Liquefied Gas Carrier

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Cargo handling, limitations & damage stability guideline for liquefied gas carriers

The master of every liquefied gas carrier should be supplied with a loading and stability information booklet. This booklet should contain details of typical service conditions, loading, unloading and ballasting operations, provisions for evaluating other conditions of loading and a summary of the ship’s survival capabilities. In addition, the booklet should contain sufficient information to enable the master to load and operate the ship in a safe and seaworthy manner. The master should be aware of the worst damage stability condition in the stability book.

Stability and cargo loading limitations

(1) Stability calculations and, where applicable, stress calculations, should be performed for the current cargo operation.
(2) The cargo watch officers should perform the stress and stability calculation at least once a watch.

Liquefied natural gas ship underway 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.

Loading Computer

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 operator.

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 operations.

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.

Cargo handling equipment of a typical 138,000m3 LNG tanker

The ship has four membrane type cargo tanks. A tripod mast is built inside each tank with the filling line and two 1700 m3/h electric, submersible pumps attached. Complete discharge can be done within 12 hours, with loading handled by shore pumps after the tanks have been pre-cooled by a LNG spray. Vapour produced during these processes is returned to shore using two 32,000 m3/h compressors, a 26,000 kg/h main vaporiser and two heaters.

Forced boil-off and delivery of gas fed to the propulsion system are achieved by two 8500 m3/h compressors and a 7600 kg/h forcing vaporiser combined with 540kW heater. Inerting of the insulated space and cargo tanks is accomplished by means of two 90 Nm3/h nitrogen generators and a 14,000 Nm3/h inert-gas generator. Two 1250 m3/h vacuum pumps, four 50 m3/h stripping/spray pumps, and a 550 m3/h emergency cargo pump are also fitted.

Related Information:

  1. Procedures for various cargo handling equipment onboard

  2. Personal protective equipments for people working onboard gas carriers

  3. Volatile nature of liquefied gases

  4. How to achieve maximum drainage of liquid during discharge

  5. The hazards of liquefied gases - Cargo information and safety factors

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

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