The Gas Codes divide gas carriers into four categories; Ship Types 1G, 2G, 2PG and 3G, according to the hazard rating of the cargoes for the carriage of which the ship is certified. For example, Type 1G ships (where the cargo tanks are located at the greatest distance from the side shell and which may also be restricted in capacity) must be used for cargoes representing the greatest hazard, such as chlorine. Ship Types 2G/2PG and 3G can carry cargoes which represent progressively decreasing environmental hazards and, therefore, progressively less stringent structural requirements in respect of damage survival capability in the event of collision or grounding.
A fully refrigerated ship, say with Type ‘A’ tanks, designed for LPG, must comply with the requirements for tank location and survival capability of a category 2G ship whereas a semi-pressurised ship, with Type ‘C’ tanks carrying LPG can comply with the requirements either of a 2G or a 2PG ship.
For the latter case, the Type ‘C’ pressure vessels must have a design pressure of at least 7 barg, and a design temperature not lower than -55°C. The 2PG category takes into account the fact that the pressure vessel design provides increased survival capability when the ship is damaged by collision or-grounding.
The Gas Codes and classification society rules should be referred to for the detailed construction requirements for each category of ship.