Tank ship

A tank ship or tankship, often referred to as a tanker is a ship designed to transport liquids in bulk. Major types of tankship include the oil tanker, the chemical tanker, and the liquefied natural gas carrier.

Background

Tankers can range in size of capacity from several hundred tons, which includes vessels for servicing small harbours and coastal settlements, to several hundred thousand tons, for long-range haulage. Beside ocean- or seagoing tankers there are also specialized inland-waterway tankers which operate on rivers and canals with an average cargo capacity up to some thousand tons. A wide range of products are carried by tankers, including:

* hydrocarbon products such as oil, liquefied petroleum gas (LPG), and liquefied natural gas (LNG)
* chemicals, such as ammonia, chlorine, and styrene monomer
* fresh water
* wine
* molasses

Tankers are a relatively new concept, dating from the later years of the 19th century. Before this, technology had simply not supported the idea of carrying bulk liquids. The market was also not geared towards transporting or selling cargo in bulk, therefore most ships carried a wide range of different products in different holds and traded outside fixed routes. Liquids were usually loaded in casks - hence the term "tonnage", which refers to the volume of the holds in terms of the amount of tuns of wine (casks) that could be carried. Even potable water, vital for the survival of the crew, was stowed in casks. Carrying bulk liquids in earlier ships posed several problems:
*The holds: on timber ships the holds were not sufficiently water oil or air-tight to prevent a liquid cargo from spoiling or leaking. The development of iron and steel hulls solved this problem.
*Loading and Discharging: Bulk liquids must be pumped - the development of efficient pumps and piping systems was vital to the development of the tanker. Steam engines were developed as prime-movers for early pumping systems. Dedicated cargo handling facilities were now required ashore too - as was a market for receiving a product in that quantity. Casks could be unloaded using ordinary cranes, and the awkward nature of the casks meant that the volume of liquid was always relatively small - therefore keeping the market more stable.
*Free Surface Effect: Describes the effect a large surface area of liquid in a ship will have on the stability of that ship. See Naval Architecture. Liquids in casks posed no problem, but one tank across the beam of a ship could pose a stability problem. Extensive sub-division of tanks solved this problem.

In the end, the tanker had its beginnings in the oil industry, as oil companies sought cheaper ways to transport their refinery product to their customers. The Oil Tanker was born. Today most liquids are cheaper to transport in bulk and dedicated terminals exist for each product. Large storage tanks ashore are used to store the product until it can be subdivided into smaller volumes for delivery to smaller customers.

Even the Guinness brewery company in Dublin had a tanker fleet to export the famous stout to the UK.

Different products require different handling and transport. Thus special types of tankers have been built, such as "chemical tankers" and "oil tankers". "LNG carriers", as they are typically known, are a relatively rare tanker designed to carry liquefied natural gas.

Among oil tankers, supertankers are designed for transporting oil around the Horn of Africa from the Middle East. The floating storage and offloading unit (FSO) "Knock Nevis," formerly the ULCC "Jahre Viking", is the largest vessel in the world. The supertanker is 458 metres (1504 feet) in length and 69 m (226 ft) wide.

Supertankers are one of the three preferred methods for transporting large quantities of oil, along with pipeline transport and rail. However such tankers can create environmental disasters from oil spills especially if an accident causes the ship to sink. See "Exxon Valdez", "Braer", "Prestige oil spill", "Torrey Canyon", and "Erika" for examples of coastal accidents.

Tanker capacity

Tankers used for liquid fuels are classified according to their capacity.

In 1954 Shell Oil developed the average freight rate assessment (AFRA) system which classifies tankers of different sizes. To make it an independent instrument, Shell consulted the "London Tanker Brokers’ Panel (LTBP)". At first, they divided the groups as "General Purpose" for tankers under . The ships became larger during the 1970s, and the list was extended, where the tons are long tons: [cite journal | last=Evangelista | first=Joe, Ed. | title=Scaling the Tanker Market | journal=Surveyor | volume= | issue=4 | pages=5–11 | publisher=American Bureau of Shipping | month=Winter | year=2002 | url=http://www.eagle.org/NEWS/pubs/pdfs/SurveyorWinter02.pdf | accessdate=2008-02-27|format=PDF]

*10,000–DWT|24,999: General Purpose tanker
*25,000–DWT|44,999: Medium Range tanker
*45,000–DWT|79,999: Large Range 1 (LR1)
*80,000–DWT|159,999: Large Range 2 (LR2)
*160,000–DWT|319,999: Very Large Crude Carrier (VLCC)
*320,000–DWT|549,999: Ultra Large Crude Carrier (ULCC)