For a ship that has triggered the CII corrective action plan requirement under MARPOL Annex VI Regulation 28.10 (rated D for three consecutive years or E for one year), the corrective action plan must project the attained CII forward over the corrective period and demonstrate return to a C rating. The projection uses two simple constant-rate equations.
Attained CII trajectory
$$ \text{CII}_a(y) = \begin{cases} \text{CII}a(y_0) & y < y\text{impl} \\ \text{CII}a(y_0) \cdot (1 - s) & y \ge y\text{impl} \end{cases} $$
The attained CII is held constant at the starting value until the implementation year, then reduced by the combined corrective measure savings s (computed using the standard non-overlapping multiplicative formula).
Required CII trajectory
$$ \text{CII}_r(y) = \text{CII}r(y_0) \cdot (1 - r\text{annual})^{(y - y_0)} $$
The Required CII is reduced by the annual reduction factor each year. The annual reduction factor is set by Resolution MEPC.338(76):
| Year | Annual reduction factor |
|---|---|
| 2023 | 5% |
| 2024 | 7% |
| 2025 | 9% |
| 2026 | 11% |
| 2027 onwards | TBD (review at MEPC 86 in 2027) |
For projection purposes the calculator uses a single annual reduction factor that the user can adjust to test sensitivity to alternative trajectories.
Rating determination
The rating for each year is determined by the ratio of attained to Required CII against the standard d-vector boundaries from Resolution MEPC.339(76):
| Rating | Upper bound (Attained / Required) |
|---|---|
| A (superior) | ≤ 0.86 |
| B (advanced) | 0.86 – 0.94 |
| C (compliant) | 0.94 – 1.06 |
| D (low minor) | 1.06 – 1.18 |
| E (inferior) | > 1.18 |
The exact d-vector values vary slightly by ship type; the values above are the typical bulk-carrier values.
Symbol legend
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
| $\text{CII}_a(y)$ | Attained CII for year y | g CO₂ / (cap·nm) | computed |
| $\text{CII}_r(y)$ | Required CII for year y | g CO₂ / (cap·nm) | MEPC.337(76) reference line |
| $y_0$ | Trigger year (most recent attained CII) | year | input |
| $y_\text{impl}$ | First year of full corrective measure implementation | year | input |
| $s$ | Combined corrective-measure savings | fraction | SEEMP combined measures calculator |
| $r_\text{annual}$ | Annual Required CII reduction factor | fraction | MEPC.338(76) |
| $d_R$ | Rating boundary multiplier for rating R | - | MEPC.339(76) |
Worked example
A 50,000 DWT bulk carrier rated D in 2023, 2024 and 2025 (trigger pulled end of 2025):
- $\text{CII}_a(2025) = 6.15$ g CO₂/dwt·nm
- $\text{CII}_r(2025) = 5.00$ g CO₂/dwt·nm
- $y_0 = 2025$
- $y_\text{impl} = 2027$ (corrective measures fully operational)
- $s = 0.26$ (26% combined improvement from slow steaming + hull cleaning + weather routing + trim + B30 biofuel)
- $r_\text{annual} = 0.05$ (5% per year, conservative post-2026 trajectory)
Year-by-year projection:
| Year | $\text{CII}_a$ | $\text{CII}_r$ | Ratio | Rating |
|---|---|---|---|---|
| 2025 | 6.15 | 5.00 | 1.23 | E |
| 2026 | 6.15 | 4.75 | 1.29 | E |
| 2027 | 4.55 (after measures) | 4.51 | 1.01 | C |
| 2028 | 4.55 | 4.29 | 1.06 | C |
| 2029 | 4.55 | 4.07 | 1.12 | D |
The plan returns the ship to C rating in 2027 (the implementation year) but slips back to D in 2029 due to the tightening Required CII trajectory. Additional measures or fuel switching would be required from 2028 onwards to maintain a C rating.
Sources
- IMO Resolution MEPC.328(76) - Amendments to MARPOL Annex VI introducing CII under Regulation 28.
- IMO MEPC.7/Circ.16 - Guidelines on the Development of a CII Corrective Action Plan.
- IMO Resolution MEPC.336(76) - 2021 Guidelines on the Operational Carbon Intensity Indicators.
- IMO Resolution MEPC.337(76) - 2021 Guidelines on the Reference Lines.
- IMO Resolution MEPC.338(76) - 2021 Guidelines on the Operational Carbon Intensity Reduction Factors.
- IMO Resolution MEPC.339(76) - 2021 Guidelines on the Operational Carbon Intensity Rating of Ships.