Architecture of the oil tanker

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Oil tankers generally have from 8 to 12 tanks. [1] Each tank is split into two or three independent compartments by fore-and-aft bulkheads. [1] 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." [1]

Contents

A cofferdam is a small space left open between two bulkheads, to give protection from heat, fire, or collision. [2] Tankers generally have cofferdams forward and aft of the cargo tanks, and sometimes between individual tanks. [3] A pumproom houses all the pumps connected to a tanker's cargo lines. [1] Some larger tankers have two pumprooms. [1] A pumproom generally spans the total breadth of the ship. [1]

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. [4] Most newer tankers are double-hulled , with an extra space between the hull and the storage tanks. [4] Hybrid designs such as double-bottom and double-sided combine aspects of single and double-hull designs. [4] All single-hulled tankers have been phased out as of 2015, in accordance with amendments to Annex I of the MARPOL Convention. [4] IMO distinguishes three categories of tankers that will be phased out: [5]

Phased out types

PreMARPOL tanker Pre-MARPOL tanker.svg
PreMARPOL tanker

PreMARPOL tanker

Category 1 tankers have been phased out in 2005. These so-called preMARPOL tankers were single hull only with some segregated ballast tanks. Around one third of the cargo tanks also acted as ballast tanks. During ballast discharge oil was released into the environment. These tankers did not extend high above the water line, allowing Hydrostatically Balanced Loading (HBL), so relatively little oil was spilled in case of bottom damage. [6]

30 percent of the side shell in way of the tanks of a MARPOL tanker should be non-cargo. MARPOL tanker.svg
30 percent of the side shell in way of the tanks of a MARPOL tanker should be non-cargo.

MARPOL tanker

Category 2 tankers were used for a number of years[ when? ] and were planned to be phased out by 2010 at the latest, depending on the year of delivery.[ citation needed ] With MARPOL tankers, it is not allowed to use ballast tanks as cargo tanks. This has reduced operational spillage drastically. The downside is designs based on MARPOL spill more oil when damaged than a preMARPOL tankers. This is due to several factors:

  1. as ballast tanks could not be used as cargo tanks anymore, cargo space was lost. To compensate for this, tanks were made taller, which means that more oil is spilled before hydrostatic balance is reached,
  2. a MARPOL rule is that 30 percent of the side shell in way of the tanks of a MARPOL tanker should be non-cargo. The cheapest way to reach this, is by making these tanks as narrow as possible. This means that centre tanks became extremely large, so in case of damage, the amount of spillage increased,
  3. in a preMARPOL tanker ballast tanks were also filled with inert gas, as these were also used as cargo tanks, which reduced corrosion. Ballast tanks of MARPOL tankers are not protective this way, causing structural failure by corrosion on the Erika , Castor and Prestige ,
  4. the painted area[ clarification needed ] tripled, increasing required maintenance and corrosion in case this maintenance is done poorly. [7]

Category 3

These small tankers will also be phased out by 2010.[ needs update ]

New types

After the Exxon Valdez disaster, public outcry became so strong that authorities were forced to come with preventive measures. Especially the double hull design was favoured and although this is not the best design in all cases, because of OPA 90, this is the only design currently in operation. [8]

Double hull

Double hull tanker Double Hull Tanker.svg
Double hull tanker

OPA 90 caused the phasing out of single hull tankers in the United States between 1997 and 2000 — apart from tankers lightering off the coast, which are allowed to be single hull until 2015. In this design, cargo tanks are protected by ballast tanks of at least 2 metres. As long as this barrier is not breached, there will be no spillage.

In 1998, the Marine Board of the National Academy of Sciences 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, [9] reduced practice of saltwater ballasting in cargo tanks decreases corrosion, [10] increased environmental protection, [10] cargo discharge is quicker, more complete and easier, [10] tank washing is more efficient, [10] and better protection in low-impact collisions and grounding. [10]

The same report lists the following as some drawbacks to the double-hull design, including more expensive to build, [11] higher canal and port expenses, [11] ballast tank ventilation difficult, [11] ballast tanks need continual monitoring and maintenance, [11] increased transverse free surface, [11] more surfaces to maintain, [11] explosion risk in double-hull spaces if vapor detection system not fitted, [12] cleaning mud from ballast spaces a bigger problem. [12]

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. [13] The safety benefits are less clear on larger vessels and in cases of high speed impact. [10]

Other downsides of this design are:

  1. as small leakages from cargo tanks do not spill in sea, they can go unnoticed for a long time. This can cause an explosive mixture in ballast tanks, as there is no requirement to connect these to the IG system,
  2. the painted area is three times as large as on a MARPOL tanker, and almost tenfold compared to a preMARPOL tanker. [14]

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 than 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. [15]

Mid-Deck Oil Tanker

Mid-Deck Tanker, damage up 3 m. Light gray is oil, dark gray is seawater. Mid-Deck Tanker.PNG
Mid-Deck Tanker, damage up 3 m. Light gray is oil, dark gray is seawater.

A Mid-Deck Tanker is a tanker design, which includes an additional deck intended to limit spills if the tanker is damaged. The extra deck is placed at about the middle of the draft of the ship.[ citation needed ]

With double hull tankers, in high energy casualties where both hulls are breached, oil can spill through the double-hull and into the sea. In grounding events of this type, a mid-deck design overcomes this by eliminating the double-bottom compartments that are void with air. Since the density of seawater is greater than that of oil, water comes into the tanks instead of oil escaping out, and rather than spilling, oil is vented upwards into overflow tanks.[ citation needed ]

If the Exxon Valdez had been a Mid-Deck ship, she would have spilled very little oil. [16]

Coulombi Egg Tanker

Coulombi Egg, damage up 3 m. Light gray is oil, dark gray is seawater. Coulombi Egg.PNG
Coulombi Egg, damage up 3 m. Light gray is oil, dark gray is seawater.

A variation on the Mid-Deck Tanker is the Coulombi Egg Tanker, which was approved by IMO as an alternative to the double hull concept. The design consists of a series of centre and wing tanks that are divided by horizontal bulkheads. The upper wing tanks form ballast tanks and act as emergency receiver tanks for cargo should the lower tanks be fractured. The lower tanks are connected to these ballast tanks by non-return valves. The United States Coast Guard does not allow this design to enter US waters, effectively preventing it from being built.[ citation needed ]

When a lower tank is damaged, the incoming sea water pushes the oil in the damaged tank up into the ballast tank. Because of the hydrostatic pressure, there is an automatic transfer out of the damaged tank. The double-hull design is aimed at the probability of zero outflow. In low energy casualties where only the outside hull is penetrated, this will be the case. However, in high energy casualties both hulls are penetrated. As the tanks of a double hull tanker are larger than those of MARPOL-tankers and preMARPOL-tankers and the height of the cargo above the water line is higher, the resulting spill can be much larger than these single hull designs. In the Coulombi Egg design spillage is greatly reduced, possibly to zero.[ citation needed ]

Where a double hull VLCC has a ballast tank coated area of about 225,000 m³, in a Coulombi Egg tanker this area is reduced to 66,000 m³. This reduces maintenance and corrosion risks, which otherwise may result in structural failure. [17]

Inert gas system

An oil tanker's inert gas system is one of the most important parts of its design. [18] Fuel oil itself is very difficult to ignite, however its hydrocarbon vapors are explosive when mixed with air in certain concentrations. [19] The purpose of the system is to create an atmosphere inside tanks in which the hydrocarbon oil vapors cannot burn. [18]

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. [20] At the same time it decreases the upper flammable limit or highest concentration at which the vapors can be ignited. [20] When the total concentration of oxygen in the tank reaches about 11%, the upper and lower flammable limits converge and the flammable range disappears. [21]

Inert gas systems deliver air with an oxygen concentration of less than 5% by volume. [18] As a tank is pumped out, it's filled with inert gas and kept in this safe state until the next cargo is loaded. [22] The exception is in cases when the tank must be entered. [22] 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%. [22] Thus, as air replaces the inert gas, the concentration cannot rise to the lower flammable limit and is safe. [22]

See also

Related Research Articles

A double-hulled tanker refers to an oil tanker which has a double hull. They reduce the likelihood of leaks occurring compared to single-hulled tankers, and their ability to prevent or reduce oil spills led to double hulls being standardized for oil tankers and other types of ships including by the International Convention for the Prevention of Pollution from Ships or MARPOL Convention. After the Exxon Valdez oil spill disaster in Alaska in 1989, the US Government required all new oil tankers built for use between US ports to be equipped with a full double hull.

Ship Large watercraft

A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research and fishing. Ships are generally distinguished from boats, based on size, shape, load capacity and purpose. Ships have supported exploration, trade, warfare, migration, colonization, and science. Ship transport is responsible for the largest portion of world commerce.

An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to as the inert gases. Inert gases are used generally to avoid unwanted chemical reactions degrading a sample. These undesirable chemical reactions are often oxidation and hydrolysis reactions with the oxygen and moisture in air. The term inert gas is context-dependent because several of the noble gases can be made to react under certain conditions.

<span class="mw-page-title-main">Merchant ship</span> Civilian boat or ship that transports cargo or carries passengers for hire

A merchant ship, merchant vessel, trading vessel, or merchantman is a watercraft that transports cargo or carries passengers for hire. This is in contrast to pleasure craft, which are used for personal recreation, and naval ships, which are used for military purposes.

A double hull is a ship hull design and construction method where the bottom and sides of the ship have two complete layers of watertight hull surface: one outer layer forming the normal hull of the ship, and a second inner hull which is some distance inboard, typically by a few feet, which forms a redundant barrier to seawater in case the outer hull is damaged and leaks.

<span class="mw-page-title-main">Chemical tanker</span>

A chemical tanker is a type of tanker ship designed to transport chemicals in bulk. As defined in MARPOL Annex II, chemical tanker means a ship constructed or adapted for carrying in bulk any liquid product listed in chapter 17 of the International Bulk Chemical Code. As well as industrial chemicals and clean petroleum products, such ships also often carry other types of sensitive cargo which require a high standard of tank cleaning, such as palm oil, vegetable oils, tallow, caustic soda, and methanol.

<span class="mw-page-title-main">Tanker (ship)</span> Ship designed to transport liquids or gases in bulk

A tanker is a ship designed to transport or store liquids or gases in bulk. Major types of tankship include the oil tanker, the chemical tanker, and gas carrier. Tankers also carry commodities such as vegetable oils, molasses and wine. In the United States Navy and Military Sealift Command, a tanker used to refuel other ships is called an oiler but many other navies use the terms tanker and replenishment tanker. Tankers were first developed in the late 19th century as iron and steel hulls and pumping systems were developed. As of 2005, there were just over 4,000 tankers and supertankers 10,000 LT DWT or greater operating worldwide.

TI-class supertanker Supertanker class

The TI class of supertankers comprises the ships TI Africa, TI Asia, TI Europe and TI Oceania, where the "TI" refers to the ULCC tanker pool operator Tankers International. The class were the first ULCCs to be built in 25 years.

<span class="mw-page-title-main">Mid-deck tanker</span>

A mid-deck oil tanker is a tanker design which includes an additional deck intended to limit spills if the tanker is damaged. The extra deck is placed at about the middle of the draft of the ship. This design is an alternative to the double-hull tanker design, and is superior in terms of spill volume.

<span class="mw-page-title-main">LNG carrier</span> Tank ship transporting liquefied natural gas

An LNG carrier is a tank ship designed for transporting liquefied natural gas (LNG).

Oil tanker Ship that carries oil

An oil tanker, also known as a petroleum tanker, is a ship designed for the bulk transport of oil or its products. There are two basic types of oil tankers: crude tankers and product tankers. 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 refined products from refineries to points near consuming markets.

<span class="mw-page-title-main">Coulombi Egg Tanker</span>

The Coulombi Egg Tanker is a design that is aimed at reducing oil spills. It was approved by the International Maritime Organization (IMO) as an alternative to the double hull concept. The United States Coast Guard does not allow this design to enter US waters, effectively preventing it from being built.

<span class="mw-page-title-main">Marine loading arm</span>

A marine loading arm, also known as a mechanical loading arm, loading arm, or MLA is a mechanical arm consisting of articulated steel pipes that connect a tankship such as an oil tanker or chemical tanker to a cargo terminal. Genericized trademarks such as Chiksan are often used to refer to marine loading arms.

<span class="mw-page-title-main">History of the oil tanker</span>

The history of the oil tanker is part of the evolution of the technology of oil transportation alongside the oil industry.

<span class="mw-page-title-main">Cargo control room</span>

The cargo control room, CCR, or cargo office of a tankship is where the person in charge (PIC) can monitor and control the loading and unloading of the ship's liquid cargo. Prevalent on automated vessels, the CCR may be in its own room, or located on the ship's bridge. Among other things, the equipment in the CCR may allow the person in charge to control cargo and stripping pumps, control and monitor valve positions, and monitor cargo tank liquid levels.

The Venpet–Venoil collision was a maritime accident involving sister supertankers; the Liberian-registered Venoil and Venpet, in dense fog off the coast of South Africa on 16 December 1977. The tankers were travelling in opposite directions; the Venoil fully laden with over 250,000 tonnes of crude oil bound for Halifax, Canada, and the Venpet, travelling in ballast, headed for Kharg Island, Iran. The Venoil ploughed into the Venpet, eventually leading to the spilling of approximately 26,600–30,500 tonnes of crude oil. The tankers were sister ships owned and operated by Bethlehem Steel Corporation. Both ships were manned by Taiwanese crews.

The Oswego-Guardian–Texanita collision was a maritime accident between two supertankers near Stilbaai, South Africa on 21 August 1972. The Texanita exploded and sank with the loss of 47 men, while a further life was lost on the Oswego Guardian. The accident was a catalyst for change to marine traffic separation procedures as well as oil tanker inerting.

<span class="mw-page-title-main">Gas carrier</span> Ship designed to transport liquefied chemical gases in bulk

A gas carrier, gas tanker, LPG carrier, or LPG tanker is a ship designed to transport LPG, LNG, CNG, or liquefied chemical gases in bulk.

Corrosion in Ballast Tanks is the deterioration process where the surface of a ballast tank progresses from microblistering, to hydroscaletric electration, and finally to cracking of the tank steel itself.

<span class="mw-page-title-main">Hydraulic tanker</span> Type of vessel

A hydraulic tanker is an oil tanker designed to use water as an incompressible fluid for loading and unloading petroleum cargo. Each cargo tank is kept full at all times so oil floating on water will be pressed against the top of the tank. A cargo tank initially filled with water is loaded with the desired quantity of oil by pumping oil into the top of the tank displacing water which overflows through an opening at the bottom of the tank. The cargo tank is unloaded by removing oil from the top of the tank as water is admitted at the bottom.

References

Notes

  1. 1 2 3 4 5 6 Turpin and McEven, 1980:8-24.
  2. ↑ Turpin and McEven, 1980:14-20.
  3. ↑ Turpin and McEven, 1980:8-25.
  4. 1 2 3 4 Hayler and Keever, 2003:14-4.
  5. ↑ IMO, Revised phase-out schedule for single-hull tankers enters into force, 4 April 2005
  6. ↑ Devanney, 2006, p. 327-336.
  7. ↑ Devanney, 2006, p. 36-42.
  8. ↑ Devanney, 2006, p. 43-49.
  9. ↑ Marine Board, 1998, p. 259.
  10. 1 2 3 4 5 6 Marine Board, 1998, p. 260.
  11. 1 2 3 4 5 6 Marine Board, 1998, p. 261.
  12. 1 2 Marine Board, 1998, p. 262.
  13. ↑ Joem K. Paik and Tak K. Lee, Damage and Residual Strength of Double-Hull Tankers in Grounding Archived October 29, 2008, at the Wayback Machine , International Journal of Offshore and Polar Engineering, Vol. 5, No. 4, December 1995.
  14. ↑ Devanney, 2006, p. 46-47.
  15. ↑ Devanney, 2006, p. 381-383.
  16. ↑ Devanney, 2006, p. 381.
  17. ↑ Devanney, 2006, p. 379-383.
  18. 1 2 3 Hayler and Keever, 2003:14-11.
  19. ↑ Turpin and McEwin, 1980:16-42.
  20. 1 2 Transport Canada, 1985:4.
  21. ↑ Transport Canada, 1985:5.
  22. 1 2 3 4 Transport Canada, 1985:9.

Bibliography

Further reading

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