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The International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) became mandatory under SOLAS Chapter VII, Part C for gas carriers constructed on or after 1 July 1986. It was comprehensively revised by Resolution MSC.370(93), adopted in 2014, with the revised Code entering into force on 1 July 2016. The revised Code applies to ships built on or after 1 July 2016; ships built before that date remain subject to the original 1983 IGC Code or the GC Code (for ships built before 1986).
The IGC Code establishes design, construction, equipment, and operational standards for gas carriers based on the properties of each cargo — its boiling point at atmospheric pressure, its vapour pressure at ambient temperature, its flammability and toxicity, and its reactivity with other substances. The Code assigns each cargo a required ship type and specifies the minimum containment system, safety equipment, and operational procedures.
Design pressure: Atmospheric only
Secondary barrier: Ship's hull structure forms the tank — no independent barrier
Typical use: Early LPG carriers; rare in modern construction
Hull stress and cargo pressure are directly linked. Only suitable for low-hazard cargoes at ambient or near-ambient temperatures. Largely superseded by Type A and membrane designs.
Design pressure: Atmospheric (low overpressure)
Secondary barrier: Full secondary barrier (the ship's inner hull). Primary barrier is a thin metallic membrane (Invar 36, or corrugated stainless steel — GTT NO96 or Mark III systems) supported by insulation.
Typical use: LNG carriers (the dominant design — Q-Flex, Q-Max, standard Moss/DFDE LNG vessels are often membrane type)
The membrane itself carries no structural load; it merely contains the cargo. The insulation system transfers hydrostatic and dynamic loads to the ship's structure. Requires excellent construction quality control and regular inspection of the membrane and insulation.
Design pressure: Near-atmospheric
Secondary barrier: Partial secondary barrier
Typical use: Some older LPG and ammonia carriers
The tank bottom and lower walls carry load; the upper corners are rounded. Less common than Type A, B, C, or membrane systems in modern fleet.
Design pressure: Atmospheric (≤ 0.7 bar gauge)
Secondary barrier: Partial secondary barrier required (drip tray or full hull)
Typical use: Traditional LPG carriers (VLGCs), butane/propane carriers
Prismatic or cylindrical tanks designed using classical ship-structural analysis. Design is based on recognised ship-structural standards. The secondary barrier covers the exposed bottom and sides of the tank.
Design pressure: Atmospheric or low pressure (≤ 0.7 bar gauge)
Secondary barrier: Partial secondary barrier (drip tray). Design by advanced analysis (finite element) to establish crack propagation characteristics — partial secondary barrier sufficient.
Typical use: Spherical Moss tanks on LNG carriers; some prismatic Type B
The defining feature of Type B is the use of detailed fatigue and fracture mechanics analysis to demonstrate that any crack would grow slowly enough to be detected before becoming critical. This permits a partial secondary barrier (a drip tray) rather than a full barrier.
Design pressure: High pressure — design pressure typically ≥ 2 bar gauge (up to ~18 bar for fully refrigerated LPG at ambient temperature)
Secondary barrier: No secondary barrier required. The tank pressure vessel design is sufficiently robust.
Typical use: Semi-pressurised LPG carriers, small LNG bunker vessels, pressurised ammonia carriers, ethylene carriers
Designed and tested as pressure vessels under recognised pressure vessel codes (e.g., ASME, EN 13445). The high pressure limits the volume-to-weight ratio, making Type C tanks common for smaller vessels and specialised cargoes.
Pressure: Atmospheric (in membrane/Moss tanks)
Hazards: Cryogenic burns; rapid phase transition (RPT) on water contact; explosive vapour (flammable limits 5–15% v/v); asphyxia in enclosed spaces
Ship type: Primarily membrane (GTT systems) or Type B spherical (Moss); Type C for small-scale LNG
Pressure: Atmospheric (refrigerated) to ~8.5 bar gauge (ambient temperature)
Hazards: Flammable (2.1–9.5% v/v); denser than air — accumulates in low spaces; cryogenic burns if refrigerated
Ship type: Type A or Type B prismatic (VLGCs/LGCs) for large fully-refrigerated; Type C for semi-pressurised
Pressure: Atmospheric (refrigerated) to ~2.5 bar gauge (ambient temperature)
Hazards: Flammable (1.4–8.5% v/v); denser than air; lower vapour pressure than propane makes it relatively easier to handle at near-ambient
Ship type: Same as propane — often carried on the same vessel (split cargo/bi-propellant trade)
Pressure: Atmospheric (refrigerated) to ~10 bar gauge (ambient)
Hazards: Acutely toxic (IDLH 300 ppm; lethal at higher concentrations); corrosive to eyes and respiratory tract; flammable (15–28% v/v); MARPOL Category Y/X depending on release scenario
Ship type: Type A (refrigerated large carriers) or Type C (semi-pressurised). Crew must hold STCW V/1-1 chemical tanker endorsement in addition to gas tanker training for ammonia.
Pressure: Atmospheric (fully refrigerated)
Hazards: Deep cryogenic; extreme flammability (2.7–36% v/v); asphyxia; requires 9% nickel steel or aluminium alloy tank materials
Ship type: Type C (small quantities) or specialised fully-refrigerated ethylene carriers with enhanced containment
Pressure: Atmospheric to ~3.4 bar
Hazards: Highly carcinogenic (IARC Group 1); flammable (3.6–33% v/v); polymerisation hazard if contaminated with oxygen or certain metals
Ship type: Type C (pressure vessels) with inhibitor injection systems to prevent polymerisation
Pressure: Atmospheric (refrigerated) to ~10 bar (ambient)
Hazards: Flammable (2–11.1% v/v); polymerisation hazard; asphyxia
Ship type: Type A, B, or C depending on scale and trade
All cryogenic liquefied gas cargoes generate boil-off gas (BOG) during the voyage as heat ingress through the insulation vaporises a small fraction of the cargo. BOG management is a core operational challenge on gas carriers:
Required on ships carrying flammable cargoes. Nitrogen or combustion products are used to displace hydrocarbon vapours from tank atmospheres during gassing-up, cooldown, and warming-up operations, and to maintain inert atmosphere in void spaces and insulation annular spaces. Continuous monitoring of oxygen content is required.
Fixed gas detection in all cargo machinery spaces, compressor rooms, control rooms, and on the main deck. Detector heads must be calibrated for the specific cargo vapour. Two alarm levels: lower explosive limit (LEL) warning and high-level shutdown.
Shore-terminal and ship ESD systems are linked at the manifold. Activation of either side shuts the manifold valves and the ship's cargo pumps within the prescribed time (typically ≤ 30 seconds for all manifold valves to close). ESD systems are tested before each cargo transfer.
Most membrane LNG carriers and some LPG carriers are equipped with a reliquefaction plant (or use boil-off gas as engine fuel in DFDE or ME-GI propulsion). The reliquefaction plant condenses boil-off gas (BOG) and returns it to the cargo tank, preventing vapour loss and maintaining tank pressure within design limits.
Required for membrane and Type A tanks. The secondary barrier must be capable of containing any cargo leakage for a minimum period (typically 15 days) without impairing structural integrity, to allow voyage completion to a repair facility. The interbarrier space is monitored for cargo vapour or liquid ingress.
Each cargo tank has one or more pressure relief valves (PRVs) set at the maximum allowable relief valve setting (MARVS). PRVs vent to the mast riser or gas combustion unit (GCU) — not directly to atmosphere where the vapour could ignite or cause harm on deck.
STCW Regulation V/1-2 requires specific training for personnel serving on gas carriers:
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