Figure 1.3 Typical oil/gas flow diagram

1.2.3 Production of chemical gases

A simplified diagram for the production of the chemical gases, vinyl chloride, ethylene and ammonia is shown in Figure 1.4. These three chemical gases can be produced indirectly from propane. The propane is first cracked catalytically into methane and ethylene. The ethylene stream can then be synthesised with chlorine to manufacture vinyl chloride. In the case of the methane stream, this is first reformed with steam into hydrogen. By combining this with nitrogen under high pressure and temperature, in the presence of a catalyst, ammonia is produced.

Figure 1.4 Typical flow diagram — production of chemical gas

1.2.4 The principal products

Whilst the hydrocarbon gases methane, ethane, propane and butane may be regarded principally as fuels, the LPGs are also important as feedstocks in the production of the chemical gases.

Liquefied Natural Gas (LNG)

Natural gas is transported either by pipeline as a gas or by sea in its liquefied form as LNG.

Natural gas comes from underground deposits as described in 1.2.1. Its composition varies according to where it is found but methane is by far the predominant con­stituent, ranging from 70 per cent to 99 per cent. Natural gas is now a major commodity in the world energy market and approximately 73 million tonnes are carried by sea each year. This is expected to increase to 100 million tonnes per year by the end of the millennium.

Natural Gas Liquids (NGLs)

Associated gas, found in combination with crude oil, comprises mainly methane and NGLs. As shown in Figure 1.1, the NGLs are made up of ethane, LPGs and gasoline.

A small number of terminals, including several facilities in Europe, have the ability to strip methane from the gas stream and to load raw NGLs onto semi-pressurised gas carriers. These ships are modified with additional compressor capacity for shipment to customers able to accept such ethane-rich cargoes. These NGLs are carried at -80°C at atmospheric pressure or at -45°C at a vapour pressure of 5 bar.

The Liquefied Petroleum Gases (LPG)

The liquefied petroleum gases comprise propane, butane and mixtures of the two. Butane stored in cylinders and thus known as bottled gas, has widespread use as a fuel for heating and cooking in remote locations. However, it is also an important octane enhancer for motor gasoline and a key petrochemical feedstock. Propane, too, is utilised as a bottled gas, especially in cold climates (to which its vapour pressure is more suited). However, LPG is mainly used in power generation, for industrial purposes such as metal cutting and as a petrochemical feedstock. About 169 million tonnes of LPG are produced each year worldwide and, of this, about 43.7 million tonnes are transported by sea.

Ammonia

With increased pressure on the world's food resources, the demand for nitrogen-containing fertilisers, based on ammonia, expanded strongly during the 1970s and 1980s. Large-scale ammonia plants continue to be built in locations rich in natural gas which is the raw material most commonly used to make this product. Ammonia is also used as an on-shore industrial refrigerant, in the production of explosives and for numerous industrial chemicals such as urea. Worldwide consumption of this major inorganic base chemical in 1996 was 120 million tonnes. About 12 million tonnes of ammonia are shipped by sea each year in large parcels on fully refrigerated carriers and this accounts for the third largest seaborne trade in liquefied gases — after LNG and LPG.

Ethylene

Ethylene is one of the primary petrochemical building blocks. It is used in the manu­facture of polyethylene plastics, ethyl alcohol, polyvinyl chloride (PVC), antifreeze, polystyrene and polyester fibres. It is obtained by cracking either naphtha, ethane or LPG. About 85 million tonnes of ethylene is produced worldwide each year but, because most of this output is utilised close to the point of manufacture, only some 2.5 million tonnes is moved long distances by sea on semi-pressurised carriers.

Propylene

Propylene is a petrochemical intermediate used to make polypropylene and poly-urethane plastics, acrylic fibres and industrial solvents. As of mid-1996, annual worldwide production of propylene was 42 million tonnes, with about 1.5 million tonnes of this total being carried by semi-pressurised ships on deep-sea routes.

Butadiene

Butadiene is a highly reactive petrochemical intermediate. It is used to produce styrene, acrylonitrile and polybutadiene synthetic rubbers. Butadiene is also used in paints and binders for non-woven fabrics and, as an intermediate, in plastic and nylon production. Most butadiene output stems from the cracking of naphtha to produce ethylene. Worldwide total production of Butadiene in 1996 was 6.9 million tonnes. About 800,000 tonnes of butadiene is traded by sea each year.

Vinyl chloride

Vinyl chloride is an easily liquefiable, chlorinated gas used in the manufacture of PVC, the second most important thermoplastic in the world in terms of output. Vinyl chloride not only has a relatively high boiling point, at -14°C, but is also, with a specific gravity of 0.97, much denser than the other common gas carrier cargoes. Worldwide production of vinyl chloride in 1996 was 22.3 million tonnes. Some 2 million tonnes of vinyl chloride is carried by sea each year.

1.3 TYPES OF GAS CARRIERS

Gas carriers range in capacity from the small pressurised ships of between 500 and 6,000 m³ for the shipment of propane, butane and the chemical gases at ambient temperature up to the fully insulated or refrigerated ships of over 100,000 m³ capacity for the transport of LNG and LPG. Between these two distinct types is a third ship type — the semi-pressurised gas carrier. These very flexible ships are able to carry many cargoes in a fully refrigerated condition at atmospheric pressure or at temperatures corresponding to carriage pressures of between five and nine bar.

The movement of liquefied gases by sea is now a mature industry, served by a fleet of over 1,000 ships, a worldwide network of export and import terminals and a wealth of knowledge and experience on the part of the various people involved. In 1996 this fleet transported about 62.5 million tonnes of LPG and chemical gases and 73 million tonnes of LNG. In the same year the ship numbers in each fleet were approximately as follows:—

LNG carriers                       105

Fully refrigerated ships       183 Ethylene carriers                100

Semi-pressurised ships      276 Pressurised ships                437

Gas carriers have certain design features in common with other ships used for the carriage of bulk liquids such as oil and chemical tankers. Chemical tankers carry their most hazardous cargoes in centre tanks, whilst cargoes of lesser danger can be shipped in the wing tanks. New oil tankers are required to have wing and double bottom ballast tanks located to give protection to the cargo. The objective in both these cases is to protect against the spillage of hazardous cargo in the event of a grounding or collision. This same principle is applied to gas carriers.

A feature almost unique to the gas carrier is that the cargo tanks are kept under positive pressure to prevent air entering the cargo system. This means that only cargo liquid and cargo vapour are present in the cargo tank and flammable atmospheres cannot develop. Furthermore all gas carriers utilise closed cargo systems when loading or discharging, with no venting of vapours being allowed to the atmosphere. In the LNG trade, provision is always made for the use of a vapour return line between ship and shore to pass vapour displaced by the cargo transfer. In the LPG trade this is not always the case as, under normal circumstances during loading, reliquefaction is used to retain vapour on board. By these means cargo release to the atmosphere is virtually eliminated and the risk of vapour ignition is minimised.

Gas carriers must comply with the standards set by the International Maritime Organization in the Gas Codes (see Chapter Three), and with all safety and pollution requirements common to other ships. The Gas Codes are a major pro-active feature in IMO's legislative programme. The safety features inherent in the Gas Codes' ship design requirements have helped considerably in the safety of these ships. Equipment requirements for gas carriers include temperature and pressure monitoring, gas

detection and cargo tank liquid level indicators, all of which are provided with alarms and ancillary instrumentation. The variation of equipment as fitted can make the gas carrier one of the most sophisticated ships afloat today.

There is much variation in the design, construction and operation of gas carriers due to the variety of cargoes carried and the number of cargo containment systems utilised. Cargo containment systems may be of the independent tanks (pressurised, semi-pressurised or fully refrigerated) or of the membrane type (see 3.2.2). Some of the principal features of these design variations and a short history of each trade are described below.

Fully pressurised ships

The seaborne transport of liquefied gases began in 1934 when a major international company put two combined oil/LPG tankers into operation. The ships, basically oil tankers, had been converted by fitting small, riveted, pressure vessels for the carriage of LPG into cargo tank spaces. This enabled transport over long distances of sub­stantial volumes of an oil refinery by-product that had distinct advantages as a domestic and commercial fuel. LPG is not only odourless and non-toxic, it also has a high calorific value and a low sulphur content, making it very clean and efficient when being burnt.

Today, most fully pressurised LPG carriers are fitted with two or three horizontal, cylindrical or spherical cargo tanks and have capacities up to 6,000 m³. However, in recent years a number of larger capacity fully-pressurised ships have been built with spherical tanks, most notably a pair of 10,000 m³ ships, each incorporating five spheres, built by a Japanese shipyard in 1987. Fully pressurised ships are still being built in numbers and represent a cost-effective, simple way of moving LPG to and from smaller gas terminals.

Semi-pressurised ships

Despite the early breakthrough with the transport of pressurised LPG, ocean move­ments of liquefied gases did not really begin to grow until the early 1960s with the development of metals suitable for the containment of liquefied gases at low temperatures. By installing a reliquefaction plant, insulating the cargo tanks and making use of special steels, the shell thickness of the pressure vessels, and hence their weight, could be reduced.

The first ships to use this new technology appeared in 1961. They carried gases in a semi-pressurised/semi-refrigerated (SP/SR) state but further advances were quickly made and by the late 1960s semi-pressurised/fully refrigerated (SP/FR) gas carriers had become the shipowners' choice by providing high flexibility in cargo handling. Throughout this book the SP/FR ships are referred to as semi-pressurised ships. These carriers, incorporating tanks either cylindrical, spherical or bi-lobe in shape, are able to load or discharge gas cargoes at both refrigerated and pressurised storage facilities. The existing fleet of semi-pressurised ships comprises carriers in the 3,000-15,000 m³ size range, although there is a notable exception — a ship of 30,000 m³ delivered in 1985.

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