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Liquefied natural gases (LNG) -marine transport & accidents in LNG tankers

Liquefied natural gas (LNG) compresses to a small fraction of its original volume (approximately 1/600) under liquefaction. With the amount of flammable material that LNG contains, it has the potential to be an extremely dangerous chemical, if handled improperly. The liquefaction of natural gas raised the possibility of its transportation to many destinations.

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.

The most severe accident that may realistically occur to a loaded LNG tanker is the breach of one or more storage tanks, with consequent discharge of liquefied natural gas outboard. No accidents leading to loss of cargo have occurred over the history of maritime liquefied natural gas transportation. This safety record is at least partially due to the double-hulled construction of LNG tankers and the separation between the LNG cargo tank and the inner hull, which effectively makes the cargo tank's wall a third safety barrier to outside penetrations.

Rapid phase transitions are physical explosions caused by rapid vaporization of liquefied natural gas that do not involve combustion or burning. When liquefied natural gas flows on water, it forms a thin vapor film that separates it from the water. In locations of vigorous mixing, this film can be breached and LNG can come into direct contact with water. Under those conditions the LNG can undergo rapid evaporation, causing a rapid phase transition. In past spill experiments, rapid phase transitions have been observed at the first point of mixing with water and at the leading edge of a spill.

Mixing is known to be the most vigorous at these two locations. Rapid phase transitions are much less energetic than combustion explosions. Unconfined rapid phase transitions are generally not considered hazardous; however, these can cause structural damage if they were to occur in a confined space.

Liquefied natural gas (LNG) is natural gas in a liquid state at a low temperature (ab.-162 deg C). In this state the LNG can be easily transported on LNG-carriers. LNG can also be used as a fuel so that ship conforms to new regulations on sulfur emissions.

LNG can be stored in different types of tanks allowed by International Maritime Organization (IMO) code, The International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk code (IGC). IMO type B spherical tanks and membrane tanks are usually used for cargo transferring, although other types of tanks are also available on the market. For ships using LNG as a fuel, the most prominent solution is IMO type C tank.

Due to its properties LNG requires special handling on every stage of LNG carrying. The tanks must be carefully supervised during dry-docking, prepared before loading, controlled for boil-off during loaded passage and ballast passage. Before the dry-docking the tanks must undergo the reverse procedure of after docking. All this is necessary to ensure the high quality of cargo. Especially the boil-off is a problem to the most of LNG carriers. The boil-off is the vapour created in tanks due to thermal losses of thermal insulation.

In IMO type C tanks the pressure is allowed to increase with no losses of cargo. In other type of tanks the pressure remains constant and the boiloff can be either used as a fuel or re-liquefied and returned to tanks. The consumption of LNG on ship can be adjusted according to ship's operator's demand from minimum boil-off to 100% LNG propulsion. The re-liquefaction can be implemented by condensing compressed LNG vapours with seawater or by cooling the LNG tanks. DNV mentions two re-liquefaction systems based on Joule- Thomson effect and nitrogen refrigerant cycle. The re-liquefaction is beneficial because it allows to deliver whole the cargo instead of burning a part of it.

The properties of LNG hide potential dangers for the crew, ship and the environment and thus requires special handling. Due to its cold temperature the LNG can cause cryogenic burns and brittle fracture to ship's structure. It also can cause asphyxiation in big concentrations. LNG consist at about 90% from methane, which is a greenhouse-effect gas. If spilled on water it explodes due it rapid expansion and in concentration between 5% and 15% it is inflammable. This means that safe handling of LNG requires measures such as good ventilation, double hull construction, minimizing the potential leakages and leakage detection systems. Till now however, these measures have been proved sufficient enough to avoid big accidents on LNG carriers.

Related Information:

  1. How to tackle fire on board LNG ship

  2. Cargo Machinery Room Precautions

  3. Fire fighting plan for LNG cargo

  4. LNG spill risk during marine transportation

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