engine?
With the low boost pressure of the turbocharger, limiting engine output to below 50% MCR leads to poor combustion quality and high temperatures. One solution could be to modify the turbocharger specification. Achieving MCR output for emergency operations after modification requires the installation of a controllable turbine bypass (waste-gate). This prevents cylinder pressure from being exceeded due to excessive boost pressure at high engine loads. Another option for reducing vessel emissions is using alternative fuels.
For bulk carriers, green methanol represents a promising option since existing bunker storage can be used by applying an appropriate coating. However, the total operational range of the vessel would be reduced due to the lower heating value of methanol, which should be considered in route planning.
With the IMO’s current TTP approach, fossil LNG is preferred to green methanol. The combustion of methane (CH4), the main component of LNG, produces less CO2 than methanol (CH3OH) combustion, which would maintain the vessel’s C rating within the current framework for just three years. Retrofitting the vessel for methanol operations will make sense only when IMO adopts the WTP approach. The technical concept for such a retrofit is already available and can be applied today.
The impact of optimized vessel operations was investigated for a capesize bulk carrier by comparing its performance in 2021 and in the first half of 2022. The aim was to define the optimum speed profile and instruction speed based on the 2021 data for reducing CII factors for 2022. Through the METIS IoT platform, which created mathematical and machine-learning models specific to the vessel, high-frequency data was taken with a sample period of 15 seconds.
The analysis of the vessel’s operations in 2021 resulted in the following key findings:
•The vessel travelled in laden condition at speeds ranging from 10 to 13.3 knots.
•The vessel was at anchorage almost 20% of the time.
A statistical analysis for several speed-over-ground ranges of the main engine’s fuel-oil consumption per hour and nautical mile revealed the problem of assessing instructed speed per voyage.
Based on the findings from 2021, the vessel’s operational profile changed as follows:
•The profile of laden speed was reduced by around 10.6 knots.
•The anchorage time was reduced by half.
•The trim was adjusted to the new instructed speed.
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As displayed above, the CII value was reduced by nearly 20%, while the travelled distance increased by 25% due to improved operation planning.
For the first half of 2022, the vessel saved 930 metric tons of fuel oil.
Ultimately, CII fulfilment requires close operational planning between charterers, owners and operators. A detailed operational data analysis with the involvement of component suppliers and service providers at the earliest possible stage can support the avoidance of issues.
Source: METIS Cyberspace Technology