IGC

Answer

The IGC Code (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk) is a mandatory framework under SOLAS Chapter VII (Carriage of Dangerous Goods). It governs gas carriers transporting liquefied gases with a vapor pressure exceeding 2.8 bar absolute at a temperature of 37.8°C.

(Liquid substances transporting in bulk with a vapor pressure less than or equal to 2.8 bar absolute at a temperature of 37.8°C (100⁰ F) are governed by the IBC Code (or the BCH Code for legacy vessels).

The threshold of 2.8 bar absolute at 37.8°C is the exact legal boundary separating liquid chemicals from liquefied gases in maritime law)

The Code covers:

1. Ship survival capability and cargo tank location
2. Ship arrangements
3. Cargo containment
4. Process pressure vessels and piping
5. Materials of construction
6. Cargo pressure/temperature control
7. Cargo vent systems
8. Environmental control
9. Electrical installations
10. Fire protection and extinction
11. Mechanical ventilation
12. Instrumentation and automation
13. Personnel protection
14. Filling limits
15. Use of cargo as fuel
16. Special requirements
17. Operating requirements
18. Summary of minimum requirements
19. List of products carried under the Code

Chapter 19 is particularly important because it lists the products and their applicable carriage requirements.

Ship Types Under the IGC Code

The Code categorizes vessels based on the severity of the hazard posed by the cargo:

• Type 1G: Designed to carry cargoes requiring maximum preventive measures against leakage (e.g., Chlorine, Methyl Bromide, Sulphur Dioxide). Requires maximum damage stability survival capability and cargo tank location set far inboard from the shell plating.
• Type 2G: Designed to carry products requiring significant preventive measures to preclude leakage (e.g., Ethylene, Refrigerated LPG, LNG).
• Type 2PG: Vessel length ≤ 150 m designed for pressurized cargoes requiring Type 2G safety levels (e.g., Semi-pressurized/Fully-pressurized LPG carrying C-type tanks with operating pressure ≥7 bar).
• Type 3G: Designed for products requiring moderate safety measures (e.g., Nitrogen, specific refrigerant gases).

Cargo Containment Systems

The IGC Code categorizes cargo tanks into two main classifications:

Cargo Containment Systems

┌──────────────────────────┴──────────────────────────┐

Independent Tanks Integrated/Membrane

(Self-supporting, not part ( Non-self-supporting,

of ship hull structure) transfers loads to hull)

│ │

┌─────────┼─────────┐ ┌─────────┴─────────┐

Type A Type B Type C Membrane Semi-Membrane

• Type A Independent: Deep-refrigerated cargoes (e.g., LPG at -50⁰C). Requires a complete secondary barrier capable of holding leaked cargo for 14 days if the primary tank fails.
• Type B Independent: Designed using advanced stress analysis ("leak-before-failure" concept, e.g., Kvaerner-Moss spherical LNG tanks). Requires only a partial secondary barrier (drip tray and splash barrier).
• Type C Independent: Pressure vessels (spherical or cylindrical, design pressure > 2bar. No secondary barrier required as catastrophic failure is considered improbable.
• Membrane Tanks: Very thin membrane (Invar, stainless steel) supported by load-bearing insulation attached directly to the inner hull (e.g., GTT No. 96, Mark III systems). Complete secondary barrier required.

Operational Safety Principles

1. Gas-Safe vs. Gas-Dangerous Spaces: Absolute physical isolation via positive-pressure airlocks (minimum 2 self-closing doors, continuous mechanical air extraction with at least 10–20 air changes/hour).
2. Inerting & Purging: Oxygen level must be maintained below 8% by volume (typically < 1–2% for cargo tanks) using an Onboard Inert Gas Plant prior to loading gas vapor.
3. Emergency Shutdown System (ESD): Linked onboard ESD valves shut down cargo operations upon detecting high tank level, low pressure, loss of control air, power failure, or manual activation at deck stations.
4. Dual-Fuel Engines (ME-GI / X-DF / Boilers): Double-walled gas fuel supply piping with inert gas (Nitrogen) purging in the outer annulus, continuously monitored for gas leaks.