Fig:LNG tank rollover
- Superheating of lower layers results in the
equilibrium of densities of the two layers.
When this occurs the interface between the
layers breaks down resulting in rapid transfer
of heat and mass within the storage tank.
- The two layers mix rapidly and the lower
layer, which has been superheated, gives off
large amounts of vapour as it rises to the
surface of the tank.
- This phenomenon is “ROLLOVER”
- The large amounts of vapour generated by
this, can cause a dramatic vapour expansion
and rapid increase in tank pressures.
It is extremely rarely that LNG is loaded into the same tank from two different sources, and
therefore with differing densities. Should this be a specific requirement of the vessel trade, then
further advice must be obtained from the managing office, charterers and terminals to minimise all
One situation where stratification may arise is if a vessel departs from a discharge port with a
considerable quantity of heel retained in one tank for an intended long voyage, and subsequent
voyage orders change to give only a short voyage. This will result in a correspondingly higher
cargo heel in one tank at the loadport there is then the possiblity that the loading of a new grade
of differing density could give rise to stratification in the tank. The loading of new LNG into the
bottom of the tank, will allow the two differing densities to be well mixed during the loading
process, reducing the risk of stratification.
Action by the ship to monitor for rollover
The cargo conditions within a tank should be closely monitored at all times.
Cargo temperatures throughout the full height of the tank should be checked for any abnormality.
Individual tank boil-off rates should be closely monitored for any unusual data.
Tank pressures should be closely monitored throughout, noting that an unexpected reduction in
boil-off gas generation may indicate stratification taking place.
It is essential therefore that all recording instruments and alarm settings are active and checked
for accuracy on a regular basis.
Action by ship staff in the event of Rollover
Rollover can cause a very rapid increase in boil-off rates and
increase in tank pressures.
Ship’s staff should be prepared to shut in to a minimum level, the vapour valves on tanks with
normal pressures, so that as much boil-off gas from affected tank(s) can be used in the boilers or
main engines, depending upon the propulsion machinery. It may be necessary to increase to full
sea speed to create additional fuel gas requirements, to assist with the control of tank pressures.
NOTE: it is extremely important that the tank pressure in any tank with valves shut in, is closely
monitored to ensure that they also do not reach a level where venting might take place.
Consideration should be given to re-circulating the cargo within a tank. The natural ship movement
whilst at sea cannot always be relied upon to mix layers of LNG with differing densities.
Should pressures continue to rise, and venting take place, then the vessel should immediately alter
course to prevent vented gas being blown down towards the accommodation, air-con vents should
be put onto recirculation maintaining a positive pressure within the accommodation and engine
room spaces. Strict No-Smoking must be enforced, until the situation is under control. Reference
should be made to the vessel’s SOPEP, which contains Emergency Procedures relating to the
unexpected venting from cargo tanks.
Procedure for loading Liquefied Gas Cargoes
Procedure for Cargo Conditioning in Liquefied Gas Carriers
Cargo Transfer between Vessels (STS Operation)
Procedure for segregation of Liquefied Gas Cargoes
Procedure for Stripping Liquefied Gas Cargoes
Procedure for Changing Liquefied Gas Cargoes
Displacing Atmosphere with Inert Gas (Inerting)
Procedure for Water washing after Ammonia Cargoes
Preparation for Cargo Transfer
Procedure for discussion prior cargo transfer
- Displacing with Vapour of the Next Cargo (Purging)
- Ice Formation in gas carrier cargo system
- Cargo emergency shutdown requirement for liquefied gas carrier
- Preventive measures against spillage of low temperature cargo
- Causes of Brittle Fracture & contermeasures
External links :
International maritime organization
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