Liquid Cargo Carriers, or tankers, are specifically designed to transport liquid cargoes in bulk. Although tankers differ greatly in size, their cargo handling equipment is similar. Specific features of the cargo handling equipment differ, however, based on the intended cargo. These differences may limit the capability of the ship to carry cargo other than that for which it was designed. Tanker capacities are stated in terms of cargo deadweight (DWT) or barrels (BBL). DWT is measured in LT of 2,240 pounds and 1 BBL equals 42 US gallons. The parameters that define a militarily useful tanker are the capability of carrying POL, a capacity within the range of 2,000 to 100,000 DWT, and a sustained speed in excess of 12 knots.
Tankers are classed by size and type of cargo. In general, smaller tankers carry "clean" cargoes (refined products, such as gasoline, diesel fuel, or jet fuel). Large tankers generally carry "dirty" (black oil or crude oil) cargoes. Tankers of less than 100,000 dwt are referred to as either "clean" or "dirty". Clean tankers carry refined petroleum products such as gasoline, kerosene or jet fuels, or chemicals. The so-called dirty vessels transport products such as heavy fuel oils or crude oil. Larger tankers usually only carry crude oil. A tanker carrying dirty cargoes will require about 2 weeks of manual labor to clean its tanks and piping before carrying clean cargo.
Oil products tankers are non-specialised tankers below 60,000 dwt, as well as coated tankers above this size. These ships carry a spectrum of cargoes, ranging from relatively unsophisticated dirty products such as fuel oil through to clean products such as naphtha. Vessels that trade at one end of this spectrum are unlikely to be able to switch easily to the other end, and "last cargo" regulations ensure that dedicated fleets become established for some cargo types. At the most sophisticated end of the fleet there is an overlap with the chemical sector, with a significant volume of "swing tonnage" that can operate in either.
Chemical tankers are modern parcel tankers with either full or partial stainless steel tanks that cover the sophisticated end of the chemicals market, for which IMO Grade I/II is required. Newer parcel tankers have the flexibility to carry many cargoes, from easy chemicals through caustic soda to methanol, requiring IMO Grade III (last cargo requirements are also enforced).
Four major groups of liquefied gases are traded by sea: LPG, ammonia, petrochemicals and LNG. LPG is co-produced with crude in most oilfields. Small volumes are also produced in the refining process. Asian markets, notably China, are also growing rapidly as LPG's properties as a "clean" alternative to traditional fossil fuels is increasingly exploited. Different sizes of LPG tankers are used in different trades. The largest vessels, which typically are around 80,000 cubic meters, are used in the deep-sea trade, for example from the Arabian Gulf to Japan. Medium sized vessels are used in the shorter haul trades, whilst the very smallest trades are used to distribute LPG and other petrochemical gases within the regions, especially Asia and Northwest Europe.
Sea transportation of natural gas requires liquefaction at -1600C and containment in insulated tanks. The first small LNG tankers were built in the mid 1960s. LNG tankers are typically 850 feet in length (about the size of a Suezmax Tanker) and capable of carrying 120,000 cbms of liquefied gas. The gas is generally liquefied in a shore based plant and transferred to the ship which is a "floating thermos flask" with heavy insulation.
- Bunker tankers supply fuel to ocean going vessels. The tanker carries out the vast majority of its bunkering operations in close vicinity to the port. The bunker tanker has been designed to carry out ship to ship transfers (STS) of oil and accordingly has a high degree of the manoeuvrability. As of 2006 the largest double hull bunker tanker in the world had a capacity of 12,000 mt and will pump at 2,200 cm per hour.
- Lighters or harbor tankers may refuel ships or move smaller amount of products. Lighterage is a method of discharging cargoes from a heavy draft vessel which cannot (because of shallow-draft port conditions) or does not (for whatever reasons) come into the port area and discharge cargoes onto the dock area. When cargoes are lightened ashore, the mother vessel discharges cargo into a smaller vessel (a lighter) and the lighter carries the cargo ashore and places the cargo onto the docks. Generally speaking, the discharge of cargoes via lighter carriers should be avoided, if possible, because individual bags of cargo are handled one additional time, thus increasing the possibility of damage. Also, in some ports, lighterage vessels do not provide adequate security for cargoes either from the elements or from pilferage.
- Handysize is a small bulk or oil tanker vessel that is suited to tie up at a T2 type pier. These vessels are a maximum of of 10,000 to 30,000 dwt. These vessels are more maneuverable and have shallower draft than larger vessels and therefore make up the majority of the world's ocean-going cargo fleet.
- Handymax is a small bulk or oil tanker vessel of 30,001 to 50,000 dwt that is a larger version of the popular Handysize vessel.
- Panamax is an ocean-going cargo vessel of the maximum size possible to pass through the locks of the Panama Canal, which are 1000ft long by 110ft wide and 85ft deep. These vessels are typically of 50,000 to 80,000 dwt, 965ft (290m) in length; 106ft. (32.3m) beam; and 39.5ft (12.04m) draft. The Ability to transit the Panama Canal fully laden affording these vessels the flexibility to trade efficiently in the Atlantic and Pacific basins. Panamax tankers are used for crude oil and petroleum products. Modern Panamax tankers employed in the crude and fuel oil trades have carved out a niche in the Caribbean and Latin American region as a result of their ability to transverse the Panama Canal and because size restrictions in certain ports restrict the use of larger tankers.
- Aframax is an ocean-going crude oil tanker vessel of standard size between 80,000 and 119,000 dwt that is the largest crude oil tanker size in the AFRA (Average Freight Rate Assessment) tanker rate system. The Aframax vessel class is considered to be the "workhorse" among tankers because of its flexibility and high potential utilization. Aframax International (AI) pool tankers are now built using the new Common Structural Rules (CSR) recently established by IACS for all tankers and bulk carriers ordered after April 1, 2006. Among other design upgrades, the new rules set forth stricter hull strength requirements and technical criteria of double hull tankers that had previously differed by each classification society. Aframax International (AI), the second-largest commercial operator of Aframax tankers in the world. The AI pool has seven members and 40 vessels, of which 97.5 percent are double hull, with an average age of 6.8 years. The pool is dedicated to serving the demand for high-quality tanker transportation of leading oil companies and charterers. As a result of their flexibility and size, Aframaxes are deployed in much more diverse trading patterns than the larger tankers and transport crude from virtually all the major crude exporting regions in both the Atlantic and Pacific. Aframax crude carriers are typically deployed on short haul or distributive routes where draft or other size restrictions prevent the use of larger tankers or where crude oil is produced or consumed in smaller quantities.
- Suezmax is an ocean-going cargo vessel of the maximum size possible to pass through the locks of the Suez Canal in Egypt. This standard has evolved over time. Prior to 1967, a Suezmax was a maximum of 80,000 dwt. The canal was closed between 1967 and 1975 because of the Israel-Arab conflict. Upon reopening in 1975, after many modifications to the locks and canal itself, the maximum was increased to 150,000 dwt.
- The shuttle tanker is a ship type that has emerged since oil exploration went offshore and into deeper and more remote waters, where pipelines to shore are neither feasible nor economical. They are largely conventional tankers which are equipped to station themselves on an offshore loading buoy far at sea and to load their cargo of crude oil directly from the oilfield, where it has been kept in a reservoir with perhaps the sulphur removed. In most respects the shuttle tanker looks similar to any other crude carrier, but the most noticeable difference is the extra equipment at the bow of the vessel for single point mooring/unloading. Shuttle tankers need to be very much more maneuvrable than comparable conventional crude carriers. Typically, a shuttle tanker is about 120,000 tons capacity and is at 16 knots rather faster than her conventional sister.
- Very Large Crude Carrier (VLCC) is an ocean-going crude oil tanker of 200,000 to 299,999 dwt. These vessels have greater flexibility than ULCCs due to their smaller size and are used extensively in the Mediterranean, West Africa and the North Sea. These vessels can sometimes be ballasted through the Suez Canal. VLCCs are typically used for the long-haul trades originating from the Arabian Gulf and West Africa, with occasional voyages from North Africa and the North Sea. Suezmax tankers are mainly employed in the Atlantic Basin for exports from West Africa, the North Sea and the FSU.
- Capesize is an ocean-going cargo vessel that is physically too large to fit through the locks of either the Panama or Suez Canals and therefore must voyage via Cape Horn at the southernmost tip of South America to get to or from the Atlantic and Pacific Oceans, or the Cape of Good Hope at the southernmost tip of South Africa to get to and from the Indian and Atlantic Oceans. Capesize vessels generally serve deepwater terminals handling raw materials, such as iron ore and coal.
- Ultra Large Crude Carrier (ULCC) is an ocean-going crude oil tanker of 300,000 to 550,000 dwt. These are the largest vessels in the world and are used for carrying crude oil on long haul routes from the Arabian Gulf to Europe, America and the Far East, via the Cape of Good Hope. These vessels require custom built terminals for loading and discharge.
The handy size tanker (6,000 to 35,000 cargo DWT, or approximately 48,000 to 280,000 BBLs) is the most militarily useful. These generally carry clean or refined products, although some may carry black oil, chemicals and, occasionally, bulk grain. The term "handy size tanker equivalent," refers to a tanker of 200,000 BBL or approximately 25,000 DWT. The military advantages of handy size tankers include their ability to enter most of the world's tanker ports, the relatively short time required for tank cleaning when required, and their overall flexibility with regard to the numbers of different cargoes they can carry. Their military disadvantages include the comparatively small capacity and limited availability in the commercial market.
The medium size tanker ranges in capacity from 35,000 to 100,000 DWT (or approximately 280,000 to 800,000 BBLs). As a general rule, those under 60,000 DWT can carry "clean" cargoes while those over 80,000 DWT will, almost exclusively, carry crude oil or other "dirty" cargoes. The military advantages of medium size tankers are that they are more readily available than handy size tankers and are capable of delivering large quantities of POL. Their military disadvantage is that it is difficult and time consuming (1-2 weeks) to clean the tanks and piping of tankers that have been transporting either crude oil or other "dirty" cargoes, so that they can be used to transport refined POL products.
Large crude carriers are the largest tanker class and are solely dedicated to the transportation of crude oil. Very Large Crude Carriers range in capacity from 100,000 to 400,000 DWT, while Ultra Large Crude Carriers have even greater capacities. None of these ships are considered militarily useful.
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