Cargo tank types that may be found on Gas Carriers
Independent Type ‘A’
Independent Type ‘B’
Independent Type ‘C’
Membrane
Other less popular types are (though fully approved, these are rarely seen in practice):
Internal insulation Type ‘1’
Internal insulation Type ‘2’
Integral
Independent tanks
Independent tanks are self-supporting; they do not form part of the ship’s hull and are not essential to the hull strength. There are three types of Independent tanks.
Type ‘A’ tanks
Type A tanks have been defined by the IGC code as being those which are designed primarily using recognized standards of classical ship structural analysis – i.e. a more detailed or accurate stress analysis has not been done. These are primarily made of flat surfaces and the code restricts the maximum permitted pressure to 0.7 bar. These are the most common types of tanks found on fully refrigerated gas carriers and typically have a pressure capability of 0.25-0.3 bar. Some vacuum resistance capability is also provided – typically 0.07 bar.
Insulation will be provided on the outside. The material used for the construction of tanks is not crack propagation resistant. In the unlikely event that the tank does crack, the surrounding space should be capable of holding the entire leakage amount under defined conditions. A secondary barrier is therefore required – since the outer hull normally fulfills this role, it should be of a material suitable for low temperatures.
The space between the tank and the hull is known as the hold space. If carrying flammable cargoes, this space must be inserted at all times to prevent a flammable atmosphere from forming in the event of a leakage in the primary barrier.
Type ‘B’ tanks
A type ‘B’ is subjected to a more detailed stress analysis compared to type ‘A’ including analysis of fatigue life and crack propagation characteristics. Due to increased reliability of design and construction and consequently reduced likelihood of leakage from the primary barrier, only a partial secondary barrier is required. This takes the form of a drip pan placed below the tank bottom. Additionally there may be splash barriers to direct small leakages to the drip pan. Insulation is applied to the outside. These tanks are most commonly found on LNG carriers. A weathertight cover is applied to the exposed part.
Prismatic type ‘B’ tanks are also in use. In this form it utilizes the hull contours and available space more efficiently and there is no protrusion above deck level. The hold space is inerted. Alternatively, it may be filled with dry air if the means of rapid inerting are available. Prismatic types ‘B’ tanks are commonly being fitted on large LPG carriers. If constructed of flat surfaces, design pressure is limited to 0.7 bar.
Type ‘C’ tanks
Type ‘C’ tanks are of a pressure vessel design with a pressure higher than 2 bar. Built largely of curved surfaces, they may be of a cylindrical or spherical design and mounted vertically or horizontally. To better utilize the hull geometry, some are of a bi-lobe design.
If fitted on fully pressurized gas carriers, typical design pressure will be of the order of 18 bar with about 50% vacuum withstanding capability. If fitted on semi pressurized, fully refrigerated gas carriers, design pressure will be of the order of 5-7 bar but with tank material suitable for the low temperature of the order of -48 °C for propane and –104 °C for ethylene. The hold space may be filled with dry inert gas or dry air. Due to the degree of high stress that is possible with this design, no secondary barrier is required.
Membrane Tanks
These are non-self-supporting tanks. The primary barrier is a thin membrane (about 1 mm thick) which is supported through the insulation by the adjacent hull structure (inner hull). A complete secondary barrier is required. The design and construction of the membrane ensure no undue stresses due to thermal expansion/contraction. The design pressure should not exceed 0.25 bar, except where increased hull scantlings and insulation strength is provided where design pressure can be increased up to 0.7 bar. Membrane tanks are usually seen on LNG carriers only. Two commercially available membrane systems are in use named after the companies which designed them.
