The Tanker; industry's ocean artery

The tanker fleet includes some of the world's biggest mechanically propelled moving objects, but the term "tanker" covers a wide variety of different types and sizes of ships designed to carry liquids or gases in bulk. Biggest of all are the Ultra Large Crude Carriers, a handful of which are able to carry half a million tonnes of crude oil, although a more normal size range is the Very Large Crude Carrier of between 250,000 and 300,000 tonnes capacity.

There is also a sizeable fleet of around 140,000 tonnes , a class of ship which is able to pass laden through the Suez Canal, and serve a range of ports which the bigger ships could not enter. Crude carriers typically carry large cargoes of crude oil from the oil producing countries to the refineries. Arabian Gulf to Singapore or Japan, or North Europe are typical trade routes.

Stenna Vision, oil tanker


VLCC Stenna Vision

Product tankers are smaller vessels, of up to around 80,000 tonnes, which carry the many petroleum products of the refineries, clean white oil cargoes like gasolene or black oils like heavy fuel oil or diesel. These vessels begin the distributive process, moving from the refineries to the tank farms in the consuming countries, from where the oil cargoes are fed by road, rail, pipeline and coastal tankers and inland tank barges to power stations, and depots close to where the products are used.

The tanker fleet is a highly adaptable, responsive floating pipeline which is capable of adjusting to considerable fluctuations in demand , as cold weather bites or industrial capacity increases. Ships can be switched to add or reduce capacity, or take advantage of price fluctuations, almost at a moment's notice. Oil being a traded commodity, the adaptability of tankers is highly important in this respect.

Tankers are enormously strong vessels, built with a honeycomb construction, a modern crude carrier typically having fifteen cargo tanks protected by a double hull structure around the sides and bottom , in which sea water ballast can be carried for the empty voyage after the oil cargo has been delivered. On such modern double hulled tankers, the oil and ballast remain separate , which greatly reduces the risk of pollution.

A tanker will normally be loaded by shore equipment , but will use her own powerful pumps to empty herself at the discharge ports. Crude oil comes in many forms, ranging from those that are light and free running, to heavy and waxy crude which require heating so they can be pumped freely. Most hydrocarbons are toxic and flammable and it is modern safety practice to keep the tanks filled with a blanket of inert gas.

Specialist tankers include vessels which are configured to carry as many as fifty different grades of chemicals and oils, each with their own separate pumping systems and capable of coping with cargoes that need to shipped hot, cold or under a blanket of nitrogen. There are ships built of stainless steel for the carriage of acids, others with sophisticated coatings for the carriage of edible oils and wine. Gas tankers carry LPG and Methane.

Tankers have had a bad press, and are associated with oil pollution, but the facts are very different, with pollution steadily reducing year on year , despite the steady increase in the quantity of oil carried by sea.





What is the technology to deal with oil spills?

The technology to deal with oil spills has not changed much since the Exxon Valdez disaster in 1989, says Toby Stone of the Maritime and Coastguard Agency (MCA). Recovering oil is still a fairly basic function, he says, but management structures and prevention have improved.


What are the options once an oil spill has happened?

Given the type of fuel - heavy oil - that spilled from the Prestige, options are limited. It is difficult to handle, hampering attempts for mechanical recovery at sea. The oil spilled while the ship was on the surface will quickly disperse to cover a large area. The International Tanker Owners Pollution Federation (ITOPF) says the best move is shoreline protection.


A chemical dispersant can be applied to lighter crude oil, breaking up the fuel much as washing up liquid would in oily dishwater. But that is not an option for the fuel spilled from the Prestige.


At other times, clean-up experts will simply monitor a slick from the air and wait for it to biodegrade into the ocean.


Authorities may choose to deflect oil from sensitive ecological or commercial areas with booms, covering another beach in oil and cleaning it up.


If a tanker's crew catch a potential spill in time, salvage experts may be able to transfer the cargo to another tanker before it spills into the sea.


What happens to oil once it is recovered?

Once oil has been scooped off a beach or skimmed off the surface of the ocean, the response teams must dispose of it. The MCA names a number of options, all of them costly:

Recover and reprocess the oil
Recycle the oil into other things, such as Tarmac
Burn it
Put it in a landfill


Why not bomb the ship to burn off the oil?

That may be a possibility with a crude oil, if the spill is concentrated and contained, according to the International Tanker Owners Pollution Federation (ITOPF). But a heavy oil will not burn unless it is heated up, and once it has mixed with water there is even less of a likelihood it will burn.


What happens to the oil left in the ship when it sinks?

If the storage compartments hold, the oil could simply remain sealed in the ship. If does leak out once the ship reaches the sea bed it will probably do so very slowly, ITOPF estimated. Salvage company Smit International reckoned the oil may freeze.


What's happened in past oil spills?

The Exxon Valdez spilled about 38,800 tonnes of crude oil into Alaska's Prince William Sound in March 1989.


Clean-up crews found the carcasses of more than 35,000 birds and 1,000 sea otters, but believed most of the animals' remains would have sunk, according to the Exxon Valdez Oil Spill Trustee Council. The council estimated the dead wildlife at 250,000 seabirds, 2,800 sea otters, 300 harbour seals, 250 bald eagles, around 22 killer whales, and billions of salmon and herring eggs.



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Will the ocean just wash away the oil?

Yes, eventually. Ocean waves probably cleaned more oil away from Alaska's beaches than the three-year clean up effort that cost Exxon $2.1bn. But in the meantime, Alaska lost about $2.8bn in hunting, fishing and holiday revenues.


Why are single hulled tankers still around?

All new oil tankers built since 1996 must have a double hull, which offers greater protection against spillage. The 158 member states of the International Maritime Organisation - a UN body - agreed last year to phase out most single hull oil tankers by 2015.


The exception: countries can register newer single hull ships if they comply with certain technical specifications. Even these tankers can only continue trading until the 25th anniversary of their delivery.


The IMO will allow other states to deny these single hull tankers entry at their ports after the 2015 cutoff. The EU has said it will deny access to these tankers.


Are oil spills becoming more common?

No, quite the opposite. ITOPF tracks oil spills and says the number of large spills (those over 700 tonnes) in the 1990s was about one third of those in the 1970s.

( The oil tanker Prestige sank off the coast of Spain on Wednesday the 20 November 2002)




Special reports
Waste and pollution
Oil and petrol

In pictures
Prestige oil tanker disaster

Where the Prestige oil slick happened

From the Guardian archive
25.03.1989: Giant oil spill imperils Alaska
08.09.1989: Aftermath to the Valdez oil spill
23.01.2001: Galapagos oil catastrophe
27.01.2001: Focus shifts to future of Galapagos Islands

World news guide
20.12.2001: Portugal

Useful links
WWF International: Spanish oil spill
Interactive guide to the spill - El Mundo (in Spanish)
Smit international news (salvage company)
World environment news - Reuters
Exxon Valdez oil spill trustee council





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Exxon Valdez stern picture




1967 Torrey Canyon UK - 119,000 tonnes

1978 Amoco Cadiz Brittany - 220,000 tonnes
1979 Atlantic Empress Tobago - 160,000 tonnes
1983 Braer Shetland Isles - 85,000 tonnes

1989 Exxon Valdez Alaska - 38,800 tonnes
1996 Sea Empress Wales - 72,000 tonnes
2002 Prestige North-west Spain - 42,000 tonnes






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