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The gas tanker is a subcategory of tankers specialized to transport different types of liquified gases via sea transportation. Gas tankers carry gases like petroleum gases, natural gases which use for energy purposes, or gases like ammonia which use as a raw material for making agricultural fertilizers. The gas carriers have special cargo handling installations to keep cargo in a liquefied form. The design and operation of liquefied gas carriers are governed by the International Gas Carrier Code (IGC Code).
These gas tankers have inbuilt tanks onboard to store gases. LNG (Liquid Petroleum Gas), LPG (Liquefied petroleum gas), and CNG (Compressed Natural Gases) are the main 3 types of gasses carried on gas tankers.
The world’s gas fleet had nearly 650 (LPG) carriers by end 2020.
Different Types of Gas Tanker Ships
Fully Pressurized ships
The specialized tankers to carry gases at their ambient temperature are the pressurised ships or pressurised gas tankers with types C tanks. Also known as the simplest design of gas carriers. In general, these ships can accommodate gases with the pressure limit of 18 bar and there are a limited number of ships in operation to handle 20 bar pressured gasses. Due to the high pressure, these ships are extremely heavy and a bit difficult to steer.
Pressurised tankers are small in size as the compressed cargo requires a small tank capacity. These ships have a capacity of about 4000 m3 and mainly carry LPG and Ammonia. High pressure supports the cargo discharging operation without thermal insulation or reliquefaction plant and can discharge using pumps or compressors.
Semi pressurised ships are mostly popular among the small gas carrier operators. These ships have a capacity of 1500 to 30,000 m3 and are widely used to carry products like LPG, vinyl chloride, propylene, and butadiene. You can see the semi pressurised ships mostly in Mediterranean and Northern Europe trade routes due to their high flexibility in use.
These vessels are similar to pressurised ships as they use type C tanks. Yet the tanks could be built with lighter thickness compared to the pressurised ships. Low-temperature steels are used to make the tanks and support to carry cargo with temperatures of -48°C making these vessels most suitable for chemical gases and LPG (Liquid Petroleum Gas).
Ethylene ships have a capacity ranging from 1,000 to 12,000 m3 and carry cargo in fully refrigerated condition with an atmospheric pressure boiling point of -104°C. Ethylene ships are fitted with cylindrical or bi-lobe, insulated, stainless, or low-temperature nickel steel cargo tanks. Further, these ships are equipped with thermal insulation and a high-capacity reliquefaction for temperature control.
If these gas tankers are built with type C tanks, no need to have a secondary barrier. If type B and A tanks are fitted, it requires partial or full secondary barriers. Secondary barriers are also known as hold space, which is the space between the cargo tank and the ship hull.
Fully refrigerated ships
Fully refrigerated LPG ships are used to carry LPG and Ammonia in large quantities. These ships carry refrigerated cargo under atmospheric pressure. Independent tanks with a double hull, Semi-membrane tanks (incorporating a double hull), Independent tanks with a single hull but the double bottom and hopper tanks, and Integral tanks (incorporating a double hull) are the 4 systems used when designing/building fully refrigerated ships.
Liquid Natural Gas carriers are the specialised gas tankers to carry LNG at its atmospheric boiling point of about -162° C. LNG ships have a cargo-carrying capacity of 125,000 to 135,000 m3. In general LNG vessels are double hull and most of the ships are equipped with steam turbine propulsion plants.
The containment system of the LNG carriers could be four types as Kvaerner Moss spherical — independent Type ‘B’, IHI SPB Tank — prismatic, Gaz Transport membrane and, Technigaz membrane.