Liquefied Gas Carrier

Home page||| LNG handling ||| LPG handling||| Other Gas products||| Fire & Safety||| Emergency response |||

Rollover effects onboard a liquefied gas carrier

A particular danger associated with cargo density is one known as rollover. Rollover is a spontaneous rapid mixing process which occurs in large tanks as a result of a density inversion, stratification develops when the liquid layer adjacent to a liquid surface becomes more dense than the layers beneath, due to boil-off of lighter fractions from the cargo. This obviously unstable situation relieves itself with a sudden mixing, which the name “rollover” aptly describes.

Liquid hydrocarbons are most prone to rollover, especially cryogenic liquids. LNG is the most likely by virtue of the impurities it contains, and the extreme conditions of temperature under which it is stored, close to the saturation temperatures at storage pressures.

There have been a significant number of rollover incidents in shore LNG tanks, however although instances on LNG tankers are relatively rare, it is important that ship’s crew know how to recognise and deal with potential rollover problems.

Rollover can then result in boil-off rates several times greater than normal, causing very rapid over-pressurisation of the tanks, with the resultant lifting of relief valves, and the venting to atmosphere of considerable quantities of vapours.

If the cargo is stored for any length of time and the boil-off is removed, evaporation can cause a slight increase in density and a reduction of temperature near the surface. The liquid at the top of the tank is therefore marginally heavier than the liquid in the lower levels. Once stratification has developed rollover can occur.

No external intervention such as vibration, stirring or introducing new liquid is required to initiate rollover. The response to a small temperature difference within the liquid (which will inevitable occur in the shipboard environment) is sufficient to provide the kinetic energy to start rollover, and release the gravitational driving forces which will invert the tank contents. The inversion will be accompanied by violent evolution of large quantities of vapour and a very real risk of tank over-pressure.

Rollover has been experienced ashore, and may happen on a ship that has been anchored for some time. If such circumstances are foreseen the tank contents should be circulated daily by the cargo pumps to prevent rollover occurring.

Rollover can occur if similar or compatible cargoes of different densities are put in the same tank. For example, if tank pressure is maintained by boil-off reliquefaction, the condensate return may be of slightly different temperature (and hence density) from the bulk liquid, and likewise if condensate from two or more cargoes is returned to one tank. In such circumstances, rollover may be prevented by returning condensate that is less dense thank the bulk liquid to the top of the tank, and condensate that is denser to the bottom of the tank.

Rollover may also occur when two part cargoes are loaded into the same tank (e.g. propane and butane). In this case there will be a large boil-off (up to 3% of the total liquid volume) due to the temperature difference between the two. For this reason, the practice is considered unsafe unless a thorough thermodynamic analysis of the process is undertaken, and the loading takes place under strictly controlled conditions.

Rollover in a ship on passage is most unlikely. Essentially, stratification and the subsequent rollover process are confined to shore LNG storage. However, if the use of LNG carriers for floating storage were to be introduced, personnel manning such vessels would need to be as aware of the problem and as vigilant to avoid rollover as their counterparts managing shore based storage.

Fig:Natural Convection within an LNG tank

Natural Convection within an LNG tank

Density Stratified Liquid Layers
Fig:Density Stratified Liquid Layers

Density Stratified Liquid Layers

LNG tank rollover
Fig:LNG tank rollover

Rollover facts

  • 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 potential risks.

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.

Related Information:

  1. Displacing with Vapour of the Next Cargo (Purging)

  2. Ice Formation in gas carrier cargo system

  3. Cargo emergency shutdown requirement for liquefied gas carrier

  4. Preventive measures against spillage of low temperature cargo

  5. Causes of Brittle Fracture & contermeasures

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

External links :

  1. International maritime organization

// Home page/// LNG handling /// LPG handling/// Sea transport /// Gas products///

Cargo work ///Fire precautions ///Health hazards ///Safety Precautions

///Emergency response ///

Copyright & copy; Liquefied Gas All rights reserved.

The content published in this website are for general reference only. We have endeavoured to make the information as accurate as possible but cannot take responsibility for any errors. For latest information please visit . Any suggestions, please Contact us !

///Links &Resources // Terms of use/// Privacy policy///Home page///