This chapter provides an overview of the liquefied gases carried by sea and it concludes with some advice on the safety issues involving the ship, the terminal and the ship/shore interface. The latter point is of the utmost importance as this is where ship and shore personnel meet to plan safe operations. Subsequent chapters provide much greater detail about gas carrier cargoes and the equipment utilised on the ship and at the terminal jetty. They also cover operational and emergency procedures. Questions of health and safety are also covered and Chapter Six is devoted exclusively to ship/shore interface matters.
A thorough understanding of the basic principles outlined in this book is recom- mended as such knowledge will help ensure safer operations, better cargo planning and the efficient use of equipment found on gas carriers and on jetties.
1.1 LIQUEFIED GASES
A liquefied gas is the liquid form of a substance which, at ambient temperature and at atmospheric pressure, would be a gas.
Most liquefied gases are hydrocarbons and the key property that makes hydrocarbons the world’s primary energy source — combustibility — also makes them inherently hazardous. Because these gases are handled in large quantities it is imperative that all practical steps are taken to minimise leakage and to limit all sources of ignition.
The most important property of a liquefied gas, in relation to pumping and storage, is its saturated vapour pressure. This is the absolute pressure (see 2.15) exerted when the liquid is in equilibrium with its own vapour at a given temperature. The International Maritime Organization (IMO), for the purposes of its Gas Carrier Codes (see Chapter Three), relates saturated vapour pressure to temperature and has adopted the following definition for the liquefied gases carried by sea:
Liquids with a vapour pressure exceeding 2.8 bar absolute at a temperature of 37.8°C
An alternative way of describing a liquefied gas is to give the temperature at which the saturated vapour pressure is equal to atmospheric pressure — in other words the liquid’s atmospheric boiling point.
In Table 1.1 some liquefied gases carried at sea are compared in terms of their vapour pressure at 37.8°C — the IMO definition — and in terms of their atmospheric boiling points.
Table 1.1 Physical properties of some liquefied gases
Vapour pressure at 37.8°C
at atmospheric pressure
* The critical temperature of methane is –82.5°C while the critical pressure is 44.7 bars. Therefore, at a temperature of 37.8°C it can only exist as a gas and not as a liquid.
On the basis of the above IMO definition, ethylene oxide (see Table 1.1) would not qualify as a liquefied gas. However, it is included in the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (the IGC Code) because its boiling point at atmospheric pressure is so low that it would be difficult to carry the cargo by any method other than those prescribed for liquefied gases.
Likewise, chemicals such as diethyl ether, propylene oxide and isoprene are not strictly liquefied gases but they have high vapour pressures coupled with health and flammability hazards. As a result of such dangers these chemicals, and several similar compounds, have been listed jointly in both the IGC Code and the Bulk Chemical Codes. Indeed, when transported on chemical tankers, under the terms of the Bulk Chemical Codes, such products are often required to be stowed in independent tanks rather than in tanks built into the ship’s structure.
1.2 LIQUEFIED GAS PRODUCTION
Figure 1.1 Constituents of natural gas
1.2.1 LNG production
Natural gas may be found in:
• Underground wells, which are mainly gas bearing (non-associated gas)
• Condensate reservoirs (pentanes and heavier hydrocarbons)
• Large oil fields (associated gas)
In the case of oil wells, natural gas may be either in solution with the crude oil or as a gas-cap above it.
Natural gas contains smaller quantities of heavier hydrocarbons (collectively known as natural gas liquids — NGLs). This is in addition to varying amounts of water, carbon dioxide, nitrogen and other non-hydrocarbon substances. These relationships are shown in Figure 1.1.
The proportion of NGL contained in raw natural gas varies from one location to another. However, NGL percentages are generally smaller in gas wells when com- pared with those found in condensate reservoirs or that associated with crude oil. Regardless of origin, natural gas requires treatment to remove heavier hydrocarbons and non-hydrocarbon constituents. This ensures that the product is in an acceptable condition for liquefaction or for use as a gaseous fuel.
Figure 1.2 Typical flow diagram for LNG liquefaction
The gas then passes to a fractionating unit where the NGLs are removed and further split into propane and butane. Finally, the main gas flow, now mostly methane, is liquefied into the end product, liquefied natural gas (LNG).
To lower the temperature of the methane gas to about –162°C (its atmospheric boiling point) there are three basic liquefaction processes in current use. These are outlined below:—
• Pure refrigerant cascade process — this is similar in principle to the cascade reliquefaction cycle described in 4.5 but in order to reach the low temperature required, three stages are involved, each having its own refrigerant, compressor and heat exchangers. The first cooling stage utilises propane,
the second is a condensation stage utilising ethylene and, finally, a sub-cooling stage utilising methane is involved. The cascade process is used in plants commissioned before 1970.
• Mixed refrigerant process — whereas with pure refrigerant process (as described above) a series of separate cycles are involved, with the mixed refrigerant process (usually methane, ethane, propane and nitrogen), the entire process is achieved in one cycle. The equipment is less complex than the pure refrigerant cascade process but power consumption is substantially greater and for this reason its use is not widespread.
• Pre-cooled mixed refrigerant process — this process is generally known as the MCR process (Multi-Component Refrigerant) and is a combination of the pure refrigerant cascade and mixed refrigerant cycles. It is by far the most common process in use today.
Fuel for the plant is provided mainly by flash-off gas from the reliquefaction process but boil-off from LNG storage tanks can also be used. If necessary, additional fuel may be taken from raw feed gas or from extracted condensates. Depending upon the characteristics of the LNG to be produced and the requirements of the trade, some of the extracted NGLs may be re-injected into the LNG stream.
1.2.2 LPG production
Liquefied petroleum gas (LPG) is the general name given for propane, butane and mixtures of the two. These products can be obtained from the refining of crude oil. When produced in this way they are usually manufactured in pressurised form.
However, the main production of LPG is found within petroleum producing countries. At these locations, LPG is extracted from natural gas or crude oil streams coming from underground reservoirs. In the case of a natural gas well, the raw product consists mainly of methane. However, as shown in Figure 1.2, in this process it is normal for NGLs to be produced and LPG may be extracted from them as a by-product.
Figure 1.3 Typical oil/gas flow diagram
1.2.3 Production of chemical gases
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 asLNG.,
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 con- stituent, 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 5bar.
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 manu- facture 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
• Fully refrigerated ships
• Ethylene carriers
• Semi-pressurised ships
• Pressurised ships
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 sub- stantial 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 move- ments 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.
Ethylene and gas/chemical carriers, Ethylene carriers are the most sophisticated of the semi-pressurised tankers and have the ability to carry not only most other liquefied gas cargoes but also ethylene at its atmospheric boiling point of –104°C. The first ethylene carrier was built in 1966 and, as of 1995, there were about 100 such ships in service ranging in capacity from 1,000 to 12,000 m3.
Of this ethylene carrier fleet, about one dozen form a special sub-group of ships able to handle a wide range of liquid chemicals and liquefied gases simultaneously. These ships feature cylindrical, insulated, stainless steel cargo tanks able to accommodate cargoes up to a maximum specific gravity of 1.8 at temperatures ranging from a minimum of –104°C to a maximum of +80°C and at a maximum tank pressure of 4 bar. The ships can load or discharge at virtually all pressurised and refrigerated terminals, making them the most versatile gas carriers in terms of cargo-handling ability.
Fully refrigerated ships, The 1960s also saw another major development in gas carrier evolution — the appearance of the first fully refrigerated ship, built to carry liquefied gases at low temperature and atmospheric pressure between terminals equipped with fully refrigerated storage tanks. The first purpose-built, fully refrigerated LPG carrier was constructed by a Japanese shipyard, to a United States design, in 1962. The ship had four prismatic-shaped (box-like) cargo tanks fabricated from 31⁄2 per cent nickel steel, allowing the carriage of cargoes at temperatures as low as –48°C, marginally below the atmospheric boiling point of pure propane. Prismatic tanks enabled the ship’s cargo carrying capacity to be maximised, thus making fully refrigerated ships highly suitable for carrying large volumes of cargo such as LPG, ammonia and vinyl chloride over long distances. Today, fully refrigerated ships range in capacity from 20,000 to100,000m3.
The main types of cargo containment system utilised on board modern fully refrigerated ships are independent tanks having rigid foam insulation. Older ships can have independent tanks with loosly filled perlite insulation. In the past, there have been a few fully refrigerated ships built with semi-membrane or integral tanks and internal insulation tanks, but these systems have only maintained minimal interest.
Liquefied natural gas (LNG) carriers, At about the same time as the development of fully refrigerated LPG carriers was taking place, naval architects were facing their most demanding gas carrier challenge. This was the transport of LNG. Natural gas, another clean, non-toxic fuel, is now the third most important energy source in the world, after oil and coal, but is often produced far from the centres of consumption. Because a gas in its liquefied form occupies much less space, and because of the critical temperature of liquefied methane, the ocean transport of LNG only makes sense from a commercial viewpoint if it is carried in a liquefied state at atmospheric pressure; as such, it represents a greater engineering challenge than shipping LPG, mainly because it has to be carried at a much lower temperature; its boiling point being –162°C.
The pioneering cargo of LNG was carried across the Atlantic Ocean in 1958 and by
1964 the first purpose-built LNG carriers were in service under a long-term gas
purchase agreement. LNG containment system technology has developed consider-
ably since those early days: now about one-half of the LNG carriers in service are fitted
with independent cargo tanks and one-half with membrane tanks. The majority of LNG
carriers are between 125,000 and 135,000 m3 in capacity. In the modern fleet of LNG
carriers, there is an interesting exception concerning ship size. This is the introduction
of several smaller ships of between 18,000 and 19,000 m3 having been built in 1994
and later to service the needs of importers of smaller volumes.
1.4 THE SHIP/SHORE INTERFACE AND JETTY STANDARDS
In comparison to most other ship types, gas carriers have a better safety record. However, casualty statistics involving gas carriers demonstrate that the risk of a serious accident is potentially greater when the ship is in port than when at sea. For this reason it is appropriate that attention should concentrate on the port facilities and the activities of ship and shore personnel involved in cargo operations.
1.4.1 Safe jetty designs
The ship/shore interface is a vital area for consideration in the safety of the liquefied gas trade. Considering jetty design (and the equipment which may be needed), safety in this area requires a good understanding of ship parameters before construction begins. In this context the following points are often addressed by terminal designers:—
• The berth’s safe position regarding other marine traffic
• The berth’s safe position in relation to adjacent industry
• Elimination of nearby ignition sources
• Safety distances between adjacent ships
• The range of acceptable ship sizes
• Ships’ parallel body length — for breasting dolphin positioning
• Suitable jetty fender designs
• Properly positioned shore mooring points of suitable strength
• Tension-monitoring equipment for mooring line loads
• Suitable water depths at the jetty
• Indicators for ship’s speed of approach to the jetty
• The use of hard arms and their safe operating envelopes
• Emergency shut-down systems — including interlinked ship/shore control
• Suitable plugs and sockets for the ship/shore link
• A powered emergency release coupling on the hard arm
• Vapour return facilities
• Nitrogen supply to the jetty
• Systems for gas-leak detection
• A safe position for ship/shore gangway
• Design to limit surge pressures in cargo pipelines
• Verbal communication systems
• The development of Jetty Information and Regulations
• Jetty life saving and fire-fighting equipment
• Systems for the warning of the onset of bad weather
• The development of Emergency Procedures
Further issues have to be considered in the port approach. These may include the suitability of Vessel Traffic Management Systems, and the sizing of fairways and turn- ing basins. However, these latter points fall outside of the scope of this publication.
1.4.2 Jetty operations
The ship/shore interface is the area where activities of personnel on the ship and shore overlap during cargo handling. Actions on one side of the interface will affect the other party and responsibility for safe operations does not stop at the cargo manifold for either ship or shore personnel. The responsibility for cargo handling operations is shared between the ship and the terminal and rests jointly with the shipmaster and
responsible terminal representative. The manner in which the responsibility is shared should, therefore, be agreed between them so as to ensure that all aspects of the operations are covered.
From an operational viewpoint it should be appreciated that at the ship/shore inter- face two differing cultures co-exist. To ensure safe operations, a proper understand- ing of the working practices of both ship and shore personnel is necessary. Equally, before and during operations, procedures of practical relevance have to be in place and jointly understood by ship and shore personnel. Most often this is best achieved by properly addressing the Ship/Shore Safety Check List (see Appendix 3) and this should be supplemented by a suitable terminal operating manual, containing Jetty Information and Regulations, which should be passed to the ship.
There is much variation in the design and operation of terminals and jetties and not all are dedicated solely to the handling of liquefied gases. Sometimes the combined nature of the products handled can complicate operations. Equally, however, variations in gas carrier and jetty construction can heighten the importance of safety issues at the interface, making them an important area requiring proper controls and good operational procedures.
LPG berths may have to handle ships of varying size and having a range of different cargo handling equipment. Jetties may be relatively new, and fitted with modern cargo facilities. Conversely, they may be relatively old using flexible hoses for cargo transfer. Of course, many jetties fall between these two extremes. At LPG berths, local design variation at the ship/shore connection may result in the need to use either hoses or all- metal hard arms. The hard arm may be hydraulically operated: it may be fitted with emergency release couplings and an emergency release system.
LNG terminals are an exception to the foregoing — they are primarily dedicated to this single product, although some LNG jetties also handle LPGs and condensates. In most cases such berths have been specially built for a particular export/import project. LNG jetties only use hard arms for cargo transfer. The hard arm is invariably hydraulically operated. Almost certainly it will be fitted with emergency release couplings and an emergency release system.
Liquefied gas cargo handling procedures can be complex and the cargo itself is potentially hazardous. For these reasons, the persons operating gas carriers and gas berths require a thorough understanding of ship and shore equipment and cargo properties. They need to have available good operating procedures so as to avoid accident and emergency plans should be in place in case an accident does occur.
Source : SIGTTO Personal Handbook.
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