WELDED JOINTS AND METHODS FOR MANUFACTURING WELDED JOINTS

Abstract

The invention relates to a welded joint of a floating offshore wind power generating apparatus comprising a tower, a floating element, and a support. The invention proposes a welded joint of increased fatigue strength at the weld with welds extended over the floating element, and a method for producing the welded joint. The welded joint of the invention consists of a first weld, a second weld, and a third weld. The first weld extends along one of the short edges of the rectangular contact surface where the support contacts the floating element. The second and third welds extend along the two long edges of the rectangular contact surface, respectively, and each weld extends over one of the beginning and end ends of the first weld, respectively, over the floating element.Furthermore, it is better if the M / N ratio of the distance M between the end of the second weld and the end of the third weld and the length N of the extended part of the second or third weld is 2.0 or less.

Description

Welded joint and its manufacturing method

[0001] The present invention relates to a welded joint and a manufacturing method thereof, and more particularly to a welded joint having excellent fatigue strength, such as a welded joint portion of a floating offshore wind power generation plant, in which floating body members and the like are welded by gas-shielded arc welding via a bracket, and a manufacturing method thereof.

[0002] In recent years, floating offshore wind turbines have been considered in the field of offshore wind power generation because of their ease of installation, even in waters 50 m or deeper. Because floating offshore wind turbines are exposed to external forces such as waves, a key issue is how to improve the fatigue life of the weld toes in the welded joints of these turbines.

[0003] Therefore, various techniques for improving the fatigue strength of welded joints in large structures such as offshore wind power generation equipment have been investigated. For example, Patent Document 1 discloses a welded girder structure consisting of ribs and face plates, in which the ends of the ribs are attached to the structural members by fillet welding on both sides, and a pair of extension beads with the structural members are formed by slightly extending the fillet weld beads on the structural members. Furthermore, Patent Document 1 discloses that the face plates are attached to the side edges of the ribs by fillet welding on both sides, and a pair of extension beads with the ribs are formed by slightly extending the fillet weld beads on both sides of the side edges of the ribs. It is claimed that these extension beads can suppress the initiation of fatigue cracks from the weld toes.

[0004] Patent Document 2 discloses a weld bead structure having two weld beads formed by butt-jointing the end face of one plate to one surface of the other plate and fillet-welding both sides, and two extended weld beads formed by extending the two weld beads from the end of the other plate onto one surface. Patent Document 2 also describes the inclusion of an end weld bead formed by additionally welding the end of one plate to the end of the other plate between the two extended weld beads. This distances the weld bead toe from high-stress areas where defects are likely to occur in the weld bead, thereby increasing the durability of the weld.

[0005] Furthermore, Patent Document 3 discloses a boxing welded joint between a gusset and a main plate, which has a first weld bead that extends onto the main plate from both sides of the short side of the gusset along the short side, and second and third weld beads that extend onto the main plate along the long side of the gusset, covering the first weld bead. This is said to result in a boxing welded joint that can inexpensively and stably improve fatigue strength.

[0006] JP 8-155634 JP 9-253843 JP 2018-158380

[0007] The methods described in the above-mentioned patent documents are said to be effective in improving the fatigue strength of welded joints. However, in the case of welded joints in large structures such as offshore wind turbines, it is not clear whether these methods are effective in improving the fatigue strength of welded joints in the tower, floating body, brackets, and other parts of floating offshore wind turbines, where structural stress concentrations are superimposed in addition to local stress concentrations.

[0008] Therefore, the inventors have sought a method for improving the fatigue strength of welded joints in floating offshore wind turbines, which are subject to both local and structural stress concentrations. As a result, they have found that fatigue strength can be improved by controlling the shape and arrangement of the weld beads.

[0009] The present invention aims to solve the problems of the prior art by extending the weld bead onto the floating body member in the weld joint of a large structure, such as a weld joint of a floating offshore wind turbine comprising a tower member, a floating body member, and a bracket. The object of the present invention is to provide a weld joint and a method for manufacturing the same that improve fatigue strength.

[0010] In order to achieve the above-mentioned object, the inventors first fabricated a simulated welded joint structure that simulated the actual structure of a floating offshore wind turbine. Two types of welded joints were fabricated for the simulated structure: one with a normal fillet weld along the member, and another with an extended weld bead. Then, fatigue tests were conducted on these welded joints. By comparing the fatigue test results, it was discovered that the fatigue strength of a welded joint with an extended weld bead was improved.

[0011] The present invention was completed based on these findings and through further study, and the gist of the present invention is as follows: [1] A welded joint including a first member, a second member abutted against a surface of the first member, and a support member in contact with the first member and the second member, the welded joint having a first weld bead, a second weld bead, and a third weld bead, the first weld bead being formed along one short side of a first rectangular abutment surface where the support member abuts against the first member, and the second weld bead and the third weld bead being formed along each long side of the first rectangular abutment surface and being formed to cover either a starting end or a terminal end of the first weld bead and extend onto the first member. [2] The welded joint according to [1], wherein M / N, which represents the ratio of the distance M between the tip of the second weld bead and the tip of the third weld bead to the length N of the extension of the second weld bead or the extension of the third weld bead, is 2.0 or less. [3] The welded joint according to [1] or [2], wherein the length N of the extension of the second weld bead and the extension of the third weld bead is in the range of 4.0 mm to 60.0 mm. [4] The welded joint according to [1] or [2], further comprising a fourth weld bead formed along a long side and a short side of a second rectangular abutment surface where the support member abuts against the second member. [5] The welded joint according to [1] or [2], which is used as a welded joint for a floating offshore wind turbine, wherein the first member is a floating body member, the second member is a tower member, and the support member is a bracket. [6] A method for manufacturing a welded joint including a first member, a second member abutted against a surface of the first member, and a support member in contact with the first member and the second member, wherein the welded joint has a first weld bead, a second weld bead, and a third weld bead, the first weld bead is formed along one short side of a first rectangular abutment surface where the support member abuts against the first member, and the second weld bead and the third weld bead are formed along each long side of the first rectangular abutment surface and are formed by covering either a part of a starting end or a part of a terminal end of the first weld bead and extending onto the first member.[7] The method for manufacturing a welded joint according to [6] above, wherein M / N, which represents the ratio of the distance M between the tip of the second weld bead and the tip of the third weld bead to the length N of the extension of the second weld bead or the extension of the third weld bead, is 2.0 or less. [8] The method for manufacturing a welded joint according to [6] or [7] above, wherein the length N of the extension of the second weld bead and the extension of the third weld bead is in the range of 4.0 mm to 60.0 mm. [9] The method for manufacturing a welded joint according to [6] or [7] above, wherein the welded joint further has a fourth weld bead, and the fourth weld bead is formed along a long side and a short side of a second rectangular abutment surface where the support member abuts against the second member.

[10] The method for manufacturing a welded joint according to [6] or [7], wherein the welded joint is used as a welded joint for a floating offshore wind turbine, the first member is a floating body member, the second member is a tower member, and the support member is a bracket.

[0012] According to the present invention, it is possible to provide a welded joint and a manufacturing method thereof that improve fatigue strength by extending the weld bead onto the floating body member in a welded joint portion of a large structure, such as a welded joint of a floating offshore wind turbine that includes a tower member, a floating body member, and a bracket. Furthermore, the present invention has significant industrial effects.

[0013] FIG. 1 is a perspective view schematically illustrating an overall image of a welded joint (simulated structure) of a floating offshore wind power generation system according to the present invention. FIGS. 2(a) and 2(b) are schematic diagrams illustrating an overall image of the welded joint (simulated structure) of FIG. 1, with FIG. 2(a) being a plan view and FIG. 2(b) being a front view. FIG. 3 is a schematic plan view illustrating an enlarged portion of the welded joint (simulated structure) of FIG. 1. FIGS. 4(a) to 4(d) are schematic diagrams illustrating an example of the steps of a manufacturing method for the welded joint (simulated structure) of FIG. 1. FIG. 5 is a schematic diagram illustrating the fixed state of the welded joint (simulated structure) of FIG. 1 in a fatigue test.

[0014] The present invention relates to a welded joint and a manufacturing method thereof, which includes a first member, a second member abutting against a surface (e.g., a front surface) of the first member, and a support member in contact with the first member and the second member, as shown in Fig. 1 and other figures. The second member is sandwiched between the pair of support members. The welded joint of the present invention is suitable for use, for example, as a welded joint for a floating offshore wind turbine. Hereinafter, the welded joint for a floating offshore wind turbine will be described as an example. In this example, the first member is a float member 3 (described below), the second member is a tower member 2 (described below), and the support member is a bracket 4 (described below).

[0015] The present invention relates to a welded joint for a floating offshore wind turbine generator in which fatigue strength is improved by extending the weld bead onto a floating body member, and a method for manufacturing the same.

[0016] Hereinafter, embodiments of the present invention will be described.

[0017] [Floating Offshore Wind Turbine] First, an overview of floating offshore wind turbines will be provided. Wind power generation is a form of renewable energy. Offshore wind turbines installed on the sea are classified into bottom-fixed and floating types, and floating offshore wind turbines are used in waters 50 meters or deeper. A floating offshore wind turbine is constructed by floating a structure carrying a wind turbine on the ocean, connected by chains or the like. The structure consists of a tower section that holds the wind turbine (wind power generator) aloft, and a floating body section that floats on the ocean and carries the tower section. The tower section and the floating body section, which serves as the base, are joined by welding, and improving the fatigue life of the welded joint is an important issue.

[0018] [Simulated structure of welded joints of a floating offshore wind turbine] In order to investigate joint structures that improve the fatigue life of the welded joints of the above-mentioned floating offshore wind turbine, we created welded joints of a simulated structure that simulated the actual structure of a floating offshore wind turbine, and conducted simulation tests to improve fatigue life. The simulated structure of the welded joint used here is shown in Figure 1. This welded joint (i.e., simulated structure) 1 has a structure in which a rectangular parallelepiped tower member 2 is placed on a floating body member 3, and two brackets 4 sandwich the tower member 2 from both sides. The joints between the members are mainly fillet welded using gas-shielded arc welding, and weld beads 5 are formed at each boundary.

[0019] The tower member 2 is a rectangular parallelepiped. There are no particular limitations on its shape, but an example of a simulated structure has a bottom long side (plate width) of 100 mm to 200 mm, a bottom short side (plate thickness) of 50 mm to 60 mm, and a height (plate length) of 250 mm to 400 mm. Examples of materials for the tower member 2 include YP460 and YP355.

[0020] The floating body member 3 is a thick plate. Again, the shape is not particularly limited, but an example of a simulated structure has a plate width (plate width) of 100 mm to 200 mm, a plate thickness (plate thickness) of 25 mm to 40 mm, and a plate length (plate length) of 800 mm to 1000 mm. The floating body member 3 may be made of the same material as the tower member 2.

[0021] The bracket 4 is a triangular prism with a right-angled bottom, and the two side surfaces that form the right angles of the triangular prism abut against the tower member 2 and the floating body member 3. FIG. 2( a) is a plan view of the simulated structure 1 viewed from above. FIG. 2( a) shows a rectangular abutment surface 4a (hereinafter sometimes referred to as a "first rectangular abutment surface") of the bracket 4 that abuts against the floating body member 3. The shape of the bracket 4 is not particularly limited, but in one example of a simulated structure, for example, the length of the long side of the rectangular abutment surface 4a of the bracket 4 is 200 mm to 400 mm, and the length of the short side of the rectangular abutment surface 4a is 30 mm to 40 mm. The material of the bracket 4 is preferably the same as that of the tower member 2 and the floating body member 3.

[0022] [Weld Bead 5] In one embodiment, weld bead 5, which is formed primarily by fillet welding, is composed of a first weld bead 5a, a second weld bead 5b, and a third weld bead 5c, which are formed in accordance with the welding procedure described below. It is preferable that weld bead 5 further includes a fourth weld bead 5d. The structure is shown in the plan view of FIG. 2(a) and the front view of FIG. 2(b). FIGS. 2(a) and 2(b) show weld bead 5 composed of the first to fourth weld beads.

[0023] In Fig. 2(a), the bracket is shown as a rectangular abutment surface (first rectangular abutment surface) 4a that abuts on the floating body member 3. The same applies to Figs. 3 and 4(a) to 4(d) described below.

[0024] The welding method is preferably gas-shielded arc welding.

[0025] [First Weld Bead 5a] The first weld bead 5a is a weld bead formed along one short side of the rectangular contact surface 4a of the bracket 4 (see FIG. 4(a) and the like).

[0026] The weld toe of the weld bead formed on the short side (i.e., the first weld bead 5a) is the part where localized stress concentration occurs most easily and fatigue cracks are likely to occur. Furthermore, in the case of a floating offshore wind turbine, which is the subject of the present invention, fatigue cracks are more likely to occur due to the superposition of structural stress concentration caused by wind power generation in addition to localized stress concentration. In order to suppress the occurrence of fatigue cracks, it is important to form the second weld bead 5b and the third weld bead 5c described below and further extend these weld beads onto the floating body member 3.

[0027] [Second weld bead 5b and third weld bead 5c] The second weld bead 5b is formed along one long side of the rectangular abutment surface 4a of the bracket 4, and is a weld bead that covers a portion of either the starting end or the ending end of the first weld bead 5a and extends onto the floating body member 3 (see FIG. 4(b) and other figures). The third weld bead 5c is formed along the other long side of the rectangular abutment surface 4a, and is a weld bead that covers a portion of either the starting end or the ending end of the first weld bead 5a and extends onto the floating body member 3 (see FIG. 4(c) and other figures).

[0028] For example, when second weld bead 5b is placed over the starting end of first weld bead 5a, third weld bead 5c is formed to cover the ending end of first weld bead 5a.

[0029] In FIG. 2( a), second weld bead 5 b and third weld bead 5 c are formed symmetrically from top to bottom, with second weld bead 5 b on the upper side of rectangular contact surface 4 a and third weld bead 5 c on the lower side, but they may be arranged upside down.

[0030] The above phrase "covering a portion of either the end or start end of the first weld bead" includes not only covering a portion of either the end or start end of the first weld bead, but also "covering" and then welding the second weld bead over a portion of the first weld bead. As shown in Figure 2(a) and other figures, second weld bead 5b and third weld bead 5c are not straight, and the bead spacing is short, thereby reducing stress concentration on the short sides of the beads.

[0031] [Distance M between the Tip of the Second Weld Bead and the Tip of the Third Weld Bead] As shown in Fig. 3 , the distance M between the tip of the second weld bead 5b and the tip of the third weld bead 5c is defined as M. Here, the distance M between the tip ends is the distance between the opposing weld toes at the tip ends of the second weld bead and the third weld bead, respectively. In order to improve the fatigue strength of the welded joint, it is preferable that the ratio M / N of this distance M to the length N of the extension of the second weld bead or the third weld bead, which will be described later, is 2.0 or less. If the ratio M / N of the distance M to the length N of the extension exceeds 2.0, fatigue cracks may occur, and fatigue strength may not be significantly improved. It is more preferable that the ratio M / N of the distance M to the length N of the extension bead is 1.8 or less.

[0032] Furthermore, even when the distance M is 0 mm or less, i.e., when the tip ends of the second weld bead and the third weld bead are in contact with each other or when the two weld beads overlap, the effect of improving fatigue strength is observed. For this reason, no lower limit value is set for the ratio M / N of the distance M to the length N of the extension portion. That is, the ratio M / N of the distance M to the length N of the extension portion is preferably 0.0 or more, and more preferably 1.0 or more.

[0033] [Length N of Second Weld Bead Extension and Third Weld Bead Extension] As shown in FIG. 3 , the length N of the extension of the second weld bead 5b and the extension of the third weld bead 5c is the length from the weld toe of the first weld bead 5a to the weld termination at the tip of the second and third weld beads. This length N is preferably 4.0 mm to 60.0 mm for both the second weld bead 5b and the third weld bead 5c. If it is less than 4.0 mm, the effect of improving fatigue life is not observed. On the other hand, if it exceeds 60.0 mm, excessive weld bead formation occurs, which is undesirable because it increases the welding cost. This length N is preferably 10.0 mm or more and 50.0 mm or less.

[0034] 2(a) and 2(b), the fourth weld bead 5d is a weld bead formed along the long and short sides of the rectangular abutment surface (hereinafter sometimes referred to as the "second rectangular abutment surface") where the bracket 4 abuts against the tower member 2, and is a weld bead for joining the entire boundary between the bracket 4 and the tower member 2. Note that welding along the long side of the rectangular abutment surface (second rectangular abutment surface) is a vertical weld. Providing this fourth weld bead 5d is preferable to improve the overall strength of the weld joint.

[0035] From the viewpoint of more effectively achieving the above-described effects, it is more preferable that d5 / a4, which represents the ratio of the length a4 of the short side of the first rectangular abutment surface 4a of the bracket 4 to the length d5 ​​of the outer flank in the short-side direction of the second rectangular abutment surface of the fourth weld bead 5d, be within the range of 1.2 to 1.5. For example, the length of the long side of the second rectangular abutment surface of the bracket 4 is 100 mm to 300 mm, and the length of the short side of the second rectangular abutment surface is 10 mm to 50 mm.

[0036] [Method for manufacturing a welded joint] Next, a specific embodiment of a welding procedure for a welding method that is a method for manufacturing a welded joint according to the present invention will be described with reference to Figures 4(a) to 4(d). Welding is performed in the order shown in Figure 4(a).

[0037] First, the second member (here, the tower member 2) is butted against the surface of the first member (here, the floating body member 3), and a support member (here, the bracket 4) is positioned so as to contact the first member and the second member.

[0038] Next, the arranged components are gas-shielded arc welded together. Note that welding in the order of (a) to (c) will result in a weld bead of the welded joint of the present invention, which has the above-mentioned characteristics. As described above, if a fourth weld bead is also provided to improve the overall strength of the welded joint, welding (c) can be performed followed by welding (d). (a) A first weld bead 5a is formed along one short side (the side farther from the tower member 2) of the rectangular abutment surface 4a where the bracket 4 abuts the floating body member 3 (see FIG. 4(a)). (b) A second weld bead 5b is formed along one long side of the rectangular abutment surface 4a, covering the first weld bead 5a and extending onto the floating body member 3 (see FIG. 4(b)). (c) Similarly, a third weld bead 5c is formed along the other long side of the rectangular abutment surface 4a, covering the first weld bead 5a and extending onto the floating body member 3 (see FIG. 4(c)). (d) Furthermore, if necessary, a fourth weld bead 5d is formed along the long and short sides of the rectangular contact surface where the bracket 4 contacts the tower member 2. That is, the fourth weld bead 5d is formed along the two long vertical sides on both sides of the rectangular contact surface to the tower member 2 and the upper short side (see FIG. 4(d)).

[0039] Although Figures 4(a) to 4(d) show only one side of the simulated weld joint structure 1, it is preferable to similarly form four weld beads around the bracket on the other side (i.e., the opposite side).

[0040] [Welding conditions for gas-shielded arc welding] Examples of welding conditions for gas-shielded arc welding are as follows: Welding current: 200 A to 400 A, welding voltage: 20 V to 50 V, welding speed: 20 cm / min to 40 cm / min, shielding gas: CO2 gas or a mixed gas of 80 vol% Ar gas and 20 vol% CO2 gas, welding wire diameter: 1.0 mm to 2.0 mm

[0041] The present invention will be further described below with reference to examples. However, the following examples are merely intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0042] First, we prepared a welded joint (i.e., a simulated structure) simulating the actual structure of a welded joint for a floating offshore wind turbine. The test material used was YP470 steel with a plate width of 100 mm. This test material was used as the float member of the floating offshore wind turbine, and a tower member and bracket made of the same material were prepared. Here, the float member had dimensions of 100 mm width, 25 mm thickness, and 800 mm length, the tower member had dimensions of 100 mm width, 50 mm thickness, and 100 mm length, and the bracket had dimensions of 200 mm long side of the first rectangular abutment surface, 25 mm short side of the first rectangular abutment surface, and 250 mm long side of the second rectangular abutment surface. These members were fillet welded by gas-shielded arc welding at a current of 230 A, a voltage of 30 V, and a welding speed of 34 cm / min to prepare six types of welded joints. The six types of joints were a conventional fillet weld (Joint No. 1) and five types with an extended weld bead (Joints No. 2 to No. 6). The Young's modulus and Poisson's ratio, which are the characteristics of the welded joints, were measured in accordance with ASTM E8M "Tensile Test Methods for Metallic Materials." All six types of joints had a Young's modulus E of 206,000 MPa and a Poisson's ratio v of 0.3.

[0043] As shown in Figure 5, fatigue tests on welded joints were conducted by fixing welded joint specimens in a chuck, which is a fixture of a fatigue testing machine (e.g., a fatigue load testing device), and applying fatigue loads (specifically, tension and compression) to the specimens as indicated by the arrows in the figure. The applied stress range was 150 MPa in all cases. The number of cycles until fracture was taken as the fatigue life.

[0044] The results of these tests are shown in Table 1.

[0045]

[0046] Joint No. 1 is a comparative example, showing test results for a normal fillet weld. Joint No. 2 shows test results for an example of the present invention in which the weld beads were extended, with the ratio M / N of the extended weld bead spacing M to the length N of the extended portion being 2.1. Joint No. 3 shows test results for an example of the present invention in which the weld beads were extended, with the ratio M / N of the extended weld bead spacing M to the length N of the extended portion being 0.3. Joint No. 4 shows test results for an example of the present invention in which the weld beads were extended, with the ratio M / N of the extended weld bead spacing M to the length N of the extended portion being 0.0. Joint No. 5 shows test results for an example of the present invention in which the weld beads were extended, with the ratio M / N of the extended weld bead spacing M to the length N of the extended portion being 2.0. In No. 6, the weld bead of the present invention was elongated, and the ratio M / N of the elongated weld bead spacing M to the elongated portion length N was 1.0.

[0047] Note that all joints from No. 2 onwards were configured with the first to fourth weld beads and tested.

[0048] Regarding the number of fractures, when the ratio M / N of the spacing M of the extended weld beads to the length N of the extended portion was 2.1 (Joint No. 2), a slight improvement in fatigue strength was observed compared to the case of a standard fillet weld (Joint No. 1). On the other hand, when the ratio M / N of the spacing M of the extended weld beads to the length N of the extended portion was 2.0 or less (Joints No. 3 to No. 6), the number of fractures was greater than the number of fractures in the case of a standard fillet weld (Joint No. 1). Therefore, extending the weld beads onto the floating body member was found to improve the fatigue strength of the welded joint. Furthermore, by narrowing the spacing M of the extended weld beads and setting the ratio M / N of the spacing M of the extended weld beads to the length N of the extended portion to 2.0 or less, an even greater improvement in fatigue strength was observed.

[0049] 1 Welded joint (simulated structure) 2 Tower member 3 Floating body member 4 Bracket 4a Rectangular abutment surface of bracket abutting against floating body member 5 Weld bead 5a First weld bead 5b Second weld bead 5c Third weld bead 5d Fourth weld bead 6 Chuck (fixture) of fatigue load test device M Distance (mm) between tip of second weld bead and tip of third weld bead N Length (mm) of extension of second weld bead and extension of third weld bead

Claims

1. A welded joint comprising a first member, a second member abutted against a surface of the first member, and a support member in contact with the first member and the second member, the welded joint having a first weld bead, a second weld bead, and a third weld bead, the first weld bead being formed along one short side of a first rectangular abutment surface where the support member abuts against the first member, and the second weld bead and the third weld bead being formed along each long side of the first rectangular abutment surface and being formed by covering a portion of either the starting end or the ending end of the first weld bead and extending onto the first member.

2. The welded joint described in claim 1, wherein M / N, which represents the ratio of the distance M between the tip of the second weld bead and the tip of the third weld bead to the length N of the extension of the second weld bead or the extension of the third weld bead, is 2.0 or less.

3. The welded joint according to claim 1 or 2, wherein the length N of the extension portion of the second weld bead and the extension portion of the third weld bead is in the range of 4.0 mm to 60.0 mm.

4. The welded joint described in claim 1 or 2, further comprising a fourth weld bead formed along the long and short sides of a second rectangular abutment surface where the support member abuts against the second member.

5. The welded joint according to claim 1 or 2, which is used as a welded joint for a floating offshore wind turbine, wherein the first member is a float member, the second member is a tower member, and the support member is a bracket.

6. A method for manufacturing a welded joint comprising a first member, a second member abutted against a surface of the first member, and a support member in contact with the first member and the second member, the welded joint having a first weld bead, a second weld bead, and a third weld bead, the first weld bead being formed along one short side of a first rectangular abutment surface where the support member abuts against the first member, and the second weld bead and the third weld bead being formed along each long side of the first rectangular abutment surface and extending onto the first member while covering either a part of a starting end or a terminal end of the first weld bead.

7. A method for manufacturing a welded joint as described in claim 6, wherein M / N, which represents the ratio of the distance M between the tip of the second weld bead and the tip of the third weld bead to the length N of the extension portion of the second weld bead or the extension portion of the third weld bead, is 2.0 or less.

8. A method for manufacturing a welded joint as set forth in claim 6 or 7, wherein the length N of the extension portion of the second weld bead and the extension portion of the third weld bead is in the range of 4.0 mm to 60.0 mm.

9. A method for manufacturing a welded joint as described in claim 6 or 7, wherein the welded joint further has a fourth weld bead, and the fourth weld bead is formed along the long sides and short sides of a second rectangular abutment surface where the support member abuts against the second member.

10. A method for manufacturing a welded joint as described in claim 6 or 7, wherein the welded joint is used as a welded joint for a floating offshore wind power generation device, the first member is a float member, the second member is a tower member, and the support member is a bracket.

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