By ShipCalculators.com · Published 8 May 2026

MEPC.391(82): IMO LCA Guidelines for marine fuels

Resolution MEPC.391(82), the 2024 Guidelines on lifecycle GHG intensity of marine fuels, adopted at MEPC 82 on 4 October 2024, is the technical rulebook that defines how every marine fuel’’s well-to-wake (WtW) GHG intensity is computed for IMO regulatory purposes. The Guidelines operationalise the lifecycle accounting that underpins the Marine Global Fuel Standard inserted as Chapter 4 ter of MARPOL Annex VI under the broader IMO Net-Zero Framework. The functional unit is grams of CO2-equivalent per megajoule on a lower-calorific-value (LCV) energy basis, and the boundary is the sum of a Well-to-Tank (WtT) upstream segment and a Tank-to-Wake (TtW) combustion-and-slip segment. GHG species are aggregated using IPCC AR5 GWP100 weights (CH4 = 28, N2O = 273, biogenic CO2 = 0). Annex 1 of the Resolution publishes default emission factors for every conventional and alternative pathway, which propagate into both the GFI attained calculator and the FuelEU GHG-intensity calculator, as well as into every per-fuel article in the WtW series (VLSFO/MGO, LNG Otto and Diesel, methanol grades, ammonia grades, hydrogen, and the blend methodology). Annex 2 supplies the verified-pathway methodology for shipowners and bunker suppliers who claim values better than the default, including the Mass Balance chain-of-custody and the additionality, temporal-correlation and geographical-correlation criteria for RFNBO electrolytic hydrogen and its derivatives. The IMO Guidelines align deliberately with FuelEU Maritime Annex II and Commission Implementing Regulation (EU) 2024/2401, which means a ship operator that satisfies the overlapping EU ETS and FuelEU regime is largely consistent with the IMO regime, with a small set of pathway-specific deltas. ShipCalculators.com tracks the Guidelines through the GFS calculators and the FuelEU compliance balance tool.

Contents

Background: lifecycle assessment in marine regulation

Before MEPC.391(82) the regulation of GHG emissions from international shipping had been almost entirely tank-to-wake (TtW): the EEDI, EEXI and CII regulations of MARPOL Annex VI Chapter 4, the IMO Data Collection System under regulations 27 and 28, and the EU MRV Regulation 2015/757 in its original form, all measured CO2 emitted from on-board combustion of fuel and ignored every gCO2eq released upstream during fuel production, processing or transport. That convention was workable while the world fleet ran on residual and distillate hydrocarbons whose upstream emissions, although non-trivial, were broadly similar across pathways. It became unworkable the moment alternative fuels entered the picture. A litre of biomethane combusted on a ship emits CO2 at the funnel, just as a litre of fossil natural gas does, but the lifecycle climate impact of the two fuels is opposite in sign: the biomethane closes a short biogenic carbon cycle, the fossil gas opens a long geological one. A pure tank-to-wake metric cannot tell those two apart. A green-hydrogen-derived ammonia and a coal-derived ammonia have identical TtW profiles (zero CO2 from combustion, but both emit N2O); their WtW profiles differ by an order of magnitude. The 2018 Initial IMO Strategy on Reduction of GHG Emissions, and even more emphatically the 2023 Revised Strategy of Resolution MEPC.377(80), accepted that the next generation of measures had to be well-to-wake if the regulator was to choose between alternative fuels rationally. The 2024 LCA Guidelines are the carrying instrument for that decision.

The Guidelines are not the first regulator-grade lifecycle framework for liquid fuels: the EU’s Renewable Energy Directive II (Directive 2018/2001) and its Annex V have applied a similar default-and-actual logic to road-transport biofuels for over a decade, the United States’ Renewable Fuel Standard runs an equivalent under the EPA, and California’s LCFS uses the GREET model in a closely related way. The IMO Guidelines borrow from those precedents, calibrate them to international shipping, and align them with the international standard family ISO 14040 / 14044 so that the methodology is recognised by every member State that already accepts ISO LCA practice in its national procurement and emissions law. The result is a regulatory implementation of ISO 14040, not a parallel construct.

Drafting history MEPC 76 to MEPC 82

The drafting history of the Guidelines spans five years and six MEPC sessions. MEPC 76 (June 2021) tasked the Intersessional Working Group on Reduction of GHG Emissions (ISWG-GHG) with developing a lifecycle methodology, in response to a joint submission by the Marshall Islands, the Solomon Islands and the Republic of Korea that flagged the inadequacy of TtW for alternative fuels. MEPC 77 (November 2021) considered a first scoping paper and agreed that the Functional Unit should be gCO2eq/MJ LCV, that the metric should aggregate CO2, CH4 and N2O using AR5 GWP100, and that the boundary should be Well-to-Wake. MEPC 78 (June 2022) approved interim Guidelines under Resolution MEPC.376(78), which were treated as a developmental text for trial application but were never operative as regulation. MEPC 79 (December 2022) and MEPC 80 (July 2023) refined the structure into the two-annex format that survives in the final Resolution: Annex 1 for default pathway values, Annex 2 for verification of actual values. MEPC 81 (March 2024) approved a near-final draft for adoption, with placeholders left in the default-value tables for ammonia (slip and N2O), methanol (specific feedstock variants) and biofuels (indirect-land-use-change considerations). MEPC 82 (30 September to 4 October 2024) adopted the final Resolution as MEPC.391(82), with the placeholder values closed and the publication of the full Annex 1 default table. Adoption came in the same session that approved the draft Chapter 4 ter for circulation to MEPC 83 (where the GFS and Net-Zero Framework were approved in April 2025). The drafting was thus deliberately sequenced so that the lifecycle methodology was complete and adopted before the regulation that would invoke it.

Adoption at MEPC 82 (October 2024)

Resolution MEPC.391(82) was adopted on the final day of MEPC 82, by consensus of the parties to MARPOL Annex VI present and voting, with no abstentions and no formal reservations. As an MEPC Resolution operationalising the existing MARPOL Annex VI, the LCA Guidelines do not require ratification by member States: they take effect immediately for every regulatory purpose for which a higher-level Annex VI provision invokes them. Because Chapter 4 ter (the GFS) cites the Guidelines as its lifecycle methodology, and because Chapter 4 ter is itself a MARPOL amendment that follows the tacit-acceptance procedure under MARPOL article 16, the Guidelines acquire binding regulatory force at the same moment the GFS amendment enters into force, currently scheduled for 1 January 2027. Member States that wish to apply the Guidelines earlier (for example to inform domestic policy or to operationalise a regional regime) are free to do so by reference; the EU has done exactly that in Implementing Regulation 2024/2401, which cites MEPC.391(82) where consistent with FuelEU Annex II. The text of the Resolution is published on the IMO website as a downloadable PDF and is also republished in the IMO MEPC document series as document MEPC 82/INF.X.

Scope: all fuels for international shipping

The Guidelines apply to all fuels and energy carriers used in international shipping for the purpose of computing GHG intensity under any IMO instrument that invokes them. In practice this means every conventional residual and distillate hydrocarbon (HFO, VLSFO, LSMGO, MGO, MDO), every alternative liquid fuel (LNG in Otto and Diesel cycles, LPG, methanol, ethanol, ammonia, hydrogen in liquid or compressed form), every drop-in or blend (FAME, HVO, bio-LNG, bio-methanol, bio-MGO), every electricity-derived synthetic fuel (RFNBO e-fuels), every recycled-carbon fuel, and the on-board electrical energy supplied through cold-ironing or stored in batteries. The scope is fuel-type-agnostic and pathway-specific: the regulator does not care that a litre of methanol exists, only that a litre of methanol from natural gas via steam reforming (grey methanol) is distinguished from a litre of methanol from biomass gasification (bio-methanol) and from a litre of methanol from CO2 and renewable hydrogen (e-methanol). The same physical fuel with different pathways is reported as different fuel records under the Guidelines.

The application boundary is defined by the regulation that invokes the Guidelines. For the GFS, the boundary is ships of 5,000 GT and above on international voyages, mirroring the existing IMO DCS scope; ships below 5,000 GT, military vessels, government non-commercial vessels, and platforms (drilling rigs, FPSOs without propulsion) are excluded. For the EU’s invocation under FuelEU, the boundary is broader on coverage but narrower on geography (100% of intra-EU energy, 50% of extra-EU energy on voyages with one EU end). For voluntary or domestic application by a member State or fuel supplier, the boundary is what that State or supplier defines.

Lifecycle accounting boundary: WtT + TtW = WtW

The accounting boundary is the Well-to-Wake sum of two segments. The Well-to-Tank (WtT) segment covers every gCO2eq released between the well-head (or the field, for a biological feedstock; or the renewable generator, for an electricity-derived fuel) and the receiving flange of the ship’s bunker manifold. It includes feedstock extraction, transport of feedstock to the processing plant, conversion of feedstock to finished fuel, transport of finished fuel through pipeline, road, rail, barge and sea-going tanker to the bunker port, and any storage losses, methane slip from storage, and venting along the way. The Tank-to-Wake (TtW) segment covers every gCO2eq released between the bunker manifold and the funnel: combustion in the main engine, auxiliary engines and boilers, plus uncombusted fuel slipping through the engine (relevant for LNG in low-pressure dual-fuel engines, ammonia in dual-fuel engines, methanol in some pre-mix designs), plus N2O formation in lean-burn or ammonia combustion, plus SOx and NOx that are reported under separate provisions but enter the GWP aggregation through their indirect effects in the few cases the Guidelines accept. The arithmetic is additive: WtW = WtT + TtW, both in gCO2eq/MJ on the LCV of the delivered fuel. The boundary is intentionally consistent with the JEC (JRC-EUCAR-CONCAWE) WtW analyses for European road transport, with the maritime adjustments needed to capture marine engine slip and bunker-supply chains.

Functional Unit: gCO2eq per MJ on LCV basis

The Functional Unit is grams of CO2-equivalent per megajoule of energy delivered, on the lower calorific value (LCV) of the fuel. Three choices in this definition matter. First, the energy-based denominator (MJ rather than tonnes or litres) makes pathways comparable across fuels of very different energy density: a tonne of ammonia carries roughly 18.6 GJ LCV against a tonne of VLSFO at 40.5 GJ LCV, and any per-mass metric would falsely flatter the ammonia. Second, lower (rather than higher) calorific value is used because LCV reflects the energy actually delivered to the engine after accounting for the latent heat of vaporisation of water in the combustion products, and is the convention in marine engineering. Third, CO2-equivalent (rather than carbon-mass-equivalent or kgC) aggregates the warming potential of all GHG species into a single number using AR5 GWP100, so that the final WtW value is directly comparable to the GFS Required GFI trajectory and the FuelEU intensity limits.

Annex 1 default values per fuel pathway

Annex 1 of MEPC.391(82) is a table of default WtW emission factors, one row per pathway, one column for WtT, one for TtW, one for the combined WtW value. The default value is what an operator may use without further evidence: it is conservative for fossil pathways (set high enough to make any actual measurement preferable when feasible) and is the standard for pathways where actual measurement is impractical or where the regulator wishes to avoid creating an incentive to game the methodology. The defaults are constructed from a JEC-aligned WtT analysis combined with a marine-engine-specific TtW model that incorporates published combustion measurements for the relevant engine cycle. Each per-fuel article in the WtW series republishes and analyses the relevant Annex 1 row with full provenance: see VLSFO and MGO, LNG in Otto and Diesel cycles, methanol grades, ammonia grades, hydrogen, HFO, HVO, FAME, LPG, bio-LNG, e-fuel, and the blend methodology that combines per-pathway defaults across a fuel mix. The Annex 1 defaults flow directly into the GFI attained calculator and the FuelEU GHG-intensity calculator, so the operator can pick a pathway, enter the energy in MJ, and read out the WtW gCO2eq.

A few representative defaults in gCO2eq/MJ on LCV: VLSFO at 91.6, MGO at 90.8, LNG-Otto at 76.1, LNG-Diesel at 70.8, grey methanol at 99.5, blue methanol at about 50, e-methanol at near zero, grey ammonia (SMR) at about 121, blue ammonia (SMR plus CCS) at about 65, green ammonia at near zero, grey hydrogen at about 132, green hydrogen at near zero, FAME at about 28 to 53 depending on feedstock, HVO at about 17 to 30, bio-LNG at about minus 14 to plus 20 depending on feedstock and credit treatment.

Annex 2 verification methodology for certified pathways

Annex 2 is the procedural complement to Annex 1: it sets out how an operator or fuel supplier may obtain a certified actual value that supersedes the Annex 1 default for a specific batch of fuel. The procedure has three pillars. First, the actual value must be calculated using the same boundary and Functional Unit as Annex 1 (WtW, gCO2eq/MJ, LCV, AR5 GWP100), so that defaults and actuals are commensurable. Second, the calculation must be verified by an independent third party against a recognised voluntary scheme (CertifHy for hydrogen, RSB or ISCC EU for biofuels and RFNBOs, Bonsucro for sugarcane-derived bioethanol and bio-methanol) or directly against Annex 2’s own audit protocol. Third, the chain of custody from feedstock origin through processing plant to bunker barge must be documented under a Mass Balance scheme: every input batch is logged with its lifecycle GHG intensity, every output batch’s intensity is the volume-weighted average of inputs at that node, and the documentary chain is reconstructable from the Bunker Delivery Note backwards to the feedstock origin. Annex 2 is the single most operationally consequential annex for the alternative-fuel supply chain: without a robust Mass Balance the supplier cannot deliver an actual value, and the buyer is forced to settle for the Annex 1 default, which for any non-trivial alternative pathway is materially worse than what a properly certified supply can demonstrate.

GHG accounting: IPCC AR5 GWP100

The Guidelines aggregate GHG species using the IPCC AR5 GWP100 metric, with carbon-cycle feedbacks excluded. The relevant values are CO2 = 1, CH4 = 28, N2O = 273, hydrofluorocarbons and other industrial species at their AR5 values, and biogenic CO2 = 0 for fuels whose carbon was withdrawn from the atmosphere within the human-relevant timescale (annual or decadal crops, forestry residues under sustainable management, anaerobic digestion of organic waste). The choice of AR5 over AR6 was deliberate: AR6 GWP100 figures for CH4 are higher (CH4 = 29.8 with carbon-cycle feedbacks, or 27.0 without) but AR5 is the metric already in use in UNFCCC national-inventory reporting under the Paris Agreement Enhanced Transparency Framework, and consistency with national inventories was prioritised. The Guidelines anticipate a future revision once the UNFCCC moves to AR6, and the periodic-review clause is the mechanism for that update. The choice of GWP100 over GWP20 is also deliberate: GWP20 would assign methane a coefficient of around 84 and would heavily penalise LNG and bio-LNG pathways relative to liquid fuels; GWP100 is the convention in the climate-policy community and is the metric that the GFS Required GFI trajectory was calibrated against in the 2023 Revised IMO GHG Strategy.

System boundaries: extraction, transport, processing, distribution, combustion

The WtW boundary breaks down into six successive stages, each of which contributes a definable share of the total. Stage 1, feedstock extraction covers crude oil and natural gas extraction, biomass cultivation and harvest, water electrolysis or biomass gasification for the renewable feedstock, and the direct emissions of the extraction process (flaring, fugitive methane from gas wells, diesel for harvest equipment, indirect land-use change for biofuels). Stage 2, feedstock transport covers movement of the extracted feedstock to the processing plant: pipeline transport for natural gas and crude, road or rail for biomass, electricity transmission losses for power-to-x feedstock electricity. Stage 3, fuel processing covers refining of crude into VLSFO, MGO and HFO; liquefaction and Fischer-Tropsch of natural gas into LNG and synthetic distillates; reforming of natural gas to grey ammonia or grey hydrogen; electrolysis to green hydrogen; methanol synthesis from syngas or from CO2 plus hydrogen; transesterification of vegetable oil to FAME; hydrotreating of waste oil or vegetable oil to HVO; anaerobic digestion plus upgrading to bio-LNG. Stage 4, fuel distribution covers movement of finished fuel from the production plant to the bunker port: ocean-going product tanker for HFO and distillates, LNG carrier for LNG and bio-LNG, ammonia carrier for ammonia, road tanker for last-mile MGO and methanol. Stage 5, bunkering and storage covers handling losses at the bunker port (vapour displacement, pumping energy, dock electricity). Stage 6, on-board combustion plus slip is the TtW: combustion CO2 stoichiometric to the carbon content of the fuel, plus methane slip on LNG and ammonia engines, plus N2O on lean-burn and ammonia engines, plus uncombusted hydrogen for the few hydrogen engines and fuel cells in the fleet. Stages 1 to 5 sum to the WtT segment; Stage 6 is the TtW segment.

Cut-off vs system expansion methodology

LCA practice offers two main methods for handling co-products and waste-derived fuels, and the Guidelines adopt a defined hierarchy. The default is the cut-off (or recycled-content) method: the input flow into a process is allocated by its energy or mass share, and a waste or residue that is the input to a fuel pathway carries no upstream burden (its prior life is “cut off” at the gate of the fuel plant). This is the convention applied to FAME from used cooking oil, HVO from animal-fat residues, bio-LNG from manure, methanol from waste pulp liquor, and any RFNBO that takes captured CO2 from a non-fossil industrial source. For pathways where a co-product has substantial economic value and cannot reasonably be treated as a residue, system expansion (substitution credit) is used: the avoided emissions of the displaced product are subtracted from the fuel’s lifecycle. This is applied conservatively for refinery co-products (the heavy cut allocated to bunker fuel takes its share of refinery WtT in proportion to energy content, with no avoided-emissions credit), and more liberally for bio-refinery co-products such as glycerol from FAME or DDGS from bioethanol, where the displaced industrial chemical has a defined market. The Guidelines explicitly disallow system expansion where the displacement claim is not robust, and they explicitly require cut-off where the input is a regulatory-defined waste under the EU Waste Framework Directive or equivalent national law.

RFNBO additionality criteria (cross-link)

For Renewable Fuels of Non-Biological Origin (RFNBO), primarily green hydrogen, green ammonia, e-methanol and other electricity-derived synthetics, the Guidelines impose additionality, temporal correlation and geographical correlation criteria adapted from the EU Delegated Regulation 2023/1184. Additionality requires that the renewable electricity used for the electrolysis is incremental to what would otherwise have been built: a new wind farm or solar plant whose generation is contractually allocated to the electrolyser, not the existing renewable share of the grid. Temporal correlation requires that production of the renewable electricity and operation of the electrolyser occur in the same hourly window (with a transitional monthly window until 2030, then hourly). Geographical correlation requires that both the generator and the electrolyser sit in the same bidding zone (or in interconnected zones that satisfy a defined grid-balance condition). A pathway that fails any of the three criteria is treated as grid-electrolysis and inherits the average grid carbon intensity of the relevant zone, which can take a green-hydrogen pathway from near-zero gCO2eq/MJ to well above grey hydrogen for a coal-heavy grid. The deep treatment of the RFNBO multiplier and the additionality regime is in /wiki/fueleu-rfnbo-multiplier.

Temporal and geographical correlation rules

Beyond RFNBO, the Guidelines apply temporal-correlation and geographical-correlation rules across every pathway to prevent double-counting and to ensure that a claimed low-carbon batch is physically associated with the claimed feedstock and the claimed processing route. Temporal correlation in the broad sense means that the lifecycle inventory used for the calculation must reflect the period in which the fuel was produced: a methanol batch claimed for 2027 cannot be calculated against the 2018 grid-intensity of the producing country if that grid has since shifted appreciably. Geographical correlation means that the inventory used must reflect the actual production location: a batch of LNG bunkered in Singapore that originated in Qatar takes its WtT from the Qatari supply chain, not a default Western Australian or US-Gulf chain, where the supplier can document the origin under the Mass Balance. Where origin cannot be documented, the most conservative default for a plausible origin is applied. Both rules apply to the WtT segment specifically; the TtW segment is determined by engine cycle and fuel-batch composition and is not subject to a geographical-correlation rule.

EU-IMO regulatory alignment

A material policy choice in the drafting of MEPC.391(82) was to align as closely as possible with FuelEU Maritime Annex II and with the EU’s Renewable Energy Directive II Annex V, so that a ship calling at both EU and non-EU ports under both regimes does not have to maintain two parallel sets of WtW calculations for the same fuel. The alignment is substantial: Functional Unit identical (gCO2eq/MJ LCV), boundary identical (WtW), GHG aggregation identical (AR5 GWP100), default-and-actual structure identical (Annex 1 defaults and Annex 2 verification on the IMO side, FuelEU Annex II defaults and Article 10 verification on the EU side). It is not absolute: the two regimes diverge on a small number of pathway-specific defaults (notably for some biofuel feedstocks where FuelEU recognises a higher indirect-land-use-change penalty, and for grid electricity used in cold-ironing where FuelEU uses an EU-grid average and the IMO uses a global default) and on the precise wording of the additionality criteria (FuelEU is more prescriptive on hourly matching). The deeper comparison is in /wiki/eu-ets-fueleu-double-regulation, which traces the operational consequences of the small divergences for a ship that has to report under both regimes.

Relationship to FuelEU Maritime (cross-link)

The relationship between MEPC.391(82) and Regulation (EU) 2023/1805 is best framed as one of regulator-implementer parity. The IMO is the standard-setter for international shipping; the EU is implementing a regional regime in advance of, and largely in parallel with, the IMO regime. Where the EU and the IMO use the same per-fuel default (the typical case), the operator computes the same WtW number for both regimes from the same input data. Where they diverge, the operator computes both. The mapping is mechanical: every default in MEPC.391(82) Annex 1 has a counterpart in FuelEU Annex II (or in Implementing Regulation 2024/2401 for the verification details), and the per-fuel WtW articles in the ShipCalculators series document both alongside each other. The deep formula breakdown for the FuelEU side is in /wiki/fueleu-intensity-formula-breakdown, and the FuelEU GHG-intensity calculator uses the FuelEU defaults where they differ from the IMO defaults.

Member State default-pathway override

The Guidelines empower a member State to override an Annex 1 default value with a State-specific value for fuels produced and bunkered within its jurisdiction, provided the State-specific value is calculated to the same boundary and Functional Unit and is published with full methodological transparency. The mechanism is intended for States with materially different feedstock conditions: a State with abundant low-carbon hydroelectricity, for example, can publish a grid-electricity intensity below the global default and apply that to e-fuels produced domestically. The overriding value is binding only for fuels supplied within that State’s jurisdiction; it does not propagate internationally and does not substitute the default for fuels supplied elsewhere. The State must notify the IMO Secretariat and provide the supporting calculation. The mechanism is rarely invoked because Annex 2 verified-actual values give a finer-grained route to the same outcome at the supplier level, but it remains a backstop for jurisdictions with thin certification infrastructure.

Certified-pathway approval: CertifHy, RSB, Bonsucro, ISCC EU

Annex 2 verification is in practice channelled through a small number of recognised voluntary schemes that the IMO accepts as equivalent to its own audit protocol. CertifHy is the European Hydrogen Association’s voluntary scheme for low-carbon and renewable hydrogen, with CertifHy Green Hydrogen and CertifHy Low-Carbon Hydrogen as the two main labels; it covers the WtT of hydrogen and ammonia derivatives. RSB (Roundtable on Sustainable Biomaterials) covers the full sustainability and lifecycle GHG calculation for biofuels and RFNBOs, with a maritime-specific addendum that maps RSB Standards onto FuelEU and the IMO Guidelines. Bonsucro covers sugarcane-derived bioethanol, bio-methanol and bio-LNG from sugarcane bagasse and vinasse. ISCC EU (International Sustainability and Carbon Certification) covers a wide spread of biomass and recycled-carbon feedstocks and is the most widely used scheme for bunker-grade biofuels in European bunker ports. Other schemes (Bonsucro EU, REDcert, 2BSvs) are accepted under the EU regime and are functionally equivalent for IMO purposes once the supplier maps the certificate onto the MEPC.391(82) Functional Unit. The supplier obtains a certificate per batch, includes the certified value on the BDN, and the ship’s verifier traces the certificate back to the scheme registry at the IAPP renewal survey.

Periodic review every 4 years

The Guidelines include a periodic-review clause under which the IMO commits to revisit the methodology no later than four years after entry into force, and at four-year intervals thereafter. The review covers Annex 1 default values (recalibration against new pathway data), Annex 2 verification rules (where field experience identifies gaps), the GWP convention (whether to migrate from AR5 to AR6 once the UNFCCC does), and treatment of pathways that were not commercial at the time of adoption (CCS-equipped fossil fuels, hydrogen by-product from chlor-alkali and steam-cracker industry, low-CO2 nuclear-derived hydrogen and ammonia). The first scheduled review is at MEPC 84 in October 2025, with substantive amendments expected in the 2027 to 2029 window. Between scheduled reviews, the Secretariat may issue corrigenda for typographical errors and may circulate addenda for newly recognised certification schemes, but substantive parameter changes wait for the formal review.

Relationship to ISO 14040/14044 LCA standards

ISO 14040:2006 (Principles and Framework) and ISO 14044:2006 (Requirements and Guidelines) define the international standard for LCA practice and are the reference for any regulator-grade lifecycle methodology. MEPC.391(82) is best understood as a regulatory implementation of ISO 14040 / 14044 with the goal-and-scope decisions, the Functional Unit, the system boundary, the impact assessment method, and the data-quality and uncertainty rules all locked in to make the methodology fit-for-regulation rather than fit-for-comparative-analysis. The locking-in is what distinguishes the IMO Guidelines from a free-form ISO LCA: an ISO LCA practitioner has discretion over the Functional Unit, the boundary and the allocation method; a MEPC.391(82) practitioner does not. The trade-off is reduced flexibility for the practitioner against gained comparability across pathways and across reporting entities, which is the right trade-off for a regulatory metric. The Guidelines cite ISO 14040 / 14044 explicitly in their methodological annexes and require Annex 2 verifiers to be ISO 14064-3 accredited (or equivalent) so that the audit chain is itself an ISO-compliant chain.

Recordkeeping: BDN chain-of-custody + Annex 2 Mass Balance

The recordkeeping regime is layered. At the top, every fuel transfer to the ship is documented by a Bunker Delivery Note (BDN) under MARPOL Annex VI Regulation 18, which after MEPC.391(82) carries an additional field for the WtW gCO2eq/MJ value (default or certified) and the certificate identifier where a certified value is claimed. Below the BDN, the fuel supplier maintains a Mass Balance ledger under Annex 2 that traces every output batch back to its input batches and their certified values, with input batches in turn traced to feedstock origin through the recognised voluntary scheme. On the ship, the master keeps the BDN and certificate copies for not less than three years. At the verifier, the IAPP renewal survey reviews the BDN file, samples the certificates, and cross-references the fuel mix against the Annex 1 defaults plus any certified actuals. The chain-of-custody is the principal point of audit failure under the regime: a missing certificate or a broken Mass Balance link forces the ship’s verifier to fall back to the Annex 1 default for that batch, which can wipe out the WtW advantage of an alternative-fuel investment.

Commercial implications for newbuild design

The Guidelines reshape the economics of newbuild design in three ways. First, fuel-flexibility becomes a hedging asset: a dual-fuel methanol or ammonia engine that can also burn diesel preserves operating optionality across a range of fuel-availability and fuel-price futures, but the methanol or ammonia path only delivers WtW value if the supply chain can certify a low-carbon batch under Annex 2; designers who optimise for the dual-fuel hardware without the supply-chain analysis underestimate the residual exposure. Second, methane-slip and N2O minimisation move from a noise-floor concern to a first-order design parameter: an engine that nominally runs LNG-Otto at 76 gCO2eq/MJ on the default but in practice slips at 4% loses 28 times that mass to GWP100, easily pushing the realised WtW above 100 gCO2eq/MJ; ammonia engine N2O at the 273 GWP100 weight is similarly material. Third, the supply-chain certification capability of bunker ports becomes a port-selection criterion: a ship that wants to claim Annex 2 actuals for its e-methanol bunker can only do so where the port’s supplier holds an RSB or ISCC EU certificate for that batch, which narrows the routing flexibility. The GFS compliance calculator and the fuel-mix WtW blend calculator are designed to surface these effects at the planning stage.

Comparison with EU 2024/2401 GHG Methodology Regulation

Commission Implementing Regulation (EU) 2024/2401, published in late 2024, is the EU’s parallel instrument: it sets the verification rules, the Mass Balance procedure, and the calculation steps for FuelEU Maritime in the same way that MEPC.391(82) Annex 2 does for the IMO regime. The two instruments are 95% aligned by design, and the EU explicitly references MEPC.391(82) where the alignment holds. Where they diverge: 2024/2401 enforces hourly temporal-correlation for RFNBO from 2030, with monthly until then, against the IMO’s monthly-with-flexibility regime; 2024/2401 sets EU-grid average factors for cold-ironing electricity at the EU member-State zone level, against an IMO global default; 2024/2401 imposes specific EU-recognised certification schemes (RED-recognised schemes only) where the IMO accepts a wider list. An operator that complies with 2024/2401 is broadly compliant with MEPC.391(82); an operator that complies only with MEPC.391(82) defaults may need to upgrade certification chains to satisfy 2024/2401. The mapping table is published in the EU’s FuelEU implementation guidance and is reproduced in summary form in the FuelEU formula breakdown.

Open issues for MEPC 84 review

MEPC 84 in October 2025 was the first scheduled review of the Guidelines. Three classes of open issue dominate the agenda. CCS-equipped fossil fuels (blue ammonia, blue methanol, blue LNG with end-to-end CO2 capture and storage) require a methodology for quantifying the captured-and-stored fraction and its permanence, with a default discount factor for non-permanence; the draft text under consideration assigns 90 to 95% credit to geological storage with multi-decadal monitoring and a much lower credit to mineral or biogenic storage. Hydrogen by-product (the hydrogen released from chlor-alkali electrolysis, steam-cracker tail gas and the like) has a near-zero marginal GHG cost because the producing process exists for non-hydrogen reasons; the draft text proposes a cut-off treatment that assigns the by-product hydrogen a fraction of the carrier process’s WtT, with the rest borne by the primary product. Low-CO2 nuclear-derived hydrogen and ammonia raise a separate set of questions around the LCA treatment of nuclear-electricity inputs, with the draft proposing equivalence to renewable inputs at a defined grid-intensity threshold; the nuclear WtW article tracks the development. Resolution of these issues at MEPC 84 was partial: CCS gained a draft Annex 1 row, hydrogen by-product was admitted as a recognised pathway, and nuclear was held over for further study. The next scheduled review is in 2029.

Formula, assumptions, and limits

Formula

The general WtW formula:

\text{EF}_{\text{WtW},f} = \text{EF}_{\text{WtT},f} + \text{EF}_{\text{TtW},f} \quad \text{(gCO}_2\text{eq/MJ)}

GHG aggregation across species:

\text{EF}_{\text{TtW}} = \sum_g \text{EF}_g \cdot \text{GWP}_{100,g}

Where $g \in \{\text{CO}_2, \text{CH}_4, \text{N}_2\text{O}\}$ and GWP100 values from IPCC AR5: CH4 = 28, N2O = 273. The same aggregation applies to the WtT segment, with each upstream stage contributing its own per-species emissions to the sum.

Derivation

The starting point is a per-pathway lifecycle inventory: a list of mass and energy flows in and out of each of the six lifecycle stages (extraction, transport, processing, distribution, bunkering, combustion-plus-slip). For each flow, the inventory records the species emitted (CO2, CH4, N2O, plus indirect contributions from electricity and heat inputs) per MJ LCV of finished fuel. The aggregation collapses the per-species emissions to a CO2-equivalent using AR5 GWP100. The stage-wise sum gives the WtT and TtW segments separately, and the WtW is the arithmetic sum of the two segments. Where the inventory at a stage includes a co-product, the cut-off or system-expansion rule allocates the share of the stage’s emissions to the fuel of interest.

Assumptions

The principal assumptions are: (i) AR5 GWP100 metrics are the convention, with biogenic CO2 = 0; (ii) LCV is the energy basis, not HCV; (iii) the boundary is WtW with the six stages defined above and no others; (iv) co-product allocation uses cut-off as the default and system expansion only where displacement is robust; (v) RFNBO additionality, temporal correlation and geographical correlation are required for the renewable-electricity input; (vi) Mass Balance is the chain-of-custody convention; (vii) the Functional Unit is gCO2eq/MJ and never per tonne or per litre. Each per-fuel article in the WtW series republishes the pathway-specific assumptions; the blend methodology handles fuel mixes.

Worked example

Consider a 5,000-tonne bunker of bio-methanol claimed under an ISCC EU certificate at a verified actual WtW of 19 gCO2eq/MJ (Annex 1 default for bio-methanol from agricultural-residue gasification is 28 to 35 gCO2eq/MJ; the supplier has documented a forestry-residue feedstock with lower upstream emissions). The LCV of methanol is 19.9 MJ/kg, so the energy delivered is 5,000,000 kg × 19.9 MJ/kg = 99.5 GJ × 1,000 = 99,500 GJ = 99.5 TJ. The total WtW for this batch is 99,500 GJ × 19 gCO2eq/MJ × 1,000 MJ/GJ = 1,890,500,000 gCO2eq = 1,890.5 tonnes CO2eq. Without the ISCC EU certificate the operator would have had to use the Annex 1 default of 30 gCO2eq/MJ, yielding 2,985 tonnes CO2eq, a 1,094-tonne CO2eq disadvantage on the same physical fuel. Multiplied by the GFS Tier 2 Remediation Unit price, the certification value is several hundred thousand US dollars on a single bunker.

Edge cases and limits

The Guidelines have a finite set of edge cases that operators must handle deliberately. Very small bunkers below the Annex 2 minimum threshold revert to the Annex 1 default regardless of the supplier’s certification capacity. Mixed batches where two streams of different certified intensity are commingled in a shore tank inherit the volume-weighted average, not the better of the two. Methane slip and N2O are characterised by engine-specific factors that vary with load and temperature; the default Annex 1 figure assumes a representative load profile, and operators with strong test-bed data can apply for an actual under Annex 2. Cold-ironing electricity is treated under a separate sub-rule that uses port-grid intensity, with a global default for non-EU ports and a member-State default for EU ports. Bunker losses and tank-cleaning residue are treated under the WtT supplier-side, not the TtW ship-side, so the ship sees only the loaded volume net of losses.

Regulatory basis

The regulatory basis is Resolution MEPC.391(82) itself, adopted at MEPC 82 on 4 October 2024 under MARPOL Annex VI as a Resolution operationalising Chapter 4 ter (the GFS) once that Chapter enters into force. Supporting instruments include MEPC.376(78) (the interim 2022 Guidelines, now superseded), MEPC.377(80) (the 2023 Revised IMO GHG Strategy, the policy umbrella), MEPC.392(82) and MEPC.393(82) (the GFS draft text and supporting Resolutions, adopted at the same session), and Regulation (EU) 2023/1805 with Implementing Regulation 2024/2401 on the EU side. The text of MEPC.391(82) is published on the IMO website as a downloadable PDF.

Common errors

The most frequent errors in MEPC.391(82) calculations: (i) using HCV instead of LCV as the energy basis, which inflates the denominator and understates intensity; (ii) using GWP100 from AR4 (CH4 = 25) or AR6 (CH4 = 27 to 30) instead of AR5 (CH4 = 28), which is a small but defined error; (iii) treating biogenic CO2 as non-zero because the regulator has not been updated to the LCA convention; (iv) computing on a per-tonne or per-litre basis and forgetting to divide by LCV; (v) applying an Annex 1 default to a batch for which the supplier holds a certificate, missing the lower certified value; (vi) applying a certificate without verifying that the certificate covers the boundary, the GHG species and the AR5 weighting that MEPC.391(82) requires; (vii) commingling certified and uncertified batches in a shore tank and claiming the certified value for the commingled output; (viii) double-counting an RFNBO multiplier in FuelEU and then using the same multiplied value as if it were the IMO LCA value, when the IMO regime does not apply the FuelEU multiplier; (ix) failing to update the WtW value when an LNG carrier or methanol tanker burns its boil-off as marine fuel, because the boil-off carries the ship’s own pathway not the carrier’s loaded cargo. The GFI attained, GFI compliance, FuelEU GHG intensity, FuelEU compliance balance and fuel-mix WtW blend calculators bake these conventions in and surface the inputs that operators most frequently get wrong.

References

The text of MEPC.391(82) and its Annexes 1 and 2 are published on the IMO website. The drafting record (MEPC 76 to MEPC 82) is in the MEPC document series. The EU implementation is published in the Official Journal as Regulation (EU) 2023/1805 and Implementing Regulation (EU) 2024/2401. ISO 14040 and ISO 14044 are available from the International Organization for Standardization. The IPCC AR5 GWP100 metrics are published in the AR5 Working Group I report. The recognised voluntary schemes maintain their own public registries: CertifHy at certifhy.eu, RSB at rsb.org, ISCC EU at iscc-system.org, Bonsucro at bonsucro.com.