Liquefied Gas Carrier

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What is LNG ? Physical properties and composition of LNG

Natural gas is a mixture of hydrocarbons which, when liquefied, form a clear colourless and odourless liquid; this LNG is usually transported and stored at a temperature very close to its boiling point at atmospheric pressure (approximately – 160 degree C).

LNG is very cold liquid form of natural gas - the fuel that's burned in gas stoves, home heaters, and electric power plants. When it warms back up, LNG becomes natural gas again. You can't liquefy natural gas without cooling it. Many countries export and many others import LNG by ship; the United States does both.

LNG does not burn itself .LNG needs to be in vapor form and mixed with air to burn .Is combustible in the range of 5% to 15% volume concentrations in air .Combustible mixtures in confined space will burn explosively .

LNG is a cryogenic substance and physical contact or spillage constitute a personnel and equipment hazard . Natural gas presents an asphyxiation hazard. Its main component is methane. It gasifies violently when directly introduced into a cargo tank at ambient temperature, rapidly increases the internal pressure of the cargo tank and makes the atmosphere into a flammable condition.

In addition, the cargo tank is rapidly cooled, resulting tremendous thermal stress on cargo tank skins and cargo piping systems. To avoid such damages, the preparatory work for cargo loading after dry docking must be followed.

During dry dock all the compartments of an LNG carrier are kept gas free. After leaving the dry dock the vessel has to be prepared to load cargo, for that the following points to be considered with priority.

Liquefied natural gas (LNG) compresses to a small fraction of its original volume (approximately 1/600) under liquefaction. Liquefaction reduces the volume of natural gas making it much more economical to transport. With the amount of flammable material that LNG contains, it has the potential to be an extremely dangerous chemical, if handled improperly.
LNG tanker at sea
Fig: LNG tanker at sea

LNG is not as dense as petroleum, requiring between double and triple the space for the fuel tank. To alleviate the loss of space, some Norwegian designers locate the tanks under accommodation spaces, building protective coffers to hold the tanks. Others have placed tanks on deck where they can vent into the atmosphere should a spill occur. IMO guidelines do not prohibit placement of LNG tanks under accommodation, however, the USCG feels the decision requires careful analysis.

Composition of LNG
Fig: Composition of LNG

The actual composition of LNG will vary depending on its source and on the liquefaction process, but in all cases the major constituent will be methane with small percentages of the heavier hydrocarbons such as ethane, propane, butane and pentane. In addition, small quantities of nitrogen may be present.

However for custody transfer purposes when accurate calculation of the heating value and density is required the specific properties based on actual component analysis must be used. During a normal sea voyage, heat is transferred to the LNG cargo through the cargo tank insulation, causing vaporization of part of the cargo, i.e. boil-off.

The composition of the LNG is changed by this boil-off because the lighter components, having lower boiling points at atmospheric pressure, vaporize first. Therefore the discharged LNG has a lower percentage content of Nitrogen and Methane than the LNG as loaded, and a slightly higher percentage of Ethane, Propane and Butane, due to Methane and Nitrogen boiling off in preference of the heavier gases.

The flammability range of Methane in air (21% Oxygen) is approximately 5.3 to 14% (by volume). To reduce this range, the air is diluted with Nitrogen until the Oxygen content is reduced to 2% prior to loading after dry-dock. In theory, an explosion cannot occur if the O2 content of the mixture is below 13% regardless of the percentage of Methane, but for practical safety reasons, purging is continued until the O2 is below 2%.

The boil-off vapour from LNG is lighter than air at vapour temperature above –110 degree C or higher depending on LNG composition Variation of Density with Temperature, therefore when vapour is vented to atmosphere, the vapour will tend to rise above the vent outlet and will rapidly disperse. When cold vapour is mixed with ambient air the vapour air mixture will appear as a readily visible white cloud due to the condensation of the moisture in the air. It is normally safe to assume that the flammable range to vapour-air mixture does not extend significantly beyond the perimeter of the white cloud.

The auto-ignition temperature of Methane, i.e. the lowest temperature to which the gas needs to be heated to cause self-sustained combustion without ignition by spark or flame is 595 degree C.

Fig: Physical properties of LNG

What happens with if LNG spilled on water?

Below is our guideline for handling LNG cargo:

  1. LNG vessel cargo containment system

  2. There are two basic types of cargo containment systems which are generally referred to as incorporating either membrane or Moss Rosenberg technology. In both cases, the containment system is designed to serve two purposes: i. To contain LNG cargo at cryogenic temperatures (-160 degree C). ii. To insulate the cargo from the hull structure.....

  3. LNG spill risk during marine transportation and hazards associated

  4. The potential hazards of a large LNG spill over water includes asphyxiation, cryogenic burns, and cryogenic damage to the ship from the very cold LNG, dispersion, fires, and explosions. ....

  5. Preparation for loading LNG

  6. Loading LNG cargo after dry docking : LNG is a cryogenic substance and its main component is methane. It gasifies violently when directly introduced into a cargo tank at ambient temperature, rapidly increases the internal pressure of the cargo tank and makes the atmosphere into a flammable condition. .....

  7. Procedures for LNG cargo loading

  8. Before loading operations begin, the pre-operational ship/shore procedures must be thoroughly discussed and followed. Appropriate information exchange is required and the relevant parts of the ship/shore safety check list should be completed.....

  9. Procedures for LNG cargo discharging

  10. Liquid is pumped ashore by use of two submerged pumps installed at the bottom of each tank. In the process, the cargo tank pressure shows a decreasing tendency as the LNG level drops in the tank, resulting from the discharge of LNG. Conversely, shore tank pressure shows an increasing tendency with the receipt of LNG. .....

  11. How LNG transferred from shore to ships cargo tanks ?

  12. During loading, cargo is transferred from shore through the appropriate mid-ships or stern manifolds, and led into the cargo tanks via the filling lines, which usually terminate close to the tank bottoms.....

  13. LNG vessel cargo handling equipment

  14. Liquefied gas carriers fitted with cargo, booster, ballast and stripping pumps, lines, eductors and their associated instrumentation and controls should be in good order and evidence of regular testing should be kept. Instrumentation, valves and pipe work should be clearly marked to indicate their service and where applicable the compartment to which they relate. ....

  15. LNG vessel cargo piping system

  16. Liquid lines are comprised of butt welded, cryogenic stainless steel pipeline connecting each of the cargo tanks to the cargo manifolds by means of a common line. At each tank, there is a manifold which connects to the loading and discharge lines from the tank to allow for the loading and discharge of cargo......

  17. Gas measuring equipment test and calibration

  18. All vessels are supplied with portable gas measuring equipment, according to the specific requirements of the vessel owners. Personnel must fully understand the purpose and limitations of vapour detection equipment, whether fixed or portable. .....

  19. Precautions for LNG carrier during loaded passage

  20. During loaded voyage, LNG carriers always burn not only fuel oil but also vapors generated as mentioned above, so as to maintain cargo tank pressure. It may be noted that the principal operation of the Cargo Department on loaded voyage are the maintenance of cargo tank pressure and the control of cargo liquid temperature. .....

  21. Precautions for LNG carrier during ballast passage

  22. Depending upon ship design, it may be necessary to undertake loading / de-ballasting or discharging / ballasting operations simultaneously. If this is the case, consideration should be given to the stability of the ship, especially to free surface effect in tanks, correct use of cargo tank centerline bulkhead valves, and cargo and ballast distribution to ensure adequate stability. Care should also be taken to ensure that the weight distribution does not lead to excessive trim, list or stress in transverse and longitudinal directions.....

  23. Drydocking preparations for LNG vessel

  24. Preparatory operations for drydocking start with the disposal of all remaining liquid and ends with the creation of the tank atmosphere meeting all criteria a dockyard requires. On a voyage just prior to dry-docking, cargo should be discharged as much as possible to leave less residual cargo on board, and such liquid left over should be disposed of by gasification. ....

  25. Risk of sloshing effect on LNG cargo tanks

  26. Sloshing is a problem which effects membrane constructed tanks. Independent containment systems such as the spherical Moss design and the IHI prismatic designs are not subject to the same sloshing impacts. Partial loading at any tank filling level is inherent in the design of Moss design tanks, giving them distinct advantages over membrane containment systems, when handling spot trades and offshore loading/unloading. ....

  27. Liquefied gas carrier safety training

  28. The LNG fleet delivers more than 110 million metric tons annually to ports around the world. Accidental spillage of liquefied natural gases are rare with no significant cargo losses. . This safety record is attributable to continuously improving tanker technology, tanker safety equipment, comprehensive safety procedures, training, equipment maintenance, and effective administration oversight. ....

  29. Inert Gas Hazards and Precautions - Atmosphere Control For Gas Carrier

  30. When carrying a flammable cargo, the cargo system contains liquid and vapour. The atmosphere around the cargo tanks is normally inerted to prevent the formation of flammable mixtures. The IMO Codes use the term “environmental control” to describe this process. Some careful considerations are needed to ensure safety . ....

  31. Stability and cargo loading limitations - use of loading computer

  32. The master of every liquefied gas carrier should be supplied with a loading and stability information booklet. This booklet should contain details of typical service conditions, loading, unloading and ballasting operations, provisions for evaluating other conditions of loading and a summary of the ship’s survival capabilities.....

  33. How to avoid structural steels suffering brittle fracture at low temperatures ?

  34. 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 the reduction in temperature changes the material’s crystal structure. ....

  35. How to tackle LNG fire ? Available fire fighting agents & safety aspects

  36. Natural gas contains numerous component gases but by far the greater percentage is methane (CH4), which represents between 60 and 95 per cent of the total volume. This fact is important when considering the safety aspects for fire-fighters tackling an LNG fire. ....

External links :

The Center for Liquefied Natural Gas : ( CLNG ) - A trade association of LNG producers, shippers, terminal operators and developers, energy trade associations and natural gas consumers

Reference publications

Related Information:

  1. How to tackle fire on board LNG ship

  2. Fire fighting plan for LNG cargo

  3. Increased Cargo Capacity for LNG ships & Advantages of the dual fuel diesel electric propulsion

LNG operating instructions- various important terms related with cargo handling

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