Method for assembling a tubular floating structure and use thereof

Abstract

The invention provides a method of assembling a tubular floating structure and use thereof. A first tubular member and a second tubular member (11, 12) are welded to opposite sides of a reinforcing ring (10) to obtain a double segment (12 / 11). Additional segments (13, 14) can be added to form a tubular multi-segment with reinforcing rings (10) between adjacent 5 tubular members (11, 12, 13). The resulting multi-segment (12 / 11 / 13) is used as a tubular buoyant module in a floating offshore structure.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method of assembling a tubular floating structure for an offshore support, for example for an offshore wind turbine. BACKGROUND

[0002] Offshore wind turbine support platforms, for example as disclosed in US patent application US2020 / 269960, as well as other types of support platforms in offshore industry, for example a support platform for a floating fish farm as disclosed in international patent application WO2021 / 053361, comprise buoyancy tanks which are moored to the seabed to keep the structure in the desired position.

[0003] For large platforms, the buoyancy tanks are correspondingly large and their manufacture can be challenging. Typically, the buoyancy tanks are manually assembled and welded from curved steel plates supported by stiffening rings and longitudinal beams in a process similar to shipbuilding and highly labor-intensive. In a more industrialized process, the buoyancy tanks can be manufactured by assembling and welding complete prefabricated tubular elements which are welded together edge-to-edge in rows so that they form a tank with the required tubular length. In order for the tubular members to maintain their tubular shape when in water, stiffening rings are inserted into the tubular members and welded to the inner surface of the tubular members as a measure against hydrostatic and hydrodynamic pressure when installed at sea.

[0004] The stiffening rings are heavy, add to the cost and require time for installation, so it is desirable to reduce their number and facilitate production as well as shorten the production time. Furthermore, the insertion of the stiffening rings requires that the inner circle of the tubular members matches the outer circle of the stiffening rings with small tolerances. However, due to the large size of the tubular members, there is a risk that their diameter varies or that the shape of the segments deviates slightly from the circular shape and becomes elliptical, for example caused by handling and transportation of the segments. Such deviations in diameter and from the circular shape can cause difficulties when inserting the circular stiffening rings. This is especially problematic when the segments are oriented with their central axis horizontal. Therefore, it is preferable to insert the rings when the axis of the segments is oriented vertically. However, in this case, one of the welds has to be made from below the ring when welding along both edges of the ring, which increases the difficulty of the welding process. Each method has its disadvantages.

[0005] Therefore, there is a need to find an improved assembly method which overcomes these disadvantages.

[0006] In other technical fields, assembling tubular members into elongated tanks is implemented as standard, for example when assembling silos in agriculture. In some silo constructions, the segments are provided with inner rings for assembly with bolts. Other examples can be found in French patent document FR 2 395 903, in which the segments are provided with rings at their opposite ends, which are then stacked step by step and the adjacent rings are welded to each other by welding from the side. However, the side welding of such rings is difficult, especially if the welding has to be watertight. Thus, the methods known from silo production do not seem to be successfully applied to the production of large buoyancy tanks for floating offshore structures.

[0007] Therefore, other and improved assembly principles are needed. SUMMARY

[0008] It is therefore an object of the present invention to provide an improvement in the art. In particular, it is an object to provide an assembly method for buoyancy tanks in offshore structures, in particular for buoyancy tanks for offshore wind turbines. This object and other advantages are achieved by the assembly method as described below and in the claims, and the use thereof.

[0009] Briefly, two tubular members are welded to opposite sides of a reinforcing ring to obtain a double segment. Further segments can be added to form a tubular multi-segment with reinforcing rings between adjacent tubular members, which are structural stability rings. The resulting multi-segment serves as a tubular buoyancy module in a floating offshore structure.

[0010] In the following, the term tank is used together with the term tubular buoyancy module, as this is the terminology in the technical field. Similarly, the segments for the tubular buoyancy module will also be referred to as tank segments or simply segments.

[0011] The assembly method comprises the following two main steps:

[0012] 1. Manufacturing of tank segments

[0013] 2. Assembling the tank segments to form a complete tank

[0014] The tank segment comprises a cylindrical tubular member and a reinforcing ring. The tubular member forms the wall of the segment and of the final complete tank. Both components are made of steel. The tubular member has a first edge and an opposite second edge at opposite ends of the tubular member. The reinforcing ring has a first surface and an opposite second surface at opposite sides of the reinforcing ring. The reinforcing ring further has an outer circumference provided to be larger than the outer circumference of the first edge of the tubular member, and an inner circumference provided to be smaller than the inner circumference of the first edge of the tubular member. This way, there is some tolerance when positioning the tubular member on the reinforcing ring.

[0015] In the manufacture of the tank segment, the stiffening ring is placed horizontally, with its central axis oriented vertically, with its first surface facing upwards and resting on its second surface. The tubular member is then placed from above onto the stiffening ring, with its vertical axis concentric or approximately concentric with the vertical central axis of the stiffening ring. Due to the concentricity of the vertical axes of the tubular member and the stiffening ring, the tubular member will rest in a position on the first surface of the stiffening ring in which the outer circumference of the first edge of the tubular member is within the outer circumference of the first surface of the stiffening ring and the inner circumference of the first edge of the tubular member is outside the inner circumference of the first surface of the stiffening ring. In this position, the tubular member and the stiffening ring are joined by welding, which forms a watertight joint.

[0016] Due to the outer circumference of the first edge of the tubular member being within the outer circumference of the first surface of the stiffening ring and the inner circumference of the first edge of the tubular member being outside the inner circumference of the first surface of the stiffening ring, the welding is of the fillet type, which can be performed with full or incomplete penetration. Due to the vertical axis orientation of the tubular member and the stiffening ring, the welding is performed in a horizontal position, which is very convenient to perform.

[0017] After the manufacture of the tank segment is completed, in which the tubular member and the stiffening ring have been joined, the tank segment is rotated 180 degrees around the horizontal axis. While the stiffening ring has been placed below the tubular member during the manufacture of the segment, it is positioned above the tubular member after the rotation. The segment now rests on the downwardly facing second edge of the tubular member and the second surface of the stiffening ring now forms the upper surface of the tank segment.

[0018] The assembly of the tank segment is performed by placing a second segment on top of the first segment. The second segment is placed from above, with its vertical axis concentric or approximately concentric with the vertical axis of the first segment, and with its second edge of the tubular member resting on the second surface of the stiffening ring of the first segment. Due to the concentricity of the vertical axes, the second segment will be placed in a position in which the outer circumference of the second edge of the second tubular member is within the outer circumference of the second surface of the stiffening ring of the first segment and the inner circumference of the second edge of the tubular member of the second segment is outside the inner circumference of the second surface of the stiffening ring of the first segment. In this position, the tubular member of the second segment and the stiffening ring of the first segment are joined by welding, which forms a watertight joint.

[0019] Due to the outer circumference of the second edge of the tubular member of the first segment being within the outer circumference of the second surface of the stiffening ring of the second segment and the inner circumference of the second edge of the tubular member of the first segment being outside the inner circumference of the second surface of the stiffening ring of the second segment, the welding is of the fillet type, which can be performed with full or incomplete penetration. Due to the vertical axis orientation of the tubular member and the stiffening ring, the welding is performed in a horizontal position, which is very convenient to perform.

[0020] This method of manufacturing and assembling of the tank segments has many advantages over conventional techniques.

[0021] As the segments are welded on the corresponding reinforcing rings, instead of inserting the rings into the segments, time is saved and the tolerance requirements between the tubular members and the reinforcing rings are less, which greatly simplifies the manufacturing.

[0022] Similarly, as the tubular members are placed on top of the reinforcing rings during the manufacturing of the segments, the tolerance requirements between the individual tubular members are significantly reduced. In conventional manufacturing methods where the tubular members are placed directly on top of each other, the diameter and roundness of two adjacent tubular members need to be identical in order to obtain a joint that will not create a stress concentration due to misalignment between the tubular members. In the manufacturing method according to the present application, some variation in the diameter and roundness of two adjacent tubular members is allowed, as the reinforcing rings inserted between the tubular members will significantly reduce the stress concentration created due to misalignment between the tubular members.

[0023] In summary, the method is an improvement on the assembly of buoyancy elements for offshore structures, for example when used for assembling buoyancy modules for tubular floating wind turbine offshore structures.

[0024] In some specific embodiments, a first welding station is provided, comprising a plurality of rollers arranged in a circular configuration and configured for rotationally supporting the first reinforcing ring in horizontal orientation on the rollers during welding between the first tubular member and the first reinforcing ring. As will become apparent below, this first station is used for welding a subsequent tubular member to the respective ring, in order to provide additional segments for assembling a multi-segmented tube. In the first welding station, one or more welding machines are provided for welding.

[0025] In some embodiments, the one or more welding machines are placed on the outside and inside of the tubular member, facilitating welding on both sides of the weld simultaneously.

[0026] The welding machines are advantageously provided at fixed positions of the first welding station, and the welding of the tubular member to the reinforcing ring is done while the combination of the tubular member and the reinforcing ring is rotated around its vertical central axis, instead of moving the head along the edge.

[0027] An advantage of this practical embodiment is that the fixed welding machines require minimal and only simple mechanical support compared to welding machines that move around the ring, as precision can be a challenge in case of large movements of the head. This is especially true when the diameter of the segments is in the range of 5 to 15 meters.

[0028] In an advantageous embodiment, a second welding station is provided, which comprises a plurality of rollers arranged in a circular configuration and configured for rotationally supporting the second edge of the first segment with its second ring-free edge when the respective first ring is facing upwards and in a horizontal orientation.

[0029] In practice, after welding the first tubular member to the first reinforcing ring in the first welding station so as to obtain a first segment, the resulting first segment is inverted so that the first reinforcing ring faces upwards, and the first segment is lifted onto the second welding station, wherein the rollers rotationally support the second edge of the first segment.

[0030] Optionally, in order to protect the rollers from potentially sharp downward-facing edges, a support ring or support plate is provided between the downward-facing edge and the rollers.

[0031] After welding the second tubular member to the second reinforcing ring in the first welding station, the resulting second segment is inverted so that the second ring faces upwards. The second segment is lifted in the second welding station onto the first ring of the first segment to form a double segment.

[0032] Advantageously, as explained for the first welding station, one or more welding machines are placed on the outside and inside of the tubular members, thereby facilitating simultaneous welding on both sides of the weld seam. The welding machines are advantageously provided at fixed positions of the second welding station, and the welding of the segments to the rings is done while the two segments are rotated around their vertical axis, rather than moving the head along the edge.

[0033] Optionally, the height of the welding head of the welding machine is adjustable, for example by being mounted on an arm of the welding robot. Thereby, the height of the welding head can be adjusted to match the height of the individual segments. However, if all segments have a single standardized height, there is no need to adjust the height of the head.

[0034] In order to even further extend the length of the multi-segment structure, which is typically done in practice to construct a buoyancy module with a desired height, an additional segment can be manufactured and joined to the segments that have already been joined by welding.

[0035] In a preferred embodiment, the additional segment is manufactured using the same method as for the first segment. After manufacturing, the additional segment is inverted and lifted onto the second welding station. The already assembled structure comprising two or more segments is then placed from above on top of the additional segment, with its vertical axis concentric or approximately concentric with the vertical axis of the additional segment. In this position, the tubular members of the first segments and the reinforcing rings of the second segments are joined by welding, the welding forming a watertight joint.

[0036] This method can be repeated with as many relevant segments as desired in order to achieve the desired height.

[0037] It is noted here that the method of lifting a multi-segment already assembled with two or more segments onto an additional segment can seem counter-intuitive compared to the method of lifting individual segments one by one onto the top of a multi-segment already welded. However, as will be explained below, this arrangement is well justified because the welding head does not have to be lifted upwards with each added segment. Instead, by first lifting the multi-segment already welded to a storage station, and then always only lifting onto a single segment for the next welding, the welding head does not need to be lifted higher than the height of a single segment, which is typically in the range of 2 to 10 meters.

[0038] As mentioned above, to protect the rollers, the second welding station optionally comprises a support ring or support plate between the rollers and the second downwardly facing edge of the single segment that is lifted off the first welding station and onto the second welding station. This is more important the more segments that are welded together into an elongated tube, as the weight increases proportionally.

[0039] Advantageously, one, but not two, welding stations not only comprises a first roller supporting the combination of segments and rings from below, but also a second roller laterally abutting the combination for correctly positioning and guiding the rings and tubular members during rotation and simultaneous welding. BRIEF DESCRIPTION OF DRAWINGS

[0040] The application will be explained in more detail with reference to the accompanying drawings, in which

[0041] Figure 1 A rotating support device for a welding station is shown;

[0042] Figure 2 Three stations for welding are shown;

[0043] Figure 3A A first assembly step is shown in perspective view; Figure 3B A first assembly step is shown in side view;

[0044] Figures 4 to 9 A subsequent step for assembly of a double segment is shown;

[0045] Figure 10A Another assembly step is shown in perspective view, with a third segment shown in side view;

[0046] Figure 10B Another assembly step is shown in perspective view, with the third segment in preparation;

[0047] Figure 11A The assembly of three segments is shown in side view;

[0048] Figure 11BA perspective view showing the assembly of three segments;

[0049] Figure 12 A side view showing the movement of the three segments to the storage station;

[0050] Figure 13 A side view showing the three segments in the storage station;

[0051] Figure 14 A perspective view showing the three segments in the storage station and the preparation of the fourth segment in the first welding station;

[0052] Figure 15A A perspective view showing the welding machines in the first and second welding stations;

[0053] Figure 15B An enlarged cross-section showing the welding machine in the first welding station. DETAILED DESCRIPTION

[0054] Figure 1 A rotating support device for the welding station 1 is shown. It comprises a plurality of first rollers 5 on a respective first frame 4 for rotatably supporting the circular tubular segments from below for welding. The first frame 4 and the first rollers 5 are arranged in a circular configuration for supporting the respective circular structure. It further comprises second rollers 6 on a respective second frame 7 for abutting such circular tubular structures from the side and correctly guiding their rotational movement around a vertical central axis.

[0055] Optionally, the second rollers 6 can be radially moved in and out in the horizontal direction. This is useful if the segment diameter changes and also to more easily fit the annular segments into the roller station 1, even if they slightly deviate from the circular shape.

[0056] Figure 2 Three stations for welding are shown. In addition to the third station, which is the storage station 1", a first welding station 1 and a second welding station 1' are provided, the latter comprising a fixed support ring 9'. The support ring 9 is provided on the second welding station 1', for example similar to the support ring 9' of the storage station 1". Further reference is made to Figure 1 , the support ring 9 is supported by the first rollers 5 and fixed to be guided into a circle by the second rollers 6. Instead of the support ring 9, a support plate can optionally be used on the second welding station 1'.

[0057] Figure 3A A perspective view and Figure 3B A side view showing the first assembly step using the first welding station 1 and the second welding station 1'. The reinforcing ring 10 is placed on the support rollers 5 of the first welding station 1 to rotate around the vertical axis X, guided by the second rollers 6 abutting its outer circumference.

[0058] Figure 4It is shown that a first tubular member 11 is placed onto the reinforcement ring 10 with the first edge 11A of the first tubular member 11 abutting the upper surface of the reinforcement ring 10. Advantageously, the first edge 11A has a smaller diameter than the reinforcement ring 10 to ensure that the entire first edge 11A rests on the reinforcement ring 10 and leaves enough space for the fillet weld, even if the first edge 11A is slightly off the circular shape, for example due to deformation caused by transport and handling. The orientation of the first tubular member 11 on top of the reinforcement ring 10 has the advantage that the welding can be performed from above, which is easier than welding from below, in particular due to the melting of the metal during the welding process.

[0059] Figure 5A It is shown that the first segment, manufactured by welding the tubular member 11 and the reinforcement ring 10, is rotated and moved from the first welding station 1 onto the second welding station 1’. Figure 5B The final result of the rotation of the first segment onto the second welding station 1’ is shown in Figure 5A the inverted orientation in which the arrow 8 is pointing downwards. When the first edge 11A of the first tubular member 11 is welded to the first reinforcement ring 10, the second edge 11B of the first tubular member 11 rests on the support ring 9, but is not fastened to the support ring 9, as the support ring 9 remains on the rollers 5 of the second support station 1’ for supporting the individual segments, which are subsequently placed on the support ring 9 with their downward facing edge.

[0060] Figure 6 It is shown that the second segment is prepared, in which the first edge 12A of the second tubular member 12 is welded to the second reinforcement ring 10 in the first welding station 1. In the present exemplary case, the second reinforcement ring 10 is similar in size to the first reinforcement ring 10, but this is not strictly necessary. The process is identical to the process of the first segment shown in Figure 4 and explained above.

[0061] As shown in Figure 7A once the welding in the first welding station 1 is completed, the second segment is rotated and lifted onto the top of the first segment in the second welding station 1’, so that the second downward facing edge 12B of the second segment 12 abuts the first reinforcement ring 10 of the first segment 11 for welding, which is shown in Figure 7B By the welding in the second welding station 1’, the two segments form a double segment 12 / 11, in which the two tubular segments 11, 12 are fastened to each other, with the first reinforcement ring 10 as a single ring between the two mutually adjacent tubular members 11, 12, and the second reinforcement ring 10 on top. The second edge 11B of the first segment 11 rests on the support ring 9.

[0062] In the illustrated process, the first segment is lifted onto the second welding station 1'and the second segment is lifted onto the top of the first segment. Alternatively, the first segment is lifted away from the first welding station 1 and onto the storage station 1 " and the second segment after welding is lifted from the first welding station 1 onto the second welding station 1'after which the first segment is lifted from the storage station 1 " onto the second segment in the second welding station 1 '. This is possible but not preferred as it requires an extra lifting operation. It is noted that the segments are of large size, typically having a diameter of 5-10 meters, favoring a minimization of the number of operations.

[0063] Subsequently, the double segment 12 / 11 is lifted away from the second welding station 1'and, as Figure 8 illustrated, is stored in the storage station 1 " with the downward facing second edge 11B of the first tubular member 11 resting on the support ring 9' of the storage station 1 ".

[0064] Figure 9 The welding of the third tubular member 13 with the third reinforcing ring 10 is illustrated. As Figure 10A illustrated, the resulting third segment is lifted away from the first welding station 1, turned and lifted with the downward facing second edge 13B of the third tubular member 13 onto the upper surface of the support ring 9 of the second welding station 1'with its reinforcing ring 10 facing upwards as Figure 10B illustrated.

[0065] As Figure 11A illustrated, the double segment 12 / 11 is then lifted away from the storage station 1 " and the downward facing second edge 11B of the first tubular member 11 is lifted onto the third support reinforcing ring 10 on top of the third tubular member 13, thereby forming the configuration as Figure 11B illustrated. In the second welding station 1 ', the downward facing second edge 11B of the first tubular member 11 is welded to the third support reinforcing ring 10 on top of the third tubular member 13 to form the triple segment 12 / 11 / 13.

[0066] As Figure 12 illustrated, the triple segment 12 / 11 / 13 is lifted away from the second welding station 1'and, as Figure 13 illustrated, is stored in the storage station 1 ". As Figure 14 illustrated, a fourth segment is produced in the first welding station 1 by using another tubular member 14 and another reinforcing ring 10 and the illustrated process is repeated for the required number of segments to form the final buoyant structure.

[0067] The shown process has some advantages compared to an alternative in which the double segment 12 / 11 is not lifted away from the second welding station 1', but the third segment 13 is lifted onto the top of the double segment 12 / 11 and the fourth segment 14 is lifted onto the top of the triple segment. By lifting and moving the double segment 12 / 11 and the corresponding triple segment 12 / 11 / 13 twice, i.e. from the second welding station 1' to the storage station 1" and back to the second welding station 1' when another single segment has been placed in the second welding station 1', the lowest segment in the second welding station 1' is always welded. Therefore, the welding head in the second welding station 1' does not need to be lifted upwards due to added segments. Since the tubular members 11, 12, 13, 14 of the segments are generally of the same height, there is no need to move the welding head for assembly upwards with each added ring, which minimizes the complexity and speed of construction and operation, especially since the lifting and welding at the two stations 1, 1' can be done simultaneously.

[0068] The welding machine 15 is shown in Figure 15A and in a close-up view in Figure 15B . As shown in Figure 15A , the welding of the third tubular member 13 to the corresponding reinforcement ring 10 can be done simultaneously with the welding of the second tubular member 12 to the reinforcement ring 10 of the first tubular member 11 already welded to the first segment.

[0069] As shown in Figure 15B , the welding machine 15 has a fixed base 16 and welds the third tubular member 13 from above to the third reinforcement ring 10. The welding machine 15 has a link arm 17 that moves the welding head 18 to the welding position. The distance of the welding head 18 from the fixed base 16 is adjustable to slightly change the welding position and angle, but the welding machine 15 is not configured to reach the upper edge 13B of the third tubular member 13. This is explained above with the purpose of simplifying the welding station 15. However, as explained in detail above, once this additional single segment has been manufactured in the first welding station 1 and moved to the second welding station 1', the longer multi-segment has to be moved to the storage station 1" and then from the storage station 1" to the top of the reinforcement ring 10 with only a single tubular member of a segment.

Claims

1. A method for assembling buoyancy modules of a tubular floating offshore structure, wherein the method comprises providing a first tubular member (11) and a first stiffening ring (10) for a first segment and a second tubular member (12) and a second stiffening ring (10) for a second segment, each of the first and second tubular members (11, 12) comprising a first edge (11A, 12A) and an opposite second edge (11B, 12B); wherein the method further comprises, for each of the first and second segments, providing a respective stiffening ring (10) laid flat and having an outer circumference larger than the circumference of the first edge (11A, 12A) of the respective tubular member (11, 12), placing the tubular member (11, 12) with its first edge (11A, 12A) onto the stiffening ring (10) within the outer circumference, and welding the first edge (11A, 12A) to the stiffening ring (10) from above; after inverting the two welded segments so that their respective stiffening rings (10) face upwards, lifting one of the segments with its second edge (11B, 12B) of the tubular member (11, 12) onto the stiffening ring (10) of the other of the segments, and welding the second edge (11B, 12B) to the stiffening ring (10) from above, to form a double segment (12 / 11), wherein one of the stiffening rings (10) is on top and the second edge (11B, 12B) of the stiffening ring (10) facing downwards is on the bottom.

2. The method according to claim 1, wherein the method comprises providing a third stiffening ring (10) laid flat and having a third outer circumference larger than the circumference of a first edge (13A) of a third tubular member (13), placing the third tubular member (13) with its first edge (13A) onto the third stiffening ring (10) within the third outer circumference, and welding the first edge (13A) of the third tubular member (13) to the third stiffening ring (10) to provide a third segment; after inverting the third segment so that the third stiffening ring (10) faces upwards, lifting the double segment (12 / 11) with the second edge (11B, 12B) of the lower one of the segments onto the third stiffening ring (10), and welding it to the third stiffening ring (10) from above, to provide a triple segment (12 / 11 / 13).

3. The method according to claim 2, wherein the method comprises providing a first welding station (1) comprising a plurality of rollers (5, 6) arranged in a circular configuration and configured for rotationally supporting the first reinforcing ring (10) in horizontal orientation on the rollers (5, 6) during the welding between the first tubular member (11) and the first reinforcing ring (10), wherein the method comprises holding a first welding machine (15) for the welding at a fixed position of the first welding station (1) and welding the first tubular member (11) to the first reinforcing ring (10) while rotating the first tubular member (11) around its vertical central axis (X).

4. The method according to claim 3, wherein the method comprises providing a second welding station (1') comprising a plurality of further rollers (5, 6) arranged in a circular configuration and configured for rotationally supporting the first tubular member (11) with its second edge (11B) facing downwards when the respective first reinforcing ring (10) is facing upwards; wherein the method comprises, - after welding the first tubular member (11) to the first reinforcing ring (10) in the first welding station (1) to provide the first segment, lifting the first segment away from the first welding station (1) and inverting the first segment so that the first reinforcing ring (10) is facing upwards; - after welding the second tubular member (12) to the second reinforcing ring (10) in the first welding station (1) to provide the second segment, lifting the second segment away from the first welding station (1) and inverting the second segment so that the second reinforcing ring (10) is facing upwards; - after the turning, lifting the first segment and the second segment one on top of the other onto the second welding station (1') and forming a double segment (12 / 11) having the upper segment and the lower segment by welding the second edge (11B, 12B) of the upper segment of the two segments (11, 12) to the reinforcing ring (10) of the lower segment of the two segments (11, 12) in the second welding station (1'), wherein the rollers (5, 6) rotationally support the second edge (11B, 12B) of the lower segment of the two segments (11, 12); - holding the second welding station (1') at a fixed position of the second welding station (1') and welding from above the lower reinforcing ring of the two reinforcing rings (10) while rotating the double segment (12 / 11) around its vertical central axis.

5. The method according to claim 4, the method comprising - lifting the double segment (12 / 11) away from the second welding station (1') and onto a storage station (1''), - after the third tubular member (13) has been welded to the third reinforcing ring (10) in the first welding station (1) to provide the third segment, and after the third segment has been inverted with the third reinforcing ring (10) facing upwards, lifting the third segment onto the second welding station (1'), - then, in the second welding station (1'), lifting the double segment (12 / 11) off the storage station (1") onto the third reinforcing ring (10), and welding the second edge (11B, 12B) of the lower tubular member of the tubular members (11, 12) of the double segment (12 / 11) onto the third reinforcing ring (10), and forming a triple segment (12 / 11 / 13) of the first segment, the second segment and the third segment, - lifting the formed triple segment (12 / 11 / 13) off the second welding station (1').

6. The method according to claim 4 or 5, wherein the second welding station (1') comprises a support ring (9) or support plate between the rollers (5, 6) and the second edge (11B) of the first tubular member (11), wherein the method comprises, before welding the double segment (12 / 11) in the second welding station (1'), lifting the first segment off the first welding station (1) and inverting the first segment and lifting the first segment onto the support ring (9) or support plate of the second welding station (1').

7. The method according to claim 6, wherein the rollers (5, 6) of at least one of the first welding station (1) and the second welding station (1') comprise a first roller (5) supporting a combination of a tubular member (11, 12, 13, 14) and a reinforcing ring (10) from below and a second roller (6) abutting the combination from the side, and wherein the method comprises guiding the combination by the second roller (6) during rotation and simultaneous welding.

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