Modern liquefied gas carriers

Modern liquefied gas carriers Gas carriers range in capacity from the small pressurised tankers of between 500 and 6,000 m3 for shipment of propane, butane and the chemical gases at ambient temperature up to the fully insulated or refrigerated seagoing tankers of over 100,000 m3 capacity for the transport of LNG and LPG. Between those two distinct types is a third tanker type – semipressurised gas carrier.

These very flexible tankers are able to carry many cargoes in a fully refrigerated condition at atmospheric pressure or at temperatures corresponding to carriage pressure of between five and nine bar. The movement of liquefied gases by waterways is now a mature industry, served by a fleet of many tankers, a network of export and import terminals and a wealth of knowledge and experience on the part of various people involved. Gas carriers have certain features common with other tankers used for the carriage of bulk liquids such as oil and chemical tankers. A feature almost unique to the gas carrier is that the cargo is kept under positive pressure to prevent air entering the cargo system. This means that only cargo liquid and cargo vapour are present in the cargo tank and flammable atmospheres cannot develop. Furthermore all gas carriers utilise closed cargo systems when loading or discharging, with no venting of vapour being allowed to the atmosphere. In the LNG trade, provision is always made for the use of a vapour return line between tanker and shore to pass vapour displaced by the cargo transfer. In the LPG trade this is not always the case as, under normal circumstances during loading, reliquefaction is used to retain vapour on board. By these means cargo release to the atmosphere is virtually eliminated and the risk of vapour ignition is minimised. Gas carriers are divided into two main groups. Liquefied Petroleum Gas (LPG) Carriers, which are designed to carry mainly butane, propane, butadiene, propylene, vinyl chloride monomer (VCM) and are able to carry anhydrous ammonia. Liquefied Natural Gas (LNG) Carriers, which are designed to carry liquefied natural gas (which is mostly methane).

Fig:LNG ship underway

Gas carriers are classed in three types based on hazard potential: i) type 1G, designed to carry the most hazardous cargoes ii) type 2G and 2PG, designed to carry cargoes having a lesser degree of hazard iii) type 3G, designed to carry cargoes of the least hazardous nature. Gas carrier types All gas cargoes are transported in liquid form (ie they are not carried as a gas in its vapour form) and, because of their physical and chemical properties, they are carried either at: - pressures greater than atmospheric, or at - temperatures below ambient, or a combination of both. Therefore, gas carriers are generally grouped as follows: i) Fully Pressurised ii) semi-pressurised and refrigerated iii) fully refrigerated Note. These grouping names are more prevalently used when discussing the classes and types of LPG carriers rather than LNG carriers.