Receptacles – For cryogenic content
Reexamination Certificate
1999-02-24
2004-05-11
Newhouse, Nathan J. (Department: 3727)
Receptacles
For cryogenic content
C220S653000
Reexamination Certificate
active
06732881
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention relates to liquefied gas storage tanks and in one aspect relates to a tank especially adapted for storing cryogenic liquefied gases (e.g., liquefied natural gas (“LNG”)) at cryogenic temperatures at near atmospheric pressures in areas susceptible to earthquake activity.
2. Background
Liquefied natural gas (LNG) is typically stored in double walled tanks or containers. The inner tank provides the primary containment for the LNG while the outer shell holds the insulation in place and protects the inner tank and the insulation from the adverse effects of the environment. Sometimes, the outer tank is also designed to provide a secondary containment of LNG and associated gas vapor in case the inner tank fails. Typical sizes of onshore tanks at import or export terminals range from 50,000 to 100,000 cubic meters although tanks as large as 200,000 cubic meters have been built or are under construction.
Two distinct types of tank construction are widely used for storing LNG at onshore locations. The first of these comprise a flat-bottomed, cylindrical, self-standing tank which typically uses a 9% nickel steel for the inner tank and carbon steel, 9% nickel steel, or reinforced/prestressed concrete for the outer shell. The second type is a membrane tank wherein a thin (e.g. 1.2 mm thick) metallic membrane is installed within a cylindrical concrete structure which, in turn, is built either below or above grade on the ground. A layer of insulation is interposed between the stainless steel or Invar membrane and the load bearing concrete cylindrical walls and flat floor.
Recently, radical changes have been proposed in the construction of LNG terminals, especially import terminals. One such proposal involves the building of the terminal a short distance offshore where the LNG will be off-loaded from a transport vessel, stored, retrieved, and regasified before it is piped to shore for sale or use. Possibly one of the more promising of this type of terminals is one where the LNG storage tanks and regasification equipment will be installed on gravity base, box-shaped, barge-like structures (GBS) similar to certain concrete gravity structures now installed on the seafloor and being used as platforms for producing petroleum in the Gulf of Mexico.
Unfortunately, neither cylindrical tanks nor membrane tanks are considered as being particularly attractive for use in storing LNG on GBS terminals. Cylindrical tanks take up too much room on the GBS in relation to the volume of LNG which can be stored therein and are difficult and expensive to construct on such. Further the size of such tanks must be limited (e.g. 50,000 cubic meters) so that the GBS structures can be fabricated economically with readily available fabrication facilities. This necessitates a multiplicity of storage units to satisfy particular storage requirements which is not desirable from cost and operational safety considerations.
A membrane-type tank system, on the other hand, can be built inside the GBS to provide a relatively large storage volume. However, a membrane-type tank requires a sequential construction schedule wherein the outer concrete structure has to be completely built before the insulation and the membrane can be installed within a cavity within the outer structure. This normally requires a long construction period which adds substantially to the costs. Further, membrane-type tanks are designed by principles known as “experimental design” wherein the guarantee of satisfactory performance of a particular tank and its safety are based on historical experience and laboratory studies rather than on rigorous demonstration by analysis and quantified experience. Where new shapes and sizes are required or when different environmental and/or seismic loading conditions are to be encountered, the satisfactory performance of membrane-type tanks at various LNG levels is difficult to insure.
Accordingly, a tank system is needed for near offshore storage of LNG which alleviates the above-discussed disadvantages of both cylindrical tanks and membrane-type tanks. Such a tank is a polygonal-shaped, box-like, structure which can be fitted into a space within a steel or concrete GBS and which is capable of storing large volumes (e.g. 100,000 cubic meters and larger) of LNG at cryogenic temperatures. The tank should also perform safely at various LNG levels in areas where seismic activity (e.g. earthquakes) is encountered and where such activity may induce liquid sloshing and associated dynamic loads within the tank.
Similar box-shaped, polygonal tanks have been used for storing LNG aboard sea-going, transport vessels. One such tank, popularly known as the “Conch” tank, (e.g. see U.S Pat. No. 2,982,441) has been built from 9% nickel steel or aluminum alloys. In its original design as proposed by the above referenced patent, the tank is constructed of six plate panels (i.e. the four sides, the top or roof, and the bottom or floor of the tank) which are reinforced or “stiffened” only by horizontal beams and stiffeners or the like. According to the inventors, vertical stiffening is deliberately omitted in order to eliminate or reduce thermal stresses due to thermal gradients in the vertical direction as the volume of LNG in tank changes.
In the “Conch” tank, horizontal tie rods may be provided (a) at the corners at the vertical interfaces of the walls to strengthen the corners; and/or (b) as connections between the opposite faces of the walls to lessen the panels deflections. Nonetheless, horizontally-stiffened wall panels and two-way stiffened floor and roof plate panels, as embodied in the above referenced patent, basically provide the structural strength and stability for the tank. The original tanks built with this concept are reported to be less than 10,000 cubic meter in capacity.
When the Conch design (as illustrated in U.S. Pat. No. 2,982,441) is extended to larger tanks, a design similar to
FIG. 1
can be expected (i.e. a known, prior-art, prismatic tank developed by IHI Co., Inc. of Tokyo, Japan). Modern materials and design methods do not restrict provision of vertical stiffening by consideration of thermal gradient as the liquid level of LNG changes. Consequently, the illustrated prismatic tank consists of wall plate panels that are stiffened by both horizontal and vertical beams/stiffeners. But even for a relatively small size of 23,500 cubic meters, to achieve satisfactory strength and stiffness during construction handling and operational use, the “Conch” tank must be provided with intermediate stiffened panel bulkheads and diaphragms, as illustrated by a vertical bulkhead in each of the length and width directions of the IHI tank. This type of design is believed to be good only for tanks having a relatively small storage capacity.
A larger tank suitable for use on a modern terminal and designed in accordance with the prior art would need still more bulkheads to support the roof structure and to provide structural strength and stability of the tank in operational use (e.g. see FIG.
2
). Accordingly, a typical large storage tank might in effect be considered as consisting of several of the smaller Conch-type tanks aligned wherein a common wall between adjacent tanks forms a horizontal or transverse bulkhead within the overall storage volume of the complete storage system.
For applications on ships and other transport vessels, the bulkheads within the tanks not only provide strength and stability for a relatively large, storage tank but also reduce the dynamic loads on the tank due to any sloshing of the LNG within the tank caused by movement of the floating vessel during transport. The dynamic excitation of the storage tank due to the oscillatory motion of the ship caused by wind and wave action, has relatively large periods (e.g. 6-12 seconds). Fundamental periods of liquid sloshing within small cells created by bulkheads within the tank are relatively small thus avoiding resonance and amplification of sloshing loads. While the bulkhead construction makes such tanks suit
Faulconer Drude
Merek Joseph C
Mobil Oil Corporation
Newhouse Nathan J.
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