Liquefied Gas Carrier

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

Risk of Overfilling of Cargo Tanks only applies when vessel is alongside Terminal. The chances of overfilling a cargo tank are improbable as all LNG carrier cargo tanks are fitted with High and Very High level safety devices. The High level alarm will be activated when the loading level has reached 98.8% (typically) of tank capacity. The activation of the alarm immediately closes the filling valve of the tank in question, stopping cargo flowing into the tank.

The Very High (VH) alarm/Emergency Shut down (ESD) alarm will be activated if cargo reaches 99.5% tank capacity and will stop all cargo operations. The set point of the safety devices may vary from vessel to vessel. As general rule High level is 98.8% and Very High level is 99.5%. However, there are vessels that load more than 98.5% and then the safety devices are set with different values. In those cases High level may be 99.5% and Very High level at 99.7%.

Should the alarms fail, overfilling during loading cannot directly rupture a cargo tank as the tanks also contain safety valves that are designed to prevent the build up of excess vapour pressure. If the pressure increases beyond a preset level, a diaphragm in the safety valve ruptures and vents the excess cargo through the tank mast. When this happens, the main hazards are:

The risk of the vessel's hull or cargo tank covers cracking

The flammable gases present in the atmosphere may create an explosive environment.

Gas carrier sea passage
Fig:LNG carrier underway

Following actions recommended in an emergency:

i) Watch keeper patrolling flying passage to advise CCR as soon as cargo is seen overflowing, coming out from vent mast.

ii) Activate ESDS immediately. Stop cargo operations.

iii) Activate general alarm and advise by public address nature of incident and location

iv) Sound predefined signals in order to warn Terminal

v) Check for any injuries or fatalities

vi) Treat injured people, and evacuate them as necessary. Advise agent/terminal if external medical assistance is required

vii) Advise terminal the nature of incident and present situation

viii) Activate Spray system on tank domes, manifolds, accommodation, compressor house in order to protect steel and vaporise the liquid

ix) Prepare fixed Dry Powder Fire Extinguishing System in case fire occurs due to external ignition source or static electricity

x) Consider - transfer cargo in order to lower down level in overfilled tank

xi) Consider - disconnect loading arms. Activation of PERC

xii) Consider - leave the jetty / terminal. Tugs/pilot required

xiii) Once the situation has stabilised, consider - continue loading operations, keeping in mind that both cargo tank safety valves are leaking through

xiv) Inform Owners, Insurance, P&I, Local Authorities

Related Information:

  1. Leaks on the Cargo System, Continuous Flow - how to prevent

  2. Compressed air system - Gas carrier immediate actions

  3. Leaks from a Loading Arm due to Tidal or Current Effects

  4. LNG tank leaks and immediate action by gas carriers

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

Minor or major leaks from LNG tanks

Emergency Procedures for rescue

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