EP2064024A1

EP2064024A1 - Method of welding sheet-like components together and welding jig to be used in the method

Info

Publication number: EP2064024A1
Application number: EP07803726A
Authority: EP
Inventors: Pekka Kallio; Antero Tanska; Kalle Pohjola
Original assignee: Rannikon Konetekniikka Oy
Current assignee: Rannikon Konetekniikka Oy
Priority date: 2006-08-29
Filing date: 2007-08-29
Publication date: 2009-06-03
Also published as: WO2008025882A1; FI20065535A; UA94766C2; RU2420381C2; FI20065535A0; FI120530B; RU2009109835A

Abstract

The invention relates a method of welding together structures, consisting of sheet-like components such as bottom plates (100) and side plates (102) of casing-like structures, which are to be welded to the bottom plates. In the invention, the bottom plate and the side plates to be welded to the bottom plates are fixed, for the welding, to a separate welding jig that is made for this purpose and that is rotatable about a longitudinal rotational axis. The sheet-like components are fixed to the welding jig in such a way that their common center-of-gravity axis lies as close to the rotational axis of the welding jig as possible. Preferably, the sheet-like components are disposed substantially symmetrically relative to the rotational axis of the welding jig. In a preferred embodiment of the invention, at least two bottom plates and side plates to be welded to these bottom plates are fixed to the welding jig at a time. The bottom plates are removably fixed on opposite sides of the rotational axis of the welding jig by means of first support members (20) and the side plates are removably fixed to the welding jig by means of second and third support members (22, 24). The welding jig has heating resistors to heat the components to be welded together to a desired temperature, and it can be bearingly supported on end supports at its ends in order to rotate the welding jig about the rotational axis during the welding. In the method according to the invention, mechanized welding methods, such as submerged arc welding, are preferably used in the welding together of the bottom plates and side plates. The method according to the invention is especially suitable for welding together sheet-like components of demanding structures, such as off-shore structures.

Description

Method of welding sheet-like components together and welding jig to be used in the method

The invention relates to a method of welding sheet-like components together, in which method the sheet-like components are fixed to a welding jig for the welding, which welding jig is rotatable about a longitudinal rotational axis during the welding. The invention also relates to a welding jig to be used in the method.

In heavily loaded steel structures it is common to use casing-like structures constructed from sheet-like components. The casing structures often have a substantially rectangular cross-section, i.e. they have a bottom plate, a cover plate parallel to the bottom plate as well as substantially parallel or symmetrically inclined side plates connecting the edges of the bottom plate and the cover plate together. The bottom plate, cover plate and side plates are elongated components connected together at their edges by means of an internal and/or external weld seam. The plates can be of the same or a different steel grade and of a different thickness. Inside the casing structures evenly spaced sheet-like bulkheads are often mounted and welded to the inner surface of the bottom plate or side plates at their edges. The cover plate of the casing structure is joined to the edges of the side plates by means of an external weld seam.

Many problems are related to the assembly-welding of casing-like structures. The heavy sheet-like components must be supported firmly for the welding in order to keep them steady during the welding. In the welding of the side plates a tensile stress is generated in the weld seam, tending to bend the plates towards each other, i.e. the angle between the plates becomes smaller. The tensile force of the weld seam increases with the dimensions of the seam. The components must therefore be supported very firmly when welding plates having a thickness of several centimeters. Frequently the sheet-like components of casing-like structures need to be welded at an elevated temperature, such as at +70 degrees Celsius. To heat the components of the casing-like structure to a desired temperature and to keep them at this temperature throughout the welding, separate heating resistors are usually arranged at the components. Installing the heating resistors and bringing them into an operable condition slows down the welding step, in addition to which the separate resistors and the wires therefrom easily suffer damages as the heavy plate components are moved. It is slow and cost-increasing to support the components to be assembly-welded for the welding and to release them when proceeding to the next processing step and/or work station.

In order to prevent the side plates from bending inwards, the casing structures are usually assembly-welded by first binding the side plates to bottom plate, into the right places, by means of small bond welds and by then putting the bulkheads into the right places between the side parts. The immobility of the bulkheads is ensured by small weld seams. Thereafter the actual welding of the side parts is performed wherein the bulkheads prevent the side plates from bending inwards. The weakness of this method is that all the internal welding of the side plates must be done manually because the stationary bulkheads disturb the use of mechanized welding methods, such as submerged arc welding and electron beam welding. As manual welding is slow, the labor costs are high and the total manufacturing time of the structure is prolonged considerably. Besides, manual welding easily causes work errors and deviations from quality, thus making the quality control harder and making it more difficult to keep to the fixed quality requirements. It is also very uncomfortable to weld components with a high temperature by hand.

US 2 240 739 describes a manipulator by means of which it is possible to support large structures for welding. The manipulator comprises a pit having pedestals placed at the opposite edges thereof for supporting a cradle across the pit. The cradle has two parallel beams and connectors connecting the beams together at their ends. The connectors are rotatably fixed to the pedestals so as to allow the cradle to rotate about the longitudinal axis of the cradle . The beams support a ring turn-table to which the article to be welded is fixed. The weakness of this solution is that it only has one mounting substrate to receive the articles to be welded. Besides, in this solution, the articles to be welded that are mounted to the turntable are not symmetrically disposed relative to the rotational axis of the cradle. Owing to the asymmetrical load, the cradle and the pedestals must have a robust support arrangement between themselves. Furthermore, a great deal of force is needed to rotate the cradle about the longitudinal axis and to retain the cradle in a suitable welding position.

The object of the invention is to provide a method of welding casing-like structures and a jig to be used in the method by means of which the disadvantages and drawbacks related to the known welding methods can be reduced significantly.

The objects of the invention are achieved by a method and a welding jig which are characterized by what is disclosed in the independent claims. In the dependent claims a few preferable embodiments of the invention are disclosed.

The invention relates to a method of welding a bottom plate and side plates, which are to be secured to the bottom plate, of structures consisting of sheet-like components. In the method, the components to be welded together are supported firmly into place before the welding, the actual welding being performed thereafter. The components are supported in order to reliably keep the components in their intended positions throughout the welding. The components need to be supported firmly enough so as to be able to take the stresses, such as heat stresses, that are generated in the components to be welded together during the welding and the tensions that are generated in the weld seam and that tend to incline the side plates. In the invention, the bottom plates and the side plates to be welded to the bottom plate are fixed to a separate welding jig for the welding, the welding jig being made for this purpose and being rotatable about a longitudinal rotational axis during the welding. The sheet-like components are fixed to the welding jig in such a way that their common center-of-gravity axis lies as close to the rotational axis of jig as possible. Preferably, the sheet-like components lie substantially symmetrically relative to the rotational axis of the welding jig.

In a preferred embodiment of the method according to the invention, at least one bottom plate is fixed to the welding jig and at least two side plates are welded to bottom plate, to opposite sides thereof. In a second preferred embodiment of the method according to the invention, at least two bottom plates and the side plates to be welded to these bottom plates are fixed to the welding jig at a time. The fixing of the plates is performed by means of removable fixing members forming part of the welding jig. Preferably, at least one first bottom plate is fixed to the welding jig, on a first side of the rotational axis, and at least one second bottom plate is fixed to the welding jig, on a second of the rotational axis.

In a third preferred embodiment of the method according to the invention, the bottom plates are fixed to welding jig by means of removably fixable first support members and the side plates are fixed to the welding jig by means of removably fixable second and third support members. Preferably, the side plates to be welded to the first bottom plate and the side plates to be welded to the second bottom plate, which first and second bottom plates are fixed on different sides of the rotational axis of the welding jig, are supported and fixed into place by means of the same second fixing members.

In a fourth preferred embodiment of the method according to the invention, the components fixed to the welding jig are heated to a desired temperature by heating resistors provided in the welding jig.

In a fifth preferred embodiment of the method according to the invention, the welding jig is rotated about the rotational axis during the welding, in order to weld the bottom plates and side plates fixed on different sides of the rotational axis.

In a sixth preferred embodiment of the method according to the invention, the bottom plates and side plates are welded together using an at least partly mechanized welding method, such as submerged arc welding.

In a seventh preferred embodiment of the method according to the invention, components of casing-like structures consisting of a bottom plate and two side plates to be welded to the edges of the bottom plate are welded together by the method. Preferably, said structures are oil-rig leg modules and the method is used for welding the internal weld seams of the casing-like structure. The welding jig to be used in the method according to the invention comprises a first end part and a second end part as well as body pipes connecting the end parts together. The welding jig is supported at its ends in such a way that it is rotatable about a longitudinal rotational axis during the welding. The welding jig comprises at least one fixing plane for fixing the sheet-like components to welding jig in such a way that the center-of-gravity axis of the sheet-like components lies as close to the rotational axis of the welding jig as possible. Preferably, the sheet- like components can be fixed to the welding jig in substantially symmetrical positions relative to the rotational axis of the welding jig.

In a preferred embodiment of the welding jig according to the invention, there are two adjacent, substantially parallel body pipes, the rotational axis being located between the body pipes.

In a second preferred embodiment of the welding jig according to the invention, there is at least one fixing plane in order to fix the bottom plates in such a way that the bottom plate can be fixed between the body pipes. Preferably, the welding jig also comprises support members for removably fixing the bottom plates to the end parts of the welding jig.

In a third preferred embodiment of the welding jig according to the invention, there are at least two fixing planes for simultaneously fixing at least two bottom plates, wherein the first fixing plane for fixing the first bottom plate is located on a first side of the plane defined by the body pipes and wherein the second fixing plane for fixing the second bottom plate is located on a second side of the plane defined by the body pipes. Preferably, the jig also comprises removable support members for removably fixing the bottom plate, and the side plates to be welded to the bottom plates, to the welding jig.

In a fourth preferred embodiment of the welding jig according to the invention, said removable support members comprise first support members to be removably fixed to the body pipes for the purpose of fixing the bottom plates, as well as second support members to be removably fixed to the body pipes and third support members to be removably fixed to the second support members, for the purpose of fixing the side plates. Preferably, said second support members are substantially symmetrical members having a first side for supporting and fixing a side plate to be welded to a first bottom plate and a second side for supporting and fixing a side plate to be welded to a second bottom plate, which first and second bottom plates are arranged to be fixed on fixing planes extending on different sides of the rotational axis of the welding jig.

In a fifth preferred embodiment of the welding jig according to the invention, heating resistors are arranged inside the body pipes to heat the components fixed to the welding jig to a desired temperature.

In a sixth preferred embodiment of the welding jig according to the invention, the end parts of the welding jig have a bearingly functioning support element for mounting the welding jig on separate end supports in a way that enables rotation about the longitudinal axis.

The method and welding jig according to the invention have the advantage that they enable the use of mechanized welding methods, such as submerged arc welding, even in the welding of the internal weld seams of a casing-like structure. By using mechanized welding methods it is possible to shorten the manufacturing time of casing-like structures, obtain labor cost savings and make a weld seam of uniform and high quality.

The invention also has the advantage that it makes it possible to support the sheet-like components of the casing-like structure for the welding in a faster and more simply way. The components of the casing-like structure stay supported by the same welding jig during several welding steps, and the welding jig can be moved from one work station to another without releasing the components.

Another advantage of the invention lies in that it makes it possible to simultaneously fix components of more than one casing-like structure to the same welding jig, thus increasing the number of welding operations that can be performed at one work station at a time. At the same, the number of welding jigs needed in the welding of the plurality of casing-like structures is kept small. The welding jig according to the invention has the advantage that it is structurally light and that only a little force is needed to rotate and support it into place during the welding. In use, the welding jig is supported by simply setting it to rest freely on the end supports at its ends which makes it easy to move it from work station to another.

An embodiment of the welding jig has the special advantage that it makes it possible to heat those components of the casing structure that are fixed to welding jig to a desired temperature and to maintain them at the desired temperature without separate heaters.

In the following, the invention is explained in detail. In the description reference is made to the appended drawings in which

Figure 1 shows, by way of example, a welding jig to be used in the method according to the invention, in a side view obliquely from above,

Figure 2a is an enlarged view of a part of the welding jig shown in Figure 1 ,

Figure 2b is a cross-sectional view of the welding jig shown in Figure 1 ,

Figure 3 shows, by way of example, a welding jig according to the invention supported on an end support, in an end view,

Figure 4 shows, by way of example, a preferred embodiment of the welding jig according to the invention, in a side view obliquely from above, and

Figure 5 is a side and exploded view of the welding jig shown in Figure 4.

Figure 1 shows, by way of example, a welding jig to be used in the method according to the invention, in a side view obliquely from above. The welding jig is an elongated structure having two end parts and two adjacent and parallel body pipes 8 connecting the end parts together. The end parts at the ends of the welding jig are of a substantially similar construction but they are mirror images of each other. The end part comprises a square-shaped end plate of steel 10 having a metal cylinder 12 welded to its first surface directed away from the middle part of the welding jig. Preferably, the size and thickness of the end plate are 850 ^* 850 mm and 20 mm, respectively. Preferably, the length, diameter and wall thickness of the cylinder are 600 mm, 800 mm and 10 mm, respectively. The metal cylinder forms a support element by which the welding jig can be supported on separate end supports in a way described below. The corners of the end plate comprise holes 14 which make it possible to grip the welding jig by a lifting device, e.g. by crane hooks. The body pipes 8 of the welding jig are attached to the end plates at their ends. The body pipes are hollow metal pipes having a preferably rectangular cross-section. Preferably, the body pipes are 100 ^* 100 mm metal pipes having a wall thickness of 10 mm. Preferably, the distance between the body pipes is 400 mm. The center line between the body pipes defines the rotational axis of the welding jig, and the longitudinal axes of the body pipes define a symmetry plane in relation to which the welding jog is symmetrical. The end parts 10 are attached to the body pipes in such a way that the center axes of the cylinders 12 lie at the rotational axis. The surfaces of the body pipes define a first fixing plane on a first side of the symmetry plane, and a second fixing plane on a second side of the symmetry plane, for fixing the sheet-like components.

The outwards directed edges of the body pipes comprise suitably spaced fixing members 16. The fixing members are shaped metallic parts having a base portion and a head portion provided with a throughhole for a locking wedge 18. The fixing members are welded to the outward directed edges of the body pipes at their base portion, their head portions being directed away from the body pipes in a parallel relationship to the symmetry plane. The welding jig also comprises first support members 20, second support members 22 and third support members 24 by means of which the bottom plate 100 and the side plates 102 of the casing structure to be assembly-welded are reliably fixed into place in the welding jig for the assembly-welding. The first, second and third support members are removably fixed to the fixing members 16 of the body pipes and to each other by means of the locking wedges 18.

Figure 2a and Figure 2b are an enlarged view of a part of the welding jig shown in Figure 1 and a cross-sectional view of the welding jig shown in Figure 1 , respectively. The Figures show a bottom plate 100 and two side plates 102 of a casing structure fixed to the welding jig as well as the first, second and third support members used in the fixing thereof. The first support members consist of two adjacent U-shaped sheet-like components connected together by bolts in such a way that a gap of the same size as the width of the head portion of the fixing member 16 is left between the sheet-like components. The base portions of the first support members have a hole for a locking wedge 18. The first support members are put into place in such a way that the edges of the bottom plates placed on both sides of the body pipes are left in an U-shaped slot in the support member and the head portion of the fixing member 16 is seated in the gap between the sheet-like components of the first support member. The first support members are locked into place by inserting a locking wedge 18 into the holes extending through the base portion of the support member and the head portion of the fixing member 16. The support member is thus locked into place and, at the same time, the edges of the bottom plate 100 on both sides of the body pipes 8 of the welding jig are left in the slot of the support member with the result that they are unable to move. If needed, a tensioning wedge 26 can be inserted between the inner edge of the slot of the support member and the surface of the bottom plate to guarantee that the bottom plate keeps steady. The first support members ensure that the bottom plate 100 is reliably fixed to the welding jig. Preferably, one first support member is placed in the proximity of each corner of the bottom plate.

The second support members 24 consist of two adjacent shaped elongated sheet- like components connected together by bolts in such a way that a gap of the same size as the width of the head portion of the fixing member 16 is left between the sheet-like components. The middle part of the second support members comprises a slot 28 having a width substantially equal to the sum of two plate thicknesses of the bottom plate and the height of a body pipe. At the slot there is a hole extending through the sheet-like components for a locking wedge 18. A second support member is fitted into place in the welding jig by setting it at the fixing member of a body pipe in such way that the head portion of the fixing member is seated between the sheet-like components of the second support member and the edges of the bottom plate are seated in the slot 28. The second support member is locked stationary by putting into place a locking wedge 18 extending through the hole in the sheet-like components and the hole in the head portion of the fixing member.

The edges of the slot 28 in the middle part of the second support member 22 form a support holding the bottom plate 100 steady, whereas the sides defined by the ends of the slot and the support member form supports immobilizing the side plates 102 of the casing structure. Thus, one support member supports side plates fixed on both sides of the symmetry plane of the welding jig. The edge faces 30 of the sides of the support member, directed toward the inner part of the welding jig, are shaped correspond to the cross-sectional shape of the mechanized side plate 102 as accurately as possible, whereby the side plate, when set against the edge face, is supported by the support member over the entire width thereof. Preferably, the shape of the edge faces is adapted to follow the shape of bevels machined in the edges of the side parts, forming a welding slot. The side plates can also be secured to the second support members in such a way that a suitable air gap is left between the edge of the side plate and the surface of the bottom plate. Besides, an outward-directed pre-inclination can be arranged ^' in the second support members to compensate for the inward-inclination of the side plate caused by the tensile stress generated in the weld seam.

The ends of a second support member also comprise third support members 24 to ensure that the edge parts of the casing-like structure do not move in their position against the edge faces 30 of the second support members. The third support members are U-shaped separate sheet-like components having a first leg to be set against a side plate 102 and a second leg to be supported on the head portion of the second support member. The head portions of the second support member comprise two throughholes for locking wedges 18. The third support members are put into place in such a way that the edge of the side plate and the locking wedges fitted into the holes in the heads of the second support member are seated between the legs. Thereafter, the locking wedges are finally tapped into place, resulting in that the first leg of the third support member is tightly pressed against the side plate. Consequently, the edge of the side plate 102 of the casing structure is left tightly pressed between the leg of the third support member 24 and the edge face 30 of the second support member 22, thus locking the side plate to the welding jig immobily. The welding jig is equipped with such a number of second support members that they support the side parts sufficiently. The number of support members needed depends, among other things, on the welding method used and on the size of the weld seam connecting the side parts and bottom parts together which has a direct impact on the magnitude of the tensile stress generated in the weld seam.

In a preferred embodiment of the welding jig according to the invention, electrically operated heating resistors 32 are mounted inside the body pipes 8 by means of which the body pipes and the components of the casing structure resting on them can be heated to a desired temperature for the welding. The heating resistors inside the body pipes are protected from hits and other mechanical stresses and are therefore not damaged when the components to be welded together are moved. The end plates 10 of the welding jig comprise, at the ends of the body pipes 8, holes (not shown) opening into the cylinder 12 through which the ends of the body pipes extend. The supply of electricity to the heating resistors is arranged through the open ends of the body pipes. The power regulation of the heating resistors is controlled by a thermostat, the components to be welded together thus maintaining the right temperature throughout the welding.

Figure 3 shows, by way of example, a welding jig according to the invention. The invention comprises two separate end supports 40 on which the welding jig is supported at its both ends for the fixing and assembly-welding of the components of the casing structure. Each end support has a frame 42 consisting of pipe-like components on which two rotatably bearing-mounted support rings 44 are mounted adjacent to each other. The size of the support rings and the distance between them is chosen with the object of supporting the cylinder 12 of the end part of the welding jig, which is to be mounted on the support rings, on the support rings at its outer edge. Preferably, the diameter of the support rings and the distance between their midpoints are 40 cm and 50 cm, respectively. Thanks to the bearings of the support rings, the welding jig is rotatable on the support rings by a small amount of force, the welding rotating then about its rotational axis.

Preferably, some of the support rings are motorized to facilitate the rotation of the support rings. The appropriate length of the welding jig depends on the application, i.e. on the length of the components to be welded together by means of the welding jig. However, the advantages attainable by the welding jig are more pronounced when the welding jig is relatively long, preferably several meters, because the welding then requires long weld seams that can be made economically by using mechanized welding methods, such as submerged arc welding.

In the method according to the invention, the assembly-welding of casing-like structures takes place in the following way. First, the components of the casing structures to be assembly-welded are fixed to the welding jig at a first work station. The fixing of the components is started by setting a first bottom plate 100 on the supports and by lifting the welding jig onto the bottom plate. Thereafter, a second bottom plate is lifted onto the body pipes and a necessary number of first support members 20 is fixed to the fixing members 16 of the body pipes by means of locking wedges 18, thus unmovably fixing the bottom plates to the body pipes, on both fixing planes. Preferably, at least four first support members are used in the fixing of a bottom plate, placed in the proximity of the corners of the bottom plate.

Next, a necessary number of second support members 22 is secured to both body pipes 8 by means of locking wedges 18, in the way explained above. After mounting the second support members, the side plates 102 of the casing structure are set against their edge faces and locked to the second support members by means of third support members 24 and locking wedges 18. The side plates are mounted on both sides of the welding jig.

After fixing the side plates, the side plates and bottom plates are assembly-welded by making a weld seam inside the casing structure, extending in the longitudinal direction of the casing structure. For the welding of the side plates the welding jig is moved to another work station with welding equipment. The welding jig is moved by a lifting device having grippers to grip the end plates of the welding jig. Preferably, the assembly-welding is performed by using a mechanized welding method, such as submerged arc welding. It is possible to use mechanized welding methods because, when using the welding jig according to the invention, no bulkheads are needed between the side plates to prevent the side plates from bending towards under the influence of the tensile forces of the weld seam. The assembly-welding is first performed on those components of the casing structure that are fixed on the first fixing plane of the welding jig. Then, the welding jig is rotated about the rotational axis so as to get the components fixed on the second fixing plane within the reach of the welding equipment. If needed, the heating resistors provided in the welding jig may heat the components of the casing structure to a desired temperature for the welding.

After welding of the side plates and the bottom plate together, the bulkheads to be secured to the side plates and the bottom plate are welded to the casing structure (the bulkheads are not shown in the Figures). Since the welding of the bulkheads requires several divergent weld seams extending along the edges of the bulkheads, it is usually most practical to perform it by manual welding, e.g. MIG welding, or by robotized welding. For the welding of the bulkheads the welding jig is taken to a third work station where the welding of the bulkheads takes place.

After welding of the bulkheads, the second and third support members can be entirely removed. Then the external seam between the side plates and the bottom plate is welded. During this welding, the bottom plate is still fixed to the welding jig by the first support members. The casing structure can be demounted from the welding jig after the above-mentioned welding operations. Preferably, the welding jig is moved to a fourth work station for demounting the casing structure. The demounting of the casing structure is performed by simply removing the locking wedges holding the first support members, whereby* the support members are released from the body pipes. At the same time, the casing structure is released from the welding jig. The welding jig can now be taken back to the first work station where new components of a casing structure are secured thereto.

The casing structure demounted from the welding jig is not completely finished yet because it does not have a cover plate fixed to the side plates. In the welding of the cover plate the welding jig according to the invention is not necessary but it can be performed using the known methods. First, the cover plate is put into the right place, onto the edges of the side plates, and then preliminarily fixed into place by means of short weld seams. After this the cover plate is assembly-welded by means of external weld seams. Preferably, the welding of the cover plate is performed using a mechanized welding method, such as submerged arc welding.

If the welding of the cover plate is performed at an elevated temperature, the temperature of the casing structure and the cover plate preliminarily bound thereto is first risen to an appropriate welding temperature. Preferably, the components to be welded together are heated to the desired temperature in a furnace into which the components are placed for a suitable period of time. When the temperature of the components is at the required level, they are taken out of the furnace and the welding is performed in the way described above.

It is also possible to perform the welding of the cover plate using the welding method and welding jig according to the invention. Like in the case of the bottom plate, the cover plates can be mounted on both fixing planes of the welding jig by means of the first support members, whereafter the part of the casing structure constructed from the side plates, bottom plates and bulkheads using the above- described method is set onto the cover plate and fixed into place by means of short weld seams. Then the final welding of the side plates and cover plate is performed. When fixing cover plates and bottom plates of different thickness to the welding jig, first support members differing from each other in their slot width can be used. Preferably, first support members having their slot width dimensioned according to the thickness of the plate to be fixed are always used in the fixing. During the welding of the cover plates, the cover plates can be heated to an elevated temperature by the heating resistors provided in the welding jig, if needed, and the welding jig supported on the end supports can be rotated about the rotational axis.

After fixing of the cover plate, the casing structure is allowed to cool, and then it can be subjected to machinings, such as possible toothings of the edge of the cover plate and finishing operations, as required.

The method and apparatus according to the invention are especially suitable for assembly-welding of casing-like structures constructed from a bottom plate, side plates, bulkheads and a cover plate. Such structures include several meters high oil-rig leg modules wherein the thickness of the sheet-like components of the casing-like structure is many meters and wherein different steel grades can be used. Besides, the method and welding jig according to invention can be used for welding other than casing-like structures. For example, the method according to the invention can be used for manufacturing robust U profiles from one bottom plate and two side plates or robust L profiles from a bottom plate and one side plate.

In the embodiment explained above, the welding jig has two fixing planes, both of them being intended for the welding of the components of one structure. A person skilled in the art understands that, when welding shorter structures, more than one bottom plate and side plate can be mounted on one fixing plane one after another and welded together in the above-described way. This makes it possible to effectively use mechanized welding methods in the manufacture of smaller sheet- like structures. It is also feasible that more body pipes lying on different levels are arranged in the welding jig, thus forming several, preferably three or four, fixing planes located symmetrically round the rotational axis of the welding jig, in the welding jig.

Figure 4 shows, by way of example, a preferred embodiment of the welding jig according to the invention, in a side view obliquely from above. Figure 4 shows the welding jig with the components to be welded together mounted in the welding jig. Figure 5 shows the welding jig shown in Figure 4 from the side, without the components to be welded together and disassembled into two components.

In the embodiment shown in Figures 4 and 5, the body pipes 8 consist of first outer pipes 8a secured to an end plate 10 of a first end part and of second outer pipes 8b secured to an end plate of a second end part as well as of inner pipes 9 arranged inside the outer pipes (Figure 5). Preferably, the summed length of a first and a second outer pipe substantially equals to the length of an inner pipe. The inner pipe may be fixed to the first or the second outer pipe or it can be a completely free part. The end plates 10 comprise two rectangular sheet-like fourth support members 25 fixedly mounted, preferably by weld attachment, to the end plate at one of their edges. The fourth support members are fixed to the end plates, between the body pipes, into an essentially parallel position next to each other, with a gap left therebetween having a width at least somewhat greater than the thickness of the bottom plate to be fixed to the welding jig. The ends of the fourth support members have holes 27 for a locking pin 29.

Figure 5 shows the welding jig in a disassembled state with the components of the welding jig completely separated from each other. A gap capable of receiving a bottom plate 100 is then opened between the first outer pipes of the body pipes. Upon mounting the bottom plate, the welding jig is assembled by inserting the free ends of the inner pipes 9 into the free ends of the second outer pipes and by pushing the ends of the welding jig towards each other. The first and the second end of the bottom plate are then seated between the fourth support members in the first end part of the welding jig and between the fourth support members in the second end part, respectively. By placing the end of the bottom plate in the gap between the support members a torsionally rigid joint is created between the end part of the welding jig and the bottom plate.

Figure 4 shows the welding jig in a loaded state with the bottom plate mounted into place. In the embodiment shown in Figure 4, a toothing is provided in both longitudinal edges of the bottom plate 100. The bottom plate is locked into place by inserting the locking pin 29 into the holes 27 in the fourth support members, whereby the recesses of the toothing in the bottom plate receive the locking pin and the bottom plate is locked into place. At the same time, the components of the welding jig are coupled to each other through the bottom plate. The above- described way of fastening carried out by means of a locking pin and an indentation in the bottom plate is just one preferred way of fastening the bottom plate. A person skilled in the art understands that other ways of fastening are possible.

In the above-described preferred embodiment of the welding jig, the fixing plane for the bottom plate is created between the body pipes and the rotational axis of the welding jig extends longitudinally "through" the bottom plate mounted on the fixing plane. The aim is to position the bottom plate between the body pipes in such a way that the center-of-gravity axis of the bottom plate and the rotational axis of the welding jig lie as close to each other as possible in order to achieve the most symmetrical loading possible for the welding jig. Preferably, the bottom plate is mounted in the welding jig in such a way that the center-of-gravity axis of the bottom plate and the rotational axis of the welding jig lie on the substantially same line.

After fixing of the bottom plate, the side plates 102 are fixed to the outer faces of the bottom plate. In the embodiment shown in Figure 4, the side plates are elongated sheet-like components molded into a chute-like shape which are welded to the bottom plate at their longitudinal edges. In Figure 4, there are two side plates, and they are welded symmetrically on both sides of the bottom plate. The chute-like side plates are set, in the position shown in Figure 4, onto the bottom plate and preliminarily fixed by means of small bond welds. Thereafter, the final welding of the bottom plate and the side plates is performed using the above- described method. Preferably, the final welding is performed using a mechanized welding method, such as submerged arc welding. In this preferred embodiment of the invention, the welding jig is moved from one work station to another, the welding jig is rotated during the welding and the components to be welded together are heated by means of heating resistors installable in the body pipes exactly in the same way as explained in the description of Figures 1 to 3.

In the above description the chute-like side plates were fixed to the bottom plate after the bottom plate was fixed to the welding jig. A person skilled in the art understands that these operational steps also can be carried out in another order, i.e. by first fixing the side plates to the bottom plate by means of small bond welds and by then fixing the bottom plate to the welding jig. Furthermore, it will be appreciated that this preferred embodiment of the invention is not limited to the welding of chute-shaped side plates but it also is useful for welding side plates and bottom plates of different shape.

The foregoing describes preferred embodiments of the method and welding jig according to the invention. The invention is not restricted to the solutions just described but the inventive idea is applicable in numerous ways within the scope of the Claims.

Claims

1. A method of welding sheet-like components (100, 102) together, in which method the sheet-like components are fixed to a welding jig for the welding, which welding jig is rotatable about a longitudinal rotational axis during the welding, characterized in that the sheet-like components are fixed to the welding jig in such a way that their common center-of-gravity axis lies as close to the rotational axis of the welding jig as possible.

2. A method according to Claim 1 , characterized in that the sheet-like components (100, 102) are fixed to the welding jig in such a way that they are located substantially symmetrically relative to the rotational axis of the welding jig.

3. A method according to Claim 1 or 2, characterized in that at least one bottom plate (100) is fixed to the welding jig and that at least two side plates (102) are welded to the bottom plate, on opposite sides thereof.

4. A method according to Claim 1 or 2, characterized in that at least two bottom plates and side plates to be welded to these bottom plates are fixed to the welding jig at a time and that the bottom plates and side plates are fixed to the welding jig by means of removable support members (20, 22, 24).

5. A method according to Claim 4, characterized in that at least one first bottom plate (100) is fixed on a first side of the rotational axis of the welding jig and that at least one second bottom plate is fixed on a second side of the rotational axis of the welding jig.

6. A method according to Claim 4 or 5, characterized in that the bottom plates (100) are fixed to the welding jig by means of removably fixable first support members (20) and that the side plates are fixed to the welding jig by means of removably fixable second and third support members (22, 24).

7. A method according to Claim 6, characterized in that the side plates (102) to be welded to the first bottom plate (100) and the side plates to be welded to the second bottom plate, which first and second bottom plates are fixed on different sides of the rotational axis of the welding jig, are supported and fixed into place by means of the same second fixing members (22).

8. A method according to any of Claims 1 to 7, characterized in that the components fixed to the welding jig are heated to a desired temperature by means of heating resistors (32) provided in the welding jig.

9. A method according to any of Claims 1 to 8, characterized in that the welding jig is rotated about the rotational axis during the welding in order to weld together the sheet-like components (100, 102) fixed on different sides of the rotational axis.

10. A method according to any of Claims 1 to 9, characterized in that the sheet- like components (100, 102) are welded together using an at least partly mechanized welding method, such as submerged arc welding.

11. A method according to any of Claims 1 to 10, characterized in that components of a casing-like structure consisting of a bottom plate (100) and at least one side plate (102) to be welded to the bottom plate are assembly-welded using the method.

12. A method according to Claim 11 , characterized in that weld seams located inside casing structures, between a bottom plate (100) and a side plate (102), are welded using the method.

13. A method according to any of Claims 1 to 12, characterized in that oil-rig leg modules are assembly-welded using the method.

14. A welding jig for welding sheet-like components (100, 102) together, which welding jig has a first end part and a second end part as well as body pipes (8) connecting the end parts together, and which welding jig is rotatable about a longitudinal rotational axis during the welding, characterized in that it comprises at least one fixing plane in order to fix the sheet-like components to the welding jig in such a way that the common center-of-gravity axis of the sheet-like components lies as close to the rotational axis of the welding jig as possible.

15. A welding jig according to Claim 14, characterized in that it has two adjacent, substantially parallel body pipes (8) and that the rotational axis of the welding jig lies between the body pipes.

16. A welding jig according to Claim 14 or 15, characterized in that the sheet- like components (100, 102) are to be fixed to the welding jig in such a way that they lie substantially symmetrically relative to the rotational axis of the welding jig.

17. A welding jig according to any of Claims 14 to 16, characterized in that it has at least one fixing plane in order to fix the bottom plates (100) in such a way that the bottom plate is to be fixed between the body pipes.

18. A welding jig according to Claim 17, characterized in that it comprises support members (25) for removably fixing the bottom plates (100) to the end parts of the welding jig.

19. A welding jig according to any of Claims 14 to 16, characterized in that it has at least two fixing planes for simultaneously fixing at least two bottom plates (100), the first fixing plane for fixing the first bottom plate being located on a first side of the plane defined by the body pipes and the second fixing plane for fixing the second bottom plate being located on a second side of the plane defined by the body pipes.

20. A welding jig according to Claim 19, characterized in that it comprises removable support members (20, 22, 24) for removably fixing at least two bottom plates (100) and side plates (102) to be welded to the bottom plates to the welding jig-

21. A welding jig according to Claim 20, characterized in that said removable support members (20, 22, 24) comprise first support members (20) to be removably fixed to the body pipes (8) for the purpose of fixing the bottom plates (100), as well as second support members (22) to be removably fixed to the body pipes and third support members (24) to be removably fixed to the second support members, for the purpose of the fixing the side plates (102).

22. A welding jig according to Claim 21 , characterized in that said second support members (22) are substantially symmetrical members having a first side for supporting and fixing a side plate (102) to be welded to the first bottom plate (100) and a second side for supporting and fixing a side plate to be welded to the second bottom plate, which first and second bottom plates are arranged to be fixed on fixing planes located on different sides of the rotational axis of the welding jig-

23. A welding jig according to any of Claims 14 to 22, characterized in that heating resistors (32) are arranged inside the body pipes (8) to heat the components fixed to the welding jig to a desired temperature.

24. A welding jig according to any of Claims 14 to 23, characterized in that the end parts of the welding jig comprise a bearingly functioning support element (12) for mounting the welding jig on separate end supports (40) rotatably about the rotational axis.