Cargo tank rupture caused by increase of pressure - Emergency procedure for gas carriers
Cargo tank ruptures due to increased pressure are highly improbable with the existing tank protection systems. There are multiple devices/systems protecting the cargo tanks - it is almost impossible for all devices to fail at the same time. Additionally, cargo tanks may support much higher pressure than the actual settings of the relief valves. However in this scenario we have two different situations - whether the affected tank is loaded or empty: .
If the tank is empty the risk is less as no liquid cargo will be released. In that case, the hold space should be inerted at once.
If the tank contains cargo and it is liquid cargo that is released:
A minor leak may be controlled by via the hold space ejector
LNG tanks internal barrier
A major leak may be impossible to control. In that case the two main options available are either abandonment or the jettisoning of the cargo
In either case the first indication will be by the one or more of the detection systems - the gas detector, level switch or temperature sensor.
Major Leak
If a major leak occurs and it can not be confined then jettisoning should be considered. Inerting of hold spaces shall be completed as described for a minor leak.
Sound general alarm
Stop cargo operation. Activate ESDS
Disconnect loading arms. Activate PERC
Leave jetty
As safety measure - Inert 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 hold space vent in order to avoid overpressure in the hold space
Transfer cargo to other tanks in order to empty the tank. Considering stability and stress factors
Consider - jettisoning. Remember two cargo pumps are required in order to have proper pressure
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 where liquid will be transferred by the eductor back to the tank. Driving liquid to the eductor is supplied by spray pump. (for Moss type LNG Carriers)
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 such kind of leakage for 15 days without transferring cargo to another cargo tank.
Release pressure in order to avoid further damages
Isolate the rest of the hold spaces
Transfer cargo to other tanks if possible, normally loaded passage all tanks full
The majority of liquefied gases are clean, non-polluting, products and
create no danger to the marine environment. If however certain
liquefied gases spill on to the sea you should be aware that they may:
create large quantities of vapour sea water rapidly vapourises
the liquid gas- which may cause a fire or explosion or a health
hazard.
generate toxic vapours, which can drift, sometimes over a
considerable distance.
dissolve in seawater and cause local pollution
The Data Sheets will give information on pollution, if any
Pollution is most likely to occur during cargo or bunkering operations:
if the operation is not correctly monitored
if the cargo hose or loading arm connections are not properly
made
when disconnecting cargo lines that have not been drained.
if moorings are not checked and excessive strain is placed on the
cargo connections or the ship "breaks out" of the berth.
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