IGC Code: Gas Carrier Construction and Equipment

The IGC Code, formally the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk, is the International Maritime Organization instrument that governs the construction, equipment, and operation of ships carrying liquefied gases in bulk. Its scope covers LNG carriers, LPG carriers, ethylene carriers, ammonia carriers, and chemical-gas carriers across every segment of the global gas trade.

The Code was first adopted by IMO Resolution MSC.5(48) in 1983 and entered into force 1 July 1986. It is mandatory under SOLAS Chapter VII Part C. A root-and-branch rewrite was adopted by Resolution MSC.370(93) at the 93rd session of the Maritime Safety Committee in May 2014; the revised Code entered into force 1 July 2016, applying to gas carriers with keel laid on or after that date.

The Code defines four hazard-based ship types (1G, 2G, 2PG, 3G), four cargo containment system types (independent Type A, Type B, Type C, and membrane), the MARVS pressure-relief regime, approved cryogenic materials, cargo-handling and gas-management systems, process-safety architecture, and the survey and certification regime that produces the Certificate of Fitness for the Carriage of Liquefied Gases in Bulk.

The MARVS Safety Margin (IGC 8.2) calculator and the IGC Tank Type classifier at ShipCalculators.com implement the two core IGC compliance checks. The full suite of LNG operational calculators, from boil-off rate to cool-down time and heel estimation, is listed in the See also section.

Background and legal basis

Origin

Gas-carrier regulation before the 1970s was a patchwork of class-society rules and national requirements with no coherent international baseline. The first attempt at a common standard was the Gas Carrier Code (GC Code), adopted by IMO Resolution A.328(IX) in 1975, applying to ships built between October 1976 and 1 July 1986. Ships built between 1968 and 1976 fell under the Existing Gas Carrier Code (EGC Code) of Resolution A.329(IX). The three-tier split reflects how quickly LNG and LPG technology developed in a single decade: the design of an LNG carrier launched in 1972 had almost nothing in common with one launched in 1982.

The International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk was adopted by IMO Resolution MSC.5(48) in 1983 and entered into force 1 July 1986, applying to new gas carriers built from that date. The GC and EGC Codes continue to govern the small residual fleet built before July 1986.

Mandatory force: SOLAS Chapter VII Part C

The IGC Code derives its mandatory force from SOLAS Chapter VII Part C, specifically Regulation 11, which requires every ship carrying liquefied gases in bulk to comply with the IGC Code or its predecessor. SOLAS Chapter VII itself was adopted in 1983 as part of the same IMO conference package; it covers dangerous goods (Part A), solid bulk cargoes (Part B), liquefied gases in bulk (Part C), and nuclear materials (Part D).

SOLAS Chapter VII Part C does not spell out the technical requirements; it mandates compliance with the appropriate code by reference. This approach let IMO update the technical code without amending the Convention.

The 2014 rewrite and the in-force date

By the mid-2000s the original IGC Code had accumulated three decades of piecemeal amendments that made its structure difficult to apply coherently. IMO undertook a root-and-branch rewrite beginning in 2009. The rewritten Code was adopted by IMO Resolution MSC.370(93) at the 93rd session of the Maritime Safety Committee in May 2014.

The revised IGC Code entered into force 1 July 2016. The application thresholds are:

• Ships with building contract placed on or after 1 January 2016
• Ships without a building contract, with keel laid on or after 1 July 2016
• Ships delivered on or after 1 January 2020 (catch-all for ships with long build cycles)

Ships that do not meet any of these thresholds remain under the pre-2014 IGC Code. A substantial portion of the LNG fleet, including most of the Q-Flex and Q-Max vessels, was delivered before 2016 and operates under the earlier Code. New-build LNG carriers contracted from 2016 onward comply with the MSC.370(93) revision.

The rewrite preserved the chapter numbering and four-tier ship-type/containment-type taxonomy of the original Code but tightened several provisions: more explicit secondary-barrier definitions, mandatory two-stage emergency shutdown (ESD-1 and ESD-2), expanded segregation requirements between cargo systems and machinery spaces, and explicit coverage of dual-fuel propulsion using cargo boil-off gas as fuel.

Relationship to the IGF Code

The IGF Code (International Code of Safety for Ships Using Gases or Other Low-Flashpoint Fuels) is a distinct instrument. It was adopted by Resolution MSC.391(95) at the 95th session of the MSC in June 2015 and entered into force 1 January 2017 via a parallel amendment to SOLAS Chapter II-1 Part G.

The distinction matters:

• IGC Code: governs ships that carry liquefied gas as cargo (LNG carriers, LPG carriers, ammonia carriers).
• IGF Code: governs ships that use gas or other low-flashpoint fuel for propulsion on any ship type (LNG-fuelled container ships, dual-fuel ferries, methanol-fuelled tankers).

A modern LNG carrier with dual-fuel propulsion burns its cargo boil-off gas as engine fuel. That ship carries gas as cargo and uses gas as fuel, so it must comply with both the IGC Code (for the cargo containment and gas-handling systems) and the IGF Code (for the fuel-gas supply system feeding the engines). The two codes share many design principles (gas-tight segregation, leak detection, ESD architecture) and explicitly cross-reference each other. The IGF Code article covers the fuel-side regime.

Scope: what the IGC Code covers and what it does not

The Code applies to ships carrying any cargo listed in Chapter 19 of the Code. The Chapter 19 cargo list covers:

• Hydrocarbons: methane (LNG), ethane, propane, butane, isobutane, ethylene, propylene, butadiene, isoprene, vinyl chloride monomer (VCM)
• Chemical gases: ethylene oxide, propylene oxide, vinyl methyl ether, vinyl ethyl ether, dimethyl ether
• Inorganic gases: ammonia, chlorine
• Liquefied industrial gases: nitrogen, argon, hydrogen (liquefied), carbon dioxide

Each cargo entry specifies the minimum ship type required, the containment-system type where restricted, and any special carriage conditions (maximum loading temperature, materials restrictions, additional inerting requirements, or inhibitor requirements).

The IGC Code does not apply to chemicals carried in liquid form at ambient temperature and pressure, those fall under the IBC Code (SOLAS Chapter VII Part B). It also does not apply to fuel oil or liquid petroleum products at ambient conditions. The IGC Code’s defining criterion is cryogenic or pressurised carriage of substances gaseous at standard conditions, substances whose atmospheric boiling point is below their intended carriage temperature.

The four ship types

The IGC Code assigns every approved cargo a minimum ship type (1G, 2G, 2PG, or 3G) based on the hazard that cargo presents to persons, ship, and environment if it escapes. A vessel’s Certificate of Fitness specifies which ship type it satisfies; it may carry any cargo requiring that type or less hazardous.

Type 1G: maximum hazard

Type 1G is the most stringent classification, required for cargoes whose uncontrolled release poses the greatest risk of severe harm. The Code imposes:

• Maximum cargo-tank capacity of 1,250 m³ per tank, limiting the consequence of any single failure
• Cargo tanks located at least B/5 from the ship’s side shell and B/15 above the keel (where B is moulded breadth), keeping tanks away from collision and grounding penetration zones
• Two independent cargo-handling systems (separate pump circuits, separate piping)
• Stricter damage-survival requirements than Types 2G and 3G

In practice Type 1G is required for chlorine carriage. Chlorine has a narrow dedicated fleet of perhaps a dozen vessels worldwide; it is the most demanding cargo from a regulatory perspective. No other commercially traded gas currently requires Type 1G status.

Type 2G: the dominant type

Type 2G covers hazardous cargoes that do not meet the extreme-hazard threshold of Type 1G. The principal requirements:

• Maximum cargo-tank capacity 8,000 m³ per tank
• Tank location B/5 inboard, B/15 above keel (same shell-clearance requirement as Type 1G)
• Two independent cargo-handling means
• Survival criteria less stringent than Type 1G but still requiring the ship to remain stable after defined flooding

Type 2G is the classification for LNG carriers (methane), ethylene carriers, and ammonia carriers. The roughly 650 LNG carriers in service as of 2026 are all Type 2G, as are the world’s 30-plus large ammonia carriers and all ethylene carriers. This makes Type 2G the regulatory classification covering the highest aggregate cargo throughput of any gas-carrier type.

Type 2PG: pressurised variant

Type 2PG is a sub-category of Type 2G reserved for ships that carry their cargo exclusively in independent Type C tanks (pressure vessels), with cargo design pressures at or above 7 bar gauge and design temperatures at or above -55°C. The “P” denotes that pressure-vessel design replaces the need for the tank-inboard location requirements applicable to refrigerated Type A and Type B tanks.

Type 2PG applies to semi-refrigerated and fully pressurised LPG carriers. Many mid-size LPG carriers carrying propane and butane trade under the 2PG classification. The pressurised-vessel design of the Type C tanks provides inherent robustness that justifies the relaxation of the tank-position clearances.

Type 3G: least hazardous

Type 3G applies to ships carrying cargoes assessed as the least hazardous in the IGC Chapter 19 list, such as some grades of propane, butane, and butadiene under certain temperature and pressure conditions. Damage-stability and tank-position requirements are less stringent than Type 2G. Type 3G ships are uncommon; most vessels in the LPG and chemical-gas fleet are certified at Type 2G or 2PG to give themselves flexibility across cargo grades.

Cargo-to-ship-type matching

The IGC Tank Type calculator implements the classification logic for the containment-system types (A, B, C, membrane), which are a separate dimension from the four ship types. A vessel must satisfy both: the minimum ship type for its approved cargo and the containment-system type specified in Chapter 19 for that cargo where applicable.

Cargo containment systems

The IGC Code recognises four containment-system types. The choice is driven by cargo temperature (LNG at -163°C vs LPG at -42°C vs pressurised gas at ambient), trade-route cargo volumes, shipyard licensing, and the secondary-barrier requirements that follow from each type’s structural analysis method.

Tank type comparison

Independent Type A tanks

Type A tanks are prismatic, self-supporting tanks designed to ordinary allowable-stress methods using the classification society’s structural rules. The design method gives conservative stress limits but does not demonstrate the leak-before-break behaviour required for Type B classification. As a consequence, Type A tanks require a complete secondary barrier: an outer containment capable of holding the full cargo volume for at least 15 days if the primary tank fails.

Type A tanks were used in some early LNG and LPG carriers. They are now uncommon in new construction because the full secondary barrier is space-intensive and expensive. Standard materials for LNG service include 9% nickel steel (ASTM A553 or equivalent); for higher-temperature LPG service, lower-nickel grades and structural steel with impact testing are acceptable.

Independent Type B tanks

Type B tanks use refined structural analysis (finite element analysis and fracture mechanics) to demonstrate that any credible defect in the tank will leak before it ruptures. That demonstration justifies the IGC Code reducing the secondary-barrier requirement to a partial barrier (drip tray and spray shield rather than a full second tank).

Two Type B implementations have entered commercial service:

• Moss spherical tank: developed by Moss Maritime / Kvaerner. The sphere is supported on a single equatorial skirt; aluminium alloy 5083 is standard. The recognisable above-deck sphere is visible on roughly 80 LNG carriers, most built before 2005, including the original Algerian LNG fleet and the early Qatari tonnage.
• IHI Self-Supporting Prismatic Type B (IHI SPB): a prismatic shape designed to the Type B leak-before-break criteria, using 9% nickel steel. The SPB design improves on the Moss sphere’s space efficiency while retaining the Type B partial-barrier benefit. A small number of IHI SPB vessels entered service with Japanese operators from the 1990s onward.

Independent Type C tanks

Type C tanks are pressure vessels designed to ASME Boiler and Pressure Vessel Code Section VIII or an equivalent national pressure-vessel standard. They operate at gauge pressures typically from 5 to 20 bar for LPG service and require no secondary barrier because the pressure-vessel code’s design margin provides the equivalent structural assurance.

Type C tanks are the standard containment for LPG carriers across all temperature categories: fully pressurised (ambient temperature, ~8 bar gauge), semi-refrigerated (intermediate temperature and pressure), and fully refrigerated (-42°C, ~0.3 bar gauge with insulation). They are also used in small-scale LNG carriers and LNG bunkering vessels where cargo capacity is below roughly 30,000 m³, a range where the pressure-vessel geometry is still economical. Ammonia carriers in large numbers use Type C tanks, though the growing trade in ammonia as a clean energy carrier is driving designs with membrane containment at larger capacities.

The IGC Type C Tank MARVS Check calculator implements the pressure-relief sizing check for Type C installations.

Membrane tanks

Membrane tanks are structurally integrated with the ship’s inner hull: a thin metal membrane lines the insulation, which is fixed to the inner hull plating. The hull provides all structural strength; the membrane provides the cryogenic-tight surface. This arrangement delivers the highest space efficiency of any containment type because the cargo fills more of the hull’s cross-section than either spherical or cylindrical pressure vessels.

Both commercially dominant membrane systems come from Gaztransport & Technigaz (GTT) of France:

• GTT Mark III: corrugated austenitic stainless steel primary membrane (1.2 mm thick), reinforced polyurethane foam panels for insulation, triplex laminated composite secondary membrane. Mark III Flex is a variant with thicker foam for improved thermal performance. Mark III is the most widely specified membrane system in post-2000 LNG carrier orders.
• GTT NO96: Invar (36% nickel-iron alloy) primary membrane and Invar secondary membrane, plywood-boxed insulation panels filled with perlite. The near-zero thermal expansion coefficient of Invar eliminates the corrugations needed in stainless-steel membranes. NO96 has been in production since the 1970s; NO96 Max is the current high-performance variant.

Both systems require a complete secondary membrane acting as the IGC Code’s secondary barrier, making them fully compliant without an additional outer tank. Membrane technology drove the growth of LNG carrier capacity from the ~125,000 m³ of the first Moss-sphere vessels to 175,000-180,000 m³ for Q-Flex carriers and 266,000 m³ for the largest Q-Max vessels trading from Qatar.

MARVS and pressure relief

Maximum Allowable Relief Valve Setting

MARVS (Maximum Allowable Relief Valve Setting) is the IGC Code’s central cargo-pressure parameter, defined in Chapter 8 of the Code. It is the highest pressure at which a cargo tank’s pressure-relief valve may discharge during normal or abnormal operation. The tank’s structural design is qualified to withstand at least MARVS without yielding.

MARVS by containment type:

• Type A and Type B: typically 0.25 bar gauge (atmospheric-pressure design with limited overpressure allowance)
• Membrane: typically 0.25 to 0.7 bar gauge depending on the system
• Type C: depends on the pressure-vessel design; 5 to 18 bar gauge for LPG service; 3 to 7 bar gauge for small LNG

The IGC Code Chapter 8 requires two independent pressure-relief means, each capable of passing the full relief flow at MARVS, with discharges piped to the vent mast above the cargo area. The MARVS Safety Margin (IGC 8.2) calculator implements the IGC 8.2 check for the required relief capacity.

Vacuum relief

The Code also requires vacuum-relief protection to prevent tank collapse during cargo cooldown or rapid discharge. Vacuum-relief valves (or vacuum-breaker arrangements) admit nitrogen or boil-off gas to maintain positive internal pressure. This is particularly critical during LNG cooldown, when large volumes of vapour condense rapidly and can pull the tank toward vacuum if the inlet flow is not controlled.

Operating pressure regime

In normal LNG carrier operation, cargo tanks run substantially below MARVS, typically 0.05 to 0.15 bar gauge. Boil-off gas is continuously removed to the engine room fuel-gas system or, when production exceeds fuel demand, to the gas combustion unit (GCU). The pressure-relief valve provides final protection only against failure of the entire boil-off management system. For LPG Type C carriers, operating pressure is typically maintained at 5 to 15 bar gauge depending on cargo and ambient temperature, and the vapor-return system to shore manages loading-port pressure.

Materials of construction

LNG service: cryogenic-grade requirements

LNG tanks operate at approximately -163°C (the atmospheric boiling point of methane). Standard structural steel undergoes a ductile-to-brittle transition at around -50°C and cannot be used at LNG temperatures. The IGC Code Chapter 6 specifies approved materials with Charpy impact-test results validated at design temperatures:

• 9% nickel steel (ASTM A553 Type I or II; or equivalent EN/JIS grades): the standard for LNG Type A tanks and IHI SPB Type B tanks. Impact-tested at -196°C; yield strength 585 MPa minimum; well-established welding procedures using Inconel filler.
• Invar (Fe-36%Ni): near-zero coefficient of thermal expansion at LNG temperatures, making expansion joints unnecessary. Used in GTT NO96 membrane systems as both primary and secondary membranes.
• Aluminium alloy 5083: cryogenic-rated; used in Moss spherical tanks. Density advantage reduces the structural loads on the equatorial skirt; weldable but requires different procedures from steel.
• Austenitic stainless steel 304L / 304LN: cryogenic-rated; used in GTT Mark III primary membranes in corrugated sheet form. Lower yield strength than 9% nickel steel, but the corrugated geometry accommodates thermal strain without stress concentration.

LPG service: a less demanding regime

LPG tanks operate at much higher temperatures: propane has an atmospheric boiling point of -42°C and butane of -0.5°C. These temperatures are cold but not cryogenic in the metallurgical sense:

• 3.5% nickel steel is standard for fully refrigerated LPG at -42°C. Impact-tested at -60°C.
• Standard carbon-manganese structural steel with enhanced impact testing is acceptable for semi-refrigerated and fully pressurised LPG where design temperatures are above -10°C.
• Carbon steel pressure-vessel grades (ASTM A516 or equivalent) are standard for fully pressurised LPG Type C tanks operating at ambient temperature.

Ammonia service: material exclusions

Ammonia (atmospheric boiling point -33°C) is corrosive to copper, copper alloys, brass, and zinc. The IGC Code Chapter 19 entry for ammonia explicitly bans these materials from any wetted-surface application. Carbon steel with attention to stress-corrosion cracking (including limits on H₂S contamination if present) is the standard ammonia-service material. Post-weld heat treatment is required to relieve residual stresses that would otherwise drive stress-corrosion cracking in ammonia service.

Cargo handling and gas management

Cooldown before first loading

A warm LNG cargo tank must be cooled gradually from ambient to near -163°C before bulk loading can begin. Thermal shock, a sudden large temperature differential across the tank structure, would induce stresses that exceed the material’s yield point. The cooldown procedure involves:

1. Inerting the tank to below 5% oxygen with nitrogen.
2. Introducing LNG spray through dedicated cooldown nozzles at the tank top at a controlled flow rate.
3. The LNG evaporates on contact with warm surfaces, absorbing heat and progressively cooling the structure.
4. Continuous monitoring of tank temperature at multiple points (top, mid, bottom, dome) to verify uniform cooling and detect gradients that indicate uneven spray distribution.
5. Continuation until the tank reaches approximately -155°C to -160°C, at which point bulk loading is safe.

Cooling a 170,000 m³ LNG tank from +30°C to cargo temperature takes 24 to 48 hours, consuming 100 to 300 tonnes of LNG as coolant depending on tank thermal mass. The LNG Tank Cool-Down Time calculator estimates cooldown duration based on insulation characteristics, spray flow rate, and heat capacity.

Boil-off gas management

Heat ingress through the insulation continuously evaporates a fraction of the LNG cargo. The boil-off rate (BOR) on modern Mark III or NO96 carriers runs at 0.085 to 0.15% of cargo volume per day, depending on insulation thickness, ambient sea temperature, cargo level, and whether the ship is laden or in ballast (empty tanks have more metal surface per unit of remaining cold). Older Moss-sphere vessels ran higher BORs, typically 0.15 to 0.20%, because aluminium has lower insulation performance than foam-panel systems.

The LNG Boil-Off Rate from Heat Ingress calculator implements the BOR estimation from first-principles heat-ingress through each tank surface.

The IGC Code requires that every gas carrier have at least one means of managing boil-off gas to prevent cargo tanks from rising above MARVS. Modern LNG carriers typically combine:

• Dual-fuel main engines (MEGI, XDF, or slow-speed two-stroke dual-fuel types) burning BOG as fuel. This is the dominant modern approach; it converts an operational liability into propulsion energy.
• Gas combustion unit (GCU): a thermal oxidizer that burns BOG to CO₂ and water vapour when BOG production exceeds engine fuel demand (port stay, slow steaming at minimum load). The LNG GCU Required Capacity calculator sizes the GCU for the maximum BOG production scenario.
• Reliquefaction plant: common on LPG carriers; less common on LNG carriers because the energy cost of liquefying methane at -163°C is high compared to LPG at -42°C.

The LNG BOG Compressor Shaft Power calculator supports sizing of the BOG compressor for the fuel-gas supply system.

Reliquefaction

LPG carriers routinely carry on-board reliquefaction plant: a refrigeration cycle (typically propane or mixed-refrigerant cascade) that condenses BOG from the cargo tanks and returns it to liquid. Reliquefaction is the standard BOG management method for LPG carriers because the cycle temperatures are moderate (-42°C for propane) and the system is economical at the scales involved. The LPG Reliquefaction COP calculator estimates the coefficient of performance of the reliquefaction cycle based on suction and condensing conditions.

For LNG, reliquefaction requires refrigerating to -163°C, which demands a complex multi-stage cycle (the same technology used in LNG export liquefaction plants but scaled to ship dimensions). A small number of LNG reliquefaction carriers entered service in the 2000s, but the wide adoption of dual-fuel engines made reliquefaction less attractive commercially and the fleet share remains low.

Heel for return voyage

LNG carriers on a return ballast voyage from the discharge port to the loading terminal retain a small quantity of LNG, the heel, for two purposes: keeping cargo tanks cold to avoid the long cooldown before next loading, and providing BOG fuel for the engines. A typical LNG carrier on a 20-day return voyage retains around 3,000 to 5,000 m³ of LNG heel, representing about 2 to 3% of cargo capacity. The LNG Heel for Return Voyage calculator estimates required heel volume from voyage duration, BOR, and engine fuel consumption.

Gas freeing and inerting

When a gas carrier needs to enter drydock or to change cargo grade, the cargo tanks must be gas-freed: progressively purged with inert gas (nitrogen or CO₂) and then air until the atmosphere is safe for entry. The IGC Code requires a documented gas-freeing procedure with continuous gas monitoring at multiple sample points before any hot work or tank entry is permitted.

The reverse operation, inerting before returning to cargo service, replaces air with nitrogen and then cargo vapour, ensuring no air mixes with the incoming cargo. The LNG Tank Inerting Dilution Purge Volume and LNG Tank Displacement Purge Volume calculators implement the purge-volume calculations per the two purging methods used in the industry.

Process safety architecture

Cargo area segregation

The IGC Code prescribes strict physical and atmospheric separation between the cargo area (cargo tanks, cargo piping, cargo pumps, gas compressors, cargo control systems) and the accommodation, machinery, and service spaces. The boundaries must be gastight (Class A-60 fire bulkheads in SOLAS terminology; vapour-tight closures on all openings). Ventilation from the cargo area must exhaust to a safe location, not to the accommodation or engine room.

Cofferdams are required between cargo tanks and adjacent machinery spaces, typically filled with nitrogen and equipped with heating elements to prevent condensation against the cold tank surfaces and to maintain positive pressure. The Cofferdam Heating calculator estimates the heating duty and surface-temperature profile for cargo cofferdams.

Cargo and boil-off gas piping crossing from the cargo area into the machinery space must pass through a defined double-block-and-bleed isolation arrangement so that any leak in the piping does not reach the machinery space atmosphere.

Leak detection

The IGC Code requires continuous gas detection at a defined set of locations:

• Ullage spaces of each cargo tank (for tanks with gas-return piping)
• Enclosed spaces in the cargo area that contain cargo-handling equipment
• Vent-mast outlets and gas-riser enclosures
• Air intakes for the machinery space, accommodation, and service spaces
• Any inter-barrier spaces (for Type A and membrane systems with secondary barriers)

Detection technology is typically catalytic-bead or infrared sensors with continuous readout at the cargo control room. For flammable gases, the standard alarm threshold is 30% of the lower flammable limit (LFL); for toxic cargoes such as ammonia and chlorine, the threshold is the immediately dangerous to life and health (IDLH) concentration.

Emergency shutdown (ESD)

The IGC Code’s two-stage ESD architecture is a feature introduced explicitly in the MSC.370(93) rewrite:

• ESD-1 (initial isolation): closes all cargo manifold valves and isolates cargo flow at the ship/shore interface without stopping running cargo pumps. ESD-1 can be triggered manually or by a shore-terminal ESD signal arriving via the ship-shore link (typically a fibre-optic ESD cable connected at the manifold during cargo operations).
• ESD-2 (full shutdown): stops all running cargo pumps, closes all cargo and stripping valves throughout the ship, and triggers immediate isolation of the vapour-return line. ESD-2 is the full cargo emergency stop.

ESD-2 activates automatically on high-high cargo level, on loss of cargo control room power, on gas detection above the alarm threshold at defined locations, and manually from the cargo control room and from emergency stations distributed through the vessel. Modern ship-terminal interfaces match the two ESD stages to the corresponding shore-terminal activation levels so that a terminal ESD-1 triggers ship ESD-1, and terminal ESD-2 triggers ship ESD-2, eliminating the previous ambiguity of a single undifferentiated ESD signal.

High-high level alarm

Each cargo tank carries an independent high-high level switch separate from the continuous level gauge (radar ullage or float system). The high-high switch activates when cargo reaches the maximum allowable filling limit, typically 98.5% of the tank’s certified liquid volume, and directly initiates ESD-2. The requirement for independence ensures that a failure of the continuous gauging system does not disable this final protection layer.

IGC Code Chapter 15 sets the filling limits by cargo density and tank design temperature. For LNG at design temperature, the filling limit is typically 98.5% by volume. For higher-density cargoes or lower temperatures (where thermal expansion may push a full tank over-pressure), Chapter 15 specifies reduced filling limits.

Survey and certification

Certificate of Fitness for the Carriage of Liquefied Gases in Bulk

The regulatory outcome of IGC Code compliance is the Certificate of Fitness for the Carriage of Liquefied Gases in Bulk. The flag state issues the certificate (or delegates issuance to a recognised classification society). A gas carrier without a current, valid Certificate of Fitness cannot carry IGC cargoes under any SOLAS-signatory flag.

The certificate records:

• The vessel’s IGC ship type (1G, 2G, 2PG, or 3G)
• The list of approved cargoes, referenced to IGC Chapter 19
• Any voyage-area or operational restrictions
• The expiry date (maximum five years from issue)
• Any special conditions applied by the administering authority

Survey schedule

IGC Code survey requirements parallel the standard SOLAS certification cycle:

• Initial survey: full inspection of the entire ship against every applicable IGC chapter requirement, completed before the first Certificate of Fitness is issued.
• Renewal survey: same scope as the initial survey, conducted within 3 months of the fifth anniversary of the certificate.
• Intermediate survey: conducted between the second and third anniversary dates; focused on cargo-tank integrity, pressure-relief systems, ESD systems, gas detection, and safety equipment.
• Annual survey: verifies no major changes have been made since the last periodical survey; spot-checks operational records and safety equipment.

ABS, Bureau Veritas, ClassNK, DNV, Korean Register, Lloyd’s Register, and RINA conduct most surveys on behalf of their respective flag states under IMO’s recognized organization arrangements. The class society files a survey report; the flag state reviews and issues or endorses the certificate.

IACS coordination

The International Association of Classification Societies publishes Unified Requirements that govern gas-carrier construction across the member societies. IACS UR S26 covers cargo-tank scantling design; UR W1 and W11 govern welding procedure qualification and steel grade selection for cryogenic service. Class rules for gas carriers (ABS Rules for Building and Classing Gas Carriers; DNV Rules for Classification, Gas Carriers; LR Rules for Gas Tankers) interpret and implement the IGC Code requirements at the design level, typically adding more detail than the Code itself.

Training requirements under STCW

STCW Section A-V/1-2

The STCW Convention mandates two levels of gas-tanker training under Section A-V/1-2:

• Basic Liquefied Gas Tanker Familiarisation (STCW A-V/1-2, para. 1): a shore-based course (typically one week) covering cargo properties, containment-system types, cargo-handling principles, hazards (flammability, toxicity, cryogenic burns), gas detection, and emergency procedures. Required for all officers and ratings assigned cargo-related duties on a liquefied gas tanker before they may serve on such a vessel.
• Advanced Liquefied Gas Tanker Training (STCW A-V/1-2, para. 2): a more detailed course (typically two weeks) required for masters, chief engineers, chief officers, and deck officers in charge of cargo operations. Covers cooldown procedures, BOG management system operation, ESD testing, Certificate of Fitness verification, IGC Code chapter-by-chapter requirements, and cargo calculation methods.

Both courses require approval by the national maritime administration and include both theoretical instruction and practical exercises or simulator time. Flag-state and port-state control inspections regularly audit training records.

Vessel-specific familiarisation

Beyond STCW certificates, IGC Code Chapter 18 requires that crew be familiar with the specific systems on their vessel before performing cargo operations. In practice this means a vessel-specific familiarisation period of two to four weeks on first joining a new ship type, covering the particular cargo-control system software, the ESD panel layout, the gas-detection console, and the valving and piping arrangement of that ship. Operators document this familiarisation in shipboard training records.

Annual emergency drills, including ESD activation under simulated cargo-leak scenarios and muster drills for toxic cargoes such as ammonia, are mandatory under SOLAS and the ISM Code and are specifically referenced in IGC Code Chapter 18.

Recent amendments and emerging cargoes

Amendments since July 2016

The IGC Code as adopted by MSC.370(93) has been amended by subsequent MSC resolutions:

• MSC.411(97): adopted at MSC 97 (November 2016), minor corrections and clarifications to Chapter 19 cargo entries.
• MSC.475(101): adopted at MSC 101 (June 2019, entered into force 1 January 2021), amendments to Chapter 19 adding or revising entries for several cargoes including updated entries for ethylene oxide and propylene oxide carriage conditions.

Ammonia as a fuel carrier

Ammonia has been an IGC Code cargo since 1986. The post-2020 interest in ammonia as a zero-carbon energy carrier is intensifying pressure on the Code’s ammonia provisions. Larger dedicated ammonia carriers, several in the 85,000 m³ range, have been ordered from 2022 onward. The IMO is working on amendments to Chapter 19 to address large-scale ammonia carriage requirements for the energy trade, which differs in some respects from the fertilizer trade for which the original ammonia provisions were written.

Ammonia-fuelled propulsion is an IGF Code matter, not an IGC Code matter. The IGC Code governs the ammonia as cargo; the IGF Code would govern its use as fuel. Both frameworks are under active development at the MSC.

CO₂ carriage for CCS

Liquid CO₂ carriage for carbon capture and storage projects involves transporting captured CO₂ in liquefied form (approximately -50°C and 7 to 15 bar) from industrial capture sites to offshore geological storage. This is a Type C pressure-vessel application under the IGC Code. Amendments to Chapter 19 covering CO₂ at higher purities and larger volumes than the existing food-grade CO₂ trade are under discussion at the IMO MSC Sub-Committee on Ship Systems and Equipment. Northern Lights (Norway) and several other CCS projects have early-stage CO₂ carrier orders pending regulatory finalisation.

Hydrogen (liquefied)

Liquid hydrogen (boiling point -253°C, near absolute zero) presents more extreme material and insulation challenges than LNG. IGC Code Chapter 19 includes a hydrogen entry but for relatively small volumes. The Suiso Frontier pilot voyage (2022, Australia to Japan) demonstrated liquid hydrogen carriage; the IGC Code’s provisions for large-scale liquefied hydrogen carriage are under revision to support commercial-scale hydrogen shipping.

Notable casualties and the Code’s safety record

El Paso Sonatrach, 1971

The El Paso Sonatrach LNG fire in Boston Harbor in January 1971 killed 37 workers during a cooldown procedure. The incident was caused by LNG spilling from a pump-well opening onto warm surfaces, generating a rapid vapour cloud that ignited. It drove early IMO development of cargo-area segregation requirements, inerting standards, and the prohibition on accommodating cargo-handling personnel in spaces adjacent to cargo tanks.

Yuyo Maru No. 10, 1974

The fully laden LPG carrier Yuyo Maru No. 10 was struck by the bulk carrier Pacific Ares in Tokyo Bay on 9 November 1974. The resulting fire and explosion killed 33 people and burned for two weeks. The casualty directly influenced the IGC Code’s requirements on damage stability, tank inboard position (keeping tanks away from the side shell), and the secondary-barrier regime for Type A tanks.

Safety record since 1986

The relatively short casualty list for IGC-era gas carriers reflects the Code’s prescriptive framework. No LNG carrier has suffered a major cargo release at sea since the Code entered force in 1986. The multi-layer protection concept, cargo containment plus secondary barrier plus cargo-area segregation plus ESD plus gas detection plus MARVS-governed pressure relief, has produced a safety record for the LNG trade that is substantially better than the tanker industry average for oil spills per tonne-mile of cargo carried.

Limitations

Code does not cover the terminal interface in detail

The IGC Code governs the ship. The loading arm, jetty, and shore tank design are covered by separate standards (OCIMF and SIGTTO guidelines, national terminal codes). Disputes over cargo quality, overfill from shore, or terminal-side ESD failures require reference to the ship-shore interface agreement and applicable terminal code alongside the IGC Code itself.

Pre-2016 fleet operates under earlier requirements

A significant fraction of the global LNG fleet, including all vessels delivered before the July 2016 cut-off, operates under the pre-2014 IGC Code. The earlier Code lacks the explicit two-stage ESD requirement and some of the tighter segregation provisions of the MSC.370(93) revision. Port-state control officers and charterers who inspect against the current Code should verify which edition applies to a given vessel.

Chapter 19 cargo list is not exhaustive

New chemicals not in IGC Chapter 19 require IMO approval (via the Evaluation of Safety and Pollution Hazards of Chemicals (ESPH) Working Group) before a ship can carry them commercially. The approval process for a novel cargo can take two to four years through the IMO cycle, which constrains how quickly ships can be positioned for new chemical-gas trades.

Training coverage varies between flag states

STCW A-V/1-2 sets minimum standards; implementation and examination depth vary across national maritime administrations. Crew certified under some flags may have completed shorter or less rigorous courses than equivalent certifications under others. Charterers and operators sometimes require supplemental vessel-specific competence assessments beyond the STCW certificate.

The Code does not replace class-society design rules

The IGC Code is a minimum regulatory floor. Classification society rules for gas carriers (ABS, DNV, LR, BV, KR, ClassNK) typically impose additional or more detailed requirements above the Code, particularly on structural design, fatigue analysis, cargo piping flexibility, and liquefaction-plant integration. A ship that complies with the IGC Code minimum may not yet meet its class society’s rules for a specific design.

See also

• LNG Cargo Containment Systems
• LNG Carrier
• LNG as Marine Fuel
• LNG Fuel System
• IGF Code: Low-Flashpoint Fuel Ships
• SOLAS Chapter VII: Carriage of Dangerous Goods
• STCW Convention
• IBC Code
• Marine Inert Gas Systems
• Marine Cargo Tank Heating Systems
• IMDG Class 2 Gases

Calculators:

• IGC Tank Type A/B/C/Membrane classifier
• MARVS Safety Margin (IGC 8.2)
• IGC Type C Tank MARVS Check
• LNG Boil-Off Rate from Heat Ingress
• LNG Tank Cool-Down Time
• LNG Heel for Return Voyage
• LNG Tank Inerting Dilution Purge Volume
• LNG Tank Displacement Purge Volume
• LNG BOG Compressor Shaft Power
• LNG GCU Required Capacity
• Cofferdam Heating
• LPG Reliquefaction COP
• IMO IGC Code cargo lookup
• IMO IGF Code lookup