Method for producing a large arcuate tank and a tank produced by the method.

(13) B

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(di) EXPLANATORY STATEMENT (19) NO (11) 178656 (si) Int Cl⁶ B21 D 11/20, 51/08

NORWAY

The Board of Industrial Justice (21) Application no. 931723 (22) Starting day 12.05.93 (24) Running day 12.05.93 (41) Alm. avail. 15.11.93 (44) Exposure date 29.01.96 (86) Int. Date of entry and application number (85) Continuation day (30) Priority 14.05.92, FI, 922191

<td>(71) Patent applicant (72) Inventor</td><td>Kvaemer Masa-Yards OY, Munkkisaarenkatu 1, SF-00150 Helsinki, Fl Jari Anttila, Turku, Fl Jukka Gustafsson, Mynåmaki, Fl Matti Heinakari, Turku, Fl Jukka Linja, Merimasku, Fl Matti Vaihinen, Turku, Fl</td>

<td>(74) Agent</td><td>Oslo Patentkontor AS, Oslo</td>

(54) Designation Method for the preparation of a spherical LNG tank (56) Published publications NO B 152324, DE B2 2240949, DE Al 3124514, US 3938363 (57) Summary A large spherical tank is manufactured by welding commercially available large planes metal plates (1a, 1b, 1c) to form a composite flat plate blank (1), to cut the composite flat plate blank into a shape which can be adapted to the desired spherical tank surface, and then molding the sheet metal into the required curved shape, preferably by hot forming.

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The present invention relates to a method for producing a large spherical LNG tank and a tank made according to the method. In the present invention, the term curved is intended to encompass the shape of the whole or any portion of the surface of a sphere.

The liquid natural gas (LNG) temperature is approx. -163 ° C. This entails special requirements for the choice of material in a tank where LNG is stored, for the structure of the tank and for the method of making the tank. Furthermore, a tank containing LNG must be self-supporting to minimize heat transfer to the tank contents. The cross section (eg diameter) of a large spherical LNG tank is approx. 40 m. A tank suitable for transporting and storing LNG is usually also suitable for transporting and storing other fluids, provided that the pressure inside the tank is not too high. Because the use of tanks for transport and storage of LNG imposes stricter requirements, the invention is described below with reference to the requirements explicitly made by LNG, but this does not preclude the use of the invention for tanks for other suitable fluid contents.

An LNG tank is suitably made of aluminum sheets because the extremely low temperature does not adversely affect the strength of aluminum. Alternatively, however, special steel alloys can be used for tank plates, but this is considerably more expensive, and molding a steel plate into curved form is more difficult than forming an aluminum plate into curved form.

Any point on a spherical surface can be arbitrarily referred to as a pole. When the radius of curvature of the spherical surface is known, it is possible to define longitude and latitude lines of the spherical surface with respect to the pole.

Flat, rectangular plates suitable for use in the construction of spherical tanks are commercially available from various sources. The largest of such plates available from a particular source will here be referred to as a standard metal plate for convenience.¹. Such a standard metal plate will be made by rolling to provide a uniform piece having substantially homogeneous composition. Even the largest commercially available standard metal plate suitable for the construction of a spherical tank is of very small size relative to the surface area of ​​a large spherical tank. Accordingly, about 100 such standard metal plates are typically required to produce a large spherical tank.

Typically, a large spherical tank is made of commercially available standard metal plates by cutting each plate to the desired peripheral shape to form a plate blank, bending the sheet blank into curved shape, and welding the curved plate blank butt-in-butt. This procedure is very demanding because it is difficult to ensure that the bent plate blanks have the correct curved shape, and deviations from the determined curved shape affect the welding procedure. Furthermore, handling is considerably more difficult when it comes to a curved workpiece than a flat workpiece. Even more important, however, is the fact that it is difficult to weld curved plates, and the smaller the plate blanks, the more welding sons there must be between the curved plates.

US-A-3 938 363 discloses a method of forming a plate into a curved shape using a mold comprising a lower convex matrix and an upper concave matrix. According to this method, a sheet of aluminum alloy is heated to a temperature of approx. 500 ° C and placed over the lower matrix. The upper matrix is ​​lowered to the hot aluminum plate and the weight of the upper matrix causes the plate to form to the desired curvature.

The lower matrix shown in US-A-3,938,363 is constructed from a steel plate framework which limits rectangular cells and the cells are filled with a refractory mass. The upper surface of the refractory mass which fills the cells of the lower matrix is ​​curved, the upper surface of the refractory being about 5 cm above the upper edges of the steel plates which confine the cells. The concave matrix is ​​of the same mainly cell-filled construction as the convex matrix and is made by using the convex matrix as its shape.

One object of the invention is to reduce the number of operations for handling curved plates in the assembly of large spherical tanks.

What constitutes the invention in its broadest aspect will be apparent from claim 1.

Preferably, some or all of the largest available standard metal sheets are welded together in planar form to form a significantly larger composite sheet. Conventional technique can be used for welding the plates (or plate parts). The area of ​​the composite plate is several, preferably at least three times the area of ​​a large standard metal plate. If the composite plate does not have the required shape after welding, the composite plate is cut to form a large plate blank whose peripheral shape is such that once bent to curved shape it will conform to the plate pattern selected for the spherical tank without further cutting. Eg. For example, the planar plate blank can be manufactured or cut so that its edges will define longitude and latitude lines in the final spherical tank. In this way, the plate blank is arranged to facilitate the construction of a spherical tank. Only after the welding of sub-plates to form the large plate blank is the latter bent into curved form, after which it can be used as part of a spherical tank without further processing. In this way, the number and length of weld seams required to weld together curved workpieces will be significantly reduced, greatly reducing the production cost of a spherical tank.

If the large plate blank manufactured in the first step is designed so that the length and width are substantially equal, a particularly suitable production method is provided. The result is, of course, dependent on the dimensions of the standard metal plates, so essentially equal will also include a difference between length and width of several meters. It has been found to be particularly convenient if the large flat plate blank assembled by welding has a size of approx. 100 m². Obviously, the purpose is to produce as large a flat plate blank as possible, but if the size of the blank is considerably larger than 100 m², bending this to a curved shape will involve disproportionately high costs.

Before the large flat plate blank is bent into curved shape, edge bevels may be provided to facilitate a later welding phase, such edge forming being easier to perform on a plane than on a curved plate blank.

The molding of the flat plate blank into curved form is suitably carried out in the case of the preferred aluminum sheets by hot molding at a temperature in the range of 350 ° C to 460 ° C, and most conveniently the molding temperature is in the range of 400 ° C. 430 ° C. In the latter of these temperature ranges, an aluminum plate suitable for the manufacture of a spherical tank can be bent into curved form in a relatively simple device.

The hot forming may be carried out by using an oven which encloses the large flat plate blank and its forming device. The oven is suitably positioned by lowering it down over the molding device. When the plate blank has reached the desired temperature in the furnace room, it must be kept constant under molding pressure for about one hour, preferably for about two hours. In this way, an efficient molding is obtained and the stresses caused by the molding are smoothed.

One form of exerting forming pressure on the large planar plate blank may be formed by convex and concave matrices which serve as forming tools between which the plate blank is formed into a curved shape. Each matrix may consist of plates arranged on the upright to form an open grid, where the edges of the plates forming the grid determine the curved shape of the matrix. Preferably, each plate of the convex matrix and a corresponding plate in the concave matrix are made by cutting a curved slit into a single large plate. The width of the slot must at least substantially correspond to the thickness of the sheet blanks to be bent using the mold. The slit in each slab can be interrupted by short bridges holding the two parts of the slab together at respective opposite sides of the slit. Two groups of plates can be used, one group to be used as longitudinal plates in the two grids and one group to be used as transverse plates in the two grids. The distance between the bridges in the longitudinal plates is suitably between 1 and 2 meters. The distance between the transverse plates is appropriate so that two bridges will lie between two adjacent plates when the grid is assembled. The longitudinal plates are suitably used as they are to form the grid, but the transverse plates are preferably cut to fit as transverse inserts in the grid, each with two bridges in the curved slot. The slit in each slab is special and each slit has its own special radius of curvature which is necessary to give the matrices the required curved shape. The bridges can be quite short, each with a length of approx. 3 cm.

The longitudinal and transverse plates are assembled to form a lattice and are welded together at each intersection of the lattice. The bridges are then cut to separate the grid structure into a grid for a convex matrix and a grid for a concave matrix. In this way, a perfect mutual fit of the two matrices is obtained, and very little sheet material becomes scraped. It is important that the device for shaping the plate blanks is not so expensive that the cost of the device represents a significant addition to the cost of the spherical tank, thus eliminating the savings obtained by reducing the length of the necessary weld seams between the curved plates.

A molding matrix produced in this way is relatively inexpensive because the desired curved shape is obtained by cutting a relatively small number of sheets along a curved curve, which is a fairly simple process. The openings of the matrix grid can be relatively large. For example, the distance between the plates can be over half a meter. In those regions of the matrix where at least one edge of the large plate blank will be located, it is advisable, at least on the curved surface of the concave matrix, to provide an additional support member which does not match the grid pattern of the matrix, but spans the spaces in the lattice, as the said at least one edge region of the plate blank may otherwise not be designed efficiently and regularly enough, but may have a slight wave shape, which is a significant disadvantage when the plate blanks with curved shape are to be joined during welding.

In general, the necessary forming force will be readily obtained by gravity at the weight of the upper matrix. If this weight were to be insufficient, additional weight could be added during the forming phase, or e.g. be able to use hydraulic devices to increase the downward force. However, the use of additional weight is a simple and inexpensive solution. If additional weight is used, it is appropriate to arrange it so that the additional weights are placed outside the oven compartment and act from there on the upper matrix. In this way, no heat energy is wasted to heat the additional weights, and in addition, the forming force can easily be controlled from the outside of the furnace room. Furthermore, because the mold and plate are heated simultaneously in the oven, it is easy to ensure that the plate has a uniform temperature when the molding force is applied. In addition, the unwanted possibility of local cooling of the plate is avoided because it is brought into contact with a relatively cold matrix.

The invention also relates to an LNG tank or the like, which is made using the methods described.

The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which: FIG. 1 schematically shows a mold and how a large plate blank to be bent into curved mold can be placed in the mold; FIG. 2 shows schematically the shape of a furnace room; FIG. 3A, 3B and 3C show the structural features of the mold; FIG. 4 shows a production line for hot forming large plate blanks for curved shape using both a forming furnace and a refrigerator; FIG. 5 is a plan view of a matrix used in the refrigerator of FIG. 4, and FIG. 6 is a section on the line VI - VI of FIG. 5.

In the drawings, 1 denotes a large composite plate blank assembled by welding three standard metal plates 1a, 1b and 1c. Plate blank 1 is shown in the drawing in elongated form, but this is only because the preferred almost rectangular shape is more difficult to view in perspective. The plate blank 1 is intended to later form part of a spherical surface and its edges 2 are therefore slightly curved. The edges 2 of the sheet metal are machined, typically chamfered, to form a suitable joint for a welding seam which will be formed in a subsequent welding operation.

Above the plate blank 1 is an upper matrix 3 having a concave lower surface, and below it is a lower matrix 4 having a convex upper surface and supported by a flat base (not shown). The upper matrix 3 is moved into position by means of a crane and during this transfer, the plate blank 1 is supported by beams 5 which are suspended in the upper matrix 3. After the forming operation, the curved plate blank 1 is lifted by the same beams. . The beams 5 are accommodated in recesses 6 in the upper surface of the lower matrix 4 so that they do not interfere with the forming of the plate blank 1.

Several guide posts 7 are arranged around the lower matrix 4 to control the upper matrix during the printing operation. Some of the posts 7 are provided with a detachable support element 8 which temporarily supports the upper matrix 3 in a first positioning step. During this step, the plate blank 1 rests on the top of the lower matrix 4 without load. Next, an oven, which is described in more detail with reference to FIG. 2, placed over the matrices 3 and 4 with a crane and the plate blank 1 is heated. When the required forming temperature is achieved evenly over the plate blank 1, the supporting elements 8 are released, whereby the weight of the upper matrix can freely act on the plate blank 1. If this weight should not be sufficient to achieve the necessary forming operation during an acceptable time , the upper matrix may be loaded with additional weight, e.g. one or more steel plates placed on load posts 9 attached to the die 3.

As shown in FIG. 1, each of the matrices 3 and 4 is made of a grid of plates, so that the respective concave and convex edges of the grid walls 13 determine the required partial spherical shape. A molding matrix constructed in this way, where the distance between the grid walls 13 is of the order of half a meter, is not very expensive despite its large dimensions. Because the matrix grating will usually not exactly match the dimensions of the plate blank, additional support elements 10, at least in the concave matrix 3, will be required to delineate at least one of the edge regions of the plate blank.

FIG. 2 shows the furnace 11 placed over the matrices 3 and 4. The furnaces may be a simple thermally insulated box-like structure provided with the necessary heating devices. The load posts 9 of the upper matrix pass through clearance openings at the top of the furnace so that any additional load weight 12 which is optionally placed on them is transmitted via the top of the furnace so that any additional weight remains outside the furnace space. Using the load posts 9, the upper matrix 3 can be raised and lowered while in the furnace compartment, which is necessary to release the support elements 8 and lower the upper matrix 3 to its forming position. Fig. 2 shows one support element 8 on one guide post 7 of the lower matrix in its released position, where it does not support the upper matrix 3.

Figures 3A, 3B and 3C show how the mold can be made of two sets of plates, longitudinal plates 20 and transverse plates 21, each provided with a curved slot 24. Each of the slots 24 is interrupted by short bridges 26 spaced apart slots. The width of each slit 24 corresponds approximately to the thickness of the sheet metal to be bent using the mold.

The transverse plates 21 are cut into transverse inserts 21a, each comprising two bridges 26 in its curved slot 24. The longitudinal plates 20 and the inserts 21a are assembled to form a grid within an outer frame defined by plates 28 which are also provided with the same type of curved slit 24. The plates 20 and the inserts 21a are fixedly welded at each of the grid's intersection points 23, and the bridges are then cut so that the grid is divided into two parts which form the basis for respectively the concave and the convex matrix.

In the production line shown in FIG. 4, a separate cooling furnace 30 is arranged in line with a forming furnace 11 of substantially the same type as shown in FIG. 2. The two ovens are stationary and each is provided with two sliding doors 34 at respective opposite ends. Two concave upper matrices 3a, 3b are arranged respectively. in the forming furnace 11 and the cooling furnace 30. The corresponding convex matrices 4a, 4b are mounted on respective transport carts 32a and 32b, each connected by a respective drive cable running in a loop from one of two coil drums 33 over a disk (not shown) and back to the respective drum 33. Each carriage is driven backwards and forwards into and out of the furnace (s) by means of the respective winding drum. Each furnace is provided with a device for raising and lowering the concave matrix and for raising and lowering the plate blank relative to the convex matrix. Both matrices are approx. 12 x 9 m, in plan, and the grid plates have an increase of approx. 60 cm.

In operating the production line of FIG. 4, the first flat plate blank is placed on the convex matrix 4a by means of the carriage 32a and the die 4a and the plate blank are moved into the furnace 11. The plate blank is bent into a spherical shape in the manner described with reference to FIG. . 1 and 2, the concave matrix 3a is raised and the shaped plate is lifted from the convex matrix 4a using support beams, as described in connection with FIG. 1 and 2. The carriage 32a with the matrix 4a then returns to its original position and the carriage 32b with the matrix 4b, which is similar to the matrix 4a, takes up space in the furnace 11. The shaped plate blank is lowered to the convex matrix 4b and the carriage 32b carries the matrix 4b and the formed plate blank into refrigerator 30 where plate blank is pressed between concave matrix 3b and convex matrix 4b under controlled cooling for about two hours. The concave matrix 3b is then raised and the carriage 32b transports the convex matrix 4b and the cooled shaped plate blank from the refrigerator 30. During cooling of the first plate blank in the refrigerator, a second plate blank is bent into curved shape in the forming furnace 11. using the matrices 3a and 4a.

Air supply pipes 36a, 36b and 36c are installed in one of the walls of the refrigerator 30, and air is supplied to these pipes by means of fans (not shown) via controllable throttles 46a, 46b and 46c. The air supply pipes have a diameter of 250 mm and the air flow through each air supply pipe is approx. 1 m³/ Sec. When the carriage 32b is placed in the furnace 30, the tubes 36a, 36b and 36c are provided with respective extension tubes 48a, 48b and 48c (diameter 250 mm) extending through channels formed in the die 4b by openings 38 in the grid plates. The pipes 48a, 48b and 48c are connected to additional air distribution pipes 36d with a diameter of 200 mm and then 125 mm. Each tube 36d extends substantially horizontally and passes through at least one cell of the matrix 4b, and is provided with a vertical outlet tube 36e (50 mm diameter) in each cell through which it passes, as shown in FIG. 6.

The outlet pipes 36e extend below the shaped plate, and each is provided at its upper end with a spreading means 44 for distributing the air flow leaving the outlet tube. Air flows out of the lower matrix 4b through the openings 38 and is released into the atmosphere.

The three pipe systems connected to the pipes respectively. 36a, 36b and 36c are separate and can be controlled separately. Arrows 42 show the direction of air flow.

By controlled cooling is meant that the cooling is regulated depending on the temperature of the plate blank. Thus, temperature sensors are provided to continuously measure the temperature of the plate at selected measurement points 40, and at each measurement point 40, the temperature is measured separately on each of the opposite sides of the plate 1. The operation of the fans for supplying air to the lower matrix is ​​controlled depending on the set temperature values, so that the temperature at each measurement point follows a selected time function during the cooling operation. Typically, three double-sided temperature measurement points are sufficient, one in the central region of the plate and one at each of two diagonally opposite corners, as shown by reference numerals 40 in FIG. 5.

The production line of FIG. 4 has the advantage that the forming furnace 11 and the die 3a are not cooled when the plate blank is cooled, and consequently energy is saved for heating the furnace 11 and the die 3a. Keeping the workpiece in the correct sub-spherical form under controlled cooling ensures that the workpiece continues to be in the proper shape as the holding power is removed.

The invention is not limited to the whole tank being spherical, but can e.g. is used for a tank composed of two hemispherical portions connected by a cylindrical portion.