Figure 3.1 Prismatic self-supporting Type 'A' tank — fully refrigerated LPG carrier
tank as found on a fully refrigerated LPG carrier. This is a self-supporting prismatic tank which requires conventional internal stiffening. In this example 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 ships 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 ship's hull, as shown in the figure. In general, it is this design approach which is adopted. By this means appropriate parts of the ship'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 ships, 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.
Type 'B' tanks
Type 'B' tanks can be constructed of flat surfaces or they may be of the spherical type. This type of containment system is the subject of much more detailed stress analysis compared to Type 'A systems. These controls must include an investigation of fatigue life and a crack propagation analysis.
The most common arrangement of Type 'B' tank is a spherical tank as illustrated in Figure 3.2(a). This tank is of the Kvaerner Moss design. Because of the enhanced design factors, a Type 'B' tank requires only a partial secondary barrier in the form of a drip tray. The hold space in this design is normally filled with dry inert gas. However, when adopting modern practice, it may be filled with dry air provided that inerting of the space can be achieved if the vapour detection system shows cargo leakage. A protective steel dome covers the primary barrier above deck level and insulation is applied to the outside of the tank. The Type 'B' spherical tank is almost exclusively applied to LNG ships; seldom featuring in the LPG trade.
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A Type 'B' tank, however, need not be spherical. There are Type 'B' tanks of prismatic shape in LNG service. The prismatic Type 'B' tank has the benefit of maximising ship-hull volumetric efficiency and having the entire cargo tank placed beneath the main deck. Where the prismatic shape is used, the maximum design vapour space pressure is, as for Type 'A' tanks, limited to 0.7 barg. A drawing of a self-supporting prismatic Type 'B' tank is shown in Figure 3.2(b).
Figure 3.2(a) Self-supporting spherical Type 'B' tank
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