Oil tanker

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The Knock Nevis, formerly the world's largest ship, has been converted to an oil storage terminal.
Class overview
Name:	Oil tanker
Subclasses:	Handymax, Handysize, Panamax, Capesize
Built:	c. 1863 – present
In service:	4,024 (above 10,000 long tons of deadweight (DWT)).[1]
General characteristics
Type:	Cargo ship
Capacity:	up to 550,000 DWT
Notes:	Rear house, full hull, midships pipeline

Oil tankers, also known as petroleum tankers, are ships designed for the bulk transport of oil. There are two basic types of oil tanker: the crude tanker and the product tanker.^[2] Crude tankers move large quantities of unrefined crude oil from its point of extraction to refineries.^[2] 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 long tons of deadweight (DWT) to the mammoth supertankers of 550,000 DWT.

Tankers move approximately 2 billion tons of oil every year.^[3] Second only to pipelines in terms of efficiency,^[3] the cost of tanker transport amounts to only two or three U.S. cents per gallon.^[3]

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.

Contents 1 History 1.1 The Nobel Brothers 1.2 World War I 1.3 World War II 1.4 The supertanker era 2 Size categories 3 Fleet characteristics 3.1 Cargo movement 3.2 Flag states 3.3 Vessel life cycle 3.4 Vessel pricing 4 Current architecture 4.1 Hull designs 4.2 Inert gas system 5 Cargo operations 5.1 Pre-transfer preparation 5.2 Loading cargo 5.3 Unloading cargo 5.4 Tank cleaning 6 Special-use oil tankers 6.1 Replenishment ships 6.2 Ore-bulk-oil carriers 6.3 Floating storage units 7 Pollution 8 See also 9 Notes 10 References 11 Further reading 12 External links

[edit] History

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.^[5] 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.^[6] In the 1850s, the Pennsylvania oil fields became a major supplier of oil, and a center of innovation.^[6]

Break-bulk boats and barges were originally used to transport Pennsylvania oil in 40-gallon wooden barrels.^[6] But transport by barrel had several problems. The first problem was weight: the standard empty barrel weighed 64 pounds, representing 20% of the total weight of a full barrel.^[7] Also, barrels were leaky, and could only be carried one way.^[7] 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.^[7]

The movement of oil in bulk was attempted in many places and in many ways. Modern oil pipelines have existed since 1860.^[6] In 1863, two sail-driven tankers were built on England's River Tyne.^[8] These were followed by the first oil-tank steamer, the Vaderland, which was purchased by Palmers Shipbuilding and Iron Company.^[8] 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.^[6]

[edit] The Nobel Brothers

Ludvig Nobel, brother of Alfred Nobel, was a pioneer in the development of early oil tankers. He first experimented with carrying oil in bulk on single-hulled barges.^[7] 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.^[9] Other challenges included allowing for the cargo to expand and contract due to temperature changes, and providing a method to ventilate the tanks.^[9]

The world's first successful oil tanker was Nobel's Zoroaster. He designed this ship in Lindholmen-Motala in Sweden with Sven Almqvist.^[9] The contract to build it was signed in January 1878, and it made its first run later that year from Baku to Astrakhan.^[9] The Zoroaster design was widely studied and copied, with Nobel refusing to patent any part of it.^[9] In October 1878, he ordered two more tankers of the same design: the Buddha and the Nordenskjöld.^[9]

Zoraster carried its 242 tons of kerosene cargo in two iron tanks joined by pipes.^[9] One tank was forward of the midships engineroom and the other was aft.^[9] The ship also featured a set of 21 vertical watertight compartments for extra buoyancy.^[9] The ship had a length overall of 184 feet (56 m), a beam of 27 feet (8.2 m), and a draft of 9 feet (2.7 m).^[9] 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.^[9]

Nobel then began to adopt a single-hull design, where the ship's hull forms part of its tank structure.^[9] In November 1880, he ordered his first single-hulled tanker, the Moses.^[9] Within a year, he ordered seven more single-hulled tankers: the Mohammed, Tatarin, Bramah, Spinoza, Socrates, Darwin, Koran, Talmud, and Calmuck.^[9]

The Nobel company experienced one of the first oil tanker disasters. In 1881, the Zoroaster's sister-ship, the Nordenskjöld exploded in Baku while taking on kerosene.^[9] The pipe carrying the cargo was jerked away from the hold when the ship was hit by a gust of wind.^[9] Kerosene then spilled onto the deck and down into the engineroom, where mechanics were working in the light of kerosene lanterns.^[9] The ship then exploded, killing half the crew.^[9] Nobel responded to the disaster by creating a flexible, leakproof loading pipe which was much more resistant to spills.^[10]

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.^[11] Instead of one or two large holds, Swan's design used several holds which spanned the width, or beam, of the ship.^[11] These holds were further subdivided into port and starboard sections by a longitudinal bulkhead.^[11] 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.^[12] But this approach of dividing the ship's storage space into smaller tanks virtually eliminated free-surface problems.^[12] This approach, almost universal today, was first used by Swan in the Nobel tankers Blesk, Lumen, and Lux.^[11][13]

In 1903, the Nobel brothers built two oil tankers which ran on internal combustion engines, as opposed to the older steam engines.^[8] The Vandale and Sarmat were each capable of carrying 750 tons of refined oil and powered by 360 horsepower (270 kW) diesel motors.^[14] This same firm soon went on to make much larger oil tankers, such as the Emanuel Nobel and Karl Hagelin, 4,600-ton kerosene tankers with 1,200 horsepower (890 kW) engines.^[15]

[edit] World War I

The fleet oiler USS Maumee, launched on April 17, 1915, pioneered the technique of underway replenishment.^[16] A large ship at the time, at 14,500 deadweight tons, Maumee began refuelling destroyers en route to Britain at the outset of World War I.^[16] 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.^[16] 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.^[16]

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.^[17]

[edit] World War II

Oil tankers, particularly the T-2 tanker, played an important part in World War II. The 16,613 DWT T2-SE-A1 was the most popular variant, with nearly 500 built during the war.^[18] After the war, these tankers were used commercially for decades, and many were sold on the international market.^[18]

Until 1956, tankers were designed to be able to navigate the Suez Canal.^[18] This size restriction became much less of a priority after the closing of the canal during the Suez Crisis of 1956.^[18] Forced to move oil around the Cape of Good Hope, shipowners realized that bigger tankers were the key to more efficient transport.^[18]

[edit] The supertanker era

Tankers have grown significantly in size since World War II.^[19] A typical T-2 tanker of the World War II era was 532 feet (162 m) long and had a capacity of 16,500 DWT.^[20] A modern ultra-large crude carrier (ULCC) can be 1,300 feet (400 m) long and have a capacity of 500,000 DWT.^[20] 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.^[19] Fierce competition among shipowners also played a part.^[19] 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.^[19]

In 1958, U.S. shipping magnate Daniel K. Ludwig broke the barrier of 100,000 long tons of heavy displacement.^[21] His Universe Apollo displaced 104,500 long tons, a 23% increase from the previous record-holder, Universe Leader which also belonged to Ludwig.^[21][22]

The world's largest supertanker ever was built in 1979 at as the Oppama Shipyard of Sumitomo Heavy Industries, Ltd. as the Seawise Giant. This ship was built with a capacity of 564,763 DWT, a length overall of 458.45 metres (1,504.1 ft) and a draft of 24.611 metres (80.74 ft).^[23] She has 46 tanks, 31,541 square metres (339,500 sq ft) of deck, and is too large to pass through the English Channel.^[24]

Seawise Giant was renamed Happy Giant in 1989, Jahre Viking in 1991.^[23] From 1979 to 2004, she was owned by Loki Stream AS, at which point she was bought by First Olsen Tankers Pte. Ltd., renamed Knock Nevis and converted into a permanently moored storage tanker.^[23][24]

As of 2008, the worlds four largest working supertankers are the sister ships currently known as the TI Asia, TI Europe, TI Oceania, and TI Africa.^[25][26] These ships were built in 2002 and 2003 as the Hellespont Alhambra, Hellespont Metropolis, Hellespont Tara and Fairfax for the Greek Hellespont Steamship Corporation.^[27] Hellespont sold these ships to Overseas Shipholding Group and Euronav NV in 2004.^[28]

Each of the four sister ships has a capacity of over 441500 DWT, a length overall of 308.0 metres (1,010 ft) and a cargo capacity of 3,166,353 barrels (503,409,900 l).^[29] The first ULCC tankers to be built for some 25 years, they were also the first ULCCs to be double-hulled.^[27] To differentiate them from smaller ULCCs, these ships are sometimes given the V-Plus size designation.^[30][29] 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.^[25]

With the exception of the pipeline, the tanker is the most cost-effective way to move oil today.^[31] Worldwide, tankers carry some 2,000,000,000 barrels (320,000,000,000 l) annually, and the cost of transportation by tanker amounts to only US$0.02 per gallon at the pump.^[31]

[edit] Size categories

Oil tanker size categories
Name	Size in DWT	New price[32]	Used price[33]
Seawaymax	10,000-60,000[34]	$43M	$42.5M
Panamax	55,000-80,000[35]
Aframax	80,000-120,000[36]	$58M	$60.7M
Suezmax	120,000-200,000[37]
VLCC	150,000-300,000[38]	$120M	$116M
ULCC	over 300,000[38]
V Plus	over 440,000

Merchant oil tankers carry a wide range of hydrocarbon liquids ranging from crude oil to refined petroleum products.^[2] Their size is measured in deadweight tons (DWT). Crude carriers are among the largest, ranging from 55,000 DWT Panamax-sized vessels to ultra-large crude carriers (ULCCs) of over 440,000 DWT.^[39]

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 250,000 DWT. These ships can transport two million barrels of oil.^[39] By way of comparison, the oil consumption of Spain and the UK combined is about 3.2 million barrels (510,000 m³) of oil a day.^[40]

Because of their great size, supertankers can't generally enter ports fully loaded.^[19] These ships generally take on their cargo at off-shore platforms and single-point moorings.^[19] On the other end of the journey, they often pump their cargo off to smaller tankers at designated lighterning points off-coast.^[19] 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.^[19]

Smaller tankers, ranging from well under 10,000 DWT to 80,000 DWT Panamax vessels, generally carry refined petroleum products, and are known as product tankers.^[39] The smallest tankers, with capacities under 10,000 DWT generally work near-coastal and inland waterways.^[39] Although they were in the past, ships of the smaller Aframax and Suezmax classes are no longer regarded as supertankers.^[41]

[edit] Fleet characteristics

In 2005, oil tankers made up 36.9% of the world's fleet in terms of deadweight tonnage.^[42] The world's total oil tankers deadweight tonnage has increased from 326.1 million DWT in 1970 to 960.0 million DWT in 2005.^[42] The combined deadweight tonnage of oil tankers and bulk carriers, represents 72.9% of the world's fleet.^[43]

[edit] Cargo movement

In 2005, 2.42 billion metric tons of oil were shipped by tanker.^[44] 76.7% of this was crude oil, and the rest consisted of refined petroleum products.^[44] This amounted to 34.1% of all seaborne trade for the year.^[44] Combining the amount carried with the distance it was carried, oil tankers moved 11,705 billion metric-ton-miles of oil in 2005.^[45]

By comparison, in 1970 1.44 billion metric tons of oil were shipped by tanker.^[46] This amounted to 34.1% of all seaborne trade for that year.^[47] In terms of amount carried and distance carried, oil tankers moved 6,487 billion ton-miles of oil in 1970.^[45]

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 ton-miles of carriage per ton of deadweight.^[48] In 2005, for each 1 DWT of oil tankers, 6.7 metric tons of cargo was carried.^[48] Similarly, each 1 DWT of oil tankers was responsible for 32,400 metric-ton miles of carriage.^[48]

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.^[49] 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.^[49]

[edit] Flag states

As of 2007, the United States Central Intelligence Agency statistics count 4,295 oil tankers of 1,000 long tons of deadweight (DWT) or greater worldwide.^[50] Panama was the world's largest flag state for oil tankers, with 528 of the vessels in its registry.^[50] 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).^[50] By way of comparison, the United States and the United Kingdom only had 59 and 27 registered oil tankers, respectively.^[50]

[edit] Vessel life cycle

In 2005, the average age of oil tankers worldwide was 10 years.^[51] Of these, 31.6% were under 4 years old and 14.3% were over 20 years old.^[52]

In 2005, 475 new oil tankers were built, accounting for 30.7 million DWT.^[53] The average size for these new tankers was 64,632 DWT.^[53] Nineteen of these were VLCC size, 19 were suezmax, 51 were aframax, and the rest were smaller designs.^[53] By way of comparison, 8.0 million DWT, 8.7 million DWT, and 20.8 million DWT worth of oil tanker capacity was built in 1980, 1990, and 2000 respectively.^[53]

Ships are generally removed from the fleet through a process known as scrapping.^[54] 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.^[55] In 1998, almost 700 ships went through the scrapping process at shipbreakers in places like Alang, India and Chittagong, Bangladesh.^[54] In 2004 and 2005, 7.8 million DWT and 5.7 million DWT respectively of oil tankers were scrapped.^[51] Between 2000 and 2005, the capacity of oil tankers scrapped each year has ranged between 5.6 million DWT and 18.4 million DWT.^[56] In this same timeframe, tankers have accounted for between 56.5 and 90.5 of the world's total scrapped tonnage.^[56] During this period, the average age of scrapped oil tankers has ranged from 26.9 to 31.5 years.^[56]

[edit] Vessel pricing

In 2005, the price for new oil tankers in the 32-45,000 DWT, 80-105,000 DWT, and 250-280,000 DWT ranges were US$43M, $58M, and $120M respectively.^[57] In 1985, these vessels would have cost $18M, $22M, and $47M respectively.^[57]

Oil tankers are often sold second-hand. In 2005, 27.3 million DWT worth of oil tankers were sold used.^[58] Some representative prices for that year include $42.5M for a 40,000 DWT tanker, $60.7M for a 80-95,000 DWT, $73M for a 130-150,000 DWT, and $116M for 250-280,00 DWT tanker.^[58]

[edit] Current architecture

Oil tankers generally have from 8 to 12 tanks.^[13] Each tank is split into two or three independent compartments by fore-and-aft bulkheads.^[13] 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."^[13]

A cofferdam is a small space left open between two bulkheads, to give protection from heat, fire, or collision.^[13] Tankers generally have cofferdams forward and aft of the cargo tanks, and sometimes between individual tanks.^[59]

A pumproom houses all the pumps connected to a tanker's cargo lines.^[13] Some larger tankers have two pumprooms.^[13] A pumproom generally spans the total breadth of the ship.^[13]

[edit] 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.^[60] Most newer tankers are double-hulled, with an extra space between the hull and the storage tanks.^[60] Self-descriptive hybrid designs such as double-bottom and double-sided combine aspects of single and double-hull designs.^[60]

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.^[60]

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,^[61]
• reduced practice of saltwater ballasting in cargo tanks decreases corrosion,^[62]
• increased environmental protection,^[62]
• cargo discharge is quicker, more complete and easier,^[62]
• tank washing is more efficient,^[62] and
• better protection in low-impact collisions and grounding.^[62]

The same report lists the following as some drawbacks to the double-hull design:

• more expensive to build,^[63]
• more expensive canal and port expenses,^[63]
• ballast tank ventilation difficult,^[63]
• ballast tanks need continual monitoring and maintenance,^[63]
• increased transverse free surface,^[63]
• more surfaces to maintain,^[63]
• explosion risk in double-hull spaces if vapor detection system not fitted,^[64]
• cleaning mud from ballast spaces a bigger problem.^[64]

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.^[65]The safety benefits are less clear on larger vessels and in cases of high speed impact.^[62]

[edit] Inert gas system

An oil tanker's inert gas system is one of the most important parts of its design.^[66] Fuel oil itself is very difficult to ignite, however its hydrocarbon vapors are explosive when mixed with air in certain concentrations.^[67] The purpose of the system is to create an atmosphere inside tanks in which the hydrocarbon oil vapors cannot burn.^[66]

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.^[68] At the same time it decreases the upper flammable limit or highest concentration at which the vapors can be ignited.^[68] When the total concentration of oxygen in the tank reaches about 11%, the upper and lower flammable limits converge and the flammable range disappears.^[69]

Inert gas systems deliver air with an oxygen concentration of less than 5% by volume.^[66] As a tank is pumped out, it's filled with inert gas and kept in this safe state until the next cargo is loaded.^[70] The exception is in cases when the tank must be entered.^[70] 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%.^[70] Thus, as air replaces the inert gas, the concentration cannot rise to the lower flammable limit and is safe.^[70]

[edit] Cargo operations

Operations aboard oil tankers are governed by an established body of best practices and a large body of international law.^[71]

[edit] 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.^[72] The next step before a transfer is the pretransfer conference.^[73] The pretransfer conference covers issues such as:

• What products will be moved,^[73]
• In what order will products be moved,^[73]
• Names and titles of key people,^[73]
• Particulars of shipboard and shore equipment,^[73]
• Critical stages of the transfer,^[73]
• Federal, state, and local regulations in effect,^[73]
• Emergency and spill-containment procedures,^[73]
• Watch and shift arrangements,^[73] and
• Shutdown procedures^[73]

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. ^[73] In the United States, the checklist is called a Declaration of Inspection or DOI.^[73] Outside of the U.S., the document is called the "Ship/Shore Safety Checklist."^[73] Items on the checklist include:

• Proper signals and signs are displayed,^[73]
• The vessel is securely moored,^[73]
• A language for all communication is chosen,^[74]
• All connections are secure,^[74]
• Emergency equipment is in place,^[74] and
• No repair work is taking place.^[74]

[edit] Loading cargo

Loading an oil tanker consists primarily of pumping cargo into the ship's tanks.^[74] As oil enters the tank, the vapors inside the tank must be somehow expelled.^[74] 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.^[74] It is also common for the ship to move water ballast during the loading of cargo to maintain proper trim.^[74]

Loading starts slowly at a low pressure to ensure that equipment is working correctly and that connections are secure.^[74] Then a steady pressure is achieved and held until the "topping-off" phase when the tanks are nearly full.^[74] Topping off is a very dangerous time in handling oil, and the procedure is handled particularly carefully.^[74] 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.^[74] 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.^[74]

[edit] Unloading cargo

The process of moving oil off of a tanker is similar to loading, but has some key differences.^[75] The first step in the operation is following the same pretransfer procedures as used in loading.^[76] When the transfer begins, it is the ship's cargo pumps that are used to move the product ashore.^[76] As in loading, the transfer starts at low pressure to ensure that equipment is working correctly and that connections are secure.^[76] Then a steady pressure is achieved and held during the operation.^[77] 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.^[75] 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.^[75]

[edit] 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.^[78] 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."^[78]

On most crude-oil tankers, a special crude oil washing (COW) system is part of the cleaning process.^[78] The COW system circulates part of the cargo through the fixed tank-cleaning system to remove wax and asphaltic deposits.^[78]

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.^[78] Some systems use rotating high-pressure water jets to spray hot water on all the internal surfaces of the tank.^[78] As the spraying takes place, the liquid is pumped out of the tank.^[78]

After a tank is cleaned, it may be gas-freed."^[79] This involves blowing fresh air into the tank to force accumulated gasses out.^[79] Specially trained personnel monitor the tank's atmosphere, often using hand-held gas indicators which measure the percentage of hydrocarbons present.^[79] When this percentage drops below a value specified in tank-vessel regulations, the tank is declared to be gas-free.^[79]

After a tank is gas-free, it may be further hand-cleaned in a manual process known as mucking.^[80] Mucking requires protocols for entry into confined spaces, protective clothing, designated safety observers, and possibly the use of airline respirators.^[80]

[edit] 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).

[edit] 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.^[81] Prior to underway replenishment, naval vessels had to enter a port or anchor to take on fuel.^[16] In addition to fuel, replenishment ships may also deliver water, ammunition, rations, stores and personnel.^[17]

[edit] 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.^[82] This design was intended to provide flexibility in two ways.^[83] Firstly, an OBO would be able the shift between the dry and wet bulk trades based on market conditions.^[83] 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.^[84]

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.^[84] Also, these ships have endemic maintenance problems.^[83] On one hand, due to a less specialized design, an OBO suffers more from wear and tear during dry cargo onload than a bulker.^[83] 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.^[83] These factors have contributed to a steady reduction in the number of OBO ships worldwide since the 1970s.^[84]

One of the more famous OBOs was the MV Derbyshire of 180,000 DWT which in September 1980 became the largest British ship ever lost at sea.^[82] It sank in a Pacific typhoon while carrying a cargo of iron ore from Canada to Japan.^[82]

[edit] Floating storage units

Floating storage and offloading units or FSOs 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.^[85] A similar system, the Floating production storage and offloading unit, or FPSO, has the ability to process the product while it is onboard.^[85] These floating units reduce oil production costs and offer, mobility, large storage capacity, and production versatility.^[85]

FPSO and FSOs are often created out of old, stripped-down oil tankers, but can be made from new-built hulls^[85] Shell España first used a tanker as an FPSO was in August 1977.^[86] An example of a FSO that used to be an oil tanker is the Knock Nevis.^[87] These units are usually moored to the seabed through a spread mooring system.^[85] A turret-style mooring system can be used in areas prone to severe weather.^[85] This turret system lets the unit to rotate to minimize the effects of sea-swell and wind.^[85]

[edit] Pollution

Oil spills have devastating effects on the environment. Crude oil contains polycyclic aromatic hydrocarbons (PAHs) which is very difficult to clean up, and lasts for years in the sediment and marine environment.^[89] 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 something of a threat to the environment. As discussed above, a VLCC tanker can carry 2 million barrels (320,000 m³) 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 10.8 million gallons of oil into the ocean in March of 1989. Despite efforts of scientists, managers, and volunteers over 400,000 seabirds, about 1,000 sea otters, and immense numbers of fish were killed.^[89]

But how much do tankers contribute to oil pollution in the oceans? According to the United States Coast Guard, 35.7 % of the volume of oil spilled in the U.S. 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.^[90] On the other hand, only 5% of the actual spills came from oil tankers, while 51.8% came from other kinds of vessels.^[90] 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.

U.S.C.G Spill Compendium Annual Data for 2004[91]
Source	Number of spills	% of spill incidents	Spill volume (gallons)	% of spill volume	Average spill size	Median spill size	Maximum spill size
TANKSHIP	35	0.90%	636,834	45.00%	18,195	1	329,678
TANKBARGE	143	3.70%	215,822	15.20%	1,509	3	151,200
ALL OTHER VESSELS	1527	39.20%	453,901	32.00%	297	3	335,732
FACILITIES	1099	28.20%	42,675	3.00%	39	1	2,100
PIPELINES	1	0.00%	15,000	1.10%	15,000	15,000
ALL OTHER NON-VESSEL SOURCES	37	0.90%	12,781	0.90%	345	5	12,000
UNKNOWN or OTHER	1055	27.10%	39,700	2.80%	38	1	8,000
YEAR-END STATISTICS	3897	100.00%	1,416,714	100.00%	364	2	335,732

The International Tanker Owners Pollution Federation has tracked 9,351 accidental spills that have occurred since 1974.^[92] According to this study, most spills result from routine operations such as loading cargo, discharging cargo, and taking on fuel oil.^[92] 91% of the operational oil spills are small, resulting in less than 7 tons per spill.^[92] 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 tons.^[92]

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".^[93]

[edit] See also

• Category:Tankers
• Aristotle Onassis
• Crude oil washing
• Daniel K. Ludwig
• List of oil spills
• List of replenishment ships of the Royal Fleet Auxiliary
• List of tankers
• List of Type T2 Tanker names
• Marine transfer operations
• MARPOL 73/78
• Prestige oil spill
• Replenishment oiler
• Stavros Niarchos
• T2 tanker
• Tanker (ship)

[edit] Notes

[edit] References

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• Det Norske Veritas (2008). Knock Nevis. DNV Exchange. Det Norske Veritas. Retrieved on 2008-04-08.
• Douet, M (July 1999). "Combined Ships: An Empirical Investigation About Versatility". Maritime Policy and Management 26 (3): pp. 231-248. Taylor and Francis, Ltd.. 
• Encyclopædia Britannica (1911). "Petroleum". Encyclopædia Britannica (11th edition) 21. Ed. Chisholm, Hugh. 316-322. Retrieved on 2008-02-22. 
• Encyclopædia Britannica (1911). "Ship". Encyclopædia Britannica (11th edition) 24. Ed. Chisholm, Hugh. 881-889. Retrieved on 2008-02-22. 
• European Commission / European Maritime Safety Agency (2005). Double Hull Tankers: High Level Panel of Experts Report. 
• Evangelista, Joe (2002). "WS50" (PDF). Surveyor (Winter 2002): pp. 10-11. Houston: American Bureau of Shipping. 
• Hayler, William B.; Keever, John M. (2003). American Merchant Seaman's Manual. Cornell Maritime Pr. ISBN 0-87033-549-9. 
• Huber, Mark (2001). Tanker operations: a handbook for the person-in-charge (PIC). Cambridge, MD: Cornell Maritime Press. ISBN 0-87033-528-6. 
• Institute of Shipping Economics and Logistics (2005). ISL Market Analysis p. 3. Institute of Shipping Economics and Logistics. Retrieved on 2008-04-26.
• International Chamber of Shipping (1996). International Safety Guide for Oil Tankers and Terminals (ISGOTT). New York: Hyperion Books. ISBN 1-85609-081-7. 
• Marine Board (1998). Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990 (1998), Marine Board Commission on Engineering and Technical Systems. Washington, D.C.: National Academy Press. ISBN 0-309-06370-1. Retrieved on 2007-04-10. 
• Marine Log (2008). The Liberty Ship and the T-2 Tanker (1941). Ships of the Century. Marine Log. Retrieved on 2008-04-08.
• Occupational Safety & Health Administration (OSHA) (2008-01-30). Process: Tank Cleaning. Shipbuilding and Ship Repair - Hazards and Solutions. Department of Labor. Retrieved on 2008-04-08.
• Office of Data and Economic Analysis (July 2006). World Merchant Fleet 2001–2005 (PDF). United States Maritime Administration. Retrieved on March 13, 2007.
• Overseas Shipholding Group (2008-02-22). Overseas Shipholding Group Fleet List. Overseas Shipholding Group. Retrieved on 2008-04-08.
• Overseas Shipholding Group (2008-02-28). Overseas Shipholding Group Enters FSO Market. Press Releases. Overseas Shipholding Group. Retrieved on 2008-04-08.
• Sawyer, L. A.; Mitchell, W. O. (1987). Sailing ship to supertanker: the hundred-year story of British Esso and its ships. Lavenham, Suffolk: Terence Dalton. ISBN 0-86138-055-X. 
• Singh, Baljit (July 11, 1999). "The world’s biggest ship". The Times (of India). 
• Tarman, Daniel; Heitmann, Edgar (2008-04-07). Case Study II: Derbyshire, Loss of a Bulk Carrier. Educational Case Studies. Ship Structure Committee. Retrieved on 2008-04-07.
• Tolf, Robert W. (1976). "4: The World's First Oil Tankers", The Russian Rockefellers: The Saga of the Nobel Family and the Russian Oil Industry. Hoover Press. ISBN 0817965815. 
• Transport Canada (1984). Standard for Inert Gas Systems. 
• Turpin, Edward A.; McEwen, William A. (1980). Merchant Marine Officers' Handbook, 4th, Centreville, MD: Cornell Maritime Press. ISBN 0-87038-056-X. 
• United Nations Council on Trade and Development (UNCTAD) (2006). Review of Maritime Transport, 2006. New York and Geneva: United Nations. 

[edit] Further reading

• Spyrou, Andrew G.. From T-2 to Supertanker: Development of the Oil Tanker, 1940-2000. [United States]: iUniverse, Inc. ISBN 0-595-36068-8. 
• Sullivan, George. Supertanker!: The Story of the World's Biggest Ships. New York: Dodd Mead. ISBN 0-396-07527-4. 
• Stopford, Martin (1997). Maritime economics. New York: Routledge. ISBN 0-415-15309-3. 

[edit] External links

• Bill Willis. Supertankers
• Intertanko - the society of International Tanker Operators
• The International Maritime Organization - Tanker Safety (for double-hulls)
• Largest Ship in the World - Blog covering the largest ships in the world

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