Answer
They are universally categorized into three distinct operational types, with a fourth sub-category defined by size and commercial trading profile.
1. Fully Pressurized Ships
These are the simplest gas carriers in terms of thermodynamic control, designed to carry cargo at ambient temperatures by maintaining high internal pressures.
Design Pressure (MARVS): Typically between 15 bar and 18 bar (can extend up to 20 bar).
Design Temperature: Ambient (no thermal insulation or refrigeration system required).
Containment System: Exclusively IGC Type C independent tanks. These are heavy, thick-walled cylindrical or spherical pressure vessels constructed from carbon-manganese steel.
Size & Trade Profile: Typically small, ranging from 500 m³ to 4,000 m³. They are primarily used for coastal regional trading and short-sea distribution.
Operational Advantage: Highly robust. Since there is no reliquefaction plant or insulation, maintenance is minimal, and cargo can be loaded/discharged at ambient shoreside facilities without temperature management constraints.
2. Semi-Refrigerated / Semi-Pressurized Ships
These vessels offer the highest operational flexibility in the gas fleet. They use a combination of moderate pressurization and partial refrigeration to reduce tank wall thickness and increase carrying capacity.
Design Pressure (MARVS): Typically between 5 bar and 8 bar.
Design Temperature: Minimum design temperatures down to -48°C (allowing them to carry fully refrigerated Propane).
Containment System: Primarily IGC Type C independent tanks, frequently configured as bi-lobe tanks to optimize hull space utilization. Tanks are insulated with polyurethane foam.
Reliquefaction Plant: Equipped with a medium-sized reliquefaction plant to cool and condense boil-off gas (BOG) back into the tanks.
Size & Trade Profile: Ranging from 3,000 m³ to 20,000 m³.
Operational Advantage: Extreme commercial flexibility. They can accept cargo from fully pressurized shore terminals, cool it down during the voyage, and discharge it into fully refrigerated atmospheric tanks at the destination, or vice versa.
3. Fully Refrigerated Ships (LGCs and VLGCs)
Designed to carry massive volumes of gas at atmospheric pressure by keeping the cargo cooled to its boiling point. This is the dominant ship category for trans-oceanic gas transport.
Design Pressure (MARVS): Very low, typically below 0.3 bar to 0.7 bar (essentially atmospheric).
Design Temperature: Down to -48°C for Propane and -33°C for Ammonia.
Containment System: Primarily IGC Type A independent tanks. These are prismatic, non-pressure vessels that maximize hull volume. They require low-temperature steel (e.g., fine-grained C-Mn steel) and a complete secondary barrier (the inner hull) capable of safely containing the cargo for 15 days in the event of a primary tank leak. Note: Some modern designs utilize Type B prismatic tanks to employ the "leak-before-failure" principle, requiring only a partial secondary barrier.
Reliquefaction Plant: Features massive, multi-stage cargo reliquefaction plants running continuously to handle heavy boil-off rates.
Size Profile: * Large Gas Carriers (LGC): 20,000 m³ to 50,000 m³.
Very Large Gas Carriers (VLGC): 50,000 m³ to over 85,000 m³.
Operational Advantage: High volumetric efficiency. Carrying cargo at its boiling point eliminates the dead-weight penalty of thick pressure-vessel walls, maximizing the economy of scale for long voyages.
Summary Technical Matrix for Oral Defense
| Feature | Fully Pressurized | Semi-Refrigerated | Fully Refrigerated (VLGC) |
|---|---|---|---|
| IGC Tank Type | Type C (Cylindrical / Spherical) | Type C (Cylindrical / Bi-lobe) | Type A or Type B (Prismatic) |
| Max Pressure (MARVS) | 15–18 bar | 5–8 bar | < 0.3–0.7 bar |
| Min Temperature | Ambient | -48°C | -48°C (Propane) / -33°C (Ammonia) |
| Secondary Barrier | None required | None required | Full Secondary Barrier (Type A) / Partial (Type B) |
| Reliquefaction Unit | None | Medium Capacity | Heavy-Duty Multi-Stage |
| Typical Capacity | 500 – 4,000 m³ | 3,000 – 20,000 m³ | 50,000 – 85,000+ m³ |
| Wall thickness | 20–40 mm; | 12–30 mm | 10–25 mm |
Key Cross-Examination Questions to Expect from the Surveyor
If you list these types smoothly, the MMD surveyor will likely follow up with these specific management-level engineering questions:
"Why can Type C tanks omit a secondary barrier?"
Answer: Because Type C tanks are designed as true pressure vessels under design codes that ensure stress levels remain well below the fatigue limits of the material. The probability of a catastrophic, sudden failure is statistically negligible.
"What happens to the hull structure if a Type A primary tank fails?"
Answer: If the primary tank leaks, the cargo is caught by the full secondary barrier (the inner hull). The hold space insulation and the use of specialized low-temperature steels prevent the cryogenic cargo from contacting and embrittling the ship's primary structural hull girder, avoiding structural failure.
"Can a Fully Refrigerated VLGC accept cargo from a Fully Pressurized shore tank?"
Answer: No, not directly or at a standard loading rate. The VLGC’s Type A prismatic tanks cannot withstand the vapor pressure of ambient-temperature LPG. Loading warm cargo would over-pressurize the tanks instantly and exceed the capacity of the ship’s reliquefaction plant. The cargo must be pre-cooled by a shoreside refrigeration facility before loading.