(a) Critical Temperature, Boiling Point, and Their Influence on Gas Carriage

Critical Temperature (Tc):

• The maximum temperature at which a gas can be liquefied by applying pressure alone.
• Above this temperature, no amount of pressure will convert the gas into a liquid.

Boiling Point (Tb):

• The temperature at which a liquid changes to vapour at a given pressure.
• At atmospheric pressure, this is called the normal boiling point.

Relation to Liquefied Gas Transportation

• If a gas has a critical temperature above ambient temperature, it can be liquefied by pressure alone and therefore carried in the fully pressurized condition.
  • Examples: Propane, Butane, Ammonia.
• If a gas has a critical temperature below ambient temperature, pressure alone cannot liquefy it, so it must be cooled below its boiling point and carried as a fully refrigerated liquid.
  • Examples: LNG (methane), Ethylene.

  Hence:

  Fully Pressurized Carriage

  • Gases with relatively high critical temperature and higher boiling points
  • Can remain liquid at normal temperatures under moderate pressure (~17 bar)
  • Example: Butane
    • Tc = 152°C
    • Tb = –0.5°C

    Fully Refrigerated Carriage

    • Gases with very low critical temperature and very low boiling points
    • Must be kept at low temperature and near-atmospheric pressure
    • Example: Methane (LNG)
      • Tc = –82°C
      • Tb = –162°C

      (b) Basic Construction Differences

      Feature	Fully Pressurized System	Fully Refrigerated System
      Operating Pressure	~17 bar	~0.25 bar
      Operating Temperature	Ambient	–48°C to –163°C
      Tank Type	Spherical or cylindrical independent tanks (Type C)	Prismatic independent tanks (Type A or B)
      Construction Material	High-strength carbon steel	Low-temperature steels (nickel steel, aluminum alloy)
      Insulation	Not required	Heavy insulation to limit heat ingress
      Cargo Handling Equipment	Simple piping, no refrigeration	Reliquefaction or refrigeration plant required
      Typical Vessel Size	Small (up to ~6,000 m³)	Large (up to 125,000 m³ or more)

      (c) Membrane vs Independent Tank Systems

      Feature	Membrane Tank System	Independent Tank System
      Definition	A thin membrane forms the cargo barrier, supported through insulation by the inner hull	A self-supporting tank independent of the ship’s hull
      Construction Material	Thin stainless steel or Invar (~1 mm) attached to insulation panels	Type A (prismatic), Type B (spherical), Type C (cylindrical)
      Support	Ship’s hull and insulation support the load	Tank supports its own load; hull provides only positioning/protection
      Space Efficiency	Very high — maximum utilization of hull volume	Lower — space wasted between tank and hull
      Maintenance	Repairs are complex; damage may require dry-docking	Easier to inspect and repair
      Typical Use	LNG carriers (GTT NO96, Mark III)	LPG carriers, small LNG carriers
      Examples	GTT membrane systems	Moss spherical (Type B), Type C tanks