Oil tanker

The Knock Nevis, formerly the world's largest ship, has been converted to an oil storage terminal.
General characteristics
Type: Cargo ship
Epoch: ~1863–present
Location: Worldwide
In use: 4,024 (above 10,000DWT).
Sub-types: Naval oiler
Classes: Handymax, Handysize, Panamax, Capesize
Current characteristics
Size: 10 - 650,000DWT
Materials: steel
Other: rear house, full hull, midships pipeline


Oil tankers, also known as petroleum tankers, or tankers are ships designed to transport liquids in bulk. There are two basic types of oil tanker: 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, usually much smaller, are used to move petrochemicals from refineries to points near consuming markets.

Also there are some specialized types of oil tankers that were designed for various purposes. A good example of such a vessel is the naval oiler, a tanker which fuels a moving vessel. Combination Ore-bulk-oil carriers and permanently moored floating storage units are two other examples of modifications on the standard oil tanker design.

Oil tankers are classified by their occupation and size. The size classes range from coastal or inland tankers of a few thousand tons to the mammoth supertankers of 650,000 tons.

In the past, oil tankers have been involved in a number of damaging, and sometimes catastrophic and high-profile oil spills. As a result, they are subject to strict design and operational regulations.



The technology of oil transportation changed alongside with the development of the oil industry. Even tough the use of oil reaches to prehistory, the modern exploitation of oil dates back to 1850 when James Young started using it to manufacture paraffin. Back then the oil from Upper Burma was transported by river using boats. Around the same time, Pennsylvania oil fields became a major supplier of oil and a center of innovation.

In 1863, two sail-driven tankers were built on the River Tyne. Soon after the first oil-tank steamer, named the Vaderland was built. In 1877, Ludvig Nobel designed the first successful oil tanker, the Zoroaster. This tanker had a displacement of 250 tons, with a length overall of 184 feet, a beam of 27 feet, and a draft of nine feet.

The Zoroaster sailed the Caspian Sea from the Apsheron Peninsula, near Baku, in what was then the Russian Empire. Earlier tanker designs had huge safety problems due to the free surface effect: as the tanker transported the oil across the sea, oil would move more quickly than the water, which often caused the ship to capsize. Nobel's tanker had proved itself in the unforgiving waters of the Atlantic ocean and his basic concept revolutionized the transport of oil.

At the beginning of the 20th century, the Nobel brothers built two oil tankers that used internal combustion engines. The Vandale and Sarmat were designed to carry 750 tons of oil and were powerd by 360 horsepower diesel motors. Soon they started designing and building much larger vessels, such as the Emanuel Nobel and Karl Hagelin, which were 4,600 ton tankers.

Size categories

Oil Tanker Size Categories
Photo Class Tonnage (DWT) Typical load (bbl)
The Fredericksburg was a coastal oil tanker of 40,000DWT.
The Fredericksburg was a coastal oil tanker of 40,000DWT.
Seawaymax 10,000-60,000  
Panamax 60,000-80,000  
The Aframax tanker Gerd Knutsen at the wharf in Brest.
The Aframax tanker Gerd Knutsen at the wharf in Brest.
Aframax 80,000-120,000 650,000 - 800,000
Suezmax 130,000-160,000 1,000,000
The informal term Supertanker is generally reserved for vessels over 250,000DWT.
The informal term Supertanker is generally reserved for vessels over 250,000DWT.
VLCC 200,000-320,000 2,000,000
ULCC over 320,000 over 2,000,000
V Plus over 440,000  


Merchant oil tankers carry hydrocarbon liquids ranging from crude oil to refined products. Their size is measured in deadweight tons or DWT. Crude oil carriers are among the largest tankers, ranging from 55,000 DWT Panamax vessels to Very Large Crude Carriers or VLCCs of over 300,000 DWT.

Product tankers fall under the smaller size tankers. They carry refined cargoes and range from under 10,000 DWT to Panamax size vessels. The smallest tankers, those of less then 10,000 DWT, work coastal and inland waterways.

Supertanker is an informal term coined for the largest tankers. Today the term is applied to the VLCC and ULCC class tankers above 250,000 DWT. There ships are able to transport two million barrels of oil. The previous "supertankers" of the Aframax and Suezmax classes are no longer considered to be "supertankers."

The four largest supertanker are assigned the designation V Plus, even tough ULCCs are not assigned a maximum DWT.

When the supertankers where first introduced, their size prevented them from docking in many ports around the world and they needed to unload their cargo into smaller tankers. Today some major ports have developed special offshore unloading facilities connected to the shore by pipelines. Also supertankers are extremely efficient ships, their transport costs account for only US $0.02 per gallon of gas at the pump.

Current architecture

Oil tankers usually have 8 to 12 tanks. Every one of theese tanks is split into independent compartments by fore and aft bulkheads. The tanks are assigned numbers with tank one being the forwardmost. Individual compartments are referred by their tank number and position, like "one port" or "three starboard".

A cofferdam is a space left open between bulkheads that is used to give protection from fire, heat or collision. Tankers usually have cofferdams forward and aft of the cargo tanks, and in some cases between tanks.

A pumproom is a space that houses all the pumps connected tothe ship's cargo lines. Some of the larger tankers have two pumprooms.

Hull designs

The design of the hull and outer structure is a major component of tanker architecture. Single-hulled tankers have a single outer shell between the cargo and the ocean. Most newer tankers are double-hulled, with an extra space between the hull and storage tanks. The hybrid designs like double-bottom and double-sided use aspects of single-hull and double-hull tanker designs.

All single-hull tankers are expected to be phased out by 2026, in accordance with the International Convention for the Prevention of Pollution from Ships.

In 1998,a survey of industry experts was conducted by the Marine Board of the National Academy of Science, regarding the pros and cons of double-hull tanker design. The advantages mentioned in the survey 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 survey listed the following as disadvantages to the double-hull design:

  • more expensive to build,
  • more expensive canal and port expenses,
  • ballast tank ventilation difficult,
  • ballast tanks need continual monitoring and maintenence,
  • 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.

Generally, double-hulled tankers are much safer than single-hulled in the scenario of grounding incident, especially is the shore is not very rocky. The safety benefits are smaller on larger tankers and in the cases of high speed impact.

Inert gas system

Intert gas system is one of the most important parts of the tankers design. Altough fuel oil is difficult to ignite, its hydrocarbon vapors are explosive in certain concetrations when they mix with air. The inert gas system is used to create an atmosphere in which the vapors cannot burn.

As inter gas is introduced into the tanks, it increases the lower flammable limit and at the same time it decreases the upper flammable limit of the vapors. When the total concentration of oxygen in the tank reaches 11%, the upper and lower limits converge and the flammable range of the vapors disappears.

Inert gas systems deliver air with an oxygen concentration of less than 5% by volume. As the tanks are pumped out, they are filled with inert gas to keep them in a 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-related operations

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

Pre-transfer preparation

Before any cargo transfer, a Chief officer develops a transfer plan, which details all the specifics of the operation, such as the amount of cargo about to be moved, which tanks are to be cleaned and how the ship's ballast will change. The next step is the pretransfer conferrence which covers issues such as:

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

When the conference is completed, the person in charge of the ship and the person in charge of the shore installation go over the final inspection checklist. In the U.S., this is called a Declaration of Inspection or DOI, outside of the U.S. it is called the Ship/Shore Safety Checklist. This checklist includes:

  • Proper signals and signs are displayed
  • The vessel is securely moored
  • A language for all communication is chosen
  • All connections are secure
  • Emergency equipment is in place
  • No repair work is ongoing.

Loading cargo


Oil is pumped on and off the ship by way of connections made at the cargo manifold.
Oil is pumped on and off the ship by way of connections made at the cargo manifold.

Loading an oil tanker consists primarily of pumping cargo into the ship's tanks. As oil enters the tank the vapors inside the tank are expelled depending on the local regulations. They are either expelled into the atmosphere or discharged to the pumping station. It is also common for the ship to move water balast during the loading of cargo to maintain proper trim.

Loading starts at a slow pace and at a low pressure to be sure that all the connections are secure and that the equipment is working properly. Then a steady pressure is achieved and held until the "topping-off" phase when the tanks are nearly full. 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


This cargo pump aboard a VLCC can move 5,000 cubic meters of product per hour.
This cargo pump aboard a VLCC can move 5,000 cubic meters of product per hour.

THe process of unloading oil off the tanker is similar to the loading process but it has some key differences. The first step of the unloading has the same pretransfer procedures as does the loading. While pumping the cargo, tank levels are monitored and key location are constantly watched.. 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 clearing

From time to time for various reasons it is required for the tanks to be cleaned. One of the common reasons is the changes in the type of cargo carried inside the 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."

Most crude oil tankers have a special oil washing system (COW) that is an integral part of the cleaning process. The COW system moves part of the cargo through fixed tank cleaning system to remove asphaltic deposits and wax.

Tanks that carry less viscous cargoes are washed with water. Fixed and portable water washing systems exist. 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 is sometimes freed of gases. This is done by blowing fresh air into the tank. The tank's atmosphere is monitored by specially trained personnel that use hand-held indicators which measure the hydrocarbons present in the tank. When the percentage of hydrocarbons drops below a certain value, the tank is declared gas-free.

After a tank is gas-free, it can be further hand-cleaned in a process referred to as "mucking." This task is generally done by able seamen and deck officers using tools such as mops, buckets, rags and sponges.

Special-use oil tankers

Some special types of oil tankers exist with the purpose of meeting specific military or economic needs. These special types are naval oilers, floating storage and offloading units (FSOs), floating production storage and offloading units (FPSOs) and oil-bulk-ore combination carriers.

Naval oilers

A naval oiler (also called a fleet tanker in the Royal Fleet Auxiliary) is distinguished from other oil tankers for its ability to deliver fuel while making way in a process known as underway replenishment. Before the advent of this technique, naval vessels were refuelled while in port or at anchor.

The fleet oiler USS Maumee, launched on April 17, 1915, pioneered the technique of underway replenishment. 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. 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.

Naval oilers sometimes also carry water, ammunition, rations, stores and personnel.

Ore-bulk-oil carriers


The OBO-carrier Maya. The picture is showing both the cargo hold hatches used for bulk and the pipes used for oil
The OBO-carrier Maya. The picture is showing both the cargo hold hatches used for bulk and the pipes used for oil

An Ore-bulk-oil carrier, also known as combination carrier or simply OBO, is a ship capable of carrying both wet and dry cargoes. The idea behind the design of such a ship is to reduce the number of empty (ballast) voyages, in which large ships only carry a cargo one way and return empty for another. These are features of the larger bulk trades (e.g. crude oil from the Middle East, iron ore and coal from Australia, South Africa and Brazil).

The combination carriers specialise in one trade or the other and their flexibility stays very much unused. This caused the idea to fall out of fashion since the 1980s. In the European Russia's waterways there is a fleet of smaller OBO carriers used by Volgotanker.

Floating storage units


Floating storage units, often former oil tankers, accumulate oil for tankers to retrieve.
Floating storage units, often former oil tankers, accumulate oil for tankers to retrieve.

A Floating production storage and offloading unit or FPSO is a floating tank system used by the offshore oil and gas industry. These units process and store oil or gas produced by one or more nearby platforms. The products are loaded into tankers via these units. A floating storage and offloading unit is similar but only without the possibility of processing oil or gass.

These units are often old, stripped down oil tankers.


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

Because of 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 of crude oil, or 62 million gallons. This is more than six times the amount that was 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 huge efforts of scientists, managers, and volunteers, over 400,000 seabirds, about 1,000 sea otters, and immense numbers of fish were killed.

But how much do takers really contribute to pollution of the oceans? United States Coast Guard reported that 35.7% of the volume of oil spilled in the U.S. from 1991 to 2004 came from tank vessels, 27% from facilities, 19.9% from non-tank vessels, 9.3% from pipelines and 7.4% from mystery spills. The detailed statistics for 2004 that can be seen in the table below shows that tanker vessels are responsible for less than 5% of total spills but more than 60% of the volume.

U.S.C.G Spill Compendium Annual Data for 2004
Source No. of spills % of spill incidents Spill volume (gallons) % of spill volume Ave. spill size Med. spill size Max. 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 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 tracked 9,351 accidental spills since 1974. According to their studies, most spills resulted from routine operations such as loading and offloading of cargo. 91% of the operational oil spills were small, resulting in less than 7 tons per spill. On the other hand, spills that resulted from collisions, groundings, hull failures and explosions were much larger, with 84% of these involving losses of over 700 tons.

After the Exxon Valdez oil spill, the U.S. passed the Oil Pollution Act of 1990 (OPA-90), which requires that all tankers entering its waters be double-hulled by 2015. Following the sinking of Erika and Prestige the European Union passed a similar legislation that requires 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".


Today's fleet

Flag states

As of 2007, the United States Central Intelligence Agency statistics count 4,295 oil tankers of 1,000 deadweight tons 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.