Transportation of Liquefied Gases by Sea
For economical marine transportation, gas is carried in a liquefied state. As a liquid, the volume to weight ratio at atmospheric pressure is in the range of 650 times less than in the gaseous state. That means we can carry 650 times more cargo in the liquid state as compared to a carriage in the gaseous state.
The temperature at which a gas condenses is a function of its pressure. The combination of pressurising and cooling is, therefore, fundamental to gas carrier design. Some ships carry gases liquefied under pressure & others under refrigeration. The relative densities of gases are low and vary between 0.42 (methane) and 0.97 (VCM). The cargo carrying capability is, therefore, more related to the volume capacity of the ship than deadweight capacity, and the cargo capacity is usually quoted in cubic metres cargo tank volume.
The most significant cargoes in terms of tonnages moved are methane/LNG, LPG (butane, propane and mixtures of these), and ammonia. Other cargoes of commercial significance are butadiene, butylene, ethylene, propylene, and vinyl chloride. Apart from ethylene and methane/LNG, all these gases can exist as liquids at normal ambient temperatures. They may, therefore, be transported in pressurised cargo containment systems at any temperature up to the highest expected ambient temperature.
The critical temperatures for ethylene and methane/LNG are below normal ambient temperatures. Above the critical temperature, the gas cannot be transformed into a liquid at any pressure and must, therefore, be refrigerated for shipboard carriage. Carriage of ethylene, ethane and methane/LNG requires semi-pressurised or fully refrigerated cargo containment. These considerations lead to the following options for carriage conditions:
Materials exposed to liquefied gas cargoes should be resistant to any corrosive action of the gases. For this reason, copper alloys (e.g. brass) have to be excluded from the cargo systems of ships intended for the carriage of ammonia. Details of materials of construction which should not be used for certain products are given in Chapter 17 of the IGC Code.
Some Basic Definitions
The International Maritime Organization (IMO), for the purposes of its Gas Carrier Codes, 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
2. Boiling Point
The temperature at which the vapour pressure of a liquid is equal to the pressure on its surface (the boiling point varies with pressure)
3. Cargo Area
That part of the ship which contains the cargo containment system, cargo pumps and compressor rooms, and includes the deck area above the cargo containment system. Where fitted, cofferdams, ballast tanks and void spaces at the after the end of the aftermost hold space or the forward end of the forwardmost hold space are excluded from the cargo area.
4. Cargo Containment Systems
The arrangement for containment of cargo including, where fitted, primary and secondary barriers, associated insulations, interbarrier spaces and the structure required for the support of these elements.
5. Gas-Dangerous Space or Zone
A space or zone within a ship’s cargo area which is designated as likely to contain flammable vapour and which is not equipped with approved arrangements to ensure that its atmosphere is maintained in a safe condition at all times.
6. Gas-Safe Space
A space on a ship not designated as a gas-dangerous space.
7. Hold Space
The space enclosed by the ship’s structure in which a cargo containment system is situated.
8. Interbarrier Space
The space between a primary and a secondary barrier of a cargo containment system, whether or not completely or partially occupied by insulation or other material.
This is the abbreviation for the Maximum Allowable Relief Valve Setting on a ship’s cargo tank — as stated on the ship’s Certificate of Fitness
10. Primary Barrier
This is the inner surface designed to contain the cargo when the cargo containment system includes a secondary barrier.
11. Secondary Barrier
The liquid-resisting outer element of a cargo containment system designed to provide temporary containment of a leakage of liquid cargo through the primary barrier and to prevent the lowering of the temperature of the ship’s structure to an unsafe level
12 Tank dome
It is not permitted for a cargo pump room to be placed below the upper deck, nor may cargo pipelines are run beneath deck level; therefore, deep well or submersible pumps must be used for cargo discharge. Pipelines to cargo tanks must be taken through a cargo tank dome which penetrates the deck.
1. Personnel Hazards
- Broadly, the personnel hazards of liquefied gases or their vapours may be five-fold. :
- Toxicity (poisoning)
- Asphyxia (suffocation)
- Low temperature (frostbite)
- Chemical burns
2. Other Hazards – Reactivity
A liquefied gas cargo may react in a number of ways: with water to form hydrates, with itself, with air, with another cargo or with other materials.
2.1 Reaction with Water- Hydrate Formation
Some hydrocarbon cargoes will combine with water under certain conditions to produce a substance known as a hydrate resembling crushed ice or slush. The water for hydrate formation can come from purge vapours with an incorrect dew point, water in the cargo system or water dissolved in the cargo. Care should be taken to ensure that the dew point of any Purge vapour or inert gas used is suitable for the cargo concerned, and that water is excluded from the cargo system.
Hydrates can cause pumps to seize and equipment to malfunction. Care should, therefore, be taken to prevent hydrate formation. Certain cargoes, notably LPGs, may contain traces of water when loaded. It may be permissible in such cases to prevent hydrate formation by adding small quantities of a suitable anti-freeze (e.g. methanol, ethanol) at strategic points in the system. It is emphasised that nothing whatsoever should be added to any cargo without the shipper’s permission. For LPG mixtures a small dose of anti-freeze may be permissible, but for chemical cargoes such as ethylene, the addition of even one litre per two hundred tonnes could make the cargo commercially valueless.
In the case of inhibited cargoes, the anti-freeze could adversely affect the inhibitor. If the use of anti-freeze is permitted it should be introduced at places where expansion occurs because the resultant lowering of temperature and pressure promotes hydrate formation.
Anti-freeze additives are often flammable and toxic and care should be taken in their storage and use.
Some cargoes may react with themselves. The most common form of self-reaction is polymerisation which may be initiated by the presence of small quantities of other cargoes or by certain metals. Polymerisation normally produces heat which may accelerate the reaction.
The IMO Codes require cargoes which may self-react either to be carried under an inert gas blanket or to be inhibited before shipment. In the later case a certificate must be given to the ship, stating:
- the quantity and name of the inhibitor added;
- the date it was added and how long it is expected to remain effective
- the action to be taken should the voyage exceed the effective lifetime of the inhibitor;
- any temperature limitations affecting the inhibitor.
Normally there should be no need to add any inhibitor to the cargo during the voyage. If it should become necessary, however, any such additions should be made in accordance with the shipper’s instructions.
Many inhibitors are much more soluble in water than in the cargo, so to avoid a reduction in inhibitor concentration; care should be taken to exclude water from the system. Similarly, the inhibitor may be very soluble in anti-freeze additives if these form a separate phase and the shipper’s instructions on the use of anti-freeze should be observed. If the ship is anchored in still conditions the cargo should be circulated daily to ensure a uniform concentration of inhibitor.
Certain cargoes which can self-react (e.g ethylene oxide, propylene oxide), but which cannot be inhibited, have to be carried under inert gas. Care should be taken to ensure that a positive pressure is maintained in the inerted atmosphere at all times and that the oxygen concentration never exceeds 0.2 % by volume.
2.3 Reaction with Air
Some cargoes can react with air to form unstable oxygen compounds which could cause an explosion. The IMO Codes require these cargoes to be either inhibited or carried under nitrogen or other inert gas. Care should be taken to observe the shipper’s instructions.
2.4 Reaction with Other Cargoes
Certain cargoes can react dangerously with one another. They should be prevented from mixing by using separate piping and vent systems and separate refrigeration equipment for each cargo. Care should be taken to ensure that this positive segregation is maintained. To establish whether or not two cargoes will react dangerously, the data sheet for each cargo should be consulted.
2.5 Reaction with Other Materials
The data sheets list materials which should not be allowed to come into contact with the cargo. The materials used in the cargo systems must be compatible with the cargoes to be carried and care should be taken to ensure that no incompatible materials are used or introduced during maintenance (e.g. gaskets).
A reaction can occur between cargo and purge vapours of poor quality: for instance, an inert gas with high CO2 content can cause carbonate formation with ammonia. The reaction can also occur between compressor lubricating oils and some cargoes, resulting in blockage and damage.
Some cargoes and inhibitors may be corrosive. The IMO Codes require materials used in the cargo system to be resistant to corrosion by the cargo. Care should, therefore, be taken to ensure that unsuitable materials are not introduced into the cargo system.
Corrosive liquids can also attack human tissue and care should be taken to avoid contact: reference should be made to the appropriate data sheets. Instructions about the use of protective clothing should be observed.
4. Low-Temperature Effects
As liquefied gas cargoes are often shipped at low temperatures it is important that temperature sensing equipment is well maintained and accurately calibrated.
5. Hazards associated with low temperatures include:
5.1 Brittle Fracture
Most metals and alloys become stronger but less ductile at low temperatures (i.e. the tensile and yield strengths increase but the material becomes brittle and the impact resistance decreases) because of the reduction in temperature changes the material’s crystal structure.
Normal shipbuilding steels rapidly lose their ductility and impact strength below 0°C. For this reason, care should be taken to prevent cold cargo from coming into contact with such steels, as the resultant rapid cooling would make the metal brittle and would cause stress due to contraction. In this condition, the metal would be liable to crack. The phenomenon occurs suddenly and is called ‘brittle fracture‘.
However, the ductility and impact resistance of materials such as aluminium, austenitic and special alloy steels and nickel improve at low temperatures and these metals are used where direct contact with cargoes at temperatures below -55°C is involved.
Care should be taken to prevent spillage of low-temperature cargo because of the hazard to personnel and the danger of brittle fracture. If spillage does occur/ the source should first be isolated and the spilt liquid then dispersed.
If there is a danger of brittle fracture, a water hose may be used both to vaporise the liquid and to keep the steel warm.
If the spillage is contained in a drip tray the contents should be covered or protected to prevent accidental contact and allowed to evaporate. Liquefied gases quickly reach equilibrium and visible boiling ceases this quiescent liquid could be mistaken for water and carelessness could be dangerous.
Suitable drip trays are arranged beneath manifold connections to control any spillage when transferring cargo or draining lines and connections. Care should be taken to ensure that unused manifold connections are isolated and that if blanks are to be fitted the flange surface is clean and free from frost. Accidents have occurred because cargo escaped past incorrectly fitted blanks.
Liquefied gas spilt onto the sea will generate large quantities of vapour by the heating effect of the water. This vapour may create a fire or health hazard, or both. Great care should be taken to avoid such spillage, especially when disconnecting cargo hoses.
5.3 Cool down
Cargo systems are designed to withstand a certain service temperature; if this is below ambient temperature the system has to be cooled down to the temperature of the cargo before cargo transfer.
For LNG and ethylene, the stress and thermal shock caused by an over-rapid cool down of the system could cause a brittle fracture. Cool down operations should be carried out carefully in accordance with instructions.
5.4 Ice Formation
Low cargo temperatures can freeze water in the system leading to blockage of, and damage to pumps, valves, sensor lines, spray lines etc. Ice can be formed from moisture in the system, purge vapour with incorrect dew point, or water in the cargo.
The effects of ice formation are similar to those of hydrates, and anti-freeze can be used to prevent them.
Rollover is a spontaneous rapid mixing process which occurs in large tanks as a result of a density inversion. Stratification develops when the liquid layer adjacent to a liquid surface becomes denser than the layers beneath, due to boil-off of lighter fractions from the cargo.
This obviously unstable situation relieves itself with a sudden mixing, which the name ‘rollover’ aptly describes.
Liquid hydrocarbons are most prone to rollover, especially cryogenic liquids. LNG is the most likely by virtue of the impurities it contains, and the extreme conditions of temperature under which it is stored, close to the saturation temperatures at storage pressures.
If the cargo is stored for any length of time and the boil-off is removed, evaporation can cause a slight increase in density and a reduction of temperature near the surface. The liquid at the top of the tank is therefore marginally heavier than the liquid in the lower levels. Once stratification has developed rollover can occur.
The inversion will be accompanied by the violent evolution of large quantities of vapour and a very real risk of tank over-pressure.
Rollover has been experienced ashore and may happen on a ship that has been anchored for some time. If such circumstances are foreseen the tank contents should be circulated daily by the cargo pumps to prevent rollover occurring.
Rollover can also occur if similar or compatible cargoes of different densities are put in the same tank.