Liquids in bulk
Gas tankers
As the world’s reserves of liquid crude petroleum diminish, the use of natural gas has increased. Indeed, in some areas like Russia reserves of gas are bigger than reserves of oil. Liquid natural gas is a very cleaning-burning fuel, causing little pollution from nitrogen or sulphur oxides. To transport this gas a hugely sophisticated and highly expensive type of ship has been developed, the gas carrier, sometimes known as LPG for liquefied petroleum gas or LNG for liquefied natural gas carriers. To keep the gas liquid, it has to be cooled to very low temperatures and kept under pressure. Gas carriers thus have immensely strong, very well insulated tanks. Often the tanks are cylindrical in shape to offer the maximum strength. This is often a recognition feature, as the tank rises up above deck level. Gas tankers need to be very carefully operated, as piercing the tanks in a collision or grounding would release clouds of highly-inflammable gas. That no major accidents have befallen these ships is a tribute to their crews and operators. To keep the gas at the optimum temperature, a small amount is allowed to evaporate or ‘boil off’. On early gas carriers, this boil off gas was used to generate steam to drive turbines. Indeed, gas carriers were some of the last ships to be built with steam turbines. However, even here the more economical oil engine has taken over. In oil-engined gas carriers, the boil off gas is refrigerated and returned to the tank.
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