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Boil-off & vaporized gas (BVG) management system for carrying LNG cargo

Boil-off gas (BOG) – LNG tankers are designed to carry natural gas in liquid form at a temperature of – 163°C, close to the vaporization temperature. Despite tank insulation designed to limit the admission of external heat, even a small amount of it will cause slight evaporation of the cargo. This natural evaporation, known as boil-off is unavoidable and has to be removed from the tanks in order to maintain the cargo tank pressure.

Boil-off gas (BOG) combustion system – BOG combustion systems are used only onboard LNG carriers. Excess BOG is sent to the engine room via gas heaters by low capacity compressor and is burned by the main boilers as fuel. The main boilers are capable of operating under different fuel combustion modes such as exclusively BOG mode, combined BOG and fuel oil mode, and exclusively fuel oil mode. Although steam turbine systems have been the main form of propulsion used onboard LNG carriers, diesel engines capable of using BOG as fuel have become perfect solution due to their higher operating efficiencies.

Boil-off gas (BOG) recovery system, also BOG reliquefaction plant – An onboard reliquefaction system that recovers boil-off gas in LNG carriers and returns it to the cargo tanks. The BOG, at about -143°C and atmospheric pressure, is taken from the cargo tanks and passed through a compressor, where its pressure is raised to 4.5 bar and temperature to -60°C. This is then passed through one side of a nitrogen heat exchanger where its temperature is reduced to – 160°C at between 2 and 4 bar. This process converts the gas back into liquid but it must then be passed through a separator to remove any incondensibles. The cargo can then be returned to the tanks via loading or spray lines.

Boil-off rate (BOR) – The amount of liquid that is evaporating from a cargo due to heat leakage and expressed in % of total liquid volume per unit time. Typical values are 0.15%/day and below, recent projected LNG carriers are offered with a BOR close to 0.1%. However as early as 1990, the LNG carrier EKAPUTRA was delivered with a BOR of 0.1%/day.

Marine Steam Turbine Engine

The BVG management system is a total gas flow control from cargo tanks to main boilers. The total gas flow consists of the following two kind of gas.

i) Natural boil-off gas (Natural BOG) from cargo tanks.

ii) Forcing boil-off gas (Forcing BOG) generated by forcing vaporizer.

The natural BOG is sent to the main boilers by low duty compressor (L/D compressor) through the low duty heater and the flow rate is controlled by L/D compressor speed & inlet guide vane (IGV). If natural BOG does not meet boiler demand, the forcing vaporizer will generate forcing BOG and add it to natural BOG for full speed range of ship.


i) Natural boil-off gas (Natural BOG) Burning:

During all periods that the vessel is in service and is “gassed-up”, the boil-off from the tanks will be burnt as fuel in the ship’s main generating plant or disposed of in the gas combustion unit (GCU). This gas burning operation is started on the deck but controlled by the ship’s engineers from the ECR. Under normal operating conditions when the vessel is at sea, one L/D compressor, and one fuel gas pump for the spray coolers, will be running to supply fuel gas to the main generator engine to ensure that a stable gas supply to the diesel generators is achieved.

LNG carrier underway
Fig:LNG carrier underway

If for any reason that boil-off cannot be used by the main generators, or if the volume is too great for the requirements of the generating plant, any excess gas will be burnt in the gas combustion unit.

ii) Forcing boil-off gas (Forcing BOG) Burning:

If the fuel consumption of the main generators cannot be met by the gas supplied by natural boil-off from the tanks, additional gas can be obtained by utilizing the forcing vaporizer. This is fed by the fuel gas pumps which are located in No.3 & 4 tanks, to supply the LNG liquid to the vaporizer, via the Forcing BOG mist separator to the inlet of the fuel Warm-Up/Boil-Off heaters. The forcing vaporizer is automatic in operation and will be started through the IAS if the natural boil-off rate from the cargo tanks is insufficient to maintain the system pressure and meet the generator demand.

Related Information:

  1. Use of cargo as fuel -Cargo conditioning, reliquefaction and boil-off control for LNG carriers

  2. How to tackle fire on board LNG ship

  3. Cargo Machinery Room Precautions

  4. Fire fighting plan for LNG cargo

  5. LNG spill risk during marine transportation

Inerting of Cargo Tanks prior loading LNG cargo

Gassing-up requirement for cargo tanks

Initial Cool Down of cargo tanks

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