Filter unit for monopile and method for preventing scour using same

Abstract

To provide a filter unit for a monopile and a scoring prevention method using the same, preventing a monopile structure from being unstable.SOLUTION: A filter unit 10 uses a bag body for a monopile. The body bag includes infilling materials. For the grain diameters of the infilling materials, its major axis and minor axis are 100 mm or longer and 500 mm or shorter as the central value, and variations within plus / minus 50 mm from the central value are permitted.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a filter unit for a monopile and a method for preventing scouring using the same, and more particularly to a filter unit for a monopile used in an offshore wind power generation facility and a method for preventing scouring using the same.

[0002] Conventionally, a technique for preventing scouring of piles such as monopiles has been proposed using a filter unit, which is a bag body composed of a bag material filled with a filling material. For example, JP 2013-002049 A (Patent Document 1) discloses a construction method for a seawall that can be constructed in a short period of time even when the ground is uneven and can prevent scouring. According to the publication, first, a scouring prevention body is piled up on the ground facing the sea. The scouring prevention body is a member in which a filling material is filled inside a retaining body and the upper part is closed. The filling material is, for example, crushed stone, and is preferably made of a material with a large specific gravity. Next, a nonwoven fabric is attached to a part of the piled up scouring prevention body. The nonwoven fabric is attached to at least the entire surface of the part facing the sea. Next, a water-shielding material is sprayed onto the entire surface side of the nonwoven fabric by a spraying device. It is described that the water-shielding material preferably has a predetermined strength and elasticity after solidification.

[0003] In addition, Japanese Patent Application Laid-Open No. 2003-171923 (Patent Document 2) discloses the provision of a scour prevention structure that is stable against waves and can be constructed in a short construction period, a scour prevention construction method, a covering material suitable for the structure, and a method for manufacturing the covering material. According to this publication, a continuous cylindrical covering material is prepared by filling a mesh bag with a filling material. The mesh bag is constructed by connecting multiple bags and has a substantially continuous cylindrical shape. Therefore, the filling material is less likely to move, and the continuous cylindrical covering material is stable against waves. Each tube is connected by a stitching line along the side, and is flexible. The continuous cylindrical covering material has a connection part on the side parallel to the stitching line, and multiple covering materials can be connected to form a continuous cylindrical covering material connected body of any length. It is described that by hoisting the continuous cylindrical covering material connected body and setting it in a predetermined position, construction to cover a wide area at high speed is possible. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2013-002049 A (Abstract, etc.) [Patent Document 2] JP 2003-171923 A (abstract, etc.) Summary of the Invention [Problem to be solved by the invention]

[0005] Conventional techniques for preventing scouring related to filter units for piles such as monopiles include those mentioned above. However, in conventional techniques, when a bag body is placed around a monopile, the size of the fill material in the filter unit reduces the pore water pressure in the gaps, which can lead to the suction of the geological layer soil of the monopile foundation layer. In addition, gaps are formed between filter units, and the geological layer soil of the monopile foundation layer is sucked out through the gaps, causing scouring, which causes the monopile foundation structure to become unstable.

[0006] The present invention has been made in consideration of the above-mentioned problems, and has an object to provide a filter unit for a monopile that does not destabilize the monopile structure, and a method of preventing scouring using the same. [Means for solving the problem]

[0007] The filter unit using a bag for monopiles according to the present invention includes a filling material, the particle size of which is centered on the major axis and minor axis of 100 mm or more and 500 mm or less, within ±50 mm from the center.

[0008] The filler may have the same particle size or may contain a plurality of different particle sizes.

[0009] In another aspect of the present invention, a method for preventing scouring of a monopile using a filter unit comprises stacking two or more layers of the above-mentioned bag body. Effect of the Invention

[0010] The particle size of the filling material contained in the bags is set to be 100 mm or more and 500 mm or less in terms of long and short diameter, with a deviation of within ±50 mm from the center. This reduces the amount of sand sucked out from the sandy ground on which the bags are installed and enables the bags to exert their filtering effect.

[0011] As a result, no gaps are generated between the bag bodies, so that the monopile structure does not become unstable. This makes it possible to provide a filter unit for monopiles and a scouring prevention method using the same. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is a diagram showing the structure of a filter unit. [Diagram 2] FIG. [Diagram 3] 1 is a graph showing the sand suction depth for each flow rate in a single layer when the size of the fill material is changed. [Figure 4] This is a graph showing the sand suction depth for each flow rate in two layers when the size of the fill material is changed. [Diagram 5] FIG. 4 is a diagram showing the state of filling material inside the filter unit. [Figure 6] FIG. 13 shows the change in the filter unit from a state in which it is placed on sand to a state in which it is buried in sand. [Figure 7] 1 is a graph showing the relationship between flow rate and sand surface fraction in an 8 ton filter unit. [Figure 8] FIG. 13 is a diagram for explaining the effect when two layers of filter units are provided. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] An embodiment of the present invention will be described below. Fig. 1 is a perspective view showing a filter unit for monopiles. Referring to Fig. 1, the filter unit is composed of a bag body 10. The bag body 10 includes a bag material 11 formed of a net woven with synthetic fibers, and a filling material 20 such as stone filled in the bag material 11. After the filling material 20 is contained, the opening of the bag material 11 is tied with a mouth tying rope 13 provided near the opening, and is pulled out from the opening. In addition, a relay rope 12 is provided from the bottom of the bag body 10 through the center of the bag body 10, and these are integrated together with a hanging rope 14 provided around the opening and the mouth tying rope 13 to be lifted up.

[0014] In this study, the inventors used a filter unit to conduct tests on the scouring conditions around a monopile, simulating the use of a monopile for installing an offshore wind turbine.

[0015] Next, the specific test method will be described. Figure 2 shows the test method. Figures 2(A) and 2(B) show the state before and after the test for a single layer, and Figures 2(C) and 2(D) show the state before and after the test for a two-layer.

[0016] 2(A) to (D), a cylindrical pile 32, which is assumed to be a monopile for an offshore wind turbine, is erected in a test equipment tank 31 on sand ground. Sand 33 is spread around the pile 32, and bags 10 containing filling material with different particle sizes are placed on the sand 33.

[0017] Under these conditions, a test was conducted in which water was allowed to flow at a certain velocity from one direction, and the amount of sand sucked out was measured after a certain period of time had passed. The amount of sand 33 sucked out around the pile 32 is shown by a in Fig. 2(B), and the amount of sand sucked out in the case of two layers is shown by b in Fig. 2(D).

[0018] The scale of the actual filling material used in the test and the actual size are shown in Table 1. In other words, the test was conducted on a scale of 1 / 30 using filling materials with single particle sizes of 50mm, 100mm, 150mm, 200mm, 300mm, 400mm, and 500mm, as well as a mixture of 100mm and 400mm. The results are shown in Table 1.

[0019] In this case, the filling material is in a lump shape and has a major axis and a minor axis, but the particle size of the major axis and the minor axis are within ±50 mm of the specified particle size.

[0020] Therefore, if the filling material has a single particle size of 50 mm, it includes particles with a particle size of 0 to 100 mm, if it is 100 mm, it includes particles with a particle size of 50 to 100 mm, and if it is 150 mm, it includes particles with a particle size of 100 to 200 mm.

[0021] In addition, in the case of a mixed particle size of 100 mm and 400 mm, particle sizes of 50 mm to 150 mm and 350 mm to 450 mm are included.

[0022] In addition, the mixed particle size of 50 mm was not included here because if the particle size of the fill material is smaller than 50 mm, there will be no gaps between the fill materials in the filter unit, the pore water pressure will increase, and the filter effect will not be achieved.

[0023] [Table 1]

[0024] The sand suction depth at each flow rate is shown in Tables 2 and 3.

[0025] [Table 2]

[0026] [Table 3]

[0027] In the case of a single layer, the relationship between the size of the fill material (mm), the suction depth of the sand (mm), and the flow velocity (m / s) is shown in a graph in Figure 3. Here, the size of the fill material is shown in actual dimensions, and the horizontal axis shows the flow velocity during the test. Therefore, the actual flow velocities are 0.2m / s, 0.4m / s, 0.6m / s, 0.8m / s, 1.0m / s, 1.10m / s, 2.19m / s, 3.29m / s, 4.38m / s, and 5.48m / s, as shown in Table 2.

[0028] That is, Fig. 3 is a graph showing the suction depth of sand for each flow speed when the size of the filler is changed. Referring to Fig. 3, in the case of a single particle size, as the particle size of the filler increases, the gaps between the filler materials become larger. Therefore, as the water flow speed increases, the suction depth of the sand on the bottom surface of the bag increases.

[0029] On the other hand, by using a mixed grain size, the suction depth of the sand from the bottom of the bag can be reduced to almost zero.

[0030] Therefore, when considering only the suction depth of the sand, it is found that a mixed particle size of 100 to 400 mm is most preferable because the suction depth of the sand is almost zero. On the other hand, in the case of a single particle size, it is found that smaller sizes such as 50 mm and 100 mm are preferable.

[0031] In the case of two layers, the relationship between the size of the fill material (mm), the suction depth of the sand (mm), and the flow velocity (m / s) is shown in Figure 4. That is, Figure 4 is a graph showing the suction depth of the sand for each flow velocity when the size of the fill material is changed. Here again, the size of the fill material is shown in actual dimensions, and the horizontal axis shows the flow velocity during the test. Therefore, as shown in Table 3, the actual flow velocities are 0.2m / s, 0.4m / s, 0.6m / s, 0.8m / s, and 1.0m / s, which are 1.10m / s, 2.19m / s, 3.29m / s, 4.38m / s, and 5.48m / s. Referring to Figures 3 and 4, the suction depth is smaller in the case of two layers than in the case of a single layer.

[0032] This fact is known from rubble tests, but these tests were only conducted on the bottoms of submerged breakwaters or offshore breakwaters.

[0033] 4, the greater the flow rate, the greater the depth of the sand that is sucked out. This tendency also shows that the larger the size of the filler, the greater the amount of sand that is sucked out.

[0034] Next, the particle size of the filler material will be explained. Figure 5 is a diagram showing the size of the filler material. Figure 5(A) shows the case where the single particle size is 100mm, Figure 5(B) shows the case where the single particle size is 400mm, and Figure 5(C) shows the case where the single particle size is a mixed particle size of 100mm and 400mm. As shown in the figure, the smaller the single particle size, the smaller the gaps, and the larger the single particle size, the larger the gaps, and it can be seen that the gaps become smaller when small and large particle sizes are mixed. Therefore, by using a mixed particle size, the suction depth of the sand can be reduced.

[0035] Next, the filtering effect of the filter unit will be described. Fig. 6 is a diagram for explaining the filtering effect of the filter unit. Fig. 6(A) is a diagram showing the state in which the filter unit 10 is placed on the sand 33, and Fig. 6(B) is a diagram showing the state in which the filter unit 10 is buried by the suction of the sand 33 after a predetermined time has elapsed.

[0036] That is, initially, the filter unit 10 is 100% exposed above the sand 33 to a height H0, but after a predetermined time has passed, the filter unit 10 becomes buried as the sand is sucked out, and changes to a state in which only a height H is exposed above the sand 33, as shown in Figure 6(B).

[0037] In order for the filter unit 10 to exert its scouring prevention effect, 60% or more of the original height of the filter unit 10 needs to be present above the sand 33. In other words, when the height of the filter unit 10 becomes equal to or less than H=H0×0.6, the filtering effect decreases.

[0038] Next, the dimension H above the sand in FIG. 6(B) required for the actual filter unit 10 to exhibit a scouring prevention effect will be described.

[0039] Here, we will explain the filter units 10 of 2t to 8t, which are often used in practice. Table 4 shows the height H of the filter units 10 of 2t to 8t and the dimensions required to exert a filtering effect.

[0040] [Table 4]

[0041] Table 4 is graphed in Figure 7. Figure 7 is a graph showing the change in the 8t filter unit 10, with the X-axis representing the flow velocity and the Y-axis representing the ratio H / H0 (%) of the exposed dimension above the sand. The same is true for the 2t to 6t filter units.

[0042] Here, the 2 ton filter unit contains 100mm to 200mm fill material of the same particle size, the 4 ton filter unit contains 300mm fill material of the same particle size, the 6 ton filter unit contains 400mm fill material of the same particle size, and the 8 ton filter unit contains 500mm fill material of the same particle size.

[0043] Referring to FIG. 7, if the filler material has a single uniform particle size, the ratio H / H0 (%) of the exposed dimension above the sand can exhibit a predetermined scouring prevention effect for all filter units of 2t to 8t.

[0044] When the particle size of the fill material is uniform, the size is set at 100 mm or more because, if it is smaller than this, the gaps between the fill materials in the filter unit will be small, increasing the pore water pressure, reducing the filtering effect, and increasing the possibility of scouring occurring on the outer periphery of the filter unit.

[0045] Although no data is shown here, the same effect can be achieved by using a filling material mainly of each particle size, mixed with particles of 50 mm or more.

[0046] Next, the effect of stacking two layers of filter units 10 will be described. Fig. 8 is a diagram for explaining this effect. Fig. 8(A) is a diagram showing the case where two layers of filter units 10 are stacked, and Fig. 8(B) is a diagram showing the case where there is only one layer of filter unit 10.

[0047] 8(B), if the filter unit 10 is a single layer, even if the filter unit 10 is able to trap sand, there is a possibility that the sand will escape through the gaps between the filter units 10, as indicated by the arrow n in the figure. On the other hand, if the filter units 10 are stacked in two layers as shown in Fig. 8(A), even if sand passes through the first filter unit 10, as indicated by the arrow m in the figure, the layer of filter units 10 above it can prevent the sand from escaping.

[0048] In order to obtain such an effect, it is preferable that the upper layer is disposed so as to cover the separated portion of the lower layer. The same applies to the case of three or more layers.

[0049] In the above embodiment, the 4t filter unit contains filler material of the same particle size of 300 mm, the 6t filter unit contains filler material of the same particle size of 400 mm, and the 8t filter unit contains filler material of the same particle size of 500 mm, but this is not limited to the above and the particle size may be changed to obtain similar results.

[0050] In addition, in the above embodiment, in the case of mixed particle sizes, a case was described in which a minimum particle size of 100 mm was mixed with a larger particle size, but this is not limited to this, and a larger particle size may be mixed with a smaller particle size, and similar results will be obtained.

[0051] In the above embodiment, the filter unit is used to prevent suction of monopiles, but the present invention is not limited to this and can also be applied to prevent suction of bridge pier foundations. [Industrial Applicability]

[0052] The filter unit according to the present invention is advantageously used as a filter unit for monopiles since it does not destabilize the monopile structure. [Explanation of symbols]

[0053] 10 bag body, 11 bag material, 12 joint rope, 13 tying rope, 14 hanging rope, 20 filling material, 31 test equipment water tank, 32 pile, 33 sand.

Claims

[Claim 1] A filter unit using a bag body for a monopile, The monopile bag contains a fill material, Each of the monopile bags has a scour prevention function, The particle size of the filling material is such that the major axis and minor axis are 100 mm or more and 500 mm or less, and the center is within ±50 mm from the center of the specified particle size. The center of the defined particle size is 100 mm or 400 mm; The particle size of the filling material includes a plurality of different particle sizes, The filter unit uses a bag for monopiles, and the different particle sizes are 100 mm and 400 mm.

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