tanker

tanker, a type of bulk carrier. Since time immemorial ships have carried casks of wine, water, and other liquids. Around 1860 the carriage of cased oils across the Atlantic commenced, and by 1886 the experience gained enabled two vessels dedicated to the carriage of oil, the Gluckauf and the Bakuin, to be delivered by shipyards in north-east England. It signalled the start of a massive industry.

Tankers have always been recognizable by having their machinery and bridges aft and all cargo carried amidships, in the widest and most capacious part of the ship. For stability in all conditions the cargo tanks have longitudinal bulkheads which divide the tanks athwartships into three or four and longitudinally into any number up to eleven or twelve, ensuring the ship has ‘packets’ of cargo.

The size of tankers has risen dramatically from the time of Gluckauf, which lifted 2,700 tons of oil. The blocking of the Suez Canal during the 1956 Suez war prompted much larger tankers to be built to make it economical to transport oil round the Cape of Good Hope, and by the mid-1960s, tankers began to exceed 200,000 deadweight tonnage. The deadweight is the mass of cargo carried, the two largest groups of tanker types being known as VLCC (Very Large Crude Carrier), which carry from 150,000 tonnes up to 300,000 tonnes deadweight, and ULCC (Ultra Large Crude Carrier), which carry 300,000 tonnes upwards. Today the largest tankers can take over 500,000 tonnes of cargo.

Since the Second World War (1939–45), tankers traditionally built for the oil trade, have been delivered to carry cargoes as diverse as orange juice, vegetable oil, and even beer—the Miranda Guinness, completed in Bristol in 1976, could carry 2 million pints of stout across the Irish Sea! Tankers transporting bituminous materials carry this at temperatures reaching 250 °C, and must maintain these temperatures throughout the voyage. There are also what are known as Product Tankers which are designed to carry several cargoes ranging from noxious compounds through to specialist oils. Such tankers have independent piping systems, which guarantee that each cargo is free from contamination; often they have stainless steel tanks and their construction has been monitored throughout by 100% X-rays.

The most advanced Product Tankers are the LPG (Liquid Petroleum Gas) and LNG (Liquid Natural Gas) carriers. The former carries butane, propane, and similar gases which are liquefied under pressure. But the LPG has never achieved the success of its ‘high-tech’ sister, the LNG ship, which transports methane-type gases liquefied by temperature reduction to -163 °C or 100° Kelvin. This light gas can be compressed to less than one-six hundredth of its original volume and it then forms a liquid about half the density of water enabling efficient transportation to take place. The difficulty is that at 100° Kelvin, the steel structure of the ship becomes as brittle as sheet ice, which means the tanks carrying the gas must be insulated from the structural hull of the ship.

Since the 1960s there have been several serious accidents to tankers, resulting in oil spillage which has raised serious environmental issues. After consideration, the international community, through the International Maritime Organization, has decreed that all new tankers must have double bottoms to reduce the risk of pollution following collision or grounding. This has brought other problems, in particular with corrosion, but has set new benchmarks for safety. See also red tides.

Fred M. Walker