Oil tanker

Oil tanker

An oil tanker, also known as a petroleum tanker, is a ship designed for the bulk transport of oil. There are two basic types of oil tankers: the crude tanker and the product tanker. Crude tankers move large quantities of unrefined crude oil from its point of extraction to refineries. Product tankers, generally much smaller, are designed to move petrochemicals from refineries to points near consuming markets.

Oil tankers are often classified by their size as well as their occupation. The size classes range from inland or coastal tankers of a few thousand metric tons of deadweight (DWT) to the mammoth supertankers of . Tankers move approximately 2 billion metric tons of oil every year. Second only to pipelines in terms of efficiency, the cost of tanker transport amounts to only two or three United States cents per gallon.

Some specialized types of oil tankers have evolved. One of these is the naval oiler, a tanker which can fuel a moving vessel. Combination ore-bulk-oil carriers and permanently moored floating storage units are two other variations on the standard oil tanker design. Oil tankers have been involved in a number of damaging and high-profile oil spills. As a result, they are subject to stringent design and operational regulations.


The technology of oil transportation has evolved alongside the oil industry. Although man's use of oil reaches to prehistory, the first modern commercial exploitation dates back to James Young's manufacture of parafin in 1850. In these early days, oil from Upper Burma was moved in earthenware vessels to the river bank where it was then poured into boat holds.

In the 1850s, the Pennsylvania, United States, oil fields became a major supplier of oil, and a center of innovation after Edwin Drake had struck oil near Titusville, Pennsylvania. The first oil well in the United States was dug here in 1859, initially yielding around ten barrels per day. Within two years, the Titusville field was providing 3,000 barrels per day.

The invention of oil refining led to the availability of kerosene as lamp oil, which has a smokeless combustion in contrast with the until then highly used whale oil. The lamp oil became known as Pennsylvania Kerosine. Due to overfishing, whale oil became rare and expensive. By this time, petroleum oil had already begun to supplant fish, whale, and vegetable oils for applications such as indoor and outdoor lighting, and transatlantic export had already begun.

Break-bulk boats and barges were originally used to transport Pennsylvania oil in wooden barrels. But transport by barrel had several problems. The first problem was weight: the standard empty barrel weighed , representing 20% of the total weight of a full barrel. Also, barrels were leaky, and could only be carried one way. Finally, barrels were themselves expensive. For example, in the early years of the Russian oil industry, barrels accounted for half the cost of petroleum production.

The movement of oil in bulk was attempted in many places and in many ways. Modern oil pipelines have existed since 1860. In 1863, two sail-driven tankers were built on England's River Tyne. These were followed in 1873 by the first oil-tank steamer, the Vaderland, which was built by Palmers Shipbuilding and Iron Company for the Belgium Red Star Line. The vessel's use was curtailed by U.S. and Belgian authorities citing safety concerns. By 1871, the Pennsylvania oil fields were making limited use of oil tank barges and cylindrical railroad tank-cars similar to those in use today.

The Nobel Brothers

In 1876, Ludvig and Robert Nobel, brothers of Alfred Nobel, founded Branobel (short for Brothers Nobel) in Baku, Azerbaijan. It was, during the late 19th century, one of the largest oil companies in the world.

Ludvig was a pioneer in the development of early oil tankers. He first experimented with carrying oil in bulk on single-hulled barges. Turning his attention to self-propelled tankships, he faced a number of challenges. A primary concern was to keep the cargo and fumes well away from the engine room to avoid fires. Other challenges included allowing for the cargo to expand and contract due to temperature changes, and providing a method to ventilate the tanks.

The world's first successful oil tanker was Nobel's Zoroaster. He designed this ship in Gothenburg, Sweden, with Sven Almqvist. The contract to build it was signed in January 1878, and it made its first run later that year from Baku to Astrakhan. The Zoroaster design was widely studied and copied, with Nobel refusing to patent any part of it. In October 1878, he ordered two more tankers of the same design: the Buddha and the Nordenskjöld.

Zoraster carried its 242 long tons of kerosene cargo in two iron tanks joined by pipes. One tank was forward of the midships engineroom and the other was aft. The ship also featured a set of 21 vertical watertight compartments for extra buoyancy. The ship had a length overall of , a beam of , and a draft of . Unlike later Nobel tankers, the Zoraster design was built small enough to sail from Sweden to the Caspian by way of the Baltic Sea, Lake Ladoga, Lake Onega, the Rybinsk and Mariinsk Canals and the Volga River.

Nobel then began to adopt a single-hull design, where the ship's hull forms part of its tank structure. In November 1880, he ordered his first single-hulled tanker, the Moses. Within a year, he ordered seven more single-hulled tankers: the Mohammed, Tatarin, Bramah, Spinoza, Socrates, Darwin, Koran, Talmud, and Calmuck.

Branobel experienced one of the first oil tanker disasters. In 1881, the Zoroaster's sister-ship, the Nordenskjöld exploded in Baku while taking on kerosene. The pipe carrying the cargo was jerked away from the hold when the ship was hit by a gust of wind. Kerosene then spilled onto the deck and down into the engineroom, where mechanics were working in the light of kerosene lanterns. The ship then exploded, killing half the crew. Nobel responded to the disaster by creating a flexible, leakproof loading pipe which was much more resistant to spills.

In 1883, oil tanker design took a large step forward. Working for the Nobel company, Colonel Henry F. Swan designed a set of three Nobel tankers. Instead of one or two large holds, Swan's design used several holds which spanned the width, or beam, of the ship. These holds were further subdivided into port and starboard sections by a longitudinal bulkhead. Earlier designs suffered from stability problems caused by the free surface effect, where oil sloshing from side to side could cause a ship to capsize. But this approach of dividing the ship's storage space into smaller tanks virtually eliminated free-surface problems. This approach, almost universal today, was first used by Swan in the Nobel tankers Blesk, Lumen, and Lux.

In 1903, the Nobel brothers built two oil tankers which ran on internal combustion engines, as opposed to the older steam engines. The Vandal and Sarmat — which were also the first diesel-electric ships — were each capable of carrying 750 long tons of refined oil and powered by diesel motors. This same firm soon went on to make much larger oil tankers, such as the Emanuel Nobel and Karl Hagelin, 4,600 long ton kerosene tankers with engines.

The Glückauf represented a large step forward in tanker design. Another design of Colonel Swan, the ship has been called the "true progenitor of all subsequent tanker tonnage." Its features included cargo valves operable from the deck, cargo main piping, a vapor line, cofferdams for added safety, and the ability to load seawater ballast when empty of cargo. Wilhelm Anton Riedemann, an agent for the Standard Oil Company purchased Glückauf and several of her sister ships. After the Glückauf was lost in 1893, Standard Oil purchased the sister ships.

Breaking the Standard Oil monopoly

The 1880s also saw the beginnings of the Asian oil trade. The oil industry in Azerbaijan was the largest producer in the world at that time, but was limited to the Russian market. John D. Rockefellers Standard Oil dominated the world market. The Rothschilds already had constructed a railway to the Black Sea.

The idea that led to moving Russian oil to the Far East via the Suez Canal was the brainchild of two men: importer Marcus Samuel and shipowner/broker Fred Lane — the London agent for the De Rothschild Frères. Prior bids to move oil through the canal had been rejected by the Suez Canal Company as being too risky. Samuel approached the problem a different way: asking the company for the specifications of a tanker it would allow through the canal.

Armed with the canal companies specifications, Samuel had James Fortescue Flannery designing tankers for Bnito — the Russian oil company of the Rothschilds — and ordered three tankers from William Gray in northern England. Named the Murex, the Conch and the Clam, each had a capacity of 5,010 long tons of deadweight. In 1893, the Samuel brothers founded the Tank Syndicate together with Fred Lane and Asian trading companies. In 1897 it was renamed Shell Transport and Trading company, forerunner of today's Royal Dutch Shell company.

With facilities prepared in Jakarta, Singapore, Bangkok, Saigon, Hong Kong, Shanghai, and Kobe, the fledgling Shell company was ready to become Standard Oil's first challenger in the Asian market. On August 24, 1892, the Murex became the first tanker to pass through the Suez Canal.

In the meanwhile, in 1890 the Koninklijke Nederlandsche Maatschappij tot Exploitatie van Petroleumbronnen in Nederlandsch-Indie (KNMEP) (Royal Dutch Company for the Working of Petroleum Wells in Netherlands India) — part of Royal Dutch Petroleum — was founded. In 1892 it found oil near Pangkalan Brandan on Sumatra, just some months before Samuels kerosene arrived in Singapore. At first, chartered ships were used, but in 1896 KNMEP launched its first tankers, the Besitang and Berandan. The threat of the Tank Syndicate was reduced as the Dutch government excluded them from trading in the Dutch East Indies. To prevent a hostile takeover by Standard Oil, preference shares were issued. By the time Shell merged with Royal Dutch Petroleum in 1907, the company had 34 steam-driven oil tankers. Standard Oil started building tankers the same way as Shell and by that time had four case-oil steamers and 16 sailing tankers.

El Águila (Mexican Eagle Oil Company) — founded in 1900 by Weetman Pearson to develop the newly found Mexican oil fields, nationalized in 1938 as PEMEX — also had its own tanker fleet. Standard Oil did not participate directly in the newly discovered oil fields in Texas — such as Spindletop — and Oklahoma, which gave opportunities for new oil companies as Gulf Oil and Texas Fuel Company, later Texaco. Avoiding the use of Standard Oil's pipe line system, they started using tankers to get their oil to the East Coast. In combination with the oil fields discovered in Mexico and Venezuela, this caused a rise in the demand for tankers, which gave opportunities for the first independents, such as the Norwegian Wilh. Wilhelmsen, that launched its first tanker in 1913.

World War I and interbellum

The fleet oiler , launched on April 17, 1915, pioneered the technique of underway replenishment. A large ship at the time, with a capacity of 14,500  long tons of deadweight, Maumee began refuelling destroyers en route to Britain at the outset of World War I. This technique enabled the Navy to keep its fleets at sea for extended periods, with a far greater range independent of the availability of a friendly port. This independence proved crucial to victory in World War II by the ships commanded by Fleet Admiral Nimitz who, as Maumee’s executive officer, had played a key role in developing underway replenishment.

Underway replenishment was quickly adopted by other navies. One example of this is the Australian fleet oiler HMAS Kurumba which provided underway replenishment services in the United Kingdom's Royal Navy from 1917 to 1919.

During World War I, unrestricted submarine warfare caused a shortage of tankers. The United States ambassador to the United Kingdom, Walter Hines Page, wrote "The submarines are sinking freight ships faster than freight ships are being built by the whole world. In this way, too, then, the Germans are succeeding. Now if this goes on long enough, the Allies' game is up. For instance, they have lately sunk so many fuel oil ships, that this country may very soon be in a perilous condition — even the Grand Fleet may not have enough fuel". Georges Clemenceau wrote to US president Wilson "Gasoline is as vital as blood in the coming battles…a failure in the supply of gasoline would cause the immediate paralysis of our armies". Wilson reacted strongly. The War Shipping Board commandeered all ships in the United States and also took over all yards. An unprecedented budget of $US1.3 billion was used for this end. At Hog Island, the largest shipyard in the world was built, known for the Hog Islander.

Between 1916 and 1921, 316 tankers were built with a total capacity of 3.2 million long tons of deadweight, where the entire world fleet before WW I was just above 2 million tons. In 1923 about 800,000 long tons were laid up, which gave enormous opportunities for speculators, such as Daniel Keith Ludwig. In 1925 he had bought the freighter Phoenix and put tanks in the holds. These riveted tanks leaked, which resulted in a explosive mixture. The resulting explosion killed two crew members and badly injured Ludwig. After this, he was a strong believer in welding.

The transition of ships using oil in stead of coal, mass production of automobiles and aviation taking off, all resulted in an increased demand for oil and thus oil transport.

With the right connections at the Shipping Board, ships could be bought cheaply, which caused a lot of fraud. This system was to complicated for the oil companies however, so when World War II started, independents owned 39 percent of the world tanker fleet. This was especially because oil companies needed capital to invest in the growing oil market. By not buying the ships, but chartering, the bond rating of the oil companies was not affected, because the charter hire was not recognized as debt in that time. In 1934 H.T. Schierwater founded the International Tanker Owners’ Association — later International Association of Independent Tanker Owners or INTERTANKO — to protect the interests of thes companies in a strongly fluctuating market.

World War II

When World War II broke out in 1939, Roosevelt could not support the United Kingdom directly, as in 1935 the Shipping Neutrality Act was signed to prevent Mussolini from invading Ethiopia. A solution was found by flagging out to Panama, which was under US control. The advantage for shipping companies was not having to comply with US regulations and not having to sail with US crew, that was 50 percent more expensive then European crews at that time. In 1939 there were 52 tankers with a total capacity of 700,000 long tons of deadweight under Panamanian flag, a boost for flags of convenience.

The block construction method was invented in the United States, because German U-boats were sinking more ships then could be built with existing methods. By builing in blocks and welding, the construction time could be reduced dramatically. Daniel Keith Ludwig had an important role in this, as he introduced the method at his Welding Shipyards in Norfolk as part of the Emergency Shipbuilding program.

Particularly the T2 tanker played an important part in World War II. The T2-SE-A1 with a capacity of 16,613 long tons of deadweight, was the most popular variant with nearly 500 built during the war. After the war, these tankers were used commercially for decades, and many were sold on the international market.

In this period, the World Scale was introduced to determine freight tariffs.

Until 1956, tankers were designed to be able to navigate the Suez Canal. This size restriction became much less of a priority after the closing of the canal during the Suez Crisis of 1956. Forced to move oil around the Cape of Good Hope, shipowners realized that bigger tankers were the key to more efficient transport.

The supertanker era

Where the size of tankers had been more or less the same for 25 years, after World War II they have grown in size significantly, initially slowly. A typical T2 tanker of the World War II era was long and had a capacity of . A modern ultra-large crude carrier (ULCC) can be long and have a capacity of . Several factors encouraged this growth. Hostilities in the Middle East which interrupted traffic through the Suez Canal contributed, as did nationalization of Middle East oil refineries. Fierce competition among shipowners also played a part. But apart from these considerations is a simple economic advantage: the larger an oil tanker is, the more cheaply it can move crude oil, and the better it can help meet growing demands for oil.

Where the refinery of oil had taken place near the well earlier, this moved to the consumer location in this stage. Oil production in the Middle East developed and the dominance of product tankers was replaced by crude oil carriers. Soon, Panamax tankers were built, soon followed bij Aframax and Suezmax tankers because of economics.

After the war, it was expected that a large number of tankers would be laid up, which indeed happened. The United States Maritime Commission had replaced the War Shipping Board vervangen, but fraudulent activities remained. Onassis and Niarchos used this to buy tankers cheaply. The expected economic decline did not come, amongst others due to the Marshall plan, increasing the demand for oil such, that in 1947 their was a shortage of tankers. Freight tariffs triple overnight, enabling some to recoup their investment in one voyage.

Ludwig had started Universe Tankships in 1947 and begun building larger tankers on his Welding Shipyards. The Bulkpatrol of 30,000 long tons was the largest tanker of its time. Four of the five Bulk class tankers sunk, likely because welding technology was not yet fully understood. As larger ships could not be constructed on the yard in Norfolk, Ludwig went to Japan where he introduced the block construction method at the Kure Naval Yard. Here, in 1952, he built the Petrokure of 38,000 long tons. That same year, Onassis built a tanker of 45,000 long ton and also Niarchos constructed super tankers. Both Onassis and Niarchos claimed to be the largest independent tanker owner in the world.

The Sinclair Petrolore that Ludwig built in 1955, was with 56,000 long tons not only the largest freighter in the world, but also a self-unloading ore-oil carrier, the only one of that type ever built. It exploded on 6 December 1960 near Brazil — likely because of cargo leakage in the double bottom — resulting in the largest spill until then with 60,000 tons.

In 1956, the Universe Leader of 85,000 long tons — just in time before the Suez Crisis started with the Pannegia — was built. In ten years time, tanker size had quadrupled. In 1958, Ludwig broke the barrier of 100,000 long tons of heavy displacement. His Universe Apollo displaced 104,500 long tons, a 23% increase from the Universe Leader.

In 1962 Niarchos built the Manhattan of 106,000 long tons. This is the largest merchant vessel ever built in the United States. It was converted to have ice breaking capacities in 1969 and was the first commercial ship to cross the Northwest Passage. Although the voyage was a success, a second attempt to cross the passage in winter proved impossible, and there were numerous environmental concerns with the project, so it was cancelled and the Trans-Alaska Pipeline System built.

In 1966, the Idemitsu Maru of 206,000 long tons was built. In twenty years, the size of tankers had increased tenfold.

Closure Suez Canal

The oil spill caused by the Torrey Canyon in 1967 caused a public awareness about the environmental dangers of oil tankers. Oil companies united in 1970 in the Oil Companies International Marine Forum to become involved in the following issuing of rules such as MARPOL 73. In 1968 also the International Tanker Owners Pollution Federation was founded to indemnify the victims.

For tanker owners, the Six-Day War of 1967 was of greater importance however. The Suez Canal closed until 1975 and freight rates sky rocketed because of the shortage of tonnage now ships had to pass the Cape of Good Hope. Even larger tankers were now built, as the limitations of the Suez Canal were not governing anymore. In only a couple of years the size of tankers quadrupled to more then 500,000 long tons and there were even plans for tankers of 1,000,000 long tons. In 1969 the first ULCC's were built.

The world's largest supertanker ever was built for Tung Chao Yung in 1979 at the Oppama Shipyard of Sumitomo Heavy Industries, Ltd. as the Seawise Giant. This ship was built with a capacity of , a length overall of and a draft of . She has 46 tanks, of deck, and is too large to pass through the English Channel.

Seawise Giant was renamed Happy Giant in 1989, Jahre Viking in 1991. From 1979 to 2004, she was owned by Loki Stream, at which point she was bought by First Olsen Tankers, renamed Knock Nevis and converted into a permanently moored storage tanker. The Batillus class supertankers are the biggest ships ever constructed by gross tonnage.

Although the tanker fleet increased around 12 percent annualy around 1970, a shortage on tonnage remained. In 1973 this resulted in an enormous increase in new building orders, especially from oil majors that wanted to gain on the quicker deciding independents, who could ask enormous rates for their vessels. Where the existing tanker fleet comprised of some 150 million long tons, in a quarter of a year a tonnage of 75 million was ordered, although new build prices doubled.

The increase in scale brought a new problem. Until then, the washing of tanks after cargo discharge was done by water. In December 1969 three tankers exploded during tank washing. The Dutch Shell tanker Marpessa sunk off Dakar and became the largest merchant vessel ever lost. The other two, the British Shell tanker Mactra en the Norwegian Kong Haakon VII were damged heavily, but remained afloat. Shell investigated the matter and came to the conclusion that water drops that impact steel with high velocity generates static electricity that can cause explosions in combination with cargo vapors. This only became apparent with the large sizes of the tanks of VLCC's.

The solution was found by filling the cargo tanks with inert gas (IG), reducing the oxygen level such that the tank remains below the explosive limit. The use of IG is seen as the biggest step in increasing tanker safety. Ten years later however, fifty people were killed when the Betelgeuse exploded at Whiddy Island in Bantry Bay. The Total tanker was still not fitted with inert gas. The Energy Concentration did have this system, preventing an explosion when it broke in two on 21 July 1980 during discharge at Europoort. It has also saved many lifes during the Tanker War.

Wahing with water in combination with the Load on Top system was replaced by Crude Oil Washing (COW), a method developed by BP. The advantages were cleaner tanks, no corrosive sea water in cargo tanks and no polluted sea water being pumped overboard.

Oil Crisis and consolidation

On 10 October 1973 the Yom Kippur War begun, causing the 1973 oil crisis, tripling oil prices to US$10 per barrel, halting economic growth. New build ships sometimes went straight from the yard to lay up. The situation worsened when the Suez Canal was reopened in 1975. Just when the situation begun to improve in 1979, the Iranian Revolution caused the second oil crisis, causing oil prices to rise to US$30. Ships were sometimes sent to the breakers after being in service for only ten years. It took until the end of the 1980s before any profits were being made in oil transport.

In 1979, World-Wide Shipping of Yue-Kong Pao with 204 vessels, many of them tankers, was the largest shipping company in the world with a tonnage of 20.5 million, but in the five years after that he sold some 140 vessels to manage the crisis. In 1980 Daniel K. Ludwig had the largest fleet after Y. K. Pao and C. Y. Tung and was regarded the richest man of the United States. Currently, John Fredriksen owns the largest tanker operator in the world, Frontline. He also owns almost ten percent of the largest shipping company in the world, Overseas Shipholding Group.

In 1976, the Intervention Convention was used for the first time when the US Coast Guard took on the salvage of the Argo Merchant, although the vessel was in international waters. This was the first time the monopoly of flag states was broken through.

In 1978, a number of European countries agreed in The Hague on memorandum that agreed to audit whether the labour conditions on board vessels were according the rules of the ILO. After the Amoco Cadiz sank that year, it was decided to also audit on safety and pollution. To this end, in 1982 the Paris Memorandum of Understanding (Paris MoU) was agreed upon, establishing Port State Control, nowadays 24 European countries and Canada. In practice, this was a reaction on the failure of the flag states - especially flags of convenience that have delegated their task to classification societies - to comply with their inspection duties.

The Exxon Valdez oil spill was the incentive to introduce legislation requiring tankers to have a double hull, a measure that is not seen as the best solution by all experts. Where a double hull should minimize the consequenses after a disaster, Concordia Maritime developed the Stena V-MAX, a VLCC with two propellers, two rudders and two redundant engine rooms, where a single fault does not result in loss of steering, reducing the chances of a grounding.

The size of VLCC's and ULCC's limits their sailing area and available ports. In the United States, Louisiana Offshore Oil Port (LOOP) is the only facility that is able to handle VLCC's. To overcome this, lightering from large tankers in smaller ones that are able to reach the destination port. The largest oil terminal is Ras Tanura in Saudi Arabia.

ULCC's had not been built since the beginning of the 1980s until the Greek Hellespont Steamship Corporation ordered four double hull supertankers in 1999. These ships were built in 2002 and 2003 as the Hellespont Alhambra, Hellespont Metropolis, Hellespont Tara and Fairfax. Hellespont sold these ships to Overseas Shipholding Group and Euronav in 2004. These TI class supertankers — currently known as the TI Asia, TI Europe, TI Oceania, and TI Africa — are as of 2008 the worlds four largest working supertankers.

Each of the four sister ships has a capacity of over , a length overall of and a cargo capacity of . The first ULCC tankers to be built for some 25 years, they were also the first ULCCs to be double-hulled. To differentiate them from smaller ULCCs, these ships are sometimes given the V-Plus size designation. In February 2008, their owners announced plans to convert TI Africa and the TI Asia into stationary floating storage and offloading units to be placed in the Al Shaheen oilfield near Qatar in late 2009.

With the exception of the pipeline, the tanker is the most cost-effective way to move oil today. Worldwide, tankers carry some annually, and the cost of transportation by tanker amounts to only US$0.02 per gallon at the pump.

Size categories

Oil tanker size categories
AFRA Scale Flexibel market scale
Class Size in DWT Class Size in DWT New
General Purpose tanker 10,000 - 24,999 Product tanker 10,000 - 60,000 $43M $42.5M
Medium Range tanker 25,000 - 44,999 Panamax 60,000 - 80,000
LR1 (Large Range 1) 45,000 - 79,999 Aframax 80,000 - 120,000 $58M $60.7M
LR2 (Large Range 2) 80,000 - 159,999 Suezmax 120,000 - 200,000
VLCC (Very Large Crude Carrier) 160,000 - 319,999 VLCC 200,000 - 320,000 $120M $116M
ULCC (Ultra Large Crude Carrier) 320,000 - 549,999 ULCC 320,000 - 550,000
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 ; Medium Range for ships between 25,000 and and Large Range for the then-enormous ships that were larger than . The ships became larger during the 1970s, which prompted rescaling.

Merchant oil tankers carry a wide range of hydrocarbon liquids ranging from crude oil to refined petroleum products. Their size is measured in deadweight metric tons (DWT). Crude carriers are among the largest, ranging from Panamax-sized vessels to ultra-large crude carriers (ULCCs) of over .

Supertanker is an informal term used to describe the largest tankers. Today it is applied to very-large crude carriers (VLCC) and ULCCs with capacity over . These ships can transport two million barrels of oil. By way of comparison, the combined oil consumption of Spain and the United Kingdom in 2005 was about of oil a day.

Because of their great size, supertankers can often not enter port fully loaded. These ships can take on their cargo at off-shore platforms and single-point moorings. On the other end of the journey, they often pump their cargo off to smaller tankers at designated lightering points off-coast. A supertanker's routes are generally long, requiring it to stay at sea for extended periods, up to and beyond seventy days at a time.

Smaller tankers, ranging from well under to Panamax vessels, generally carry refined petroleum products, and are known as product tankers. The smallest tankers, with capacities under generally work near-coastal and inland waterways. Although they were in the past, ships of the smaller Aframax and Suezmax classes are no longer regarded as supertankers.

Fleet characteristics

In 2005, oil tankers made up 36.9% of the world's fleet in terms of deadweight tonnage. The world's total oil tankers deadweight tonnage has increased from in 1970 to in 2005. The combined deadweight tonnage of oil tankers and bulk carriers, represents 72.9% of the world's fleet.

Cargo movement

In 2005, 2.42 billion metric tons of oil were shipped by tanker. 76.7% of this was crude oil, and the rest consisted of refined petroleum products. This amounted to 34.1% of all seaborne trade for the year. Combining the amount carried with the distance it was carried, oil tankers moved 11,705 billion metric-ton-miles of oil in 2005.

By comparison, in 1970 1.44 billion metric tons of oil were shipped by tanker. This amounted to 34.1% of all seaborne trade for that year. In terms of amount carried and distance carried, oil tankers moved 6,487 billion metric-ton-miles of oil in 1970.

The United Nations also keeps statistics about oil tanker productivity, stated in terms of metric tons carried per metric ton of deadweight as well as metric-ton-miles of carriage per metric ton of deadweight. In 2005, for each of oil tankers, 6.7 metric tons of cargo was carried. Similarly, each of oil tankers was responsible for 32,400 metric-ton miles of carriage.

The main loading ports in 2005 were located in Western Asia, Western Africa, North Africa, and the Caribbean, with 196.3, 196.3, 130.2 and 246.6 million metric tons of cargo loaded in these regions. The main discharge ports were located in North America, Europe, and Japan with 537.7, 438.4, and 215.0 million metric tons of cargo discharged in these regions.

Flag states

As of 2007, the United States Central Intelligence Agency statistics count 4,295 oil tankers of or greater worldwide. Panama was the world's largest flag state for oil tankers, with 528 of the vessels in its registry. Six other flag states had more than 200 registered oil tankers: Liberia (464), Singapore (355), China (252), Russia (250), the Marshall Islands (234) and the Bahamas (209). By way of comparison, the United States and the United Kingdom only had 59 and 27 registered oil tankers, respectively.

Vessel life cycle

In 2005, the average age of oil tankers worldwide was 10 years. Of these, 31.6% were under 4 years old and 14.3% were over 20 years old. In 2005, 475 new oil tankers were built, accounting for . The average size for these new tankers was . Nineteen of these were VLCC size, 19 were suezmax, 51 were aframax, and the rest were smaller designs. By way of comparison, , , and worth of oil tanker capacity was built in 1980, 1990, and 2000 respectively.

Ships are generally removed from the fleet through a process known as scrapping. Ship-owners and buyers negotiate scrap prices based on factors such as the ship's empty weight (called light ton displacement or LDT) and prices in the scrap metal market. In 1998, almost 700 ships went through the scrapping process at shipbreakers in places like Alang, India and Chittagong, Bangladesh. In 2004 and 2005, and respectively of oil tankers were scrapped. Between 2000 and 2005, the capacity of oil tankers scrapped each year has ranged between and . In this same timeframe, tankers have accounted for between 56.5 and 90.5 of the world's total scrapped tonnage. During this period, the average age of scrapped oil tankers has ranged from 26.9 to 31.5 years.

Vessel pricing

In 2005, the price for new oil tankers in the , , and ranges were US$43M, $58M, and $120M respectively. In 1985, these vessels would have cost $18M, $22M, and $47M respectively.

Oil tankers are often sold second-hand. In 2005, worth of oil tankers were sold used. Some representative prices for that year include $42.5M for a tanker, $60.7M for a , $73M for a , and $116M for tanker.

Current architecture

Oil tankers generally have from 8 to 12 tanks. Each tank is split into two or three independent compartments by fore-and-aft bulkheads. The tanks are numbered with tank one being the forwardmost. Individual compartments are referred to by the tank number and the athwartships position, such as "one port", "three starboard", or "six center."

A cofferdam is a small space left open between two bulkheads, to give protection from heat, fire, or collision. Tankers generally have cofferdams forward and aft of the cargo tanks, and sometimes between individual tanks. A pumproom houses all the pumps connected to a tanker's cargo lines. Some larger tankers have two pumprooms. A pumproom generally spans the total breadth of the ship.

Hull designs

A major component of tanker architecture is the design of the hull or outer structure. A tanker with a single outer shell between the product and the ocean is said to be single-hulled. Most newer tankers are double-hulled, with an extra space between the hull and the storage tanks. Hybrid designs such as double-bottom and double-sided combine aspects of single and double-hull designs. All single-hulled tankers around the world will be phased out by 2026, in accordance with the International Convention for the Prevention of Pollution from Ships, 1973 (MARPOL).

In 1998, the Marine Board of the National Academy of Science conducted a survey of industry experts regarding the pros and cons of double-hull design. Some of the advantages of the double-hull design that were mentioned include ease of ballasting in emergency situations, reduced practice of saltwater ballasting in cargo tanks decreases corrosion, increased environmental protection, cargo discharge is quicker, more complete and easier, tank washing is more efficient, and better protection in low-impact collisions and grounding.

The same report lists the following as some drawbacks to the double-hull design, including more expensive to build, more expensive canal and port expenses, ballast tank ventilation difficult, ballast tanks need continual monitoring and maintenance, increased transverse free surface, more surfaces to maintain, explosion risk in double-hull spaces if vapor detection system not fitted, cleaning mud from ballast spaces a bigger problem.

In all, double-hull tankers are said to be safer than a single-hull in a grounding incident, especially when the shore is not very rocky. The safety benefits are less clear on larger vessels and in cases of high speed impact.

Although double-hull design is superior in low energy casualties and prevents spillage in small casualties, in high energy casualties where both hulls are breached, oil can spill through the double-hull and into the sea and spills from a double-hull tanker can be significantly higher then designs like the Mid-Deck Tanker, the Coulombi Egg Tanker and even a pre-MARPOL tanker, as the last one has a lower oil column and reaches hydrostatic balance sooner.

Inert gas system

An oil tanker's inert gas system is one of the most important parts of its design. Fuel oil itself is very difficult to ignite, however its hydrocarbon vapors are explosive when mixed with air in certain concentrations. The purpose of the system is to create an atmosphere inside tanks in which the hydrocarbon oil vapors cannot burn.

As inert gas is introduced into a mixture of hydrocarbon vapors and air, it increases the lower flammable limit or lowest concentration at which the vapors can be ignited. At the same time it decreases the upper flammable limit or highest concentration at which the vapors can be ignited. When the total concentration of oxygen in the tank reaches about 11%, the upper and lower flammable limits converge and the flammable range disappears.

Inert gas systems deliver air with an oxygen concentration of less than 5% by volume. As a tank is pumped out, it's filled with inert gas and kept in this safe state until the next cargo is loaded. The exception is in cases when the tank must be entered. Safely gas-freeing a tank is accomplished by purging hydrocarbon vapors with inert gas until the hydrocarbon concentration inside the tank is under about 1%. Thus, as air replaces the inert gas, the concentration cannot rise to the lower flammable limit and is safe.

Cargo operations

Operations aboard oil tankers are governed by an established body of best practices and a large body of international law. Cargo can be moved on or off of an oil tanker in several ways. One method is for the ship to moor alongside a pier, connect with cargo hoses or marine loading arms. Another method involves mooring to offshore buoys, such as a single point mooring, and making a cargo connection via underwater cargo hoses. A third method is by ship-to-ship transfer, also known as lightering. In this method, two ships come alongside in open sea and oil is transferred manifold to manifold via flexible hoses. Lightering is sometimes used where a loaded tanker is too large to enter a specific port.

Pre-transfer preparation

Prior to any transfer of cargo, the chief officer must develop a transfer plan detailing specifics of the operation such as how much cargo will be moved, which tanks will be cleaned, and how the ship's ballasting will change. The next step before a transfer is the pretransfer conference. The pretransfer conference covers issues such as what products will be moved, the order of movement, names and titles of key people, particulars of shipboard and shore equipment, critical states of the transfer, regulations in effect, emergency and spill-containment procedures, watch and shift arrangements, and shutdown procedures.

After the conference is complete, the person in charge on the ship and the person in charge of the shore installation go over a final inspection checklist. In the United States, the checklist is called a Declaration of Inspection or DOI. Outside of the U.S., the document is called the "Ship/Shore Safety Checklist." Items on the checklist include proper signals and signs are displayed, secure mooring of the vessel, choice of language for communication, securing of all connections, that emergency equipment is in place, and that no repair work is taking place.

Loading cargo

Loading an oil tanker consists primarily of pumping cargo into the ship's tanks. As oil enters the tank, the vapors inside the tank must be somehow expelled. Depending on local regulations, the vapors can be expelled into the atmosphere or discharged back to the pumping station by way of a vapor recovery line. It is also common for the ship to move water ballast during the loading of cargo to maintain proper trim.

Loading starts slowly at a low pressure to ensure that equipment is working correctly and that connections are secure. Then a steady pressure is achieved and held until the "topping-off" phase when the tanks are nearly full. Topping off is a very dangerous time in handling oil, and the procedure is handled particularly carefully. Tank-gauging equipment is used to tell the person in charge how much space is left in the tank, and all tankers have at least two independent methods for tank-gauging. As the tanker becomes full, crew members open and close valves to direct the flow of product and maintain close communication with the pumping facility to decrease and finally stop the flow of liquid.

Unloading cargo

The process of moving oil off of a tanker is similar to loading, but has some key differences. The first step in the operation is following the same pretransfer procedures as used in loading. When the transfer begins, it is the ship's cargo pumps that are used to move the product ashore. As in loading, the transfer starts at low pressure to ensure that equipment is working correctly and that connections are secure. Then a steady pressure is achieved and held during the operation. While pumping, tank levels are carefully watched and key locations, such as the connection at the cargo manifold and the ship's pumproom are constantly monitored. Under the direction of the person in charge, crew members open and close valves to direct the flow of product and maintain close communication with the receiving facility to decrease and finally stop the flow of liquid.

Tank cleaning

Tanks must be cleaned from time to time for various reasons. One reason is to change the type of product carried inside a tank. Also, when tanks are to be inspected or maintenance must be performed within a tank, it must be not only cleaned, but made "gas-free."

On most crude-oil tankers, a special crude oil washing (COW) system is part of the cleaning process. The COW system circulates part of the cargo through the fixed tank-cleaning system to remove wax and asphaltic deposits. Tanks that carry less viscous cargoes are washed with water. Fixed and portable automated tank cleaning machines, which clean tanks with high-pressure water jets, are widely used. Some systems use rotating high-pressure water jets to spray hot water on all the internal surfaces of the tank. As the spraying takes place, the liquid is pumped out of the tank.

After a tank is cleaned, it may be gas-freed." This involves blowing fresh air into the tank to force accumulated gasses out. Specially trained personnel monitor the tank's atmosphere, often using hand-held gas indicators which measure the percentage of hydrocarbons present. When this percentage drops below a value specified in tank-vessel regulations, the tank is declared to be gas-free. After a tank is gas-free, it may be further hand-cleaned in a manual process known as mucking. Mucking requires protocols for entry into confined spaces, protective clothing, designated safety observers, and possibly the use of airline respirators.

Special-use oil tankers

Some sub-types of oil tankers have evolved to meet specific military and economic needs. These sub-types include naval replenishment ships, oil-bulk-ore combination carriers, floating storage and offloading units (FSOs) and floating production storage and offloading units (FPSOs).

Replenishment ships

Replenishment ships, known as oilers in the United States and fleet tankers in Commonwealth countries, are ships that can provide oil products to naval vessels while on the move. This process, known as underway replenishment, extends the length of time a naval vessel can stay at sea, as well as her effective range. Prior to underway replenishment, naval vessels had to enter a port or anchor to take on fuel. In addition to fuel, replenishment ships may also deliver water, ammunition, rations, stores and personnel.

Ore-bulk-oil carriers

An ore-bulk-oil carrier, also known as combination carrier or OBO, is a ship designed to be capable of carrying wet or dry bulk cargoes. This design was intended to provide flexibility in two ways. Firstly, an OBO would be able the shift between the dry and wet bulk trades based on market conditions. Secondly, an OBO could carry oil on one leg of a voyage and return carrying dry bulk, reducing the number of unprofitable ballast voyages it would have to make.

In practice, the flexibility which the OBO design allows has gone largely unused, as these ships tend to specialize in either the liquid or dry bulk trade. Also, these ships have endemic maintenance problems. On one hand, due to a less specialized design, an OBO suffers more from wear and tear during dry cargo onload than a bulker. On the other hand, components of the liquid cargo system, from pumps to valves to piping, tend to develop problems when subjected to periods of disuse. These factors have contributed to a steady reduction in the number of OBO ships worldwide since the 1970s.

One of the more famous OBOs was the MV Derbyshire of which in September 1980 became the largest British ship ever lost at sea. It sank in a Pacific typhoon while carrying a cargo of iron ore from Canada to Japan.

Floating storage units

Floating storage and offloading units (FSO) are used worldwide by the offshore oil industry to receive oil from nearby platforms and store it until it can be offloaded onto oil tankers. A similar system, the floating production storage and offloading unit (FPSO), has the ability to process the product while it is onboard. These floating units reduce oil production costs and offer, mobility, large storage capacity, and production versatility.

FPSO and FSOs are often created out of old, stripped-down oil tankers, but can be made from new-built hulls Shell España first used a tanker as an FPSO was in August 1977. An example of a FSO that used to be an oil tanker is the Knock Nevis. These units are usually moored to the seabed through a spread mooring system. A turret-style mooring system can be used in areas prone to severe weather. This turret system lets the unit rotate to minimize the effects of sea-swell and wind.


Oil spills have devastating effects on the environment. Crude oil contains polycyclic aromatic hydrocarbons (PAHs) which are very difficult to clean up, and last for years in the sediment and marine environment. Marine species constantly exposed to PAHs can exhibit developmental problems, susceptibility to disease, and abnormal reproductive cycles.

By the sheer amount of oil carried, modern oil tankers must be considered a threat to the environment. As discussed above, a VLCC tanker can carry of crude oil, or 62,000,000 gallons. This is more than six times the amount spilled in the widely known Exxon Valdez incident. In this spill, the ship ran aground and dumped of oil into the ocean in March 1989. Despite efforts of scientists, managers, and volunteers over 400,000 seabirds, about 1,000 sea otters, and immense numbers of fish were killed.

Oil tankers are only one source of oil spills. According to the United States Coast Guard, 35.7% of the volume of oil spilled in the United States from 1991 to 2004 came from tank vessels (ships/barges), 27.6% from facilities and other non-vessels, 19.9% from non-tank vessels, and 9.3% from pipelines; 7.4% from mystery spills. On the other hand, only 5% of the actual spills came from oil tankers, while 51.8% came from other kinds of vessels. The detailed statistics for 2004 shown in the table below show tank vessels responsible for somewhat less than 5% of the number of total spills but more than 60% of the volume. In summary, spills are much more rare but much more serious on tank vessels than on non-tank vessels.

The International Tanker Owners Pollution Federation has tracked 9,351 accidental spills that have occurred since 1974. According to this study, most spills result from routine operations such as loading cargo, discharging cargo, and taking on fuel oil. 91% of the operational oil spills are small, resulting in less than 7 metric tons per spill. On the other hand, spills resulting from accidents like collisions, groundings, hull failures, and explosions are much larger, with 84% of these involving losses of over 700 metric tons.

Following the Exxon Valdez spill, the United States passed the Oil Pollution Act of 1990 (OPA-90), which included a stipulation that all tankers entering its waters be double-hulled by 2015. Following the sinkings of the Erika (1999) and Prestige (2002), the European Union passed its own stringent anti-pollution packages (known as Erika I, II, and III), which also require all tankers entering its waters to be double-hulled by 2010. The Erika packages are controversial because they introduced the new legal concept of "serious negligence".

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