Answer
Physical Properties
Composition: Primarily Methane (CH4, >85–95%) with trace amounts of ethane, propane, nitrogen, and carbon dioxide.
State & Temperature: Cryogenic liquid stored at boiling point, approximately −162∘C (111 K) at atmospheric pressure.
Density: ~420–460 kg/m3 in liquid state (less than half the density of water).
Volumetric Expansion: Ratio of 1:600 (1 volume unit of liquid LNG expands into 600 volume units of Natural Gas at standard temperature and pressure).
Flammability Range: 5% to 15% concentration by volume in air. Non-flammable in pure liquid state.
Auto-ignition Temperature: ∼580∘C (significantly higher than MGO/HFO).
Vapor Density: Cold methane vapor at −162∘C is heavier than air (1.5× relative density) and stays low. As it warms above −110∘C, it becomes lighter than air (0.55× relative density) and rapidly dissipates upwards.
Specific Heat / Boiling Point: Highly prone to boil-off gas (BOG) generation due to continuous heat ingress.
Storage Systems (IGF Code Requirements) LNG fuel storage tanks are governed strictly by Chapter 6 of the IMO IGF Code (International Code of Safety for Ships Using Gases or Other Low-Flashpoint Fuels).
Type C Independent Tanks (Most Common Onboard Ships):
Pressure vessels designed using standard pressure vessel codes (typically operating up to 10 bar design pressure).
Feature a double-walled construction with vacuum insulation and perlite to minimize heat transfer.
Do not require a secondary containment barrier if designed with low stress parameters under IGF Code provisions.
Engine Room Integration & Operation
To comply with IGF Code standards (ensuring equivalent safety to conventional oil-fueled machinery spaces), engine rooms utilize specific configurations:
Gas-Safe Engine Room Concept:
The Machinery Space is designated as a safe area under normal operating conditions.
Double-Walled Gas Piping: All fuel gas supply pipes running inside the Engine Room MUST be double-walled. The inner pipe carries high- or low-pressure gas, while the outer duct is continuously continuously mechanically ventilated at 30 air changes per hour.
Gas detection sensors are installed inside the outer duct annulus. If gas leaks exceed 20% LEL (Lower Explosive Limit), automatic safety shut-off valves isolate the line.
Fuel Gas Handling & Supply Chain in ER:
Bunkering & Storage: Liquid LNG is bunkered into the Type C tank at −162∘C.
Vaporization (Fuel Gas Supply System - FGSS): Cryogenic pumps force LNG through a water-glycol heated main fuel vaporizer to warm the gas to ≈+20∘C before entry into the engine room.
Gas Valve Unit (GVU): Located in a dedicated gas-tight room or inside an ATEX-certified double-enclosure (GVU-ED) right next to the engine. Houses the Double Block and Bleed valve arrangement for immediate shutdown and line venting.
Combustion (Dual Fuel Engines): Gas is injected into 4-Stroke (Low-Pressure Otto Cycle) or 2-Stroke (High-Pressure Diesel Cycle - ME-GI at ~300 bar, or Low-Pressure X-DF at ~16 bar) engines along with a micro-pilot diesel injection for ignition.
Boil-Off Gas (BOG) Management:
Natural heat ingress continuously generates BOG inside the storage tank.
BOG is managed via:
Direct utilization in Dual-Fuel Main Engines/Auxiliary Engines or Boilers.
Gas Combustion Unit (GCU) to safely burn excess gas when engines are idle.
Onboard Re-liquefaction Plants (compressing and cooling gas back to liquid phase).
Pressure accumulation inside Type C tanks (allowing tank pressure to rise up to design limits during port stays).
- Safety Safeguards:
- Double Block and Bleed Valves: Automatically isolate gas supply and vent trapped gas in emergency shut-off conditions.
- Gas Safe Engine Room: Double-walled pipes ensure single failure does not leak gas into the space.
- ESD-Protected Engine Room: Uses gas detection to trip power and gas supply instantly in spaces with single-walled piping.
- BOG Management: Boil-Off Gas managed via Gas Combustion Units (GCU), re-liquefaction plants, or dual-fuel auxiliary engines.