Semi-membrane tanks are non-self supporting tanks in the loaded condition and consist of a layer, parts of which are supported through insulation by the adjacent hull structure, whereas the rounded parts of this layer connecting the above mentioned supported parts are designed also to accommodate the thermal and other expansion or contraction.
The design vapour pressure P0 is not normally to exceed 0,25 bar. If, however, the hull scantlings are increased accordingly, and consideration is given, where appropriate, to the strength of the supporting insulation, P0 may be increased to a higher value but less than 0,7 bar.
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 categories of independent tanks :
Type A independent tanks
are tanks which are designed primarily using classical ship-structural analysis procedures. Where such tanks are primarily constructed of plane surfaces (gravity tanks), the design vapour pressure P0 is to be less than 0,7 bar.
Type B independent tanks
Type B are tanks which are designed using model tests, refined analytical tools and analysis methods to determine stress levels, fatigue life and crack propagation characteristics. Where such tanks are primarily constructed of plane surfaces (gravity tanks) the design vapour pressure P0 is to be less than 0,7 bar.
Type C independent tanks (also referred toas pressure vessels) are tanks meeting pressure vessel criteria and having a design vapour pressure P0 not less than: