Floating offshore wind turbine system, and construction method, demolition method, and maintenance method therefor

Abstract

Provided is a floating offshore wind turbine system having a structure useful for implementing replacement of a large component of a floating offshore wind turbine. The floating offshore wind turbine system includes a floating offshore wind turbine, a main floating body, and a mooring body. The floating offshore wind turbine has: blades that receive wind; a hub to which the blades are fixed; a nacelle that houses a generator that converts rotational energy of the hub into electric power; a tower that supports the nacelle; and a sub-floating body that supports the tower. The main floating body is moored to the sea bottom by the mooring body, and has a sub-floating body insertion space into which the sub-floating body is inserted. The sub-floating body inserted into the sub-floating body insertion space is detachably connected at a connection part with the main floating body.

Description

Floating offshore wind turbine system, and its construction, dismantling and maintenance methods

[0001] The present invention relates to a floating offshore wind turbine system having a separable floating structure, and to methods for constructing, dismantling and maintaining the same.

[0002] In recent years, wind power generation has been widely adopted worldwide as one of the countermeasures against global warming. The installation locations of wind power generation facilities are expanding from land to offshore, and the offshore installation locations are also expanding from shallow waters for bottom-fixed offshore wind turbines to deep waters for floating offshore wind turbines. However, because self-elevating vessels (jack-up vessels, JUVs) cannot be used in deep waters, it has been difficult to replace large parts during maintenance of floating offshore wind turbines, as these require large cranes on the JUVs.

[0003] Therefore, the wind power generation facility of Patent Document 1 is described in the abstract as "a wind power generation facility having a tower that is installed on land or offshore and serves as a support for a generator, a nacelle that is installed on the tower and has the generator built in, and a rotor that is installed at one end of the nacelle and consists of a hub and blades that receives wind and converts it into rotational energy, wherein the wind power generation facility is characterized in that a winch is installed within the nacelle for transporting replacement parts for the generator between the land or offshore and the nacelle." This allows large parts inside the nacelle to be replaced using a winch inside the nacelle.

[0004] JP 2015-110927 A

[0005] However, the winch in Patent Document 1 is only for replacing parts inside the nacelle, and is not capable of replacing large parts outside the nacelle, such as blades.

[0006] Therefore, an object of the present invention is to provide a floating offshore wind turbine system having a structure that is useful for realizing replacement of large components of the floating offshore wind turbine.

[0007] In order to solve the above problems, the floating offshore wind turbine system of the present invention is a floating offshore wind turbine system having a floating offshore wind turbine, a main floating body, and a mooring body, wherein the floating offshore wind turbine has blades that receive the wind, a hub to which the blades are fixed, a nacelle that houses a generator that converts the rotational energy of the hub into electricity, a tower that supports the nacelle, and a sub-floor that supports the tower, and the main floating body is moored to the seabed by the mooring body and has a sub-floor insertion space into which the sub-floor is inserted, and the sub-floor inserted into the sub-floor insertion space is connected to the main floating body at a connection part that allows it to be disconnected.

[0008] According to the floating offshore wind turbine system of the present invention, it becomes possible to realize replacement of large components of the floating offshore wind turbine.

[0009] 12. A bird's-eye view of a floating offshore wind turbine system according to one embodiment (barge type). A side view of a floating offshore wind turbine system according to one embodiment. A flowchart showing a construction method for a floating offshore wind turbine system according to one embodiment. A conceptual diagram showing the state of the main floating body or the sub-floating body at each step of FIG. 3. A diagram showing a method for pulling out a non-self-supporting floating offshore wind turbine. A diagram showing a method for pulling out a self-supporting floating offshore wind turbine. A flowchart showing a turbine replacement method for a floating offshore wind turbine system according to one embodiment. A bird's-eye view of a tension leg platform type floating offshore wind turbine system. A bird's-eye view of a semi-submersible type floating offshore wind turbine system. A bird's-eye view of a spar type floating offshore wind turbine system. A flowchart showing a construction method for a floating offshore system according to one modified example. A flowchart showing a turbine replacement method for a floating offshore wind turbine system according to one modified example. A flowchart showing a construction method for a floating offshore system according to one modified example. A conceptual diagram showing the state of the main floating body or the sub-floating body at each step of FIG. 12.

[0010] Hereinafter, an embodiment of the floating offshore wind turbine system of the present invention will be described with reference to the drawings.

[0011] Figure 1 is a bird's-eye view of a floating offshore wind turbine system 100 according to one embodiment of the present invention. The floating offshore wind turbine system 100 illustrated here is an offshore power generation system that is installed in a deep sea area at a depth of about 50 to 100 meters, converts the energy of offshore wind W into electricity, and transmits it to an onshore system, and comprises a floating offshore wind turbine 1, a main float 2, a mooring body 3, an offshore cable 4, and a submarine cable 5. Each of these components will be described in detail below.

[0012] The floating offshore wind turbine 1 comprises blades 11 that receive offshore wind W, a hub 12 to which the blades 11 are fixed, a nacelle 13 that houses a generator that converts the rotational energy of the hub 12 into electricity, a tower 14 that supports the nacelle 13, and a sub-floating body 15 that is a base that supports the tower 14. Note that although the number of blades 11 in Figure 1 is three, the number of blades is not limited to this example.

[0013] The main float 2 is a float shaped to fit with the sub-float 15 of the floating offshore wind turbine 1, and is moored to the seabed by moorings 3 such as chains while floating on the sea surface SL. Note that Fig. 1 shows an example in which the main float 2 and the sub-float 15 are connected at a connecting part J to form a barge-type float. The method of connection at the connecting part J will be described later.

[0014] The marine cable 4 is an electric wire that relays the power generated by the generator in the nacelle 13, and the submarine cable 5 is an electric wire that transmits the generated power via the marine cable 4 to the land system.

[0015] FIG. 2 is a side view of the floating offshore wind turbine system 100 of FIG. 1 , showing the direction of the overturning moment M acting on the floating offshore wind turbine 1 when subjected to wind W. As shown here, when wind W blows from left to right in the figure, an overturning moment M is generated in the floating offshore wind turbine 1 when subjected to wind W, in a clockwise direction in the figure. This overturning moment M generates a large shear force at the connection J between the sub-floor 15 and the main float 2 of the floating offshore wind turbine 1. To resist this shear force, the connection J is firmly connected in the vertical direction by bolts or welding. Therefore, the entire barge-type floater, which connects the sub-floor 15 and the main float 2, is also affected by the overturning moment M. However, because the main float 2 is moored to the seabed via the mooring structures 3, the entire barge-type floater can maintain a substantially horizontal posture (specifically, with a tilt of about 5°).

[0016] <Construction Method of Floating Offshore Wind Turbine System> Next, a construction method of the floating offshore wind turbine system 100 of this embodiment will be described using the flowchart in Figure 3 and the conceptual diagram in Figure 4. Note that in Figure 4, components other than the main float 2 and the sub-float 15 are not shown.

[0017] First, in step S1 of Fig. 3, a construction company or the like installs the main float 2 in a desired sea area. Specifically, after towing the main float 2 to the desired sea area using a transport ship 6, a mooring body 3 is attached to the main float 2 and the main float 2 is moored to the seabed. Fig. 4(a) is a conceptual diagram of the main float 2 after the work of step S1 is completed, viewed from the lower left in Fig. 1 or from the left in Fig. 2. As shown there, a sub-float insertion space 21 is formed in the center of the main float 2 as a space intended for insertion of the sub-float 15. In addition, a notch 22 is formed in the upper part of the wall surface that contacts the sub-float insertion space 21. A space is formed inside the main float 2 for injecting ballast water, but at the time of step S1, no ballast water has been injected, or only enough ballast water has been injected to maintain the stability of the float, and since it is possible to inject more ballast water, the buoyancy of the main float 2 is large and the waterline (sea surface SL) of the main float 2 is at a relatively low position compared to the height of the main float 2.

[0018] Next, in step S2, the construction company or the like injects ballast water into the main float 2. This reduces the buoyancy of the main float 2. Figure 4(b) is a conceptual diagram of the main float 2 upon completion of the work in step S2. As shown here, the main float 2 sinks due to the weakening of its buoyancy, and the waterline (sea surface SL) of the main float 2 becomes higher relative to the height of the main float 2.

[0019] In step S3, the construction company or the like places the sub-float 15 of the floating offshore wind turbine 1 in a predetermined position in the sub-float insertion space 21 of the main float 2. Specifically, the construction company or the like uses the transport ship 6 to tow the floating offshore wind turbine 1 close to the main float 2, and then guides the sub-float 15 of the floating offshore wind turbine 1 to the back of the sub-float insertion space 21 of the main float 2. Figure 4(c) is a conceptual diagram of the main float 2 and the sub-float 15 upon completion of the work in step S3. As shown in this figure, a protrusion 15a is formed on the upper part of the sub-float 15 so as to fit into the cutout portion 22 of the main float 2. At this point, since the main float 2 is originally lowered, a sufficient space is formed between the underside of the protrusion 15a of the sub-float 15 and the upper surface of the cutout portion 22 of the main float 2, and there is no interference between the two. Therefore, construction workers and the like can smoothly insert the sub-float 15 into the sub-float insertion space 21 of the main float 2.

[0020] In step S4, the construction company or the like discharges ballast water from the main float 2. This causes the main float 2 to regain its original buoyancy. When the main float 2 regains its original buoyancy, the main float 2 rises relative to the sea surface SL. As a result, the cutout portion 22 of the main float 2 pushes up the protruding portion 15a of the sub-floor 15.

[0021] In step S5, the construction company or the like firmly connects the connection part J between the sub-float 15 and the main float 2 of the floating offshore wind turbine 1 using a releasable connection method such as bolts or welding. Figure 4(d) is a conceptual diagram of the main float 2 and the sub-float 15 upon completion of the work in step S5. By connecting the sub-float 15 and the main float 2 using the above procedure, it is possible to construct the floating offshore wind turbine system 100 of the form exemplified in Figures 1 and 2.

[0022] <Method for dismantling a floating offshore wind turbine system> Figures 3 and 4 have explained the construction work of the floating offshore wind turbine system 100, but if a floating offshore wind turbine 1 with a broken large component such as a blade 11 needs to be repaired in a port or other location, the floating offshore wind turbine system 100 can be dismantled in the following procedure.

[0023] That is, first, the bolts and welds are removed from the connection part J between the main float 2 and the sub-float 15 to release the connection between them, then ballast water is poured into the main float 2 to sink it, and then the sub-float 15 is pulled out of the sub-float insertion space 21 using a transport ship 6, thereby separating the floating offshore wind turbine 1 from the main float 2. Then, the floating offshore wind turbine 1 pulled out from the main float 2 is towed by the transport ship 6 to a port or a land facility, where maintenance work such as repair and replacement of large parts is carried out using large cranes installed in the port or onshore facility. This has the special effect of allowing large parts that are difficult to maintain offshore to be transported to a port or a land facility for maintenance.

[0024] Here, a method for extracting a faulty floating offshore wind turbine 1 from the main floater 2 will be described using the side views of Figures 5A and 5B. Figure 5A shows a method for extracting a floating offshore wind turbine 1 that cannot stand on its own due to a large overturning moment M. In this example, a transport ship 6 is directly connected to the front side of the floating offshore wind turbine 1, integrating the transport ship 6 and the floating offshore wind turbine 1. This allows the floating offshore wind turbine 1, which may overturn if it is not supported by the transport ship 6, to be towed to a desired location in a stable state. On the other hand, Figure 5B shows a method for extracting a self-supporting floating offshore wind turbine 1. In this example, the floating offshore wind turbine 1 will not overturn even without the support of the transport ship 6, so the floating offshore wind turbine 1 can be towed while connected to the transport ship 6 via a towing chain 61.

[0025] <Method for Replacing a Wind Turbine in a Floating Offshore Wind Turbine System> Next, a wind turbine replacement method that can be achieved by combining the construction method and dismantling method described above will be described. The main float 2, which has few moving parts, has a longer lifespan than the floating offshore wind turbine 1, which has many moving parts. Furthermore, mooring the main float 2 to the seabed is a complicated task that requires a lot of man-hours. For these reasons, if the previous main float 2 can be reused when updating the floating offshore wind turbine system 100, the man-hours required for system updating can be significantly reduced.

[0026] Below, a method for replacing an old small floating offshore wind turbine 1A incorporated in a floating offshore wind turbine system 100 with a new large floating offshore wind turbine 1B will be described using the flowchart in FIG. 6 .

[0027] First, in step S11, the construction company or the like removes bolts and welds from the connection J between the sub-float 15 and the main float 2 of the old floating offshore wind turbine 1A, thereby releasing the connection between the two.

[0028] Next, in step S12, the construction company or the like injects ballast water into the main floating body 2. This reduces the buoyancy of the main floating body 2. Therefore, the sub-floor 15 and the main floating body 2 of the old floating offshore wind turbine 1A are in the relative relationship as shown in Figure 4(c).

[0029] In step S13, the construction company or the like pulls out the sub-float 15 of the old floating offshore wind turbine 1A from the sub-float insertion space 21 of the main float 2, as shown in FIG. 5A or 5B.

[0030] In step S14, the construction company or the like places the sub-float 15 of the new floating offshore wind turbine 1B at a predetermined position in the sub-float insertion space 21 of the main float 2. This corresponds to the work of step S3 in Fig. 3 .

[0031] In step S15, the construction company or the like discharges ballast water from the main floating body 2. This corresponds to the work of step S4 in FIG.

[0032] In step S16, the construction company etc. firmly connects the sub-floor 15 of the new floating offshore wind turbine 1B to the connecting portion J of the main float 2 by bolts or welding. This corresponds to the work of step S5 in Fig. 3 .

[0033] According to the above procedure, the floating offshore wind turbine 1 of the floating offshore wind turbine system 100 can be updated from an old one to a new one without repeating the complicated task of mooring the main floating body 2 in the deep sea area.

[0034] <Modifications> In the above, an example has been shown in which a barge-type float is formed by connecting the main float 2 and the sub-float 15, but other types of floats may be formed depending on the situation. For example, as shown in the bird's-eye view of Figure 7, a tension leg platform (TLP) type float may be formed, using tension moorings 31 instead of the moorings 3, as shown in the bird's-eye view of Figure 8, a semi-submersible type float may be formed, or as shown in the bird's-eye view of Figure 9, a spar-type float may be formed.

[0035] Furthermore, although the above describes an example in which a space for injecting ballast water is formed inside the main float 2, a space for injecting ballast water may also be formed inside the sub-float 15. In this case, when constructing or dismantling the floating offshore wind turbine system 100, ballast water may be discharged from the sub-float 15 instead of injecting ballast water into the main float 2. This makes it possible to lower the relative height of the main float 2 with respect to the sub-float 15, similar to when ballast water is injected into the main float 2. Similarly, instead of discharging ballast water from the main float 2, ballast water may be injected into the sub-float 15. This makes it possible to increase the relative height of the main float 2 with respect to the sub-float 15, similar to when ballast water is discharged from the main float 2.

[0036] Furthermore, spaces for injecting ballast water may be formed inside both the main float 2 and the sub-float 15. In this case, ballast water may be discharged from the sub-float 15 at the same time as ballast water is injected into the main float 2, thereby lowering the relative height of the main float 2 with respect to the sub-float 15. Similarly, ballast water may be discharged from the main float 2 at the same time as ballast water is discharged from the main float 2, thereby raising the relative height of the main float 2 with respect to the sub-float 15.

[0037] That is, the floating offshore wind turbine system 100 may be constructed as shown in the flowchart of Figure 10. Step S21 is similar to step S1 of Figure 3, and a detailed description thereof will be omitted. In step S22, the construction company or the like adjusts the amount of ballast water inside the main float 2 and / or the sub-float 15, and adjusts the height of the main float 2 and / or the sub-float 15 relative to the sea surface to a height that allows the sub-float 15 to be placed in the sub-float insertion space 21. Step S22 may also be performed before step S21. That is, before proceeding to step S23, the height of the sub-float 15 may be adjusted to a height that allows it to be placed in a predetermined position in the sub-float insertion space 21 of the main float 2. Step S23 is similar to step S3 of Figure 3, and a detailed description thereof will be omitted.

[0038] In step S24, the construction company or the like adjusts the amount of ballast water inside the main float 2 and / or the sub-float 15, and adjusts the height of the main float 2 and / or the sub-float 15 relative to the sea surface to a height that allows the sub-float 15 to be connected to the main float 2. Step S25 is similar to step S5 in Figure 3, and a detailed description thereof will be omitted.

[0039] By connecting the sub-float 15 and the main float 2 according to the above procedure, the floating offshore wind turbine system 100 of the form exemplified in Figures 1 and 2 can be constructed.

[0040] The same can be said for a method of dismantling a floating offshore wind turbine system. That is, first, the bolts and welds are removed from the connection part J between the main float 2 and the sub-float 15 to release the connection between them, then the amount of ballast water inside the main float 2 and / or the sub-float 15 is adjusted to adjust the height of the main float 2 and / or the sub-float 15 relative to the sea surface to a height that allows the sub-float 15 to be pulled out from the sub-float insertion space 21, and then a transport ship 6 is used to pull the sub-float 15 out of the sub-float insertion space 21, thereby separating the floating offshore wind turbine 1 from the main float 2. Then, the floating offshore wind turbine 1 pulled out from the main float 2 is towed by the transport ship 6 to a port or a land facility, where maintenance work such as repair and replacement of large parts is carried out using a large crane installed in the port or on the land facility.

[0041] The same can be said for a method for replacing a wind turbine in a floating offshore wind turbine system. Below, a method for replacing an old small floating offshore wind turbine 1A incorporated in a floating offshore wind turbine system 100 with a new large floating offshore wind turbine 1B will be described using the flowchart in Figure 11.

[0042] Step S31 is similar to step S11 in FIG. 6, and a detailed description thereof will be omitted.

[0043] Next, in step S32, the construction company or the like adjusts the amount of ballast water inside the main float 2 and / or the sub-float 15, and adjusts the height of the main float 2 and / or the sub-float 15 relative to the sea surface to a height that allows the sub-float 15 to be pulled out from the sub-float insertion space 21. As a result, the sub-float 15 and the main float 2 of the old floating offshore wind turbine 1A are in the relative relationship shown in Figure 4(c).

[0044] Steps S33 and S34 are similar to steps S13 and S14 in FIG. 6, and detailed description thereof will be omitted.

[0045] In step S35, the construction company or the like adjusts the amount of ballast water inside the main float 2 and / or the sub-float 15, and adjusts the height of the main float 2 and / or the sub-float 15 relative to the sea surface to a height that allows the sub-float 15 to be connected to the main float 2. This corresponds to the work of step S24 in Figure 10.

[0046] Step S36 is similar to step S16 in FIG. 6, and a detailed description thereof will be omitted.

[0047] According to the above procedure, the floating offshore wind turbine 1 of the floating offshore wind turbine system 100 can be updated from an old one to a new one without repeating the complicated task of mooring the main float 2 in the deep sea area.

[0048] In addition, in the above example, a cutout 22 is formed in the upper part of the wall surface of the main float 2 that contacts the sub-float insertion space 21, and a protrusion 15a shaped to fit into the cutout 22 of the main float 2 is formed in the upper part of the sub-float 15. On the other hand, a protrusion 23 may be formed in the upper part of the wall surface of the main float 2 that contacts the sub-float insertion space 21, and a cutout 15b shaped to fit into the protrusion 23 of the main float 2 may be formed in the upper part of the sub-float 15. In the following, the sub-float 15 has a space inside for injecting ballast water.

[0049] A construction method for the floating offshore wind turbine system 100 of this modified example, in which a protrusion 23 is provided on the main float 2 and a cutout 15b is provided on the sub-float 15, will be described using the flowchart in Figure 12 and the conceptual diagram in Figure 13. Note that in Figure 13, configurations other than the main float 2 and the sub-float 15 are omitted from the illustration.

[0050] Step S41 in Fig. 12 is the same as step S1 in Fig. 3, and detailed description thereof will be omitted. Fig. 13(a) is a conceptual diagram of the main float 2 after the completion of the work of step S41, as viewed from the lower left in Fig. 1, or as viewed from the left in Fig. 2. As shown here, a protrusion 23 is formed on the upper part of the wall surface that contacts the sub-floor insertion space 21.

[0051] Next, in step S42, the construction company or the like discharges ballast water from the main float 2, or injects ballast water into the sub-float 15, or both. Step S42 may also be performed before step S41. That is, before proceeding to step S43, the height of the sub-float 15 may be adjusted to a height that allows it to be placed at a predetermined position in the sub-float insertion space 21 of the main float 2.

[0052] Here, because the main float 2 is moored to the seabed by the mooring bodies 3, there is little freedom to discharge ballast water from the stable moored state and raise its height relative to the sea surface. For this reason, in step S42, it is more desirable not to discharge ballast water from the main float 2, or to discharge it only to the extent that excessive load is not placed on the mooring bodies, and to inject ballast water into the sub-floor 15 and adjust the relative heights of the main float 2 and the sub-floor 15.

[0053] In other words, adjustment of the relative heights of the main float 2 and the sub-float 15 in this modified example depends heavily on the ballast adjustment function of the sub-float 15. On the other hand, in the embodiment described in Figures 3 and 4, the main float 2 is lowered from a stable state in which it is moored to the seabed, so the above-mentioned restriction on the degree of freedom is small. In other words, by providing the cutout portion 2a in the main float 2 as in the embodiment described in Figures 3 and 4, it becomes possible to adjust the relative height from a stable state by injecting ballast water into the main float 2, and the ballast water adjustment function of the sub-float 15 can be reduced. This makes it possible to reduce the cost of the floating offshore wind turbine system 100.

[0054] In step S43, the construction company or the like places the sub-float 15 of the floating offshore wind turbine 1 in a predetermined position in the sub-float insertion space 21 of the main float 2. FIG. 13( b) is a conceptual diagram of the main float 2 and the sub-float 15 upon completion of the work in step S43. As shown here, the main float 2 has been adjusted to a relatively higher position with respect to the sub-float 15 in step S42. Also, as shown here, a notch 15b is formed in the upper part of the sub-float 15, shaped to fit with the protrusion 23 of the main float 2. At this point, the main float 2 is more elevated than it should be, or the sub-float 15 is more submerged than it should be, so there is sufficient space between the underside of the protrusion 23 of the main float 2 and the upper surface of the notch 15b of the sub-float 15, and there is no interference between them. Therefore, the construction company or the like can smoothly insert the sub-float 15 into the sub-float insertion space 21 of the main float 2.

[0055] In step S44, the construction company or the like injects ballast water into the main float 2, or discharges ballast water from the sub-float 15, or both. This causes the main float 2 to sink and the sub-float 15 to float. As a result, the cutout portion 15b of the sub-float 15 pushes up the protrusion 23 of the main float 2.

[0056] Step S45 is similar to step S5 in Fig. 3, and therefore a detailed description thereof will be omitted. Fig. 13(c) is a conceptual diagram of the main float 2 and the sub-float 15 upon completion of the work in step S45. By connecting the sub-float 15 and the main float 2 according to the above procedure, it is possible to construct the floating offshore wind turbine system 100 of the form exemplified in Figs. 1 and 2.

[0057] <Effects of this embodiment> As described above, in the present invention, the floats of a floating offshore wind turbine system are composed of a main float that is moored to the seabed and a sub-float to which the wind turbine is fixed, and the main float and the sub-float are connected in a detachable manner, so that even after the floating offshore wind turbine system has been installed offshore, the wind turbine and the sub-float can be towed into a port or harbor by releasing the connection. This makes it possible to replace large parts of the wind turbine using a large crane installed in the port or harbor.

[0058] 100 Floating offshore wind turbine system 1, 1A, 1B: Floating offshore wind turbine 11: Blade 12: Hub 13: Nacelle 14: Tower 15: Sub-float 15a: Protrusion 15b: Cutout 2: Main float 21: Sub-float insertion space 22: Cutout 23: Protrusion 2A Tension leg platform type main float 2B Semi-sub type main float 2C Spar type main float 3: Mooring 31: Tension mooring 4: Marine cable 5: Submarine cable 6: Transport ship 61: Towing chain

Claims

1. A floating offshore wind turbine system having a floating offshore wind turbine, a main floating body, and mooring bodies, wherein the floating offshore wind turbine has blades that catch the wind, a hub to which the blades are fixed, a nacelle that houses a generator that converts the rotational energy of the hub into electricity, a tower that supports the nacelle, and a sub-floating body that supports the tower, wherein the main floating body is moored to the seabed by the mooring bodies and has a sub-floating body insertion space into which the sub-floating body is inserted, and the sub-floating body inserted into the sub-floating body insertion space is connected to the main floating body at a connection part so that it can be disconnected.

2. A floating offshore wind turbine system according to claim 1, characterized in that a submarine cable that transmits the electricity generated by the generator to an onshore system is connected to the main float.

3. A floating offshore wind turbine system as described in claim 1, wherein the connecting portion has a protruding portion of the sub-floor and a cutout portion of the main float that overlap vertically, and the two are connected in the vertical direction by bolts or welding.

4. A floating offshore wind turbine system according to claim 1, characterized in that the floating offshore wind turbine, which has been detached from the main float, can be towed by a transport vessel.

5. A floating offshore wind turbine system as claimed in claim 1, characterized in that by connecting the main float and the sub-float, one of a barge-type float, a tension leg platform-type float, a semi-submersible float, and a spar-type float is formed.

6. A construction method for a floating offshore wind turbine system as described in claim 1, comprising the steps of: mooring the main float to the seabed; step A of adjusting the height of the main float and / or the sub-float relative to the sea surface so that the sub-float can be placed in the sub-float insertion space; placing the sub-float in the sub-float insertion space; step B of adjusting the height of the main float and / or the sub-float relative to the sea surface so that the sub-float can be connected to the main float; and connecting the main float and the sub-float.

7. A construction method for a floating offshore wind turbine system as described in claim 6, wherein the main float has a cutout portion in the upper part of the wall surface that contacts the sub-float insertion space, the sub-float has a protrusion shaped to fit into the cutout portion, step A is a step of injecting ballast water into the main float and / or discharging ballast water from the sub-float, and step B is a step of discharging ballast water from the main float and / or injecting ballast water into the sub-float.

8. A construction method for a floating offshore wind turbine system as described in claim 6, wherein the main float has a protrusion on the upper part of the wall that contacts the sub-float insertion space, the sub-float has a cutout that fits into the protrusion, step A is a step of discharging ballast water from the main float and / or injecting ballast water into the sub-float, and step B is a step of injecting ballast water into the main float and / or discharging ballast water from the sub-float.

9. A method for dismantling a floating offshore wind turbine system as described in claim 1, comprising the steps of: disconnecting the main float from the sub-float; step C adjusting the height of the main float and / or the sub-float relative to the sea surface to a height that allows the sub-float to be pulled out from the sub-float insertion space; and pulling out the sub-float from the sub-float insertion space.

10. A method for dismantling a floating offshore wind turbine system as described in claim 9, wherein the main float has a cutout portion in the upper part of the wall surface that contacts the sub-float insertion space, the sub-float has a protrusion shaped to fit into the cutout portion, and step C is a step of injecting ballast water into the main float and / or discharging ballast water from the sub-float.

11. A method for dismantling a floating offshore wind turbine system as described in claim 9, wherein the main float has a protrusion on the upper part of the wall that contacts the sub-float insertion space, the sub-float has a cutout that is shaped to fit into the protrusion, and step C is a step of draining ballast water from the main float and / or injecting ballast water into the sub-float.

12. A maintenance method comprising towing the floating offshore wind turbine, which has been pulled out from the main float by the dismantling method described in any one of claims 9 to 11, into a harbor for maintenance.

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