What are the cargo tank types of a gas carrier, and which secondary barrier does each need?
Short answer
| Tank | Design basis | Secondary barrier | Typical use |
|---|---|---|---|
| Type A | prismatic, ship-structural design, vapour pressure below 0.7 barg | full | fully refrigerated LPG carriers |
| Type B | analysis proving leak before failure | partial (drip tray) | Moss spheres and IHI-SPB tanks on LNG carriers |
| Type C | pressure vessel with a large margin | none | pressurised and semi-refrigerated LPG and ethylene carriers, LNG fuel tanks |
| Membrane | thin Invar or stainless membrane, load through the insulation to the hull | full | GTT Mark III and NO96 LNG carriers |
- Goal (IGC chapter 4): contain the gas under all conditions; the tank type sets the secondary barrier, the material and the cargo system.
- Type A: prismatic, vapour pressure below 0.7 barg; a crack could propagate, so a full secondary barrier; fully refrigerated LPG carriers.
- Type B: analysis proving leak before failure, so a partial secondary barrier; Moss spheres and the IHI-SPB tank on LNG carriers.
- Type C: pressure vessels with a large margin, so no secondary barrier; pressurised and semi-refrigerated LPG carriers and most LNG fuel tanks (IGF chapter 6).
- Membrane: a thin Invar or stainless membrane loads the hull through the insulation, so a full secondary barrier; GTT Mark III and NO96.
- Material and ship type: LNG at -163°C needs 9% nickel steel, stainless, aluminium or Invar against brittle fracture; ship type (1G to 3G) follows the cargo hazard (IGC chapter 2).
- As Chief Engineer: reliquefaction, cargo machinery and cryogenic-material integrity all follow from the containment type.
Full answer
The full answer explains each point step by step, with the regulations and figures behind it and the discussion from other candidates. It is in the MEO Class I section of the Marine One forum, open to members enrolled for the MEO Class I exam.