When a ship operates on multiple fuels (LNG dual-fuel, methanol dual-fuel, ammonia dual-fuel, biofuel-blended bunkers, drop-in renewable diesel, hydrogen pilot trials), the attained CII is no longer the simple single-fuel calculation of the CII Attained calculator. Each fuel contributes CO₂ at its own MEPC.364(79) Cf factor, and the contributions sum (per the standard MEPC.336(76) AER formula) on a mass basis.
The fuel mix correction calculator quantifies how the actual mix changes the attained CII relative to a baseline single-fuel reference (typically VLSFO for post-2020 ships, HFO for legacy vessels). The principal use cases are biofuel pilot purchases, LNG dual-fuel operation, and dual-fuel methanol or ammonia transition planning.
Baseline (single-fuel) CII
$$ \text{CII}\text{baseline} = \frac{F\text{base} \cdot C_{f,\text{base}} \cdot 10^6}{\text{Capacity} \cdot D} $$
The baseline assumes that the entire bunker energy is delivered by the baseline fuel.
Actual (mixed-fuel) CII
$$ \text{CII}\text{actual} = \frac{\sum_j F_j \cdot C{f,j} \cdot 10^6}{\text{Capacity} \cdot D} $$
The actual CII sums the CO₂ contributions across all fuels in the mix.
Energy-equivalent mass conversion
The calculator preserves total bunker energy across the baseline and actual scenarios. From the user-input baseline mass and the user-input energy share per fuel:
$$ E_\text{total} = F_\text{base} \cdot \text{LCV}_\text{base} $$
$$ F_j = \frac{E_\text{total} \cdot s_j}{\text{LCV}_j} $$
This ensures that the fuel mix correction reflects real fuel-switching: substituting low-Cf fuel for high-Cf fuel at constant energy delivery (constant ship operation).
CII reduction
$$ \text{Reduction (%)} = \frac{\text{CII}\text{baseline} - \text{CII}\text{actual}}{\text{CII}_\text{baseline}} \times 100 $$
Symbol legend
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
| $\text{CII}_\text{baseline}$ | Attained CII if 100% baseline fuel were burned | g CO₂/(cap·nm) | computed |
| $\text{CII}_\text{actual}$ | Attained CII for the actual mix | g CO₂/(cap·nm) | computed |
| $F_\text{base}$ | Mass of baseline fuel (energy-equivalent) | t | input |
| $F_j$ | Mass of fuel $j$ in the actual mix | t | derived from energy share |
| $C_{f,\text{base}}$, $C_{f,j}$ | Tank-to-wake CO₂ conversion factor | t CO₂/t fuel | MEPC.364(79) Table 1 |
| $\text{LCV}_\text{base}$, $\text{LCV}_j$ | Lower calorific value | MJ/kg | MEPC.364(79) |
| $E_\text{total}$ | Total bunker energy | MJ | derived |
| $s_j$ | Energy share of fuel $j$ in the mix | fraction | input |
| $Capacity$ | DWT (cargo) or GT (ro-pax / cruise) | t or - | MEPC.337(76) |
| $D$ | Distance travelled in the reporting year | nm | IMO DCS |
Worked example
A Capesize bulk carrier (180,000 DWT) operating 40,000 nm/year, with annual bunker burn of 8,000 t VLSFO-equivalent. Reporting year 2025.
Baseline scenario (100% VLSFO, $C_f = 3.151$, LCV = 41.0 MJ/kg):
- $E_\text{total} = 8000 \times 1000 \times 41.0 = 3.28 \times 10^8$ MJ
- $\text{CII}_\text{baseline} = 8000 \times 3.151 \times 10^6 / (180000 \times 40000) = 3.50$ g CO₂/(dwt·nm)
Actual scenario (70% VLSFO + 30% LNG via dual-fuel HPDF, $C_f = 2.750$, LCV = 49.0 MJ/kg, slip-corrected):
- $F_\text{VLSFO} = (3.28 \times 10^8 \times 0.70) / (1000 \times 41.0) = 5,600$ t
- $F_\text{LNG} = (3.28 \times 10^8 \times 0.30) / (1000 \times 49.0) = 2,008$ t
- $\text{CII}_\text{actual} = (5600 \times 3.151 + 2008 \times 2.750) \times 10^6 / (180000 \times 40000)$
- $= (17645 + 5522) \times 10^6 / 7.2 \times 10^9 = 3.22$ g CO₂/(dwt·nm)
Reduction = (3.50 − 3.22) / 3.50 = 8.0% reduction in attained CII.
This is the headline benefit of switching to a 30% LNG energy share, before accounting for methane slip which can erode some or all of this benefit on lower-spec engines (LBSI four-stroke, LPSI). The HPDF case modelled above has minimal slip (approximately 0.3%) and the apparent 8% reduction is largely real on a WtW intensity basis as well.
Practical use cases
- Biofuel pilot purchase decisions: quantify the CII benefit of a single B30 or B100 bunker to inform the price the operator is willing to pay above conventional fuel.
- LNG dual-fuel operating mode optimisation: compare LNG-priority mode vs MGO-priority mode in different operating areas.
- Methanol dual-fuel transition planning: model the CII trajectory as the methanol energy share grows over time.
- Charter party CII clause negotiation: model the CII implications of charterer-specified fuel mix.
- SEEMP III corrective measure planning: quantify the CII contribution of fuel switching alongside other operational measures.
Sources
- IMO Resolution MEPC.336(76) - 2021 Guidelines on operational CII.
- IMO Resolution MEPC.337(76) - Reference lines.
- IMO Resolution MEPC.338(76) - Reduction factors.
- IMO Resolution MEPC.339(76) - Rating boundaries.
- IMO Resolution MEPC.364(79) - Cf factors.