LNG TANK SECURED AGAINST EARTHQUAKES

Field of invention The present invention relates to storage of LNG, i.e. liquefied natural gas, and other cryogenic liquids. More particularly, the invention relates to large tanks for storage of LNG or other cryogenic liquids, and especially means for improving the strength of such tanks against earthquakes.

Prior art and background of the invention

Storage of LNG requires tanks withstanding operating temperatures lower than -161 °C, which is the boiling point at atmospheric pressure of methane, the main component of LNG. A normal operating temperature of an LNG tank is -163 ⁰ C.

Today, LNG is usually stored in double-walled tanks at atmospheric pressure. The inner tank serves to keep the LNG content enclosed, whereas the outer tank keeps the insulating material in position, protects the inner tank and the insulation against external influence, and provides increased security in case of leakage in the inner tank. For small to medium size tanks it is not unusual to use pressure tanks or isolation provided by arranging a vacuum between the tank walls. Such arrangements are inadequate for large tanks because the walls must be made unduly solid. In order to limit the wall thickness a circular cross section of the tank is the most usual. The outer tank is usually constructed to keep LNG or gas enclosed if a leakage should occur in the inner tank, especially if the tank is located at a location having bad ventilation or in a populated area. The tanks of LNG import and export terminals may have sizes of up to about 160,000 m ³ , or even 200,000 m ³ , which may necessitate a tank diameter of about 70 m and a tank height of about 60 m.

For large LNG tanks, mainly two types of constructions are used. The first type of construction is a cylindrical, self-standing tank of which the inner tank is made of a suitable steel and the outer tank is built of steel or reinforced/prestressed concrete. The second type of construction is a membrane tank in which a thin metal membrane, for instance of a thickness of 1.2 mm, is installed in a cylindrical concrete structure which is built either below or above the ground level. An insulating layer is positioned between the metallic membrane of stainless steel and the load-bearing concrete structure.

Patent publication NO 314,814 teaches a tank for storage of cryogenic fluids, comprising a tank (11) having a bottom part (12), a vertical wall part (14), and preferably an upper delimitation (15), which tank (11) is equipped with a fluid tight barrier (26) preventing the stored fluids from seeping out from the tank (11), said fluid tight barrier (26) being preferably made of thin, joined metal plates; said tank being distinguished by said vertical wall part (14) comprising an inner structurally bearing part (24), an outer

structurally bearing part (25), and said fluid tight barrier (26) being arranged between said inner (24) and outer (25) structurally bearing parts, said structurally bearing wall parts (24, 25) constituting, together with the intermediate fluid tight barrier (26), a compact, structurally integrated and fluid tight wall part (14). The inner structurally bearing part (24) is made of multi-axially prestressed concrete, as the outer structurally bearing part (25). The fluid tight barrier (26) is made of a ductile material, such as Ni steel.

The tank according to NO 314,814 is provided with a separate outer tank (16) adapted for collecting leakages. However, the outer tank is sensitive to damages caused by natural disasters such as earthquakes, a problem needed to be solved. There is a demand for an LNG tank which is more secure in case of an earthquake or in other situations which may cause high stress to the tank.

Summary of the invention

The above-mentioned demand is met by providing a tank for storage of LNG and other cryogenic fluids, comprising an inner tank having a bottom, and an inner side wall extending upwards to an optional insulating roof, an outer tank having a bottom, an outer side wall extending upwards around the inner tank to a level above said inner tank, an outer gas-tight lining on or in the outer side wall, and an outer roof construction above said inner tank, insulating material between said inner tank and said outer tank, and at least one feedthrough (5) for filling and draining, said tank being distinguished by comprising at least one earthquake support arranged between the side walls of the outer tank and the inner tank.

The tank advantageously comprises two sets of six earthquake supports between the side walls of the outer tank and the inner tank, one set arranged at a low level and one set at a higher level, arranged symmetrically around the periphery, said earthquake supports being made of wood, and pitch in a hot state has been filled into and has solidified sealingly in intermediate spaces between side walls and earthquake supports after filling of the inner tank. Each of the earthquake supports is advantageously wedge-shaped and/or has a through-going aperture for easy filling of insulating material (for instance perlite) tightly around the earthquake supports.

The invention also provides a more general tank for storage or treatment of fluids, comprising an inner tank having a side wall, and an outer tank having a side wall, distinguished by said tank comprising at least one earthquake support between the outer tank and the inner tank, preferably arranged between the side walls of the tanks.

Preferably, a number of earthquake supports are arranged between the side walls, and optionally also between the roof constructions, so as to afford improved support against impacts induced by an earthquake. The earthquake supports are preferably made of solid

or laminated wood, due to good low temperature properties and simple implementation. The earthquake supports are preferably ring constructions, having openings for filling of insulating material between the side walls. Between the earthquake support and the outer side wall, a system of filling lines for filling of hot pitch or other suitable filling material is advantageously arranged. The above-mentioned advantageous features can also be implemented for the more general tank.

Drawings

The invention is illustrated by four drawings, of which: Figure 1 illustrates a tank according to the invention, in half section,

Figure 2 illustrates the transition between inner tank, roof and outer tank for the tank of Figure 1,

Figure 3 illustrates the transition between the side wall and bottom of the tank for the tank of Figure 1, and Figure 4 is a more detailed illustration of the earthquake support arranged in the tank illustrated in Figure 1.

Detailed description

Reference is made to Figure 1, illustrating a tank 1 according to the invention for storage of LNG or other cryogenic fluids. The tank 1 comprises an inner tank 2 having a bottom 2a, inner side walls 2b extending upwards and consisting of an inner concrete wall 2c, an intermediate low temperature ductile tight metal lining 2d, an outer wall 2e of prestressed concrete, and an insulating roof 2f.

The tank 1 further comprises an outer tank 3 having a bottom 3 a, an outer side wall 3b extending upwards around the inner tank 2 to a level above said inner tank, an outer gas tight steel lining 3 c on the inner side of the outer wall, and an outer roof construction 3d above the inner tank. An insulating material 4 is arranged between the inner tank 2 and the outer tank 3. Also, at least one feedthrough 5 is arranged for filling and emptying, which feedthrough is not shown in detail, but consists of inlet and outlet pipes, pumps and cables. Means for access of personnel are provided at the feedthrough locality.

In the tank of the present invention side walls of concrete 2c, 2e, 3b are preferably slip formed. Slip forming is advantageous because the casting time can be considerably reduced as compared to casting using climbing formwork. The tank of the present invention comprises at least one earthquake support 6 between the outer wall and the inner wall. Earthquake supports are illustrated in Figures 1, 2, 3, and 4. The earthquake supports are advantageously arranged as two sets of earthquake supports, each set having six earthquake supports, one set of earthquake supports being arranged at a low level and one set at a higher level, preferably symmetrically

arranged around the periphery of the tank, as indicated in Figure 1. The earthquake supports advantageously comprise pitch having been introduced in a hot state into the intermediate spaces between the side walls and the earthquake supports and having been solidified so as to provide support after filling of the inner tank, upon temperature contraction of the inner tank. The pitch may advantageously be melted for draining if needed. Thus, filling lines are provided and preferably also means for melting, such as heating cables, as well as a line for draining of the pitch. The earthquake support per se may advantageously be a wood construction arranged between the inner wall and the outer wall, having means for filling and optionally means for melting of the pitch and draining thereof. The earthquake support is illustrated in more detail in Figure 4, where a filling line 6a is shown. The filling line is also shown in Figure 2. Other construction materials and filling materials may also be useful. The pitch is shown as a dark mass 6b in the figures, which is seen quite clearly in Figure 4. The pitch may fill a part or all of the earthquake support 6. As an alternative, the earthquake support may be arranged as one, two or more ring constructions extending supportingly around the inner tank in the intermediate space between the inner tank and the outer tank. For said embodiment sufficient openings are needed for a tight filling of insulating material, preferably perlite, between the side walls and against the earthquake supports. A further reference is made to Figure 4, wherein sections of alternative embodiments of the earthquake support are shown, more specifically a wedge-shaped earthquake support, and an embodiment having feedthrough(s) for facilitating a tight filling of insulating material between the side walls and around the earthquake supports.