Liquefied Gas Carrier

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Why LNG is stored in a unpressurized tank ? Is it safe ?

Liquefied natural gas is transported in double-hulled ships designed to prevent leakage or rupture in an accident. For most LNG carriers, the cargo is stored in either membrane containment systems made of special materials or in spherical tanks located within the ship's inner hull. For membrane containment systems, a complete secondary containment system surrounds the primary container. The insulation space between the two has sensing equipment able to detect even the smallest presence of methane (the main component of natural gas), possibly indicating a leak of LNG .

LNG is stored unpressurized at an extremely cold temperature (-260 degrees F). Should a tank ever fail and a leak result, fire is possible, but only if there is the right concentration of liquefied natural gas vapor in the air and a source of ignition. Since such a combination rarely exists, explosions are highly unlikely. LNG is not explosive, Although a large amount of energy is stored in LNG, it cannot be released rapidly enough to cause the overpressures associated with an explosion.LNG vapors (methane) mixed with air are not explosive in an unconfined environment.

LNG carrier moss tanks
Fig:LNG carrier moss tanks

LNG ships have emergency shutdown systems that can identify potential safety problems and shut down operations. This significantly limits the amount of liquefied natural gas that could be released. Fire and gas detection and fire fighting systems help address the risk of fire. Special operating procedures, training, and maintenance further contribute to safety.

LNG vessels also have equipment to make ship handling safer. This equipment includes sophisticated radar and positioning systems that enable the crew to monitor the ship's position, traffic, and identified external hazards. A global maritime distress system automatically transmits signals if an onboard emergency occurs that requires external assistance. In addition, some LNG ships use velocity meters to ensure safe speeds when berthing. When moored, automatic line monitoring helps keep ships secure. When connected to the onshore system, the instrument systems and the shore-ship LNG transfer system act as one, allowing emergency shutdowns of the entire system both from ship and from shore.

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.

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 done in the following sequence.

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.

  1. Preparation for loading LNG cargo

  2. Procedures for LNG cargo loading

  3. Procedures for LNG cargo discharging

  4. Liquefied gases - How to remove all cargo liquid from tanks

LNG spill risk during marine transportation and hazards associated

How to tackle fire on board LNG ship

Fire fighting plan for LNG cargo

External links :

  1. Oregon LNG Import Terminal Operations

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