Per-fuel well-to-wake intensity: blend methodology

Background: when ships burn multiple fuels

A merchant ship rarely burns a single fuel across a full calendar year. The simplest case is the global cargo vessel that lifts VLSFO at sea-going speed and switches to MGO entering an Emission Control Area (ECA) under MARPOL Annex VI. The next case is the dual-fuel container ship or LNG carrier that runs on LNG at sea and on MGO during manoeuvring, with a small MGO pilot for the gas-cycle injection. The third case, increasingly common from 2025 onwards, is the operator who lifts a bio-blend (B24 or B30 FAME-VLSFO, or a 100 percent HVO drop-in cargo) for one or two voyages a quarter as a FuelEU compliance lever, then reverts to fossil bunker for the remaining voyages. The fourth case is the early adopter on methanol, ammonia, or e-fuel dual-fuel newbuilds, which always carry a fossil pilot and frequently sail on a 60-to-90 percent alternative-fuel split with a 10-to-40 percent fossil residual.

Each of these patterns produces a mixed-fuel reporting profile for the calendar year. The regulatory question is how to convert a basket of bunker uplifts (each with its own per-fuel WtW intensity, its own LCV, its own certification chain) into a single annual ship-level WtW intensity that the IMO GHG Fuel Standard (GFS) and FuelEU Maritime compare against the year’s required reduction trajectory. The answer, set out in Article 4 of FuelEU Maritime and replicated in MEPC.391(82) Annex 1 under the IMO Net-Zero Framework, is the energy-weighted average of the per-fuel WtW factors.

The energy-weighting is the only sensible mathematical choice. Mass-weighting would penalise high-density fuels arbitrarily and reward low-density alternatives that deliver less actual propulsion energy per kilogramme. Volume-weighting would have the opposite distortion. Energy-weighting on the lower calorific value (LCV) basis converts each bunker into a common currency (megajoules of usable propulsion energy) and applies the WtW intensity per megajoule, so the resulting blend intensity is the actual gCO2eq per MJ that the ship delivered to its propulsion system across the year.

The mechanics are straightforward in principle, but the practical implementation requires a tight chain of custody linking each tonne of bunker delivered into the ship’s tanks to a specific WtW value supported by a verifiable production pathway. The remainder of this article walks through the formula, the LCV mechanics, the default-versus-certified pathway choice, the mass-balance system that allows physical co-mingling, the BDN documentation infrastructure, the Article 8 and Annex 2 verification protocols, and the six canonical scenarios that operators encounter on the bunker desk in 2026.

The Article 4 / GFS energy-weighted-average formula

The formula at the heart of the blend methodology is identical in algebraic form between FuelEU Maritime Article 4 and the IMO GHG Fuel Standard. Variant subscripts and ancillary terms (the 2x multiplier for RFNBOs, the CO2 capture credit on bunkers with carbon-capture-utilisation, the wind-assist correction) differ across the two frameworks, but the core energy-weighted average is the common skeleton.

For a ship that burns $n$ different fuels in a reporting year, the annual WtW GHG intensity of the energy delivered is:

Where $E_i$ is the annual energy from fuel $i$ (MJ on LCV basis) and $\text{EF}_i^{\text{WtW}}$ is the well-to-wake emission factor for fuel $i$ in gCO2eq/MJ. The summation runs over every distinct fuel batch with its own WtW value: a single VLSFO bunker counts as one fuel, a separate VLSFO bunker from a different supplier with a different certified pathway counts as a different fuel, and a bio-blend B30 FAME counts as either two fuels (the fossil VLSFO carrier and the FAME blend component) or as a single blended product depending on the BDN reporting convention chosen.

The denominator total energy is the same total whether the operator is computing the FuelEU GHG intensity or the GFS attained GFI. The numerator differs only in the small-print application of multipliers and credits. For FuelEU, the RFNBO 2x multiplier appears as a denominator amplifier on the energy from RFNBO-eligible fuels, not as a numerator term. For GFS, no equivalent multiplier exists; the IMO regime treats RFNBO and biofuel energy at face value with no regulatory acceleration mechanism.

The formula is dimensionally consistent: numerator is grams of CO2-equivalent (the product of MJ and gCO2eq/MJ), denominator is megajoules, ratio is gCO2eq/MJ. It is invariant under proportional scaling of the energy basket, so a vessel that doubles its annual consumption in the same proportions across all fuels reports the same intensity. It is sensitive to the energy-share of each fuel and to the WtW spread between fuels, so a small high-WtW fuel can be offset by a large low-WtW fuel and vice versa.

LCV-based energy normalisation

The choice of lower calorific value (LCV) for energy normalisation is a deliberate methodological decision shared by both MEPC.391(82) Annex 1 and FuelEU Annex II. The alternative would be the higher calorific value (HCV) or gross calorific value (GCV), which includes the latent heat of condensation of the water vapour produced in combustion. Marine engines exhaust water vapour at temperatures well above the dew point, so the latent heat is not recoverable, and using HCV would over-count the available energy by 5 to 10 percent depending on hydrogen content of the fuel.

The LCV table that anchors the calculation is set out in MEPC.391(82) Annex 1 and replicated in FuelEU Annex II. The headline values are:

• HFO: 40.5 MJ/kg
• VLSFO: 41.0 MJ/kg
• MGO: 42.7 MJ/kg
• LNG: 49.1 MJ/kg
• Methanol: 19.9 MJ/kg
• Ammonia: 18.6 MJ/kg
• Hydrogen: 120.0 MJ/kg
• LPG (propane/butane): 46.0 MJ/kg
• FAME (B100): 37.2 MJ/kg
• HVO: 44.0 MJ/kg
• e-diesel: 43.0 MJ/kg

A bunker uplift of 1,000 tonnes of VLSFO therefore contributes $1{,}000 \cdot 1{,}000 \cdot 41.0 = 4.1 \cdot 10^7$ MJ or 41 TJ of energy to the annual basket. The same mass of methanol contributes only $1{,}000 \cdot 1{,}000 \cdot 19.9 = 1.99 \cdot 10^7$ MJ or 19.9 TJ, less than half on energy terms. This is why methanol bunkers must be roughly twice the mass of equivalent VLSFO uplifts to deliver the same propulsion energy, with corresponding tank-volume implications addressed in the methanol grades article.

The LCV is reported on the BDN at the time of delivery and is verified during the annual MRV cycle. Where the BDN does not state an LCV (as is permissible for legacy bunker contracts), the verifier applies the Annex II default value for the fuel grade. A producer with a verified pathway under ISCC EU, REDcert, RSB, or another recognised voluntary scheme can substitute a pathway-specific LCV that is typically within 1 to 2 percent of the default value. The default-versus-certified choice is treated in the next section.

Per-fuel default vs certified pathway

Every fuel in the MEPC.391(82) Annex 1 and FuelEU Annex II tables has a default WtW value that the operator can apply without further documentation. The default values are the conservative end of the credible range for a fuel grade, calibrated against the JEC v5 dataset for fossil fuels and against RED III Annex V for biofuels and RFNBOs. The defaults are constructed to over-state the WtW intensity by 5 to 15 percent relative to a typical certified pathway, so a producer with a verified actual value almost always benefits from substituting the actual.

The certified pathway substitution is permitted under FuelEU Article 10 and the parallel provision of MEPC.391(82). The substitution requires:

1. A recognised voluntary scheme certificate (ISCC EU, REDcert, RSB, REDISS, 2BSvs, or a national scheme recognised by the European Commission under Commission Implementing Regulation (EU) 2022/996)
2. A pathway-specific WtW value computed under the RED III Annex V Methodology for biofuels and Commission Delegated Regulation (EU) 2023/1185 for RFNBOs
3. A mass-balance accounting chain linking the certified pathway batch to the specific bunker uplift on the ship
4. A verification statement from the certification body or the FuelEU verifier confirming the chain integrity

Where any element fails (the producer is uncertified, the BDN does not link to a specific pathway batch, or the verifier rejects the chain), the operator falls back to the Annex II default. A producer can choose to remain on default values voluntarily, which is common for fossil VLSFO, MGO, and HFO bunkers where the certified pathway delivers only a marginal improvement and the certification cost outweighs the WtW saving.

For biofuels and RFNBOs, the default-vs-certified choice is more consequential. The FAME default in FuelEU Annex II is approximately 50 to 70 gCO2eq/MJ depending on feedstock category, while a verified pathway based on used cooking oil (UCO) or animal fats can deliver 10 to 25 gCO2eq/MJ. The certification overhead is recovered many times over by the lower attributed intensity. The same logic applies to HVO, bio-LNG, e-methanol, and e-ammonia: the producer who invests in voluntary-scheme certification reaps the WtW reduction that the default value forecloses.

Mass Balance system: physical mixing, separated regulatory accounting

The mass-balance system is the bookkeeping convention that allows physical co-mingling of certified and non-certified product in the same storage tank, pipeline, bunker barge, or ship’s bunker tanks while preserving the regulatory ability to attribute specific WtW values to specific bunker uplifts. The system is set out in Article 30 of RED III for biofuels and in Commission Delegated Regulation (EU) 2023/1185 for RFNBOs, with marine-specific elaboration in FuelEU Article 9 and the verification body guidance.

The principle is energy-conservation in the regulatory ledger. A terminal that receives 1,000 tonnes of certified UCO-FAME and 9,000 tonnes of conventional palm-oil FAME can blend the two into a 10,000-tonne homogeneous tank for physical handling. The regulatory ledger, however, must show that the terminal can sell up to 1,000 tonnes of the blend as certified UCO-FAME (drawing down the certified credit one tonne for each tonne sold) before the certified credit is exhausted and remaining sales must be reported at the conventional palm-oil pathway. Physical mixing is permitted; regulatory attribution is preserved.

The mass-balance period is calendar-year for biofuels under RED III and is the same for RFNBOs under the 2023/1185 rules. A terminal that receives certified product in March can sell certified credits against that product through December of the same year. Carry-over into the next year is permitted to the extent that the terminal’s closing stock supports the carried credit. Audited annual reconciliation by the certification body confirms that total certified sales across the year do not exceed total certified inputs plus carry-over.

For marine bunkers, the mass-balance chain typically runs:

1. Producer (e.g. UCO-FAME refinery in Rotterdam) issues certified product into the terminal pool with an ISCC EU certificate and a stated WtW value
2. Terminal (e.g. ARA bunker terminal) holds the certified credit in its mass-balance ledger and physically blends the product into the terminal’s storage tank
3. Bunker barge lifts product from the terminal and delivers to the ship at the bunker manifold, with the BDN stating the certified WtW value and the mass-balance chain identifier
4. Ship records the BDN in its monitoring plan, the bunker uplift in its bunker tank ledger, and the certified WtW value in the FuelEU and GFS reporting
5. Verifier audits the chain at the annual report stage, confirming that each link is supported by a valid certificate and that the total certified energy on the ship does not exceed the total certified energy bunkered

A break in the chain at any link (an uncertified producer, a terminal without ISCC accreditation, a bunker barge that does not pass the certified product through, a BDN that does not state the certified value) downgrades the bunker to the default value. The verifier flag the break in the verification report and the operator must adjust the annual intensity calculation accordingly.

BDN chain-of-custody documentation

The Bunker Delivery Note (BDN) is the single most important document in the blend methodology. MARPOL Annex VI Regulation 18 requires every bunker delivery to be accompanied by a BDN that states the supplier name, the receiving ship name and IMO number, the date and place of delivery, the product name and grade (per ISO 8217), the mass in tonnes, and a representative sample sealed for compliance reference. The 2024 amendments to ISO 8217 and the CIMAC Working Group 7 guidance extend the BDN content to include the WtW gCO2eq/MJ value, the LCV in MJ/kg, the certification scheme reference, the mass-balance chain identifier, and the biofuel or RFNBO content percentage where applicable.

The BDN is the legal evidence that a specific bunker uplift carries a specific WtW value. The verifier audits the BDN at the annual report stage; the FuelEU verifier requires the BDN to be retained for at least three years; the IMO DCS submission references the BDN through the ship’s fuel oil consumption data filing.

For a blended bunker (B30 FAME-VLSFO, B24 bio-VLSFO, B7 FAME-MGO, methanol-pilot blend, etc.), the BDN must state the constituent fractions on an energy basis and the WtW value of each constituent. The verifier may either accept a single combined WtW value for the blend or require the operator to allocate the bunker into two virtual fuel categories in the annual reporting. The latter is the preferred approach for FuelEU because it preserves the eligibility of each component for any applicable multiplier (e.g. the RFNBO 2x multiplier on the e-fuel component of a blend).

The BDN chain is reinforced by the representative sample requirement under MARPOL Annex VI. The supplier provides a sealed sample at the time of delivery; the ship retains it for one year; the flag-state inspectorate or the FuelEU verifier may request testing of the sample to confirm composition. The sample testing covers ISO 8217 quality parameters (sulphur, viscosity, FAME content, water, ash) but increasingly also extends to lifecycle parameters where the bunker is reported as bio-blend or RFNBO. Disputes over the WtW value are typically resolved by sample testing and BDN cross-reference rather than by recalculation from first principles.

FuelEU Article 8 verification of the BDN-to-WtW chain

FuelEU Maritime Article 8 sets out the monitoring and verification framework that audits the BDN-to-WtW chain at the annual report stage. The framework is built on the Monitoring Plan that each ship submits to its accredited verifier before the start of the reporting year, with the verifier role performed by an accredited verification body (e.g. DNV, Lloyd’s Register, ABS, Bureau Veritas, ClassNK, RINA) under Commission Delegated Regulation (EU) 2023/2917 and the Implementing Regulation (EU) 2023/2918.

The Monitoring Plan describes how the operator will:

1. Identify each fuel type and grade bunkered during the year, with specific reference to the VLSFO and MGO defaults, HFO defaults, FAME, HVO, bio-LNG, methanol grades, ammonia grades, hydrogen, LPG cargo-as-fuel and e-fuel categories
2. Document the BDN evidence for each bunker uplift, including supplier, date, port, mass, LCV, WtW value, and certification chain
3. Apply the energy-weighted blend formula to compute the annual WtW intensity
4. Apply any RFNBO multipliers, wind-assist credits, or carbon-capture credits that the ship is entitled to claim
5. Submit the result through THETIS-MRV for FuelEU reporting and through the IMO DCS for the GFS submission
6. Retain the underlying records for at least three years for FuelEU and ten years for the IMO DCS

The verifier’s annual review checks the Monitoring Plan against the actual bunker records, the BDN evidence, and the certification scheme references. A finding of material non-conformity (a missing BDN, a broken mass-balance chain, an unsupported pathway claim, an arithmetic error in the energy-weighted average) triggers a non-compliance report that prevents the FuelEU submission from passing. The operator must either correct the calculation (typically by falling back to default values for the affected bunkers) or provide additional evidence that closes the finding before the annual deadline.

The verification report itself is a public document under FuelEU Article 14 and is published by EMSA on the THETIS-MRV portal. Operators with consistent clean verification records benefit from supplier reputation and counterparty trust; operators with repeated non-conformity findings face increased verifier scrutiny in subsequent years.

MEPC.391(82) Annex 2 verification methodology

The IMO equivalent to FuelEU Article 8 is MEPC.391(82) Annex 2, which sets out the verification methodology for the lifecycle GHG intensity of marine fuels in support of the GFS. The Annex 2 framework is broadly aligned with the FuelEU Article 8 framework but is structured around the IMO DCS infrastructure and the flag-state verification process rather than the EMSA verifier-accreditation system.

The Annex 2 process requires:

1. The supplier to certify the WtW value of each bunker batch under a recognised scheme (ISCC EU, REDcert, RSB, RTRS, 2BSvs, or a national equivalent)
2. The ship to record the certified WtW value on the BDN and in the Bunker Receipt Record (the IMO equivalent of the FuelEU bunker tank ledger)
3. The flag-state administration or its recognised organisation (RO) to verify the annual fuel-mix submission as part of the IMO DCS audit
4. The IMO Secretariat to compile the global fleet attained GFI and publish the aggregated outcome through the GFS Trading System

The flag-state verification is typically performed by a classification society acting as RO. The classification society reviews the Bunker Receipt Records, the BDNs, the certification chain, and the energy-weighted blend computation, and issues a Verification Confirmation to the flag-state. The flag-state then submits the verified ship data to the IMO DCS.

The MEPC.391(82) Annex 2 process is less prescriptive than the FuelEU Article 8 process on the documentary chain, partly because the IMO regime is newer (adopted at MEPC 83 in April 2025) and partly because the IMO governance structure relies on flag-state delegation rather than central-authority accreditation. The 2026 and 2027 IMO MEPC sessions are expected to refine the Annex 2 verification methodology in light of operating experience, with the parallel FuelEU framework serving as a reference benchmark.

For ships that fall under both regimes (any vessel above 5,000 GT calling at EU ports), the operator typically runs a single integrated verification process with the same accredited verifier covering both submissions, with the FuelEU report triggering through THETIS-MRV and the GFS report triggering through the IMO DCS. The integrated approach reduces duplicate documentation and ensures consistency between the two regulatory submissions.

Scenario 1: classic dual-fuel VLSFO + MGO (transit vs ECA fuel switch)

The simplest and most common blend scenario is the global trader that runs on VLSFO at sea-going speed in non-ECA waters and switches to MGO entering an ECA to comply with the 0.10 percent sulphur cap under MARPOL Annex VI Regulation 14.3. The fuel-switch mechanic itself has been standard practice since the 2015 ECA amendment and since the 2020 global sulphur cap; the WtW reporting consequence is a two-component blend with VLSFO at typically 60 to 85 percent of annual energy and MGO at 15 to 40 percent.

For a 100,000 DWT bulker on a typical trade pattern with 20 percent of voyage-time in ECAs (e.g. North Sea, Baltic, North America 200 nm, Caribbean, Mediterranean from 2025):

• Annual VLSFO consumption: 8,000 tonnes at 41.0 MJ/kg LCV = 328 TJ
• Annual MGO consumption: 1,500 tonnes at 42.7 MJ/kg LCV = 64 TJ
• Total energy: 392 TJ
• VLSFO WtW: 91.0 gCO2eq/MJ
• MGO WtW: 93.3 gCO2eq/MJ
• Numerator: $328 \cdot 91.0 + 64 \cdot 93.3 = 29{,}848 + 5{,}971 = 35{,}819$ tonnes CO2eq
• Blend WtW: $35{,}819 / 392 = 91.4$ gCO2eq/MJ

The blend is dominated by VLSFO and the MGO contribution shifts the result by less than 0.5 gCO2eq/MJ. This is typical of fossil-only fuel switching: the WtW spread between VLSFO and MGO is small enough that ECA compliance does not materially change the annual GFI position. Operators targeting GFS or FuelEU compliance through fuel switching cannot rely on VLSFO-to-MGO substitution as a meaningful lever; the relevant lever is the substitution of fossil VLSFO or fossil MGO with bio or RFNBO alternatives, which is the topic of the next scenarios.

Scenario 2: bio-blend VLSFO + B30 FAME

The second scenario is the operator who lifts a B30 FAME-VLSFO blend on selected voyages as a FuelEU compliance lever. The B30 designation indicates 30 percent FAME by volume blended with VLSFO (typically as B100 FAME from a UCO or animal-fat feedstock, treated in the FAME article). The blend is supplied as a homogeneous bunker product with a single BDN that states the FAME percentage, the FAME WtW value, and the VLSFO WtW value.

For the same 100,000 DWT bulker that lifts 2,000 tonnes of B30 in addition to 6,000 tonnes of VLSFO and 1,500 tonnes of MGO:

• B30 FAME bunker mass: 2,000 tonnes
• FAME mass within blend (30 percent by volume, approximately 28 percent by mass given FAME density 880 kg/m3 vs VLSFO 970 kg/m3): 560 tonnes
• Carrier VLSFO mass within blend: 1,440 tonnes
• FAME energy: $560 \cdot 37.2 \cdot 1{,}000 = 20.8$ TJ
• Blend-carrier VLSFO energy: $1{,}440 \cdot 41.0 \cdot 1{,}000 = 59.0$ TJ
• Pure VLSFO energy: $6{,}000 \cdot 41.0 \cdot 1{,}000 = 246.0$ TJ
• MGO energy: $1{,}500 \cdot 42.7 \cdot 1{,}000 = 64.0$ TJ
• Total energy: 389.8 TJ
• FAME WtW (UCO certified): 14.0 gCO2eq/MJ
• VLSFO WtW: 91.0 gCO2eq/MJ
• MGO WtW: 93.3 gCO2eq/MJ
• Numerator: $20.8 \cdot 14.0 + 59.0 \cdot 91.0 + 246.0 \cdot 91.0 + 64.0 \cdot 93.3 = 291 + 5{,}369 + 22{,}386 + 5{,}971 = 34{,}017$ tonnes CO2eq
• Blend WtW: $34{,}017 / 389.8 = 87.3$ gCO2eq/MJ

The 560-tonne UCO-FAME component (5.3 percent of annual energy) reduces the annual WtW intensity from 91.4 to 87.3 gCO2eq/MJ, a 4.1 gCO2eq/MJ improvement or 4.5 percent. The improvement is roughly proportional to the FAME energy share times the WtW spread between FAME and VLSFO. The numerical lever is significant: the same bulker on a 2030 FuelEU compliance trajectory (target intensity 86.4 gCO2eq/MJ versus the 91.16 baseline) is brought into compliance by the modest B30 lift on a small fraction of voyages. This is why bio-blends are the most cost-effective short-term FuelEU compliance instrument, even at the FAME premium of USD 200 to 400 per tonne over VLSFO.

Scenario 3: drop-in renewable HVO replacing MGO

The third scenario is the operator who substitutes HVO (hydrotreated vegetable oil, treated in the HVO article) for MGO entirely on ECA voyages. HVO is a paraffinic distillate that meets the EN 15940 specification and is fully fungible with MGO in any compression-ignition engine. The substitution is invisible to the engine and the crew; the only difference is the BDN reference and the WtW value.

For the same 100,000 DWT bulker that substitutes 1,500 tonnes of HVO for 1,500 tonnes of MGO:

• HVO mass: 1,500 tonnes
• HVO LCV: 44.0 MJ/kg
• HVO energy: $1{,}500 \cdot 44.0 \cdot 1{,}000 = 66.0$ TJ (vs 64.0 TJ for the same mass of MGO)
• VLSFO energy: $8{,}000 \cdot 41.0 \cdot 1{,}000 = 328.0$ TJ
• Total energy: 394.0 TJ
• HVO WtW (UCO certified): 18.0 gCO2eq/MJ
• Numerator: $328 \cdot 91.0 + 66 \cdot 18.0 = 29{,}848 + 1{,}188 = 31{,}036$ tonnes CO2eq
• Blend WtW: $31{,}036 / 394 = 78.8$ gCO2eq/MJ

The HVO-for-MGO substitution drops the annual WtW intensity from 91.4 to 78.8 gCO2eq/MJ, a 12.6 gCO2eq/MJ improvement or 13.8 percent. The improvement is much larger than the B30 case because HVO replaces 100 percent of the MGO energy rather than 30 percent of a partial bunker. The HVO premium over MGO (typically USD 400 to 700 per tonne) is offset by the FuelEU compliance value and, in the integrated regime, by the EU ETS allowance saving on the MGO emissions that no longer need to be surrendered.

Scenario 4: RFNBO blend fossil + e-methanol

The fourth scenario is the operator with a methanol dual-fuel newbuild that lifts e-methanol (RFNBO-certified, treated in the e-fuel article and the methanol grades article) as the primary fuel and falls back to fossil VLSFO or fossil grey methanol when e-methanol is not available at the bunker port. The e-methanol component is eligible for the FuelEU 2x multiplier under Article 4(4), which doubles the energy attribution of the RFNBO bunker for compliance purposes.

For a 16,000 TEU container ship with a MAN B&W ME-LGIM engine that lifts 50,000 tonnes of e-methanol and 25,000 tonnes of fossil VLSFO across the year:

• e-methanol mass: 50,000 tonnes
• e-methanol LCV: 19.9 MJ/kg
• e-methanol energy (raw): $50{,}000 \cdot 19.9 \cdot 1{,}000 = 995$ TJ
• e-methanol energy (with 2x multiplier on the denominator): $2 \cdot 995 = 1{,}990$ TJ
• Fossil VLSFO mass: 25,000 tonnes
• VLSFO energy: $25{,}000 \cdot 41.0 \cdot 1{,}000 = 1{,}025$ TJ
• e-methanol WtW (verified RFNBO): 7.0 gCO2eq/MJ
• VLSFO WtW: 91.0 gCO2eq/MJ
• Numerator (using raw e-methanol energy in the numerator): $995 \cdot 7.0 + 1{,}025 \cdot 91.0 = 6{,}965 + 93{,}275 = 100{,}240$ tonnes CO2eq
• Denominator (using 2x-multiplied e-methanol energy): $1{,}990 + 1{,}025 = 3{,}015$ TJ
• FuelEU effective intensity: $100{,}240 / 3{,}015 = 33.2$ gCO2eq/MJ

Compared with the same energy basket without the multiplier ($100{,}240 / (995 + 1{,}025) = 49.6$ gCO2eq/MJ), the RFNBO multiplier reduces the FuelEU-effective intensity by a further 16.4 gCO2eq/MJ. The multiplier is the single largest regulatory lever in FuelEU and is the reason that early-mover RFNBO adopters (Maersk, CMA CGM, MSC, Wallenius Wilhelmsen) bear the green premium during 2025 to 2033. The multiplier sunset in 2034 reduces the regulatory acceleration but does not affect the underlying CO2 saving against the fossil reference.

The same calculation under the GFS uses the raw 995 TJ energy without the multiplier (the GFS does not have an RFNBO multiplier provision). The GFS attained intensity for the same ship is $100{,}240 / 2{,}020 = 49.6$ gCO2eq/MJ, well below the Tier 1 required GFI for 2030. The two-tier compliance position (deeply over-compliant under GFS, modestly over-compliant under FuelEU) is the structural shape of an early RFNBO adopter under the integrated regulatory regime.

Scenario 5: full alternative ammonia or methanol

The fifth scenario is the late-2020s and 2030s newbuild that runs on essentially pure ammonia or methanol with a small MGO pilot (typically 5 to 8 percent of energy) to drive the gas-cycle injection. The blend is a two-component mix (alternative + pilot) with the alternative dominating the energy share.

For a 5,000 DWT short-sea ammonia-fuelled coaster running on 95 percent ammonia and 5 percent MGO pilot:

• Annual ammonia consumption: 4,000 tonnes
• Ammonia LCV: 18.6 MJ/kg
• Ammonia energy: $4{,}000 \cdot 18.6 \cdot 1{,}000 = 74.4$ TJ
• Annual MGO pilot consumption: 100 tonnes
• MGO energy: $100 \cdot 42.7 \cdot 1{,}000 = 4.27$ TJ
• Total energy: 78.67 TJ (95.0 percent ammonia, 5.4 percent MGO)
• Ammonia WtW (e-ammonia, verified RFNBO): 5.0 gCO2eq/MJ
• MGO WtW: 93.3 gCO2eq/MJ
• Numerator: $74.4 \cdot 5.0 + 4.27 \cdot 93.3 = 372 + 398 = 770$ tonnes CO2eq
• Blend WtW (raw): $770 / 78.67 = 9.8$ gCO2eq/MJ

The pilot fuel residual (5 percent of energy at fossil intensity) adds approximately 5 gCO2eq/MJ to what would otherwise be a near-zero ammonia bunker. Operators targeting the deepest WtW reductions specify a bio-MGO or HVO pilot fuel, which closes the gap to approximately 6 gCO2eq/MJ. The 2x multiplier on the e-ammonia component further halves the FuelEU-effective intensity to approximately 5 gCO2eq/MJ, well below any plausible 2030s compliance trajectory.

The same calculation under the GFS without the multiplier reports approximately 9.8 gCO2eq/MJ, still well below the Tier 1 trajectory. The pure-alternative position is the maximum-compliance position under both regimes and is the reference point against which transition strategies are benchmarked.

Scenario 6: cargo-as-fuel LPG carrier

The sixth scenario is the LPG carrier that burns propane or butane from its own cargo tanks as the propulsion fuel, treated in detail in the LPG cargo-as-fuel article. The cargo-as-fuel arrangement is permitted under IGC Code Chapter 16 for LPG carriers fitted with MAN B&W ME-LGIP or WinGD X-DF-LPG dual-fuel engines, with the LPG drawn from cargo tanks via a fuel-conditioning skid and delivered to the engine as a high-pressure liquid.

The WtW reporting treats the LPG burned as fuel as a separate fuel category with a WtW value derived from the LPG production pathway (typically refinery LPG from the same crude as VLSFO, or NGL LPG from gas processing). The MEPC.391(82) Annex 1 default LPG WtW is approximately 75 to 80 gCO2eq/MJ, lower than VLSFO due to the lower carbon-to-hydrogen ratio of LPG (C3H8 or C4H10 vs heavy hydrocarbons in VLSFO).

For an 84,000 m3 VLGC running on 95 percent LPG cargo-as-fuel and 5 percent MGO pilot:

• Annual LPG consumption: 18,000 tonnes
• LPG LCV: 46.0 MJ/kg
• LPG energy: $18{,}000 \cdot 46.0 \cdot 1{,}000 = 828$ TJ
• Annual MGO pilot consumption: 500 tonnes
• MGO energy: $500 \cdot 42.7 \cdot 1{,}000 = 21.4$ TJ
• Total energy: 849.4 TJ
• LPG WtW (default, refinery LPG): 76.0 gCO2eq/MJ
• MGO WtW: 93.3 gCO2eq/MJ
• Numerator: $828 \cdot 76.0 + 21.4 \cdot 93.3 = 62{,}928 + 1{,}997 = 64{,}925$ tonnes CO2eq
• Blend WtW: $64{,}925 / 849.4 = 76.4$ gCO2eq/MJ

The cargo-as-fuel arrangement delivers a WtW position approximately 15 gCO2eq/MJ below pure VLSFO operation, without any RFNBO or biofuel premium. The LPG carrier benefits from the inherent lower carbon intensity of the cargo molecule and from the avoided boil-off venting that would otherwise occur on a non-fuel-burning LPG carrier. The same logic applies in modified form to LNG carriers burning LNG cargo (treated in the LNG dual-fuel article) and to methanol carriers burning methanol cargo.

FuelEU pooling effect on blend accounting

FuelEU Maritime Article 21 permits multiple ships under common operator control or under a pooling agreement to pool their compliance by averaging the post-blend annual intensities across the pool. The pool’s average intensity is compared against the year’s required intensity, and pool participants are deemed compliant or non-compliant collectively rather than individually. The mechanic is treated in detail in the pooling article.

For a pool of three ships with annual blend intensities of 85.0, 91.5, and 75.0 gCO2eq/MJ and respective annual energies of 200, 350, and 150 TJ, the pool average is:

The pool member with the cleanest blend (Ship 3 at 75.0) cross-subsidises the pool member with the dirtiest blend (Ship 2 at 91.5). The pool benefits when the spread of intensities is wide and when the cleanest ship has surplus compliance value to share with the dirtiest. The economics are set out in the pooling balance mechanism: the cleanest ship effectively sells compliance value to the dirtiest at an internal transfer price, with the price set by negotiation or by a standardised pool tariff.

The pool blend formula does not change the per-ship blend formula; each ship continues to compute its annual WtW intensity through the energy-weighted average of its own bunker uplifts. The pooling layer sits above the per-ship layer and aggregates the per-ship results into a pool average. Participation in a pool is a contractual decision made by the operator at the start of the reporting year and notified to EMSA through THETIS-MRV.

GFS per-ship independence from pooling

The IMO GHG Fuel Standard does not currently include a pooling provision. Each ship is assessed on its own attained GFI against the year’s required GFI, with Surplus Units (SUs) earned by over-compliant ships and Deficit Units (DUs) owed by under-compliant ships. The Surplus Units can be traded through the GFS Trading System between any two ships under any operator, but the trading mechanism is per-ship rather than per-pool.

The practical consequence is that a pool that under FuelEU shares compliance value internally must, under the GFS, transact the equivalent value through the SU/DU trading mechanism. The economics may differ depending on the SU price discovery, the pool’s internal transfer price, and the relative scarcity of compliance value in each market. Operators with both regimes in scope (any vessel above 5,000 GT calling at EU ports) typically run an integrated compliance optimisation that takes both pooling (FuelEU) and SU/DU trading (GFS) into account when sizing fuel-procurement and pool-participation decisions.

The GFS per-ship independence is also relevant for ships outside the FuelEU scope (deep-sea vessels not calling at EU ports). Such ships are subject to GFS only and rely on SU/DU trading for compliance flexibility. The pooling option is not available, so the ship’s individual blend strategy directly determines the compliance position and the SU/DU position in the trading market.

Reporting infrastructure: THETIS-MRV + IMO DCS

The two reporting infrastructures that receive the blend computation outputs are:

1. THETIS-MRV: The European Maritime Safety Agency portal that consolidates EU MRV (the original 2015 reporting regime), EU ETS (carbon market submissions), and FuelEU Maritime (intensity-and-pooling submissions). All EU-relevant reporting flows through THETIS-MRV as a single pipeline. The portal is operated by EMSA on behalf of the European Commission and is accessed by operators, verifiers, flag-state administrations, and the Commission’s enforcement teams.

2. IMO DCS: The IMO Data Collection System for fuel oil consumption of ships, established under MARPOL Annex VI Regulation 22A in 2018. The DCS submission flows from the ship to the flag-state to the IMO Secretariat and is the foundational data source for the GHG Fuel Standard attained GFI computation. From 2027 onwards, the DCS submission is extended to include the per-fuel WtW values and the energy-weighted blend intensity that drives the GFS compliance assessment.

The two reporting infrastructures share the underlying data but apply different rules and produce different outputs. The operator’s reporting workflow runs in parallel:

• March 31 of year N+1: FuelEU verified report submitted through THETIS-MRV
• March 31 of year N+1: IMO DCS report submitted through the flag-state to the IMO Secretariat
• April 30 of year N+1: EU MRV verified report submitted through THETIS-MRV (often consolidated with the FuelEU submission)
• September 30 of year N+1: EU ETS allowance surrender for the prior year’s emissions

The integrated workflow is supported by classification-society monitoring software and by third-party reporting platforms that aggregate the bunker records, BDN evidence, and verifier sign-off into a single submission package. The blend computation itself is performed in the same software stack that produces the per-ship intensity, with the energy-weighted average formula applied at the annual report stage.

Commercial implications: bunker procurement hedging

The blend methodology has direct commercial consequences for bunker procurement decisions. The operator who can mix multiple fuel categories within a single annual reporting period has optionality that the operator locked into a single fuel grade does not have. The optionality is valued through the integrated regulatory cost of each fuel:

• Fossil VLSFO at default WtW: USD 600 per tonne bunker price, plus FuelEU non-compliance cost (USD 50 to 150 per tonne CO2 above the trajectory), plus EU ETS allowance cost (USD 70 to 100 per tonne CO2)
• Bio-blend B30 FAME-VLSFO: USD 700 per tonne bunker price, partial offset of FuelEU non-compliance through certified biofuel WtW
• HVO drop-in for MGO: USD 1,200 per tonne bunker price, full offset of FuelEU non-compliance on the substituted MGO, partial offset of EU ETS allowance cost
• e-methanol RFNBO: USD 1,800 per tonne bunker price, maximum offset of FuelEU non-compliance through 2x multiplier, full offset of EU ETS allowance cost on the substituted fossil

The integrated cost minimisation across these options is the bunker desk’s central optimisation problem in 2026 and beyond. The optimal strategy typically involves a base of cheap fossil with the regulatory penalty paid in cash (FuelEU non-compliance penalty plus EU ETS allowance) supplemented by a small RFNBO-eligible component (e-methanol, e-LNG, e-ammonia) sized to capture the multiplier value. The bio-blend middle ground (B30 FAME, B7 HVO, drop-in HVO) is the cost-effective filler that bridges the gap between the cheap-fossil and the expensive-RFNBO ends.

The optimum varies by:

• Ship type and engine compatibility: A methanol dual-fuel newbuild has access to e-methanol; a steam turbine LNG carrier has access only to LNG and MGO
• Trade pattern: A short-sea coastal trader has more flexibility on bunker port choice than a deep-sea trader on long-haul Pacific or Atlantic crossings
• Bunker port supply: Rotterdam, Singapore, Houston, Algeciras have wide supply of bio-blends and growing supply of e-methanol; smaller bunker hubs may be limited to fossil grades
• Compliance position relative to the trajectory: An over-compliant ship saves marginal compliance; an under-compliant ship gains marginal compliance from the next tonne of bio or RFNBO

Operators run continuous bunker-procurement optimisation models that take all these variables into account and produce a recommended fuel-mix profile for the annual reporting period. The optimisation is supported by /calculators/fuel-wtw-blend, /calculators/fueleu-ghg-intensity, /calculators/gfi-attained, /calculators/gfi-compliance, /calculators/fueleu-pooling, and /calculators/fueleu-compliance-balance.

Verifier approval of blend WtW

The role of the verifier in the blend methodology is to convert the operator’s claimed annual intensity into a verified annual intensity that the regulator accepts as the basis for compliance assessment. The verifier reviews:

1. Each BDN for completeness and consistency with the bunker records
2. The certification chain for each non-default-WtW bunker
3. The mass-balance ledger at each upstream node (terminal, supplier, producer)
4. The energy-weighted blend computation for arithmetic correctness
5. The application of multipliers and credits for regulatory eligibility
6. The total annual energy and the total annual emissions against the bunker records and the voyage data

The verifier may adjust the operator’s claimed intensity downwards (more emissions, higher intensity) if any element of the chain fails the audit. The most common adjustments are:

• BDN does not state the certified WtW value: bunker downgraded to default
• Mass-balance chain broken at the terminal: bunker downgraded to default
• Pathway-specific WtW value not supported by ISCC EU certificate: bunker downgraded to default
• RFNBO multiplier claim not supported by additionality, temporal, geographical correlation evidence: multiplier removed
• Pool participation notification not filed by the deadline: ship reverts to per-ship assessment

The verifier may also clear the operator’s claim without adjustment if all elements of the chain are supported. A clean verification produces a verified annual report that the operator submits to THETIS-MRV (FuelEU) and to the flag-state for IMO DCS (GFS). The verified report is the legal record of the ship’s compliance position for the year.

The verifier accreditation is governed by Commission Delegated Regulation (EU) 2023/2917 for FuelEU (with implementing rules in 2023/2918) and by the flag-state recognition framework for IMO DCS (typically via IACS classification societies). The two accreditation regimes are aligned in practice, with most large classification societies holding both recognitions.

IMO Net-Zero Framework Tier 1 + Tier 2 compliance per blend

The IMO Net-Zero Framework adopted at MEPC 83 (April 2025) introduces a two-tier compliance structure that interacts with the blend methodology in a specific way. The structure is set out in detail in the Tier 1 GFI and Tier 2 GFI articles; the relevant point for blend reporting is that each ship is assessed against both tiers in the same annual cycle.

• Tier 1 (Direct Compliance Threshold): The required GFI that the ship must attain on its own without any external compliance instruments. A ship below the Tier 1 threshold is in direct compliance and earns no Surplus Units; a ship above the Tier 1 threshold owes Deficit Units that must be balanced through SU purchase or through the Net-Zero Reserve Fund.

• Tier 2 (Net-Zero trajectory): The aspirational required GFI that anchors the IMO’s net-zero-by-2050 pathway. A ship between the Tier 1 and Tier 2 thresholds is in partial compliance and owes a different set of Deficit Units, typically priced at a different rate.

The blend methodology applied to a ship determines its single attained GFI, which is then compared against both thresholds simultaneously. A ship with an attained GFI of 85.0 gCO2eq/MJ in 2030 (Tier 1 at 91.5, Tier 2 at 75.0) is below Tier 1 (direct compliance, no Tier 1 deficit) and above Tier 2 (Tier 2 deficit owed). The ship’s compliance position is therefore “Tier 1 OK, Tier 2 partial deficit” and the operator pays the Tier 2 DU price on the energy-weighted gap between the attained 85.0 and the Tier 2 75.0.

The blend strategy that minimises both tiers simultaneously is rarely the same as the strategy that minimises FuelEU alone. The IMO regime weights the deeply alternative fuels (RFNBO, e-fuel, ammonia, hydrogen) more heavily than the bio-blends because the Tier 2 trajectory requires deeper reductions than the FuelEU trajectory in the 2030s and 2040s. Operators with both regimes in scope therefore size their blend procurement against the binding constraint, which from approximately 2032 onwards is the IMO Tier 2 trajectory rather than the FuelEU 2030s and 2040s targets.

Bunker quality testing for blend stability

A practical concern at the bunker barge is the physical compatibility of the blend components. A bunker that is regulatory-compliant on WtW grounds may still be physically problematic if the components are not stable in mixture. The most common failure modes are:

• Asphaltene precipitation on VLSFO + MGO blends where the VLSFO’s stability is marginal and the MGO’s solvency power is low (the CCAI / CII index per ISO 8217 forecasts this)
• Phase separation on FAME blends where temperature drops below the cold filter plugging point (CFPP) and the FAME crystallises
• Microbial growth on biofuel-containing blends in long storage with humid bunker tanks (FAME and HVO are both susceptible)
• Methanol-water incompatibility in non-dedicated bunker tanks with residual fossil contamination
• Ammonia material compatibility in bunker pipelines with copper, brass, or zinc components (the IGF Code amendments restrict ammonia bunker hardware to compatible alloys)

The standard test procedure is the ASTM D7112 Asphaltene Precipitation test for VLSFO compatibility, the ISO 22192 (or equivalent) for FAME content, the ISO 8754 for sulphur content, the ISO 6245 for ash, and the supplier-issued certificate of quality (COQ) for blend ratio. The compatibility test is performed at the bunker barge before delivery; a failed test prevents the bunker from being lifted into the ship’s tanks.

The CIMAC Working Group 7 guidance and the ISO 8217:2024 amendments increasingly specify the bunker tank conditioning, the heating regime, and the stability monitoring required for each blend type. Operators with a diverse fuel basket implement fuel-management procedures that segregate bunker tanks by fuel type, schedule tank cleaning between incompatible products, and run continuous quality monitoring on critical voyage legs. The blend strategy on the WtW side and the blend strategy on the physical side must be coherent; a regulatory-optimal blend that fails the bunker compatibility test is not deliverable in practice.

Formula, assumptions, and limits

Formula

The annual well-to-wake GHG intensity of a ship’s bunkered energy is the energy-weighted average of the per-fuel WtW emission factors:

Where $E_i$ is the annual energy from fuel $i$ in MJ on LCV basis, and $\text{EF}_i^{\text{WtW}}$ is the WtW emission factor for fuel $i$ in gCO2eq/MJ. Energy is computed from bunker mass and the lower calorific value:

For FuelEU Maritime with the RFNBO 2x multiplier applied during 2025 to 2033:

The numerator is the same total emissions as without the multiplier; the denominator is amplified for the RFNBO-eligible component, which lowers the reported intensity.

For the IMO GFS, no multiplier applies and the formula reduces to the basic energy-weighted form. For pooled FuelEU compliance, the per-ship intensity is computed first and the pool average is the energy-weighted average of the per-ship intensities across the pool:

Derivation

The energy-weighted average is the only formulation that preserves dimensional consistency and conservation of total emissions across the basket. For a basket of fuels with masses $m_i$, LCVs $\text{LCV}_i$, and emission factors $\text{EF}_i^{\text{WtW}}$:

• Total energy delivered to propulsion: $E_{\text{total}} = \sum_i m_i \cdot \text{LCV}_i$
• Total emissions across the lifecycle: $G_{\text{total}} = \sum_i m_i \cdot \text{LCV}_i \cdot \text{EF}_i^{\text{WtW}}$
• Average intensity: $\text{EF}_{\text{avg}} = G_{\text{total}} / E_{\text{total}} = (\sum_i E_i \cdot \text{EF}_i^{\text{WtW}}) / (\sum_i E_i)$

Mass-weighted or volume-weighted alternatives would not satisfy conservation: a mass-weighted average of methanol (low energy density) and VLSFO (high energy density) would over-weight the methanol contribution to total emissions because methanol represents a smaller share of energy than of mass. The energy-weighted average is the unique formulation that returns the actual gCO2eq per MJ of usable propulsion energy.

The RFNBO multiplier in the FuelEU regime is a regulatory amplification, not a physical adjustment. It scales the denominator (energy attribution) without scaling the numerator (emissions), which produces a lower reported intensity for the same physical fuel basket. The multiplier is justified as a market-acceleration mechanism for the early adoption of RFNBOs and sunsets in 2034 once the technology is presumed to be commercially mature.

Assumptions

1. LCV and WtW values are from MEPC.391(82) Annex 1 or FuelEU Annex II defaults, or from a verified pathway under a recognised voluntary scheme. The default values are conservative and a verified pathway is preferred where available.
2. Each bunker uplift is supported by a BDN that links to a specific WtW value. A bunker without a BDN-stated WtW value is treated at the Annex II default for the fuel grade.
3. The mass-balance chain is intact from producer to bunker delivery. A break in the chain at any node (producer, terminal, bunker barge, ship) downgrades the bunker to the default.
4. The verifier accepts the chain at the annual report stage. A verifier-flagged break triggers an adjustment to the claimed intensity.
5. The energy basket is computed on the LCV basis. HCV-based or mass-based aggregation is not permitted under either MEPC.391(82) or FuelEU.
6. The RFNBO multiplier applies only during 2025 to 2033 and only on FuelEU. The multiplier sunsets in 2034 and never applies to the IMO GFS.
7. Pooling under FuelEU does not change the per-ship blend computation. The pool aggregation sits above the per-ship formula and aggregates verified per-ship intensities.

Worked example

A 100,000 DWT bulker on a global trade pattern lifts the following bunker basket in calendar year 2030:

• 6,000 tonnes VLSFO at 41.0 MJ/kg LCV and 91.0 gCO2eq/MJ WtW (Annex II default)
• 1,500 tonnes MGO at 42.7 MJ/kg LCV and 93.3 gCO2eq/MJ WtW (Annex II default)
• 2,000 tonnes B30 FAME-VLSFO blend, decomposed into 560 tonnes UCO-FAME at 37.2 MJ/kg LCV and 14.0 gCO2eq/MJ WtW (ISCC EU certified) and 1,440 tonnes carrier VLSFO at 41.0 MJ/kg LCV and 91.0 gCO2eq/MJ WtW
• 200 tonnes e-methanol at 19.9 MJ/kg LCV and 7.0 gCO2eq/MJ WtW (ISCC EU certified RFNBO, eligible for the FuelEU 2x multiplier)

Step 1: Compute the energy from each fuel:

• $E_{\text{VLSFO}} = 6{,}000 \cdot 41.0 \cdot 1{,}000 = 246.0$ TJ
• $E_{\text{MGO}} = 1{,}500 \cdot 42.7 \cdot 1{,}000 = 64.0$ TJ
• $E_{\text{FAME}} = 560 \cdot 37.2 \cdot 1{,}000 = 20.8$ TJ
• $E_{\text{VLSFO,carrier}} = 1{,}440 \cdot 41.0 \cdot 1{,}000 = 59.0$ TJ
• $E_{\text{e-methanol}} = 200 \cdot 19.9 \cdot 1{,}000 = 3.98$ TJ

Step 2: Compute the total energy:

$E_{\text{total}} = 246.0 + 64.0 + 20.8 + 59.0 + 3.98 = 393.78$ TJ

Step 3: Compute the total emissions:

$G_{\text{total}} = 246.0 \cdot 91.0 + 64.0 \cdot 93.3 + 20.8 \cdot 14.0 + 59.0 \cdot 91.0 + 3.98 \cdot 7.0 = 22{,}386 + 5{,}971 + 291 + 5{,}369 + 28 = 34{,}045$ tonnes CO2eq

Step 4: Compute the raw blend intensity (GFS basis):

$\text{EF}_{\text{blend, GFS}} = 34{,}045 / 393.78 = 86.5$ gCO2eq/MJ

Step 5: Compute the FuelEU-effective intensity with the RFNBO 2x multiplier on the e-methanol component:

$\text{Denominator, FuelEU} = 393.78 + 3.98 = 397.76$ TJ (the e-methanol energy of 3.98 TJ is added once more to apply the 2x multiplier)

$\text{EF}_{\text{blend, FuelEU}} = 34{,}045 / 397.76 = 85.6$ gCO2eq/MJ

Step 6: Compare with the regulatory thresholds:

• GFS Tier 1 required GFI for 2030: approximately 91.5 gCO2eq/MJ (5 percent reduction from 93.3 baseline). Attained 86.5, ship is in direct compliance, no Tier 1 deficit.
• FuelEU 2030 required intensity: approximately 86.4 gCO2eq/MJ (5.25 percent reduction from 91.16 baseline). Attained 85.6, ship is marginally over-compliant, small surplus available for pooling or sale.

Step 7: Compute the EU ETS allowance liability on the fossil emissions:

The fossil emissions are $246.0 \cdot 91.0 + 64.0 \cdot 93.3 + 59.0 \cdot 91.0 = 22{,}386 + 5{,}971 + 5{,}369 = 33{,}726$ tonnes CO2eq, of which the EU ETS share is approximately 70 percent on the EU-leg basis ($33{,}726 \cdot 0.70 = 23{,}608$ tonnes CO2eq). At an EU ETS allowance price of EUR 80 per tonne, the liability is EUR 1.89 million for the year.

The same ship without the bio-blend or e-methanol would attain approximately 91.4 gCO2eq/MJ (the Scenario 1 result), which is below the GFS Tier 1 trajectory but above the FuelEU 2030 trajectory by 5 gCO2eq/MJ. The B30 FAME plus 200 tonnes e-methanol close the FuelEU gap and produce a small surplus, at a marginal bunker premium of approximately USD 200,000 to 400,000 for the year. The integrated cost saving (FuelEU non-compliance avoided plus EU ETS allowance avoided on the displaced fossil) typically more than offsets the bunker premium.

Edge cases and limits

Default-only operation: A ship that bunkers exclusively on default-WtW values (no certified pathway, no biofuel, no RFNBO) cannot improve its attained intensity below the lowest fossil default in its bunker basket. For a global trader on VLSFO and MGO, the floor is approximately 91.0 gCO2eq/MJ; for an LNG dual-fuel ship without bio-LNG or e-LNG, the floor is approximately 75 to 85 gCO2eq/MJ depending on engine slip. Improving below the floor requires substituting bio or RFNBO components.

Carry-over of FuelEU surplus: A ship that over-complies in year N can carry over the surplus to year N+1 under FuelEU Article 6, but only up to a 2 percent buffer. Larger surpluses must be pooled or sold within the year. The carry-over does not apply to the IMO GFS, where surplus is monetised through SU trading rather than through carry-over.

Multiple-supplier bunker uplifts: A bunker uplift that combines product from multiple suppliers (rare but legally possible) requires the BDN to state the contribution of each supplier and the WtW value of each contribution. The blend formula is applied at the granularity of each contribution, not at the granularity of the combined uplift.

Bunker contamination: A bunker uplift that is contaminated with off-specification product (e.g. higher-than-stated FAME content, residual product from a previous bunker barge load) must be reported at the contaminated specification, which may be at a different default or pathway. The verifier’s audit of the BDN sample ties the reported value to the actual delivery.

Engine-side losses: The LCV-based energy normalisation is on the bunker side. Engine thermal efficiency varies (typically 45 to 55 percent for two-stroke marine diesels, 35 to 45 percent for four-stroke, 30 to 40 percent for gas turbines) and translates the bunker LCV into actual propulsion energy. The blend methodology is independent of engine efficiency; the engine efficiency affects the bunker volume needed to deliver a given voyage but does not change the gCO2eq/MJ at the bunker manifold.

Auxiliary engine vs main engine fuels: A ship that runs different fuels on its main engine and its auxiliary engine (e.g. main engine on LNG, auxiliary on MGO) is reported as a single bunker basket with all fuels aggregated at the ship level. The split between main and auxiliary is not material to the blend computation; the energy-weighted average treats every megajoule equally regardless of which engine consumed it.

Boil-off gas on LNG carriers: An LNG carrier with boil-off gas (BOG) consumption from the cargo tanks reports the BOG as a separate bunker entry with the LNG WtW value applied. The BOG is treated as a planned consumption from the cargo, not as a bunker uplift, and is recorded on a separate line in the Bunker Receipt Record. The blend formula treats the BOG as if it were an LNG bunker for the energy-weighted average purpose.

Cargo-as-fuel on non-LNG carriers: An LPG carrier burning LPG cargo, a methanol carrier burning methanol cargo, or an ammonia carrier burning ammonia cargo applies the same logic as LNG BOG: the cargo consumption is reported as a bunker entry with the cargo’s WtW value, separate from any external bunker uplifts.

Verifier rejection scenarios: A verifier that rejects a claimed pathway may issue a downgrade adjustment that the operator can either accept (revising the report) or contest through the Article 14 review mechanism. A successful contest restores the claimed value; an unsuccessful contest finalises the downgrade. The contest mechanism is rare in practice because verifiers typically clear documented claims and reject only clearly unsupported claims.

Multi-year offtake contracts: A multi-year RFNBO or biofuel offtake contract that commits the producer to a specified WtW value across multiple years requires re-verification each year if the upstream conditions (renewable electricity supply, feedstock origin, mass-balance ledger) change. Operators should not assume that a year-1 verified pathway carries unchanged into year 2 or beyond.

Regulatory basis

• IMO MEPC.391(82): 2023 Guidelines on Lifecycle GHG Intensity of Marine Fuels, Annex 1 default emission factors and LCV table, Annex 2 verification methodology for chain-of-custody, energy-weighted blend formula
• IMO Net-Zero Framework (MEPC 83 outcome, April 2025): Tier 1 and Tier 2 GFI structure, Surplus Unit and Deficit Unit trading, GFS attained GFI per ship per year
• MARPOL Annex VI Regulation 22A: IMO DCS data collection on fuel oil consumption, mandatory reporting per fuel type and per voyage, foundational data layer for the GFS
• MARPOL Annex VI Regulation 18: Bunker Delivery Note requirements, sealed sample retention, ISO 8217 fuel quality specification
• FuelEU Maritime (Regulation (EU) 2023/1805): Article 4 GHG intensity formula, Article 8 verification of monitoring data, Article 21 pooling, Annex I monitoring rules, Annex II default values
• EU MRV (Regulation (EU) 2015/757): Original 2015 reporting infrastructure consolidated under THETIS-MRV, voyage-level emissions data
• EU ETS Maritime (Directive (EU) 2023/959): Allowance surrender on 50 percent of intra-EU plus 100 percent of inside-EU emissions, integrated reporting through THETIS-MRV
• RED III (Directive (EU) 2023/2413): Sustainability and 70 percent GHG saving criteria for biofuels and RFNBOs, Article 30 mass-balance verification
• Commission Delegated Regulation (EU) 2023/1184: RFNBO additionality, temporal and geographical correlation rules
• Commission Delegated Regulation (EU) 2023/1185: RFNBO GHG calculation methodology
• Commission Implementing Regulation (EU) 2022/996: Voluntary scheme certification and mass-balance verification rules
• Commission Delegated Regulation (EU) 2023/2917 + Implementing Regulation (EU) 2023/2918: FuelEU verifier accreditation framework
• ISO 8217:2024: Marine fuel specifications, BDN content requirements, bio-blend categories
• CIMAC Working Group 7 (Fuels): Industry guidance on BDN, ISO 8217 sampling, bunker compatibility testing for blended fuels

Common errors

1. Using mass-weighting or volume-weighting instead of energy-weighting. The Article 4 formula is unambiguous: the weighting is on energy in MJ on LCV basis, not on mass in tonnes or volume in m3. Substituting the wrong weighting can shift the blend intensity by 5 to 20 percent.
2. Applying the RFNBO multiplier to the IMO GFS calculation. The multiplier is FuelEU-specific and does not apply to the GFS attained GFI. Operators with both regimes in scope must run two parallel calculations: GFS without the multiplier, FuelEU with the multiplier.
3. Treating a bio-blend as a single fuel category. A B30 FAME-VLSFO blend should be decomposed into the FAME component (with its certified WtW) and the carrier VLSFO component (with the fossil WtW). Treating the blend as a single product at a single weighted WtW loses the option to apply different multipliers and credits to each component.
4. Counting cargo-as-fuel as a “free” emission source. LPG cargo, LNG BOG, methanol cargo, or ammonia cargo burned as fuel must be reported with the relevant WtW value, the same as if the fuel were lifted from a bunker barge. The cargo origin does not exempt the consumption from the reporting.
5. Carrying over FuelEU surplus beyond the 2 percent buffer. Article 6 limits the carry-over to 2 percent of the prior year’s compliance value. Larger surpluses must be pooled, sold, or forfeited within the year.
6. Assuming the verifier will accept claims at face value. The verifier audits the BDN, the certification chain, the mass-balance ledger, and the multiplier eligibility. Operators that rely on unverified claims face downgrade adjustments at the annual report stage.
7. Confusing the LCV with the HCV in the energy basket. Marine engines do not recover latent heat from water vapour; the LCV is the correct basis. Using the HCV would over-state the energy by 5 to 10 percent and under-state the intensity proportionally.
8. Ignoring the pilot fuel on dual-fuel engines. A methanol or ammonia dual-fuel engine with a 5 to 8 percent MGO pilot reports the pilot as a separate fuel entry with the fossil MGO WtW. Operators that omit the pilot fuel from the blend computation under-state the actual intensity by 4 to 7 gCO2eq/MJ.

See also

• /wiki/per-fuel-wtw-vlsfo-mgo
• /wiki/per-fuel-wtw-hfo
• /wiki/per-fuel-wtw-fame
• /wiki/per-fuel-wtw-hvo
• /wiki/per-fuel-wtw-bio-lng
• /wiki/per-fuel-wtw-lng-otto-diesel
• /wiki/per-fuel-wtw-methanol-grades
• /wiki/per-fuel-wtw-ammonia-grades
• /wiki/per-fuel-wtw-hydrogen
• /wiki/per-fuel-wtw-lpg
• /wiki/per-fuel-wtw-efuel
• /wiki/marine-gfs-methodology
• /wiki/fueleu-intensity-formula-breakdown
• /wiki/fueleu-compliance-balance-pooling
• /wiki/fueleu-rfnbo-multiplier
• /wiki/imo-net-zero-framework
• /wiki/marpol-annex-vi
• /calculators/fuel-wtw-blend
• /calculators/fueleu-ghg-intensity
• /calculators/gfi-attained
• /calculators/gfi-compliance
• /calculators/fueleu-pooling
• /calculators/fueleu-compliance-balance

Related calculators

• e-Diesel / FT e-Fuel - Well-to-Wake
• VLSFO - Well-to-Wake
• LPG - Well-to-Wake
• LNG - Well-to-Wake by engine pathway
• HFO - Well-to-Wake
• MGO / MDO - Well-to-Wake
• Methanol - Well-to-Wake by pathway
• Hydrogen - Well-to-Wake by pathway

Related formulas

• fuel wtw ulend
• fuel wtw vlsfo
• fuel wtw nuclear
• fuel wtw mgo
• fuel wtw methanol
• fuel wtw lpg
• fuel wtw lng
• fuel wtw hydrogen
• Blend Compatibility (SPOT / CCAI)