Type ‘A’ Tanks:
These tanks are designed using the traditional method of ship structural design. LPG at near-atmospheric conditions or LNG can be carried in these tanks. The design pressure of Type A tanks is less than 700 mbar. The following figures show the general arrangement of a liquid methane carrier with Type ‘A’ tanks.
The most notable and distinguishing feature of Type ‘A’ tanks is that the IGC Code specifies that Type ‘A’ tanks must have a secondary barrier to contain any leakage for at least 15 days. The secondary barrier must be a complete barrier of such capacity that it is sufficient to contain the entire tank volume at any heel angle. Often, this secondary barrier comprises of the spaces in the ship’s hull as shown in the figure below.
Type ‘B’ Tanks: The concept behind the design of such tanks is to have such a structure in which a crack can be detected long before the actual failure. This allows a time margin before the actual failure occurs. The methods used for design of such tanks include determination of stress levels at various temperatures and pressures by first principle analyses, determination of fatigue life of tank structure, and study of crack propagation characteristics. This enhanced design of such tanks requires on a partial barrier.
The tank structure is spherical in shape, and it is so positioned in the ship’s hull that only half or a greater portion of the sphere is under the main deck level. The outer surface of the tank plating is provided with external insulation, and the portion of the tank above the main deck level is protected by a weather protective layer. A vertical tubular support is led from the top of the tank to the bottom, which houses the piping and the access rungs.
As evident from the layout, any leakage in the tank would cause the spill to accumulate on the drip tray below the tank. The drip pan and the equatorial region of the tank are equipped with temperature sensors to detect the presence of LNG. This acts as a partial secondary barrier for the tank.
LNG is usually carried in this type of tanks. A flexible foundation allows free expansion and contraction according to thermal conditions, and such dimensional changes do not interact with the primary hull structure, as shown in Figure.
Type ‘C’ Tanks: These tanks are designed as cryogenic pressure vessels, using conventional pressure vessel codes, and the dominant design criteria is the vapour pressure. The design pressure for these tanks is in ranges above 2000 mbar. The most common shapes for these tanks are cylindrical and bi-lobe. Though Type ‘C’ tanks are used in both, LPG and LNG carriers, it is the dominant design in LNG carriers.
The following figures show the arrangements of cylindrical and bi-lobe tank arrangements in midship view. The cylinders can be either vertically or horizontally mounted, depending on the dimensions and spatial constraints of the ship. Note, in Figure 6, that the space between the two cylinders is rendered useless. Due to this, the use of cylindrical tanks is a poor use of the hull volume. In order to circumvent this, the pressure vessels are made to intersect, or bilobe tanks are used
