Method for evaluating scouring range, method for designing scouring prevention structure, and method for constructing scouring prevention structure

Abstract

To provide an evaluation method of a scouring range capable of properly evaluating the scouring range, and a design method and a construction method of a scouring prevention structure.SOLUTION: A step S1 of providing a boring core or the like on a bottom surface of a first experimental water channel such that an upper end surface and the bottom surface of the boring core or the like taken from a water bottom ground are flush with each other, and then performing an experiment of giving a flow velocity by a water flow in the first experimental water channel to acquire a limit flow velocity that is a limit flow velocity at which the upper end surface of the boring core or the like is not scoured; This method has a step S2 for acquiring a flow velocity distribution of a bottom surface around the model, and a step S3 for evaluating a scouring range by determining a range of the occurrence of scouring based on the acquired flow velocity distribution and a critical flow velocity.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the extent of scour of the ground surrounding an underwater structure such as a monopile foundation, and a method for designing and constructing a scour prevention structure. [Background technology]

[0002] It is known that measures to prevent scouring of the seabed around monopile foundations used as the foundations of offshore wind power generation facilities against waves, tides, and tsunamis have been implemented. For example, as shown in Fig. 6, it is known to cover the surface of the seabed ground 2 around a monopile foundation 1 with a scour prevention structure 4 made of stone material 3 (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-019177 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the coverage area 5 of the stone material 3 is often set based on the scouring area evaluated by treating the seabed ground 2 as sandy ground, and even in the case of rocky ground that is not easily scouring, the scouring area is evaluated by treating it in the same way as sandy ground that is easily scouring. As a result, when the seabed ground 2 is rocky ground, the coverage area of ​​the stone material is designed to be excessive, which increases construction costs and lengthens the construction period.

[0005] The present invention has been made in view of the above, and aims to provide a method for evaluating a scour area, a method for designing a scour prevention structure, and a method for constructing the structure, which are capable of appropriately evaluating the scour area. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems and achieve the object, the method for evaluating the extent of scouring according to the present invention is a method for evaluating the extent of scouring on the bottom ground around an underwater structure installed at the bottom of a body of water, and is characterized by comprising the steps of: installing a boring core collected from the bottom ground or a sample simulating the bottom ground on the bottom surface of a first experimental flume so that the top surface of the boring core or the sample is flush with the bottom surface, and then conducting an experiment in which a water flow velocity is applied in the first experimental flume to obtain a critical flow velocity, which is the limit flow velocity at which the top surface of the boring core or the sample is not scouring; installing a model of the underwater structure on the bottom surface of a second experimental flume, and then conducting an experiment in which a water flow velocity is applied in the second experimental flume to obtain a flow velocity distribution on the bottom surface around the model; and evaluating the extent of scouring by determining the extent of occurrence of scouring based on the obtained flow velocity distribution and the critical flow velocity.

[0007] Furthermore, another method for evaluating the extent of scouring according to the present invention is characterized in that, in the above-mentioned invention, the second experimental waterway is a numerical experimental waterway that simulates the water area using a numerical model.

[0008] Furthermore, the design method for a scour prevention structure according to the present invention is characterized in that it sets a construction range for a scour prevention structure for preventing scouring around the underwater structure, based on the scour range evaluated by the above-mentioned scour range evaluation method.

[0009] Furthermore, the construction method of the scour prevention structure according to the present invention is characterized in that a construction area is set based on the scour area evaluated by the above-mentioned scour area evaluation method, and a scour prevention structure for preventing scouring around the underwater structure is constructed within the set construction area. [Effects of the Invention]

[0010] According to the method for evaluating a scour area of ​​the present invention, a method for evaluating a scouring area of ​​the bottom ground around an underwater structure installed at the bottom of a body of water includes the steps of: installing a boring core collected from the bottom ground or a sample simulating the bottom ground on the bottom surface of a first experimental flume so that the top surface of the boring core or the sample is flush with the bottom surface, and then conducting an experiment in which a water flow velocity is applied in the first experimental flume to obtain a critical flow velocity that is the limit flow velocity at which the top surface of the boring core or the sample is not scoured; installing a model of the underwater structure on the bottom surface of a second experimental flume, and then conducting an experiment in which a water flow velocity is applied in the second experimental flume to obtain a flow velocity distribution on the bottom surface around the model; and evaluating the scour area by determining the extent of scour based on the obtained flow velocity distribution and the critical flow velocity, thereby achieving the effect of being able to appropriately evaluate the scour area.

[0011] Furthermore, according to another method for evaluating the extent of scouring of the present invention, the second experimental waterway is a numerical experimental waterway that simulates the water area using a numerical model, which has the effect of enabling a large number of cases to be considered on a desk in a short period of time.

[0012] Furthermore, according to the design method of the scour prevention structure of the present invention, the construction range of the scour prevention structure for preventing scouring around the underwater structure is set based on the scour range evaluated by the above-mentioned scour range evaluation method, thereby achieving the effect of enabling the scour prevention structure to be designed economically so as not to be overdesigned.

[0013] Furthermore, according to the construction method of the scour prevention structure of the present invention, a construction area is set based on the scour area evaluated by the above-mentioned scour area evaluation method, and a scour prevention structure for preventing scouring around the underwater structure is constructed in the set construction area, thereby achieving the effect of enabling the scour prevention structure to be constructed economically. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram showing an embodiment of a method for evaluating a scour area, a method for designing a scour prevention structure, and a method for constructing the structure, according to the present invention. [Figure 2] Figure 2 is a schematic perspective view showing the experimental image, (1) is Experiment A, and (2) is Experiment B. [Figure 3] Figure 3 shows photographs of the flume in Experiment A, where (1) is a section of the flume and (2) is the bottom of the flume. [Figure 4] FIG. 4 is a diagram showing an example of the relationship between the amount of scouring and the flow velocity. [Figure 5] FIG. 5 is a diagram showing an example of the relationship between the scouring amount and the elapsed time. [Figure 6] FIG. 6 is a schematic diagram showing an example of a conventional measure to prevent scouring around a monopile foundation. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the method for evaluating the scour area, the method for designing a scour prevention structure, and the method for constructing the structure according to the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to these embodiments.

[0016] As shown in Figure 1, a scour extent evaluation method according to an embodiment of the present invention is a method for evaluating the scouring extent of the seabed ground (water bottom ground) around an underwater structure installed on the bottom of a sea area (water body), and includes step S1 of acquiring a critical scour current velocity through experiment A, step S2 of acquiring a current velocity distribution through experiment B, and step S3 of evaluating the scouring extent. The seabed ground is assumed to be rocky ground. The underwater structure is a vertical columnar monopile foundation with a circular cross section, which is driven into the seabed ground. The monopile foundation is made of steel or concrete, and its upper part protrudes above the sea surface.

[0017] The design method for a scour prevention structure according to an embodiment of the present invention sets the construction range of the scour prevention structure to prevent scouring around the monopile foundation based on the scour range evaluated in step S3 above (step S4). The construction range is assumed to be, for example, a range corresponding to the conventional covered range 5 in Fig. 6. The construction range may be a range that coincides with the evaluated scour range, or it may be a larger range that includes the scour range.

[0018] The construction method for a scour prevention structure according to an embodiment of the present invention constructs a scour prevention structure to prevent scouring around the monopile foundation in the construction area set in step S4 above (step S5). The scour prevention structure is assumed to have a structure corresponding to the conventional scour prevention structure 5 made up of multiple stones 3 shown in Fig. 6, for example.

[0019] Next, the specific contents of steps S1 to S3 constituting the method for evaluating the scour range will be described.

[0020] First, in step S1, as shown in FIG. 2(1), a borehole 14 is placed on the bottom 12 of a first experimental flume 10 so that the top surface 16 of the borehole 14, collected from the seabed to be evaluated, is flush with the bottom 12 of the experimental flume 10. Experiment A is then performed in the experimental flume 10, applying a flow velocity va due to waves, tsunamis, tidal currents, etc. to the borehole 14. Experiment A determines the critical flow velocity for scouring, which is the flow velocity at which the top surface 16 of the borehole 14 is not scoured. For example, a current meter is used to measure the flow velocity at a position several centimeters above the top surface 16 of the borehole 14, and the presence or absence of scouring of the top surface 16 of the borehole 14 is visually confirmed. The applied flow velocity va is gradually changed to obtain the flow velocities when the top surface 16 of the borehole 14 is not scoured and when it is scoured. This allows the critical flow velocity for scouring to be determined. It is preferable to apply the obtained critical scouring flow velocity for a long period of time (for example, about 180 minutes) to confirm that scouring does not occur.

[0021] The experimental flume 10 can be, for example, a two-dimensional wave flume (e.g., a rectangular open flume with a length of several tens of meters, a height of approximately 1 to 1.5 meters, and a width of approximately 0.5 to 1 meter) commonly used in hydraulic model experiments. Such a flume is equipped with a wave-making function that generates waves in one direction and a flow function that allows water to flow, making it suitable for conducting Experiment A. Figure 3 (1) shows an example of the experimental flume 10, and Figure 3 (2) shows the condition of the bottom surface 12. The bottom surface 12 of the experimental flume 10 is horizontal. Note that the inclination of the bottom surface 12 is not limited to being horizontal. For example, if the seabed to be evaluated has an inclination, the inclination gradient of the seabed and the inclination gradient of the bottom surface 12 of the experimental flume 10 may be made to match. By generating water flows, such as waves, tsunamis, or tidal currents, along the longitudinal direction of the experimental flume 10, a longitudinal water flow is generated near the bottom surface 12 where the boring core 14 is installed. This allows a constant flow velocity va to be applied to the bottom surface 12. The applied flow velocity va is preferably set to a flow velocity equal to or greater than the flow velocity generated near the seabed ground to be evaluated due to waves, tsunamis, etc.

[0022] The object to be placed on the bottom surface 12 of the experimental flume 10 is not limited to the boring core 14. For example, a sample simulating the seabed ground to be evaluated may be placed on the bottom surface 12 of the experimental flume 10. The sample may be a ground material prepared in a laboratory, or a sample such as a boring core collected from a location near or similar to the location to be evaluated.

[0023] In the next step S2, as shown in Figure 2(2), a scaled-down model 22 of the monopile foundation is placed on the bottom surface 20 of the second experimental flume 18. Experiment B is then conducted in this experimental flume 18, applying a flow velocity vb due to waves, tsunamis, tidal currents, etc., to obtain the flow velocity distribution on the bottom surface 20 around the model 22. The applied flow velocity vb is set to correspond to the actual flow velocity around the monopile foundation in the sea area being evaluated due to waves, tsunamis, tidal currents, etc. The flow velocity distribution can be obtained using an ultrasonic current meter or particle image velocimeter. Since Experiment B is conducted using a scaled-down model of a monopile foundation of actual dimensions in the actual sea area, the flow velocity obtained in Experiment B is proportional to the 1 / 2 power of the scale of the model, according to Froude's law of similarity, and is therefore smaller than the flow velocity observed in the actual sea area. Therefore, the flow velocity obtained in Experiment B is converted to the actual flow velocity in the sea area to obtain the actual flow velocity distribution.

[0024] The experimental flume 18 may be the same as the experimental flume 10. In Experiment B, a numerical experimental flume that is reproduced on a computer using flow condition reproduction analysis software (a three-dimensional flow condition analysis tool such as CADMAS3D) may be used instead of or in addition to the experimental flume 18. In this way, when there are many cases of waves and tsunamis to be examined, it is possible to shorten the period during which the experimental flume 18 is required to be used.

[0025] In the next step S3, the scour extent is evaluated by determining the range in which scouring will occur based on the critical scouring flow velocity obtained in experiment A and the flow velocity distribution obtained in experiment B. For example, if the critical scouring flow velocity is V, then the region in the flow velocity distribution obtained in experiment B where the flow velocity is V or greater is determined as the range in which scouring will occur, and the region where the flow velocity is less than V is determined as the range in which scouring will not occur. Note that the present invention is not limited to this, and it is also possible to make a conservative estimate and determine the region in the flow velocity distribution obtained in experiment B where the flow velocity is V1 or greater, which is smaller than flow velocity V, as the range in which scouring will occur, and the region where the flow velocity is less than V1 as the range in which scouring will not occur.

[0026] According to the method for evaluating the scour extent of this embodiment, the scour extent of rocky ground can be appropriately evaluated by conducting a flume experiment that combines Experiment A and Experiment B. This allows the construction area of ​​a scour prevention structure to be constructed on rocky ground to be designed economically so as not to be oversized. Compared to the conventional method for evaluating the scour extent that treated it the same as sandy ground, this method allows for an appropriate evaluation of the scour extent, reducing the amount of stone, which has tended to be overestimated in the past, and enabling cost reductions and a shorter construction period. This means that scour prevention structures can be constructed economically.

[0027] [Example] Next, an example of the method for evaluating the scour range according to the present invention will be described.

[0028] (Experiment A) In Experiment A, a boring core from the target ground was placed flush with the bottom of the experimental flume to obtain the critical flow velocity at which scouring would not occur. The experimental flume used was as shown in Figure 3. In this example, the following two experiments (A-1 and A-2) were conducted, with the water flow time being used as a parameter.

[0029] <Experiment A-1> The flow velocity on the borehole core was changed stepwise over a short period of time (e.g., about 5 minutes) to obtain the critical flow velocity at which scouring would not occur on the top surface of the borehole core. In this example, seven cases of stepwise flow velocities were performed: 2.5 m / s, 3.5 m / s, 4.5 m / s, 5.0 m / s, 6.2 m / s, 7.0 m / s, and 8.2 m / s, and the amount of scouring (cumulative) on the surface of the top surface of the borehole core was measured at each flow velocity. The measurement results are shown in Figure 4. From these results, it was determined that the critical flow velocity for scouring of the target ground was 5.0 m / s.

[0030] <Experiment A-2> The critical scouring flow velocity obtained in Experiment A-1 was allowed to act for a long period of time (for example, about 180 minutes) to confirm that scouring would not occur. In this example, since scouring was not confirmed in a short period of time at a flow velocity of 5.0 m / s in Experiment A-1, the same flow velocity of 5.0 m / s was allowed to act for a long period of time (up to 180 minutes), and the cumulative amount of scouring was measured at each arbitrary elapsed time during the process. The measurement results are shown in Figure 5. From these results, there was no tendency for the amount of scouring to increase over time, so it was determined that scouring would not occur at flow velocities of 5.0 m / s or less.

[0031] (Experiment B) In Experiment B, the flow conditions at the bottom around the monopile foundation are confirmed. In this example, confirmation is performed using a reproduction experiment using numerical analysis instead of an experimental flume. The numerical analysis uses the above-mentioned reproduction analysis software, and parameters fitted using the results of a separately conducted large-scale flume experiment. The conditions of the sea area to be evaluated are used as input values, and the maximum flow velocity at the surface of the seabed is obtained as the output value.

[0032] (Judgment method) Around the monopile foundation, if the maximum flow velocity output from the numerical analysis is 5.0 m / s or more, it is determined that scouring will occur, and if it is less than 5.0 m / s, it is determined that scouring will not occur. The area where scouring is determined to occur is set as the construction area for scour prevention structures. Areas where scouring is determined not to occur are excluded from the construction area for scour prevention structures.

[0033] In the above embodiment, the water area has been described as an ocean area, but the water area of ​​the present invention is not limited to this and may also be a river area or a lake or marsh where scouring is possible. Furthermore, while the underwater structure has been described as a monopile foundation in the form of a vertical column with a circular cross section, the underwater structure of the present invention is not limited to this and may be any other shape of underwater structure as long as it is placed on the bottom of the water. In this case, the same effects as those described above can be achieved.

[0034] As described above, the scouring extent evaluation method of the present invention is a method for evaluating the scouring extent of the water bottom ground around an underwater structure installed at the bottom of a body of water, and includes the steps of: installing a boring core collected from the water bottom ground or a sample simulating the water bottom ground on the bottom surface of a first experimental flume so that the top surface of the boring core or the sample is flush with the bottom surface, and then conducting an experiment in which a water flow velocity is applied in the first experimental flume to obtain a critical flow velocity, which is the limit flow velocity at which the top surface of the boring core or the sample is not scouring; installing a model of the underwater structure on the bottom surface of a second experimental flume, and then conducting an experiment in which a water flow velocity is applied in the second experimental flume to obtain a flow velocity distribution on the bottom surface around the model; and evaluating the scouring extent by determining the extent of scour based on the obtained flow velocity distribution and the critical flow velocity, thereby enabling the scouring extent to be evaluated appropriately.

[0035] Furthermore, according to another method for evaluating the extent of scouring of the present invention, the second experimental waterway is a numerical experimental waterway that simulates the water area using a numerical model, so that a large number of cases can be considered on the desk in a short period of time.

[0036] Furthermore, according to the design method of the scour prevention structure of the present invention, the construction range of the scour prevention structure for preventing scouring around the underwater structure is set based on the scour range evaluated by the above-mentioned scour range evaluation method, so that the scour prevention structure can be designed economically so as not to be oversized.

[0037] Furthermore, according to the construction method of the scour prevention structure of the present invention, a construction area is set based on the scour area evaluated by the above-mentioned scour area evaluation method, and a scour prevention structure for preventing scouring around the underwater structure is constructed in the set construction area, so that the scour prevention structure can be constructed economically.

[0038] The Sustainable Development Goals (SDGs) are 17 international goals adopted at the United Nations Summit in September 2015. The method for assessing the extent of scouring, and the method for designing and constructing a scour prevention structure according to this embodiment can contribute to achieving one of the 17 SDGs, for example, goal 7, "Affordable and clean energy for all." [Industrial Applicability]

[0039] As described above, the method for evaluating the extent of scour, and the method for designing and constructing a scour prevention structure according to the present invention are useful for underwater structures such as monopile foundations, and are particularly suited to appropriately evaluating the extent of scour of the ground surrounding underwater structures. [Explanation of symbols]

[0040] 10 First Experimental Channel 12,20 bottom 14 Boring Core 16 Top surface 18 Second Experimental Channel 22 Model

Claims

1. A method for evaluating a scour range that is scoured on the bottom ground around an underwater structure installed on the bottom of a water body, comprising: a step of placing a boring core collected from the bottom of the water or a sample simulating the bottom of the water on the bottom of a first experimental water channel so that the top surface of the boring core or the sample is flush with the bottom surface, and then conducting an experiment in which a water flow velocity is applied in the first experimental water channel to obtain a critical flow velocity that is the limit flow velocity at which the top surface of the boring core or the sample is not eroded; a step of providing a model of the underwater structure on the bottom surface of a second experimental flume, conducting an experiment in which a flow velocity is applied by a water current in the second experimental flume, and acquiring a flow velocity distribution on the bottom surface around the model; a step of evaluating the scour extent by determining the extent of occurrence of scouring based on the acquired flow velocity distribution and the critical flow velocity.

2. 2. The method for evaluating a scour extent according to claim 1, wherein the second experimental flume is a numerical experimental flume that simulates the water area using a numerical model.

3. 3. A method for designing a scour prevention structure, comprising: setting a construction range of a scour prevention structure for preventing scouring around the underwater structure based on the scour range evaluated by the scour range evaluation method according to claim 1 or 2.

4. 3. A construction method for a scour prevention structure, comprising: setting a construction area based on the scour area evaluated by the scour area evaluation method according to claim 1 or 2; and constructing a scour prevention structure in the set construction area to prevent scouring around the underwater structure.

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