The requirement of the cargo Emergency Shutdown (ESD) system are to stop cargo liquid and vapour flow in the event of an emergency and to bring the cargo handling system to a safe, static condition. The earlier method of cargo shut down comprise of manual trip points and automatic fire sensors that can initiate remote closure of emergency shutdown valves "for shutting down liquid and vapour cargo transfer between ship and shore"

This emergency trip, when activated, must also stop cargo pumps and compressors. However, these provisions do not necessarily provide adequate protection, particularly against overflow, during other operations involving the transfer of liquid and vapour on board. It must be recognised that operations such as reliquefaction or cargo tank spraying may be routine operations at sea. These deficiencies eliminated by intruducing cargo emergency shutdown (ESD).

The ESD system minimises potential risks during the transfer of liquefied gases between ship and shore loading and unloading installations. It provides a quick and safe means of stopping the transfer of cargo and isolating ship and shore cargo systems in a controlled manner, either manually or automatically, in the event of fault conditions that affect the ability of the operator to control safely the transfer of cargo. Most export terminals, and an increasing number of import terminals, now have a second level of protection providing for the rapid disconnection of the loading arms from the ship. These two levels of cover are known as `ESD-1' and `ESD-2'.

The emergency shutdown (ESD) system is a requirement of the IMO code for the carriage of liquefied gases in bulk and is a recommendation of SIGTTO. It is fitted to protect both the ship and terminal in the event of power loss, cryogenic or fire risks, on either the ship or in the terminal.

The system will stop the flow of LNG liquid and vapour by shutting down the pumps and gas compressors as well as manifold and shipside valves, by the activation of a single control. Shut down of the cargo system can be initiated either manually or automatically if certain off-limit conditions occur.

The ship's ESD system is active at all times, whether at sea or in port. When at sea all manifold and tank filling valves are held in the shut position and the cargo and spray pumps are held in the off position. The cargo compressors may be operated as normal, but will stop if an ESD is initiated. The shore ESD input is blocked in the At Sea DCS condition

Manual emergency shut down push buttons are situated strategically around the ship, at locations that include the wheelhouse, cargo control room, fire control station, manifold platforms and tank liquid domes. In addition, manual activation of the shore ESD system will, through the ship/ shore link, set off the ship’s ESD.

Automatic shut down for fire is initiated by fusible plugs which are generally located at each tank dome, manifold platform, and in the cargo compressor and electric motor rooms. ESD1 may also be initiated automatically under conditions such as the following:

1. Blackout of the ship.
2. Vapour header pressure falls below pre-set limit.
3. Individual tank pressure falls below pre-set limit.
4. Extreme liquid level in any cargo tank.
5. Low cargo valve hydraulic pressure.

ESD2 is normally initiated by the terminal and will result in all the actions as for ESD1, plus the initiation of a dry break of the shore arm from the ship. ESD2 may be initiated manually, for example, in the event of a terminal emergency, or automatically, for example, if the ship moves outside the movement envelope of the chicksans.

The automatic disconnection of shore arms can be a violent and potentially dangerous operation and it is important that personnel at the manifold are warned to leave the area before ESD2 activation.

Each ship must have procedures for testing the function of ESD systems which must be tested prior to arrival in port and also immediately before commencing cargo operations.

Emergency shutdown (ESD) blocking and override

The ESD system will have a facility to activate a “block” or “override”. Under normal vessel operating procedures the ESD system will be fully active, There may be occasions when it will be necessary to inhibit part or all of the system.

The “At Sea” condition” will be selected prior to the shore connection being disconnected after the cargo operations have been completed. The “At Sea” condition has the following effect:;

• Isolates the shore connection from the ESD logic
• Locks the cargo pumps in the OFF condition
• Positions the manifold valves in the CLOSED position
• Positions the cargo tank filling valves in the CLOSED position
• Allows the low duty compressors to run if the ESD or low duty system trips are not activated
• Allows the high duty compressors to run if the ESD or high duty system trips are not activated

Prior to any cargo operations in port, the “At Sea” condition must be switched to the “In Port” position to allow the ESD system to be fully active.

After any emergency shutdown of the cargo system, it may be necessary to “Override” the system. Before the system is switched to “Override” the cause of the shutdown must be determined. The “override” facility should only be used when absolutely necessary to allow recovery from an emergency condition. As soon as the emergency condition is corrected, the ESD should be returned to the normal condition and the “override” facility switched off.

Before the ESD is overridden, the Master must be fully appraised of the situation, and must give his approval for the “Override” to be switched on.

It must be noted that on any occasion that the ESD is not in its normal operational condition, any cargo related emergency situation on board the ship and or terminal, will not result in activation of the ESD, and full shut down of the cargo system will not take place.


ESD Testing

LNG vessels must always conduct pre-arrival ESD system tests 48 hours before arrival at any load or discharge port. Additionally in the event of an extended voyage, the ESD system should again be tested at intervals of not more than 30 days from the previous test.

These tests must include, but not be limited to:

• Cargo Emergency Shutdown system test, including all push buttons and trips (These may be tested in rotation).
• All Cargo and Ballast valves operated.
• Manifold valve timings checked.
• Check the operating parameters of nitrogen generators and barrier space pressures (where applicable).
• Barrier space water detection (where applicable).
• Mast riser nitrogen snuffers.
• Ship-Shore interface connection operations.

Successful completion of these tests must be logged on form LNG02 and recorded in the deck log book.
Prior to loading / discharge operations in port, additional ESD testing in both the warm and cold conditions will be carried out as part of the pre-transfer ship-shore checklist.





Related Information:

1. Procedures for LNG cargo loading



2. Manifold arrangements
   
   
3. Procedures for LNG cargo discharging



4. Boil-off & Vaporized Gas (BVG) Management System for LNG cargo



5. Gas cargo containment systems - primary barrier (the cargo tank),secondary barrier, thermal insulation and more



6. Details of various cargo handling equipment onboard



7. Procedure for loading Liquefied Gas Cargoes
   
   Procedure for Cargo Conditioning in Liquefied Gas Carriers



8. Cargo Transfer between Vessels (STS Operation)



9. Procedure for segregation of Liquefied Gas Cargoes



10. Procedure for Stripping Liquefied Gas Cargoes



11. Procedure for Changing Liquefied Gas Cargoes



12. Displacing Atmosphere with Inert Gas (Inerting)



13. Procedure for Water washing after Ammonia Cargoes



14. Preparation for Cargo Transfer



15. Procedure for discussion prior cargo transfer

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