JP7510385B2 - How to lay underwater cables on monopiles - Google Patents

Description

The present invention relates to a method for laying an underwater cable on a monopile.

Around the monopile foundations of offshore wind power generation facilities, the flow speeds due to wave and tidal forces become large, making it easy for scouring to occur on the bottom of the water. For this reason, when the ground is soft, scouring prevention materials are installed on the ground around the monopile foundations.

In addition, offshore wind power generation facilities are connected to cables for power transmission, etc., but if the cables are exposed at the bottom of the water, there is a risk that the free-span cables will be damaged by external forces such as wave and tidal forces. For this reason, the cables are protected by being inserted into a sheath pipe attached to the monopile, and scouring prevention material is further installed on the sheath pipe, which holds down the sheath pipe through which the cables are inserted, preventing the cables from becoming free-span (see, for example, Patent Document 1).

Japanese Patent No. 6460894

However, when the ground around the foundation is relatively hard, scour prevention materials are sometimes omitted to reduce costs. If scour prevention materials are omitted, the sheath pipe that protects the underwater cable is exposed underwater or on the water bottom, making the sheath pipe more susceptible to external forces such as wave and current forces, and there is a risk of the sheath pipe breaking due to a large reaction force acting on the attachment part to the monopile. Furthermore, when the sheath pipe is exposed underwater or on the water bottom, it may be damaged by anchoring or dragging of a work boat when performing maintenance on the offshore wind power generation facility with the work boat.

The present invention has been made in consideration of the above-mentioned problems, and an object of the present invention is to provide a method for laying an underwater cable on a monopile, which can prevent damage to the sheath pipe through which the cable is inserted, even when a scour prevention material is not installed.

In order to achieve the above-mentioned object, the first invention is a method for laying an underwater cable on a monopile, the monopile having a guide tube fixed inside, characterized in that it comprises the steps of: installing a curved boring machine near the top of the monopile; inserting a drilling rod into the guide tube and using the curved boring machine to perform curved boring toward the ground outside the monopile; inserting a sheath tube into the drilling rod after drilling to the water bottom surface is completed; withdrawing the drilling rod; and inserting a cable into the sheath tube.

According to the first invention, curved boring is performed toward the ground outside the monopile, the sheath pipe is buried in the water bottom, and the cable is inserted, so even if a scour prevention material is not installed, damage to the sheath pipe and cable due to external forces such as wave force and tidal force can be prevented. In addition, because the guide pipe is fixed inside the monopile, curved boring can be performed and the sheath pipe can be inserted from above the water.

It is desirable that the guide pipe be curved and fixed by a plurality of support members inside the monopile.
This allows the guide tube to be bent into an appropriate shape and supported while being fixed to the monopile.

It is desirable that the vicinity of the lower end of the guide pipe be connected to the opening of the monopile in a downward direction rather than horizontally.
This allows the drilling rod to be smoothly launched from inside the guide pipe into the ground surrounding the monopile.

A protective member may be disposed on an upper portion of the sheath pipe exposed between the water bottom surface and the monopile.
With this, when the sleeve pipe is exposed in water, the sleeve pipe can be covered and protected by the protective member.

It is desirable that the distance from the monopile to the point where the sheath pipe is exposed on the water bottom surface be approximately 50 m or more so that it is not affected by casting anchors by maintenance vessels.
This makes it possible to prevent damage to the cables caused by the work boat dropping anchor or dragging its anchor when performing maintenance on offshore wind power generation equipment supported by monopile foundations using a work boat.

A plurality of the guide pipes may be arranged so as to face in different directions, and curved boring may be performed for each of the guide pipes.
This allows multiple cables to be laid in different directions from the monopile foundation.

According to the present invention, a method for laying an underwater cable on a monopile can be provided which can prevent damage to the sheath pipe through which the cable is inserted even when no scour prevention material is installed.

A diagram showing the process of constructing a monopile foundation 3. A diagram showing a vertical cross section of the upper part of the monopile 2. A diagram showing the process of installing a curved boring machine 5. A diagram showing the process of curved boring. 13A and 13B are diagrams showing a process of inserting a sheath tube 7. A figure showing the process of pulling out the drilling rod 6. 4A to 4C are diagrams showing the process of inserting the cable 8. 4A to 4C are diagrams showing the process of inserting the cable 8. FIG. 13 is a diagram showing an example in which two guide tubes 4 are fixed. 1A is a diagram showing an example of using a work stage 10a, and FIG. 1B is a diagram showing an example of casting a monopile 2 using the core excavation method.

Below, a preferred embodiment of the present invention will be described in detail with reference to the drawings.

Figure 1 shows the process of constructing a monopile foundation 3, and shows a vertical cross section. Figure 2 shows a vertical cross section of the upper part of a monopile 2.

In the process shown in Figure 1, a large SEP 10 installed on the water bottom surface 12 is used to install a monopile 2 on the ground 1, constructing a monopile foundation 3 for an offshore wind power generation facility. The SEP (Self-Elevating Platform) is a self-elevating work platform, and during installation, the lifting legs are placed on the ground 1 and the platform is jacked up above the water.

As shown in FIG. 2, the monopile 2 has a guide tube 4 fixed inside it. The guide tube 4 is curved and fixed inside the monopile 2 by a number of support members 41. The support members 41 are, for example, horizontal members in a cross shape, and each of the crosses has a different intersection position. The guide tube 4 is fixed and supported at the intersections of the support members 41, and is fixed inside the monopile 2 in a bent state with an appropriate shape. The monopile 2 has an opening 21 on its periphery, and the lower end 42 of the guide tube 4 is connected to the opening 21 at an angle downward from the horizontal direction.

The monopile 2 is driven into the ground 1, for example, by a hammering method. The monopile 2 is driven to a depth such that the opening 21 reaches just above the water bottom surface 12. The guide pipe 4 and the support member 41 are preferably made of steel or the like so that they can withstand the impact when the monopile 2 is driven.

Figure 3 shows the process of installing the curved boring machine 5, and shows a vertical cross section. In the process shown in Figure 3, the curved boring machine 5 is installed on the extension 11 that extends above the top of the monopile 2 from the large SEP 10. The curved boring machine 5 is installed in a position where the drilling rod 6, which will be described later, can be inserted into the guide pipe 4 from above.

Figure 4 shows the process of curved boring, where FIG. 4(a) shows a vertical cross section, and FIG. 4(b) shows a cross section taken along A1-A1 in FIG. 4(a). In the process shown in FIG. 4, a hollow drilling rod 6 with a drilling bit at its tip is inserted from the upper end of the guide pipe 4, and the drilling bit coming out of the lower end 42 of the guide pipe 4 is launched toward the ground 1 outside the monopile 2 by the curved boring machine 5. Then, the curved boring machine 5 performs curved boring in the ground 1 in a direction away from the monopile 2 as shown by the arrow B1, and when the tip 61 of the drilling rod 6 comes out of the water bottom surface 12, the curved boring is completed. The curvature of the curved boring does not need to match the curvature of the guide pipe 4, and can be changed as appropriate. If improvement of the ground 1 is required, an injection tube is inserted into the drilling rod 6 to inject chemicals.

Figure 5 shows the process of inserting the sheath pipe 7, with Figure 5(a) showing a vertical cross section and Figure 5(b) showing a cross section taken along A2-A2 in Figure 5(a). In the process shown in Figure 5, the sheath pipe 7 is inserted into the drilling rod 6. The sheath pipe 7 is inserted from the top side of the monopile 2 in the direction toward the tip 61 of the drilling rod 6 as indicated by arrow B2, and insertion is complete when the sheath pipe 7 emerges from the tip 61. The sheath pipe 7 is a durable and flexible PVC pipe or the like.

Figure 6 shows the process of pulling out the drilling rod 6, with Figure 6(a) showing a vertical cross section and Figure 6(b) showing a cross section taken along A3-A3 in Figure 6(a). In the process shown in Figure 6, the drilling rod 6 is pulled out from the ground 1 and guide tube 4 by the curved boring machine 5. The drilling rod 6 is dismantled as it is pulled out in the direction above the water, as indicated by arrow B3. The sheath tube 7 is left behind with its tip 71 protruding above the water bottom surface 12.

Figures 7 and 8 show the process of inserting cable 8, with Figure 7(a) showing a vertical cross section, Figure 7(b) showing a cross section taken along A4-A4 in Figure 7(a), and Figure 8 showing a cross section taken along C-C in Figure 7(a). In the process shown in Figure 7, cable 8 is inserted into sheath tube 7. Cable 8 may be pushed into sheath tube 7 from tip 71 toward the water, or cable 8 may be connected to a rope pre-installed inside sheath tube 7 at tip 71 and pulled up from above the water.

As shown in Figures 7 and 8, when the sheath pipe 7 slightly exposed between the water bottom surface 12 and the monopile 2 is expected to be affected by wave force or tidal force, a protective member 9 is placed on the upper part of the exposed sheath pipe 7 as necessary to cover the sheath pipe 7 so that it is not subjected to external forces such as wave force or tidal force. The protective member 9 may be, for example, a net bag filled with crushed stone, and may also serve as a scour prevention material. As shown in Figure 8, the horizontal distance 13 from the peripheral surface of the monopile 2 to the position 14 where the sheath pipe 7 is exposed on the water bottom surface 12 is desirably set so that the cable 8 is not damaged by an anchor or the like when a ship for maintenance or the like is moored near the monopile 2. For example, a ship for transporting people that does not require the capacity of a tugboat or pushboat, which is called a domestic traffic ship, is considered to be used for maintenance such as inspection, and the length of the ship is about 10m + α. Also, since the applicable water depth for monopile wind turbines is thought to be around 30m, if the anchor is drifted from the mooring position to an angle of 45°, it will be at a position of 30m. Therefore, if the bow of the work boat is in contact with the monopile for boarding and disembarking, the boat length from there is 10m + α + 30m, and assuming α is 10m or less, it is thought to be around 50m. For this reason, it is desirable for the horizontal distance 13 to be around 50m or more.

As described above, according to this embodiment, curved boring is performed toward the ground 1 outside the monopile 2, the sheath pipe 7 is buried in the water bottom, and the cable 8 is inserted, so even if no scour prevention material is installed, damage to the sheath pipe 7 and cable 8 due to external forces such as wave force and tidal force can be prevented. In addition, because the guide pipe 4 is fixed inside the monopile 2, curved boring can be performed with the drilling rod 6 from the top side of the monopile 2 using the guide pipe 4, and the sheath pipe 7 can be inserted, minimizing the work of divers.

In this embodiment, the guide pipe 4 is curved by a plurality of support members 41 and fixed inside the monopile 2, and the lower end 42 of the guide pipe 4 is connected to the opening 21 of the monopile 2 downwardly relative to the horizontal direction, so that the drilling rod 6 can be smoothly launched from inside the guide pipe 4 into the ground 1 around the monopile 2. At this time, since the guide pipe 4 is curved rather than straight, the guide pipe 4 can receive the reaction force when curved boring is performed by the drilling rod 6.

In addition, a protective member 9 is placed as necessary on the sheath pipe 7 located between the water bottom surface 12 and the monopile 2, so that the sheath pipe 7 can be protected from external forces such as wave force and tidal force.

Furthermore, by setting the distance 13 between the monopile 2 and the exposed position 14 of the sheath pipe 7 to approximately 50 m or more, damage to the sheath pipe 7 and cable 8 on the seabed due to anchoring or dragging of a work boat can be prevented when the offshore wind power generation equipment supported by the monopile foundation 3 is maintained by a work boat.

In this embodiment, one guide tube 4 is fixed inside the monopile 2, but multiple guide tubes 4 may be arranged facing in different directions. Figure 9 is a diagram showing an example in which two guide tubes 4 are fixed. In the example shown in Figure 9, two guide tubes 4-1 and 4-2 are arranged facing in different directions. This allows curved boring to be performed in the directions shown by dotted lines for the guide tubes 4-1 and 4-2, respectively, to lay multiple underwater cables in different directions. This method is useful when connecting the cable to cables of other monopile foundations, etc.

The details of each process are not limited to those described above. Figure 10(a) is a diagram showing an example using a work stage 10a. In the above embodiment, the curved boring machine 5 is installed on the large SEP 10, but in the example of Figure 10(a), after the monopile 2 is cast using the large SEP 10, the large SEP 10 is evacuated and the curved boring machine 5 is installed on the work stage 10a fixed to the monopile 2 to carry out the subsequent process. This allows the usage period of the large SEP 10 to be shortened.

Figure 10(b) is a diagram showing an example of constructing the monopile 2 using the pre-drilling and erecting method. In the above embodiment, the monopile 2 was installed using the hammering method, but in the example shown in Figure 10(b), the monopile 2 is inserted to a depth where the opening 21 reaches below the water bottom surface 12 and the lower end 42 of the guide pipe 4 is placed in the ground 1 using the pre-drilling and erecting method in which a hole is pre-drilled in the ground 1 and then the monopile 2 is erected. In other words, the lower end of the guide pipe 4 is located deeper than the water bottom surface 12. As a result, the drilling rod 6 can be launched into the ground 1 from the lower end 42 of the guide pipe 4, so that the sheath pipe 7 is not exposed between the water bottom surface 12 and the monopile 2, and the protective member 9 is not required.

The above describes preferred embodiments of the present invention with reference to the attached drawings, but the present invention is not limited to these examples. It is clear that a person skilled in the art can come up with various modified or revised examples within the scope of the technical ideas disclosed in this application, and it is understood that these also naturally fall within the technical scope of the present invention.

For example, the shape of the support member 41 is not limited to a cross shape, and may be any shape that allows the guide pipe 4 to be fixed inside the monopile 2 in a curved state. The lower end 42 of the guide pipe 4 may be sharply shaped to protrude from the opening 21 of the monopile 2, and the lower end 42 of the guide pipe 4 may be inserted into the water bottom surface 12 when the monopile 2 is cast. This allows the lower end of the guide pipe 4 to be positioned deeper than the water bottom surface 12, so that the drilling rod 6 can be launched into the ground 1 from the lower end 42 of the guide pipe 4 even in the case of a casting method. Furthermore, by making the guide pipe 4 and the support member 41 into a structure that can be attached after the monopile 2 is cast, it can also be applied to a center excavation method in which a center excavator is inserted into the monopile 2 during casting.

1 ... Ground 2 ... Monopile 3 ... Monopile foundation 4, 4-1, 4-2 ... Guide pipe 5 ... Curved boring machine 6 ... Drilling rod 7 ... Sheath pipe 8 ... Cable 9 ... Protective material 10 ... Large SEP
10a: Working stage 11: Projection 12: Water bottom surface 13: Distance 14: Position 21: Opening 41: Support member 42: Lower end 61, 71: Tip

Claims (6)

A method for laying an underwater cable on a monopile, comprising the steps of:
The monopile has a guide tube fixed therein,
A step of installing a curved boring machine near the top of the monopile;
A step of inserting a drilling rod into the guide pipe and performing curved boring toward the ground outside the monopile using the curved boring machine;
After the drilling to the water bottom surface is completed, inserting a sheath pipe into the drilling rod;
withdrawing the drilling rod;
inserting a cable into the sleeve tube;
A method for laying an underwater cable on a monopile, comprising: The method for laying an underwater cable on a monopile according to claim 1, characterized in that the guide pipe is curved and fixed inside the monopile by a plurality of support members. The method for laying an underwater cable on a monopile according to claim 1 or 2, characterized in that the lower end of the guide pipe is connected to the opening of the monopile in a direction downward rather than horizontally. A method for laying an underwater cable on a monopile according to any one of claims 1 to 3, characterized in that a protective member is placed on the upper part of the sheath tube exposed between the water bottom surface and the monopile. A method for laying an underwater cable on a monopile according to any one of claims 1 to 4, characterized in that the distance from the monopile to where the sheath pipe is exposed on the water bottom surface is 50 m or more. A method for laying an underwater cable on a monopile according to any one of claims 1 to 5, characterized in that a plurality of the guide pipes are arranged facing in different directions, and curved boring is performed for each of the guide pipes.

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