The invention relates to a tank for very cold fluids, especially liquid natural gas, as indicated in the introduction to claim 1.

It is known in the prior art that liquid natural gas, LNG, can be transported in ships in spherical tanks which are supported by means of a cylindrical plate construction or skirt. This skirt is connected to the tank along a horizontally extending great circle and rests on the bottom of the ship.

The spherical tanks are advantageous, particularly because their geometry is simple, which permits an accurate calculation to be made of the stresses which arise in the tank material under different operating conditions.

^• For spherical tanks it is advantageous for the tank to be supported by means of a skirt which extends around the tank, the skirt causing only small local bending stresses in the tank and permitting practically free thermal movement of the tank. Hull deformations are transferred to the tank to only a minor extent due to the rigidity of the skirt.

It is known in the prior art that relatively small cylindrical tanks for liquid petroleum gas, LPG, with a horizontally extending longitudinal axis can be supported by means of U-shaped cradles.

This kind of support presents difficulties in the case of large tanks, since major local bending stresses can occur in the tank wall in the region of the cradles. Moreover, it is difficult to position the tank correctly against the cradle for all the different load conditions which may occur.

However, the demands made on tanks for carrying LNG are quite different to those for tanks for carrying LPG, and it is not obvious that tanks of a cylindrical shape will be suitable for carrying LNG, even though such tanks are used for LPG. This is not obvious particularly if those tanks which have to transport LNG also have to have a volume which is five times larger than the tanks for LPG.

When using spherical tanks the volumetric utilization of the ship's cargo hold is low. Thus the object of the invention is to provide a tank of the type mentioned in the introduction which is encumbered in a less degree with this disadvantage.

The characteristics of the tank according to the invention are indicated by the characteristic features in the claims presented.

The invention will now be described in more detail with reference to the drawing which schematically illustrates an embodiment of a tank according to the invention.

Fig. 1 is a front view of a tank according to the invention.

Fig. 2 is a side view of the tank which is shown in fig. 1.

As illustrated in the figures, a tank 1 according to the invention comprises two hemispherical end pieces 2,3 which are connected to each other via a cylindrical tank section 4 with the same diameter as the end pieces 2,3. The tank is therefore rotationally symmetrical about a horizontally extending axis 5 which, when the tank is installed in the ship, normally extends in its longitudinal direction. The tank diameter is usually between 30 m and 45 m.

The cylinder section of cylindrical tanks normally requires a considerably greater wall thickness than the end pieces of the tanks. Surprisingly, however, it has been shown that if the length of the cylindrical section is less than approximately 1/3 of its diameter, it can have a wall thickness which is substantially less than the wall thickness of tanks with long cylindrical sections.

The improved volumetric efficiency which is obtained with tanks designed in this manner compared to tanks of a purely spherical shape is of great importance for operating economy. In this context it can be mentioned that it is essential with a tank of this type that the different areas of the tank wall can be designed in such a manner that the tank can obtain a "Type B" classification according to the regulations of the International Maritime Organization, only tanks with this classification being of interest to the industry, and that it is surprising that a tank with a cylindrical section with reduced wall thickness as stated above can obtain such a classification.

The tank is supported by a skirt 10 which can extend continuously around the tank 1. The upper section 12 of the skirt is connected to the tank 1 near the line of intersection between the tank and a horizontal plane 11 which comprises the tank's axis of symmetry 5, and the skirt's longitudinal axis extends vertically downwards. At the upper skirt section 12 there extend plate sections of this approximately tangentially in relation to the adjacent tank sections. The skirt's lower section 13 rests on a base deck, the ship's bottom 14 or the like. The sides of the skirt also extend vertically.

The skirt can comprise vertical and/or horizontal stiffening elements and include cut¬ outs in order to achieve an optimum weight/strength ratio. Since the curved sections of the skirt 10 carry more than the straight sections, the skirt's straight section at the cylindrical tank section 4 can be stiffened, thus correspondingly relieving the pressure on the curved sections at the end pieces 2,3.

Alternatively the straight section of the skirt 10 can have a cut-out 20 as illustrated in fig. 2, since this straight section with little buckling strength under any circumstances is capable of bearing only a small proportion of the tank's weight.

Instead of the skirt's upper section 12 being connected to the tank at the above- mentioned line of intersection and the skirt's side extending vertically, the skirt's upper section can be connected to the tank 1 along a line of intersection between the tank and a horizontal plane under the tank's axis of symmetry and the skirt can extend tapered downwardly, a design which is advantageous, since it enables the plate areas near the upper skirt edge to also extend tangentially in relation to adjacent tank areas.