Method of closing a tank with a cover

Abstract

The present invention relates to a method of closing a tank (12) with a cover (14), the method comprising providing a cover having an opening (20) and a tank, Attaching a plurality of anvil segments (22) to an edge of the cover or to an edge of the tank, the segments forming a ring inside the edge and having a part protruding from the edge, Fixing the tank to a welding apparatus, Assembling the edge of the cover in contact to the edge of the tank, the part of the segments protruding from the edge of the cover or of the edge of the tank being inserted into the other of the cover or the tank, Friction stir welding of the edge of the cover and the edge of the tank together, from outside the cover and tank, Dismantling the segments through opening, Removing segments through opening.

Description

[0001] METHOD OF CLOSING A TANK WITH A COVER

[0002] Technical field

[0003] The invention pertains to a method of closing a tank with a cover. In particular, the closure is performed by friction stir welding.

[0004] Prior art

[0005] There is a need for a method of closing a tank with a cover, while fixing the cover to the tank without changing the microstructure of the tank and cover. This is particularly the case for aerospace and space applications.

[0006] Disclosure of the invention

[0007] An object of the invention is to provide a method of closing a tank with a cover. To that end, the invention relates to a method of closing a tank with a cover, the method comprising

[0008] Providing a cover having an opening and a tank,

[0009] Attaching a plurality of anvil segments to an edge of the cover or to an edge of the tank, the segments forming a ring inside the edge and having a part protruding from the edge,

[0010] Fixing the tank to a welding apparatus,

[0011] Assembling the edge of the cover in contact to the edge of the tank, the part of the segments protruding from the edge of the cover or of the edge of the tank being inserted into the other edge of the cover or the tank,

[0012] Friction stir welding of the edge of the cover and the edge of the tank together, from outside the cover and tank,

[0013] Dismantling the segments through opening,

[0014] Removing segments through opening.

[0015] In an embodiment, the segments are attached to the tank or to the cover that has the smaller diameter.

[0016] In an embodiment, some of the segments are ring segments and another of the segments is a plug segment with tapered sides, the plug segment connectingtwo facing ring segments, the segment plug closing the ring formed by the ring segments.

[0017] In an embodiment, a connection fixes the plug segment to two facing ring segments.

[0018] In an embodiment, the tank is fixed to the welding apparatus by inserting the tank into a base ring and by pressing fingers against the tank, the fingers being supported by the base ring.

[0019] In an embodiment, a clamping ring is positioned over the fingers, the fingers being pressed against the tank by tightening the clamping ring to the base ring.

[0020] In an embodiment, after assembling the edge of the cover in contact to the edge of the tank, the connection and the clamping ring are tightened until the edges of the cover and tank are aligned.

[0021] In an embodiment, a pushing ring is positioned on the cover, the edge of the cover being pushed against the edge of the tank by urging the pushing ring against the cover.

[0022] In an embodiment, the pushing ring is urged against the cover by tightening bolts or by pressing a press against the pushing ring.

[0023] In an embodiment, friction stir welding of the edges of the cover and the tank together is performed in one run or by portions all around the cover and tank.

[0024] In an embodiment, the friction stir welding starts and stops in a thicker area of the cover and tank, the thicker areas are then machined after welding.

[0025] In an embodiment, a circular backing strip is positioned between the segments and the cover or the tank and / or a rubber insert is positioned between the fingers and the tank and / or a rubber insert is positioned between the pushing ring and the cover.

[0026] In an embodiment, after removing segments though the opening, the method further comprising a step of closing the opening.

[0027] In an embodiment, closing the opening occurs by fitting or welding a pipe on a flange of the cover that surrounds the opening.

[0028] In an embodiment,the cover is provided with a bimetal interface around the opening, the bimetal interface has a first metal away from the opening and has a second metal around the opening, the second metal being different from the first metal, the first metal being friction stir welded to the rest of the cover,

[0029] wherein, closing the opening occurs by

[0030] TIG welding a portion to the second metal to close the tank.

[0031] In the framework of this document, the use of the indefinite article “a”, “an” or the definite article “the” to introduce an element does not exclude the presence of a plurality of these elements. In this document, the terms “first”, “second”, “third” and the like are solely used to differentiate elements and do not imply any order in these elements.

[0032] In the framework of the present document, the use of the verbs “comprise”, “include”, “involve” or any other similar variant, as well as their conjugational forms, cannot exclude the presence of elements other than those mentioned. When the verb “comprise” is used for defining an interval by the terms “comprised between” two values, these two values should not be interpreted as excluded from the interval.

[0033] Brief description of the figures

[0034] Other characteristics and advantages of the present invention will appear on reading the following detailed description, for the understanding of which, it is referred to the attached figures where:

[0035] - Figure 1 illustrates an equipment to which the method is applied;

[0036] - Figure 2 illustrates a detail of the method;

[0037] - Figure 3a and figure 3b illustrate a detail of the method;

[0038] - Figure 4(a), figure 4(b), figure 4(c), figure 4(d), figure 4(e), figure 4(f), figure 4(g), figure 4(h) illustrate various steps of the method;

[0039] - Figure 5(a), figure 5(b), figure 5(c), figure 5(d), figure 5(e), figure 5(f) illustrate various steps of the method;

[0040] - Figure 6(a), figure 6(b), figure 6(c) illustrate various steps of the method;

[0041] - Figure 7 illustrates a welding step;- Figure 8(a), figure 8(b) illustrate other details of the invention;

[0042] - Figure 9 illustrates a step of removal of the anvil;

[0043] - Figure 10 illustrates a step of closure of the cover;

[0044] - Figure 11 illustrates a step of closure of the cover.

[0045] The drawings in the figures are not scaled. Similar elements can be assigned by similar references in the figures. In the framework of the present document, identical or analogous elements may have the same references. The presence of reference numbers in the drawings cannot be considered to be limiting, in particular if these numbers are indicated in the claims.

[0046] Description of specific embodiments of the invention

[0047] Description of preferred embodiments of the present invention are hereafter described with references to figures, but the invention is not limited by these references. In particular, the drawings or figures described below are only schematic and are not limiting in any way.

[0048] The invention relates to a method of assembling two workpieces and closing one of the workpieces with the other. For example, the invention relates to a method of assembling a tank with a cover and closing the tank with the cover. The method comprises providing a cover having an opening and a tank, attaching a plurality of anvil segments to an edge of the cover or of the tank, the segments forming a ring inside the edge and having a part protruding from the edge. The tank is fixed to a welding apparatus. The method comprises assembling the edge of the cover in contact to the edge of the tank, the part of the segments protruding from the edge of the cover or of the tank being inserted into the other of the cover or the tank. Then, the edges of the cover and the tank are friction stir welded together, from outside the cover and tank. Then, the segments are dismantled and removed through cover opening. The invention makes it possible to apply friction stir welding. This provides welding with high quality than other welding methods, and that can be applied to lightweight equipment for aerospace or space applications. This save weight and this also allows use of strong and light alloys that cannot correctly support other welding methods.The figures show the various steps in the method for closing a piece of equipment. The method applies to equipment that needs to be closed, for example, but whose interior is not easily accessible. This could be workpieces such as a tank that is closed with a cover. The method is not only applicable to such equipment, nor only to its closure. Figure 1 shows a possible realization of equipment 10. Equipment 10 comprises a tank 12 and a cover 14. The tank 12 may comprise a body 16 and a bottom 18. The bottom 18 may be of the cover 14 type. The tank 12 and the cover 14 may be rotationally symmetrical. Tank 12 and cover 14 may form a cylindrical assembly, by way of example. Cover 14 and bottom 18 can be domeshaped. The bottom 18 can be fixed to one end of the body 16 prior to the application of the present method. The bottom 18 can be attached by welding, for example, by friction stir welding. Access to the welding zone, which is the interface between the facing edges of the body 16 and the bottom 18, is not a problem, as the tank 12 remains open at its other end (intended to receive the cover 14). The assembly of the cover 14 to the tank 12 thus forms a vessel.

[0049] The method involves closing the tank 12 with the cover 14. The cover 14 is provided with an opening 20. The opening 20 of the cover 14 is a small hole compared to the surface area of the cover 14. For example, the opening 20 represents less than 30%, preferably less thanl 0%, of the surface area of the cover 14. The opening 20 can be smaller in the field of aerospace and the opening 20 can be larger for trunks. As will be explained later, the opening 20 can be closed or used to receive accessories, such as a pipe.

[0050] The method comprises attaching an anvil to an edge 141 of the cover 14 or to an edge 121 of the tank 12. The anvil is attached to one of the two workpieces to be assembled. The anvil can comprise a plurality of anvil segments 22. The segments 22 are thus attached (or positioned) to the edge of the cover 14 or to the edge of the tank 12. The segments 22 form a ring inside the edge 121 or 141. The segments 22 have a part protruding from the edge 121 or 141. The segments 22 are partly in the tank 12 or cover 14 and partly outside the tank 12 or cover 14. The segments 22 are partly inside a geometric envelope of the tank 12 or cover 14 and partly protruding from the geometric envelope of the tank 12 or cover 14. Figure 2shows the attachment of the segments 22 to the tank 12 or the cover 14 and figure 5 (figure 5(a)-(f)) show the attachment of the segments 22 to the tank 12. The diameters of the cover 14 and of the tank 12 are targeted to be the same. However there may be a small difference. It is then easier to position the anvil segments 22 on the workpiece that has the smallest inside diameter. This then makes it easier to assemble the other, larger-diameter workpiece to the workpiece carrying the segments 22. Also, placing the anvil ring on the tank 12 instead of on the cover 14 may be easier. A circular backing strip 23 (in one or more parts) may be provided and positioned between the anvil segments 22 and the cover 14 or the tank 12 for extra grip. The backing strip 23 supports the welded areas (of aluminum or other materials).

[0051] The method may also include fixing the tank 12 to a welding apparatus. The order of the steps is not limited, as this step may be implemented beforehand or at a later stage. Figure 8 shows an example of a welding apparatus support 44.

[0052] Figure 6 shows assembling the edge 141 of the cover 14 in contact to the edge 121 of the tank 12. The part of the segments 22 protruding from the edge 141 , 121 of the cover 14 or of the tank 12 is inserted into the other of the cover 14 or the tank 12. Figure 7 then shows the welding of the cover 14 and the tank 12 together, enabling the two workpieces to be secured together. This closes the tank 12. The welding is friction stir welding. The edges 141, 121 of the cover 14 and tank 14 are friction stir welded together. According to figure 7, a tool 24 rotates at constant speed on the line of contact of the edges 121, 141 between the workpieces 12, 14 to be welded. The tool 24 intimately mixes the materials of tank 12 and cover 14 in a welding zone 25. Complete assembly is achieved as the tool 24 progressively runs along the entire interface between the edges 121 , 141 to be welded. Welding occurs from outside the cover 14 and tank 12, i.e. orbital welding. Finally, the segments 22 are dismantled and removed through the opening 20, as shown in figure 9. The opening in the cover 14 is thus sized to allow access to the anvil for dismantling, and to allow removal of the segments 22 from the tank 12, after the friction stir welding process is finished.The closing weld is performed by a welding technology where the tank 12 and cover 14 material does not melt, contrarily to welding technology like TIG welding (standing for Tungsten Inert Gas welding) or Electron Beam welding. The closing weld connects the cover 14 to the tank 12 (irrespectively of the presence of the opening 20). Unlike these welding technologies, the method of the invention prevents changes of the microstructure of the material of the tank 12 and the cover 14 that would lead to make it weaker. Thanks to the invention it is not necessary to make the tank 12 and cover 14 structure thicker and more heavy. The method makes it possible to use tank 12 and cover 14 made of, for example, aluminum, titanium and alloys with superior strength and low weight (such as Al-Cu-Li alloys) that cannot be welded by TIG or Electron Beam welding.

[0053] The anvil comprises a plurality of segments 22 that can be dismantled and removed through the opening 20. As a matter of example, the anvil may comprise five segments 22 (the anvil may be segmented in another amount than five segments). Some of the segments are ring segments. For example, four out of five segments 22 are ring segments. The ring segments 22 may constitute a ring anvil straddling both edges 121 , 141. Another of the segments may be a plug 221 with tapered sides, the plug segment 221 connecting two facing ring segments. The plug segment 221 closes the ring formed by the ring segments 22.

[0054] Figure 3 shows how the segments 22 are attached to the cover 14 or tank 12. A connection 26 secures the plug segment 221 to two facing ring segments 22. Connection 26 comprises a frame 261 and fastening means such as bolts 262. A center bolt 262 is screwed to the plug segment 221 and two further bolts 263, 264 are screwed to segments on either side of the plug segment 221. The center bolt 262 is tightened strongly enough for keeping the anvil ring formed by the segments 22 in the cover 14 or in the tank 12. To achieve this, the tightening of the center bolt 262 pushes the center bolt 262 into the plug segment 221 , tending to spread the facing ring segments 22 apart.

[0055] Further steps are shown in figure 4 (figure 4(a)-(g)), figure 5 (figure 5(a)-(f)) and figure 6 (figure 6(a)-(c)). The result is a workpieces and tooling assembly 11 ready for welding. The difference between figure 4 and figure 5 is that the segments22 are attached to the cover 14 in figure 4 and are attached to the tank 12 in figure 5. Furthermore, in figure 5, the various steps visible in figure 4 are not shown in sequence. The description in figure 4 also applies to figure 5. Figure 4 and figure 5 show the tank 12 and the cover in perspective and cross-section, as appropriate, for a better understanding.

[0056] The tank 12 may be fixed to the welding apparatus by inserting the tank 12 into a base ring 28 (such as on figure 4(a) and figure 4(b)). The base ring 28 surrounds the body (16) of the tank 12. The base ring 28 is connected to the welding apparatus by a support 44 (see figure 8, such as a rigid frame). Then fingers 30 are placed around the tank 12 and are pressed against the tank 12 (such as on figure 4(c) and figure 4(d)). The fingers 28 are positioned on the base ring 28. The fingers 30 are supported by the base ring 28, for example by placing the fingers 30 in a groove 32 of the base ring 28. In addition, the fingers 30 may be provided with a rubber insert 34 for extra grip. The rubber insert 34 may be positioned between the fingers 30 and the tank 12. Then a clamping ring 36 is positioned over the fingers 30 (such as on figure 4(e) and figure 4(f)). The fingers 30 may be pressed against the tank 12 by tightening the clamping ring 36 to the base ring 28. For example the clamping ring 36 is pulled to the base ring 28 by loosely tightening bolts 38 (such as on figure 4(g) and figure 4(h)). Alternatively, the external clamping system comprising the clamping ring 36 and the fingers 30 may be replaced by a bayonet lock, for example.

[0057] Same description applies to figure 5 (figure 5(a)-(f)). Figure 5(a)-(f) show that the sequence of steps is not strict. It is possible to attach the segments 22 to the cover 14 or to the tank 12 before or after (or in parallel) to the positioning of the base ring 28, fingers 30, etc. to the tank 12.

[0058] Figure 6 (figure 6(a)-(c)) show the assembling of the edge 141 of the cover 14 in contact to the edge 121 of the tank 12. Figure 6 shows the tank 12 and the cover in perspective and cross-section. Figure 6(a) shows the cover 14 positioned on the assembled tank of figure 4 and figure 5 - either way, whether the segments 22 are attached to the cover 14 or to the tank 12. The cover 14 is pushed to the tank 12 by a pushing ring 40 (such as on figure 6(b)). The pushing ring 40 is positioned on thecover 14, the edge 141 of the cover 14 being pushed against the edge 121 of the tank 12 by urging the pushing ring 40 against the cover 14. The cover 14 is pushed to the tank 12 by the pushing ring 40 in such a way that the edges 121, 141 make contact. The pushing ring 40 may be provided with a rubber insert 42 for extra grip (such as on figure 6(a)). The rubber insert 42 is positioned between the pushing ring 40 and the cover 14.

[0059] On figure 6, assembly 11 comprises the two workpieces 12, 14 to be assembled and welded and all of the tooling, i.e. (but not limited to this list) the anvil segments 22, the base ring 28, the fingers 30, the clamping ring 36 and the pushing ring 40 (and all related tightening means such as bolts, etc.).

[0060] After assembling the edge 141 of the cover 14 in contact to the edge 121 of the tank 12, the connection 26 and the clamping ring 36 are tightened until the edges 141, 121 of the cover 14 and tank 12 are aligned. This is achieved by tightening the bolt 38 of the clamping ring 36 and by expanding the anvil ring by tightening the bolt 262 of the connection 26 until the edges of the cover 14 and tank 12 are aligned. The edges 121 , 141 of the cylindrical part of the tank 12 and of the cover 14 are well aligned after assembly to improve the quality of the welding. Bolt 262 is accessible through the opening 20 (such as on figure 6(c)). The segments 22 are positioned on either the cover 14 or the tank 12 first and are then slightly expanded a first time to keep it in place. After the other workpiece is mounted on the assembled segments, the segments 22 are further expanded (expanded a second time). The fingers 30 push the tank edge 121 firmly to the segments 22. The pushing ring 40 may be supported by a rigid frame that is connected to the welding apparatus (not included in the drawing).

[0061] This method allows the anvil (formed by the segments 22) to be held immobile by the connection 26 (the opening 20 even allows access to adjust the connection 26 once the cover 14 is in place). The method thus enables both the edge 121 of the tank and the edge 141 of the cover 14 to be pressed against the immobile anvil. This enables friction stir welding to be carried out with quality, even though the inside of the tank 12 is no longer accessible (by the welding apparatus) once the cover 14 is in place on the tank.During the friction stir welding step, the workpiece and tooling assembly 11 may rotate continuously or stepwise between welding operations. The friction stir welding of the edges 141 , 121 of the cover 12 and the tank 14 together is performed in one run or by portions all around the cover 14 and tank 12. The welding can be done in one run when the assembly 11 is rotating. Alternatively, the assembly 11 is fixed to the welding apparatus and is only welded on portions or areas that can be reached by the welding tool 24. In that case the assembly 11 is loosened, rotated and fixed again on the welding apparatus. Then a new portion or area can be welded. This process is repeated until the whole edges 121 , 141 are welded (welding zone 25). In both cases the welding is overlapped and a welding tool 24 with retractable pin is used. The way of proceeding depends on the welding apparatus. The present method makes it possible to run the welding in different manners and offers flexibility. The friction stir welding may start and stop in a thicker area of the cover 14 and tank 12, the thicker areas are then machined after welding. Starting at thicker areas eliminates the zone where the start / stop causes reduced properties of the welded material.

[0062] According to figure 7, welding takes place on the assembly 11 described with a vertical position of tank 12 and cover 14 (cover 14 is above tank 12). According to figure 8 (figure 8(a), figure 8(b)), welding takes place with assembly 11 in a tilted position. Figure 8 shows assembly 11 in perspective (figure 8(b)) and cross-section (figure 8(a)). Assembly 11 is positioned on welding apparatus support 44. Assembly 11 is held in a tilted position of approximately 90° with respect to figure 7. Then the welding is realized in segments and the assembly 11 is turned in between the segment welds. In this way it is possible to weld larger tanks because the weld segments will be easier to reach by the apparatus.

[0063] Also shown in figure 8 is how to push the cover 14 to the tank 12 using the pushing ring 40 (this is applicable irrespective of the tilted position of assembly 11). To this end, the pushing ring 40 is urged against the cover 14 by tightening bolts 46. Bolts 46 extend from the pushing ring 40, through the clamping ring 36, to the base ring 28. By tightening bolts 46, the pushing ring 40 approaches the base ring 28 and the edge 141 of the cover 14 is urged against the edge 121 of the tank 12.Alternatively or in combination, one can consider using a press pressed against the pushing ring. On figure 8, a press would press the cover 14 from the right to the left (or vertically from up to down on figure 6 for example). These possibilities of pushing the cover 14 to the tank improve the quality of the welding due to close contact between the edges 121, 141.

[0064] Other objects that feed through the wall of workpieces 12, 14 (such as pipes, sensors, etc.) are assembled in and to the workpieces beforehand. For example, on figure 8, a hole 48 is foreseen to assemble a pipe.

[0065] Figure 9 shows the removal of tooling, such as segments 22 (ring segments and plug segment), connection 26, backing strip 23. The size of the opening 20 is defined to allow such a removal. All or part of these tooling may be connected by ropes for pulling the tooling out of the vessel after welding the tank 12 and cover 14 together.

[0066] After removing tooling such as segments 22 though the cover opening 20, the method further comprising a step of closing the cover opening 20. That can be performed in different ways according to different embodiments of the cover 14. Figure 10 and figure 11 depict different ways to close opening 20. On both figures, the cover 14 is depicted before closing (upper part of the figures) and after closing (lower part of the figures).

[0067] According to figure 10, a possibility is to fit or weld a pipe 50 (or a cap) to the opening 20. To that end, a flange 52 surrounding the opening 20 of the cover 14 is provided.

[0068] According to figure 11, another possibility is to close the opening 20 by welding. To this end, the cover 14 is made from different materials (different metals or alloys). This can be realized by welding a separate piece 54 in the center of the cover 14 during the manufacturing process of the cover 14, around the opening 20. The separate piece 54 can be another aluminum alloy that can be TIG welded to the rest of the cover 14, around the opening 20. Alternatively the separate piece 54 can be a bimetal interface 54 (such as 54a, 54b on figure 11) that is manufactured by explosive welding. The separate piece 54 in the form of bimetal interface is welded to the rest of the cover 14, around the opening 20. For example, the bimetalinterface 54 is friction stir welded 56 to the rest of the cover 14 (preferably, before assembling the cover 14 to the tank 12). The bimetal interface 54 has a first metal away from the opening 20 and has a second metal around the opening 20 (at the edge of the opening 20). The second metal is different from the first metal (the second metal can be e.g. stainless steel), the first metal being friction stir welded 56 to the rest of the cover. Irrespective of the separate piece 54, the method comprises welding 60 (such like TIG welding) a portion 58 to the separate piece 54 to close the cover (and the vessel formed by the tank 12 and cover 14). With the separate piece 54 in the form of bimetal interface 54, the portion 58 is welded to the second metal.

[0069] The method is especially suitable for welding on metal components for aerospace and / or space applications. In particular, the invention is suitable for the manufacture of tanks (and vessel) for liquid hydrogen in air and space craft. Since hydrogen tanks have to be lightweight, aluminum can be used and the method of the invention makes it possible to use alternative light materials. Thanks to the method, the fixture in the form of the connection 26 and the anvil in the form of the segments 22 allow friction stir welding of orbital welds for closing the tank and the opening 20 allows the removal of the connection 26 and segments 22 out of the tank.

[0070] The present invention has been described in relation to the specific embodiments which have a value that is purely illustrative and should not be considered to be limiting. The skilled person will notice that the invention is not limited to the examples that are illustrated and / or described here above. The invention comprises each of the new technical characteristics described in the present document, and their combinations.

Claims

Claims1. A method of closing a tank with a cover, the method comprising- Providing a cover (14) having an opening (20) and a tank (12),- Attaching a plurality of anvil segments (22) to an edge (141 ) of the cover (14) or to an edge (121 ) of the tank (12), the segments (22) forming a ring inside the edge (121 , 141) and having a part protruding from the edge (121 , 141),- Fixing the tank (12) to a welding apparatus,- Assembling the edge (141 ) of the cover (14) in contact to the edge (121 ) of the tank (12), the part of the segments (22) protruding from the edge (141 ) of the cover (14) or of the edge (121 ) of the tank (12) being inserted into the other edge of the cover (14) or the tank (12), - Friction stir welding of the edge (141 ) of the cover (14) and the edge (121 ) of the tank (12) together, from outside the cover (14) and tank (12), - Dismantling the segments (22) through opening (20),- Removing segments (22) through opening (20).

2. The method according to the preceding claim, wherein the segments (22) are attached to the tank (12) or to the cover (14) that has the smaller diameter.

3. The method according to one of the preceding claims, wherein some of the segments (22) are ring segments and another of the segments (22) is a plug segment (221) with tapered sides, the plug segment (221) connecting two facing ring segments, the segment plug closing the ring formed by the ring segments.

4. The method according to the preceding claim, wherein a connection (26) fixes the plug segment (221) to two facing ring segments.

5. The method according to one of the preceding claims, wherein the tank (12) is fixed to the welding apparatus by inserting the tank (12) into a base ring (28) and by pressing fingers (30) against the tank (12), the fingers (30) being supported by the base ring (28).

6. The method according to the preceding claim, wherein a clamping ring (36) is positioned over the fingers (30), the fingers (30) being pressed against the tank (12) by tightening the clamping ring (36) to the base ring (28).

7. The method according to the two preceding claims when depending on claim 4, wherein, after assembling the edge (141 ) of the cover (14) in contact to the edge (121 ) of the tank (12), the connection (26) and the clamping ring (36) are tightened until the edges (141, 121) of the cover (14) and tank (12) are aligned.

8. The method according to one of the preceding claims, wherein a pushing ring (40) is positioned on the cover (14), the edge (141) of the cover (14) being pushed against the edge (121 ) of the tank (12) by urging the pushing ring (40) against the cover.

9. The method according to the preceding claim, wherein the pushing ring (40) is urged against the cover (14) by tightening bolts or by pressing a press against the pushing ring.

10. The method according to one of the preceding claims, wherein friction stir welding of the edges (141, 121) of the cover (14) and the tank (12) together is performed in one run or by portions all around the cover and tank.

11. The method according to one of the preceding claims, wherein the friction stir welding starts and stops in a thicker area of the cover (14) and tank (12), the thicker areas are then machined after welding.1512. The method according to one of the preceding claims, wherein a circular backing strip (23) is positioned between the segments (22) and the cover (14) or the tank (12) and / or a rubber insert (34) is positioned between the fingers (30) and the tank (12) and / or a rubber insert (42) is positioned between the pushing ring (40) and the cover (14).

13. The method according to one of the preceding claims, wherein, after removing segments (22) though the opening (20), the method further comprising a step of closing the opening (20).

14. The method according to the preceding claim, wherein closing the opening (20) occurs by- Fitting or welding a pipe (50) on a flange (52) of the cover (14) that surrounds the opening (20).

15. The method according to one of the preceding claims,- the cover (14) is provided with a bimetal interface (54) around the opening (20), the bimetal interface (54) has a first metal away from the opening (20) and has a second metal around the opening (20), the second metal being different from the first metal, the first metal being friction stir welded to the rest of the cover,wherein, closing the opening (20) occurs by- TIG welding a portion (58) to the second metal to close the tank (12).

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