Fig:Liquefied natural gas ship on sea passage
Free of inherent intact stability problems
Vessels which have large width tanks will be subject to reductions of intact stability due to free surface. Although such vessels may meet IMO intact stability criteria when in fully loaded or ballasted conditions, they may be unstable when multiple tanks are slack during cargo or ballast transfer operations, or in intermediate states of loading. Trim and stability manuals generally deal only with arrival and departure conditions and operators are not made aware that stability problems may exist at intermediate stages during cargo transfers.
(1) If a vessel has either large width cargo tanks, U section ballast tanks, or double bottom tanks without watertight centerline bulkheads, attempt to ascertain if the vessel meets IMO intact stability criteria by requesting the chief officer to demonstrate, using the loading instrument, the intact stability of the worst case condition (all tanks slack and maximum free surface).
(2) If there is no suitable loading instrument and adequate instructions are not available, the key question should be answered ‘No’, unless there is satisfactory proof that the vessel is free of inherent stability problems.
(3) If cargo tanks are fitted with centre line bulkhead valves, these should normally be kept closed and only used for leveling. No more than 50% of the valves should be open at any one time.
This instrument is provided to supplement the stability booklet for the vessel. It allows the Officer
responsible, to carry out the various complex calculations required to ensure that the ship is not
overstressed or damaged during the carriage of the nominated cargoes. It will also permit the
assessment of damage stability. The Master and Chief Officer will make themselves aware of the
worst case damage stability condition within the stability booklet.
It must be remembered that a loading computer, as with navigation aids, is only an aid to the
It relies on human input of data, and more importantly the human interpretation of the output
data. If the input data is incorrect, the output data will also be incorrect. Used correctly it will
ensure the safe operation of the ship for all conditions of loading, discharging, ballasting and at all
stages of the voyage.
It is a requirement that where such equipment is provided to a ship, test conditions must also be
supplied for use in verifying the accuracy of the equipment.The test
conditions must be run and records of results maintained as soon as possible after a change of
Chief Officer and at least every three months and in any case prior to the vessel proceeding to
drydock. The frequency and records of such tests are to be recorded in the vessel’s planned
maintenance system. Where the running of these reveal significant errors the Company is to be
advised immediately with a request for attention.
Where online gauging of tank contents is not fitted the loading computer must be regularly
updated in order that stresses, draft and trim can be monitored throughout the discharging
Requirement of class approved loading computer
(1) Class requirements are that ships of more than 65 meters in length shall be provided with an approved type loading instrument. Ships with very limited possibilities for variations in the distribution of cargo and ballast and ships with a regular or fixed trading pattern may be exempt from the requirement.
(2) If a class approved loading computer is not available, record as to how stress and stability calculations are performed.
(3) The loading instrument is to be capable of calculating shear forces and bending moments, in any load or ballast condition at specified readout points and is to indicate the permissible values.
(4) The operational accuracy of the load computer should be tested regularly.
(5) Class approved data should be used and the tests should be carried out at least quarterly.
(6) Longitudinal stresses, where applicable, should be maintained within design limits throughout.
More Info pages
Procedure for cargo planning in Liquefied Gas Carriers
Details of various cargo handling equipment onboard
Cargo piping layout for LNG carriers
Procedure for commissioning the cargo system
Preparation for Cargo Transfer
Procedure for discussion prior cargo transfer
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 with Vapour of the Next Cargo (Purging)
Procedure for Water washing after Ammonia Cargoes
Details of various cargo handling equipment onboard
- Procedures for various cargo handling equipment onboard
- Personal protective equipments for people working onboard gas carriers
- Volatile nature of liquefied gases
- How to achieve maximum drainage of liquid during discharge
- The hazards of liquefied gases - Cargo information and safety factors
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