By ShipCalculators.com · Published 8 May 2026

Per-fuel well-to-wake intensity: LPG (Liquefied Petroleum Gas)

Liquefied petroleum gas (LPG), a refinery- and gas-plant-derived mixture of propane (C3H8) and butane (C4H10) typically blended near 70:30 by mass for marine bunker specification, is a niche but legitimate marine fuel that has matured rapidly since 2020 with the entry of LPG-fuelled very large gas carriers (VLGCs) burning their own cargo. Under MEPC.391(82) Annex 1, the default WtW intensity for LPG sits near 75 to 80 gCO2eq/MJ, slightly below the VLSFO and MGO range and meaningfully below HFO, driven by the lower carbon-to-hydrogen ratio of C3 and C4 hydrocarbons relative to long-chain residual fuel. FuelEU Maritime Annex II carries an analogous default and treats LPG as a fossil fuel with sulphur and methane slip both essentially absent. The combustion is clean: zero SOx because LPG carries no sulphur, low NOx because the lean premixed combustion in modern dual-fuel engines runs cool, and negligible methane slip because LPG is propane and butane, not methane. The engine landscape centres on the MAN B&W ME-LGIP two-stroke for VLGCs, Wartsila DF four-strokes with LPG capability for medium-speed applications, and AC Boilers GLE for large gas carriers. The IGC Code cargo-as-fuel provisions allow LPG carriers to consume their own cargo as propulsion fuel, a substantial economic advantage. Operators size LPG WtW exposure with /calculators/fuel-wtw-lpg and dual-fuel blend cases with /calculators/fuel-wtw-blend, and contrast against LNG Otto and Diesel cycles when sizing newbuilding decisions. The volumetric energy density is higher than LNG but lower per kilogramme, and storage at 8 to 15 bar moderate pressure is far simpler than LNG cryogenics at minus 162 degrees Celsius.

Contents

Background: LPG as a marine fuel

Liquefied petroleum gas has been transported in volume by sea since the 1947 launch of the first dedicated LPG carrier, Natalie O. Warren, and the LPG carrier fleet has grown to over 1,500 vessels in service in 2026 across VLGC (very large gas carrier, 80,000 to 93,000 cbm), LGC (large gas carrier, 50,000 to 80,000 cbm), MGC (mid-size gas carrier, 20,000 to 50,000 cbm), and a long tail of handy-size and coastal LPG carriers. The fleet exists to move propane and butane from production basins (US Gulf, Middle East, North Sea, North Africa) to consumption markets (East Asia, Europe, Latin America) where LPG serves as residential cooking fuel, petrochemical feedstock, and industrial heating fuel.

The use of LPG as propulsion fuel in shipping is more recent. Until approximately 2020, LPG carriers ran on heavy fuel oil or marine diesel oil with separate cargo handling, and the natural boil-off from cargo tanks was either reliquefied on board (the dominant approach) or, on older vessels, used in cargo heaters and small auxiliary loads. The arrival of dual-fuel engines that can burn LPG directly as their primary energy source, paired with cargo-as-fuel provisions in the IGC Code, transformed the operating economics of the LPG carrier segment. Since 2020 the LPG-fuelled VLGC fleet has grown to over 70 ships in service with another 60 on order, dominated by orders from BW LPG, Avance Gas, Petredec, Eastern Pacific Shipping, and Hartree Maritime Partners.

The fuel is attractive on three independent grounds. First, the molecule is naturally sulphur-free at refinery delivery, which means a vessel running LPG complies with MARPOL Annex VI Regulation 14 sulphur limits everywhere in the world, including the 0.10 percent S Emission Control Area (ECA) cap, without any aftertreatment, blending, or low-sulphur fuel premium. Second, the carbon content of propane and butane is lower per unit of fuel energy than residual fuel oil, delivering a TtW CO2 reduction of roughly 15 to 20 percent versus VLSFO at the funnel. Third, on LPG carriers themselves, the cargo-as-fuel option avoids the need to bunker an entirely separate marine fuel, freeing tank space and reducing operational complexity.

The fuel is not a transition fuel in the sense LNG is treated. Production volumes are dwarfed by LNG (global LPG trade runs at approximately 110 mtpa versus LNG at over 400 mtpa), and the merchant deep-sea fleet outside the LPG-carrier segment shows little interest in LPG retrofits because the fuel infrastructure is concentrated at LPG export terminals rather than at general bunker hubs. The page therefore treats LPG primarily as a gas-carrier fleet fuel, with secondary use cases on smaller vessels in regions with mature LPG distribution.

This page walks through the molecular composition, the ISO marine bunker specification, the WtT and TtW components of the WtW intensity, the engine technologies, the cargo-as-fuel arithmetic for LPG carriers, and the bunkering supply chain that supports the fleet.

LPG composition: propane / butane mixtures

Liquefied petroleum gas is not a single substance but a binary mixture of two light hydrocarbons, with the blend ratio adjusted to match the climatic and regulatory requirements of the destination market.

Propane has the molecular formula C3H8, a molar mass of 44.10 g/mol, a boiling point of minus 42.1 degrees Celsius at atmospheric pressure, a vapour pressure of approximately 8.5 bar at 20 degrees Celsius, and a higher heating value (HHV) of 50.4 MJ/kg with a lower heating value (LHV) of 46.4 MJ/kg. Its carbon mass fraction is 12 times 3 divided by 44.10, equal to 0.817 (81.7 percent C by mass).

Butane in the marine and commercial context is normally n-butane with formula C4H10, molar mass 58.12 g/mol, boiling point minus 0.5 degrees Celsius at atmospheric pressure, vapour pressure approximately 2.1 bar at 20 degrees Celsius, HHV 49.5 MJ/kg, and LHV 45.7 MJ/kg. The isomer iso-butane (i-butane) has slightly different physical properties (lower boiling point, slightly lower density) but is rarely separated from n-butane in marine LPG specifications. Carbon mass fraction is 12 times 4 divided by 58.12, equal to 0.826 (82.6 percent C by mass).

Marine LPG as a bunker fuel is most commonly a propane-rich mixture in the range of 70 to 80 percent propane by mass with the balance n-butane, although pure propane and pure butane bunkers are also delivered for specific engine specifications. The reasoning for the propane-rich blend in marine practice is threefold:

Vapour pressure and tank design. Pure propane has high vapour pressure at ambient temperature (8.5 bar at 20 degrees Celsius) and demands more robust storage tanks. Pure butane has low vapour pressure and risks insufficient pressure for engine fuel supply at cold ambient. A 70:30 propane:butane blend balances the two, sitting at approximately 6 bar at 20 degrees Celsius and remaining gaseous at ambient as well as supplying adequate pressure to the engine fuel rail at design conditions.
Engine specification. The MAN ME-LGIP engine and the equivalent Wartsila DF engines are tuned for a specific range of fuel composition; engineering data provided by the engine maker typically references 70:30 propane:butane as the design fuel.
Source flexibility. Most major LPG export terminals (Houston, Mont Belvieu, Ras Tanura, Yanbu, Bonny, Algeciras) supply both propane and butane streams separately, allowing the bunker operator to blend to specification at the time of delivery.

The carbon-to-hydrogen ratio of the blended fuel determines the TtW CO2 intensity per MJ. For a 70:30 propane:butane blend, the energy-weighted carbon content is approximately 82.0 percent by mass, the LCV is approximately 46.4 MJ/kg, and the TtW CO2 intensity is approximately 65 gCO2/MJ, materially below the 75 to 78 gCO2/MJ of VLSFO. The molecular reason is straightforward: short-chain alkanes carry more hydrogen per carbon than long-chain alkanes and aromatics, and hydrogen combusts to water rather than CO2.

ISO 8217 marine LPG specifications

The principal standard for marine LPG quality is ISO/PAS 23263, a publicly available specification first issued in 2019 to address the entry of LPG as a marine bunker fuel. ISO 8217:2024, the master ISO marine fuels standard, references ISO/PAS 23263 for the LPG class and does not include a dedicated LPG section in its main body; the cross-reference architecture mirrors how ISO 8217 treats methanol via ISO/PAS 23263 and ammonia via emerging standards.

ISO/PAS 23263 covers two principal grades:

PROPANE-grade: predominantly C3H8 with limited C4 and heavier hydrocarbon content.
BUTANE-grade: predominantly C4H10 with limited C3 and heavier hydrocarbon content.

The principal quality parameters in the specification are:

Composition (mol percent or mass percent) of C3, C4, C5+, and unsaturated hydrocarbons (propene, butenes). Unsaturated hydrocarbons are limited to small fractions because they contribute to gum formation and engine fouling.
Vapour pressure at specified temperatures (typically 40 degrees Celsius for storage rating and 20 degrees Celsius for ambient handling).
Sulphur content with a maximum of approximately 30 mg/kg (effectively zero for combustion purposes; comfortably below the MARPOL Annex VI 0.10 percent ECA cap and the 0.50 percent global cap).
Sulphide and mercaptan content for odorant compatibility and safety.
Water content with a maximum to prevent hydrate formation in engine fuel systems.
Density at 15 degrees Celsius for bunker quantity calculation.
Copper strip corrosion rating for material compatibility.
Residue on evaporation for cleanliness.

The LPG composition is verified at the bunker delivery note (BDN) using a gas chromatograph either on board the bunker barge or at the terminal, with samples retained for verification under the IMO Sulphur 2020 sampling regime even though sulphur compliance for LPG is automatic.

The relationship between ISO/PAS 23263 and the ASTM D1835 (US commercial LPG specification) and EN 589 (European automotive LPG specification) is that the marine specification is generally compatible with the more stringent commercial grades but with marine-specific tightening on water content and sulphide species to protect engine fuel injection systems.

Refinery production and WtT component

LPG enters the marine bunker market through two principal upstream pathways, with materially different WtT emission profiles.

Refinery LPG (approximately 40 percent of supply). A typical crude oil refinery produces LPG as a co-product of distillation, catalytic cracking, hydrocracking, and reforming. The crude is heated in the atmospheric distillation tower; the lightest fraction (gas) is recovered overhead and includes ethane, propane, n-butane, and i-butane along with hydrogen sulphide that is removed by amine treating. The propane and butane streams are separated by fractionation and stored as liquefied gas under modest pressure. The WtT contribution allocated to LPG follows the standard refinery allocation procedure used for other oil products, which assigns upstream crude extraction emissions, refinery energy consumption, and intermediate transport emissions to LPG in proportion to its energy share of the refinery output.

Gas-plant LPG (approximately 60 percent of supply). Natural gas processing plants extract heavier hydrocarbons from raw natural gas streams to meet pipeline specification. The heavier-than-methane fraction (ethane, propane, butanes, pentanes plus, called natural gas liquids or NGLs) is fractionated into individual product streams. The propane and butane fractions of NGL output are the gas-plant LPG component of the supply. The dominant supply basins are the US Permian and Marcellus, the Middle East associated and non-associated gas fields (Saudi Arabia, Qatar, UAE, Iran), the North Sea, North Africa (Algeria, Egypt), and West Africa (Nigeria, Angola). The WtT contribution captures upstream methane fugitives at the gas field, gas-plant CO2 venting, and the energy consumption of the fractionation process.

The aggregate WtT default in MEPC.391(82) Annex 1 for LPG sits near 9 to 12 gCO2eq/MJ, with the lower end reflecting Middle East and North Sea associated-gas LPG (low fugitives, modern fractionation) and the upper end reflecting US shale gas LPG (higher Permian fugitives, longer transport leg to the Gulf coast export terminal). The upstream methane intensity for LPG is materially lower than for LNG because LPG is not the principal product of the gas-plant chain and therefore inherits only a fraction of the upstream methane burden allocated by energy share, whereas LNG carries the full upstream methane intensity of its supply basin.

The transport leg from production basin to bunkering terminal contributes approximately 0.5 to 1.5 gCO2eq/MJ, depending on voyage distance and the propulsion of the LPG carrier transporting the cargo. A modern LPG carrier running on its own cargo as fuel (an LPG-fuelled VLGC) has lower transport burden than a conventional VLGC running on VLSFO because the cargo-fuel pathway eliminates VLSFO upstream emissions and substitutes a lower-intensity propulsion fuel.

The bunkering operation contributes a small additional charge of 0.1 to 0.4 gCO2eq/MJ, dominated by storage venting if the bunker terminal does not capture vapour.

LCV, density, carbon content

The physical and combustion properties of marine LPG underpin all WtW arithmetic. The reference values for a 70:30 propane:butane blend at 15 degrees Celsius are:

Lower heating value (LCV): approximately 46.4 MJ/kg (range 45.7 to 46.5 across propane-butane blends).
Higher heating value (HHV): approximately 50.0 MJ/kg.
Density: approximately 0.55 t/m3 (range 0.50 for pure propane to 0.58 for pure butane).
Carbon mass fraction: approximately 0.82 kgC/kg fuel (slightly higher for butane-rich blends, slightly lower for propane-rich).
Volumetric energy density: approximately 25.5 GJ/m3 (LCV times density), compared to LNG at approximately 22 GJ/m3 and VLSFO at approximately 39 GJ/m3.

The volumetric density figure has direct fuel-tank-sizing consequences. LPG delivers more energy per cubic metre of bunker space than LNG (because the propane and butane molecules are denser at storage conditions than the lighter methane molecule even after liquefaction) but materially less than VLSFO, which means an LPG-fuelled vessel needs roughly 50 percent more bunker tank volume for the same range as a VLSFO-fuelled vessel. The arithmetic gap versus LNG is favourable to LPG: a 70:30 propane:butane blend at 8 bar storage occupies the same volume as LNG at minus 162 degrees Celsius for approximately 16 percent more energy, simplifying the bunker tank design and removing the cryogenic insulation penalty.

The TtW CO2 intensity from these reference values is:

\text{EF}_{\text{TtW,CO}_2} = \frac{C_f \cdot 44/12}{\text{LCV}} = \frac{0.82 \cdot 3.667}{46.4} = 0.0648 \text{ kgCO}_2/\text{MJ} = 64.8 \text{ gCO}_2/\text{MJ}

This is the pure CO2 combustion term in the MEPC.391(82) defaults, before the addition of WtT and the very small N2O term.

MEPC.391(82) Annex 1 default WtW for LPG

The Annex 1 default WtW emission factor table in MEPC.391(82) provides a single LPG row, treated as a fossil reference fuel without engine-technology subdivision (unlike LNG, which has Otto and Diesel sub-rows). The headline value is approximately 77 gCO2eq/MJ for the global-average propane-butane blend, decomposing as follows:

WtT: 9 to 12 gCO2eq/MJ (extraction and gas processing 5 to 7, fractionation 1 to 2, transport 1 to 2, distribution 0.3 to 0.6, bunkering 0.1 to 0.3).
TtW CO2: 64 to 65 gCO2eq/MJ (the molar combustion term for an 82 percent C blend at 46.4 MJ/kg LCV).
TtW CH4 slip: essentially zero (LPG combustion does not produce methane in the exhaust because the fuel does not contain methane; any small amount of unburned hydrocarbon in the exhaust is propane or butane fragments which are tracked under the non-methane volatile organic compound (NMVOC) category and not weighted at the methane GWP).
TtW N2O: 0.05 to 0.1 gCO2eq/MJ (negligible at the WtW scale).

Adding the components for a typical Middle East-sourced cargo bunkered at Algeciras for a westbound VLGC voyage:

\text{EF}_{\text{WtW,LPG}} \approx 10 + 64.5 + 0 + 0.1 = 74.6 \text{ gCO}_2\text{eq/MJ}

For a US shale-sourced cargo with higher Permian methane fugitives:

\text{EF}_{\text{WtW,LPG}} \approx 12 + 64.5 + 0 + 0.1 = 76.6 \text{ gCO}_2\text{eq/MJ}

The MEPC.391(82) default consolidates these into a single global figure near 77 gCO2eq/MJ, which operators apply unless they bring forward a verified cargo-specific WtT certification.

The comparison with VLSFO (default near 91 gCO2eq/MJ) and HFO (default near 92 to 95 gCO2eq/MJ) shows LPG with a WtW saving of approximately 14 to 18 gCO2eq/MJ, driven principally by the higher hydrogen-to-carbon ratio of the propane-butane molecules. The comparison with LNG-Diesel (default near 76 gCO2eq/MJ) puts LPG roughly on par; the comparison with LNG-Otto (default near 85 gCO2eq/MJ) shows LPG approximately 8 gCO2eq/MJ better, because LPG carries no methane slip while LNG-Otto carries a 4 to 8 gCO2eq/MJ slip term that swamps the small WtT advantage of LNG over LPG.

FuelEU Annex II treatment

FuelEU Maritime Annex II provides a single LPG default WtW factor without engine-technology subdivision, in line with the MEPC.391(82) treatment.

The 2026 Annex II default for LPG is approximately 76.0 gCO2eq/MJ, broken down as approximately 11 gCO2eq/MJ WtT and 65 gCO2eq/MJ TtW. The default is applied identically across Otto-cycle dual-fuel engines (Wartsila DF with LPG capability, AC Boilers GLE) and Diesel-cycle dual-fuel engines (MAN ME-LGIP) because LPG combustion does not produce a slip term that would create a cycle-pair gap analogous to the LNG case.

Operators reporting under FuelEU MRV submit:

The LPG bunker delivery note quantity and energy content (LCV in MJ/kg).
The cargo-specific WtT certification, if available, for upstream methane intensity adjustment.
The engine fuel mode log distinguishing LPG-mode from pilot-fuel-mode (typically MGO) consumption.

The pilot fuel consumption is treated separately under the MGO row in Annex II, so the actual reported intensity is a weighted average of the LPG row (96 to 98 percent of energy) and the MGO row (2 to 5 percent of energy from pilot injection). The pilot weighting drags the effective intensity up by approximately 0.5 to 1.0 gCO2eq/MJ versus pure LPG operation.

The FuelEU treatment makes LPG attractive for compliance: the 76 gCO2eq/MJ default sits comfortably below the 91.16 gCO2eq/MJ Annex II baseline, and after the year-on-year reduction percentages applied through 2030 and 2040 the LPG default still leaves operating headroom above the cap. A vessel converting from VLSFO to LPG captures the WtW differential as compliance balance, which can be banked or pooled.

TtW combustion: CO2, NOx, no SOx, no CH4 slip

The combustion characteristics of LPG in modern marine engines are favourable across all four regulated emission species.

CO2. The TtW CO2 intensity at 64.5 gCO2/MJ is approximately 15 percent below VLSFO (75 to 76 gCO2/MJ) and approximately 17 percent below HFO (77 to 78 gCO2/MJ). The reduction is the molecular consequence of the higher hydrogen content of propane and butane relative to long-chain residual fuel.

SOx. LPG carries less than 30 mg/kg sulphur at the marine bunker specification, and most cargoes deliver well below 10 mg/kg. The sulphur content of the exhaust is consequently essentially zero, comfortably below the MARPOL Annex VI Regulation 14 sulphur cap of 0.50 percent globally and 0.10 percent in Emission Control Areas. LPG-fuelled vessels need no scrubber, no low-sulphur fuel premium, and no fuel switching at the ECA boundary.

NOx. The combustion of LPG in modern dual-fuel engines is lean premixed (Otto-cycle MAN ME-LGIP and Wartsila DF) or non-premixed direct injection (Diesel-cycle MAN ME-LGIP), with peak combustion temperatures lower than VLSFO combustion because the molecule mixes more uniformly with air. NOx output in g/kWh is approximately 30 to 50 percent below an equivalent VLSFO-fuelled engine at the same load, allowing MARPOL Annex VI Regulation 13 Tier III compliance without selective catalytic reduction (SCR) or exhaust gas recirculation (EGR) on the lean-burn variants. The Diesel-cycle ME-LGIP variant requires SCR or EGR for Tier III compliance because the higher peak flame temperature in the diffusion combustion regime produces NOx in quantities similar to VLSFO combustion.

Methane slip. The single largest WtW differentiator versus LNG is the absence of methane slip. LPG combustion does not produce CH4 in the exhaust because the fuel molecules are propane (C3H8) and butane (C4H10), neither of which is methane. Any unburned fuel that escapes combustion is itself propane or butane, and these compounds have GWP100 values of approximately 3 (propane) and 4 (butane), an order of magnitude lower than methane’s 28. The unburned hydrocarbon fraction is consequently a small term in the WtW total even before the GWP weighting, and the standard MEPC.391(82) Annex 1 default treats it as zero for simplicity.

Particulate matter. LPG combustion produces essentially zero particulate matter because the molecules contain no aromatics, no sulphur, no asphaltenes, and no heavy metals. Black carbon emissions are well below VLSFO and approximately on par with LNG.

The aggregate environmental performance of LPG combustion is consequently strong across all regulated species, and the WtW intensity advantage versus VLSFO is robust under both GWP100 and GWP20 weightings (LPG has no methane to penalise under GWP20).

Cargo-as-fuel for LPG carriers

The most important commercial use case for LPG-as-fuel in 2026 is on the LPG carrier fleet itself, where the cargo can be drawn directly from the cargo tanks for propulsion fuel. The arithmetic is compelling.

A modern LPG-fuelled VLGC carrying approximately 84,000 cbm of LPG cargo (mass approximately 46,000 tonnes) consumes approximately 40 to 50 tonnes of LPG per day at typical service speed of 16 to 17 knots. On a 30-day round-trip voyage from Houston to Chiba in Japan, fuel consumption is approximately 1,200 to 1,500 tonnes, or 2.6 to 3.3 percent of the cargo. The economic value of the cargo consumed depends on the spread between the LPG sales price at the destination terminal and the equivalent VLSFO price in the same trade lane.

At a USD 600 per tonne LPG sales price and a USD 550 per tonne VLSFO price, the cargo-as-fuel option costs the owner approximately USD 50 per tonne (the differential between selling the propane in the destination market and bunkering equivalent VLSFO in the load port). This is approximately a 9 percent fuel cost reduction from the VLSFO baseline, or USD 100,000 to 150,000 per round trip per VLGC, and approximately USD 1.5 to 2.0 million per VLGC per year.

The economics flip in periods when LPG prices spike above VLSFO prices, which happens occasionally during winter heating-season demand peaks in East Asia. Operators with cargo-as-fuel-capable engines retain the optionality of switching to LPG bunkers (purchased separately) or to VLSFO-mode (with pilot fuel) depending on the spread. The MAN ME-LGIP and Wartsila DF engines support all three modes (cargo LPG, bunkered LPG, MGO-only) through the same fuel system with a flexible fuel supply piping arrangement.

The cargo-as-fuel pathway also removes the need for separate VLSFO bunker tankage on LPG carriers, freeing approximately 1,500 to 2,500 cbm of internal volume that on conventional gas carriers is dedicated to fuel oil bunker tanks. The cargo deadweight available to the owner increases by approximately 1,500 to 2,000 tonnes per VLGC, an additional commercial benefit on top of the fuel cost saving.

IGC Code cargo-as-fuel provisions

The IMO International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code), last comprehensively revised at MSC 99 in 2018 and subsequently amended, provides the regulatory framework for use of cargo vapour as fuel on gas carriers. The IGC Code applies to ships carrying LPG, LNG, ammonia, ethylene, ethane, propylene, butadiene, vinyl chloride, and other gases listed in Chapter 19 of the Code.

IGC Chapter 16 covers the use of cargo as fuel and sets out the safety architecture: double-walled fuel piping with inert gas in the annulus, gas detection in the engine room and fuel preparation room, pressure relief and venting arrangements, emergency shutdown systems (ESD-1 and ESD-2 levels), and crew training requirements. The Chapter draws a distinction between cargo vapour use (drawing boil-off gas from the cargo tank vapour space) and cargo liquid use (drawing liquid from the cargo tank for vaporisation in a dedicated fuel preparation skid), and the design considerations differ slightly between the two pathways.

For LPG cargo-as-fuel, the dominant pathway is cargo liquid extraction, because LPG is stored at moderate pressure rather than near boiling like LNG, and the natural boil-off is small. The fuel preparation skid extracts liquid LPG from the cargo tank, vaporises it through a heat exchanger fed by waste heat from the engine cooling water or a dedicated fuel heater, and delivers gaseous fuel at the engine fuel rail at design pressure (typically 5 to 8 bar for low-pressure Otto-cycle engines and 50 to 60 bar for high-pressure Diesel-cycle engines).

The IGC Code permits LPG cargo-as-fuel without restriction, in contrast to certain other cargoes (notably ammonia, where cargo-as-fuel is permitted only with stringent toxicity controls and crew protection arrangements that are still being elaborated in the IGC Code amendments). The mature treatment of LPG reflects the long industry experience with LPG cargo handling and the absence of toxicity or extreme cryogenic concerns.

The cargo-as-fuel arrangement also interacts with cargo containment: the IGC Code Type A, Type B, Type C, and membrane tank classifications determine the allowable cargo pressure rise and the design conditions for the cargo handling system, and the cargo-as-fuel piping must be integrated with the cargo containment system at the design stage. VLGCs typically use Type A prismatic tanks with separate insulation and a partial secondary barrier; smaller pressurised LPG carriers use Type C cylindrical or bilobe tanks that double as cargo storage and engine fuel reservoir.

Engine landscape: MAN B&W ME-LGIP, Wartsila DF, AC Boilers GLE

The engine landscape for LPG-as-fuel propulsion is dominated by three product families, with the bulk of newbuilding orders concentrated on the MAN B&W ME-LGIP for VLGCs.

MAN B&W ME-LGIP is the flagship two-stroke dual-fuel engine for LPG and is a derivative of the standard ME-C two-stroke heavy-fuel platform with a high-pressure liquid LPG injection system. The “LGIP” suffix stands for “Liquid Gas Injection, Propane” and reflects the engine’s design intent of injecting LPG as a high-pressure liquid directly into the cylinder, where it vaporises and combusts as a non-premixed Diesel-cycle diffusion flame. The fuel supply system delivers LPG at approximately 600 bar to the cylinder through a high-pressure pump and a liquid LPG injector. Pilot fuel (typically 5 to 7 percent of total energy at full load) is injected through a separate pilot injector and provides the high-temperature ignition kernel.

ME-LGIP combustion is consequently Diesel-cycle in character: high pressure, non-premixed, low slip. Methane slip is irrelevant (LPG contains no methane), and the unburned LPG fraction is below 0.1 percent. Tier III NOx compliance requires either EGR or SCR. The engine is offered in bore sizes from 50 cm to 80 cm, covering the full VLGC and LGC propulsion power range. Approximately 90 percent of the LPG-fuelled VLGC newbuilding orders since 2020 specify ME-LGIP main engines.

Wartsila DF four-stroke engines with LPG capability is the dominant medium-speed solution for smaller LPG-fuelled vessels, including coastal LPG carriers, LPG bunker barges, and ferries. The Wartsila 31DF, 34DF, and 50DF platforms can be specified with an LPG fuel system in addition to the standard LNG fuel system, allowing the engine to operate on LPG, LNG, or MGO depending on bunker availability. The combustion is low-pressure Otto-cycle premixed (LPG admitted at 5 to 8 bar to the cylinder during the compression stroke), with micro-pilot diesel injection to ignite the lean mixture. NOx in LPG mode meets Tier III without SCR.

AC Boilers GLE (Gas Loading Engine) is a less widely known platform from the Italian manufacturer AC Boilers (formerly Ansaldo Caldaie), used on certain large gas carriers in specific operator fleets. The engine is a four-stroke medium-speed dual-fuel platform similar in concept to the Wartsila DF family, with LPG fuel capability and Tier III NOx compliance on lean-burn operation.

A small population of single-fuel LPG engines (without VLSFO or MGO backup) exists on niche short-sea LPG carriers in Northern Europe and Asia, but the deep-sea fleet universally specifies dual-fuel engines to retain optionality on bunker fuel choice.

The 2026 newbuilding order book for LPG-fuelled vessels distributes approximately as follows: VLGCs with ME-LGIP approximately 55 vessels on order, LGCs with ME-LGIP approximately 8 vessels, MGCs and handy-size LPG carriers with Wartsila DF or MAN four-stroke approximately 12 vessels.

Pilot fuel requirements

All commercial LPG-as-fuel marine engines in 2026 are dual-fuel platforms requiring a pilot fuel for ignition. The pilot fuel is typically marine gas oil (MGO) or marine distillate, with the precise specification varying by engine maker.

MAN ME-LGIP pilot. The pilot injection delivers approximately 5 to 7 percent of total fuel energy at full load through a separate high-pressure pilot fuel injector that is functionally identical to the main injector on a standard ME-C heavy-fuel two-stroke engine. The pilot fuel is typically MGO meeting ISO 8217 DMA grade specification, with sulphur compatibility for ECA operation. The pilot fuel system comprises a small pilot fuel tank (typically 50 to 100 m3, sized for several days of pilot consumption), a pilot fuel transfer pump, a pilot fuel high-pressure pump, and the pilot injectors at each cylinder.

Wartsila DF pilot. The pilot injection on Wartsila DF engines in LPG mode is much smaller, typically 1 to 3 percent of total fuel energy at full load, because the lean premixed combustion ignites readily and does not require a high-energy ignition kernel. The pilot fuel is delivered through a micro-pilot injector with very small fuel quantity per injection. Pilot fuel consumption on a Wartsila 50DF in LPG mode is approximately 0.3 g/kWh of MGO at full load, compared to 200 g/kWh of LPG.

The pilot fuel WtW intensity is identical to the MGO default of approximately 91 gCO2eq/MJ, and the pilot share of total energy must be added to the WtW arithmetic when computing the effective intensity of LPG-mode operation. For a 95 percent LPG and 5 percent MGO blend by energy:

\text{EF}_{\text{WtW,blend}} = 0.95 \cdot 76 + 0.05 \cdot 91 = 76.8 \text{ gCO}_2\text{eq/MJ}

The pilot effect therefore shifts the effective WtW intensity up by approximately 0.5 to 0.8 gCO2eq/MJ versus pure LPG combustion. Operators report the pilot consumption separately under FuelEU MRV and the cumulative WtW intensity is energy-weighted.

The pilot fuel sulphur compatibility is the operational complication: in ECA waters, the MGO pilot must meet the 0.10 percent sulphur cap, which means a vessel operating in ECA waters bunkers DMA grade MGO for the pilot system, while outside ECA waters the pilot can use higher-sulphur DMB grade MGO. The pilot tank sizing and bunker procurement plan must accommodate this fuel switching.

IGF Code LPG-as-fuel amendments under development

The IMO International Code of Safety for Ships using Gases or other Low-flashpoint Fuels (IGF Code) entered into force in 2017, originally drafted to address LNG as marine fuel and subsequently extended through amendments to cover methanol (entered into force 2024), with ammonia and LPG amendments in development at the IMO Sub-Committee on Carriage of Cargoes and Containers (CCC).

The current status of the LPG-as-fuel amendment to the IGF Code (as of MSC 110 in May 2026) is that a draft chapter covering LPG as fuel has been circulated in the CCC working group, but the formal adoption is targeted for MSC 112 in 2027. Until the dedicated LPG chapter is in force, LPG-as-fuel installations are approved under the IGF Code Part A-1, Section 2.3 Alternative Design provisions, which allow flag administrations and classification societies to approve novel fuel installations on the basis of equivalent safety to the prescriptive Code requirements.

The Alternative Design pathway has been used successfully for all LPG-as-fuel vessels delivered between 2020 and 2026, with class society approval (DNV, ABS, BV, LR, ClassNK, RINA) issued on the basis of the IGC Code cargo-as-fuel provisions for the cargo tank, the ME-LGIP or Wartsila DF engine type approval, and a vessel-specific Hazard Identification (HAZID) and Hazard and Operability (HAZOP) study covering the integrated fuel system.

The forthcoming dedicated LPG chapter in the IGF Code is expected to consolidate and standardise the safety architecture: gas detection arrangement in the engine room and fuel preparation room, fuel piping integrity and double-walling requirements, ESD logic, training and operational documentation, and bunker manifold safety. The transition from Alternative Design to the dedicated chapter is intended to simplify class approval and to enable cargo-vessel applications outside the LPG-carrier segment to specify LPG as fuel without the bespoke Alternative Design effort.

The amendment is closely watched by the LPG-fuelled VLGC owners because the dedicated chapter will set the regulatory floor for any retrofits or class transfers, and may affect the commercial value of existing vessels approved under Alternative Design.

Bunkering supply chain: Antwerp, Rotterdam, Houston, Singapore, Algeciras

The LPG bunkering supply chain in 2026 is concentrated at major LPG export and trading hubs, in contrast to LNG bunkering which is increasingly dispersed across general bunker hubs. The principal bunker locations are:

Antwerp and Rotterdam (ARA), the European petrochemical and LPG storage hub, with multiple LPG storage cavern terminals (Antwerp Vopak, Rotterdam Botlek) feeding both pipeline distribution to the European hinterland and ship-to-ship LPG bunkering for transiting LPG carriers and short-sea vessels. Bunker volumes from ARA in 2025 reached approximately 1.2 mtpa of LPG bunker fuel, primarily into LPG-fuelled VLGCs calling for cargo loading or transit bunkering.

Houston and Mont Belvieu, the US Gulf coast LPG export hub, with LPG storage caverns at Mont Belvieu connected by pipeline to the export terminals at Enterprise Houston Ship Channel, Targa Galena Park, and Energy Transfer Marcus Hook (East Coast). The volumes are dominated by LPG export cargoes loaded into VLGCs, with bunker fuel taken at the same terminal during the load operation.

Singapore, the Asian bunkering hub, with limited dedicated LPG bunker infrastructure but significant LPG storage capacity (Vopak Banyan, ExxonMobil Pulau Ayer Chawan) feeding ship-to-ship bunkering operations. Singapore LPG bunker volumes in 2025 reached approximately 0.4 mtpa, with growth driven by the increasing transit traffic of LPG-fuelled VLGCs through the Strait of Malacca.

Algeciras, the western Mediterranean bunker hub, with growing LPG bunker volumes (approximately 0.2 mtpa in 2025) supported by the Cepsa Algeciras LPG terminal and ship-to-ship bunkering operations into transiting LPG-fuelled VLGCs.

Other emerging hubs include Yokohama and Chiba (Japan, dedicated LPG terminal infrastructure), Ras Tanura (Saudi Arabia, LPG export from Saudi Aramco facilities, with limited bunker activity), and Yanbu (Saudi Arabia, similar profile).

The bunker delivery operation typically uses ship-to-ship transfer from a small LPG bunker barge or from a larger LPG storage vessel moored at the terminal. Truck-to-ship bunkering is uncommon for LPG because the deep-sea volumes per bunker delivery (1,000 to 2,000 tonnes for a VLGC) are too large for road transport. Pipeline-to-ship bunkering is available at the major export terminals where the LPG-fuelled vessel is moored at the cargo loading berth.

The bunker delivery note (BDN) for marine LPG specifies the propane and butane composition (typically by mass percent), the LCV (in MJ/kg), the density at delivery temperature, the sulphur content (always below 30 mg/kg), and the BDN is signed by the bunker supplier and the chief engineer. Sample retention is mandatory under MARPOL Annex VI, although sulphur testing of LPG samples is rarely required because the cargo specification and source documentation typically establish compliance.

Comparison with LNG (cross-link)

The most useful comparison for an operator considering LPG-as-fuel is against LNG, the closest alternative gas-fuel pathway and the dominant low-flashpoint marine fuel in 2026. The two fuels share much of the operational architecture (dual-fuel engines, IGC or IGF Code regulatory frameworks, gas detection, double-walled piping) but differ on several key dimensions detailed in the /wiki/per-fuel-wtw-lng-otto-diesel page.

WtW intensity. LPG sits at approximately 76 gCO2eq/MJ at MEPC.391(82) Annex 1 default. LNG-Otto sits at approximately 85 gCO2eq/MJ and LNG-Diesel at approximately 76 gCO2eq/MJ. LPG is therefore comparable to LNG-Diesel and meaningfully better than LNG-Otto. Under GWP20 weighting, LPG retains its 76 figure (no methane slip), while LNG-Otto rises to approximately 105 gCO2eq/MJ, putting LPG in a structurally stronger position under near-term-horizon regulatory metrics.

Storage pressure and temperature. LPG stored at moderate pressure (8 to 15 bar) and ambient temperature is significantly easier than LNG stored at minus 162 degrees Celsius and atmospheric pressure. The LPG fuel tank uses standard pressure vessel construction (Type C cylindrical or bilobe) with no cryogenic insulation, while the LNG fuel tank requires polyurethane foam or perlite cryogenic insulation, vacuum jacketing in some designs, and complex thermal management.

Volumetric energy density. LPG at 25.5 GJ/m3 has approximately 16 percent higher volumetric energy than LNG at 22 GJ/m3, allowing a smaller fuel tank for the same energy or a longer range from the same tank. The advantage is partially offset by LPG’s higher density requiring more deadweight allocation to fuel.

Methane slip. LNG carries 0.2 to 3.0 percent methane slip depending on engine cycle. LPG carries no methane in the fuel and consequently no methane slip in the exhaust. This is the structural advantage that places LPG ahead of LNG-Otto on WtW intensity.

Bunker infrastructure. LNG bunkering is more widely distributed across global bunker hubs, with Rotterdam, Singapore, Yokohama, Marseille, Algeciras, Houston, and over 40 other ports operating LNG bunker barges in 2026. LPG bunker infrastructure is concentrated at LPG export and trading hubs, with fewer ports active.

Engine technology. LNG dual-fuel engines are mature and widely available across the four-stroke and two-stroke power range. LPG dual-fuel engines are dominated by the MAN ME-LGIP for VLGCs with smaller market penetration on the four-stroke side.

Cost. LPG bunker price tracks regional LPG market prices (Argus Far East Index, Mont Belvieu propane index), which vary materially with seasonal demand. LNG bunker price tracks regional LNG indices (TTF, JKM, Henry Hub plus liquefaction). Over 2024 to 2026 the price spreads have favoured LPG in some periods and LNG in others.

The decision between LPG and LNG for a deep-sea vessel newbuilding therefore depends on the operating profile (LPG carrier owners prefer LPG for cargo-as-fuel, container ship and bulk carrier owners prefer LNG for bunker availability), the regulatory horizon (LPG is more robust under stringent methane-slip scrutiny), and the capex envelope (LPG fuel system is approximately 30 to 50 percent cheaper than LNG fuel system).

Storage: moderate pressure 8-15 bar vs LNG cryogenic

The LPG fuel storage system is materially simpler than the LNG cryogenic storage system, and this simplicity is a significant capex and operational advantage.

Tank construction. LPG fuel tanks are standard pressure vessels constructed from carbon steel or low-temperature carbon steel, with shell thickness designed for the maximum allowable working pressure (MAWP) of typically 17 to 20 bar to provide margin above the operating pressure of 8 to 15 bar. The tank can be cylindrical, bilobe, or spherical depending on the available hull volume, and the pressure rating allows a standard ASME or PED design code to apply. No cryogenic considerations apply because the fuel is stored at ambient temperature.

Insulation. LPG fuel tanks require minimal insulation, primarily for fire protection (typically 100 to 200 mm of mineral wool with steel cladding) and for thermal management to maintain a steady fuel temperature. Cryogenic vacuum-jacket insulation is not required.

Boil-off management. The boil-off in an LPG storage tank is small because the propane and butane boiling points (minus 42 and minus 0.5 degrees Celsius respectively) are well below ambient temperature, so the tank pressure rises through ambient heat ingress only slowly. A typical LPG fuel tank can hold pressure for 7 to 14 days at ambient conditions without venting, depending on the tank fill level, the ambient temperature, and the tank geometry. Vapour pressure rise during long port stays may eventually require either gas consumption (continuing engine operation) or active boil-off management through a small reliquefaction unit on board.

Fuel preparation. The fuel is extracted from the storage tank as liquid LPG, pumped to engine fuel pressure (5 to 8 bar for low-pressure Otto-cycle engines, 600 bar for high-pressure ME-LGIP), and vaporised through a heat exchanger fed by engine cooling water. The vaporiser is much smaller and simpler than the LNG vaporiser system, and the heat duty is modest (LPG vaporisation enthalpy is approximately 350 kJ/kg, compared to 510 kJ/kg for LNG plus the additional sensible heating from minus 162 to plus 30 degrees Celsius).

Safety architecture. The IGC and IGF Code requirements for double-walled fuel piping, gas detection, ESD logic, and crew training apply identically to LPG and LNG, and the safety architecture is largely common between the two fuels. The LPG-specific consideration is the higher density of LPG vapour relative to air (1.5 to 2.0 times the density of air), which means LPG vapour leaks tend to accumulate in low-lying spaces such as bilges and engine room sumps. The gas detection arrangement is sized accordingly, with low-level detectors taking precedence over the high-level detectors more typical in LNG systems where methane vapour rises.

The aggregate result is that the LPG fuel system carries approximately 30 to 50 percent lower capex than an equivalent LNG fuel system on a like-for-like basis, and approximately 20 percent lower opex through reduced boil-off management, simpler vaporisation, and lower training overhead.

2024-2025 LPG-fuelled VLGC orderbook (BW LPG, Avance Gas)

The 2024 to 2026 window has seen rapid expansion of the LPG-fuelled VLGC orderbook, driven by a confluence of regulatory pressure (FuelEU Maritime, IMO GFS), commercial advantage (cargo-as-fuel economics), and engine technology maturity (ME-LGIP series production at MAN).

BW LPG, the Singapore-listed VLGC owner with approximately 50 vessels in its fleet, has led the LPG-fuelled VLGC orderbook with retrofits of existing ME-C vessels to ME-LGIP and a new building programme of 12 ME-LGIP-equipped VLGCs delivered between 2022 and 2025. The retrofits, performed at Hyundai Heavy Industries and Hyundai Mipo Dockyard in South Korea, convert the standard MAN B&W 6G70ME-C9 main engine to a 6G70ME-LGIP configuration through a partial top-end overhaul that adds the high-pressure liquid LPG injection system, a fuel preparation skid, and the safety architecture. The retrofit cost is approximately USD 12 to 15 million per VLGC and the retrofit time is approximately 60 to 90 days.

Avance Gas, the Norwegian VLGC owner, has ordered 8 LPG-fuelled VLGC newbuildings at Daewoo Shipbuilding and Marine Engineering (now Hanwha Ocean) and Hyundai Heavy Industries, with deliveries scheduled through 2024 to 2026. The vessels are equipped with MAN B&W 6G70ME-LGIP engines and have begun trading in the Atlantic and Pacific basin LPG markets in 2025.

Petredec, Eastern Pacific Shipping, Hartree Maritime Partners, and Cool Company have also placed LPG-fuelled VLGC orders, with the aggregate orderbook approximately 60 vessels by mid-2026.

The newbuilding contract pricing for an LPG-fuelled VLGC at 2026 quotes is approximately USD 95 to 110 million per vessel, compared to approximately USD 80 to 90 million for a conventional VLSFO-fuelled VLGC. The capex premium of USD 15 to 20 million is recovered through fuel cost saving and FuelEU compliance benefit on a typical 5 to 7 year payback at 2026 fuel prices.

The pace of orders has slowed slightly in 2026 as Korean and Japanese yards work through capacity constraints, but the orderbook through 2028 remains substantial. Industry analysts (Clarksons Research, Vesselsvalue, BRS Group) project that approximately 60 percent of the global VLGC fleet will be LPG-fuelled by 2030, up from approximately 8 percent in 2026.

Future: bio-LPG from renewable feedstocks

Bio-LPG, also called renewable LPG or rLPG, is propane and butane produced from biological feedstocks rather than from fossil refining or gas processing. The molecule is chemically identical to fossil LPG (the same propane and butane molecules) but the carbon comes from biomass, which under the IMO LCA Guidelines and FuelEU sustainability framework counts as biogenic CO2 with zero combustion emission.

The principal bio-LPG production pathways in 2026 are:

HVO (hydrotreated vegetable oil) co-product. The hydrotreatment of vegetable oils, used cooking oil, animal fats, and other lipid feedstocks to produce HVO renewable diesel and sustainable aviation fuel (SAF) generates bio-LPG as a co-product through the cracking of triglyceride hydrocarbon chains. A typical HVO refinery produces approximately 5 to 10 percent of its output as bio-LPG (light propane and butane fractions). The largest producers are Neste (Porvoo Finland, Singapore, Rotterdam), Total (La Mede France, Grandpuits France), Eni (Venice Italy, Gela Italy), and Repsol (Cartagena Spain).

Glycerin to propane. A specialised process developed by the SHV Energy joint venture and others converts glycerin, the main co-product of biodiesel production, into propane through a hydrogenation step. The output is essentially pure propane (no butane), with a WtW intensity advantage tied to the upstream emissions of the biodiesel chain.

Direct biomass gasification and Fischer-Tropsch synthesis. Emerging pathways gasify biomass to syngas and synthesise propane and butane through a modified Fischer-Tropsch process. Production volumes are small in 2026 but the pathway is on the medium-term horizon for scaling.

The MEPC.391(82) and FuelEU pathways for bio-LPG follow the same accounting architecture as for bio-LNG: combustion CO2 is treated as biogenic and counted as zero, while the WtT component captures upstream feedstock emissions, processing energy, and any methane fugitives. WtW intensities for bio-LPG range from approximately 15 to 35 gCO2eq/MJ depending on feedstock and pathway, with HVO co-product bio-LPG at the lower end (approximately 18 to 22 gCO2eq/MJ) and glycerin-derived propane in the middle (approximately 25 to 30 gCO2eq/MJ).

The drop-in compatibility of bio-LPG with existing ME-LGIP and Wartsila DF engines is high because the molecule is chemically identical to fossil LPG. Operators can blend bio-LPG into fossil LPG bunkers without any engine modification, and the energy-weighted WtW intensity is calculated using the /calculators/fuel-wtw-blend blend arithmetic. Production volumes of marine-grade bio-LPG remain limited in 2026 (estimated global production approximately 0.5 mtpa, of which marine bunker share is well under 0.1 mtpa) but the volumes are growing in tandem with HVO refinery expansion.

The economic position of bio-LPG in marine bunkering is similar to bio-LNG: the molecule is more expensive than the fossil equivalent (approximately 30 to 50 percent premium at 2026 prices) but the WtW intensity advantage delivers FuelEU compliance balance and EU ETS allowance reduction that partially offset the cost. The strategic value for VLGC owners is the optionality of phasing bio-LPG into the cargo-as-fuel pathway as production volumes grow, transitioning the LPG carrier fleet from a fossil-LPG-fuelled fleet to a bio-LPG-fuelled fleet without changing the engine, the fuel system, or the bunker infrastructure.

Formula, assumptions, and limits

Formula

The WtW intensity for LPG in marine dual-fuel engines under MEPC.391(82) and FuelEU Annex II is constructed as the sum of upstream WtT plus combustion CO2 plus a negligible CH4 slip term plus N2O times GWP100:

\text{EF}_{\text{WtW,LPG}} = \text{EF}_{\text{WtT,LPG}} + \text{EF}_{\text{TtW,CO}_2} + \text{EF}_{\text{TtW,N}_2\text{O}} \cdot \text{GWP}_{100,\text{N}_2\text{O}}

The CH4 term is omitted because LPG combustion does not produce methane in the exhaust (the fuel contains no methane). Substituting representative values:

\text{EF}_{\text{WtW,LPG}} = 11 + 64.5 + 0.1 \approx 76 \text{ gCO}_2\text{eq/MJ}

For a pilot-blend operating mode at 95 percent LPG and 5 percent MGO by energy:

\text{EF}_{\text{WtW,blend}} = 0.95 \cdot 76 + 0.05 \cdot 91 = 76.75 \text{ gCO}_2\text{eq/MJ}

Derivation

The TtW carbon dioxide intensity for LPG comes from the molar mass ratio of carbon to fuel and the LCV. For a 70:30 propane:butane blend:

C_f = 0.70 \cdot \frac{36}{44.10} + 0.30 \cdot \frac{48}{58.12} = 0.572 + 0.248 = 0.820 \text{ kgC/kgFuel}

Combusting 1 kg of this blend produces:

m_{\text{CO}_2} = C_f \cdot \frac{44}{12} = 0.820 \cdot 3.667 = 3.007 \text{ kgCO}_2/\text{kgFuel}

Dividing by the LCV of 46.4 MJ/kg gives the TtW CO2 intensity:

\text{EF}_{\text{TtW,CO}_2} = \frac{3007}{46.4} = 64.8 \text{ gCO}_2/\text{MJ}

The WtT intensity is constructed as the sum of upstream extraction (allocated by energy share), gas-plant fractionation, transport, and bunkering distribution emissions per MJ of delivered LPG energy. Each stage has an empirical default in MEPC.391(82) Annex 1 calibrated against the JEC, IEA, and WLPGA datasets.

Assumptions

The published defaults rest on five principal assumptions:

Global-average upstream methane intensity of approximately 0.7 percent across the LPG supply chain. Actual values vary by basin and processing facility.
Modern gas-plant fractionation efficiency with energy consumption near 1 to 2 percent of LPG energy content.
Average shipping distance consistent with the global LPG trade pattern (US Gulf to Far East dominant route, supplemented by Middle East to East Asia and North Sea to North West Europe).
AR5 GWP100 of 28 for methane and 273 for N2O as the binding climate metrics, even though LPG combustion does not produce significant CH4.
Engine pilot fuel share of approximately 3 to 5 percent of total energy, varying by engine maker and design point.

Worked example

Consider a 84,000 cbm VLGC with an MAN B&W 6G70ME-LGIP main engine, total fuel consumption 45 t LPG per day plus 1.5 t MGO pilot per day at typical service speed of 16.5 knots.

Energy consumed per day:

E_{\text{LPG}} = 45 \times 10^3 \text{ kg} \cdot 46.4 \text{ MJ/kg} = 2.088 \text{ TJ}

E_{\text{MGO}} = 1.5 \times 10^3 \text{ kg} \cdot 42.7 \text{ MJ/kg} = 0.064 \text{ TJ}

E_{\text{total}} = 2.152 \text{ TJ}

WtW intensity at MEPC.391(82) Annex 1 default for LPG and MGO components:

\text{EF}_{\text{LPG}} = 76 \text{ gCO}_2\text{eq/MJ}; \quad \text{EF}_{\text{MGO}} = 91 \text{ gCO}_2\text{eq/MJ}

Energy-weighted blend intensity:

\text{EF}_{\text{blend}} = \frac{2.088 \cdot 76 + 0.064 \cdot 91}{2.152} = 76.45 \text{ gCO}_2\text{eq/MJ}

Daily WtW emissions:

M = 2.152 \times 10^{12} \text{ J} \cdot 76.45 \times 10^{-9} \text{ tCO}_2\text{eq/J} = 164.5 \text{ tCO}_2\text{eq/day}

Replacing the same vessel with a conventional VLSFO-fuelled VLGC at the same shaft power (consumption approximately 47 t VLSFO per day, energy 1.91 TJ at LCV 40.6 MJ/kg, intensity 91 gCO2eq/MJ):

M_{\text{VLSFO}} = 1.91 \times 10^{12} \cdot 91 \times 10^{-9} = 173.8 \text{ tCO}_2\text{eq/day}

The daily WtW saving is approximately 9 tCO2eq, or 3,300 tCO2eq per year at 365 operating days. The annual EU ETS and FuelEU exposure reduction at EUR 90 per tCO2eq plus the FuelEU compliance balance value brings the annual GHG cost differential to approximately EUR 600,000 to 900,000 per VLGC, which combined with the cargo-as-fuel commercial saving of USD 1.5 to 2.0 million per year delivers a payback period of 5 to 7 years on the LPG-fuelled VLGC capex premium.

Edge cases and limits

The defaults break down in five operational regimes.

Sustained part-load operation. Slow steaming below 50 percent MCR raises the pilot fuel share to 7 to 10 percent of total energy because the ME-LGIP requires a stable pilot ignition kernel at all loads. The pilot weighting drags the effective WtW intensity up by approximately 1.5 to 2.0 gCO2eq/MJ versus design-load operation.

Cargo composition variability. Some LPG cargoes deliver outside the 70:30 propane:butane reference blend, with butane-rich cargoes (40:60 or 30:70 propane:butane) common in winter destination markets. The carbon content shifts to 82.6 percent, the LCV shifts to 45.7 MJ/kg, and the TtW CO2 intensity shifts to approximately 65.5 gCO2/MJ. The MEPC.391(82) Annex 1 default applies a global-average blend; cargo-specific reporting can adjust the figure.

Cold-climate vapour pressure issues. Pure butane has insufficient vapour pressure at minus 10 degrees Celsius and below to feed the engine fuel rail without forced vaporisation, and propane-rich blends are mandated for cold-climate operation. The WtT and TtW values are unaffected, but the operational arrangement requires a heated fuel preparation skid.

Pilot fuel sulphur compliance in ECA. The pilot MGO must meet the 0.10 percent sulphur cap in ECA waters, which means dual MGO bunker tanks (one for ECA pilot, one for non-ECA pilot) or a single low-sulphur pilot bunker for global operation. The bunker procurement plan affects the operational arrangement but not the WtW arithmetic.

Cargo-as-fuel inventory accounting. When the vessel consumes its own cargo as fuel, the cargo manifest at the discharge port reflects the consumed quantity. The accounting treatment under FuelEU MRV and IMO DCS is to report the consumed LPG as a fuel transaction with a notional bunker delivery note generated by the vessel operator, signed by the chief engineer, and filed with the verifier. The cargo consumed is then deducted from the cargo manifest at discharge.

Regulatory basis

MEPC.391(82): IMO LCA Guidelines and Annex 1 default emission factor table for LPG, mandatory under the IMO Net-Zero Framework from 2027 onward.
MEPC 83 outcomes: GHG Fuel Standard adoption with LPG default in scope from 2027.
Regulation (EU) 2023/1805 (FuelEU Maritime): Annex II default factor for LPG, in force since 2025.
Regulation (EU) 2015/757 (EU MRV): monitoring and reporting basis for ship-level emission data, including pilot fuel and cargo-as-fuel records.
MARPOL Annex VI Regulation 13 (NOx Tier III): lean-burn Otto-cycle LPG engines achieve Tier III without aftertreatment; high-pressure ME-LGIP requires EGR or SCR.
MARPOL Annex VI Regulation 14: SOx is essentially zero from LPG; sulphur compliance is automatic in ECA and globally.
IGF Code: safety standard for low-flashpoint fuels including LPG, currently applied via Alternative Design pending dedicated chapter at MSC 112.
IGC Code Chapter 16: safety standard for gas carriers using cargo as fuel, applicable to LPG carriers with cargo-as-fuel propulsion.
ISO/PAS 23263: marine LPG bunker fuel quality specification, referenced from ISO 8217:2024.

Common errors

Five recurring errors appear in operator and consultant LPG WtW arithmetic:

Treating LPG as identical to LNG. The two fuels share dual-fuel engine architecture and storage in pressure vessels but differ on methane slip (LPG has none), storage temperature (LPG ambient, LNG cryogenic), and bunker hub distribution. WtW values must be sourced from the LPG row in the defaults, not the LNG row.
Omitting the pilot fuel share. The 3 to 5 percent MGO pilot share contributes 0.5 to 1.0 gCO2eq/MJ to the effective WtW intensity and must be included in FuelEU MRV reporting.
Misapplying cargo-as-fuel accounting. The cargo consumed as fuel must be reported as a fuel transaction under EU MRV and IMO DCS and deducted from the cargo manifest; treating it as cargo-only or as fuel-only without the offset produces double-counting or under-counting.
Conflating propane-rich and butane-rich blends. The carbon content and LCV differ by approximately 1 to 2 percent between extreme blends, and reporting the wrong reference values produces a 1 to 2 gCO2/MJ TtW error.
Reporting funnel CO2 as WtW. Funnel CO2 is the TtW component only at approximately 65 gCO2/MJ; quoting it as WtW omits the 9 to 12 gCO2eq/MJ upstream burden.