Answer
Under Chapter 4 of the IMO IGC Code, independent tanks are defined as self-supporting cargo containment systems that do not form part of the ship’s hull and do not contribute to the hull girder strength.
They are categorized into Type A, Type B, and Type C based on their design text, structural analysis, thermodynamic operating limits, and the requirements for a secondary barrier.
1. Type A Independent Tanks
Type A tanks are traditional, non-pressure vessels designed using classical ship structural/naval architecture principles.
Design Criteria: Designed primarily using standard structural analysis (mechanics of materials and classification society rules) rather than extensive fatigue lifecycle modeling.
Design Pressure (MARVS): Very low. The Maximum Allowable Relief Valve Setting is typically less than 0.3 bar (can occasionally be up to 0.7 bar under strict conditions).
Secondary Barrier: Mandatory FULL Secondary Barrier. Because the structural analysis cannot definitively guarantee that a crack won't propagate catastrophically, the ship must have a complete secondary containment system (often the inner hull jacket combined with special cryogenic insulation). It must be capable of safely containing the entire volume of leaked liquid cargo for a minimum of 15 days.
Shape: Primarily Prismatic (box-like with sloped wings), geometrically optimized to maximize the internal volumetric efficiency of the cargo hold.
Application: Dominant in Fully Refrigerated Very Large Gas Carriers (VLGCs) carrying Propane (at -42°C) or Ammonia (at -33°C).
Operational Note: The hold space surrounding a Type A tank must be completely inerted (filled with Nitrogen) if carrying flammable cargoes, to prevent an explosive atmosphere in case of a primary barrier leak.
2. Type B Independent Tanks
Type B tanks are highly sophisticated containment systems designed using advanced analytical tools, finite element analysis (FEA), and extensive model testing.
Design Criteria: Built on the "Leak-Before-Failure" principle. The design uses fracture mechanics to prove that if a fatigue crack develops in the tank shell, the rate of crack propagation will be exceptionally slow. This ensures the leak will be detected by the gas monitoring system long before it can cause a catastrophic failure of the tank structure.
Design Pressure (MARVS): Low pressure, typically less than 0.3 bar for prismatic types, though spherical types can handle slightly higher pressures.
Secondary Barrier: PARTIAL Secondary Barrier Only. Because a sudden, massive failure is ruled out by the "Leak-Before-Failure" analytics, a full secondary barrier is not legally required. Instead, it requires a partial secondary barrier consisting of a Drip Tray beneath the tank, accompanied by splash/spray shields to guide any minor leak safely into the tray without touching the primary ship structure.
Shape: Can be Spherical (the iconic Moss-Rosenberg systems used on LNG carriers) or Prismatic (such as the IHI SPB design used in large LNG/LPG installations).
Application: Widely used on large LNG carriers and specialized, modern VLGCs where structural reliability is paramount.
Operational Note: The interbarrier hold spaces are continuously monitored for hydrocarbon vapors. Since a leak would be small, the gas detection system acts as the primary safety sentinel.
3. Type C Independent Tanks
Type C tanks are built as true, heavy-duty pressure vessels complying with recognized international engineering standards (such as ASME Section VIII or BS 5500).
Design Criteria: Designed using modified pressure vessel formulas where the internal design vapor pressure dominates the structural dimensions. Dynamic loads from ship motions are factored in, but stress limits are kept so conservative that crack propagation is statistically negligible.
Design Pressure (MARVS): High pressure. Typically ranges from 5 bar to 8 bar on semi-refrigerated ships, and up to 15 bar to 18 bar on fully pressurized vessels.
Secondary Barrier: NONE Required. Because these tanks are designed and constructed as robust pressure vessels with high safety margins, the probability of a sudden or progressive failure is considered negligible by the IGC Code.
Shape: Cylindrical, Spherical, or Bi-lobe (two intersecting cylinders, which optimize the bottom hull space on semi-ref ships).
Application: Used exclusively on Fully Pressurized and Semi-Refrigerated gas carriers, as well as modern LNG/LPG fuel tanks for dual-fuel vessels (FGDS).
Operational Note: Because there is no risk of a liquid leak compromising the hull structure, the hold spaces surrounding Type C tanks do not require nitrogen inerting; they can be filled with dry air, making physical entry and inspections much simpler for the ship's crew.
Comparison Matrix for MMD Oral Defense
If the surveyor asks you to summarize, this table provides the exact technical cross-comparison they expect:
| Technical Parameter | Type A | Type B | Type C |
|---|---|---|---|
| Design Standard | Classical Structural Rules | Fracture Mechanics & FEA | Pressure Vessel Codes (ASME) |
| Core Philosophy | Standard Structural Margin | "Leak-Before-Failure" | High-Pressure Containment |
| MARVS (Vapor Pressure) | Low (<0.3 bar) | Low (<0.3 bar) | High (5 to 18+ bar) |
| Secondary Barrier | Full Secondary Barrier (15-day hold) | Partial Secondary Barrier (Drip tray/spray shields) | None Required |
| Common Geometry | Prismatic | Spherical or Prismatic (SPB) | Cylindrical, Spherical, Bi-lobe |
| Hold Space Atmosphere | Inert Gas (Nitrogen) | Inert Gas or Dry Air | Dry Air (Accessible) |
| Primary Ship Type | VLGCs (Fully Refrigerated LPG) | Moss LNG Carriers / Large Prismatic | Fully Pressurized / Semi-Refrigerated |
Direct Examination "Trap" to Avoid
Surveyor Follow-up: "Can you carry a cargo at −42∘C in a Type C tank?"
Correct Response: "Yes, Sir. On Semi-Refrigerated ships, the Type C tanks are constructed using low-temperature carbon-manganese steel and insulated. This allows them to carry fully refrigerated Propane at ambient atmospheric pressure (and −42∘C), while still possessing the structural strength to handle up to 5 to 8 bar of pressure when the reliquefaction plant is secured or when loading from a pressurized terminal."