Independent Tanks

These are self-supporting and independent of the hull. There are three types:
‘Type A’, which are designed primarily using standard traditional methods of ship-structural analysis. LPG at or near atmospheric pressure of LNG may be carried in such tanks .
‘Type B’, which are designed using more sophisticated analytical tools and methods to determine stress levels, fatigue life and crack propagation characteristics. The overall design concept of these tanks is based on the so called ‘crack detection before failure principle’ which permits their use with a reduced secondary barrier (see Figure 25). LNG is normally carried in such tanks.
‘Type C’, which are designed as pressure vessels, the dominant design criteria being the vapour pressure. Normally used for LPG and occasionally ethylene.

Integral Tanks

Those tanks which form a structural part of the ships hull and are influenced in the same manner and by the same loads which stress the adjacent hull structure. These are used for the carriage of LPG at or near atmospheric conditions, butane for example, where no provision for thermal expansion and contraction of the tank is necessary.