LNG fuel system: onboard storage to gas supply

An LNG fuel system is the onboard installation that receives liquefied natural gas at bunkering, stores it in cryogenic tanks at about minus 162 C, manages the boil-off gas that heat ingress generates, and conditions and delivers gas to the ship’s dual-fuel or gas engines at the pressure each combustion cycle requires. It is governed by the IGF Code , IMO Resolution MSC.391(95), adopted 11 June 2015 and effective from 1 January 2017.

The system burns LNG as fuel and does not carry it as cargo. That distinction is what separates it from the containment plant of an LNG carrier , which is regulated instead by the IGC Code , and it changes almost every design driver: the inventory is sized by bunker interval rather than by cargo parcel, the tank is usually a pressure vessel rather than a membrane, and the fuel is conditioned for an engine rather than preserved for a buyer.

The Code binds ships through SOLAS Chapter II-1 Part G. Regulation II-1/56 sets a three-limb application test on contract date, keel date or delivery date, and Regulation II-1/57 requires any ship in scope to comply with the Code. There is no tonnage figure anywhere in Part G or in the Code itself.

Five subsystems make up the installation, and each is treated in turn below: fuel containment, boil-off and pressure control, the fuel gas supply train, the bunkering interface, and the safety, detection and shutdown systems that tie them together.

What an LNG fuel system is and what it is not

The installation spans the bunker manifold to the gas admission valve on each engine, and it is defined by the hazard it manages rather than by the equipment it contains. Natural gas has no practical liquid-phase flashpoint, is lighter than air on warming, and is stored at a temperature that embrittles ordinary hull steel on contact. Every architectural choice in the Code follows from those three facts.

The five subsystems

Fuel containment is the tank, its supports, its insulation and the tank connection space that encloses every penetration. Pressure and temperature control is whatever holds the tank below its relief valve setting as heat leaks in. The fuel gas supply system, usually abbreviated FGSS, raises pressure, adds heat and regulates flow to the engine. The bunkering interface is the manifold, the transfer hose or arm, the emergency release arrangement and the ship-shore link. Control, monitoring and safety covers gas detection, ventilation, the emergency shutdown chain and the hazardous area classification that decides what equipment may be installed where.

Gas-fuelled, gas-ready and dual-fuel are three different things

A gas-fuelled ship has a complete, certified installation and burns gas today. A ship with a gas ready notation has some subset of the structural, piping and electrical provision for a later conversion, and the notation level decides how much of the conversion has genuinely been bought in advance. Dual-fuel describes the engine rather than the ship: it can run on gas with a liquid pilot or on liquid fuel alone. Most gas-fuelled ships are dual-fuel, but a small number of ferries and harbour craft use single-fuel gas engines with no liquid fallback, which raises the redundancy requirement on the supply train considerably.

The boundary against cargo containment

An LNG carrier with dual-fuel propulsion can be subject to both codes at once, complying with the IGC Code, made mandatory by SOLAS chapter VII part C , for the cargo and drawing fuel gas from the cargo system. SOLAS Regulation II-1/56.4 excludes from Part G those gas carriers that use their cargoes as fuel in compliance with the IGC Code. Whether that exclusion reaches a gas carrier burning methanol, ammonia or hydrogen is unresolved: MSC 109 could not conclude the question and deferred it. Cargo containment types are treated separately under LNG cargo containment systems .

The IGF Code and the chain that makes it mandatory

The Code is not a stand-alone instrument. It acquires force through SOLAS Chapter II-1 Part G, created by Resolution MSC.392(95) at the same 2015 session, and its amendments take effect through the SOLAS tacit acceptance procedure. Regulation II-1/57 is a single sentence requiring compliance; Regulation II-1/56 does the work of deciding who is caught.

Which ships are caught, and when

Regulation II-1/56.1 applies Part G to ships using low-flashpoint fuels for which the building contract is placed on or after 1 January 2017, or, absent a contract, whose keels are laid or which are at a similar stage of construction on or after 1 July 2017, or whose delivery is on or after 1 January 2021. The three limbs are alternatives, so a ship contracted in 2016 and delivered in 2022 is caught by the third.

The widely repeated statement that the Code applies at 500 gross tonnage is a shorthand, and stating its derivation is what makes it usable. The threshold appears nowhere in Part G or in the Code. It is inherited from SOLAS Chapter I Regulation 3(a)(vi), which excludes cargo ships of less than 500 gross tonnage from the Convention altogether. Resolution MSC.391(95) separately invites administrations to apply the Code voluntarily below that size, which is an invitation rather than an obligation.

Two further limbs catch existing tonnage. Regulation II-1/56.2 treats a ship of any date of construction that converts to low-flashpoint fuel on or after 1 January 2017 as a ship using low-flashpoint fuels from the date conversion commenced, which is the rule that governs every LNG dual-fuel retrofit conversion . Regulation II-1/56.3 applies the same treatment where a ship undertakes to use a different low-flashpoint fuel from the one it was approved for. Regulation II-1/56.5 exempts government non-commercial ships.

How the Code is organized

The Code has five parts, and knowing which is which saves time in an approvals argument.

There is no Part E, F or G. Part A-1 is where an engineer spends almost all of their time, and Part C-1 chapters 17 and 18 together with Part D chapter 19 fall outside the scope of classification in the rules of at least one major society, which puts them squarely with the flag state and the operator.

The amendment chain, and one attribution that is widely repeated and wrong

Six amending resolutions have touched the Code. The set that took effect on 1 January 2026 is the largest since adoption and is not yet reflected in most secondary material.

A caution worth recording, because the error is common in secondary sources and was previously carried on this page: MSC.477(102) is not an IGF Code instrument. It is the 2020 amendment to the IMDG Code, Amendment 40-20. The IGF amendments adopted at MSC 102 are MSC.475(102), and they concern pressure relief and fire safety rather than risk assessment methodology.

The alternative design route, and what the Code does not cover

The Code’s prescriptive engineering in Part A-1 addresses natural gas only. Every other low-flashpoint fuel, including methanol, ammonia, hydrogen and fuel cell installations, is approved through the alternative design and arrangements provision of SOLAS Regulation II-1/55 together with the relevant IMO interim guidelines, demonstrating equivalence to the natural gas requirements. The interim guidelines for methyl and ethyl alcohol are MSC.1/Circ.1621 of 7 December 2020; for fuel cell power installations MSC.1/Circ.1647 of 15 June 2022; for LPG under the IGF Code MSC.1/Circ.1666; and for ammonia MSC.1/Circ.1687. A ship burning methanol as a marine fuel or ammonia as a marine fuel is therefore an IGF ship approved by equivalence, not a ship with its own prescriptive Code part.

What IACS actually publishes on gas-fuelled ships

The harmonized class layer is the UI GF series, the IACS Unified Interpretations of the IGF Code, running from GF1 of January 2017 through GF18 of February 2019, with GF13 revised in May 2023 to reflect MSC.458(101) and MSC.475(102). Subjects include drip trays and hull steel protection, the tank connection space and fuel preparation room, hazardous zones in open-deck fuel preparation rooms, and the loading limit relaxation. On the machinery side the relevant Unified Requirement is UR M78, covering engines using gas and other low-flashpoint fuels.

A correction that matters for anyone citing this area: UR Z23 is Hull Survey for New Construction, not a gas fuel instrument. It has no bearing on gas detection, ventilation, emergency shutdown architecture or safety-case documentation, and material attributing those requirements to it, including material previously on this page, is unfounded. Attaching a real requirement number to the wrong subject is the single most common failure mode in secondary regulatory writing, and it is why the IACS publications register is worth checking directly.

Fuel containment: the tank types and how each is used

IGF Code 6.4.15 defines the tank types within the Code itself rather than deferring to the IGC Code , so an IGF ship is approved against 6.4.15 even though the text tracks the cargo code closely. The secondary barrier table at 6.4.3 is what drives the commercial choice: membrane and Type A need a complete secondary barrier, Type B a partial one, and Type C none at all.

Type C pressure vessels

Type C dominates fuel service because it removes the secondary barrier and because its pressure capability turns boil-off into a pressure rise rather than an immediate disposal problem. IGF Code 6.4.15.3.1.1 states the design basis explicitly: pressure vessel criteria modified to include fracture mechanics and crack propagation, with the minimum design pressure of 6.4.15.3.1.2 set so that an initial surface flaw will not propagate more than half the shell thickness over the tank lifetime.

Design vapor pressure is calculated rather than selected. The formula at 6.4.15.3.1.2 gives

where \(A\) is a function of the ratio of material allowable stress to allowable dynamic membrane stress, \(C\) is a characteristic tank dimension taken as the greatest of the height, three quarters of the width or 0.45 of the length, and \(\rho_r\) is the relative density of the fuel. The allowable dynamic membrane stress is 55 N/mm² for ferritic-perlitic, martensitic and austenitic steel and 25 N/mm² for aluminium alloy 5083-O.

Minimum shell thickness under 6.4.15.3.2.1 is 5 mm for carbon-manganese and nickel steels, 3 mm for austenitic steels and 7 mm for aluminium alloys, after forming and including corrosion allowance. Construction is normally 9 percent nickel steel or an austenitic stainless grade, with the tank supported so that thermal contraction is accommodated without transmitting unacceptable load into the hull.

Membrane fuel tanks

Where the required inventory exceeds what pressure vessel construction can economically deliver, membrane containment adapted from cargo practice becomes the space-efficient answer, at the cost of a complete secondary barrier and the fuel storage hold space arrangement required by IGF Code 5.3.5, with a double bottom and longitudinal bulkheads forming side tanks.

The reference application is the CMA CGM Jacques Saade class of 23,000 TEU container ships , which carry a single GTT Mark III membrane fuel tank of 18,600 cubic metres gross capacity, about 52 m wide, sited below the accommodation block, driving a WinGD X92DF low-pressure two-stroke main engine. That single tank is larger than most complete Type C installations and is the clearest demonstration that membrane fuel containment scales.

Type B tanks in fuel service

Type B tanks use refined structural analysis, model testing, fatigue assessment and crack propagation analysis to justify a partial secondary barrier with a protection system under 6.4.15.2.1.2. That is a different thing from the drip trays required separately by 5.10, and conflating the two is a common error. In fuel service the self-supporting prismatic type B concept was developed jointly for a container ship newbuilding programme and received approval in principle from a major society for the bunker tank concept, so its fuel-service history is shorter than its cargo history.

Sizing the tank

Tank sizing follows from the design bunker interval, the engine gas consumption at service power and the lower calorific value of the delivered LNG, which itself varies with composition. The mass and volume relation is direct:

Density at storage conditions is not a constant either, and the calculation procedure for static tank measurement is the basis for converting a gauged volume into the mass and energy that actually matter:

The volumetric penalty against fuel oil is the design consequence that reaches furthest into the rest of the ship. LNG carries roughly the same energy per unit mass as distillate but far less per unit volume, and the tank must also carry insulation, a tank connection space and access. On a container ship that space comes out of slot capacity; on a cruise ship it competes with revenue area.

Tank location: the deterministic and probabilistic rules

The Code will not let the tank sit where it is convenient. IGF Code 5.3.3 sets the deterministic rule: the tank is placed at least B/5 or 11.5 m, whichever is less, inboard from the ship side at the summer load line draught, and never closer to the shell or the aft terminal than a set of minimum distances.

For passenger ships the minimum is B/10 with a floor of 0.8 m, which need not exceed B/15 or 2 m where the shell is already inboard of B/5 or 11.5 m. For cargo ships the minimum scales with tank capacity Vc, taken as 100 percent of the gross design volume of the individual tank at 20 C including domes and appendages:

The lowermost boundary sits above B/15 or 2.0 m, whichever is less, from the bottom shell moulded line at the centreline, and the tank is abaft 0.08L from the forward perpendicular on passenger ships or abaft the collision bulkhead on cargo ships.

IGF Code 5.3.4 offers a probabilistic alternative for a designer who cannot meet the deterministic distances. The calculated value fCN, the product of longitudinal, transverse and vertical damage factors drawn from the damage stability machinery of SOLAS Regulations II-1/7-1 and II-1/7-2, must fall below 0.02 for passenger ships and 0.04 for cargo ships. Multiple non-overlapping tanks are summed and asymmetric arrangements averaged port and starboard. The minimum distances still apply on either route, so the probabilistic path relieves the B/5 rule rather than the local clearances. This is where the fuel system and subdivision and damage stability design meet, and it is normally an iteration rather than a single calculation.

Holding time, boil-off and the four permitted pressure control methods

Heat flows into a cryogenic tank continuously, and a fraction of the liquid vaporizes. On a ship under way with the engines burning gas, demand usually exceeds generation and tank pressure falls. The problem cases are the ones where it does not: a multi-day port stay, an anchorage, extended low-load running, and the period shortly after bunkering when a warm delivery raises the generation rate before the contents settle.

The regulatory quantity is a holding time, not a boil-off rate

This is the single most important reframing in the subject, and it is where most secondary writing goes wrong by quoting a percentage per day. IGF Code 6.9.1.1 requires that the chosen method hold tank pressure below the relief valve set pressure for 15 days, assuming a full tank at normal service pressure and the ship in idle condition, generating only power for domestic load. IGF Code 6.9.2.1 fixes the ambient design condition for worldwide service at sea 32 C and air 45 C, adjusted for particularly hot or cold trading zones.

As amended by MSC.551(108), the requirement expressly excludes tanks designed to withstand the full gauge vapor pressure of the fuel at the upper ambient design temperature, and permits one or more of the four methods rather than a single one.

Holding time is not a fixed property of a tank. It falls as the fill level rises toward the loading limit, because there is less vapor space to absorb the pressure rise, and it improves with subcooling on delivery. The 15 days is a design gate at the idle, full, 45 C condition, and an operator planning a long port stay should compute it from the actual condition rather than assume the design figure.

Two related 15-day criteria appear elsewhere in the Code and must not be conflated with it: 6.4.4.1 requires a secondary barrier to contain envisaged leakage for 15 days, and 6.4.12.2.7 requires the predicted remaining failure development time from leak detection to critical state to be not less than 15 days.

Venting is prohibited

IGF Code 6.9.1.2 is unambiguous: venting of fuel vapor for control of tank pressure is not acceptable except in emergency situations. IGF Code 3.2.9 requires the system to be designed to prevent venting under all normal operating conditions including idle periods, and 6.9.2.2 requires the pressure control system to hold pressure within design conditions without venting. Where a reliquefaction plant produces a methane-bearing waste stream during normal pressure control, 6.9.3.2.4 requires it to be disposed of without venting as far as reasonably practicable.

The four methods

Pressure accumulation is the first line and costs nothing to install beyond the tank’s own design pressure. It is what makes Type C attractive and what carries a ship through a short layover.

Consumption by the engines is the preferred disposal route when there is load to absorb it. The difficulty is that the condition which generates the most surplus, a long stay at low or zero load, is the condition in which the engines can absorb the least.

Thermal oxidation covers both consumption in the ship’s own consumers and a dedicated gas combustion unit . IGF Code 6.9.4.1 requires the capacity of the oxidation system to be demonstrated for the required quantity of vapors, and states that periods of slow steaming and of no consumption from propulsion or other services shall be considered. That is the sizing case, and a unit sized only for the sea-going condition will not hold the tank in the condition that matters.

Reliquefaction returns the vapor to the tank as liquid at the cost of the power the refrigeration cycle consumes. It earns its capital where port dwell is long and the hotel load is large, which is why it appears on cruise ships and large ferries more often than on cargo tonnage. The trade against the other routes is treated under boil-off gas reliquefaction plant and, for the cargo case, boil-off gas management .

Fuel cooling is the fourth listed method and is the least common on fuel installations.

The underlying heat ingress and the resulting vapor generation follow directly from the insulation performance and the temperature difference:

Filling limits

IGF Code 6.8.1 sets the filling limit at 98 percent at the reference temperature, with the loading limit at the actual bunkering temperature derived as LL = FL multiplied by the ratio of reference to actual liquid density. IGF Code 6.8.2 permits a higher loading limit than that calculation gives, but never above 95 percent, and only where insulation and location make fire heating very improbable or a second pressure maintenance system is fitted. Where tank pressure can be maintained only by the consumers, 6.8.1 governs and the relaxation is unavailable. IACS UI GF16 interprets the relaxation.

The relationship is frequently stated backwards, with 95 percent presented as the Code maximum. It is a ceiling on a relaxation from a 98 percent baseline, which is a different thing.

The fuel gas supply system and how its architecture follows the engine cycle

The FGSS connects the tank to the gas admission point on each engine, and its architecture is decided almost entirely by one question: what pressure does the combustion cycle need. That single choice cascades into the pump type, the machinery space concept, the piping design pressure and the achievable methane slip.

Low-pressure supply

A low-pressure dual-fuel engine takes gas at a pressure a Type C tank can reach directly or with modest assistance. The pressure build-up unit is a small heat exchanger that circulates liquid from the tank sump through a warm service medium, vaporizing a controlled fraction to raise or hold tank pressure at the supply level. Where natural boil-off alone meets consumption, the unit idles.

The vapor path runs from the tank vapor dome or the build-up unit through the main isolation valve, filtration and the valve train to the engine, entering the machinery space through the enclosure required by IGF Code 9.6. A final heat exchanger conditions gas temperature to the engine builder’s window.

High-pressure supply

A high-pressure diesel-cycle engine injects gas directly into the cylinder late in compression, which requires delivery well above peak firing pressure. The MAN B and W ME-GI takes a maximum of 300 bar at the engine inlet, with the safety relief valve at 350 bar, pulsation held within plus or minus 2 bar, and maximum flow specified at 100 percent of the specified maximum continuous rating and 315 bar.

Gas temperature at the engine inlet is 45 C plus or minus 10 C, with alarms at 35 C and 55 C and shutdown at 30 C and 60 C. The builder gives the reasons plainly: reduce condensation on the outer wall of the inner pipe, protect engine performance, limit thermal load on the piping, keep gas density uniform, and hold blow-off gas temperature within the material limits of the piping.

Reaching that pressure is done in the liquid phase. A cryogenic reciprocating plunger pump raises the liquid from tank pressure to delivery pressure, and a high-pressure vaporizer then adds heat from a glycol water circuit. The builder’s standardized package uses a three-cylinder reciprocating pump actuated by linear hydraulic pistons with a printed circuit heat exchanger, and individual cold-end control allows operation on two of three cold ends. Centrifugal machinery cannot deliver that differential, which is why the architecture is reciprocating.

At 300 bar methane is above its critical pressure of 45.99 bar, and above its critical temperature of minus 82.59 C it has no distinct liquid or vapor phase, so the fluid delivered to the injector is supercritical rather than a gas in the ordinary sense.

Low-pressure and high-pressure compared

The table below is a Shipping-Wiki.com construction. Each figure is drawn from the engine builder’s published project guide or datasheet, or from the regulatory instrument named, and the columns are not directly comparable between builders because test conditions differ.

The methane number row is the one that most often surprises an operator moving between platforms. A premixed charge knocks, so the delivered gas composition constrains the achievable load; a diffusion flame does not, so it does not. The consequence is that a low-pressure ship has an acceptance test to run on every delivery and a high-pressure ship largely does not. The knock resistance metric itself is treated under methane number , and the combustion-side detail under pilot injection in dual-fuel engines , WinGD X-DF dual-fuel architecture and Wartsila 50DF .

One platform-specific operating mode deserves naming because it is poorly documented elsewhere. The ME-GA offers a specified dual-fuel mode running a gas and fuel oil mixture at a gas-to-diesel ratio the builder expects to be realistic at 10 to 30 percent above 40 percent engine load. It carries reduced efficiency, increased methane slip and increased NOx, Tier III compliance is not guaranteed and the mode is not IMO certified. Entering it raises an alarm and notifies the bridge.

Nitrogen oxide compliance differs by platform and is worth stating per engine rather than as a property of gas operation. The Wartsila 50DF meets Tier III in gas mode as standard and Tier II on liquid fuel. The WinGD X62DF-2.1 datasheet claims Tier III without external exhaust gas treatment in both gas and diesel mode. The ME-GA reaches Tier III in dual-fuel and fuel-oil-only mode using its standard exhaust gas recirculation , which is the alternative to selective catalytic reduction on this class of engine. The governing instrument in each case is MARPOL Annex VI Regulation 13 and the NOx Technical Code 2008 .

Gas composition acceptance

Two properties decide whether a delivered parcel can be burned as supplied. The Wobbe index measures combustion interchangeability and must fall inside the engine builder’s window:

The high-pressure engine’s guiding fuel gas specification is a useful worked example of what a builder actually constrains: lower calorific value at least 38 MJ/kg, methane at least 82 percent mol, ethane at most 15 percent mol, propane and butane together at most 5 percent mol, pentanes and heavier at most 1 percent mol, and hydrogen sulphide plus carbonyl sulphide at most 5 mg/Nm³.

The gas valve unit and the safety barriers between tank and engine

The Code does not use the term gas valve unit. It requires a master gas fuel valve and a double block and bleed, and the industry packages both into a certified skid that class and the builders variously call a gas valve unit or a gas valve train. Citing the paragraphs rather than the trade name is what makes a specification argument tractable, and the equipment itself is treated under gas valve unit .

IGF Code 9.4.2 places the master gas fuel valve outside the machinery space containing the gas consumers and as near as possible to the gas heating installation where one is fitted, in series with a manual stop valve. It must be operable from safe locations on escape routes inside the machinery space, from the engine control room, from outside the machinery space and from the navigation bridge.

IGF Code 9.4.4 requires a double block and bleed per gas consumer: two shutoff valves in series with a bleed valve venting the section between them to a safe location in the open air, or a single body combining the functions. Under 9.4.5 the two shutoff valves are fail-to-close and the ventilation valve fail-to-open, so a loss of power isolates the consumer and vents the trapped section. Under 9.4.6 the same arrangement performs the normal engine stop, which means the barrier is exercised every time the engine comes off gas rather than only in a fault.

Two changes took effect on 1 January 2026. Regulation 9.4.7 now requires the pipe between the master gas fuel valve and the double block and bleed and between the block and the consumer to be automatically vented on master valve shutdown. Regulation 9.4.8 makes the manual shutdown valve upstream of the block a per-consumer rather than a per-engine item.

For a main gas line entering an ESD-protected machinery space and for lines to high-pressure installations, IGF Code 9.4.10 requires rapid rupture detection with automatic shutoff before or as close as possible to the point of entry.

Machinery space concepts: gas safe and ESD protected

IGF Code 5.4.1 offers exactly two ways to arrange a space containing gas consumers, and the difference between them is frequently stated backwards.

A gas safe machinery space is one in which a single failure cannot release fuel gas into the space. It is achieved under 5.5.2 by enclosing all fuel piping within the machinery space boundaries in a gastight enclosure meeting 9.6, and the space is treated as non-hazardous in normal operation. This is the architecture that carries no zone classification burden across the engine room.

An ESD-protected machinery space is non-hazardous under normal conditions but may become hazardous under certain abnormal ones, because a single failure may release gas into the space. It buys relief from full pipe enclosure and pays for it in three ways. IGF Code 5.6.1 limits the concept to machinery spaces certified for periodically unattended operation. IGF Code 5.6.3 requires the engines to be in two or more machinery spaces with no common boundaries, minimum equipment in the space, and a fixed gas detection system that shuts down the gas supply and disconnects electrical equipment not of a certified safe type. Equipment required to operate after gas detection must be certified for zone 1 under 12.5.2.7. Spaces separated by a single bulkhead must withstand a local gas explosion in either, under 5.6.5.

The decisive constraint is pressure. IGF Code 9.7.1 caps the gas fuel supply pressure in an ESD-protected machinery space at 1.0 MPa, with 9.7.2 requiring the supply lines to be designed for not less than that. A high-pressure diesel-cycle installation delivering at 300 bar therefore cannot use the ESD-protected concept at all, and the choice of engine cycle decides the machinery space concept rather than the other way round.

Piping arrangements follow from the concept. IGF Code 9.5.1 requires gas piping through enclosed spaces to be in a secondary enclosure, ventilated duct or double wall, mechanically underpressure ventilated at 30 air changes per hour with gas detection. IGF Code 9.6.1 gives the gas safe machinery space two options: an annulus pressurized with inert gas above the gas fuel pressure with loss-of-pressure alarms, or a ventilated pipe or duct at at least 30 air changes per hour, reducible to 10 where automatic nitrogen filling on gas detection is arranged. Ducting extends to the connection at the gas injection valves and covers all gas pipes on the engine itself up to injection.

Fuel piping is kept not less than 800 mm from the ship’s side under 5.7.1, and must not pass through accommodation, service spaces, electrical equipment rooms or control stations under 5.7.2.

Hazardous area classification and gas detection

Hazardous area classification decides what may be installed where, and it is a design control recorded on an approved drawing rather than an operational judgement. The framework question is worth settling first: ATEX, Directive 2014/34/EU, is an EU product directive and a named regional overlay, not the SOLAS baseline. The Code requires equipment of a certified safe type to a recognized standard, which in marine practice means the IEC 60079 series together with IEC 60092-502. An owner trading to EU ports may face ATEX conformity in addition. The general subject is covered under hazardous area classification on ships .

IGF Code 12.5 sets the zones for a gas-fuelled ship.

Zone 0 covers the interiors of fuel tanks, pressure relief and venting pipework, and pipes and equipment containing fuel. From 1 January 2026, MSC.551(108) adds interbarrier spaces to zone 0.

Zone 1 covers tank connection spaces and fuel storage hold spaces, the fuel preparation room, and defined radii around release points: within 3 m of a fuel tank outlet, gas or vapor outlet, bunker manifold valve, other fuel valve, fuel pipe flange, fuel preparation room ventilation outlet or thermal relief opening; within 1.5 m of fuel preparation room entrances, its ventilation inlets and other zone 1 openings; within spillage coamings around bunker manifold valves and 3 m beyond to 2.4 m above deck; and, other than for Type C tanks, within 2.4 m of a weather-exposed containment system surface.

Zone 2 covers areas within 1.5 m surrounding open or semi-enclosed zone 1 spaces, and the space containing a bolted hatch to a tank connection space. Ventilation ducts take the classification of the space they serve.

Gas detection coverage is prescribed at 15.8.1 and includes the tank connection space, all ducts around fuel pipes, machinery spaces containing gas piping or consumers, compressor rooms and fuel preparation rooms, interbarrier and fuel storage hold spaces of independent tanks other than Type C, airlocks, gas heating circuit expansion tanks, motor rooms, and accommodation and machinery ventilation inlets where the risk assessment requires.

The alarm and action levels are fixed. IGF Code 15.8.6 requires an audible and visible alarm at 20 percent of the lower explosive limit and activation of the safety system at 40 percent LEL at two detectors. IGF Code 15.8.7 allows 30 percent and 60 percent respectively for ventilated ducts around gas pipes in machinery spaces containing gas-fuelled engines. Detection must be continuous and without delay under 15.8.9, and each ESD-protected machinery space carries redundant detection under 15.8.2.

Ventilation rates come from several specific paragraphs rather than one general rule: 13.4.1 for the tank connection space, 13.5.2 for an ESD-protected machinery space at 30 air changes per hour or 15 with automatic increase to 30 on gas detection, 13.6.1 for the fuel preparation room, and 13.8.1 for ducts and double pipes. Fan capacity must not fall by more than 50 percent with one fan or one common-circuit fan group inoperable.

Access control is structural. IGF Code 5.11.1 forbids direct access from a non-hazardous to a hazardous area. Where an airlock is used, MSC.551(108) now fixes its geometry at 5.12.1: two substantially gastight doors 1.5 m to 2.5 m apart, sill height to the hazardous side not less than 300 mm, self-closing with no hold-back arrangement.

The emergency shutdown system

The emergency shutdown chain isolates the gas supply automatically on a safety-critical exceedance and manually from several stations, and it is designed fail-safe so that loss of power or signal drives every actuator closed. The subject in full is treated under ESD systems on gas-fuelled ships .

During bunkering the system extends beyond the ship. IGF Code 8.5.7 requires a ship-shore link or equivalent for automatic and manual emergency shutdown communication to the bunkering source, so activation on either side propagates to the other.

Two shutdown stages are used in practice, and they come from industry guidance carried into ISO 20519:2021 rather than from the Code’s own vocabulary. ESD-1 stops the transfer: manifold valves close on both sides and the pumps or compressors associated with the transfer shut down, with any LNG trapped between closed valves relieved to a safe location rather than to atmosphere. ESD-2 escalates to actuate the emergency release system, whose emergency release coupling contains interlocked valves closing on both sides for a dry disconnect.

A passive dry-break breakaway coupling is a different device. It cannot be remotely actuated and is not part of the linked ESD system, so treating breakaway activation as an ESD initiator is correct only for the actuated coupling.

Valve closing times are prescribed and are shorter than the figures usually quoted. IGF Code 8.5.8 read with 16.7.3.7 requires the bunkering line valve to close in 3600U/BR seconds or 5 seconds, whichever is the least, where U is the ullage volume at the operating signal level in cubic metres and BR the maximum agreed bunkering rate in cubic metres per hour, with the bunkering rate adjusted to limit surge pressure on closure. Separately, IGF Code 16.7.3.6 requires emergency shutdown valves in liquefied gas piping to close fully and smoothly within 30 seconds of actuation, with the closure time verifiable, repeatable and documented on board.

LNG bunkering

Bunkering is where the fuel system meets a second organization, a second set of equipment and usually a port authority, and it is the operation with the highest procedural content in the whole subject. The generic fuel-oil case is covered under bunkering operations ; what follows is what differs.

The four transfer modes

Ship to ship uses a dedicated LNG bunkering vessel alongside at berth or at anchorage, and is the mode that makes deep-sea LNG fuelling practical because it removes dependence on a fixed shore connection at each port. Truck to ship delivers from road tankers through a quay manifold and dominates at smaller ports without bunker vessel service. Shore to ship uses a fixed cryogenic loading arm or hose from a terminal and offers the highest sustained rate and the best custody transfer metering. Portable tank exchange lifts an ISO-format Type C tank aboard and suits very small vessels and emergency supply.

The sequence, and the shipboard rules that govern it

Compatibility check. The supplier’s certificate of quality gives the composition in mole percent, from which the Wobbe index, calorific value and methane number are derived and checked against the engine acceptance window before any connection is made.

Written agreement. IGF Code 18.4.1.1 requires the master or a nominated representative and the person in charge at the bunkering source to agree in writing the transfer procedure including cool-down and gassing up, the maximum transfer rate at all stages and the volume, and the emergency actions, and to complete and sign the bunker safety checklist.

Connection. IGF Code 8.3.1.1 requires the bunkering station to be on open deck for sufficient natural ventilation, with a closed or semi-enclosed station subject to special consideration in the risk assessment. Hoses must have a bursting pressure of not less than five times the maximum pressure during bunkering under 8.3.2.2. From 1 January 2026, IGF Code 8.4.1 as amended permits three connection arrangements: a dry-disconnect and connect coupling to a recognized standard, a manual or hydraulic connect coupler to the receiving manifold presentation flange, or a bolted flange-to-flange assembly, the latter two combined with procedures ensuring dry disconnect and supported by a design-stage risk assessment documented in the fuel handling manual. New 8.4.3 requires an emergency release coupler or system, or an equivalent, unless installed on the supply side.

Inerting and purging. IGF Code 8.5.1 requires an arrangement for purging bunkering lines with inert gas, and 8.5.5 requires the lines to be arranged for inerting and gas freeing and to be free of gas when not bunkering unless the consequences have been evaluated and approved. Nitrogen displaces air before any gas is admitted, and the residual concentration after a given purge is what confirms the atmosphere is outside the flammable range:

Cool-down. Liquid is circulated at a controlled rate to bring the transfer line and manifold from ambient down toward the delivery temperature, because introducing LNG to warm steel produces a pressure surge and risks damage to valves and fittings. The thermal mass of the section sets the liquid consumed and the time taken:

Bulk transfer. Level, tank pressure and manifold temperatures are monitored continuously against the filling limit of IGF Code 6.8.1. Transfer duration follows from the volume and the agreed rate:

No discharge to atmosphere. IGF Code 8.5.2 requires the bunkering system to be arranged so that no gas is discharged to the atmosphere during filling of storage tanks. A vapor return line is the customary means of complying, and the Code mandates the outcome rather than the equipment, which is a distinction worth preserving when reading a specification.

Draining and disconnection. IGF Code 8.5.4 requires means for draining fuel from the bunkering pipes on completion, and 8.5.6 requires isolation against inadvertent cross-over transfer.

Custody transfer and the delivery note

LNG is bought as energy, not volume, and the conversion chain runs from a gauged volume or a metered mass through composition to calorific value. Tank gauging, Coriolis metering at the manifold, gas chromatography for composition and calorific value calculation from composition are the four elements, and ISO 20519:2021 clause 6.2.2 requires disclosure of whether a flowmeter conforms to ISO 21903.

A point that surprises most people coming from fuel oil: MARPOL Annex VI does not require a bunker delivery note for an LNG bunker. Regulation 18.4 disapplies the delivery note and retained sample provisions to gas fuels including LNG, leaving only a duty on the supplier to document the sulphur content. The note comes instead from ISO 20519:2021, which makes it one of the standard’s five elements, and from the annex to Part C-1 of the IGF Code, with IGF Code 18.4.1.2 requiring the ship to receive and sign one. Where the fuel is non-fossil, FuelEU Maritime Annex I adds further content requirements covering lower calorific value and the emission factor or production pathway. The oil-fuel regime it displaces is covered under MARPOL Annex VI Regulation 18 and bunker delivery note .

Standards, guidance and the safety zone

ISO 20519:2021, the second edition dated December 2021, is the governing bunkering specification for vessels not covered by the IGC Code. It addresses five elements: liquid and vapor transfer hardware, operational procedures, the provider’s obligation to supply an LNG bunker delivery note, personnel training and qualification, and facility standards. It replaced ISO 20519:2017, aligns dry couplings to ISO 21593 and flowmeters to ISO 21903, and is adopted regionally as EN ISO 20519:2022.

A terminological caution: ISO 20519:2021 uses controlled zone, established by risk assessment, which is not the same concept as a hazardous area zone under IEC 60079-10-1. The two are frequently used interchangeably and should not be. The area control around the operation is treated under LNG bunkering safety zone .

Industry guidance sits with SGMF, whose FP07-01 LNG as a Marine Fuel: Safety and Operational Guidelines, Bunkering, version 3.0 of 2021, is the flagship document, with FP02-01 covering controlled zones and FP08-01 covering simultaneous operations. GIIGNL publishes the LNG Custody Transfer Handbook, sixth edition 2021, which is a practice reference rather than a standard.

Cryogenic materials and spill protection

Ordinary shipbuilding steel loses ductility well above LNG temperature, and contact with liquid at minus 162 C would produce brittle fracture propagating across welds and structural connections. That single property, the ductile to brittle transition, decides the material list for everything in contact with the fuel or credibly exposed to a spill of it. The subject in full is covered under cryogenic materials for marine service .

Qualified materials for fuel service include austenitic stainless grades such as 304L and 316L, 9 percent nickel steel, and aluminium alloys in the 5083 and 6061 families. From 1 January 2026, MSC.524(106) added high manganese austenitic steel to Table 7.3 of the Code for cryogenic service, which is the first substantial addition to the marine cryogenic material set in many years and is relevant to anyone specifying a tank today. Weld procedure qualification at service temperature, not only at ambient, is what makes the material selection meaningful.

The structure beneath is protected by containment rather than by material choice. IGF Code 5.10 requires drip trays where leakage may damage ship structure or where spill area limitation is needed, made of suitable material, thermally insulated from the ship’s structure so that the surrounding hull or deck is not exposed to unacceptable cooling, fitted with a drain valve to discharge rainwater over the side, and sized for the maximum spill identified by the risk assessment. IGF Code 8.3.1.5 requires that hull and deck structures at the bunkering station not be exposed to unacceptable cooling, and 8.3.1.3 requires safe management of spilled fuel.

Fire safety

Fire safety is the part of the Code most changed by recent amendments and the part most often missing from secondary descriptions of the subject.

Regulation 11.8, introduced by MSC.475(102) for ships constructed on or after 1 January 2024, requires a fixed fire-extinguishing system in a fuel preparation room containing pumps, compressors or other ignition sources. The relationship to the wider fixed-installation regime is covered under the FSS Code and marine fire detection and fixed fire-fighting systems .

From 1 January 2026, MSC.551(108) amended regulation 11.3.1 so that for ships constructed on or after that date, fuel preparation rooms are regarded as a machinery space of category A for the purposes of SOLAS Regulation II-2/9. That is a change in fire integrity and structural boundary standard, not merely a labelling change, and it connects the fuel system to SOLAS Chapter II-2 .

The same resolution amended regulation 11.6.2 to require a portable dry powder extinguisher of at least 5 kg near the bunkering station and in the fuel preparation room, and this one reaches existing ships, which must comply by the first survey on or after 1 January 2026. It is a live retrofit obligation in the current survey cycle rather than a newbuilding requirement.

The fuel preparation room and the tank connection space

Two enclosed spaces carry most of the Code’s arrangement requirements, and neither is named in most general descriptions of an LNG fuel system.

The tank connection space is the gastight enclosure surrounding all fuel tank connections: fill lines, vapor lines, instrumentation penetrations and safety valve outlets. It is zone 1, mechanically ventilated at 30 air changes per hour under 13.4.1, continuously gas detected, and arranged so that leakage vents to a safe area rather than into the ship. Access is normally through a bolted hatch, and the space containing that hatch is itself hazardous. Its importance to redundancy is direct: IGF Code 9.3.3 accepts a single fuel tank on a single-fuel ship only where it is a Type C tank served by two completely separate tank connection spaces.

The fuel preparation room houses the pumps, vaporizers, pressure regulation, metering and boil-off compressor. IGF Code 5.8 requires it to be on open deck unless arranged and fitted in accordance with the requirements for tank connection spaces. It is zone 1 under 12.5.2.2, ventilated at 30 air changes per hour under 13.6.1, and from 1 January 2026 it is a category A machinery space for SOLAS Regulation II-2/9. The term fuel gas handling room is not a Code term and is best avoided in a specification.

Certification, survey and the fuel handling manual

There is no standalone IGF Code certificate. Resolution MSC.392(95) amended the appendix to the SOLAS annex so that compliance with Chapter II-1 Part G is endorsed on the ship’s existing statutory certificate, the Cargo Ship Safety Construction Certificate or the Passenger Ship Safety Certificate. The survey regime follows from SOLAS Chapter I regulations 7 to 10 for that certificate together with the rules of the classification society acting for the flag, and the relevant class notations are assigned by each society under its own gas-fuelled ship rules. The major societies each publish a gas-fuelled notation and a separate readiness notation, and the notation string rather than the marketing phrase is what tells an owner what has been certified. DNV , Lloyd’s Register , ABS , Bureau Veritas and ClassNK each carry their own, and the exact strings move between rule editions, so they are worth reading from the current rule set rather than from a summary.

For the containment system specifically, IGF Code 18.3.2 ties in-service survey, maintenance and testing to the inspection and survey plan required by 6.4.1.8, which is approved as part of the design. That is the citation to use when asking what the survey scope actually is, because it is ship-specific by construction. Type C tanks are pressure vessels in continuous cryogenic service and their examination follows the pressure vessel route covered under marine pressure vessel inspection .

Maintenance of electrical equipment in explosion hazardous spaces is tied by IGF Code 18.3.3 to a recognized standard, which in practice is the inspection and maintenance part of the IEC 60079 series. Certified equipment must be maintained with certified parts, because substituting an uncertified component silently invalidates the zone compliance of the installation.

IGF Code 18.2.3 requires a fuel handling manual, and 18.4.2.1 enumerates its contents: dry-dock to dry-dock operation, cool-down and warm-up, bunkering, sampling, inerting and gas freeing, temperature and pressure control, and the system limitations including cool-down rates, maximum tank temperatures before bunkering, minimum fuel temperatures, maximum tank pressures, transfer rates, filling limits and sloshing limitations, inert gas system operation and the firefighting and emergency procedures. Chapter 17 separately requires drills and emergency exercises. Tank entry for maintenance follows the enclosed space regime, with purging confirmed by air changes or by measurement, and is treated generically under marine confined space entry and tank inspection . The inert gas used for purging a fuel system is nitrogen and is a different arrangement from the flue gas inert gas systems fitted to oil and chemical tankers.

Crew competence under STCW

Personnel certification is a separate limb from ship certification and has its own instruments. STCW Regulation V/3 was introduced by Resolution MSC.396(95) and the competence standards in STCW Code section A-V/3 by Resolution MSC.397(95), both adopted 11 June 2015 and in force 1 January 2017, the same date as the Code. Chapter 19 of the IGF Code sets the goal and refers competence to the STCW Convention and Code rather than prescribing a syllabus itself.

Familiarization applies to all personnel on IGF ships before assignment to shipboard duties. Basic training, to the standard in table A-V/3-1, applies to seafarers responsible for designated safety duties associated with the care, use or emergency response to the fuel. Seafarers already qualified and certificated on liquefied gas tankers under Regulation V/1-2 are deemed to meet the basic standard.

Advanced training, to table A-V/3-2, applies to masters, engineer officers and all personnel with immediate responsibility for the care and use of fuels and fuel systems. It requires completion of approved advanced training plus at least one month of approved seagoing service including a minimum of three bunkering operations on IGF Code ships, of which two of the three may be replaced by approved simulator training. A certificate of proficiency is issued and must be held before assignment. Holders should undertake refresher training at intervals not exceeding five years or demonstrate the standard achieved within that period. The wider engineering certification framework sits under STCW Chapter III engine officers and STCW Chapter V special training , and the procedural layer aboard under the ISM Code .

Methane slip and how each regulator counts it

Methane slip is unburned methane leaving in the exhaust, and it matters to the fuel system because the supply pressure and the combustion cycle largely determine how much of it there is. The mechanism and the abatement options are treated in depth under methane slip and methane slip and nitrous oxide ; what belongs here is which instrument counts it and how.

Three instruments do not count it at all

The CII does not. Resolution MEPC.352(78) defines the attained CII as the total mass of CO2 emitted divided by transport work, and the guidelines contain no methane or nitrous oxide term anywhere. Methane slip has no effect on a CII rating in either direction, and no correction factor exists for it.

The EEDI and EEXI do not. The current calculation guidelines, MEPC.364(79), contain no CH4 or N2O term, and EEXI is built on the EEDI machinery.

The DCS does not. MARPOL Annex VI Regulation 27 collects fuel consumption by type, distance and hours under way, and collects no emission species. LNG is reportable as a fuel type because Regulation 2.1.14 defines fuel oil to include gas, but the slip is not.

The carbon conversion factor for LNG under MEPC.364(79) section 2.2.1 is 2.750 tonnes CO2 per tonne of fuel, against a reference lower calorific value of 48,000 kJ/kg and carbon content 0.7500. It is a single value for every installation and does not vary by engine cycle.

Two instruments do count it, with different numbers

The IMO LCA Guidelines, resolution MEPC.391(81) of 22 March 2024, apply GWP100 values of CH4 = 28 and N2O = 265, the IPCC AR5 set, with an optional GWP20 calculation using 84 and 264 for comparison only. Its default slip coefficients are 3.5 percent of fuel mass for LNG Otto dual fuel medium speed, 1.7 percent for Otto slow speed, 0.15 percent for Diesel slow speed and 2.6 percent for lean-burn spark-ignited engines.

FuelEU Maritime, Regulation (EU) 2023/1805, applies GWP100 values of CH4 = 25 and N2O = 298, taken from Directive (EU) 2018/2001 Annex V Part C paragraph 4, which are IPCC AR4 values. Its Annex II defaults are 3.1, 1.7, 0.2 and 2.6 percent for the same four converter classes, calculated at 50 percent engine load.

The two regimes therefore diverge on the medium-speed Otto and slow-speed Diesel factors and on the GWP vintage, and neither uses the IPCC AR6 fossil-methane value of 29.8 that is widely quoted in commentary. Three vintages are in circulation and the choice moves the answer by roughly 20 percent, so a methane figure without its regulatory frame is not usable.

An owner may substitute a measured value for a default under both regimes. The IMO route is resolution MEPC.402(83), adopted 11 April 2025, which sets the test-bed and onboard measurement protocol with flame ionization detection plus a non-methane cutter as the reference method. FuelEU permits certified actual values, although the fossil well-to-tank factors in Annex II remain mandatory defaults.

The compliance arithmetic on the FuelEU side runs through the attained intensity against a limit that starts at 2 percent below the 91.16 gCO2eq/MJ reference and tightens in six steps to 2050:

Under the EU ETS , methane and nitrous oxide enter the maritime scope from 1 January 2026 under the amended Annex I to Directive 2003/87/EC. Monitoring under the MRV Regulation began with emissions released from 2024, so there was a two-year window in which an LNG-fuelled ship reported its slip but surrendered no allowances for it. The surrender phase-in is 40 percent of verified 2024 emissions, 70 percent of 2025 and 100 percent from 2026, and the geographic scope is 100 percent intra-EEA and at berth with 50 percent of voyages into and out of the EEA. The scope and phase-in and the allowance liability are treated separately.

The status of the IMO’s own market measure needs stating precisely because it changes often. The Net-Zero Framework was approved at MEPC 83 in April 2025 and its adoption was adjourned by vote at the second extraordinary MEPC session held from 14 to 17 October 2025. It is not adopted. The IMO Net-Zero Framework article carries the current position.

Limitations

This article states requirements from the IGF Code and the instruments named in it, and reports engine and equipment figures only where a builder’s published project guide or datasheet carries them. Several limits on its usefulness should be read alongside it.

Rule editions move. The Code has been amended six times, twice with effect from 1 January 2026 and once with effect from 1 January 2028. Class rules and IACS interpretations are revised more often than that. Any paragraph number cited here should be checked against the edition applicable to the ship’s construction date, because the Code applies by construction date and a ship contracted in 2019 is not governed by the 2026 text except where an amendment is expressly retrospective.

Boil-off rate is a design value, not a guarantee. No IMO instrument, IACS Unified Requirement or ISO standard prescribes a percentage per day for a fuel tank, and published bands trace to vendor literature rather than to a regulatory source. Boil-off is a strong function of fill level, tank geometry, insulation condition and ambient state. The regulatory quantity is the 15-day holding time at the idle, full, 45 C condition, and this article leads with that deliberately.

Methane slip figures are declared under test conditions. Published values come from builder guarantees and type approval testing at defined loads. Service values vary with load profile, and independent measurement campaigns published since 2024 disagree with each other and with the regulatory defaults, in both directions, largely according to whether measurement was by exhaust plume or onboard sampling and at what engine load. Where a figure is needed for a compliance calculation, the default in the applicable instrument is the defensible number unless a measurement certified under MEPC.402(83) or the FuelEU equivalent exists.

Well-to-wake comparisons are not settled. Studies published between 2020 and 2026 reach materially different conclusions about whether LNG reduces life-cycle greenhouse gas emissions against distillate, and the disagreement is structural rather than arithmetic: it turns on the GWP horizon, the GWP vintage, the assumed engine slip and the assumed upstream leakage. No single number is presented here as the answer.

Fleet and infrastructure counts differ by basis. Published counts of LNG-fuelled ships and bunkering ports vary by a factor of more than two depending on whether LNG carriers, LNG-capable hulls and vessels on order are included. Any figure quoted from this article should carry its publisher and date.

Port and flag requirements are additional. The bunkering safety zone, permission for simultaneous operations, and local notification requirements are set by the port authority and the coastal state, not by the IGF Code, and they differ substantially between the major bunkering ports. Port state control inspection practice for gas-fuelled ships is still developing.

No state investigation report into an IGF Code fuel system failure has been published. The operational record is short. This article therefore grounds its safety treatment on the Code’s design provisions and the class survey scope rather than on incident statistics, and readers should treat any claim about the comparative safety record of LNG fuel systems, favourable or unfavourable, as unsupported by published casualty investigation at the time of writing.

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