Gases can be liquefied by using one of the following methods:
1. Pressurization under normal temperature.
2. Refrigeration and pressurization.
3. Refrigeration under atmospheric temperature.
Ships that carry gas products under state 1 above are referred to as “fully pressurized gas carriers”, while those that carry gas product under state 2 are known as “semi-refrigerated gas carriers”. Those under state 3 above are called “refrigerated gas carriers” and this type is often used in design of large LPG and LNG carriers.
Ad 1. Pressure gas carriers, also fully-pressurized ships – These ships are the simplest of all gas carriers in terms of containment systems and cargo-handling equipment and carry their cargoes at ambient temperature. Independent pressure vessels with a typical design vapour pressure of 17.5 bar are used as cargo tanks (type C tanks). Ships with higher design vapour pressure are in service; 18 bar is quite common – a few ships can accept pressure up to 20 bar. No thermal insulation or a reliquefaction plant is necessary and cargo can be discharged by either pumps or compressors.
Because of their design pressure the tanks are extremely heavy. As a result, fully pressurised ships tend to be small with maximum cargo capacities of about 4000 m3 and they are used to carry primarily LPG and ammonia. See also CNG concept.
Ad 2. Semi-refrigerated gas carriers – Constructed in the size range of 1500 to 30,000 m3, this type of gas carrier evolved as the optimum means of transporting the variety of gases, from LPG and VCM to propylene and butadiene. Today, this type of ship is the most popular amongst operators of “smaller-size” gas carriers.
Semi-refrigerated gas tankers use pressure vessel tanks designed for design vapour pressure in the range of 4-8 bar. The tanks are made either from low temperature steels for carriage temperature of –48°C which is suitable for most LPG and chemical gas cargoes, or from special alloyed steels to allow the carriage of ethylene at –104°C. See also Semi-refrigerated gas carrier NORGAS ORINDA.
Ad 3. Fully-refrigerated gas carriers – They can carry cargoes at approximately atmospheric pressure and are generally designed to transport large quantities of LPG and ammonia. Different cargo containment systems have been used in FR ships. The most widely used arrangement is the independent tanks with single side shell. Type A prismatic freestanding tanks capable of withstanding a maximum design vapour pressure of 0.7 bar are used. A complete secondary barrier is required and the hold spaces must be inerted when carrying flammable cargoes.