A method for designing a tower structure, a method for manufacturing a tower structure, and a tower structure

Abstract

The present invention provides a method for designing a tower structure, a method for manufacturing a tower structure, and a tower structure that allows for the longest possible length of the steel pipes constituting the tower structure, while also providing a rational structure that responds to the design external forces that change in the height direction of the tower structure, thereby reducing the amount of steel used in the tower structure. A design method for a tower structure in which multiple steel pipes stacked in the height direction are integrated by welding, wherein the first yield strength σ y1 By using a first type of steel having the above characteristics for all of the plurality of steel pipes, and designing the tower structure using the allowable stress design method under design conditions such that the difference in plate thickness between two vertically adjacent steel pipes is less than or equal to a predetermined limit value, the plate thickness t of each of the plurality of steel pipes c1 A preliminary design step to calculate the first yield strength σ; a region setting step to set a region in the height direction of the tower structure that includes a reference height h0 which is the height at which the bending moment is maximum in the bending moment distribution in the height direction of the tower structure designed by the preliminary design step, and which is in the range of 0.8 to 2.4 times the maximum outer diameter of the tower structure; and a region setting step in which, among the plurality of steel pipes, the steel pipe in the region is determined to have the first yield strength σ y1 The second yield strength σ is greater than y2 By using a second type of steel having the above characteristics, and using the first type of steel for steel pipes outside the above region, and designing the tower structure using the allowable stress design method under design conditions such that the difference in plate thickness between two vertically adjacent steel pipes is less than or equal to the above limit value, the plate thickness t of each of the multiple steel pipes c3 A design method for a tower structure, including the main design step of calculating [a certain value].

Description

[Technical Field]

[0001] The present invention relates to a method for designing tower structures such as monopiles installed on the seabed to support offshore wind power generation facilities, a method for manufacturing tower structures, and tower structures themselves. [Background technology]

[0002] In recent years, the use of renewable energy has been promoted as a solution to the challenge of reducing greenhouse gas emissions such as carbon dioxide caused by the use of fossil fuels. Wind power generation facilities, which use wind energy, a type of renewable energy, to rotate wind turbines and generate electricity using the resulting kinetic energy, are being used worldwide.

[0003] Wind power generation facilities can be installed on land or offshore. In the latter case, there are no obstructions, the site area is vast, and large wind turbines can be installed, so offshore wind power generation has been promoted in particular in recent years.

[0004] One type of foundation used to support offshore wind power generation facilities is the monopile foundation. Figure 1 schematically shows the overall configuration of an offshore wind power generation facility using a monopile foundation. Figure 2 shows an enlarged cross-sectional view of the monopile that makes up the monopile foundation. Furthermore, Figure 3 shows an enlarged cross-sectional view of the joint between the steel pipes that make up the monopile.

[0005] As shown in Figures 1 to 3, the monopile 1 installed on the seabed to support offshore wind power generation equipment is constructed by stacking and integrating multiple steel pipes (in Figure 2, some of the multiple steel pipes 11 to 14 are shown) in the height direction. As described in Patent Document 1, the monopile 1 supporting the wind turbine 2 of the offshore wind power generation equipment is formed with an outer diameter and wall thickness that can withstand loads such as earthquakes, wind, waves, and its own weight. When the wind turbine 2 becomes large, steel pipes with an outer diameter of 10 m and a plate thickness of 100 mm or more may be used as the monopile 1.

[0006] As shown in FIG. 1, the tower 20, which is a support part of the wind turbine 2, is connected to the upper end of the monopile 1 via a steel pipe called a transition piece 3. Between the transition piece 3 and the tower 20 of the wind turbine 2, and between the transition piece 3 and the monopile 1, it is generally joined by bolt joining or end bearing joining by injecting grout.

[0007] In the design of a monopile, it is common to design the monopile based on allowable stress against loads such as earthquakes, wind, waves, and self-weight. Here, the magnitude of the design external force acting on the monopile changes in the height direction of the monopile. That is, depending on the height at which a plurality of steel pipes constituting the monopile are provided in the monopile, the cross-sectional performance required for these plurality of steel pipes is different from each other. Since it is common for the steel type of the steel pipes constituting the monopile to be of one type, as described in Non-Patent Document 1 for example, it is common to set the plate thicknesses of the plurality of steel pipes to be different according to the change in the design external force in the height direction of the monopile.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Non-Patent Documents

[0009]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

[0010] Here, the length of the steel pipes that make up a monopile is usually around 2 to 4 meters. If the length of the steel pipes that make up the monopile is shortened and stacked, the plate thickness of the monopile can be continuously changed in accordance with the design external force that changes in the height direction of the monopile, and the amount of steel material used in the monopile can be reduced. However, this increases the number of welding joints, which greatly increases the construction load of the monopile and lengthens the manufacturing time. Therefore, it is now common practice to manufacture steel pipes with the longest possible length and then weld these steel pipes together to produce the monopile. In this case, the location where the plate thickness of the steel pipes can be changed is limited to the connection points between the steel pipes. Furthermore, in order to suppress stress concentration caused by the difference in plate thickness of the steel pipes at the connection points between the steel pipes, there is a limit to the difference in plate thickness, and for example, Non-Patent Literature 2 recommends that this difference in plate thickness be 7 mm or less. For this reason, if the length of the steel pipes that make up the monopile is made as long as possible, there will be a large portion of each steel pipe where there is a margin of error in plate thickness relative to the design external force, which leads to the problem of increased steel material usage.

[0011] Furthermore, since monopiles are large components and their manufacturing plants are limited, the transportation distance to the offshore wind power generation facility installation site tends to be long. The CO2 emissions from logistics when transporting monopiles from the manufacturing plant to the offshore wind power generation facility installation site can be calculated, for example, using the following formula. CO2 emissions (g-CO2) = Transport weight (t) x Transport distance (km) x CO2 emission intensity (g-CO2 / t km) This can be calculated using the following formula. Here, the CO2 emission intensity is a value set according to the means of transport, and is set to around 20 when the means of transport is by ship. Since the CO2 emissions when transporting one monopile from the manufacturing plant to the installation site often exceed 2000 tons, even a 1% reduction in the weight of the monopile has a significant effect in reducing CO2 emissions.

[0012] In view of the above-mentioned problems, the present invention aims to provide a method for designing a tower structure, a method for manufacturing a tower structure, and a tower structure that can reduce the amount of steel material used in a tower structure, such as a monopile, by making the length of the steel pipes constituting the tower structure as long as possible while providing a rational structure that responds to the design external forces that change in the height direction of the tower structure. [Means for solving the problem]

[0013] To solve the above problems, the present invention has the following features. [1] A design method for a tower structure in which multiple steel pipes stacked in the height direction are welded together, wherein the first yield strength σ y1 By using a first type of steel having the above characteristics for all of the plurality of steel pipes, and designing the tower structure using the allowable stress design method under design conditions such that the difference in plate thickness between two vertically adjacent steel pipes is less than or equal to a predetermined limit value, the plate thickness t of each of the plurality of steel pipes c1 A preliminary design step to calculate the first yield strength σ; a region setting step to set a region in the height direction of the tower structure that includes a reference height h0 which is the height at which the bending moment is maximum in the bending moment distribution in the height direction of the tower structure designed by the preliminary design step, and which is in the range of 0.8 to 2.4 times the maximum outer diameter of the tower structure; and a region setting step in which, among the plurality of steel pipes, the steel pipe in the region is determined to have the first yield strength σ y1 The second yield strength σ is greater than y2 By using a second type of steel having the above characteristics, and using the first type of steel for steel pipes outside the above region, and designing the tower structure using the allowable stress design method under design conditions such that the difference in plate thickness between two vertically adjacent steel pipes is less than or equal to the above limit value, the plate thickness t of each of the multiple steel pipes c2 A design method for a tower structure, including the main design step of calculating [a certain value]. In this invention, the term "tower structure" is not limited to the entire structure, but also includes a part of the structure that is installed on the ground, such as a monopile used as a foundation to support offshore wind power generation equipment. [2] The method for designing a tower structure according to [1], wherein the region set in the region setting step is symmetrically set vertically from the reference height h0. [3] The method for designing a tower structure according to [1] or [2], wherein the limit value is set to 7 mm or less. [4] The second yield strength σ y2 is set to be 40 N / mm y1 or more greater than the first yield strength σ 2 The method for designing a tower structure according to [1] or [2]. [5] A method for manufacturing a tower structure in which a plurality of steel pipes laminated in the height direction are integrated by welding, using a first steel type having a first yield strength σ y1 for all of the plurality of steel pipes, and designing the tower structure by the allowable stress design method under the design condition that the difference in the plate thickness between two vertically adjacent steel pipes is equal to or less than a preset limit value, thereby calculating the plate thickness t c1 of each of the plurality of steel pipes in a preliminary design step; a region setting step including a reference height h0 which is the height at which the bending moment is maximum in the bending moment distribution in the height direction of the tower structure designed in the preliminary design step, and setting a region in the range of 0.8 to 2.4 times the maximum outer diameter of the tower structure in the height direction of the tower structure; and among the plurality of steel pipes, using a second steel type having a second yield strength σ y1 greater than the first yield strength σ y2 for the steel pipes in the region, using the first steel type for the steel pipes outside the region, and designing the tower structure by the allowable stress design method under the design condition that the difference in the plate thickness between two vertically adjacent steel pipes is equal to or less than the limit value, thereby calculating the plate thickness t c2 of each of the plurality of steel pipes in a final design step; and manufacturing the tower structure by setting the plate thickness and steel type of each of the plurality of steel pipes as designed in the final design step. A method for manufacturing a tower structure. [6] A tower structure comprising multiple steel pipes stacked in the height direction and integrated by welding, wherein among the multiple steel pipes, a reference height h1 which is the height at which the bending moment is maximum in the bending moment distribution in the height direction of the tower structure, and a steel pipe in the height direction of the tower structure in the range of 0.8 to 2.4 times the maximum outer diameter of the tower structure, has a second yield strength σ y2 It is composed of a second type of steel having the first yield strength σ, and the steel pipe outside the aforementioned region has the first yield strength σ y1 It is composed of a first type of steel having the second yield strength σ y2 This is the first yield strength σ y1 40 N / mm 2 A tower structure that is large enough, with a difference in plate thickness of 7 mm or less between two adjacent steel pipes, one above the other. [Effects of the Invention]

[0014] According to the present invention's method for designing a tower structure, a method for manufacturing a tower structure, and a tower structure, by combining steel grades with different yield strengths, the plate thickness of the steel pipes constituting the tower structure can be reduced compared to the case where the tower structure is constructed using steel pipes made of a single grade of steel with a single yield strength. In this case, the first yield strength σ y1 The second yield strength σ is greater than y2 The steel grade having this property can be used in which of the multiple steel pipes constituting the tower structure is used, and the plate thickness of each of the multiple steel pipes can be determined in a simple manner.

[0015] This allows for a reduction in the amount of steel used in a tower structure compared to constructing it with a single grade of steel having a single yield strength, resulting in cost reduction, easier transportation by crane or ship, reduced required bearing capacity, and reduced construction load.

[0016] The present invention relates to a method for designing a tower structure, a method for manufacturing a tower structure, and, when the tower structure is applied to a monopile that constitutes a monopile foundation supporting an offshore wind power generation facility, the amount of steel used in the tower structure can be reduced, thereby reducing the CO2 emissions from logistics when transporting the monopile from the monopile manufacturing plant to the installation site of the offshore wind power generation facility.

[0017] Furthermore, while maximizing the length of the steel pipes that make up the tower structure, it is possible to create a rational structure for the tower structure according to the bending moment distribution in the height direction of the tower structure. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 is a schematic side view showing the overall configuration of an offshore wind power generation facility using a monopile foundation. [Figure 2] Figure 2 is an enlarged cross-sectional view of a monopile that makes up a monopile foundation. [Figure 3] Figure 3 is an enlarged cross-sectional view of the joint between segments that make up a monopile in a monopile foundation. [Figure 4] Figure 4 shows an example of the distribution of plate thickness t1 in the height direction of a tower structure, calculated by the preliminary design step of the tower structure design method according to the present invention. [Figure 5] Figure 5 is a graph showing the reduction rate of the total weight of a tower structure designed using the tower structure design method according to the present invention, in comparison to the size of the area set in the area setting step. [Modes for carrying out the invention]

[0019] The design method for a tower structure, the manufacturing method for a tower structure, and one embodiment of the tower structure of the present invention will be described in detail below with reference to the drawings.

[0020] In this embodiment, a monopile used as a foundation to support offshore wind power generation equipment will be used as an example to describe one form of tower structure.

[0021] Figure 1 schematically shows the tower structure 1 of the first embodiment installed on the seabed, and the offshore wind power generation facility supported by the tower structure 1. As shown in Figure 1, the tower structure 1 of the first embodiment is a monopile installed on the seabed to support the wind turbine 2 of the offshore wind power generation facility. The tower 20, which is the support part of the wind turbine 2, is connected to the upper end of the tower structure 1 via a transition piece 3. The transition piece 3 and the tower 20 of the wind turbine 2, and the transition piece 3 and the tower structure 1 are joined by bolt connections or bearing connections by grout injection.

[0022] Figure 2 shows an enlarged cross-sectional view of a monopile that makes up a monopile foundation. Figure 3 shows an enlarged cross-sectional view of the joint between segments that make up the monopile of the monopile foundation.

[0023] As shown in Figures 2 and 3, the tower structure 1 is constructed by stacking multiple steel pipes in the height direction, and these multiple steel pipes are integrated by welding. Specifically, the tower structure 1 is constructed by stacking multiple steel pipe segments from the bottom upwards. Each of these steel pipe segments is constructed by arranging multiple single steel pipes (in Figure 2, some of the multiple steel pipes 11-14 are shown) that are approximately 3-4m in length, and integrating them by welding. The tower structure 1 is then constructed by arranging and welding these steel pipe segments together.

[0024] The tower structure design method of this embodiment includes a preliminary design step, a region setting step, and a final design step. Each of these steps will be described below. (Preliminary design step) In the tower structure design method according to this embodiment, first, in the preliminary design step, the first yield strength σ y1The tower structure 1 is designed using the allowable stress design method, with the design condition that a first type of steel having the following properties is used for all of the multiple steel pipes constituting the tower structure 1. Here, if the difference in plate thickness between two vertically adjacent steel pipes is large, there is a risk that plastic deformation or fracture of the steel pipes may occur prematurely due to stress concentration. Therefore, in the preliminary design step, the design condition is set so that the difference in plate thickness between two vertically adjacent steel pipes is less than or equal to a predetermined limit value Δt0. This ensures that the plate thickness t of each of the multiple steel pipes c1 Calculate.

[0025] At this time, the first yield strength σ y1 For example, 235~325 N / mm 2 It is preferable to set it within the range.

[0026] Furthermore, it is preferable to set the above limit value Δt0 to 7 mm or less. As described in Non-Patent Literature 2, when joining steel pipes of different thicknesses by welding, it is necessary to avoid stress concentration at the joint. For example, it is necessary to provide a thickness change section where the ratio (gradient) of the change in wall thickness of the steel pipe to the axial length of the steel pipe is 1 / 4, thereby eliminating the difference in wall thickness between the steel pipes to be welded together. Therefore, by setting the above limit value Δt0 to 7 mm or less, the amount of cutting required to create the taper in the thickness change section can be kept below a certain amount, and stress concentration caused by shape discontinuity due to the difference in plate thickness can be mitigated. (Area setting step) Next, in the domain setting step, a reference height h0 is identified, which is the height at which the bending moment is maximum in the bending moment distribution in the height direction of the tower structure 1 designed in the preliminary design step described above. Specifically, external forces such as seismic forces and wind forces are applied to the tower structure 1 using frame analysis, FEM analysis, etc., and the reference height h0, which is the height at which the bending moment is maximum, is obtained from the bending moment distribution obtained from the analysis results. Then, as shown in Figure 4, a domain is set that includes the reference height h0 and is in the height direction of the tower structure 1 in the range of 0.8 to 2.4 times the maximum outer diameter of the tower structure 1.

[0027] In this case, it is preferable that the region set in the region setting step be set symmetrically above and below the reference height h0. This allows the region to be appropriately set in the range of the tower structure 1 where the bending moment is large. (This design step) Next, in this design step, the tower structure 1 is designed using the allowable stress design method under different design conditions than those in the preliminary design step, thereby determining the plate thickness t of each of the multiple steel pipes. c2 The first yield strength σ is calculated. In this design step, some of the steel pipes among the multiple steel pipes that make up the tower structure 1 are given a first yield strength σ. y1 The second yield strength σ is greater than y2 A second steel grade having the following characteristics shall be used. Specifically, the steel pipes in the region set in the region setting step, which includes the reference height h0 and is in the height direction of the tower structure 1 and is in the range of 0.8 to 2.4 times the maximum outer diameter of the tower structure 1, shall have a second yield strength σ y2 A second steel grade having the following characteristics will be used. In this design step, the design condition is set such that the difference in plate thickness between two vertically adjacent steel pipes is less than or equal to the above limit value Δt0.

[0028] At this time, the second yield strength σ y2 For example, 385~485 N / mm 2 It is preferable to set it within the range.

[0029] Also, the second yield strength σ y2 This is the first yield strength σ y1 40 N / mm 2 It is preferable to set the yield strength to a larger value than the above. By doing so, by combining two types of steel with different yield strengths, the plate thickness of the steel pipes constituting the tower structure can be reduced by about 5 to 10 mm compared to when the tower structure is constructed using steel pipes made of a single type of steel with a single yield strength. This significantly reduces the plate thickness.

[0030] This concludes the design method for the tower structure according to this embodiment.

[0031] Furthermore, the manufacturing method of the tower structure in this embodiment is achieved by setting the plate thickness and steel type of each of the multiple steel pipes constituting the tower structure 1 as designed by the tower structure design method described above, and then manufacturing the tower structure 1.

[0032] Furthermore, the tower structure 1 of this embodiment can be manufactured by the method for manufacturing a tower structure described above. That is, the tower structure 1 is constructed by stacking a plurality of steel pipes in the height direction and integrating them by welding. Of these plurality of steel pipes, the steel pipes that include a reference height h1, which is the height at which the bending moment is maximum in the bending moment distribution in the height direction of the tower structure 1, and that are in a region in the height direction of the tower structure that is in the range of 0.8 to 2.4 times the maximum outer diameter of the tower structure, have a second yield strength σ y2 It is composed of a second type of steel having the above-mentioned yield strength σ. y1 It is composed of a first type of steel having the second yield strength σ y2 This is the first yield strength σ y1 40 N / mm 2 The above is larger. Also, the difference in plate thickness between two adjacent steel pipes, one above the other, is 7 mm or less. Note that the reference height h1, which is the height at which the bending moment is actually maximum in the height-direction bending moment distribution of the tower structure 1, does not exactly coincide with the reference height h0 specified in the domain setting step of the tower structure design method, but it is close to the reference height h0. [Examples]

[0033] The following describes an example of designing a tower structure using the tower structure design method of the present invention.

[0034] This embodiment describes an example of designing a tower structure 1 with a length of 90m and a maximum diameter of 10m to support a 15MW wind turbine, using the tower structure design method of the present invention. In this embodiment, of the 90m length of the tower structure 1 to be designed, the lower 50m portion is inserted into the ground, and the upper 40m portion is located in the sea. Furthermore, the outer diameter of the tower structure 1 is continuously reduced from 10m to 8m in the upper 35m portion of the tower structure 1 so that it can be joined to a transition piece 3.

[0035] In this embodiment, the length of each steel pipe constituting the tower structure 1 was set to 4m, which is the maximum length achievable during manufacturing. Furthermore, the plate thickness of each steel pipe constituting the tower structure 1 was set so that the ratio of outer diameter to plate thickness was 120 or less, in order to prevent local buckling. The limit value Δt0 for the difference in plate thickness between two vertically adjacent steel pipes was set to 5mm. (Preliminary design step) First, in the preliminary design step, the first yield strength σ y1 The tower structure 1 was designed using the allowable stress design method, with design conditions that used a first type of steel having the specified properties for all of the multiple steel pipes constituting the tower structure 1.

[0036] In this example, the tower structure 1 was designed using steel grade A, which is equivalent to steel grade SM520 as specified in Japanese Industrial Standard JIS G3106, as the first steel grade. The design standard strength of SM520 is 325 N / mm² for plate thicknesses exceeding 70 mm and not exceeding 100 mm. 2 , 315 N / mm² in the range of plate thickness exceeding 100 mm 2 And this value is the first yield strength σ y1 It was used as such.

[0037] In the preliminary design step, the design condition was set such that the difference in plate thickness between two adjacent steel pipes, one above the other, would be less than or equal to the above-mentioned limit value Δt0 (=5mm).

[0038] Figure 4 shows the plate thickness t in the height direction of the tower structure 1, calculated by designing the tower structure 1 using the allowable stress design method under the design conditions described above. c1The distribution of, that is, the plate thickness t of each of the multiple steel pipes that make up the tower structure 1 c1 This is shown. The values ​​on the vertical axis in Figure 4 are displayed with sea level height set to 0m. (Area setting step) Next, in the region setting step, a reference height h0 was identified, which is the height at which the bending moment is maximum in the bending moment distribution in the height direction of the tower structure 1 designed in the preliminary design step described above. Then, as shown in Figure 4, regions were set up that include the reference height h0 and are within the range of 8m (Example 1), 16m (Example 2), 24m (Example 3), and 30m (Comparative Example 2) in the height direction of the tower structure 1. These correspond to ranges of 0.8 times (Example 1), 1.6 times (Example 2), and 2.4 times (Example 3) the maximum outer diameter of the tower structure 1, respectively, and are within the range of 0.8 to 2.4 times the maximum outer diameter of the tower structure 1, which is a requirement of the present invention described above.

[0039] For comparison, a range was set that includes the reference height h0 and extends to 0m (Comparative Example 1) and 30m (Comparative Example 2) in the height direction of the tower structure 1. These ranges correspond to 0 times (Comparative Example 1) and 3.0 times (Comparative Example 2) the maximum outer diameter of the tower structure 1, respectively, and are outside the range of 0.8 to 2.4 times the maximum outer diameter of the tower structure 1, which is a requirement of the present invention as described above.

[0040] The lower end of the region set in the region setting step was set to the height at which there is a difference in plate thickness t1 between two adjacent steel pipes, among the plate thickness t1 of multiple steel pipes calculated in the design step, that is lower than the above reference height h0 and closest to the above reference height h0. In this embodiment, the height of the lower end of the region was sea level - 11m. The upper end of the region set in the region setting step was set to heights of 0m (Comparative Example 1), 8m (Inventive Example 1), 16m (Inventive Example 2), 24m (Inventive Example 3), and 30m (Comparative Example 2) in the height direction upward from the lower end of the tower structure 1. (This design step) Next, in this design step, the tower structure 1 is designed using the allowable stress design method under different design conditions than those in the preliminary design step, thereby determining the plate thickness t of each of the multiple steel pipes. c2The yield strength of steel type A (first yield strength σ) was calculated for some of the steel pipes among the multiple steel pipes that make up the tower structure 1. y1 ) The second yield strength σ is greater than y2 Steel type B, which has the following properties, was chosen.

[0041] In this embodiment, steel grade B is 550 N / mm 2 Tower structure 1 was designed assuming the use of graded steel. 550 N / mm 2 The design strength of grade steel is 385 N / mm². 2 And this value is the second yield strength σ y2 It was used as the second yield strength σ. y2 This is the first yield strength σ y1 40 N / mm 2 The above is set to a large value.

[0042] Then, in the region set in the region setting step, that is, the region including the reference height h0 and in the height direction of the tower structure 1, the steel pipes in the range of 0 times (Comparative Example 1), 0.8 times (Inventive Example 1), 1.6 times (Inventive Example 2), 2.4 times (Inventive Example 3), and 3.0 times (Comparative Example 2) of the maximum outer diameter of the tower structure 1 were to be made of steel type B.

[0043] Specifically, in Invention Example 1, the starting point is sea level -11m, which is the lower end of the area set in the area setting step, and steel type B is used for the steel pipes in the area within a range of 4m above and below this point, i.e., the range from sea level -7m to -15m.

[0044] In Invention Example 2, the starting point for the region defined in the region setting step is sea level -11m, and steel type B is used for the steel pipes in the region ranging from sea level -3m to -19m, which is an 8m range above and below this point.

[0045] In Invention Example 3, the starting point is sea level -11m, which is the lower end of the region set in the region setting step, and steel type B is used for the steel pipes in the region ranging from sea level +1m to -23m, with a range of 12m above and below this point.

[0046] In Comparative Example 2, the starting point was sea level -11m, which is the lower end of the area set in the area setting step, and steel type B was used for the steel pipes in the area ranging from sea level +4m to -26m, with a range of 15m above and below this point.

[0047] In Comparative Example 1, the area set in the area setting step is within the range of 0m in the height direction of the tower structure 1. Therefore, in the main design step, among the multiple steel pipes constituting the tower structure 1, there are no steel pipes using steel type B, and all steel pipes use steel type A. In other words, in Comparative Example 1, the design conditions in the main design step are the same as the design conditions in the preliminary design step, and this corresponds to designing the tower structure 1 using only the preliminary design step without performing the area setting step or the main design step.

[0048] Figure 5 shows the reduction rate of the total weight of Tower Structure 1 designed using the tower structure design method described above, in comparison to the size of the area set in the area setting step. In Figure 5, the reduction rate of the total weight of the monopile is shown as a relative value based on the total weight of Tower Structure 1 (Comparative Example 1), which was designed using only the preliminary design step without performing the area setting step or the main design step.

[0049] As shown in Figure 5, when the size of the region set in the region setting step was in the range of 0 times (Comparative Example 1) to less than 0.8 times (Inventive Example 1) the maximum outer diameter of the tower structure 1, the overall weight of the designed tower structure 1 was hardly reduced.

[0050] When the size of the area set in the area setting step was in the range of 0.8 times (Example 1) to 2.4 times (Example 3) the maximum outer diameter of the tower structure 1, the greater the size of the area, the greater the effect of reducing the overall weight of the designed tower structure 1. The reduction rate of the overall weight of the designed tower structure 1 was approximately 0.99 to 0.92, and the overall weight of the designed tower structure 1 was reduced by approximately 1 to 8%.

[0051] When the size of the area set in the area setting step exceeds 2.4 times the maximum outer diameter of the tower structure 1 (Example 3 of Invention), the reduction rate of the overall weight of the designed tower structure 1 plateaus at around 0.9, and the effect of reducing the overall weight of the designed tower structure 1 does not increase any further.

[0052] Thus, the size of the region set in the region setting step is such that the steel pipes have a yield strength of steel type A (first yield strength σ) in the range of 0.8 to 2.4 times the maximum outer diameter of the tower structure 1. y1 ) The second yield strength σ is greater than y2 It was confirmed that by using steel grade B, which has [specific properties], the overall weight of the tower structure 1 can be effectively reduced compared to using only steel grade A. [Explanation of symbols]

[0053] 1. Tower structure (monopile) 11~14 Steel pipe 2. Offshore wind power generation facilities 20 Towers 3 Transition Pieces h0, h1 reference height

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