Liquefied Gas Carrier

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How LNG transferred from shore to ships cargo tanks ? - Liquefied gas carriers guideline

During loading, cargo is transferred from shore through the appropriate mid-ships or stern manifolds, and led into the cargo tanks via the filling lines, which usually terminate close to the tank bottoms. If the tank has not been cooled down it is normal to bypass some of the incoming liquid through the tank spray, if fitted, to reduce the temperature gradient from tank top to bottom, and to even out the rate of boil-off. The loading rate is determined by the rate of change of the tank pressure.

As the liquid level in the tank rises, the tank pressure is increased by:

(1) Vapour pressure of the “warm” cargo;

(2) Vapour displaced by the incoming liquid;

(3) Vapour generated by heat transfer through the tank walls to the liquid; and

(4) Vapour generated by heat transfer from the ship and shore pipelines and the shore pumps. On fully or semi-pressurized ships the vapour pressure increase during loading can be reduced by spray loading, provided the cargo temperature will give a saturation pressure safety below the relief valve set pressure. With fully or semi-pressurized tanks, the boil-off and displaced vapour is either returned to shore or condensed by the ship’s reliquefaction plant. Venting during loading must be avoided. In the case of LNG the boil-off cannot normally be condensed and the ship will be dependent on full vapour return to shore.

During normal operations the vessel’s tank pressure is not to exceed 90% of the pressure at which the safety valve lifts. Should the tank pressure increase to above this pressure then you are to use all possible methods to reduce the pressure to below the 90% level. This may include stopping operations such as ESD tests, burning gas in the boiler or sending ashore. When agreeing the plan of operation with the terminal you are to ensure that we have in place a 10% safety factor. Should the vessel be pressured at any time to exceed the 90% pressure you are to discuss the matter with the office first.
The responsible officer should ensure that the following precautions are observed:

(1) In the event of an emergency, the emergency shutdown procedures should be implemented.

(2) All fixed gas detection equipment should be operated throughout all loading operations.

(3) During the early stages of loading, the incoming liquid may be relatively warm and generate quantities of vapour in excess of capacity of the reliquefaction plant or vapour return line.

i) The tank pressure should be regularly observed during loading and the loading rate reduced in good time before approaching safety valve set pressure.

ii) If reducing the loading rate does not reduce the pressure rise, loading should cease immediately, and the terminal should be notified to enable proper steps to be taken in the event of hazard to the adjacent shore areas.

iii) If venting occurs it will cause self-refrigeration, thus reducing the cargo temperature and pressure.

Gas carrier sea passage
Fig:LNG carrier underway

(4) Filling of the cargo tanks may cause a significant loss of pressure in the hold or inter-barrier spaces, depending upon the cargo system design. This should be continuously monitored and pressure maintained by the addition of supplementary inert gas, dry air or dry nitrogen.

(5) Filling limit regulations should be observed. The maximum liquid level in each tank should be calculated before or during the early stages of loading.

(6) If large quantities of vapour are being generated (i.e. the cargo liquid is boiling rapidly) the bubbles created will increase the liquid volume. To measure tank contents accurately under such circumstances it is recommended that vapour removal should be reduced temporarily to allow the liquid level to stabilize.

(7) In all tanks, whether or not being loaded, the liquid level and pressure readings should be monitored throughout loading. A reading which does not change as expected may indicate a fault which should be investigated.

(8) When liquid flow is diverted from one tank to another the valves on the tank about to receive cargo should be fully opened before those on the tank being isolated are shut.

(9) On completion of loading all ship’s lines should be drained into the cargo tanks using the facilities provided. Liquid in hoses or loading arms should also be drained to the cargo tanks, if possible, or blown to shore and pressed past shore valves by vapour pressure. If possible the ship-shore connection should be purged before being disconnected. Ship-shore connections should not be broken until it has been ascertained that all liquid has been removed and the lines are depressurized. Adjacent isolating valves on ship and shore and any other relevant valves should then be closed before connections are broken.

(10) Bonding wires, if fitted, should not be disconnected until after the hoses have been disconnected.

(11) The relief valves of some ships have dual or multiple settings, either for operational purposes or to meet differences in national regulations. Changes to the relief valve setting should be carried out in accordance with the procedures specified and under supervision of the master. Changes should be recorded in the ship’s log and a sign posted at the relief valve and in the cargo control room, if provided, stating the set pressure.

Preparation for loading LNG

It is assumed that all preparatory tests and trials have been carried out on the ballast voyage prior to arrival at the loading terminal.

All operations for the loading of cargo are controlled and monitored from the ship’s CCR. The loading of LNG cargo and simultaneous de-ballasting are carried out in a sequence to satisfy the following:
An officer responsible for the operation (OOW) must be present in the CCR when cargo is being transferred. A deck watch is required for routine checking and/or any emergency procedures that must be carried out on deck during the operation.

During the loading operations, communications must be maintained between the ship’s CCR and the terminal by telephone and radio. The ESD system must remain in contact with the terminal in order to ensure signals for the automatic actuation of the Emergency Shutdown from or to the ship.

At all times when the ship is in service with LNG and mainly during loading, the following are required:
  1. The pressurization system of the insulation spaces must be in operation with its automatic pressure controls.
  2. The secondary Level Indicating system should be maintained ready for operation.
  3. The temperature recording system and alarms for the cargo tank barriers and double hull structure should be in continuous operation.
  4. The gas detection system and alarms must be in continuous operation.
  5. Normally when loading cargo, vapour is returned to the terminal by means of the H.D. compressors or shore compressor. The pressure in the ship’s vapour header is maintained by adjusting the compressor flow.
  6. The cargo tanks must be maintained in communication with the vapour header on deck, with the vapour valve on each tank dome open.
  7. A suitable Vent Mast is maintained ready during the loading operation, for emergency venting.
  8. If the tanks have not been previously cooled down, LNG spraying is carried out.
Alongside of terminal as applicable

Related Information:

  1. Liquefied gas Cargo safe handling & responsibility on board

  2. Cargo emergency shutdown requirement for liquefied gas carrier

  3. Gas analyzing equipment

  4. Custody Transfer Measurement (CTM) System

  5. Details of various cargo handling equipment onboard

  6. Tank construction materials for a modern gas tanker

  7. LNG vessel construction -Advantages of membrane technology

  8. Advantages of Moss rosenberg cargo containment system

  9. Type of gas carriers - variation in the design, construction and operation

  10. Transporting liquefied natural gases by LNG ships

  11. Cargo conditioning, reliquefaction and boil-off control requirement for a liquefied gas carrier

  12. The sea transport of liquefied gases in bulk -Where do the products come from ?

Gas carrier cargo containment procedure

Cargo conditioning, reliquefaction and boil-off control requirement for a liquefied gas carrier

Cargo Containment Systems in Liquefied Gas Carriers

cargo emergency shutdown requirement

damage stability guideline for liquefied gas carriers

Various Cargo handling equipments onboard

Cargo hoses connection guideline

Documents accompanying a liquid gas cargo

How LNG transferred from shore to ships cargo tanks ?

Cargo operation guideline onboard a liquefied gas carrier

Cargo piping Systems in Liquefied Gas Carriers

cargo planning requirement

cargo and pumproom safety precautions

cargo stripping guideline

Emergency response for cargo system leaks

Emergency response for cargo tank rupture

Risk of overfilling of cargo tank during loading onboard a liquefied gas carrier

Preparation for cargo transfer

cargo transfer between vessels- safety guideline

Gas carrier cargo handling additional guidelines

  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

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