Fully refrigerated tankers that carry LPG, Ammonia & Vinyl chloride over long distances
Fully refrigerated tankers are built to carry liquefied gases at low temperature and atmospheric pressure between terminals equipped with fully refrigerated storage tanks. The tankers have prismatic-shaped cargo tanks fabricated from 3.5% nickel steel, allowing the carriage of cargoes at temperatures as low as –48°C, marginally below the boiling point of pure propane.
Prismatic tanks enabled the ship’s cargo carrying capacity to be maximized, thus making fully refrigerated ships highly suitable for carrying large volumes of cargo such as LPG, ammonia and vinyl chloride over long distances.
The economic advantages of transporting LPG and ammonia in a fully refrigerated, non-pressurised condition are more evident for longer haul and larger quantity cargoes. The self-supporting prismatic shape of the cargo tanks allows for a better utilisation of the available hold space than the type of ships described previously. The tanks are usually designed for a maximum working pressure of about 0.28 kg/cm2 (280 milibars) and a minimum working temperature of -50oC making them suitable for the carriage of butane, butadiene, VCM, ammonia, propane and propylene.
Fig:Fully refrigerated LPG carrier at sea
The ships are typically in the range 15,000 m3 – 85,000 m3, with three common sizes for LPG/Ammonia trades of 30,000 m3, 52,000 m3 and 80,000m3.
The trend for longer voyages has imposed a demand larger ships, and with the increasing size of the ship, the pumping and refrigeration plant capacity has increased proportionally.
The tanks nearly extend to the full width of the ship, with ballast in the double bottom and upper hopper or wing tanks. These tanks normally have a centreline bulkhead fitted with two equalizing valves. You should be cautious should these vessels develop a list alongside as the tanks carry a large free surface area and if the vessel has problems with the ballast or levelling the cargo during load they can quickly list over to 2 or 3°.
Type 'A' tanks
Type 'A' tanks are constructed primarily of flat surfaces. The maximum allowable tank design pressure in the vapour space for this type of system is 0.7 barg; this means cargoes must be carried in a fully refrigerated condition at or near atmospheric pressure (normally below 0.25 barg).
Figure shows a section through this type of tank as found on a fully refrigerated LPG carrier. This is a self-supporting prismatic tank which requires conventional internal stiffening. The tank is surrounded by a skin of foam insulation. Where perlite insulation is used, it would be found filling the whole of the hold space.
The material used for Type 'A' tanks is not crack propagation resistant. Therefore, in order to ensure safety, in the unlikely event of cargo tank leakage, a secondary containment system is required. This secondary containment system is known as a secondary barrier and is a feature of all tankers with Type 'A' tanks capable of carrying cargoes below -10°C.
For a fully refrigerated LPG carrier (which will not carry cargoes below -55°C) the secondary barrier must be a complete barrier capable of containing the whole tank volume at a defined angle of heel and may form part of the tanker's hull, as shown in the figure.
In general, it is this design approach which is adopted. By this means appropriate parts of the tanker's hull are constructed of special steel capable of withstanding low temperatures. The alternative is to build a separate secondary barrier around each cargo tank.
The IGC Code stipulates that a secondary barrier must be able to contain tank leakage for a period of 15 days. On such tankers, the space between the cargo tank (sometimes referred to as the primary barrier) and the secondary barrier is known as the hold space. When flammable cargoes are being carried, these spaces must be filled with inert gas to prevent a flammable atmosphere being created in the event of primary barrier leakage.
Fig:Fully refrigerated gas carrier underway
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