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Encyclopedia of Marine and Energy Technology

Gas carrier cargo handling


Typical operation cycle of most refrigerated type liquefied gas carriers includes drying, inerting, cooling down, preparation work prior to loading, loading, laden voyage, unloading, ballast voyage, stripping, warming up and gas freeing.

- Drying  – In order to prevent ice formation during cooling and to maintain the quality of products, moisture in all cargo tanks and associated piping is to be eliminated. Drying is achieved by introducing dry air into areas to be dried.

- Inerting – Inerting is done by supplying an inert gas to cargo tanks and associated piping in order to produce non-explosive atmosphere in these areas. Two types of inert gas are commonly used: gas produced with an inert gas generator and nitrogen gas.

- Cooling down – In order to avoid any damage to cargo tanks and piping due to thermal shock as well as to reduce the generation of boil-off gases (BOG) during loading operations, cargo tanks and piping are gradually cooled down. This procedure often consists of introducing low temperature cargo vapour into the cargo tanks and piping or of spraying the tanks with liquefied cargo.

- Loading – Cargoes are loaded into cargo tanks through cargo liquid lines by means of shore side pumps. Cargo vapour present in tanks is returned to the shore terminal via cargo vapour lines using compressors on board. Since the vapour is not vented at any time during this operation, this process is often referred to as a closed cycle one.

- Laden voyage – Except for pressure type gas carriers, cargo temperature and pressure control is mainly carried out at sea. Boil-off gases produced at sea are sent to reliquefaction plant and returned to the cargo tanks after being re-liquefied. In the case of LNG carriers, BOG may either be reliquefied or sent to the main engines as fuel for propulsion.

- Unloading – Cargoes are discharged using cargo pumps on board and cargo gases are supplied from the shore side terminal. This operation is carried out in a closed cycle. A sufficient portion of the cargo is retained in one of the cargo tanks in order to carry out cooling down operation during ballast voyage.

- Stripping – All cargo in the tanks is completely discharged as much as possible before any warming up and gas-freeing operations are undertaken.

- Warming up – It consists of raising the temperature of the cargo tanks and their piping in order to carry out gas-freeing operation. During warming up all remaining cargo is vaporized and heated by a cargo vaporizer and heater installed onboard. The heated gas then circulates through tanks and piping to raise the temperature of the entire system.