JP7627017B2 - Filter unit for monopile and method for preventing scour using same - Google Patents

Description

This invention relates to a filter unit for monopiles and a method for preventing scouring using the same, and in particular to a filter unit for monopiles used in offshore wind power generation facilities and a method for preventing scouring using the same.

Conventionally, a technique for preventing scouring of piles such as monopiles has been proposed, using a filter unit, which is a bag body made 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 this publication, first, scouring prevention bodies are piled up on the ground facing the sea. The scouring prevention body is a member in which the inside of a retaining body is filled with a filling material and the top is closed. The filling material is, for example, crushed stone, and is preferably made of a material with a high specific gravity. Next, nonwoven fabric is attached to a part of the piled 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.

JP 2003-171923 A (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 together and has a generally continuous cylindrical shape. This makes it difficult for the filling material 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 that is 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 that covers a wide area at high speed is possible.

JP 2013-002049 A (Abstract, etc.) JP 2003-171923 A (Abstract, etc.)

Conventional technologies for preventing scouring in relation to filter units for piles such as monopiles include those mentioned above. However, with conventional technologies, when placing a bag around the 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 geological soil in the monopile foundation layer. In addition, gaps can form between filter units, which can cause the geological soil in the monopile foundation layer to be sucked out through the gaps, resulting in scouring and destabilizing the monopile foundation structure.

This invention was made in consideration of the above problems, and aims to provide a filter unit for monopiles that does not destabilize the monopile structure, and a method for preventing scouring using the same.

The filter unit using a monopile bag according to this invention includes a filling material, the particle size of which is centered on the major and minor axes of 100 mm or more and 500 mm or less, within ±50 mm from the center.

The particle size of the filling material may be the same or may contain multiple different particle sizes.

In another aspect of the invention, a method for preventing scouring of monopiles using a filter unit involves stacking two or more layers of the above-mentioned bag body.

The particle size of the filling material contained in the bag is set so that the major and minor axes are 100 mm or more and 500 mm or less, with the center being within ±50 mm from the center, which reduces the amount of sand sucked out from the sandy ground where the bag is installed and also enables the bag to exert its filtering effect.

As a result, no gaps are created between the bags, so the monopile structure does not become unstable, making it possible to provide a filter unit for monopiles and a method for preventing scouring using the same.

FIG. 2 is a diagram showing the structure of a filter unit. FIG. 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. 1 is a graph showing the sand suction depth for different flow rates in two layers when the size of the fill material is changed. FIG. 4 is a diagram showing the state of filling material inside the filter unit. 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. 1 is a graph showing the relationship between flow rate and sand surface fraction in an 8 ton filter unit. FIG. 13 is a diagram for explaining the effect of providing two layers of filter units.

An embodiment of the present invention will be described below. Figure 1 is a perspective view showing a filter unit for monopiles. Referring to Figure 1, the filter unit is composed of a bag body 10. The bag body 10 includes a bag material 11 formed of a mesh woven from 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 tie rope 13 provided near the opening, and is pulled out from the opening. In addition, a relay rope 12 is provided that runs from the bottom of the bag body 10 through the center of the bag body 10, and these are integrated together with the hanging rope 14 and the tie rope 13 provided around the opening and lifted up.

The inventors used the filter unit to test the scouring conditions around a monopile, simulating the monopile used to install an offshore wind turbine.

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.

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

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

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

The filling material is in a lump shape and has a long and short diameter, but the particle size is set so that the long and short diameters are within ±50 mm of the specified particle size.

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

In addition, in the case of a mixed particle size of 100 mm and 400 mm, this includes particles with particle sizes of 50 mm to 150 mm and 350 mm to 450 mm.

The reason why 50 mm is not included as a mixed particle size here is that 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.

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

In the case of a single layer, Figure 3 shows a graph of the relationship between the size of the fill material (mm), the suction depth of the sand (mm), and the flow velocity (m/s). 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 0.2 m/s, 0.4 m/s, 0.6 m/s, 0.8 m/s, and 1.0 m/s, as shown in Table 2, and are 1.10 m/s, 2.19 m/s, 3.29 m/s, 4.38 m/s, and 5.48 m/s.

That is, Figure 3 is a graph showing the depth of sand sucked out for each flow speed when the size of the filler is changed. Referring to Figure 3, in the case of a single particle size, as the particle size of the filler increases, the gaps between the filler particles become larger. Therefore, as the water flow speed increases, the depth of sand sucked out from the bottom surface of the bag increases.

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.

Therefore, if we only consider the suction depth of the sand, it is clear that a mixed particle size of 100 to 400 mm is most preferable because it results in almost no suction depth of the sand. On the other hand, in the case of a single particle size, it is clear that smaller sizes such as 50 mm or 100 mm are more preferable.

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 0.2 m/s, 0.4 m/s, 0.6 m/s, 0.8 m/s, and 1.0 m/s, which are 1.10 m/s, 2.19 m/s, 3.29 m/s, 4.38 m/s, and 5.48 m/s. Referring to Figures 3 and 4, the suction depth is smaller in the case of two layers compared to a single layer.

This is known from rubble tests, but these tests were only conducted on the bottoms of submerged breakwaters and offshore breakwaters.

Referring to Figure 4, as the flow velocity increases, the depth of the sand sucked out increases. This tendency also shows that the larger the size of the fill material, the greater the amount of sand sucked out.

Next, the particle size of the filler material will be explained. Figure 5 shows the size of the filler material. Figure 5(A) shows the case where the single particle size is 100 mm, Figure 5(B) shows the case where the single particle size is 400 mm, and Figure 5(C) shows the case where the single particle size is a mixed particle size of 100 mm and 400 mm. 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.

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

That is, initially, the filter unit 10 is 100% exposed above the sand 33 to a height H0, but after a certain time has passed, the filter unit 10 is 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 FIG. 6(B).

For the filter unit 10 to have a scouring prevention effect, 60% or more of the original height of the filter unit 10 must be above the sand 33. In other words, if the filter unit 10 falls below H = H0 x 0.6, the filtering effect will decrease.

Next, we will explain the dimension H above the sand in Figure 6 (B) that is required to achieve the scouring prevention effect in an actual filter unit 10.

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.

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 applies to the 2t to 6t filter units.

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

Referring to Figure 7, if the fill material is of a single uniform particle size, the ratio of exposed dimensions above the sand, H/H0 (%), can achieve a desired anti-scouring effect for all filter units from 2t to 8t.

When the particle size of the fill material is uniform, the size must be 100 mm or more because if it is smaller than this, the gaps between the fill materials in the filter unit will be smaller, the pore water pressure will increase, the filtering effect will be reduced, and there will be a higher possibility of scouring occurring on the outer periphery of the filter unit.

Although no data is shown here, the same effect can be achieved by using a filling material that is mainly composed of each particle size and mixing it with particles with a particle size of 50 mm or more.

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

Referring to FIG. 8(B), if the filter unit 10 is a single layer, even if the filter unit 10 is able to trap the sand, there is a possibility that the sand may escape through the gaps between the filter units 10, as shown 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 shown by the arrow m in the figure, the layer of filter units 10 above it can prevent the sand from escaping.

In order to achieve this effect, it is preferable that the upper layer is arranged so as to cover the separated portion of the lower layer. The same applies when there are three or more layers.

In the above embodiment, the 4t filter unit contains filler material with the same particle size of 300 mm, the 6t filter unit contains filler material with the same particle size of 400 mm, and the 8t filter unit contains filler material with 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.

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 the above, and a larger particle size may be mixed with a smaller particle size, and similar results will be obtained.

In addition, in the above embodiment, the filter unit is described as being used to prevent suction from monopiles, but it can also be used to prevent suction from bridge pier foundations.

The filter unit of this invention is advantageously used as a filter unit for monopiles because it does not destabilize the monopile structure.

10 bag body, 11 bag material, 12 intermediate rope, 13 tying rope, 14 hanging rope, 20 filling material, 31 test equipment tank, 32 pile, 33 sand.

Claims (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 .