IGF Code: Low-Flashpoint Fuel Ships

Background

Why the IGF Code exists

By the late 2000s several distinct trends were converging to make a dedicated regulatory framework for gas-fuelled ships necessary. Stricter sulphur emissions regulations (MARPOL Annex VI Chapter 3 and the Emission Control Areas) made low-sulphur compliance economically attractive, and natural gas, methanol, and ammonia all combust with substantially lower SOx than conventional residual fuel oil. The Norwegian short-sea industry had been operating LNG-fuelled ferries since the early 2000s under flag-state-specific approvals, and the industry was pushing for an internationally-harmonised regime to enable LNG-fuelled ships to operate worldwide. Container shipping consolidation with major operators (Maersk, MSC, CMA CGM) ordering very large gas-capable newbuilds for the 2020s required a stable regulatory framework before commitment of multi-billion-dollar fleet renewal programmes.

The IMO’s Maritime Safety Committee initiated work on a unified gas-as-fuel code in 2009, and after six years of drafting (involving input from class societies, gas-industry stakeholders, the maritime authorities of LNG-importing and LNG-exporting nations, and the established European LNG-ferry operators), the IGF Code was adopted on 11 June 2015 by Resolution MSC.391(95) and entered into force on 1 January 2017.

Legal status: SOLAS Chapter II-1 Part G

The IGF Code derives its mandatory force from a new section of SOLAS, Chapter II-1 Part G (Ships Using Low-flashpoint Fuels), added at the same 2015 conference. SOLAS Regulation II-1/57 (the lead provision) requires that any ship using gases or other low-flashpoint fuels comply with the IGF Code. The IGF Code is therefore not a stand-alone instrument but is incorporated by reference into the binding SOLAS framework.

Scope: what counts as a “low-flashpoint fuel”

The IGF Code defines low-flashpoint fuel as any fuel with a flashpoint below 60°C, distinguishing it from conventional residual marine fuels (heavy fuel oil with flashpoint typically 60-110°C), distillate marine fuels (marine gas oil and marine diesel oil with flashpoint 60-65°C), and methane (which has no liquid-phase flashpoint at any practical temperature, it is gaseous at all relevant marine conditions, but the IGF Code categorises it as low-flashpoint by convention because of its overall fire-and-explosion hazard profile).

The Code currently provides detailed prescriptive requirements for natural gas (Part A-1 and Chapter 6 detailed prescriptions) and methanol (Chapter 6A added by MSC.481(102), in force 2024). Other fuels (ethanol, hydrogen, ammonia, LPG, fuel cells using any low-flashpoint fuel) are addressed under the Code’s risk-based design provisions (Part A and Chapter 4) where the designer demonstrates that the proposed fuel system meets the same safety goals as the prescriptive natural-gas requirements.

IGF vs IGC: a crucial distinction

A common source of confusion is the relationship between the IGF and IGC Codes. They cover different regulatory subjects:

• The IGC Code governs ships carrying gas as cargo, the LNG carriers, LPG carriers, ammonia carriers, ethylene carriers and chemical-gas carriers. The cargo is the commodity being transported.
• The IGF Code governs ships using gas as fuel, including any ship type whose propulsion or auxiliary power uses LNG, methanol, hydrogen, ammonia, or another low-flashpoint fuel. The fuel is consumed during the voyage, not delivered to a customer.

A vessel can be subject to both codes simultaneously: a modern LNG carrier with dual-fuel propulsion uses some of its cargo as fuel, complying with the IGC Code for the cargo function and (for ships subject to it) the IGF Code for the fuel function. Most LNG carriers built since 2010 are dual-fuel, drawing fuel-gas via a vapour return line from the cargo system. Older LNG carriers built before 2010 used steam turbines fired by boil-off gas, these vessels were not subject to the IGF Code (which post-dates them) and were governed entirely by the IGC Code with extensions for the fuel application.

For non-gas-cargo vessels using gas as fuel (LNG-fuelled container ships, LNG-fuelled cruise ships, methanol-fuelled tankers) only the IGF Code applies. The fuel is bunkered like marine diesel: from a bunker barge, truck, or shore terminal, into dedicated fuel storage tanks distinct from any cargo system.

Goal-based design philosophy

Why goal-based

The IGF Code’s drafters faced a fundamental difficulty: the universe of potential low-flashpoint fuels is extensive (natural gas, LPG, methanol, ethanol, butanol, di-methyl ether, ammonia, hydrogen, biofuels, methanol-water blends, dual-fuel arrangements running multiple fuels), each with distinct hazard properties (toxicity, flammability range, flame speed, ignition temperature, density of vapour, behaviour in the event of leakage, low-temperature behaviour, water-reactivity). Drafting prescriptive requirements for every combination would have produced an unmanageably long Code.

The chosen alternative is a goal-based structure (Part A) that articulates safety functional requirements with which any fuel system must comply, supplemented by detailed prescriptive provisions for natural gas (Part A-1 and Chapter 6) where industry experience permits prescriptive specification. The goal-based provisions provide a flexible regulatory pathway for novel fuels while ensuring all installations meet the same safety targets.

Functional requirements (Chapter 4)

The IGF Code Chapter 4 establishes functional requirements that any low-flashpoint fuel system must meet:

1. Safety equivalent to a conventional oil-fuelled ship, the IGF system must not produce a higher fire-and-explosion risk than a conventional oil-fuelled equivalent.
2. Containment of any fuel leakage, leakage must not migrate to manned spaces or to ignition sources.
3. Detection of any fuel leakage, gas-detection systems with continuous monitoring at all leakage-prone locations.
4. Mitigation of any leakage event, emergency shutdown, ventilation, and isolation systems.
5. Limitation of damage propagation, leak events must not cascade to multiple system failures.
6. Crew protection, personal protective equipment and procedures appropriate to the fuel.
7. Asset protection, fire and explosion protection equivalent to conventional fuels.

These requirements are tested via the Code’s risk assessment process (Chapter 4 paragraph 4.2), which requires designers to perform a HAZID/HAZOP-style hazard study for the specific fuel system and demonstrate to the flag-state administration (and class society on its behalf) that all credible failure modes are addressed.

Prescriptive natural-gas provisions (Chapter 6)

Where the goal-based provisions allow flexibility, the natural-gas-specific Chapter 6 provides detailed prescriptive requirements that have been industry-vetted through the early generation of LNG-fuelled vessels. These provisions cover:

• Fuel containment system design (Section 6.4)
• Material requirements for cryogenic service (Section 6.5)
• Bunkering systems (Section 6.7)
• Fuel supply systems (Section 6.8)
• Power generation including gas-fuel main and auxiliary engines (Section 6.9)
• Fire safety (Section 6.10)
• Explosion prevention (Section 6.11)
• Ventilation (Section 6.12)
• Electrical installations (Section 6.13)
• Control, monitoring, and safety systems (Section 6.14)

A vessel using natural gas as fuel may comply by following Chapter 6 prescriptively (the most common path, used for the majority of LNG-fuelled newbuilds) without invoking the goal-based provisions.

Fuel containment systems

Type C pressure vessels

The dominant fuel-containment arrangement for LNG-fuelled ships is the Type C pressure vessel, the same containment type extensively used in LPG carriers and small LNG carriers under the IGC Code. Type C tanks are pressure vessels designed to ASME Boiler and Pressure Vessel Code Section VIII or equivalent national standards, with significant gauge pressure capability (typically 5-10 bar gauge for LNG-fuel applications) which allows passive boil-off-gas accumulation without immediate pressure-relief activation.

Type C tanks for LNG fuel service are typically:

• Cylindrical or bilobe in cross-section
• Capacity 50-1,500 m³ for fuel applications (vs much larger for cargo applications)
• 9% nickel steel construction with internal and external insulation
• Insulation system delivering boil-off rate around 0.10-0.20% per day depending on size and insulation quality

Some applications use Type A prismatic tanks where the cargo capacity is large enough that pressure-vessel construction becomes uneconomical (large cruise ships, very large container ships). Type A applications require full secondary barriers and additional protection, more closely paralleling the IGC Code arrangements for cargo Type A tanks.

Tank Connection Space (TCS)

The IGF Code requires the Tank Connection Space (TCS), a gas-tight enclosed space surrounding all fuel-tank connections (fill lines, vapour lines, instrumentation penetrations, safety-valve outlets). The TCS is mechanically ventilated at high air-change rate (typically 30+ air changes per hour), gas-detected continuously, and designed so that any leakage from a tank connection vents to the safe-area atmosphere rather than into the ship.

Fuel storage hold space

The fuel tanks themselves typically reside within a fuel storage hold space, an enclosed compartment with gas-tight bulkheads on all sides, water-tight subdivision per the SOLAS damage stability requirements applicable to the vessel, and the IGF-Code requirements for cofferdams between the fuel space and adjacent machinery and accommodation spaces. The fuel storage hold space is treated as a gas-hazardous area and follows the Code’s electrical-installation requirements (intrinsically-safe equipment, gas-tight cable penetrations).

Fuel preparation room

Function

The fuel preparation room houses the equipment that takes the stored cryogenic LNG fuel and converts it into the form delivered to the engines: typically warm gas at 5-10 bar pressure for medium-pressure dual-fuel engines (Wärtsilä low-pressure dual-fuel, MAN ME-GI gas-engine concept variants), or vaporised gas at lower pressure for boilers. Equipment in the preparation room includes:

• Fuel-pump room or in-tank pumps
• Fuel vapouriser (typically a glycol-water heated heat exchanger, sometimes seawater-heated)
• Fuel-pressure regulation and metering
• Fuel valve unit (FVU), the engineered junction between the fuel preparation system and the engine fuel-train
• Boil-off-gas (BOG) compressor (where applicable) for warming and compressing tank vapour
• Pressure relief equipment

IGF Code requirements for the fuel preparation room

The preparation room is treated as a hazardous area with Code requirements including:

• Gas-tight enclosure with vapour-tight closures on doors and ventilation openings.
• Mechanical ventilation at a minimum 30 air changes per hour.
• Gas detection with continuous read-out at the cargo control room and machinery space.
• Electrical equipment rated for the hazardous-area classification.
• Emergency shutdown triggered by gas detection above threshold (typically 30% LFL).
• Bilge wells that drain to a dedicated bilge system isolated from the conventional engine-room bilge.

Fuel piping

Double-walled requirement

The IGF Code requires double-walled fuel piping for any portion of the fuel-gas system passing through manned spaces or unprotected machinery spaces. The inner pipe carries the fuel; the outer pipe is a continuous secondary barrier with continuous gas detection in the annular space and ventilation at sufficient airflow to dilute any inner-pipe leakage to safe levels. Activation of gas detection in the annular space initiates emergency shutdown.

The outer pipe specification depends on the fuel state:

• Cryogenic LNG fuel in liquid form: outer pipe must be cryogenic-rated (9% nickel steel or stainless steel) because a leak from the inner pipe rapidly chills the outer pipe.
• Vapourised gas at moderate temperature: outer pipe can be standard structural steel.
• Methanol fuel (warmer cryogenic case applies less): outer pipe specifications are correspondingly relaxed.

Single-wall exception: ventilated trunk

A specific allowed exception to the double-wall rule is the use of a ventilated trunk, a continuously-ventilated enclosure that surrounds single-wall piping and extracts any leakage to a safe outboard atmosphere. The trunk arrangement is functionally equivalent to the outer pipe of a double-walled arrangement and satisfies the Code’s containment-of-leakage goal.

Engine fuel valve unit

At the interface between the fuel piping and each engine, the fuel valve unit (FVU) provides:

• A double block-and-bleed valve arrangement isolating the engine from the fuel supply
• Pressure regulation and metering
• Flame arrestor (in some configurations)
• Connection to the engine’s emergency shutdown chain

A single FVU per engine is the standard arrangement; multi-engine systems often share a primary fuel manifold with individual FVUs at each engine.

Bunkering procedures

Three principal bunkering modes

LNG and methanol fuel can be bunkered to a vessel via three principal arrangements:

• Truck-to-Ship (TTS): a road tanker positions on the quay alongside the vessel and discharges via flexible cryogenic hoses. Smallest delivery rate (typically 30-60 m³ per hour); used for small bunker volumes (<200 m³) or where shore terminal infrastructure is unavailable. Common for small LNG-fuelled coastal and harbour vessels.
• Ship-to-Ship (STS): a dedicated bunker barge or small bunker ship comes alongside the receiving vessel and transfers via cryogenic hoses. Medium-to-high delivery rate (200-2,000 m³ per hour). Common at major bunker hubs (Singapore, Rotterdam, Zeebrugge, Norwegian fjords). The bunker barge itself is an IGC Code vessel (carrying LNG as cargo to deliver as fuel).
• Shore-to-Ship (STS shore): a permanent dock-side bunker terminal connects via cryogenic loading arms or hoses. Highest delivery rate (1,000+ m³ per hour). Common at LNG export and import terminals where a permanent installation justifies the capital cost.

Bunkering plan and ship-shore link

The IGF Code requires a bunkering plan prepared collaboratively between the vessel and the bunker provider for each bunkering operation. The plan documents:

• Bunker quantity and quality (LNG specifications including methane number, calorific value)
• Transfer rate and pressure
• Tank fill levels and high-level alarm settings
• Communications protocol between vessel and bunker source
• Emergency shutdown procedures (ESD) including the ship-shore link

The ship-shore link is a fibre-optic or pneumatic emergency-shutdown signal cable connected at the manifold during the operation. ESD activation on either side propagates immediately to the other; both sides stop their pumps and close their valves within the IGF Code-specified maximum response time.

Simultaneous Operations (SIMOPS)

A particularly contentious area of IGF Code application is simultaneous operations, performing bunkering at the same time as cargo operations, passenger embarkation, or other activities in the same dock area. The Code requires a documented SIMOPS risk assessment for any such operation, with restrictions including:

• Defined exclusion zones around the manifold during transfer
• Suspension of welding, hot work, and ignition-source activities in the exclusion zone
• Coordinated communication between bunkering, cargo operations, and passenger embarkation teams
• Specific port-state and flag-state approvals where required

Many ports prohibit SIMOPS during LNG bunkering for safety reasons; the trend in major bunker hubs has been gradual relaxation as operational experience accumulates.

Bunker custody transfer measurement (BCTM)

Bunker custody transfer measurement for LNG fuel is more complex than for conventional fuel oil because LNG is a low-density, low-temperature, multi-component liquid whose volume varies with temperature, pressure, and composition. The IGF Code requires:

• Tank gauging by independent radar and float-type gauges
• Mass-flow metering at the manifold (typically Coriolis meters)
• Cargo composition measurement via gas chromatograph
• Density correction using ISO 6976 calorific-value calculations

The result of a bunker transfer is documented in the Bunker Delivery Note (LNG) with both volume (m³) and energy content (MJ or MMBtu), since LNG quality varies between sources and the customer is buying energy, not volume.

Leak detection and emergency shutdown

Leak detection coverage

The IGF Code requires continuous gas detection at every location where leakage could plausibly occur:

• Tank Connection Space (TCS) of every fuel tank
• Fuel preparation room (multi-point, including ventilation outlet)
• Annular space of double-walled fuel piping
• Engine fuel valve unit (FVU)
• Engine room (where any single-walled fuel piping is permitted, including ventilated trunks)
• Accommodation block air intakes
• Machinery space ventilation outlets
• Bunkering manifold area

Detection is typically by infrared (IR) point sensors for hydrocarbons (LNG, methanol, LPG) or catalytic-bead sensors for hydrogen. Open-path IR sensors scanning across the manifold area are common at bunkering stations.

Alarm thresholds

Standard alarm thresholds:

• Low alarm at 20% of LFL (initiating verification of leak source)
• High alarm at 40% of LFL (initiating ESD-1)
• High-high alarm at 60% of LFL (initiating ESD-2)

For toxic fuels (ammonia, methanol vapour), thresholds are also set by the immediately-dangerous-to-life-and-health (IDLH) value: typically 30 ppm for ammonia, 6,000 ppm for methanol vapour.

Emergency shutdown architecture

Like the IGC Code, the IGF Code requires a two-stage emergency shutdown system:

• ESD-1: isolation of fuel flow at the fuel valve unit (FVU); engines transition to backup fuel (typically MGO) and continue propulsion.
• ESD-2: full shutdown of the fuel system including BOG compressors, fuel pumps, and tank isolation valves; all engines stop.

ESD activation triggers include:

• Gas detection above ESD-1 / ESD-2 thresholds (automatic)
• Manual activation from any of multiple emergency stations
• Loss of control room power
• Fire detection in the fuel preparation room or fuel storage hold space
• High-high level alarm in any fuel tank
• Loss of communication with the bunker provider during bunkering

Relationship to the propulsion control system

A key engineering consideration is the interface between the fuel system ESD and the propulsion/electrical management. ESD-1 must allow the engines to transition to backup fuel without losing propulsion (because losing propulsion at an inopportune moment (entering port, in heavy weather) could itself be a safety hazard). ESD-2 must cleanly stop the fuel system; the engines must accept this and trigger their own controlled shutdown without engine damage.

Crew training

STCW Section V/3

The STCW Convention Section A-V/3 (added by the 2014 STCW amendments and refined since) sets training requirements for officers and ratings serving on IGF Code vessels:

• Basic Training for Service on Ships Subject to the IGF Code (Section A-V/3-1), required for all crew with any responsibility for the fuel system.
• Advanced Training for Service on Ships Subject to the IGF Code (Section A-V/3-2), required for officers in charge of the fuel system, including masters, chief engineers, chief officers and watch-keeping engineers.

Both certifications are typically classroom + simulator + sea-time-experience based, lasting from one week (Basic) to several weeks of structured training plus a documented seagoing experience period (Advanced).

IGC vs IGF training

The IGC Code and IGF Code training requirements (STCW V/1-2 and V/3 respectively) are independent. A seafarer holding only IGC Code training cannot serve as IGF officer of the watch on an IGF-Code vessel; conversely an IGF-trained seafarer cannot serve as cargo officer on an IGC-Code gas carrier without the additional IGC certification. Many seafarers in the gas-fuel sector hold both certifications.

Specific fuel-type provisions

Natural gas (LNG and CNG)

LNG and CNG are governed in detail by Chapter 6 of the IGF Code (and its prescriptive provisions across the rest of the Code). The natural-gas pathway has the deepest body of operational experience and the most mature regulatory framework. The vast majority of IGF-Code vessels in service today (2020s era) are LNG-fuelled.

Methanol (Chapter 6A)

Resolution MSC.481(102) added Chapter 6A to the IGF Code in 2020, with entry into force on 1 January 2024 (subsequently amended in detail). The methanol provisions adapt the Code’s general framework to methanol’s specific characteristics:

• Liquid at ambient temperature (no cryogenic complications)
• Substantially lower flashpoint (12°C, vs methane’s effective non-flashpoint and conventional fuels’ 60°C+)
• Toxic to humans (acute exposure threshold), unlike LNG which is asphyxiation hazard primarily
• Substantially less explosive than gas-phase methane
• Carbon-bearing, combustion produces CO₂ but no SOx or particulates

See Methanol as Marine Fuel for the broader trade context.

Ammonia and hydrogen (under development)

IMO MSC has approved interim guidelines for ammonia-fuelled and hydrogen-fuelled ships pending formal IGF Code amendments. The interim approach is to apply the Code’s risk-based provisions (Chapter 4) with reference to project-specific risk assessments. Several first-generation ammonia-fuelled ships are entering service from 2025-2027 under this regime; commercial-scale hydrogen-fuelled deep-sea vessels are not yet in service but feasibility studies are advanced.

The complexity of ammonia is its toxicity (ammonia at parts-per-million concentrations is acutely toxic to humans), which drives stringent leak-containment and ventilation provisions distinct from the explosive-vapour focus of natural gas.

The complexity of hydrogen is its wide flammable range (4-75% in air vs methane’s 5-15%), low minimum ignition energy, and low molecular weight (resulting in rapid leak dispersion but also rapid pressure changes in any containment).

Fuel cells

The IGF Code addresses fuel cells under its general framework with cross-reference to the underlying fuel (LNG, methanol, hydrogen, ammonia). Fuel cells using natural gas via on-board reforming follow Chapter 6’s natural-gas provisions for the fuel side and the fuel-cell module’s own safety case for the electrochemical side. The Fuel Cell System Efficiency calculator implements the basic energy-balance.

Bunkering infrastructure

Class society bunker-ready notations

To support investment in IGF-Code-ready vessels before LNG bunkering infrastructure was widely available, several classification societies developed bunker-ready notations that allow a vessel to be fully built and certified except for the actual fuel-installation, with provisions for retrofitting the IGF system later when bunkering becomes economical at the vessel’s trading routes.

• ABS LNG Bunker Ready Notation
• LR LNG-Ready Notation
• DNV “Gas Fuelled Ready” and similar notations from BV and ClassNK

These notations have driven thousands of “ready” newbuilds since 2018, with progressive conversion to actual gas-fuel as bunkering grows.

Major bunker hubs

The principal LNG-fuel bunkering hubs as of the mid-2020s are:

• Rotterdam, Netherlands, multiple bunker barges, dedicated LNG-bunker terminal capacity
• Antwerp-Zeebrugge, Belgium
• Singapore, multiple bunker barges since 2017, expanding rapidly
• Norwegian coast (Bergen, Stavanger, Mongstad), mature LNG-fuel network supporting the Norwegian short-sea fleet
• Yokohama and Tokyo Bay, Japan
• Houston, Galveston, Tampa Bay (US Gulf)
• Shanghai and other major Chinese ports, rapid expansion
• Korean major ports (Busan, Ulsan)

Methanol bunkering infrastructure is at an earlier stage; truck-to-ship from chemical terminals is the current norm with permanent terminal investments in early planning at Rotterdam, Singapore, and the US Gulf.

Casualties and operational experience

The IGF era is young

The IGF Code has been in force for less than a decade (2017+) and the major IGF-Code vessel deliveries (large container ships, cruise ships, tankers) are concentrated in 2020-2025. Casualty experience is therefore limited but growing. The relatively short IGF era has produced no major casualties of the magnitude seen in older fuel-oil and LNG-cargo casualties; minor leakage events have been reported but contained by the multiple barriers built into the Code.

Operational lessons

The principal operational learnings from the first generation of IGF-Code vessels:

• Bunkering operations are slower than fuel-oil bunkering, the IGF Code’s safety provisions (ship-shore link, gas-tight closures, manifold isolation, ESD verification) extend bunkering time substantially. Plan voyages accordingly.
• Gas-detection false alarms are common and operationally disruptive, cleaning, calibration, and inspection of gas-detector heads is routine.
• Crew training is the load-bearing element, IGF systems are sufficiently different from oil-fuel systems that hands-on simulator and on-the-job training is critical.
• Fuel quality varies between bunker sources, methane number, LNG composition, and trace contaminants (sulphur, mercury, nitrogen) vary between sources and affect engine performance.
• Methanol corrosion considerations are emerging as the methanol fleet grows; standard fuel-oil materials are sometimes inadequate for methanol service.

IGF Code in the decarbonisation regulatory landscape

Connection to MARPOL Annex VI

The IGF Code is fundamentally enabling for compliance with MARPOL Annex VI Chapter 4 (energy efficiency for ships) and the IMO greenhouse-gas reduction strategy. Conventional residual fuel oil produces approximately 3.114 grams of CO₂ per gram of fuel combusted; LNG produces approximately 2.75 g CO₂/g fuel (with combustion-efficiency advantages partly offset by methane slip from incomplete combustion); methanol produces 1.375 g CO₂/g fuel (lower than diesel because methanol contains oxygen in its molecular structure); ammonia and hydrogen produce zero direct CO₂ in combustion (though their lifecycle emissions depend on production pathway). The IGF Code makes it operationally practical for vessels to access these lower-CO₂-intensity fuels.

The EEXI (Energy Efficiency Existing Ship Index) and CII (Carbon Intensity Indicator) mechanisms under MEPC.328(76) and MEPC.336(76) measure CO₂ emissions on a per-tonne-mile basis. Vessels using LNG as fuel typically benefit from a 20-25% reduction in attained EEXI and CII values relative to oil-fuelled equivalents (with the magnitude depending on the engine type’s methane-slip characteristics and the fuel’s actual lifecycle CO₂). The IGF Code is therefore one of the principal compliance pathways for these instruments.

EU MRV, EU ETS shipping, and FuelEU Maritime

The European Union has implemented its own maritime greenhouse-gas regime layered on top of the IMO framework:

• EU MRV (Monitoring, Reporting, Verification) since 2018, quantifies emissions from voyages calling at EU ports.
• EU ETS (Emissions Trading System) extension to shipping since 1 January 2024, assigns financial cost to CO₂ emissions from EU-arriving and EU-departing voyages.
• FuelEU Maritime since 1 January 2025, sets per-vessel greenhouse-gas intensity limits with progressively tightening trajectory, with explicit recognition of bio-LNG, e-methanol, e-ammonia, and other low-carbon fuel pathways.

The IGF Code provides the regulatory mechanism by which vessels can comply with these instruments using gas, methanol, ammonia, or hydrogen fuels. The class-society LNG-ready and gas-fuelled notations (referenced earlier) are expected to convert progressively to actual gas-fuel use as the EU regime tightens through 2030.

IMO Net-Zero Framework

The IMO Net-Zero Framework (MEPC.391(81), entering into force 2027) introduces a global GHG fuel intensity (GFI) regulation and a worldwide GHG emissions pricing mechanism that will further drive uptake of IGF-Code fuels. The framework specifically credits well-to-wake (WtW) emissions, meaning bio-LNG and e-methanol from renewable energy sources receive substantially lower GFI values than fossil-derived equivalents, making the choice of fuel pathway as well as the choice of fuel chemistry a regulatory and economic decision.

Class-society LNG-ready and gas-fuelled notations in detail

The class-society notations referenced earlier (ABS LNG Bunker Ready, LR LNG-Ready, DNV Gas Fuelled Ready) typically come in tiered forms:

• Level 1 (or “Concept Approved”): vessel design includes notional space allocation for IGF-Code retrofit; structural reservations made for tank installation; piping run reservations made for fuel supply.
• Level 2 (or “Engineered Ready”): detailed design of the IGF-Code retrofit complete; tank and piping foundations partially fitted; key penetrations through bulkheads pre-engineered.
• Level 3 (or “Fitted Ready”): substantial portions of the IGF-Code installation actually fitted, with only the cryogenic tank and final fuel-system commissioning required for activation.

The tiered approach allows owners to commit to the LNG-ready position with varying levels of capital exposure, then convert to active gas fuel when bunkering economics justify it. As of the mid-2020s, several thousand newbuilds across all major classification societies hold one of these notations.

See also

• IGC Code (carriage of liquefied gases as cargo)
• LNG as Marine Fuel
• LNG Fuel System
• Methanol as Marine Fuel
• LNG Cargo Containment Systems
• LNG Carrier
• STCW Convention
• SOLAS Chapter VII Carriage of Dangerous Goods
• MARPOL Annex VI Prevention of Air Pollution
• Marine Inert Gas Systems

References

• International Maritime Organization. International Code of Safety for Ships Using Gases or Other Low-Flashpoint Fuels (IGF Code), 2017 edition (consolidating MSC.391(95) with amendments through MSC.481(102)). Part A general requirements (Chapters 1-5), Part A-1 specific requirements (Chapter 6 natural gas, Chapter 6A methanol), Part B (additional requirements).
• IMO Resolution MSC.391(95) adopting the IGF Code (June 2015, in force 1 January 2017).
• IMO Resolution MSC.481(102) adopting Chapter 6A (Methanol) of the IGF Code (in force 1 January 2024).
• IMO Resolution MSC.392(95) adopting consequential amendments to SOLAS Chapter II-1 Part G.
• International Convention for the Safety of Life at Sea, 1974 (SOLAS), Chapter II-1 Part G Ships Using Low-flashpoint Fuels, Chapter II-2 Fire Safety.
• International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW), Section A-V/3 Specific Training Requirements for Personnel on Ships Subject to the IGF Code.
• IMO MSC.1/Circ.1599 (Interim Guidelines for the Safety of Ships Using Methyl/Ethyl Alcohol as Fuel), MSC.1/Circ.1455 (Guidelines for the Approval of Alternatives and Equivalents), and MSC-MEPC.5/Circ.15 (Guidelines for Risk-Based Design).
• ISO 20519 (Ships and marine technology, Specification for bunkering of liquefied natural gas-fuelled vessels), ISO 19000-series LNG measurement standards, ISO 6976 (Natural gas, Calculation of calorific values, density, relative density and Wobbe index).
• IACS Recommendation No. 142 on Risk Assessment as Required by the IGF Code.
• IACS Unified Requirement UR M55 (Engine room and fuel-oil installations), and class society rules for gas-fuelled ships (ABS Guide for LNG Fuel Ready Vessels; DNV Rules for Gas Fuelled Ships; LR Provisional Rules for the Use of Methanol or Ethanol as Fuel; ClassNK Guidelines for Ships Using Alternative Fuels; BV NR 547 LNG Bunkering Ship Rules; KR LNG-Fuelled Ships Rules).
• SIGTTO Guidelines for the Alleviation of Excessive Surge Pressures on ESD (gas-fuel context).
• SGMF (Society for Gas as a Marine Fuel), industry guidance documents on LNG bunkering, fuel-supply system design, and crew training.
• OCIMF and SIGTTO joint guidelines on Ship/Shore Interface for LNG Operations (also relevant to LNG fuel bunkering by ship-to-ship from LNG-cargo bunker barges).
• Marine Accident Investigation Branch (UK) and counterpart national investigation reports for early IGF-era operational events.