Vessel Types - Oil Gas Lpg Lng Chemical Tankers

**VESSEL TYPES - OIL/GAS/LPG/LNG/CHEMICAL TANKERS**

Liquid/Hydrate Gas Tankers Liquefied gas tankers are oceangoing ships designed to carry one or more types of liquefied gas cargo—gas which has been cooled, pressurized, or both, below its boiling point so it can be shipped as a liquid. Such liquefied gas cargoes include butane and propane (both referred to as liquefied petroleum gases, or LPG), liquefied natural gas (LNG), butadiene, propylene, ethylene, ethylene, vinyl chloride, methyl chloride, ammonia and propylene oxide. Liquefied gas tankers consist of several large and separate onboard tanks which may be pressurized, refrigerated, and insulated to accommodate different cargo needs. International International shipping codes impose extensive standards for the construction and operation of these vessels. Their cargo tanks must be built to withstand high pressures or l ow temperatures, as necessary. Therefore, these tanks are robust and resistant to impact damage, or flexible and able to distort without failure. The vessels are also “double-hulled,” with cargo tanks located above a double bottom and inboard of the outer hull, independent of the tankers’ outer hull structures. Consequently, liquefied gas tankers possess a level of  structural integrity greater than that found in most other classes of ship, which makes them highly resistant to grounding and collision damage. LNG tankers carry only LNG. Other liquefied gas tankers may simultaneously carry a combination of different cargoes, such as butane and propylene, in different storage tanks. Such combination cargoes create potentially unique multi-chemical hazards.

Pressurised LPG carriers

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Last Updated: 25/4/2006

Regional and coastal cargoes are often carried in such craft with the cargo fully pressurised at ambient temperature. Accordingly, the tanks are built as pure pressure vessels without the need for any extra metallurgical consideration appropriate to colder temperatures. Design pressures are usually for propane (about 20 bar) as this form of LPG gives the highest vapour pressure at ambient temperature. The ship design comprises outer hull and an inner hold containing the pressure vessels. These rest in saddles built into the ship's structure. Double bottoms and other spaces act as water ballast tanks.

Liquid gas carriers carriers, especially the pressurised type, are characterised by their tanks, usually cylindrical of spherical which are prominent along the deck of the vessel. Non pressurised vessels may look more like a refined products carrier as the box shaped tanks are not so visible.

**VESSEL TYPES - OIL/GAS/LPG/LNG/CHEMICAL TANKERS**

Semi pressurised LPG carriers In these ships, sometimes referred to as 'semi-refrigerated', the cargo is carried in pressure vessels usually bi-lobe in cross section, designed for operating pressures of up to 7 bars. The tanks are constructed of special grade steel suitable for the cargo carriage temperature. The tanks are insulated to minimise heat input to the cargo. The cargo boils off causing generation of vapour, which is reliquefied by refrigeration and returned to the cargo tanks. The required cargo temperature and pressure is maintained by the reliquefaction plant. These ships are usually larger than the fully pressurised types and have cargo capacities up to about 20,000 m3. As with the fully pressurised ship, the cargo tanks are of pressure vessel construction and similarly located well inboard of the ship's side and also protected by double bottom ballast tanks. This arrangement again results in a very robust and inherently buoyant ship.

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This site generally details the types of vessels that visit the Solent area (the ports of  Southampton and Portsmouth in the UK), but as there has been worldwide interest in vessel types, I have included types that may not visit this area.

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Fully refrigerated LPG carrier These are generally large ships, up to about 100,000 m3 cargo capacity, those above 70,000 m3 being designated as VLGCs. Many in the intermediate range (say 30,000 m3 to 60,000 m3) are suitable for carrying the full range of hydrocarbon liquid gas from butane to propylene and may be equipped to carry chemical liquid gases such as ammonia. Cargoes are carried at near ambient pressure and at temperatures down to -48ºC. Reliquefaction plants are fitted, with substantial reserve plant capacity provided. The cargo tanks do not have to withstand high pressures and are therefore generally of the free standing prismatic type. The tanks are robustly stiffened internally and constructed of special low temperature resistant steel. All ships have substantial double bottom spaces and some have side ballast tanks.

In all cases the tanks are protectively located inboard. The ship's structure surrounding or adjacent to the cargo tanks is also of special grade steel, in order to form a secondary barrier to safely contain any cold cargo should it leak from the cargo tanks. All c argo tanks, whether they be of the pressure vessel type or rectangular, are provided with safety relief valves amply sized to relieve boil-off in the absence of reliquefaction and even in conditions of surrounding fire.

Ethylene, one of the chemical gases, is the premier building block of  the petrochemicals industry. It is used in the production of 

**VESSEL TYPES - OIL/GAS/LPG/LNG/CHEMICAL TANKERS**

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dichloride, ethanol, styrene, glycols and many other products.

Ethelyne Carrier Storage is usually as a fully refrigerated liquid at -104ºC. Ships designed for ethylene carriage also fall into the semipressurised class. They are relatively few i n number but are among the most sophisticated ships afloat. In the more advanced designs they have the ability to carry several grades. Typically this range can extend to ethane, LPG, ammonia, propylene butadiene and vinyl chloride monomer (VCM). To aid in this process several independent cargo systems co-exist onboard to avoid cross contamination of the cargoes, especially for the reliquefaction process. The ships range in size from about 2,000 m3 to 15,000 m3 although several larger ships now trade in ethylene. Ship design usually includes independent cargo tanks (Type-C), and these may be cylindrical or bi-lobe in shape constructed from stainless steel. An inert gas generator is provided to produce dry inert gas or dry air. The generator is used for inerting and for the dehydration of the cargo system as well as the interbarrier spaces during voyage. For these condensation occurs on cold surfaces with unwanted build-ups of ice. Deck tanks are normally provided for changeover of cargoes. The hazards associated with the cargoes i nvolved are obvious from temperature, toxic and flammable concerns. Accordingly, the safety of all such craft is criti cal with good management and serious personnel training remaining paramount.

LNG carrier with membrane tanks.

General arrangement vessel types roro pcc pctc vehicle carrier pictures escort tractor tugs voith schneider cycloidal propulsion offshore supply ships oil tankers chemical LNG LPG

**VESSEL TYPES - OIL/GAS/LPG/LNG/CHEMICAL TANKERS**

LNG carrier with type B tanks. LNG carrier All LNG carriers have a watertight inner hull and most tank designs are required to have a secondary containment capable of safely holding any leakage for a period of 15 days. Because of the simplicity and reliability of stress analysis of the spherical containment designs, a full secondary barrier is not required but splash barriers and insulated drip trays protect the inner hull from any leakage that might occur in operation.

Existing LNG carriers do not reliquefy boil-off gases, they are steam ships and the gas is used as fuel for the ship's boilers. The first ships to burn this gas in medium speed diesel engines will be delivered in 2005/6, and ships with reliquefaction plant and conventional slow speed diesel engines will enter service late in 2007. It is likely that gas turbine propelled ships may appear soon after this. Natural gas hydrate carrier. (NGH) In comparison with the conventional Liquefied Natural Gas (LNG) which is produced and stored at extremely low temperature of -162°C , the production cost for NGH is much lower. Although the transportation amount per unit energy of  NGH is more than that of LNG, the facilities of NGH for production, storage, transport and gasification can be simplified because NGH requires a milder condition i.e. -20°C under atmospheric pressure. Eventually, the total cost of  NGH system including production, storage, transportation and gasification therefore is much cheaper than that of  LNG.

Chemical Tankers

**VESSEL TYPES - OIL/GAS/LPG/LNG/CHEMICAL TANKERS**

From the outside chemical tankers look similar to refined products carriers. The difference lies with the tanks which must be lined with a suitable material which is inert to the cargo and may be capable of heating or cooling the cargo. Also the hold may be divided up into many well isolated tanks to allow the carriage of many types of chemical simultaneously (parcel tankers) The manifold where the ship is unloaded tends to be complicated and the decks are sometimes covered with pipe work, valves, vents and processing plant.

Chemicals were once carried around the world as deck cargo in drums and glass carboys on conventional ships, stowed near the rail so they could be easily jettisoned if acid started to leak over the deck or fire broke out and never carried in the holds. But chemical shipments grew substantially, with more demanding products, and gradually a class of ship designed to carry chemicals in bulk was evolved. These are highly sophisticated ships - some people describe them as "liquid liners" with the most complex vessels able to carry up to sixty different types of chemicals, each in its own carriage

**VESSEL TYPES - OIL/GAS/LPG/LNG/CHEMICAL TANKERS**

conditions, with its own piping and pumping system. Some cargoes will require heating, others refrigerating, some are so volatile that they must be kept safe under a blanket of inert nitrogen, others react violently with water, so must be handled in ultra-dry conditions. Some are desperately corrosive and require tanks of the highest quality of stainless steel , while others must have tank coatings of a precise specification. Complex manifold arrangement - typical of chemical tankers with segregated tanks. Some tankers are specifically designed for one type of cargo. A molten sulphur carrier is such a vessel. Sulphur remains liquid at temperatures from 240 to 310F, but quickly increases in viscosity above 320F and freezes at 240F. The cargo is maintained at 278F.

**VESSEL TYPES - OIL/GAS/LPG/LNG/CHEMICAL TANKERS**

Chemical parcel tankers are versatile vessels designed to carry a wide range of liquid and chemical cargoes. Externally, they appear similar to petroleum product/chemical tankers, but typically can carry 10 to 60 separate cargo tanks to simultaneously accommodate multiple cargoes or “parcels.” They range in total cargo capacity from approximately 3,000 to 50,000 tons, although most are well under 50,000 tons.

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Some cargoes must be kept in motion, lest they settle out, and others can only be carried in tanks which have never carried cargoes with which they are incompatible. Some cargoes can be tainted by the residue of a previous cargo, even after a stainless steel tank has been scrupulously cleaned and purged. Some cargoes react violently to others, or to exposure to the atmosphere. Many are flammable, explosive or give off noxious vapours so safety will always be an important consideration. Some are edible. Many of these chemical cargoes are immensely valuable, demand fantastic standards of cleanliness to maintain their product purity and must be discharged to the last drop, with none left remaining on board. Special pumps are installed to assist in this. The stowage and handling of one of these chemical parcel tankers (so called because a tank constitutes a "parcel" of cargo), demands enormous care and precision, meticulous planning and a great deal of specialist knowledge additional to the normal skills of the navigator. A multiple port schedule, with perhaps several different specialist berths to load and discharge in the same port is not unusual. The use of computers for planning and stowage has been invaluable in modern times; indeed it is difficult to recall how the business was undertaken without such electronic assistance. Special training and certification is required for officers in these ships, who need to have a good knowledge, not merely of these cargoes and their characteristics, but of all the trade names given to the varied products, which may be presented in port. New products are being developed all the time by the chemical industry throughout the world, and being offered for shipment. Keeping up with the scientists is a special responsibility for this sector of the shipping industry, which, despite the often hazardous cargo carried, enjoys an excellent safety record with a modern fleet of double-hulled ships which range from 3000 tonne acid tankers to 80,000 tonne chemical parcel vessels.

Here a list of some of substances that are carried by chemical/edible products tankers: Ethane Propene (Propylene) Vinyl Ethyl Ether Pentane Styrene Latex Vegetable oil Molasses

Anhydrous Ammonia Propene Oxide Xylene Ammonia n-Butane Acetaldehyde Dimethylamine Ethyl chloride Diethyl ether Isoprene (Monomer) Beer Fruit Juice Asphalt

Isopropyl amine Methyl chloride Monoethylamine Benzene Alcohols Vinyl chloride Monore (VCM) i-Butane Butene (Butylene) Butadiene Ethene Oxide/Propene Oxide Commercial Propane Wine Acid

**VESSEL TYPES - OIL/GAS/LPG/LNG/CHEMICAL TANKERS**