Liquefied Gas Carriers & Dual-Fuel Ships · IGC Code & IGF Code

IGC Code Gas Carrier & IGF Code LNG Bunkering Safety Checklist (MSC.370(93) & MSC.391(95))

Q: How do you calculate the maximum loading limit (LL) of a liquefied gas tank under IGC Code Chapter 15?

Under IGC Code Chapter 15 (IMO Resolution MSC.370(93)), the default Filling Limit (FL) is 98% of tank volume at the reference temperature (the temperature corresponding to the saturated vapour pressure of the cargo at the pressure relief valve set pressure, MARVS). When loading at a cooler cargo temperature where liquid density (ρ_L) is higher than the density at the reference temperature (ρ_R), the maximum allowable Loading Limit is LL = FL × (ρ_R / ρ_L), ensuring the tank never exceeds 98% liquid full if the cargo warms up to the relief valve set pressure.

Q: What is the maximum ESD valve closure time required by the IGC Code and IGF Code during gas transfer or LNG bunkering?

Both the 2014 Revised IGC Code (§ 18.10.2.1.3, IMO Resolution MSC.370(93)) for gas carriers and the International Code of Safety for Ships Using Gases or Other Low-Flashpoint Fuels (IGF Code § 16.7.3.6, IMO Resolution MSC.391(95)) for LNG-fuelled vessels require Emergency Shutdown (ESD) valves in liquid piping to close fully and smoothly within 30 seconds of actuation. The IGC Code also requires the ESD system to be designed to avoid surge pressures in the cargo transfer pipework, and the ship’s ESD to include a ship-shore link.

Q: What STCW endorsements are required for crew serving on IGC gas carriers versus IGF gas-fuelled ships?

Officers and ratings with immediate responsibility for cargo or cargo equipment on liquefied gas tankers must hold an STCW Regulation V/1-2 Certificate of Proficiency in Basic or Advanced Training for Liquefied Gas Tanker Cargo Operations (STCW Code Section A-V/1-2). Seafarers serving on non-tanker ships fuelled by LNG or other low-flashpoint fuels covered by the IGF Code must hold Basic or Advanced Training for Ships Subject to the IGF Code under STCW Regulation V/3 (STCW Code Section A-V/3), though a valid STCW Reg. V/1-2 liquefied gas tanker certificate is recognized under Reg. V/3.10 as meeting the IGF training standard.

Q: What specific information must be recorded on an LNG Bunker Delivery Note (LNG-BDN) under the IGF Code?

Under IGF Code Part C-1 § 18.4 (IMO Resolution MSC.391(95)) and MARPOL Annex VI Regulation 18.4, the LNG Bunker Delivery Note (LNG-BDN) signed before and after bunkering must document the sulphur content, density, lower calorific value (MJ/kg), Methane Number (MN — critical to prevent knock and misfire in dual-fuel engines), and molar composition percentages of methane, ethane, propane, butane, nitrogen and any contaminants, together with pre-bunkering ESD link test records and purge verification.

Liquefied gas carriers (LNG, LPG, ethylene, ammonia and petrochemical gases) and dual-fuel ships using LNG or low-flashpoint fuels are audited under two tightly linked mandatory SOLAS codes: the 2014 Revised IGC Code (IMO Resolution MSC.370(93), mandatory under SOLAS Chapter VII Part C) and the IGF Code (IMO Resolution MSC.391(95), mandatory under SOLAS Chapter II-1 Part G). This guide covers Ship Types 1G–3G, Type A/B/C and membrane containment barriers, the 98% loading limit formula, independent overflow and 30-second ESD valve tests, 30% LFL gas detection, 15-day IGF tank hold time, and LNG bunkering ship-shore checklists.

Updated 22 October 2026 · Written by SeaReady in our own words. Not affiliated with OCIMF, RightShip, CDI, any MoU or any administration — always check the current official documents.

1. IGC Code Ship Types (1G, 2G, 2PG & 3G) & Containment Systems

Every gas carrier engaged on international voyages must hold a valid International Certificate of Fitness for the Carriage of Liquefied Gases in Bulk (IGC Code Chapter 1 § 1.4, IMO Resolution MSC.370(93)) listing each approved liquefied gas product from Chapter 19 and any operational temperature/pressure conditions. The IGC Code classifies gas carriers into four survival and tank-location standards based on product hazard:

IGC Code Ship Type	Product Hazard Scope (Ch. 19 Column c)	Tank Location & Damage Survival Standard
Type 1G Gas Carrier (IGC Code Ch. 2 § 2.1.2.1)	Products requiring maximum preventive measures to preclude the escape of such cargo (most severe environmental/toxicity/flammability hazard)	Cargo tanks must be located at least B/5 or 11.5 m (whichever is less) inboard from the side shell at the summer load line, and at least B/15 or 2.0 m above the moulded baseline, with full Type 1G two-compartment / high-energy damage survival capability across the ship's length.
Type 2G Gas Carrier (IGC Code Ch. 2 § 2.1.2.2)	Products requiring significant preventive measures to preclude the escape of such cargo (e.g. LNG, LPG, ethylene, ammonia, vinyl chloride)	Cargo tanks must be located at least 0.8 m (800 mm) inboard from the side shell and at least B/15 or 2.0 m above the baseline. Standard containment for large membrane and independent Type A / Type B / Type C LNG and LPG carriers.
Type 2PG Gas Carrier (IGC Code Ch. 2 § 2.1.2.3)	Ship of 150 m in length or less designed to carry products required to be carried in a Type 2G ship inside Independent Type C pressurized tanks	Applies where cargo is carried in Independent Type C tanks designed for a MARVS of at least 0.7 MPa (7 bar gauge) and a cargo containment system design temperature of -55°C or above, allowing reduced survival extent compared to large Type 2G vessels.
Type 3G Gas Carrier (IGC Code Ch. 2 § 2.1.2.4)	Products requiring moderate preventive measures to preclude the escape of such cargo (e.g. refrigerated nitrogen or lower-hazard gases)	Cargo tanks located at least 0.8 m (800 mm) inboard from the side shell and at least B/15 or 2.0 m above the baseline, with damage survival standards tailored to ships ≥ 80 m or < 80 m.

Under IGC Code Chapter 4, cargo containment systems are categorized as Independent Tanks Type A (prismatic self-supporting tanks designed by classical structural analysis, requiring a complete secondary barrier when cargo temperature is below -10°C), Independent Tanks Type B (spherical Moss or prismatic IHI-SPB tanks designed using fracture mechanics and crack-propagation model tests, requiring a partial secondary barrier and small-leak protection system), Independent Tanks Type C (pressurized cylindrical or bilobe vessels designed to pressure-vessel criteria where crack propagation is precluded, requiring no secondary barrier), and Membrane Tanks (thin primary metallic membrane 0.7–1.5 mm Invar or corrugated stainless steel supported through load-bearing insulation by the inner hull with a complete secondary barrier).

2. 98% Filling Limit, Independent Overflow, 30 s ESD & Gas Detection

Both Port State Control officers (under psc:cert-gs01 and psc:cargo-gs01..gs09) and vetting inspectors (SIRE 2.0 and CDI) test the physical safety barriers that prevent liquid overfill, cryogenic hull embrittlement, and vapour ignition:

Safety Barrier & Code Reference	Statutory Threshold / Timing	Operational & PSC Verification
Filling Limit & Loading Limit Formula (IGC Code Ch. 15 § 15.3–15.5 / IGF Code § 6.8.1–6.8.2)	98% default filling limit at reference temperature (LL = FL × ρ_R / ρ_L); any specially allowed higher IGF loading limit is never above 95%	Under IGC Code Ch. 15 (IMO Res. MSC.370(93)), the default filling limit (FL) of a cargo tank is 98% at the reference temperature (the temperature corresponding to the cargo vapour pressure at the PRV set pressure, MARVS); a higher filling limit is permitted only under the conditions in § 15.4. The maximum loading limit is LL = FL × (ρ_R / ρ_L) (§ 15.5.1). Under IGF Code § 6.8.1 (IMO Res. MSC.391(95)), liquefied gas fuel tanks are not filled beyond 98% at the reference temperature, with a loading limit curve from the same formula; § 6.8.2 lets the Administration specially allow a higher loading limit where external fire heating is very unlikely or a second pressure-maintenance system is fitted, but never above 95%.
Independent High-Level Alarm & Overflow Shutdown (IGC Code Ch. 13 § 13.3 / IGF Code § 15.4.2)	High liquid level alarm + a separate independent sensor that automatically closes the shutoff valve before the tank becomes liquid full	Every liquefied gas cargo tank and IGF LNG bunker tank must be fitted with a high liquid level alarm operating independently of other liquid level indicators, plus a second independent sensor that automatically actuates the shutoff valve in a manner that avoids excessive surge pressure in the loading/bunkering pipeline and prevents the tank from becoming liquid full.
Emergency Shutdown (ESD) System & Fire Detection (IGC Code Ch. 18 § 18.10 / IGF Code § 16.7.3.6)	ESD valves in liquid lines close fully and smoothly within 30 seconds of actuation; fire detection on deck (e.g. fusible plugs) triggers the ESD	Under IGC Code § 18.10.2.1.3 and IGF Code § 16.7.3.6, ESD valves in liquid (liquefied gas) piping must close fully and smoothly within 30 seconds of actuation, and information on their closure time must be kept on board. The ESD stops cargo pumps and compressors, the ship’s ESD includes a ship-shore link (IGC § 18.10.1.4), and the system is designed to avoid surge pressures in the transfer pipework. Besides the manual ESD pushbuttons, fusible plugs, electronic point temperature monitoring or area fire detection on deck can trigger the ESD.
Fixed & Portable Gas Detection & Hold/Interbarrier Pressure Hold (IGC Code Ch. 13 § 13.6 / IGF Code § 6.9.1.1 & § 15.8)	Fixed gas detection alarm at ≤ 30% LFL (shutoff at ≤ 60% LFL in double-wall gas piping); 15-day IGF fuel tank pressure hold without venting	Fixed gas detection in hold spaces, interbarrier spaces, compressor rooms, electric motor rooms, cargo control rooms and airlocks must activate audible/visual alarms at ≤ 30% LFL. Under IGF Code § 6.9.1.1, Liquefied gas fuel tanks must be capable of holding boil-off pressure for at least 15 days from full filling level without venting to atmosphere.

You can verify exact cargo tank loading limits (LL = 98% × ρ_R / ρ_L), liquid mass, and vapour space mass before every loading operation in the Liquefied Gas Cargo Calculator.

3. IGC Code Chapter 14 Personnel Protection, Airlocks & Water Spray

Water-Spray System (IGC Code Ch. 11 § 11.3): Gas carriers must have a fixed water-spray system covering exposed cargo tank domes, liquid/vapour manifolds, control valves, and boundaries of superstructures, compressor rooms and cargo control rooms facing the cargo area (delivering ≥ 10 L/m²/min on horizontal surfaces and ≥ 4 L/m²/min on vertical surfaces), plus a fixed dry chemical powder system (§ 11.4) protecting the manifold and deck cargo piping area.
Motor-Room Airlocks & Differential Pressure (IGC Code Ch. 3 § 3.6): Access between a gas-dangerous zone on the open weather deck and a gas-safe space (such as an electric motor room or cargo control room) must be through a mechanically ventilated two-door airlock fitted with audible and visual alarms on both sides that sound if more than one door is moved from the closed position or if positive overpressure is lost (which triggers automatic electrical de-energization of non-certified equipment).
Safety Equipment Sets & Medical First Aid (IGC Code Ch. 14): Ships must carry sufficient protective clothing for crew engaged in loading/discharging, plus at least 3 complete sets of safety equipment (Wait-independent SCBA of ≥ 1,200 L free air capacity, tight-fitting goggles, gas-tight chemical/cryogenic protective suit, electrically non-conductive boots and gloves, steel-core safety belt/lifeline, and explosion-proof lamp) together with either 8 fully charged spare air bottles or a special low-pressure air line + breathing-air compressor, plus respiratory/eye escape sets in every crew cabin for toxic cargoes (Ch. 19 Column n) and freeze-protected decontamination showers and eyewash on deck.

4. IGF Code LNG Bunkering Safety Checklist (SOLAS II-1 Part G)

For dual-fuel container ships, car carriers, bulkers, tankers, and passenger vessels operating under the IGF Code (IMO Resolution MSC.391(95)), pre-bunkering and in-service machinery inspections focus on:

Pre-Bunkering Ship-Shore / Ship-to-Ship Safety Checklist (IGF Code § 18.4): Verify the controlled Bunkering Safety Zone around the bunker station, drip-tray water curtain / cryogenic hull protection beneath the manifold, electrical insulation flange or bonding Policy, nitrogen purging and leak testing of hoses/arms, and live testing of the linked optical/electric/pneumatic ESD1 (manifold shutdown within ≤ 30 s) and ESD2 (emergency dry-break coupler release) links.
Double-Wall Gas Piping & Fuel Preparation Room Ventilation (IGF Code Ch. 9 & Ch. 13): All gas fuel piping passing through enclosed machinery spaces must be enclosed in a ventilated double-wall pipe or duct (extracted at ≥ 30 air changes/hour with nitrogen inerting or continuous gas detection triggering automatic Master Gas Valve and double-block-and-bleed shutdown at ≤ 60% LFL).
LNG Bunker Delivery Note (LNG-BDN) & STCW Reg. V/3 Certificates: Retain the signed LNG-BDN on board recording sulphur content, density, lower calorific value, gas composition, and Methane Number (MN), and verify that all officers and ratings assigned fuel-handling or engine watchkeeping duties hold valid STCW Regulation V/3 Basic or Advanced IGF Code Certificates of Proficiency in the PSC Inspection Workspace.

For machinery-side troubleshooting on 4-stroke dual-fuel engines, Gas Valve Units (GVU), and Fuel Gas Supply Systems (FGSS), see Marvel Solutionz’s Dual-Fuel Engine Gas-to-Diesel Switching & Fast Gas Trip Checklist (opens in a new tab) and LNG FGSS & GVU Double-Block-and-Bleed Purge Checklist (opens in a new tab).

Questions seafarers ask

How do you calculate the maximum loading limit (LL) of a liquefied gas tank under IGC Code Chapter 15?

Under IGC Code Chapter 15 (IMO Resolution MSC.370(93)), the default Filling Limit (FL) is 98% of tank volume at the reference temperature (the temperature corresponding to the saturated vapour pressure of the cargo at the pressure relief valve set pressure, MARVS). When loading at a cooler cargo temperature where liquid density (ρ_L) is higher than the density at the reference temperature (ρ_R), the maximum allowable Loading Limit is LL = FL × (ρ_R / ρ_L), ensuring the tank never exceeds 98% liquid full if the cargo warms up to the relief valve set pressure.

What is the maximum ESD valve closure time required by the IGC Code and IGF Code during gas transfer or LNG bunkering?

Both the 2014 Revised IGC Code (§ 18.10.2.1.3, IMO Resolution MSC.370(93)) for gas carriers and the International Code of Safety for Ships Using Gases or Other Low-Flashpoint Fuels (IGF Code § 16.7.3.6, IMO Resolution MSC.391(95)) for LNG-fuelled vessels require Emergency Shutdown (ESD) valves in liquid piping to close fully and smoothly within 30 seconds of actuation. The IGC Code also requires the ESD system to be designed to avoid surge pressures in the cargo transfer pipework, and the ship’s ESD to include a ship-shore link.

What STCW endorsements are required for crew serving on IGC gas carriers versus IGF gas-fuelled ships?

Officers and ratings with immediate responsibility for cargo or cargo equipment on liquefied gas tankers must hold an STCW Regulation V/1-2 Certificate of Proficiency in Basic or Advanced Training for Liquefied Gas Tanker Cargo Operations (STCW Code Section A-V/1-2). Seafarers serving on non-tanker ships fuelled by LNG or other low-flashpoint fuels covered by the IGF Code must hold Basic or Advanced Training for Ships Subject to the IGF Code under STCW Regulation V/3 (STCW Code Section A-V/3), though a valid STCW Reg. V/1-2 liquefied gas tanker certificate is recognized under Reg. V/3.10 as meeting the IGF training standard.

What specific information must be recorded on an LNG Bunker Delivery Note (LNG-BDN) under the IGF Code?

Under IGF Code Part C-1 § 18.4 (IMO Resolution MSC.391(95)) and MARPOL Annex VI Regulation 18.4, the LNG Bunker Delivery Note (LNG-BDN) signed before and after bunkering must document the sulphur content, density, lower calorific value (MJ/kg), Methane Number (MN — critical to prevent knock and misfire in dual-fuel engines), and molar composition percentages of methane, ethane, propane, butane, nitrogen and any contaminants, together with pre-bunkering ESD link test records and purge verification.

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