[edit] Fleet characteristics
In 2005, oil tankers made up 36.9% of the world's fleet in terms of deadweight tonnage.[61] The world's total oil tankers deadweight tonnage has increased from 326.1 million DWT in 1970 to 960.0 million DWT in 2005.[61] The combined deadweight tonnage of oil tankers and bulk carriers, represents 72.9% of the world's fleet.[62]
[edit] Cargo movement
In 2005, 2.42 billion metric tons of oil were shipped by tanker.[3] 76.7% of this was crude oil, and the rest consisted of refined petroleum products.[3] This amounted to 34.1% of all seaborne trade for the year.[3] Combining the amount carried with the distance it was carried, oil tankers moved 11,705 billion metric-ton-miles of oil in 2005.[63]
By comparison, in 1970 1.44 billion metric tons of oil were shipped by tanker.[64] This amounted to 34.1% of all seaborne trade for that year.[65] In terms of amount carried and distance carried, oil tankers moved 6,487 billion metric-ton-miles of oil in 1970.[63]
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.[66] In 2005, for each 1 DWT of oil tankers, 6.7 metric tons of cargo was carried.[66] Similarly, each 1 DWT of oil tankers was responsible for 32,400 metric-ton miles of carriage.[66]
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.[67] 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.[67]
[edit] Flag states
As of 2007, the United States Central Intelligence Agency statistics count 4,295 oil tankers of 1,000 long tons deadweight (DWT) or greater worldwide.[68] Panama was the world's largest flag state for oil tankers, with 528 of the vessels in its registry.[68] 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).[68] By way of comparison, the United States and the United Kingdom only had 59 and 27 registered oil tankers, respectively.[68]
[edit] Flags of convenience
The first open register was Panama in 1916. Fear for political instability and high and excessive consular fees led the president of Liberia, William Tubman, in 1948 to start an open register with the help of Edward Stettinius, Jr.. The World Peace of Stavros Niarchos was the first ship in that register. In 1967 Liberia passed the United Kingdom as the largest register. Nowadays, Panama — currently the largest register — and Liberia have one third of the world fleet under their flag.[69]
Flags of convenience have lower standards for vessel, equipment, and crew the traditional maritime countries and often have classification societies certify and inspect the vessels in their registry, instead of by their own shipping authority. This made it attractive for ship owners to change flag, whereby the ship lost the economic link and the country of registry. With this, also the link between classification society and traditional maritime country became less obvious - for instance Lloyd's with the United Kingdom and ABS with the United States. This made it easier to change class and introduced a new phenomena; class hopping. A ship owner that is dissatisfied with class can change to a different class relatively easily. This has led to more competition between class societies and to a relaxation of the rules.[25] This has led to the shipping industry losing confidence in the classification societies, but also by the European Commission.[70]
To counteract class hopping, the IACS has established TOCA (Transfer Of Class Agreement).
In 1978, a number of European countries agreed in The Hague on a 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 consisting of 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.
[edit] Vessel life cycle
Tankers may carry unusual cargoes such as grain on their final trip to the scrapyard.
In 2005, the average age of oil tankers worldwide was 10 years.[71] Of these, 31.6% were under 4 years old and 14.3% were over 20 years old.[72] In 2005, 475 new oil tankers were built, accounting for 30.7 million DWT.[73] The average size for these new tankers was 64,632 DWT.[73] Nineteen of these were VLCC size, 19 were suezmax, 51 were aframax, and the rest were smaller designs.[73] 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.[73]
Ships are generally removed from the fleet through a process known as scrapping.[74] 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.[75] In 1998, almost 700 ships went through the scrapping process at shipbreakers in places like Alang, India and Chittagong, Bangladesh.[74] In 2004 and 2005, 7.8 million DWT and 5.7 million DWT respectively of oil tankers were scrapped.[71] Between 2000 and 2005, the capacity of oil tankers scrapped each year has ranged between 5.6 million DWT and 18.4 million DWT.[76] In this same timeframe, tankers have accounted for between 56.5 and 90.5 of the world's total scrapped tonnage.[76] During this period, the average age of scrapped oil tankers has ranged from 26.9 to 31.5 years.[76]
[edit] Vessel pricing
| Size |
1985 |
2005 |
| 32–45,000 DWT |
US$18M |
$43M |
| 80–105,000 DWT |
$22M |
$58M |
| 250–280,000 DWT |
$47M |
$120M |
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.[77] In 1985, these vessels would have cost $18M, $22M, and $47M respectively.[77]
Oil tankers are often sold second-hand. In 2005, 27.3 million DWT worth of oil tankers were sold used.[78] 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.[78]
[edit] Current structural design and engineering
Oil tankers generally have from 8 to 12 tanks.[16] Each tank is split into two or three independent compartments by fore-and-aft bulkheads.[16] 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."[16]
A cofferdam is a small space left open between two bulkheads, to give protection from heat, fire, or collision.[16] Tankers generally have cofferdams forward and aft of the cargo tanks, and sometimes between individual tanks.[79] A pumproom houses all the pumps connected to a tanker's cargo lines.[16] Some larger tankers have two pumprooms.[16] A pumproom generally spans the total breadth of the ship.[16]
[edit] Hull designs
Single hull, Double bottom, and Double hull ship cross sections. Green lines are watertight; black structure is not watertight
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.[80] Most newer tankers are double-hulled, with an extra space between the hull and the storage tanks.[80] Hybrid designs such as double-bottom and double-sided combine aspects of single and double-hull designs.[80] 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).[80] The United Nations has decided to phase out single hull oil tankers by 2010.[81]
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,[82] reduced practice of saltwater ballasting in cargo tanks decreases corrosion,[83] increased environmental protection,[83] cargo discharge is quicker, more complete and easier,[83] tank washing is more efficient,[83] and better protection in low-impact collisions and grounding.[83]
The same report lists the following as some drawbacks to the double-hull design, including higher build costs,[84] greater operating expenses (e.g. higher canal and port tariffs),[84] difficulties in ballast tank ventilation,[84] the fact that ballast tanks need continuous monitoring and maintenance,[84] increased transverse free surface,[84] the greater number of surfaces to maintain,[84] the risk of explosions in double-hull spaces if a vapor detection system not fitted,[85] and that cleaning ballast tanks is more difficult for double hull ships.[85]
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.[86] The safety benefits are less clear on larger vessels and in cases of high speed impact.[83]
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.[87]
[edit] Inert gas system
An oil tanker's inert gas system is one of the most important parts of its design.[88] Fuel oil itself is very difficult to ignite, however its hydrocarbon vapors are explosive when mixed with air in certain concentrations.[89] The purpose of the system is to create an atmosphere inside tanks in which the hydrocarbon oil vapors cannot burn.[88]
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.[90] At the same time it decreases the upper flammable limit or highest concentration at which the vapors can be ignited.[90] When the total concentration of oxygen in the tank reaches about 11%, the upper and lower flammable limits converge and the flammable range disappears.[91]
Inert gas systems deliver air with an oxygen concentration of less than 5% by volume.[88] As a tank is pumped out, it's filled with inert gas and kept in this safe state until the next cargo is loaded.[92] The exception is in cases when the tank must be entered.[92] 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%.[92] Thus, as air replaces the inert gas, the concentration cannot rise to the lower flammable limit and is safe.[92]
[edit] Cargo operations
Cargo flows between a tanker and a shore station by way of
marine loading arms attached at the tanker's cargo manifold.
Operations aboard oil tankers are governed by an established body of best practices and a large body of international law.[93] 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.[94] 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.[95] Lightering is sometimes used where a loaded tanker is too large to enter a specific port.[95]
[] 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.[96] The next step before a transfer is the pretransfer conference.[97] 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.[97]
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. [97] In the United States, the checklist is called a Declaration of Inspection or DOI.[97] Outside of the U.S., the document is called the "Ship/Shore Safety Checklist."[97] Items on the checklist are[98]:
- Appropriate personal notified that the cargo operation are about to commence.
- Sufficient personal available for cargo watch to manifold.
- Warning notified displaces no smoking & no naked light.
- Fire application ready for immediate used.
- Mooring tight and emergency towing wire correct positioned.
- Agree ship/shore communication system working.
- No unauthorized work to the carried out.
- No unauthorized persons on board.
- Safe lighting available.
- Galley precaution observed.
- Cargo tank lead closed.
- Manifold drip trays in use.
- Tank inert and inert gas system checked.
- All doors and ports to accommodation closed.
- Air conditioning unit on internal air recycling.
- Safe ship/shore access.
- Tank ventilation system checked.
- Ship ready to move under own power.
- Emergency shutdown understood.
- Appropriate flag and light signal to be showed.
- All deck scuppers plugged to prevent oil leaks over board.
- No unauthorized craft alongside.
- Cargo line properly set, all valves not in use closed and lashed.
- All necessary spark arresters in good condition in place.[98]
[ Loading cargo
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.[99] As oil enters the tank, the vapors inside the tank must be somehow expelled.[99] 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.[99] It is also common for the ship to move water ballast during the loading of cargo to maintain proper trim.[99]
Loading starts slowly at a low pressure to ensure that equipment is working correctly and that connections are secure.[99] Then a steady pressure is achieved and held until the "topping-off" phase when the tanks are nearly full.[99] Topping off is a very dangerous time in handling oil, and the procedure is handled particularly carefully.[99] 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.[99] 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.[99]
[] Unloading cargo
This cargo pump aboard a VLCC can move 5,000 cubic meters of product per hour.
The process of moving oil off of a tanker is similar to loading, but has some key differences.[100] The first step in the operation is following the same pretransfer procedures as used in loading.[101] When the transfer begins, it is the ship's cargo pumps that are used to move the product ashore.[101] As in loading, the transfer starts at low pressure to ensure that equipment is working correctly and that connections are secure.[101] Then a steady pressure is achieved and held during the operation.[102] 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.[100] 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.[100]
[] Tank cleaning
The nozzle of an automated tank cleaning machine
Tanks must be cleaned from time to time for various reasons. One reason is to change the type of product carried inside a tank.[103] 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."[103]
On most crude-oil tankers, a special crude oil washing (COW) system is part of the cleaning process.[103] The COW system circulates part of the cargo through the fixed tank-cleaning system to remove wax and asphaltic deposits.[103] 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.[103] Some systems use rotating high-pressure water jets to spray hot water on all the internal surfaces of the tank.[103] As the spraying takes place, the liquid is pumped out of the tank.[103]
After a tank is cleaned, it may be "gas-freed".[104] This involves blowing fresh air into the tank to force accumulated gasses out.[104] Specially trained personnel monitor the tank's atmosphere, often using hand-held gas indicators which measure the percentage of hydrocarbons present.[104] When this percentage drops below a value specified in tank-vessel regulations, the tank is declared to be gas-free.[104] After a tank is gas-free, it may be further hand-cleaned in a manual process known as mucking.[105] Mucking requires protocols for entry into confined spaces, protective clothing, designated safety observers, and possibly the use of airline respirators.[105]
[] 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, 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.[106] Prior to underway replenishment, naval vessels had to enter a port or anchor to take on fuel.[21] In addition to fuel, replenishment ships may also deliver water, ammunition, rations, stores and personnel.[22]
[] 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
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.[107] This design was intended to provide flexibility in two ways.[108] Firstly, an OBO would be able to shift between the dry and wet bulk trades based on market conditions.[108] 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.[109]
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.[109] Also, these ships have endemic maintenance problems.[108] On one hand, due to a less specialized design, an OBO suffers more from wear and tear during dry cargo onload than a bulker.[108] 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.[108] These factors have contributed to a steady reduction in the number of OBO ships worldwide since the 1970s.[109]
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.[107] It sank in a Pacific typhoon while carrying a cargo of iron ore from Canada to Japan.[107]
[] Floating storage units
Floating storage units, often former oil tankers, accumulate oil for tankers to retrieve.
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.[110] A similar system, the floating production storage and offloading unit (FPSO), has the ability to process the product while it is onboard.[110] These floating units reduce oil production costs and offer, mobility, large storage capacity, and production versatility.[110]
FPSO and FSOs are often created out of old, stripped-down oil tankers, but can be made from new-built hulls[110] Shell España first used a tanker as an FPSO was in August 1977.[111] An example of a FSO that used to be an oil tanker is the Knock Nevis.[112] These units are usually moored to the seabed through a spread mooring system.[110] A turret-style mooring system can be used in areas prone to severe weather.[110] This turret system lets the unit rotate to minimize the effects of sea-swell and wind.[110]
[edit] Pollution
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.[114] 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 2 million barrels (320,000 m3) of crude oil, or 62,000,000 gallons. This is about six times the amount spilled in the widely known Exxon Valdez incident. In this spill, the ship ran aground and dumped 10,800,000 US gallons (41,000 m3) 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.[114] Considering the volume of oil carried by sea, however, tanker owners' organisations often argue that the industry's safety record is excellent, with only a tiny fraction of a percentage of oil cargoes carried ever being spilled. INTERTANKO - the International Association of Independent Tanker Owners - has observed that "accidental oil spills this decade have been at record low levels - one third of the previous decade and one tenth of the 1970s - at a time when oil transported has more than doubled since the mid 1980s."
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.[115] On the other hand, only 5% of the actual spills came from oil tankers, while 51.8% came from other kinds of vessels.[115] 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.[116] According to this study, most spills result from routine operations such as loading cargo, discharging cargo, and taking on fuel oil.[116] 91% of the operational oil spills are small, resulting in less than 7 metric tons per spill.[116] 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.[116]
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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