Insulated tanks for liquefied gases
This is a continuation of application Ser. No. 804,257 filed June 7, 1977, now U.S. Pat. No. 4,141,465, issued Feb. 27, 1979. 
    
    
     This invention relates to an improvement in the insulated tanks used to hold liquefied gases, where the tank structure includes a support in the form of a vertical skirt which forms a structural unity with the wall of the tank. 
     In the known spherical tanks which are supported by skirts on board ships, for example, the tank itself and the top portion of the skirt are thermally insulated, however, there is still some heat leak into the tank despite the insulation, such heat leak being by way of conduction by the skirt itself from the supporting hull structure below. This results in so-called &#34;boil-off&#34; of the cargo. 
     On insulated cargo tanks which are designed to transport methane, the maximum boil-off has been calculated to be 0.25% per 24 hours. Increasingly stringent terms in freighting contracts have resulted in a need for reducing this boil-off. On large aluminum tanks having the skirt construction and insulation used at present, the heat flow through the skirt constitutes approximately 35% of the total heat leak into the spherical tank itself, the other 65% being directly from the tank. By improving the insulation of the tank within the limits of what seems practically possible today, the heat leak directly from the tank can be reduced by approximately 30% out of that 65%. This means that the percentage of heat leak due to the skirt will then be 50%. A further reduction of the heat leak can best only be accomplished by reducing the heat flow through the skirt. 
     The tank and skirt constitute a monolithic or integral structure. This construction principle provides advantages, both technically and from a safety point of view. It is therefore undesirable to introduce an &#34;insulator&#34; into the skirt, which would mean that the principle of structural unity would be violated. 
     Calculations show that a thickening of the insulation on the skirt would result in only small changes in the rate of heat leak (temperature gradient). In addition, it should be noted that the material that is used today for the tank wall and at least the upper part of the skirt is aluminum, a material which has relatively high heat conductivity. 
     Therefore, the aim of the invention is to introduce a kind of heat brake or temperature brake into the skirt without violating the previously mentioned principle of structural unity. That result is achieved according to the invention in that the vertical skirt has an intermediate belt or skirt portion in the region between the transition to the tank wall and the support structure below which is referred to herein as the bottom skirt portion. Said intermediate skirt portion is made of a material which has the properties of poor or low heat conductivity relative to that of the material comprising the tank walls and the rest of the skirt, and a thermal expansion coefficient which lies between the values for the other skirt portions above and below it, and the ability to withstand low temperatures said intermediate skirt portion, being thermally insulated. 
     One material which would satisfy these conditions is stainless steel, for example, 18-8 SS. 
     When the skirt is provided with the heat brake of the invention, the heat flow through the skirt can be reduced by 40%-50%. This means a 15%-25% reduction of the total heat leak, depending on the tank insulation. 
    
    
     The invention will be explained further with reference to the drawings, where 
     FIG. 1 is a cross section through one known embodiment of the skirt, 
     FIG. 2 is a cross section through an embodiment of the skirt according to the invention, and 
     FIG. 3 is a comparative graph of the temperature distributions for the skirts of FIG. 1 and FIG. 2, i.e., without the improvement of the invention and including said improvement. 
    
    
     FIG. 1 shows how the skirt is constructed with a lower skirt portion 1 made of a suitable steel material, and an upper skirt portion 2 made of aluminum. The two skirt portions 1 and 2 are welded together at 3 in an appropriate manner. A portion of the wall of the spherical tank is shown on FIG. 1, designated by 4. The spherical tank wall and the upper part of the skirt are insulated as shown by reference numbers 5 and 6, respectively. 
     FIG. 2 shows a similar cross section through a new embodiment of the skirt. The lower skirt portion 1&#39; of the skirt is also in this case made of a suitable steel material, while the skirt&#39;s upper skirt portion 2&#39; is made of aluminum. The spherical tank wall 4, as in FIG. 1, is made of aluminum. The tank&#39;s insulation is designated by 5. 
     Between its lower skirt portion 1&#39; and its upper skirt portion 2&#39;, the skirt is provided with an intermediate belt or skirt portion 7 in accordance with the invention, the skirt portion 7 in this case being made of stainless steel. An example of a suitable material for the zone 7 would be 18-8 stainless steel. The skirt portion 7 is welded into the skirt and thus constitutes a structural load bearing part of the skirt. In this way, one retains the important principle of structural unity mentioned previously. The skirt&#39;s insulation 6&#39; is extended down so that it also covers the intermediate skirt portion 7. The insulation in this zone enhances the effect of the heat brake to reduce the gross heat input to the tank structure, from the hull of the ship, by isolating the intermediate zone 7 from the effects of the ambient temperature, thereby to keep the temperature of the skirt&#39;s upper zone 2&#39; and of the intermediate zone 7 as low as possible. The insulation in this zone enhances the effect of the heat brake or temperature brake to reduce the gross heat input to the tank structure by isolating the intermediate skirt portion 7 from the effects of the ambient temperature, thereby to keep the temperature of the skirt&#39;s upper portion 2&#39; and of the intermediate portion 7 as low as possible. 
     The temperature distribution in the new skirt construction is shown on FIG. 3, where the principal structural components of the skirt have been drawn in on the diagram. The upper curve shows the temperature distribution on the skirt lacking the heat brake of the present invention invention, while the lower curve shows the temperature distribution on the skirt provided with the heat brake of the invention. 
     These curves also generally represent an approximation of the deflection of the skirt as a result of the temperature conditions therein. Accordingly, it will be appreciated that with the heat brake or the temperature brake of the present invention, the upper skirt portion 2&#39; will remain more nearly tangent to the tank than will the upper portion of the skirt which does not contain the insulated heat brake system of the present invention. This reduces stress transmittal to the point of juncture between the skirt and the tank. 
     When the tank is at the ambient temperature, the entire skirt has a cylindrical configuration. However, when the tank is placed into service, the entire tank and the top skirt portion and the top of the intermediate skirt portion are cooled to or toward the temperature of the liquefied gas, whereas the bottom edge of the intermediate skirt remains at substantially the ambient temperature. That change in temperature causes shrinkage in the diameter of the tank and in the skirt, the amount of shrinkage at each portion of the skirt being a function of its change in temperature. Hence, the intermediate skirt portion shrinks the greatest at its top edge and shrinks the least at its bottom edge. The top edge of the lower portion of the skirt will also shrink to the extent that its temperature is reduced. Between the top and bottom edges of the intermediate skirt portion, the rate of reduction in temperature is at a substantially uniform rate. Therefore, the intermediate skirt portion changes from its substantially true cylindrical configuration to a somewhat frusto-conical configuration. As shown in FIG. 3, the temperature of the lower edge of the upper skirt portion is substantially above the temperature of its upper edge and of the tank. The structural unity of the three skirt portions is maintained throughout the entire range of the change in temperature of the tank and of the skirt. 
     These curves also generally represent an approximation of the deflection of the skirt as a result of the temperature conditions therein. Accordingly it will be appreciated that with the heat brake of the present invention, the upper skirt zone 2&#39; will remain more nearly tangent to the tank than will the upper zone of the skirt which does not contain the insulated heat brake system of the present invention. This reduces stress transmittal to the point of juncture between the skirt and the tank.