Fig:LNG carrier underway
Under normal operational conditions primary and secondary barrier spaces are continually swept
with nitrogen. A small leakage of methane vapour through the membrane may not be readily
obvious. However, indications are likely to be:
Leak from an LNG Tank – Ignited (for LNG Carriers)
- A sudden rise in the percentage of methane vapour in one primary insulation space. Some
porosity in the primary barrier weld will allow the presence of methane vapour in the
primary insulation space. The amount of this vapour should be kept to a minimum by
- If a porosity fracture occurs in the primary membrane barrier below the level of the liquid in
the tank, the vapour concentration will increase slowly and steadily.
- If the fracture is above the liquid level, the concentration will exhibit a fluctuating increase.
- An increase in pressure due to vapour leakage will be less obvious than an increase due to
liquid leakage. This is because the volume of vapour passing through a fracture is small
compared with the volume of liquid, which subsequently vaporises, passing through the
same fracture. In both cases, volumes are likely to be small in comparison with the volume
of the inter barrier space.
- A fracture above the liquid level in a cargo tank will allow direct flow of vapour into the
primary insulation space. This flow will vary according to the pressure in the tank.
- A porosity fracture in the primary membrane below the liquid level in a cargo tank,
resulting in a small amount of liquid vaporising as it passes through the fracture, will cause
a small increase in pressure. This increase is dependent upon the height of liquid above the
fracture and the pressure in the tank.
- No temperature change will be obvious, unless the fracture is in the immediate vicinity of
the sensors below the cargo tank.
- Leakage of methane vapour into the primary insulation space presents no immediate
danger to the tank or vessel. As much as possible information concerning the fracture and
leak should be obtained and recorded.
Ascertain whether the risk is increasing, as follows:
- After the leak is detected, and without changing the flow of nitrogen to the primary
insulation space, record the gas concentration and primary space temperature every hour
for eight hours.
- Then, if necessary, adjust the flow of nitrogen to maintain the gas concentration below
30% (vol) and record the gas concentration and primary insulation space temperature
every four hours.
- In conjunction with the above, record all pressure changes occurring in the cargo tank and
primary insulation space.
Where there has been an LNG vapour leakage to a primary insulation space, the nitrogen supply
controller should be set to the appropriate pressure setting and the space purged.
An Ignited leak from LNG tank will result in fire or explosion.
The most effective fire extinguishing agent for fires involving gas is dry powder. Following actions are recommended :
1) Raise the Fire Alarm (if it has not been detected by fire detecting system). Inform bridge, CCR.
2) Muster at fire stations. Check for missing or injured crew/personnel
3) Advise terminal and sound the pre-defined signal
4) Stop all cargo operations. Activate ESDS
5) Stop dual firing. FO only, isolate engine room from cargo side
6) Activate spray system
7) Consider - reduce speed, change course, head to deeper waters away from coast in order to avoid damage to third parties
8) Leave berth as soon as possible. Activate PERC system to release loading arms
9) Attend any injured people
10) Close the relevant isolation valves to prevent the spread of fire to the other cargo tanks and cargo pipes
11) Smoke divers should be ready and prepared as soon as possible
12) Chief Officer shall advise the Bridge (Master) regarding situation and possibilities of fire developing
13) Squad No.1 shall proceed to fire / explosion location in order to recover any missing crew/personnel
14) Squad No.2 shall cool down surroundings of fire location by means of fire hoses
15) Squad No.1 shall proceed to the location of the fire in order to tackle it. Technical Squad will be back-up team
16) Consider – whether any dangerous material is involved in the fire; consider the dangers of toxic fumes, flammable vapours, etc.
17) Shut down all fans and fire dampers/flaps, and water tight doors
18) Use water fire hoses for radiation protection in the approach to shut-off valves or to leaking jet
19) If fire cargo is involved in the fire do not apply water directly to the fire since it will provide a heat source for more rapid vaporisation of the liquid and therefore increase the fire
20) Consider - is external assistance needed
21) Consider - if we have to evacuate the area and activate fixed CO2 fire extinguishing system
22) If CO2 decided to be used. Raise CO2 alarm
23) Make sure nobody is in the location where CO2 will be released and all dampers, flaps and doors are closed and fans stopped
24) Chief Engineer will release the fixed CO2 system with authorisation of the Master
25) Consider - abandon vessel
26) Inform Owners, Insurance/ P&I
27) Consider – whether there is any possibility of oil spill or leakage. Try to minimise, confine.
28) Consider - running inert gas into void spaces
LNG liquid leakage to primary barrier (MEMBRANE)
A major failure in the primary membrane, allowing liquid into the primary insulation space, will be
indicated as follows:
- A rapid increase in the methane content of the affected space.
- A rise in pressure in the primary insulation space nitrogen header, accompanied by
continuous increased venting to atmosphere.
- Low temperature alarms at all temperature sensors in the insulation below the damaged
- A general lowering of inner hull steel temperatures.
If a major failure of the membrane occurs, liquid from the tank will flow into the primary insulation
space until the levels in both compartments are equal. When the contents of the cargo tank are
discharged, unless the LNG in the primary insulation space can drain sufficiently quickly to the
cargo tank, a differential liquid head will build up, tending to collapse the membrane of the tank.
Before discharging a tank with major failure in the primary membrane, it is essential that the
primary membrane is punched so that liquid can freely flow back into the tank from the primary
insulation space. In this way, no hydrostatic head occurs in the primary insulation space which
could cause damage to the primary membrane support.
NOTE – UNDER NO CIRCUMSTANCES IS THE VESSEL TO OPERATE THE PUNCHING
DEVICE WITHOUT PRIOR AUTHORISATION. AS THE USE OF THE PUNCHING DEVICE
WILL RESULT IN EXTENSIVE REPAIRS, THE DECISION TO USE THE PUNCHING DEVICE
WILL BE GIVEN AFTER CONSULTATION BETWEEN THE VESSEL, V.SHIPS AND THE
The punching device is ship specific but often consists of a 30kg messenger which is dropped down
the float gauge tube so as to punch a hole in the primary membrane at the base. The base of the
float gauge tube is fitted with a split perforated base to allow the messenger to penetrate through
to the membrane. The membrane is fitted with a thin diaphragm and the plywood insulation boxes
are thinner than normal to allow the messenger to penetrate fully as per Barrier Punching
Procedure. The tank can then be discharged safely.
The operating procedure for draining the inter barrier space is as follows:
Water leakage to barrier (MEMBRANE)
- Operate the barrier punch system. It should be noted that during the discharge and until
repairs have been completed, the primary insulation space of the damaged tank is to
remain isolated from all other spaces and allowed to vent freely to atmosphere.
- Isolate the primary insulation space nitrogen supply to affected tank and allow venting to
atmosphere via the vent mast.
- Discharge to another tank using the spray pump and spray main. A cargo pump could be
used to discharge affected tank, but at a very slow rate.
- Start the affected tank spray pump in sequence, once the pump is running, slowly open the
spray pump discharge valve into the spray main.
Ballast water leakage from the wing tanks to the insulation spaces can occur through fractures in
the inner hull plating. If the leakage remains undetected and water accumulates in these spaces,
ice accumulation can occur and cause deformation and possible rupture of the insulation. The
resultant cold conduction paths forming in the insulation will cause cold spots to form on the inner
The pressure differential caused by the head of water building up in the insulation space may be
sufficient to deform or even collapse the membrane into the cargo tank. To reduce the risk of
damage from leakage, each cargo insulation space is provided with water detection units. A bilge
piping system is used for the removal of any water.
Bilge wells are provided in each of the transverse cofferdam spaces and are fitted with
independent level alarms. The tank secondary insulation space is connected to the bilge wells by
means of a drain pipe. The bilge wells are connected to a draining pipe system with a dedicated
bilge pump, of sufficient capacity, situated in each of the cofferdams for discharging the water to
deck level and then overboard. If ballast water is suspected of having leaked into a secondary
insulation space, appropriate arrangements must be made, such as the following:
1. Pump out the ballast water from adjacent wing tanks with consideration to vessel’s stability
and stress condition.
2. Ventilate the required cofferdam and carry out normal enclosed space entry.
3. Connect a flexible hose to the pump outlet valve in order to drain the water discharge
overboard via a scupper pipe.
4. Open the pump outlet valve on the deck.
5. Start the pump and continue pumping until the maximum amount of water has been
6. Carry out an inner hull inspection to determine the cause of the leak, with particular
reference to a safe atmosphere in the ballast tank space.
7. After the maximum possible amount of water has been discharged from the insulation
space, appreciable moisture will remain in the insulation and over the bottom area.
Increasing the flow of nitrogen through the space can assist in drying out of the insulation.
LNG liquid leakage to drip pan (MOSS)
A serious failure of the tank structure, allowing liquid into the insulation space or hold space, will
be indicated as follows:
- A rapid increase in the methane content of the affected space.
- Low temperature alarms at the temperature sensors in the surrounding areas.
- A general lowering of inner hull steel temperatures.
- Possible liquid alarms in the surrounding areas.
- Possible gas alarms in the surrounding areas.
In case of leakage from the northern hemisphere, it will drain by gravity and collect in the drain
channel at the wedge space, namely, the skirt to tank joint. From this drain channel, it passes the
wedge space via numerous drain holes cut in the skirt and gravitates to the southern hemisphere.
The leakage will eventually collect in the drain tube which contains the rupture disc and, from
there, after the rupture of the disc, to the drip pan.
Any leakage from the southern hemisphere will drain directly by gravity between the tank and the
insulation to the drain tube.
If leakage of cargo into the hold space occurs, the oxygen content of the space must be reduced to
below the explosive limit by the use of inert gas or nitrogen. Inert gas is introduced via the hold
space aeration header to the hold space bottom and is exhausted to the atmosphere from the top
of the hold space.
Water leakage to cargo hold space (MOSS)
Ballast water leakage from the wing tanks to the hold spaces can occur through fractures in the
inner hull plating. If the leakage remains undetected and water accumulates in these spaces, ice
may be formed. Ice accumulation can cause deformation and possible rupture of the tank
insulation. The resultant cold conduction paths forming in the insulation may cause cold spots to
To reduce the risk of damage from leakage, detector units are located in each cargo tank hold
The leakage protection system also includes a method of collecting and accumulating small leaks of
liquid cargo as well as water. The water is collected in a well at the after end of the hold space
under the cargo tank. The water leakage collects in this sump where monitoring equipment in the
form of liquid and temperature indicators is installed. In addition, gas detectors are situated in
various locations within the hold space.
Any liquid collecting in the sump will raise a liquid alarm and, by observing temperature and gas
indicators, it is possible to determine whether the liquid is cargo due to tank leakage or water due
to leakage from the ballast tanks.
A bilge ejector is installed in the sump to empty any water leakage from the hold space.
Leakgae of liquefied gas into hold spaces
The Master must judge whether liquefied gas has leaked into a hold space, paying attention to a
measured value by a gas detecting device, changes in hold pressure and temperature, and take
the following response actions in consultation with the Company.
The hold space where a gas leak is detected, must be isolated from other holds and inerted
immediately. The inert gas generator (IGG) must be continuously operated so as to lower the
Oxygen concentration below 8%.
Handling with cargo eductor
When it is considered transfer of the leaked liquefied gas is necessary, the leaked liquefied gas
must be transferred to other cargo tanks by a cargo eductor in the following procedure.
Shifting to another cargo tank
- Establish which tank will receive the spillage by checking the liquid levels and pressures of
- Align the eductor driving line and delivery line as described in the "Cargo Operating
Manual" of the vessel and purge the cargo space.
- Start the cargo pump or the spray pump and supply liquefied gas to the eductor driving line
to entrain the leaked liquefied gas to shift to another tank.
The liquefied gas contained in a cargo tank from which liquefied gas has leaked, must be shifted to
another cargo tank with sufficient ullage by use of a cargo pump.
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
generate toxic vapours, which can drift, sometimes over a
- 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
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.
if cargo equipment is not properly maintained
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 Defining various gas carrier types
Fuel flexibility of LNG ships
LNG ship spillage risk
Initial Cool Down of cargo tanks
Leaks on the Cargo System, Continuous Flow - how to prevent
LNG tank leaks and immediate action by gas carriers
Leaks from a Loading Arm due to Tidal or Current Effects
Minor or major leaks from LNG tanks
Procedures for LNG cargo loading
Procedures for LNG cargo discharging
- Leaks from a Loading Arm due to Tidal or Current Effects
- Low temperature effects of Liquefied gases
- Minor or major leaks from LNG tanks
- Connection and disconnection of cargo hoses and hard arms
External links :
International maritime organization
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