Minor or major leaks from LNG tanks & dealing with vapour clouds

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Minor or major leaks from LNG tanks & dealing with vapour clouds

An emergency can occur at any time and in any situation. Effective action is only possible if pre-planned and practical procedures have been developed and are frequently exercised. The Contingency Plan provides guidelines and instructions that assist in making an efficient response to emergency situations onboard ships.

Major Leak from an LNG Tank- Not Ignited

If a major leak occurs and it can not be confined then jettisoning should be considered. Inerting of hold spaces shall be done same way as in case of minor leak.

i) Sound general alarm

ii) Stop cargo operation. Activate ESD

iii) Disconnect loading arms. PERC

iv) Leave jetty

LNG bunkering

v) As safety measure - Inert the hold space where leakage (Cargo Tank rupture) has been detected until the O2% is reduced to 2%. Continue blowing inert gas to hold space in order to keep temperature as high as possible. Remember to open the vent for hold space in order to avoid overpressure in the hold space

vi) Transfer cargo to other tanks in order to empty the tank.

vii) Consider - stability and stress factors.

viii) Consider - jettisoning. Remember two cargo pumps are required in order to have proper pressure in the nozzle.

ix) Consider - external assistance

x) Prepare fire fighting equipment

xi) Consider - abandonment. Prepare life-rafts, lifeboats

Minor Leaks from an LNG Tank- Not Ignited

If a small leak occurs the vessel will be able to handle the leak through the drip pan. The liquid will flow to the drip pan to be transferred by the eductor (by means of a spray pump), back to the tank. Driving liquid to the eductor is supplied by spray pump. Data to remember: The tank designer advises that if a cargo tank cracks an average of 8 Liter/hour will be released. The Drip pan will be able to contain this kind of leakage for 15 days without transferring cargo to another cargo tank.

i) Release pressure in order to avoid further damages

ii) Isolate the rest of the hold spaces

iii) Transfer cargo to other tanks, if possible. Note that this is not normally possible on a normally loaded voyage as all tanks are full.

Vapour clouds

If there is no immediate ignition of an LNG spill, a vapour cloud may form. The vapour cloud is long, thin, cigar shaped and, under certain meteorological conditions, may travel a considerable distance before its concentration falls below the lower flammable limit. This concentrate is important, for the cloud ignite and burn, with the flame travelling back towards the originating pool.

The cold vapour is denser that air and thus, at least initially, hugs the surface. Weather conditions largely determine the cloud dilution rate, with a thermal inversion greatly lengthening the distance travelled before the cloud becomes non-flammable.

The major danger from an LNG vapour cloud occurs when it is ignited. The heat from such a fire is a major problem. A deflagration (simple burning) is probably fatal to those within the cloud and outside buildings but is not a major threat to those beyond the cloud, though there will be burns from thermal radiations.

When loaded in the cargo tanks, the pressure of the vapour phase is maintained as substantially constant, slightly above atmospheric pressure.

The external heat passing through the tank insulation generates convection currents within the bulk cargo, heated LNG rises to the surface and boils.

The heat necessary for the vaporization comes from the LNG and long as the vapour is continuously removed by maintaining the pressure as substantially constant, the LNG remains at the boiling temperature

If the vapour pressure is reduced by removing more vapour than generated, the LNG temperature will decrease. In order to make up the equilibrium pressure corresponding to its temperature, the vaporization of LNG is accelerated, resulting in an increase heat transfer from LNG to vapour.

Related Information:

Leaks from a Loading Arm due to Tidal or Current Effects

Compressed air system - Gas carrier immediate actions

LNG tank leaks and immediate action by gas carriers

Connection and disconnection of cargo hoses and hard arms

Risk of Overfilling of Cargo Tank during Loading

Cargo tank ruptures due to increased pressure - emergency procedure for gas carriers

Gas carrier Emergency Procedures for rescue

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