Welding method

Abstract

To provide a method for easily welding both sides of a leg or a brace or an intersection point with the brace.SOLUTION: This welding method of an ocean windmill jacket structure 1 including a leg 10 which is a cylindrical steel pipe and a first brace 21 which is a cylindrical steel pipe and which abuts at the end against the side surface of the leg 10 comprises: a first inner side welding step of welding a first intersection point C between the leg 10 and the first brace 21 from the inside of the first brace 21; a gouging step of gouging the first intersection point C1 from the outside of the first brace 21 after the first inner side welding step; and a first outer side welding step of welding the first intersection point C1 from the outside of the first brace 21 after the gouging step.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a welding method.

Background Art

[0002] Conventionally, welding joints of legs and braces included in a jacket structure for an offshore wind turbine have been performed. Patent Document 1 discloses performing double-sided welding at the intersection of a first diagonal brace and a second diagonal brace included in a jacket structure for an offshore wind turbine, and performing double-sided welding at the intersection of the first diagonal brace and a leg included in the jacket structure for an offshore wind turbine.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in Patent Document 1, there was room for improvement in the welding method of these double-sided welds.

[0005] The present disclosure has been made in view of the above circumstances, and an object thereof is to provide a method for easily performing double-sided welding at the intersection of a leg or a brace and a brace.

Means for Solving the Problems

[0006] <1>The welding method according to Embodiment 1 of the present disclosure is a welding method for an offshore windmill jacket structure including a leg that is a cylindrical steel pipe and a first brace that is a cylindrical steel pipe and whose end abuts against the side surface of the leg, and includes a first inner welding step of welding a first intersection point that is an intersection of the leg and the first brace from the inside of the first brace, a gouging step of gouging the first intersection point from the outside of the first brace after the first inner welding step, and a first outer welding step of welding the first intersection point from the outside of the first brace after the gouging step.

Effect of the Invention

[0007] According to the present disclosure, it is possible to provide a method for easily performing double-sided welding at the intersection of a leg or a brace and a brace.

Brief Description of the Drawings

[0008]

Figure 1

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Mode for Carrying Out the Invention

[0009] (First Embodiment) Hereinafter, with reference to the drawings, a welding method for a jacket structure for an offshore wind turbine according to an embodiment of the present disclosure will be described. The welding method according to the present embodiment is performed when joining components constituting the jacket structure for an offshore wind turbine by welding during manufacturing of the jacket structure for an offshore wind turbine at a yard on the ground. In explaining the welding method for the jacket structure for an offshore wind turbine according to the present embodiment, first, the structure of the jacket structure for an offshore wind turbine will be described.

[0010] (Structure of Jacket Structure for Offshore Wind Turbine) FIG. 1 is a perspective view of a jacket structure 1 for an offshore wind turbine according to an embodiment. FIG. 2 is a front view of the jacket structure 1 for an offshore wind turbine according to an embodiment. FIG. 3 is an enlarged view of part III shown in FIG. 1. FIG. 4 is a perspective view showing the arrangement step according to the first embodiment. FIG. 5 is a plan view showing the arrangement step according to the first embodiment. FIG. 6 is a cross-sectional view taken along the line VI-VI of FIG. 5. FIG. 7 is a perspective view showing a state in which the inversion step is performed in the first embodiment. FIG. 8 is a plan view showing a state in which the inversion step is performed in the first embodiment. FIG. 9 is a cross-sectional view taken along the line IX-IX of FIG. 8. As shown in FIG. 1, the jacket structure 1 for an offshore wind turbine includes legs 10, braces 20, and a transition piece 30. The leg 10 is a cylindrical steel pipe extending in the vertical direction. The lower end of the leg 10 is connected to a pile (not shown) driven into the seabed ground. The upper end of the leg 10 is connected to the transition piece 30. At this time, the leg 10 may be appropriately bent or curved according to the dimensions of the transition piece 30 connected to the upper end and the interval between the steel pipe piles (not shown) connected to the lower end. In this embodiment, the jacket structure 1 for an offshore wind turbine includes a plurality of legs 10. Specifically, the jacket structure 1 for an offshore wind turbine has four legs 10.

[0011] The brace 20 connects the four legs 10 to each other along the circumferential direction with respect to the vertical direction of the jacket structure 1 for an offshore wind turbine. In this embodiment, the brace 20 is joined to the leg 10 by welding. Thus, the brace 20 reinforces the structure composed of the four legs 10. As shown in FIG. 2, the brace 20 includes a first brace 21 and a second brace 22. Each of the first brace 21 and the second brace 22 is a cylindrical steel pipe. Both ends of the first brace 21 are respectively connected to the legs 10. That is, both ends of the first brace 21 abut against the side surfaces of the legs 10. In this state, both ends of the first brace 21 are welded to the legs 10 respectively. One end of the second brace 22 is connected to the leg 10, and the other end is connected to the first brace 21. That is, one end of the second brace 22 abuts against the side surface of the leg 10, and the other end abuts against the side surface of the first brace 21. In this state, one end of the second brace 22 is welded to the leg 10, and the other end of the second brace 22 is connected to the first brace 21.

[0012] As shown in FIG. 2, between two legs 10, two second braces 22 are provided for one first brace 21. Thus, the first brace 21 and the second braces 22 form an X shape between the two legs 10. The X shape is provided, for example, in two vertical stages between the two braces 20. Alternatively, the X shape may be provided in only one stage or three or more stages between the two braces 20.

[0013] The transition piece 30 is a portion of the jacket structure 1 for an offshore wind turbine to which an offshore wind turbine (not shown) is connected. Specifically, the lower end of an offshore wind turbine tower (not shown) is connected to the transition piece 30. Further, the transition piece 30 is supported by the four legs 10 described above. With each of the above configurations, the jacket structure 1 for an offshore wind turbine supports an offshore wind turbine (not shown).

[0014] (Welding method for jacket structure for offshore wind turbine) Next, a welding method for the jacket structure 1 for an offshore wind turbine according to the present embodiment will be described. In the present embodiment, in particular, a method for welding a first intersection C1, which is an intersection between the leg 10 and the first brace 21, will be described. In the present embodiment, the first intersection C1 is a contact surface between an end portion of the first brace 21 and a side surface of the leg 10. That is, in the present embodiment, the end portion of the first brace 21 has a shape that is cut in an arc along the outer peripheral surface of the leg 10 as shown in FIG. 3. Thereby, the end portion of the first brace 21 contacts along the outer peripheral surface of the leg 10.

[0015] The welding method for the first intersection C1 includes an arrangement step, a first inner welding step, a gouging step, and a first outer welding step. That is, first, in the placement step, as shown in FIG. 4 or FIG. 5, the leg 10 and the first brace 21 before being connected to each other are placed on the jig J in a state where they are appropriately aligned. Next, in the first inner welding step, the first intersection C1 is welded from the inside. Then, after performing caulking on the first intersection C1 in the caulking step, in the first outer welding step, the first intersection C1 is welded from the outside. By welding the first intersection C1 in the above-described process, that is, when welding the first intersection C1, by first welding from the inside of the first brace 21, the caulking step can be performed from the outside of the first brace 21. This makes it easier to secure a larger working space compared to the case where the caulking step is performed from the inside of the first brace 21. Therefore, when performing double-sided welding of the first intersection C1 between the leg 10 and the first brace 21, the caulking step can be easily performed. Thus, double-sided welding of the intersection between the leg 10 and the first brace 21 can be easily carried out. Hereinafter, details of each step of the welding method according to the present embodiment will be described.

[0016] (Placement Step) The placement step is a step of placing the leg 10 and the first brace 21 in a state where they can be welded to each other. That is, in the placement step, as shown in FIG. 4 or FIG. 5, the leg 10 and the first brace 21 are placed on the jig J in a state where they are appropriately aligned. In the present embodiment, in the placement step, the leg 10 and the first brace 21 are arranged such that the tube axis of the leg 10 is along a substantially horizontal direction and the tube axis of the first brace 21 is along a substantially horizontal direction. In the present embodiment, the substantially horizontal direction means, for example, a direction within a range of ±10° with respect to the horizontal direction with the horizontal direction as a reference. Specifically, with each of the leg 10 and the first brace 21 laid down, after appropriately adjusting the intersection angle between the leg 10 and the first brace 21, the end of the first brace 21 is brought into contact with the side surface of the leg 10.

[0017] In the present embodiment, when the leg 10 and the first brace 21 are arranged as described above by the arranging step, as shown in FIG. 6, the portion below the first intersection C1 is referred to as the first lower portion U1. Further, when the leg 10 and the first brace 21 are arranged as described above by the arranging step, one of the portions that become the side faces of the first intersection C1 is referred to as the first side face S1, and the other is referred to as the second side face S2. In other words, for example, when the first intersection C1 is divided into four in the circumferential direction, the region facing downward is referred to as the first lower portion U1, and the directions that become the side faces are referred to as the first side face S1 and the second side face S2, respectively. When the first intersection C1 is divided into four in the circumferential direction, the region facing the first lower portion U1 is referred to as the second lower portion U2 (details will be described later).

[0018] In the present embodiment, as shown in FIG. 5, when viewed from a direction orthogonal to both the pipe axis of the leg 10 and the first brace 21 in the state where the leg 10 and the first brace 21 are arranged as described above by the arranging step, the side where the intersection angle between the leg 10 and the first brace 21 forms an acute angle is referred to as the acute angle side A1, and the side where the intersection angle between the leg 10 and the first brace 21 forms an obtuse angle is referred to as the obtuse angle side A2.

[0019] As described above, depending on the arranging step, the leg 10 and the first brace 21 are arranged substantially horizontally with respect to each other. For this reason, in the arranging step, the central axis of the leg 10 and the central axis of the first brace 21 are substantially horizontal as shown in FIG. 4 or FIG. 5. That is, in the arranging step, the central axis of the leg 10 and the central axis of the first brace 21 are substantially orthogonal to the vertical line. In the present embodiment, being substantially orthogonal to the vertical line means, for example, that the intersection angle with a vertical line parallel to the vertical direction is within the range of 90° ± 10°. Therefore, the above-described acute angle side A1 and obtuse angle side A2 are defined when viewing the leg 10 and the first brace 21 arranged by the arranging step from the vertical direction. In the present embodiment, as shown in FIGS. 5 and 6, the first side face S1 refers to the side face on the acute angle side A1 among the portions that become the side faces of the first intersection C1 when the leg 10 and the first brace 21 are arranged as described above by the arranging step. In the present embodiment, as shown in FIGS. 5 and 6, the second side surface portion S2 refers to the side surface portion on the side located on the obtuse angle side A2 among the portions that become the side surface portion of the first intersection point C1 when the leg 10 and the first brace 21 are arranged as described above in the arrangement step.

[0020] (First inner welding step) The first inner welding step is a process of welding the first intersection point C1 from the inside of the first brace 21. In the first inner welding step, the first intersection point C1 is welded all around from the inside of the first brace 21. By this, the quality of the welding of the first intersection point C1 from the inside of the first brace 21 is improved. In the first inner welding step, for example, arc welding is performed.

[0021] Here, as shown in FIG. 5, the intersection angle between the outer peripheral surface of the leg 10 and the inner peripheral surface of the first brace 21 becomes an obtuse angle on the acute angle side A1 and an acute angle on the obtuse angle side A2. Also, when welding the first intersection point C1 from the inside of the first brace 21, since it is difficult to bring the welding tool close to the portion where the intersection angle between the outer peripheral surface of the leg 10 and the inner peripheral surface of the first brace 21 is an acute angle, higher welding skills are required than for the portion where the intersection angle is an obtuse angle. That is, when welding the first intersection point C1 from the inside of the first brace 21, relatively high welding skills are required for welding on the obtuse angle side A2 compared to welding on the acute angle side A1. Therefore, in the present embodiment, in the first inner welding step, the acute angle side A1 of the first intersection point C1, which is the side where the intersection angle between the outer peripheral surface of the leg 10 and the inner peripheral surface of the first brace 21 forms an obtuse angle, is pre-welded from the inside of the first brace 21. And the obtuse angle side A2 of the first intersection point C1, which is the side where the intersection angle between the outer peripheral surface of the leg 10 and the inner peripheral surface of the first brace 21 forms an acute angle, is post-welded from the inside of the first brace 21. That is, in the first inner welding step, when welding the first intersection point C1, welding is first performed from the acute angle side A1, and then the obtuse angle side A2 is welded. In this way, by welding first from the acute angle side A1 where the intersection angle between the outer peripheral surface of the leg 10 and the inner peripheral surface of the first brace 21 is an obtuse angle, when welding the obtuse angle side A2 where the intersection angle between the outer peripheral surface of the leg 10 and the inner peripheral surface of the first brace 21 is an acute angle, the positions of the leg 10 and the first brace 21 can be made substantially fixed. Therefore, it is possible to easily suppress the possibility that the positions of the leg 10 and the first brace 21 shift during welding.

[0022] In the present embodiment, the welding amount in the first inner welding step is less than the welding amount in the first outer welding step. That is, the amount of the welding bead when welding the first intersection point C1 from the inside of the first brace 21 is less than the amount of the welding bead when welding from the outside. In this way, it is preferable to increase the welding bead when welding from the outside to easily ensure the quality of the welding.

[0023] Hereinafter, in the first inner welding step, two examples of the procedure for welding the first intersection point C1 will be described. Note that the two examples of the first inner welding step described below each include a first downward posture welding step, a first vertical posture upward welding step, a turning step, a second downward posture welding step, and a second vertical posture upward welding step. The two examples of the first inner welding step differ in the order in which these steps are performed. In the present embodiment, in the first inner welding step, either of the following two examples may be appropriately selected and implemented.

[0024] (First example of the first inner welding step) First, the first example of the first inner welding step will be described. Hereinafter, the first example of the first inner welding step will be simply referred to as the first example. In the first example, the first inner welding step is performed in the order of the first downward posture welding step, the first vertical posture upward welding step, the turning step, the second downward posture welding step, and the second vertical posture upward welding step.

[0025] In the first example, the first downward position welding step is performed after the placement step. In the first downward position welding step, the first lower part U1 is welded in a downward position from the inside of the first brace 21. In the first downward position welding step, for example, welding is performed from the center of the first lower part U1 in the circumferential direction of the first intersection C1 to the portion corresponding to the end of the first side surface part S1. Thereafter, welding is performed from the center of the first lower part U1 in the circumferential direction of the first intersection C1 to the portion corresponding to the end of the second side surface part S2.

[0026] In the first example, the first vertical position upward welding step is performed after the first downward position welding step. In the first vertical position upward welding step, the first side surface part S1 is welded in an upward vertical position from the inside of the first brace 21. In the first vertical position upward welding step, from the end of the first lower part U1 welded in the first downward position welding step, the first side surface part S1 is welded by upward welding to the portion corresponding to the end of the second lower part U2.

[0027] In the first example, the inversion step is performed after the first vertical position upward welding step. In the inversion step, the upper and lower parts of the leg 10 and the first brace 21 arranged such that their pipe axes are respectively along the substantially horizontal direction by the placement step are inverted. By this, the second lower part U2 becomes the lower part of the first intersection C1.

[0028] In the first example, the second downward position welding step is performed after the inversion step. In the second downward position welding step, the second lower part U2 is welded in a downward position from the inside of the first brace 21. In the second downward position welding step, for example, welding is performed from the center of the second lower part U2 in the circumferential direction of the first intersection C1 to the end of the first side surface part S1 welded in the first vertical position upward welding step. Thereafter, welding is performed from the center of the second lower part U2 in the circumferential direction of the first intersection C1 to the portion corresponding to the end of the second side surface part S2.

[0029] In the first example, the second vertical upward welding step is performed after the second downward welding step. In the second vertical upward welding step, the second side surface portion S2 is welded in an upward direction in a vertical posture from the inside of the first brace 21. In the second vertical upward welding step, from the end portion of the second lower portion U2 welded in the second downward welding step, the second side surface portion S2 is welded by upward welding to the end portion of the first lower portion U1. By the above steps, the first inner welding step according to the first example is performed.

[0030] (Second example of the first inner welding step) Next, a second example of the first inner welding step will be described. Hereinafter, the second example of the first inner welding step will simply be referred to as the second example. In the second example, the first inner welding step is performed in the order of the first vertical upward welding step, the first downward welding step, the inversion step, the second vertical upward welding step, and the second downward welding step.

[0031] In the second example, the first vertical upward welding step is performed after the placement step. In the first vertical upward welding step, the first side surface portion S1 is welded in an upward direction in a vertical posture from the inside of the first brace 21. In the first vertical upward welding step, upward welding is performed from the portion corresponding to the end portion of the first lower portion U1 to the portion corresponding to the end portion of the second lower portion U2.

[0032] In the second example, the first downward posture welding step is performed after the first upward vertical posture welding step. In the first downward posture welding step, the first lower part U1 is welded in a downward posture from the inside of the first brace 21 and from the portion welded in the first upward vertical posture welding step. In the first downward posture welding step, the first lower part U1 is welded from the end of the first side surface part S1 welded in the first upward vertical posture welding step to the portion corresponding to the end of the second side surface part S2. At this time, the welding may be performed separately from the end of the first side surface part S1 to the center of the first lower part U1 in the circumferential direction of the first intersection C1 and from the center of the first lower part U1 to the portion corresponding to the end of the second side surface part S2. Alternatively, the first lower part U1 may be continuously welded from the end of the first side surface part S1 to the portion corresponding to the end of the second side surface part S2.

[0033] In the second example, the inversion step is performed after the first downward posture welding step. The inversion step is performed in the same manner as in the first example.

[0034] In the second example, the second upward vertical posture welding step is performed after the inversion step. In the second upward vertical posture welding step, the second side surface part S2 is welded in an upward vertical posture from the inside of the first brace 21. In the second upward vertical posture welding step, the welding is performed upward from the portion corresponding to the end of the second lower part U2 to the end of the first lower part U1 welded in the first downward posture welding step.

[0035] In the second example, the second downward position welding step is performed after the second vertical upward position welding step. In the second downward position welding step, the second lower part U2 is welded in a downward position from inside the first brace 21 and from the portion welded in the second vertical upward position welding step. In the second downward position welding step, the second lower part U2 is welded from the end of the second side surface part S2 welded in the second vertical upward position welding step to the end of the first side surface part S1 welded in the first vertical upward position welding step. At this time, the welding from the end of the second side surface part S2 to the center of the second lower part U2 in the circumferential direction of the first intersection point C1 and the welding from the center of the second lower part U2 to the end of the first side surface part S1 may be performed individually. Alternatively, the second lower part U2 may be continuously welded from the end of the second side surface part S2 to the end of the first side surface part S1. By the above process, the first inner welding step according to the second example is performed.

[0036] (Gouging step) The gouging step is a process of gouging the first intersection point C1 from the outside of the first brace 21 after the first inner welding step described above. That is, the gouging step is a process of removing the welding bead that has penetrated from the inside to the outside of the first brace 21 among the welding beads generated in the first inner welding step. In the gouging step, for example, a method by arc gouging is preferably used. In the present embodiment, the gouging step is performed from the outside of the first brace 21. By this, it is possible to easily secure a working space as compared with the case where the gouging step is performed from the inside of the first brace 21.

[0037] Here, in the gouging step, for example, since it is easier to bring a tool for performing gouging closer to the welded part, the work on the obtuse angle side A2 is easier than that on the acute angle side A1. In this embodiment, the gouging step may start from any part at the first intersection C1. For example, when the first intersection C1 is viewed from a direction orthogonal to both the pipe axis of the leg 10 and the pipe axis of the first brace 21, it is preferably started from the side where the outer peripheral surface of the leg 10 and the outer peripheral surface of the first brace 21 form an obtuse angle. By doing this, after performing the work on the acute angle side A1 where the work is relatively easy first, when performing gouging on the obtuse angle side A2, the work can be performed in a relatively familiar state.

[0038] (First outer welding step) The first outer welding step is a process of welding the first intersection C1 from the outside of the first brace 21 after the gouging step. The first outer welding step may start from any part at the first intersection C1. In the first outer welding step, the welding operation may be continuously performed over the circumferential direction of the first intersection C1, or may not be continuously performed. Through the above steps, the first intersection C1 is welded.

[0039] As described above, according to the welding method of the jacket structure 1 for an offshore wind turbine according to this embodiment, after the first inner welding step, it includes a gouging step of gouging the first intersection C1 from the outside of the first brace 21. Then, after the gouging step, in the first outer welding step, the first intersection C1 is welded from the outside of the first brace 21. In this way, by first welding the first intersection C1, which is the intersection of the leg 10 and the first brace 21, from the inside of the first brace 21, the gouging work can be performed from the outside of the first brace 21. Therefore, compared with the case where the gouging work is performed from the inside of the first brace 21, it is easier to secure a larger working space. Thus, when performing double-sided welding at the first intersection C1 between the leg 10 and the first brace 21, the gouging step can be made easier to perform. Therefore, it is possible to easily perform double-sided welding at the intersection between the leg 10 and the first brace 21.

[0040] In addition, the first inner welding step includes an inversion step of inverting the vertical positions of the leg 10 and the first brace 21 after the first downward position welding step and the first vertical position upward welding step. Then, after the inversion step, a second downward position welding step and a second vertical position upward welding step are performed. By providing the inversion step in this way, when welding the first intersection C1 from the inside of the first brace 21, the work performed by the welder can be limited to only one of upward welding in the vertical position or welding in the downward position. Therefore, the skills required of the welder can be kept low by eliminating upward welding work and downward welding work in the upward position, which require relatively high skills.

[0041] In addition, the first inner welding step includes an inversion step of inverting the vertical positions of the leg 10 and the first brace 21 after the first vertical position upward welding step and the first downward position welding step. Then, after the inversion step, a second vertical position upward welding step and a second downward position welding step are performed. By providing the inversion step in this way, when welding the first intersection C1 from the inside of the first brace 21, the work performed by the welder can be limited to only one of upward welding in the vertical position or welding in the downward position. Therefore, the skills required of the welder can be kept low by eliminating upward welding work and downward welding work in the upward position, which require relatively high skills.

[0042] Here, when welding the first intersection C1 from the inside of the first brace 21, a portion where the intersection angle between the outer peripheral surface of the leg 10 and the inner peripheral surface of the first brace 21 is an acute angle requires higher welding skills than a portion where the intersection angle is an obtuse angle. Therefore, in the first inner welding step, among the first intersection points C1, the side where the intersection angle between the outer peripheral surface of the leg 10 and the inner peripheral surface of the first brace 21 forms an obtuse angle is pre-welded from the inside of the first brace 21, and among the first intersection points C1, the side where the intersection angle between the outer peripheral surface of the leg 10 and the inner peripheral surface of the first brace 21 forms an acute angle is post-welded from the inside of the first brace 21. In this way, by welding the side that requires relatively less welding skill first, when welding the side that requires relatively high welding skill, the positions of the leg 10 and the first brace 21 can be kept in a substantially fixed state. Therefore, it is easier to suppress the possibility that the positions of the leg 10 and the first brace 21 shift during welding. Therefore, it is possible to make it easier to perform the welding operation at the first intersection point C1.

[0043] Also, in the first inner welding step, the first intersection point C1 is welded all around from the inside of the first brace 21. Thereby, the weld beads formed inside the first intersection point C1 can be continuously provided over the entire circumference. Therefore, the welding quality can be further improved.

[0044] Here, when performing the gouging step from the outside of the first brace 21, for the portion where the intersection angle between the outer peripheral surface of the leg 10 and the outer peripheral surface of the first brace 21 forms an obtuse angle, it is easier to bring the tool for gouging closer to the welded part than the portion where the intersection angle forms an acute angle, and the work can be performed relatively easily. Therefore, the gouging step starts from the side where the outer peripheral surface of the leg 10 and the outer peripheral surface of the first brace 21 form an obtuse angle when the first intersection point C1 is viewed from the direction orthogonal to both the pipe axis of the leg 10 and the first brace 21. In this way, by starting the gouging step from the side where the work can be performed relatively easily, for example, when the operator performs gouging on the portion where the intersection angle between the outer peripheral surface of the leg 10 and the outer peripheral surface of the first brace 21 forms an acute angle, the work can be performed in a relatively proficient state. Therefore, it is easier to improve the efficiency and quality of the gouging work.

[0045] Here, for example, in the first inner welding step, it is conceivable that the welder's welding posture is poor and the visibility is not good. Therefore, the welding amount in the first inner welding step is less than that in the first outer welding step. By doing this, it is possible to suppress the welding amount in the first inner welding step and easily ensure the quality of the first inner welding step. Further, when welding the leg 10 and the first brace 21, by increasing the number of outer welding beads, the work amount of inner welding, for which it is relatively difficult to ensure quality, can be reduced, and the work amount of outer welding, for which it is relatively easy to ensure quality, can be increased, so that it is possible to more reliably ensure the welding quality.

[0046] (Second Embodiment) Next, the welding method according to the second embodiment of the present disclosure will be described with reference to FIG. 10. In this second embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted, and only the differences will be described. FIG. 10 is a perspective view showing the arrangement step according to the second embodiment. FIG. 11 is a front view showing the arrangement step according to the second embodiment. The welding method according to the second embodiment is different from the first embodiment in that, in the arrangement step, the leg 10 and the first brace 21 are arranged as follows.

[0047] In the second embodiment, in the arrangement step, the leg 10 and the first brace 21 are arranged such that the tube axis of the leg 10 is along the substantially horizontal direction and the tube axis of the first brace 21 is along the substantially vertical direction. That is, in the arrangement step in the second embodiment, it is the same as the first embodiment in that the leg 10 is in a laid-down state, but is different from the first embodiment in that the first brace 21 is in an upright state. Note that arranging the tube axis of the first brace 21 along the substantially vertical direction means that while the first brace 21 is in an upright state, the intersection angle with respect to the laid-down leg 10 is adjusted so that the tube axis of the first brace 21 is inclined from the vertical direction.

[0048] In the second embodiment, after the placement step, in the first inner welding step, the first intersection point C1 is welded from the inside of the first brace 21. At this time, since the leg 10 and the first brace 21 are arranged in the above-described state in the placement step, in the placement step of the second embodiment, when viewed from the axial direction of the leg 10, the axial line of the first brace 21 is substantially vertical. In other words, the axial line of the first brace 21 is substantially parallel to the vertical line when viewed from the axial direction of the leg 10. In the present embodiment, being substantially parallel to the vertical line means, for example, that the intersection angle with the vertical line parallel to the vertical direction is within the range of ±10°. That is, the first intersection point C1 faces substantially in the horizontal direction. Thereby, in the first inner welding step according to the second embodiment, the welding operation can be performed only in the downward posture or only in the upright posture over the circumferential direction of the first intersection point C1. In the second embodiment, the gauging step and the first outer welding step are performed in the same manner as in the first embodiment.

[0049] As described above, according to the welding method of the jacket structure 1 for an offshore wind turbine according to the second embodiment, in the placement step, the leg 10 and the first brace 21 are arranged such that the axial line of the leg 10 is along the substantially horizontal direction and the axial line of the first brace 21 is along the substantially vertical direction. By arranging the first brace 21 and the leg 10 in this way, the first intersection point C1 is arranged to face substantially in the horizontal direction. Therefore, when welding the first intersection point C1 in the first inner welding step, the welding operation can be performed in the same posture over the entire circumference of the first intersection point C1. That is, for example, the welding operation can be performed in the downward posture or the upright posture over the entire circumference of the first intersection point C1. Thus, the welding operation can be made easier. Also, for example, the welding operation in the upward posture, which requires relatively high skills, is not required, and the skills required for the welder can be kept low.

[0050] (Third Embodiment) Next, the welding method of the third embodiment according to the present disclosure will be described with reference to FIG. 12. In addition, in this third embodiment, the same components as those in the first and second embodiments are denoted by the same reference numerals, and the description thereof is omitted, and only the differences will be described. FIG. 12 is a plan view showing a first example of the arrangement step according to the third embodiment. FIG. 13 is a perspective view showing a second example of the arrangement step according to the third embodiment. The welding method according to the third embodiment is a method of welding the second intersection C2 (first intersection), which is the intersection of the first brace 21 and the second brace 22, among the jacket structures 1 for offshore wind turbines.

[0051] The welding method of the second intersection C2 includes an arrangement step, a first inner welding step, a gouging step, and a first outer welding step. That is, the welding of the second intersection C2 is performed in the same manner as the welding of the first intersection C1 described in the first and second embodiments. Specifically, in the welding method of the second intersection C2, the first brace 21 corresponds to the leg 10 in the first and second embodiments, and the second brace 22 corresponds to the first brace 21 in the first and second embodiments.

[0052] More specifically, first, in the arrangement step, the first brace 21 and the second brace 22 are arranged on the jig J in a state where they are aligned with each other. The positional relationship between the first brace 21 and the second brace 22 may be the same as the form described in the first or second embodiment. That is, for example, as shown in FIG. 12, each of the first brace 21 and the second brace 22 may be laid down, or as shown in FIG. 13, the first brace 21 may be laid down and the second brace 22 may be set upright. Hereinafter, when viewed from a direction orthogonal to both the pipe axis of the first brace 21 and the pipe axis of the second brace 22 in a state where the first brace 21 and the second brace 22 are aligned in a state where they can be welded to each other by the arrangement step, the side where the intersection angle between the pipe axis of the first brace 21 and the pipe axis of the second brace 22 forms an acute angle is referred to as the acute angle side A1, and the side where the intersection angle between the pipe axis of the first brace 21 and the pipe axis of the second brace 22 forms an obtuse angle is referred to as the obtuse angle side A2.

[0053] Next, in the first inner welding step, weld the second intersection C2 from the inside of the second brace 22. In the first inner welding step, perform full circumferential welding of the second intersection C2 from the inside of the second brace 22. In the third embodiment, in the first inner welding step, on the acute angle side A1 of the second intersection C2, which is the side where the intersection angle between the outer peripheral surface of the first brace 21 and the inner peripheral surface of the second brace 22 forms an obtuse angle, perform pre-welding from the inside of the second brace 22. Then, on the obtuse angle side A2 of the second intersection C2, which is the side where the intersection angle between the outer peripheral surface of the first brace 21 and the inner peripheral surface of the second brace 22 forms an acute angle, perform post-welding from the inside of the second brace 22. That is, in the first inner welding step, when welding the second intersection C2, weld first from the acute angle side A1, and then weld the obtuse angle side A2. Then, in the gouging step, after gouging the second intersection C2 from the outside of the second brace 22, in the first outer welding step, weld the second intersection C2 from the outside of the second brace 22. Through the above steps, the second intersection C2 is welded. In the third embodiment, matters other than those described above are the same as in the first embodiment or the second embodiment, and thus the description is omitted.

[0054] As described above, according to the welding method of the offshore windmill jacket structure 1 according to the third embodiment, after the first inner welding step, it includes a gouging step of gouging the second intersection C2 from the outside of the second brace 22. Then, after the gouging step, in the first outer welding step, weld the second intersection C2 from the outside of the second brace 22. In this way, by first welding the second intersection C2 between the first brace 21 and the second brace 22 from the inside of the second brace 22, the gouging work can be performed from the outside of the second brace 22. Therefore, compared with the case where the gouging work is performed from the inside of the second brace 22, it is easier to secure a wider working space. Thus, when performing double-sided welding of the second intersection C2 between the first brace 21 and the second brace 22, the gouging step can be easily performed. Therefore, it is possible to easily perform double-sided welding of the intersection between the first brace 21 and the second brace 22.

[0055] Also, in the first inner welding step, the second intersection C2 is welded all around from the inside of the second brace 22. As a result, a weld bead formed inside the second intersection C2 can be continuously provided over the entire circumference. Therefore, the welding quality can be further improved.

[0056] Here, when welding the second intersection C2 from the inside of the second brace 22, a portion where the intersection angle between the outer peripheral surface of the first brace 21 and the inner peripheral surface of the second brace 22 is an acute angle requires a higher welding skill than a portion where the intersection angle is an obtuse angle. Therefore, in the first inner welding step, among the second intersection C2, the side where the intersection angle between the outer peripheral surface of the first brace 21 and the inner peripheral surface of the second brace 22 forms an obtuse angle is pre-welded from the inside of the second brace 22, and among the second intersection C2, the side where the intersection angle between the outer peripheral surface of the first brace 21 and the inner peripheral surface of the second brace 22 forms an acute angle is post-welded from the inside of the second brace 22. In this way, by welding the side that does not require a relatively high welding skill first, when welding the side that requires a relatively high welding skill, the positions of the first brace 21 and the second brace 22 can be made substantially fixed. Therefore, it is easier to suppress the possibility that the positions of the first brace 21 and the second brace 22 shift during welding. Therefore, it is easier to perform the welding operation of the second intersection C2.

[0057] Note that the technical scope of the present disclosure is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present disclosure. For example, when welding the leg 10 and the first brace 21, before performing the first inner welding step, after adjusting the position of the first brace 21 with respect to the leg 10, it may be temporarily fixed by assembly welding or tack welding. Since the gouging step is performed from the outside of the first brace 21, the assembly welding is preferably performed from the outside of the first brace 21. Also, even when welding the first brace 21 and the second brace 22, before performing the first inner welding step, after adjusting the position of the second brace 22 with respect to the first brace 21, it may be temporarily fixed by assembly welding or tack welding. In addition, since the gouging step is performed from the outside of the second brace 22, it is preferable to perform the assembly welding from the outside of the second brace 22. The above-mentioned assembly welding or tack welding may be performed in any welding posture.

[0058] Also, the welding operations of the leg 10 with the first brace 21 and the first brace 21 with the second brace 22 may be performed simultaneously in parallel with each other. In this case, for example, the inversion step, which is a common working process, can be completed in one go. Also, appropriate grooves may be provided in each welded part described in the embodiment. When providing a groove, the groove formed inside the first brace 21 or the second brace 22 is preferably smaller than the groove formed outside. In this way, by making the outer groove larger, it is possible to reduce the amount of work for inner welding, which is relatively difficult to ensure quality, and increase the amount of work for outer welding, which is relatively easy to ensure quality, so that it is easier to ensure the quality of welding. Also, the leg 10 and the second brace 22 may be connected by the welding method of the leg 10 and the first brace 21 described in the first embodiment. Also, the gouging step may start, for example, from the side where the outer peripheral surface of the leg 10 and the outer peripheral surface of the first brace 21 form an acute angle, which is the side where the work is relatively difficult. In this way, the relatively difficult work may be performed when the operator has enough physical strength. Also, regardless of the above, the gouging step may be performed in any working order based on any other judgment. In the first inner welding step, the obtuse angle side A2 of the first intersection point C1, which is on the side where the intersection angle between the outer peripheral surface of the leg 10 and the inner peripheral surface of the first brace 21 forms an acute angle, may be pre-welded from the inside of the first brace 21, and the acute angle side A1 of the first intersection point C1, which is on the side where the intersection angle between the outer peripheral surface of the leg 10 and the inner peripheral surface of the first brace 21 forms an obtuse angle, may be post-welded from the inside of the first brace 21. That is, in the first inner welding step, when welding the first intersection point C1, welding may be performed first from the obtuse angle side A2, and then the acute angle side A1 may be welded. By doing this, relatively difficult work may be performed when the operator has enough physical strength.

[0059] In addition, within the scope not departing from the gist of the present disclosure, it is possible to appropriately replace the components in the above-described embodiment with well-known components, and the above-described modified examples may also be appropriately combined.

[0060] (Appendix) The welding method according to the above-described embodiment is understood as follows, for example. <1> A welding method according to an aspect of the present disclosure is a welding method for an offshore windmill jacket structure including a leg that is a cylindrical steel pipe and a first brace that is a cylindrical steel pipe and whose end abuts against the side surface of the leg. The welding method includes a first inner welding step of welding a first intersection point, which is an intersection point between the leg and the first brace, from the inside of the first brace, a gouging step of gouging the first intersection point from the outside of the first brace after the first inner welding step, and a first outer welding step of welding the first intersection point from the outside of the first brace after the gouging step.

[0061] The welding method of the jacket structure for the above-mentioned offshore wind turbine includes a gouging step of gouging the first intersection from the outside of the first brace after the first inner welding step. Then, after the gouging step, in the first outer welding step, the first intersection is welded from the outside of the first brace. In this way, by first welding the first intersection, which is the intersection of the leg and the first brace, from the inside of the first brace, the gouging work can be performed from the outside of the first brace. Therefore, compared with the case where the gouging work is performed from the inside of the first brace, it is easier to secure a wider working space. Thus, when performing double-sided welding of the first intersection of the leg and the first brace, the gouging step can be facilitated. Therefore, it is possible to easily perform double-sided welding of the intersection of the leg and the first brace.

[0062] <2>In the welding method according to <1>, an arrangement step of arranging the leg and the first brace is further provided such that the pipe axis of the leg is along a substantially horizontal direction and the pipe axis of the first brace is along a substantially horizontal direction. The first inner welding step includes, after the arrangement step, a first downward posture welding step of welding a first lower part, which is a part below the first intersection, from the inside of the first brace in a downward posture, a first upright posture upward welding step of welding a first side face part, which is one of the side face parts of the first intersection, from the inside of the first brace in an upright posture and upward after the first downward posture welding step, a reversing step of reversing the up and down of the leg and the first brace after the first upright posture upward welding step, a second downward posture welding step of welding a second lower part, which is a part below the first intersection, from the inside of the first brace in a downward posture after the reversing step, and a second upright posture upward welding step of welding a second side face part, which is the other of the side face parts of the first intersection, from the inside of the first brace in an upright posture and upward after the second downward posture welding step. A configuration characterized by including these steps may be adopted.

[0063] The first inner welding step includes an inversion step of inverting the vertical positions of the leg and the first brace after the first downward posture welding step and the first upward progression welding step in the vertical posture. After the inversion step, a second downward posture welding step and a second upward progression welding step in the vertical posture are performed. By providing the inversion step in this way, when welding the first intersection point from the inside of the first brace, the work performed by the welder can be limited to either welding in the downward posture or upward progression welding in the vertical posture. Therefore, it is possible to reduce the skills required of the welder by eliminating the upward posture welding work and downward welding that require relatively high skills.

[0064] <3>In the welding method according to <1> or <2> above, an arrangement step of arranging the leg and the first brace is further provided such that the pipe axis of the leg is along a substantially horizontal direction and the pipe axis of the first brace is along a substantially horizontal direction. The first inner welding step includes, after the arrangement step, a first upward progression welding step of welding, in an upward vertical posture from the inside of the first brace, a first side surface portion which is one of the side surface portions of the first intersection point; after the first upward progression welding step, a first downward posture welding step of welding, in a downward posture from the inside of the first brace and from the portion welded in the first upward progression welding step, a first lower portion which is the lower portion of the first intersection point; after the first downward posture welding step, an inversion step of inverting the vertical positions of the leg and the first brace; after the inversion step, a second upward progression welding step of welding, in an upward vertical posture from the inside of the first brace, a second side surface portion which is the other of the side surface portions of the first intersection point; and after the second upward progression welding step, a second downward posture welding step of welding, in a downward posture from the inside of the first brace and from the portion welded in the second upward progression welding step, a second lower portion which is the lower portion of the first intersection point. A configuration characterized by including these steps may be adopted.

[0065] The first inner welding step includes an inversion step of inverting the up and down of the leg and the first brace after the first vertical upward welding step and the first downward welding step. Then, after the inversion step, a second vertical upward welding step and a second downward welding step are performed. By providing the inversion step in this way, when welding the first intersection point from the inside of the first brace, the work performed by the welder can be limited to only either upward welding in the vertical position or welding in the downward position. Therefore, it is possible to eliminate the need for upward welding work in the vertical position or downward welding that requires relatively high skills, and keep the skills required for the welder low.

[0066] <4>In the welding method according to any one of the above <1> to <3> aspects, an arrangement step of arranging the leg and the first brace is further provided such that the pipe axis of the leg is along a substantially horizontal direction and the pipe axis of the first brace is along a substantially vertical direction, and the first inner welding step welds a first intersection point between the leg and the first brace from the inside of the first brace after the arrangement step. A configuration characterized by this may be adopted.

[0067] In the arrangement step, the leg and the first brace are arranged such that the pipe axis of the leg is along a substantially horizontal direction and the pipe axis of the first brace is along a substantially vertical direction. By arranging the first brace and the leg in this way, the first intersection point is arranged to face substantially horizontally. Therefore, when welding the first intersection point in the first inner welding step, the welding work can be performed in the same position around the entire circumference of the first intersection point. That is, for example, the welding work can be performed in a downward position or a vertical position around the entire circumference of the first intersection point. Therefore, the welding work can be made easier. Also, for example, it is possible to eliminate the need for upward welding work in the vertical position that requires relatively high skills, and keep the skills required for the welder low.

[0068] <5>In the welding method according to any one of <1> to <4> above, in the first inner welding step, among the first intersections, the side where the intersection angle between the outer peripheral surface of the leg and the inner peripheral surface of the first brace forms an obtuse angle is pre-welded from the inside of the first brace, and among the first intersections, the side where the intersection angle between the outer peripheral surface of the leg and the inner peripheral surface of the first brace forms an acute angle is post-welded from the inside of the first brace. A configuration characterized by this may be adopted.

[0069] Here, when welding the first intersection from the inside of the first brace, a higher welding skill is required for the portion where the intersection angle between the outer peripheral surface of the leg and the inner peripheral surface of the first brace is acute than for the portion where the intersection angle is obtuse. Therefore, in the first inner welding step, among the first intersections, the side where the intersection angle between the outer peripheral surface of the leg and the inner peripheral surface of the first brace forms an obtuse angle is pre-welded from the inside of the first brace, and among the first intersections, the side where the intersection angle between the outer peripheral surface of the leg and the inner peripheral surface of the first brace forms an acute angle is post-welded from the inside of the first brace. In this way, by welding the side that requires relatively less welding skill first, when welding the side that requires relatively higher welding skill, the positions of the leg and the first brace can be made in a substantially fixed state. Therefore, it is easier to suppress the possibility that the positions of the leg and the first brace shift during welding. Therefore, it is easier to perform the welding operation of the first intersection.

[0070] <6>In the welding method according to any one of <1> to <5> above, in the first inner welding step, the first intersection is welded all around from the inside of the first brace. A configuration characterized by this may be adopted.

[0071] In the first inner welding step, the first intersection is welded all around from the inside of the first brace. Thereby, the weld bead formed inside the first intersection can be continuously provided over the entire circumference. Therefore, the welding quality can be further improved.

[0072] <7>In the welding method according to any one of the above <1> to <6> aspects, the gouging step is started from the side where the outer peripheral surface of the leg and the outer peripheral surface of the first brace form an obtuse angle when viewed from a direction orthogonal to both the pipe axis of the leg and the pipe axis of the first brace. A configuration characterized by this may be adopted.

[0073] Here, when performing the gouging step from the outside of the first brace, the portion where the intersection angle between the outer peripheral surface of the leg and the outer peripheral surface of the first brace is an obtuse angle makes it easier for the tool for gouging to approach the welded part than the portion where the intersection angle is an acute angle, and the work can be performed relatively easily. Therefore, the gouging step is started from the side where the outer peripheral surface of the leg and the outer peripheral surface of the first brace form an obtuse angle among the first intersections when viewed from a direction orthogonal to both the pipe axis of the leg and the first brace. In this way, by starting the gouging step from the side where the work can be performed relatively easily, for example, when the operator performs gouging on the portion where the intersection angle between the outer peripheral surface of the leg and the outer peripheral surface of the first brace is an acute angle, the work can be performed in a relatively skilled state. Therefore, it is easy to improve the efficiency and quality of the gouging work.

[0074] <8>A welding method according to one aspect of the present disclosure is a welding method for an offshore wind turbine jacket structure including a first brace that is a cylindrical steel pipe and a second brace that is a cylindrical steel pipe and whose end abuts against the side surface of the first brace. The method includes a first inner welding step of welding a first intersection, which is an intersection between the first brace and the second brace, from the inside of the second brace, a gouging step of gouging the first intersection from the outside of the second brace after the first inner welding step, and a first outer welding step of welding the first intersection from the outside of the second brace after the gouging step.

[0075] The welding method of the jacket structure for the above-mentioned offshore wind turbine includes a gouging step of gouging the second intersection from the outside of the second brace after the first inner welding step. And after the gouging step, in the first outer welding step, the second intersection is welded from the outside of the second brace. In this way, by first welding the second intersection of the first brace and the second brace from the inside of the second brace, the gouging work can be carried out from the outside of the second brace. Therefore, compared with the case where the gouging work is carried out from the inside of the second brace, it is easier to secure a wider working space. Therefore, when performing double-sided welding at the second intersection of the first brace and the second brace, the gouging step can be made easier. Therefore, it is possible to easily perform double-sided welding at both sides of the intersection of the first brace and the second brace.

[0076] <9>In the welding method according to <8> above, in the first inner welding step, it may be adopted to adopt a configuration characterized in that the first intersection is welded all around from the inside of the second brace.

[0077] In the first inner welding step, the second intersection is welded all around from the inside of the second brace. Thereby, the weld bead formed inside the second intersection can be continuously provided over the entire circumference. Therefore, the quality of welding can be further improved.

[0078] <10>In the welding method according to <8> or <9> above, in the first inner welding step, among the first intersections, the side where the intersection angle between the outer peripheral surface of the first brace and the inner peripheral surface of the second brace forms an obtuse angle is pre-welded from the inside of the second brace, and among the first intersections, the side where the intersection angle between the outer peripheral surface of the first brace and the inner peripheral surface of the second brace forms an acute angle is post-welded from the inside of the second brace. It may be adopted to adopt a configuration characterized by this.

[0079] Here, when welding the second intersection from the inside of the second brace, a higher welding skill is required for the portion where the intersection angle between the outer peripheral surface of the first brace and the inner peripheral surface of the second brace forms an acute angle than for the portion where the intersection angle forms an obtuse angle. Therefore, in the first inner welding step, among the second intersections, the side where the intersection angle between the outer peripheral surface of the first brace and the inner peripheral surface of the second brace forms an obtuse angle is pre-welded from the inside of the second brace, and among the second intersections, the side where the intersection angle between the outer peripheral surface of the first brace and the inner peripheral surface of the second brace forms an acute angle is post-welded from the inside of the second brace. In this way, by welding the side that requires relatively less welding skill first, when welding the side that requires relatively high welding skill, the positions of the first brace and the second brace can be kept substantially fixed. Therefore, it is possible to easily suppress the possibility that the positions of the first brace and the second brace shift during welding. Therefore, it is possible to more easily perform the welding operation at the second intersection.

[0080] <11>In the welding method according to any one of the above aspects <1> to <10>, a configuration may be adopted in which the welding amount in the first inner welding step is less than the welding amount in the first outer welding step.

[0081] Here, for example, in the first inner welding step, it is considered that the welder's welding posture is poor and the visibility is not good. Therefore, the welding amount in the first inner welding step is less than the welding amount in the first outer welding step. By doing this, it is possible to suppress the welding amount in the first inner welding step and easily ensure the quality of the first inner welding step. Also, when welding the leg and the first brace, by increasing the number of outer welding beads, the amount of work for inner welding, which is relatively difficult to ensure quality, can be reduced, and the amount of work for outer welding, which is relatively easy to ensure quality, can be increased, so that it is possible to more reliably ensure the quality of welding.

Explanation of Signs

[0082] 1 Jacket structure for offshore wind turbine 10 Leg 20 Brace 21 First brace 22 Second brace 30 Transition piece A1 Acute angle side A2 Obtuse angle side C1 First intersection point C2 Second intersection point S1 First side face S2 Second side face U1 First lower part U2 Second lower part

Claims

1. A leg that is a cylindrical steel pipe, A first brace that is a cylindrical steel pipe, wherein an end portion thereof abuts against a side surface of the leg, A welding method for a jacket structure for an offshore wind turbine, comprising: A first inner welding step of welding a first intersection, which is an intersection of the leg and the first brace, from inside the first brace; After the first inner welding step, a gouging step of gouging the first intersection from outside the first brace; After the gouging step, a first outer welding step of welding the first intersection from outside the first brace; A welding method characterized by comprising the above.

2. An arrangement step of arranging the leg and the first brace such that the pipe axis of the leg is along a substantially horizontal direction and the pipe axis of the first brace is along a substantially horizontal direction, Further comprising: The first inner welding step includes: After the arrangement step, a first downward posture welding step of welding a first lower part, which is a part below the first intersection, from inside the first brace in a downward posture; After the first downward posture welding step, a first upward vertical posture welding step of welding a first side part, which is one of the side parts of the first intersection, from inside the first brace in an upward vertical posture; After the first upward vertical posture welding step, a reversing step of reversing the up and down of the leg and the first brace; After the reversing step, a second downward posture welding step of welding a second lower part, which is a part below the first intersection, from inside the first brace in a downward posture; After the second downward posture welding step, a second upward vertical posture welding step of welding a second side part, which is the other of the side parts of the first intersection, from inside the first brace in an upward vertical posture; The welding method according to claim 1, characterized by including the above.

3. An arranging step of arranging the leg and the first brace such that the pipe axis of the leg is along a substantially horizontal direction and the pipe axis of the first brace is along a substantially horizontal direction; further comprising: The first inner welding step includes: a first vertically upward welding step of performing upward welding in a standing posture from the inside of the first brace on a first side surface which is one of the portions forming the side surface of the first intersection after the arranging step; a first downward welding step of performing welding in a downward posture from the inside of the first brace and from the portion welded in the first vertically upward welding step on a first lower portion which is the portion forming the lower part of the first intersection after the first vertically upward welding step; an inversion step of inverting the leg and the first brace up and down after the first downward welding step; a second vertically upward welding step of performing upward welding in a standing posture from the inside of the first brace on a second side surface which is the other of the portions forming the side surface of the first intersection after the inversion step; a second downward welding step of performing welding in a downward posture from the inside of the first brace and from the portion welded in the second vertically upward welding step on a second lower portion which is the portion forming the lower part of the first intersection after the second vertically upward welding step; The welding method according to claim 1, characterized by including the above.

4. An arranging step of arranging the leg and the first brace such that the pipe axis of the leg is along a substantially horizontal direction and the pipe axis of the first brace is along a substantially vertical direction; further comprising: In the first inner welding step, after the arranging step, the first intersection of the leg and the first brace is welded from the inside of the first brace. The welding method according to claim 1, characterized by the above.

5. In the first inner welding step, Among the first intersections, the side where the intersection angle between the outer peripheral surface of the leg and the inner peripheral surface of the first brace forms an obtuse angle is pre-welded from the inside of the first brace. Among the first intersections, the side where the intersection angle between the outer peripheral surface of the leg and the inner peripheral surface of the first brace forms an acute angle is post-welded from the inside of the first brace. The welding method according to claim 1, characterized in that.

6. In the first inner welding step, the first intersection is welded all around from the inside of the first brace. The welding method according to any one of claims 1 to 5, characterized in that.

7. The gouging step is started from the side where the outer peripheral surface of the leg and the outer peripheral surface of the first brace form an obtuse angle when the first intersection is viewed from a direction perpendicular to both the pipe axis of the leg and the pipe axis of the first brace. The welding method according to claim 1, characterized in that.

8. A first brace that is a cylindrical steel pipe, A second brace that is a cylindrical steel pipe, and the end of which abuts against the side surface of the first brace, A welding method for an offshore windmill jacket structure including: A first inner welding step of welding a first intersection, which is an intersection between the first brace and the second brace, from the inside of the second brace; A gouging step of gouging the first intersection from the outside of the second brace after the first inner welding step; A first outer welding step of welding the first intersection from the outside of the second brace after the gouging step; A welding method characterized by comprising.

9. In the first inner welding step, the first intersection is welded all around from the inside of the second brace. The welding method according to claim 8, characterized in that.

10. In the first inner welding step, Among the first intersections, the side where the intersection angle between the outer peripheral surface of the first brace and the inner peripheral surface of the second brace forms an obtuse angle is pre-welded from the inside of the second brace, Among the first intersections, the side where the intersection angle between the outer peripheral surface of the first brace and the inner peripheral surface of the second brace forms an acute angle is post-welded from the inside of the second brace, The welding method according to claim 8, characterized in that.

11. The welding amount in the first inner welding step is less than the welding amount in the first outer welding step, The welding method according to any one of claims 1 to 5, characterized in that.

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