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Instruments failure during unloading operations - Recommended actions by liquefied gas carriers

Liquefied natural gas (LNG) compresses to a small fraction of its original volume (approximately 1/600) under liquefaction. With the amount of flammable material that LNG contains, it has the potential to be an extremely dangerous chemical, if handled improperly. The liquefaction of natural gas raised the possibility of its transportation to many destinations.

An emergency can occur at any time and in any situation. Effective action is only possible if pre-planned and practical procedures have been developed and are frequently exercised. Cargo Pumps Failure

If one cargo pump fails it is possible to maintain the discharge with one pump in one tank. A problem arises when both cargo pumps in a cargo tank fail.

Consider - Emergency Discharge. Refer to the vessel's Cargo Handling Manual for further information on 'Pressure Discharge/Liquid Transfer'.

Capacitance Level Gauge (Transonic) Failure

Note 1: In this scenario, the High level alarm is disabled and possibly the Very High level alarm will not function (even if it has independent sensor).

Note 2: In this scenario, the trips for cargo/stripping pumps will not work as they should.

Monitor the level readings in tank(s) closely, particularly in the first stages of the unloading operations Monitor the Whessoe tape level reading closely, particularly when tanks are coming to the end Prepare Whessoe readings for commencement of ramping down and final level

ESD Failure

Try to disable ESD (normally optical) and go with the back-up (pneumatic). If both ESD fail, stop cargo operations Try to detect the problem and solve it. Do not resume loading operations until ESD is functioning

Liquefied gas cargo loading

Gas Detection System Failure

Stop cargo operations, Resolve the problem, Restart cargo operations

Hydraulic System for Cargo Valves Failure

Stop cargo operations. ESD will be activated if hydraulic press reaches a low limit Advise the Terminal as to the nature of the problem. Try to repair hydraulic system. Check hydraulic oil tank level. If loss is noticed check for leaks. Check filter. Resume unloading when problem has been solved

Return Gas Blower Failure

Reduce unloading rate. Advise Terminal as to reason for reduction in unloading rate. Use Vaporiser in order to supply vapour to cargo tanks. If necessary, drop cargo tanks pressure to 50 Mbar If Vaporiser fails and RGB is out, stop unloading immediately. Advise Terminal before unloading is stopped. Resume unloading if one of them (Vaporiser/RGB) may be used.

Safety Valves in Cargo Tank Failure

Stop cargo operations, Advise Terminal, Evaluate reason for failure. If Pilot valve suspected to be failing, replace pilot valve, Ensure Cargo Tank is under positive pressure, Continue discharging and monitor tank atmosphere pressure if Terminal allows to do it, If not, vessel should leave berth and try to blank defective safety valve or replace it with a spare

Water Curtain Failure

This can happen either before unloading commences or during the unloading operation. Any unloading operations must not commence until the water curtain is operational. It is advisable to have a flange fitted with a fire hose connection ready to use in case of system failure.

If necessary, stop cargo operations
Restore the water curtain. Consider - connecting a fire hose to one end of the pipe curtain.
Restart cargo operations

Related Information:

Cargo tank ruptures due to increased pressure - emergency procedure for gas carriers

Loss of power supplies - emergency actions

Risk and hazards of Nitrogen Loss

Gas carriers Loss of Instrumentation during Loading Operations

Gas carriers Structural Damage due to Incorrect Loading/Unloading Sequence

Emergency Procedures for rescue

Procedure for Water washing after Ammonia Cargoes

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

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