Floating platform, anchor container body, floating body, and method for installing floating platform on water

Abstract

A floating platform (2) comprises: a floating body (3) that is configured to support a tower (1) and that is disposed on the water by tension mooring; an anchor container body (4) provided with a plurality of containers (17); and a plurality of tethers (5) that connect the floating body (3) and the anchor container body (4). Each of the plurality of containers (17) is fixed to at least one of the plurality of tethers (5) and is installed on the seabed. Each of the plurality of containers (17) is provided with a bottom, a container outer wall provided to surround the outer periphery of the bottom, and a container inner wall provided to face the container outer wall. A storage space is formed by the bottom, the container outer wall, and the container inner wall. A heavy object including at least one of soil, sand, gravel, crushed stone, and concrete pieces is stored in the storage space.

Description

Floating platform, anchor vessel body, float, installation method for installing floating platform on water

[0001] The present disclosure relates to a floating platform, an anchor vessel body, a float, and an installation method for installing a floating platform on water.

[0002] A tension mooring method is known in which a float floating on the water surface is connected to a pile driven into the water bottom down to the bedrock below, or to a heavy object placed on the water bottom, with a tether such as a chain, wire or rope, and the float is pulled into the water with a force stronger than the weight of the tether itself, thereby suppressing the movement of the float.

[0003] For floating platforms supporting tall towers, minimal motion is an important quality requirement. For example, when installing towers for offshore wind power generation, this significantly impacts power generation efficiency and stable operation of the facility. In this respect, tension mooring is an ideal mooring method. Furthermore, minimizing the area occupied by the entire floating platform—including not only the surface of the water where the float is located but also the bottom and underwater areas where the mooring devices are installed—is also important in minimizing the impact on local fishing communities that originally used the same waters. Tension mooring, in which mooring devices such as chains are placed only directly below the float and do not extend widely over the seabed or underwater, is a highly effective mooring method in this respect as well.

[0004] Japanese Patent No. 5565803

[0005] “TLP concept of FOWT suitable for characteristics of Japan”, 7th July 2022 MODEC Inc. GICON-SOF, “A modular and cost competitive TLP Solution”, GICON

[0006] Tension mooring requires a strong foundation on the seabed that can not only support the initial tension on the tether but also withstand buoyancy and drift forces generated by waves, swells, and tsunamis. Conventionally, as described in Non-Patent Document 1, a foundation has been created by driving multiple piles deep into the ground from the seabed. However, driving piles into the seabed in deep water is difficult and complex, resulting in high construction costs. Gravity tension mooring devices, described in Patent Documents 1 and 2, were devised to address this issue. However, these gravity tension mooring devices are heavy and massive concrete structures, resulting in high manufacturing and installation costs. In particular, when installing towers for offshore wind power generation, the most important load to consider is the horizontal wind pressure acting on the tower top during typhoons, etc. To support this load, a massive concrete structure must be installed on the seabed to withstand a large overturning moment. The costs of manufacturing and installing such huge concrete structures are a problem, and therefore the gravity-type tension mooring devices disclosed in Patent Document 1 and Non-Patent Document 2 have not been put into practical use.

[0007] An object of the present disclosure is to provide a floating platform and an installation method for installing a floating platform on water that can reduce manufacturing costs and installation costs.

[0008] A floating platform according to one aspect of the present disclosure includes a floating body configured to support a tower and positioned on the water by tension mooring, an anchor vessel body having a plurality of containers, and a plurality of tethers connecting the floating body and the anchor vessel body. Each of the plurality of containers is fixed to at least one of the plurality of tethers and is installed on the bottom of the water. Each of the plurality of containers includes a bottom, an outer container wall surrounding the outer periphery of the bottom, and an inner container wall facing the outer container wall. The bottom, the outer container wall, and the inner container wall define the storage space. The storage space is configured to store heavy objects including at least one of soil, sand, gravel, crushed stone, and concrete pieces.

[0009] The bottom may have an opening. The container inner wall may be provided to surround an inner periphery of the bottom. Each of the plurality of containers may further include a tether support member formed on the container inner wall and fixed to the at least one tether.

[0010] The inner wall of the container may have a cavity that is in communication with the opening and is open at the top.

[0011] The inner surface of the container inner wall facing the container outer wall may be inclined with respect to a height direction of the container, and the distance between the container inner wall and the container outer wall in a horizontal direction perpendicular to the height direction may gradually decrease toward the bottom.

[0012] The tether support member may also include a first tether support member formed on the inner wall of the container and fixed to one of the plurality of tethers, and a second tether support member formed on the inner wall of the container and fixed to another one of the plurality of tethers.

[0013] The floating body may include a central pillar configured to support the tower, a plurality of arms extending radially from the central pillar or a vicinity of the central pillar, a stage fixed to the central pillar, a plurality of first braces each connected to a corresponding one of the plurality of arms and the stage or the central pillar, a plurality of second braces each connected to two adjacent arms of the plurality of arms, a plurality of tether fixing devices configured to fix a corresponding one of the plurality of tethers, and a plurality of tether winches configured to wind up a corresponding one of the plurality of tethers.

[0014] The float may further include a plurality of grips, each connected to an end of one of the plurality of arms, an axial direction of each of the plurality of grips being approximately perpendicular to the axial direction of the corresponding arm, and a plurality of guides, each disposed on a corresponding one of the plurality of grips, each configured to contact a corresponding one of the plurality of tethers and change the extending direction of the corresponding one tether. The tether fixing device and the tether hoist may be disposed on the stage.

[0015] The tower may also have a nacelle and blades attached thereto. The floating platform may be an offshore wind power generation platform.

[0016] An anchor vessel body according to one aspect of the present disclosure is configured to support a tower and is connected to a floating body placed on the water by tension mooring via a plurality of tethers. The anchor vessel body includes a plurality of vessels. Each of the plurality of vessels is fixed to at least one of the plurality of tethers and is installed on the bottom of the water. Each of the plurality of vessels includes a bottom, an outer vessel wall surrounding the outer periphery of the bottom, and an inner vessel wall facing the outer vessel wall. The bottom, the outer vessel wall, and the inner vessel wall form the storage space. The storage space stores heavy objects including at least one of soil, sand, gravel, crushed stone, and concrete pieces.

[0017] A float according to one aspect of the present disclosure is placed on water by tension mooring and configured to support a tower. The float includes a central pillar configured to support the tower, a plurality of arms extending radially from or near the central pillar, a plurality of grips each connected to an end of one of the arms, the axial direction of each of the grips being approximately perpendicular to the axial direction of the corresponding arm, a stage fixed to the central pillar, a plurality of first braces each connected to a corresponding one of the arms and the stage or the central pillar, a plurality of second braces each connected to two adjacent arms of the arms, and a plurality of tether fixing devices configured to fix a corresponding one of a plurality of tethers.

[0018] An installation method for installing the floating platform on water by tension mooring according to one aspect of the present disclosure includes the steps of: towing the float to a planned installation location with the anchor container body equipped with the plurality of containers positioned directly below the float and the float floating at a first waterline; placing the plurality of containers on the bottom of the water at the planned installation location by letting out the plurality of tethers into the water; pouring the heavy objects including at least one of soil, sand, gravel, crushed stone, and concrete pieces into the storage spaces of each of the plurality of containers positioned on the bottom of the water; reeling in the plurality of tethers; sinking the float to a second waterline deeper than the first waterline; adjusting the tension of the plurality of tethers so that the tension of the plurality of tethers is a predetermined value; and fixing each of the plurality of tethers to the float.

[0019] According to the present disclosure, it is possible to provide a floating platform and an installation method for installing a floating platform on water that can reduce manufacturing costs and installation costs.

[0020] 1 is an overall view of a floating platform that supports a tower according to this embodiment. FIG. 2 is a perspective view of a floating platform according to this embodiment. FIG. 3 is a perspective view of an anchor vessel body according to this embodiment. FIG. 4 is a perspective view of a vessel. (a) is a plan view of the vessel. (b) is an A-A cross-sectional view of the vessel. (c) is a B-B cross-sectional view of the vessel. FIG. 5 is a diagram for explaining the propagation of load within the vessel. FIG. 6 is a diagram for explaining the fishing reef effect of the vessel. FIG. 7 is a side view of a float. FIG. 8 is a perspective view showing the main parts of the float. FIG. 9 is a diagram showing the stored position of the anchor vessel body when launching and towing. FIG. 10 is a diagram for explaining a method of installing a floating platform according to this embodiment. FIG. 11 is a diagram showing an example of a heavy load insertion device.

[0021] Hereinafter, the floating platform according to this embodiment will be described with reference to the drawings.

[0022] 1 is an overall view of a floating platform 2 supporting a tower 1 according to this embodiment. In this example, the floating platform 2 supports a large offshore wind turbine generator and floats with the water surface at a second waterline 27. A nacelle 42 and blades 40 may be attached to the tower 1.

[0023] FIG. 2 is a perspective view of a floating platform 2 supporting a tower 1. The floating platform 2 includes a float 3 configured to support the tower 1, an anchor vessel body 4 equipped with multiple containers 17 (four containers 17 in this example), and multiple tethers 5 connecting the float 3 and the anchor vessel body 4. In this embodiment, each of the four containers 17 is connected to the float 3 via two tethers 5. The floating platform 2 is moored using a gravity tension mooring method, in which the float 3 and the anchor vessel body 4, containing heavy loads such as soil and sand, are connected by multiple tethers 5, and initial tension is applied to the tethers 5 to retract the lower part of the float 3 into the sea. In this manner, the float 3 is placed on the water while being moored using the tension mooring method. Here, the heavy loads include at least one of soil, sand, gravel, crushed stone, and concrete fragments. The conventional gravity tension mooring devices described in Patent Document 1 and Non-Patent Document 2 are heavy and huge concrete structures, and the cost of producing the concrete structure itself is high, including the cost of materials such as the concrete and the rebars that support it, the cost of producing and installing the formwork for pouring the concrete, and the cost of pouring the concrete. On the other hand, the unit price of heavy materials such as earth and sand is generally a fraction of that of concrete, so the method of securing weight by filling the container 17 of the anchor container body 4 installed on the seabed with earth and sand is effective in reducing production costs.

[0024] The tether 5 may be a rope, a wire, or a chain depending on the maximum tension that can be generated.

[0025] (Anchor vessel body 4) Next, the structure of the anchor vessel body 4 will be described below with reference to Figure 3. Figure 3 is a perspective view of the anchor vessel body 4. As shown in Figure 3, the anchor vessel body 4 is connected to the floating body 3 via a tether 5 and installed on the seabed. The anchor vessel body 4 includes four vessels 17. Each vessel 17 contains heavy loads including at least one of soil, sand, gravel, crushed stone, and concrete pieces. The vessels 17 are connected to one another by connecting lines 18. The role of the connecting lines 18 is to prevent the four vessels 17 from shifting relative to one another. When lowering the anchor vessel body 4 to the seabed and installing it, first, a number of tugboats equal to the number of vessels 17 are prepared, and the tugboats and vessels 17 are tied together with ropes or the like and pulled outward from the center of the anchor vessel body 4, applying tension to the connecting lines 18 while the vessels 17 are lowered into the sea. This allows the vessels 17 to be installed on the seabed while maintaining their relative positions relative to one another.

[0026] (Container 17) Next, the structure of the container 17 will be described below with reference to Figures 4 and 5. Figure 4 is a perspective view of the container 17. Figure 5(a) is a plan view of the container 17. Figure 5(b) is an A-A cross-sectional view of the container 17. Figure 5(c) is a B-B cross-sectional view of the container 17. In the container 17, the outer periphery of the bottom 19 is surrounded by an outer container wall 20, and the top is open, allowing heavy objects such as soil and sand to be poured into the container 17 from above. Note that Figure 5 shows X, Y, and Z axes set for the container 17. One of the X, Y, and Z axes is orthogonal to the other axes. The Z-axis direction corresponds to the height direction of the container 17. The horizontal direction perpendicular to the Z-axis, which corresponds to the height direction of the container 17, is parallel to the X and Y axes.

[0027] As shown in Figures 4 and 5, the container 17 includes a bottom 19, a container outer wall 20, a container inner wall 21, and a pair of tether support members 22. The bottom 19 has a circular opening 190. The container outer wall 20 is provided to surround the outer periphery of the bottom 19. The container inner wall 21 is provided to surround the inner periphery of the bottom 19 (in other words, the outer periphery of the opening 190) and to face the container outer wall 20. The pair of tether support members 22 are formed on the container inner wall 21. At least one tether 5 (one tether 5 in this example) is fixed to the tether support member 22. The bottom 19, the container outer wall 20, and the container inner wall 21 define a storage space S. The storage space S stores a heavy object including at least one of soil, sand, gravel, crushed stone, and concrete pieces.

[0028] (Container Inner Wall 21) The container inner wall 21 communicates with the opening 190 in the bottom 19 and has a hollow portion 36 that is open at the top. The hollow portion 36 provides a place for small fish and other creatures to take refuge from strong currents, and is also expected to be effective as a fishing reef. The inner surface 26 of the container inner wall 21, which faces the container outer wall 20, is inclined with respect to the Z-axis direction (height direction) of the container 17 (see FIG. 5(a)). The distance between the container inner wall 21 and the container outer wall 20 in the horizontal direction (XY direction) perpendicular to the Z-axis direction gradually decreases as the container approaches the bottom 19. Here, as the position in the Z-axis direction approaches the bottom 19, the diameter of the container inner wall 21 gradually increases, and therefore the distance between the container inner wall 21 and the container outer wall 20, which corresponds to the difference between the radius of the container outer wall 20 and the radius of the container outer wall 20, also gradually decreases (see FIGS. 5(a) and 5(b)).

[0029] If the container 17 does not have an inner wall, the distance from the side wall near the center of the bottom 19 becomes large, necessitating support from below by a large strength member. Therefore, by providing a container inner wall 21 near the center that forms the side of a cylinder, rectangular pillar, cone, pyramid, truncated cone, or truncated pyramid, the container inner wall 21 itself becomes a strong strength member, and at the same time, the frame 25 that supports the bottom 19 from below can be made short and thin, leading to a reduction in the weight of the frame 25. Note that if the maximum tension applied to the tethers 5 connected to the container 17 differs for each tether 5, shifting the position of the container inner wall 21 toward the tether 5 with a relatively weaker tension is effective in reducing the amount of heavy material required, such as soil and sand.

[0030] (Tether Support Member 22) One of the pair of tether support members 22, tether support member 22a (an example of a first tether support member) and the other tether support member 22b (an example of a second tether support member), are provided on the inner surface 26 of the container inner wall 21 and face each other via a cavity 36 formed in the container inner wall 21. The number of tether support members 22 provided in the container 17 is not particularly limited. By providing the tether support members 22 on the container inner wall 21, a strong mooring force can be transmitted to the container 17. A tether fixing portion 23 for fixing the tether 5 is provided on the upper surface of the tether support member 22. When heavy loads such as earth and sand are dumped from a ship 32 into a container 17 installed on the seabed using a heavy load dumping hose 30, the tether 5 extending straight from the container 17 toward the floating body 3 on the water surface may get in the way (see FIG. 12 , etc.). By providing the tether support member 22 having the tether fastening part 23 that retracts the float 3 on the container inner wall 21 rather than the container outer wall 20, when dumping heavy loads such as soil and sand, the heavy load injection hose 30 and the like can be positioned outside the container inner wall 21, reducing the possibility of the heavy load injection hose 30 and the tether 5 coming into contact with each other. Furthermore, by locating the place where heavy loads such as soil and sand are dumped outside the tether fastening part 23, the volume for dumping the required amount of heavy loads such as soil and sand can be easily adjusted by changing the diameter of the container outer wall 20 at the design stage, regardless of the shape of the float 3 or the attachment positions and number of tethers 5.

[0031] (Strength Effectiveness of Tether Support Member 22) Next, the strength effectiveness of the tether support member 22 will be described below with reference to FIG. 6 . FIG. 6 is a diagram illustrating how a load resulting from the tension of the tether 5 propagates through the container 17. The large tension applied to the tether 5 is transmitted to the tether support member 22 via the tether fastening portion 23, then to the bone 25 via the container inner wall 21, and then to the bottom 19, where it becomes a force that supports a heavy load, such as soil and sand, from below. The entire series of these paths requires strength sufficient to withstand a large load sufficient to lift a heavy load, such as soil and sand, that has been poured into the container 17. It is possible to support the load of the tether 5 throughout the entire tether support member 22 by using a large casting around the tether fastening portion 23 in the tether support member 22. However, because most of the container 17 is made of steel plate, it is necessary to prevent damage to the container 17 by distributing the load throughout the entire series of paths described above.

[0032] In this embodiment, the tether support member 22 is connected to the vessel inner wall 21, which has a truncated conical side, via a long weld line 24. The vertical tension of the tether 5 is transmitted directly to the tether support member 22 as a vertical force, but this force is dispersed over the entire area of ​​the long weld line 24. Moreover, this force can be considered by decomposing it into an in-plane component and an out-of-plane component of the vessel inner wall 21. Since plates are generally extremely strong against in-plane forces, the in-plane component of force can be ignored in terms of strength design. On the other hand, with regard to the out-of-plane component of force, if the vessel inner wall 21 has a curved surface or bent edges, the buckling strength is significantly improved compared to a simple flat surface, allowing for a reduction in the amount of reinforcing material.

[0033] Next, the force transmitted to the container inner wall 21 is transmitted to the ribs 25. In this case, since the ribs 25 are connected to the container inner wall 21 as a cantilever beam, it is necessary to provide a bracket 29 inside the container inner wall 21. The ribs 25 are arranged between the container inner wall 21 and the container outer wall 20, and the length of the ribs 25 can be significantly shorter than in the case where the container inner wall 21 is not present and the container outer wall 20 is connected to the opposite container outer wall 20. This reduces the size of the ribs 25 and reduces the amount of steel used. Finally, the ribs 25 support heavy loads such as soil and sand from below via the bottom 19.

[0034] (Holding force by ribs 25) Reinforcing ribs 25 are arranged radially on the underside of the bottom 19, and these ribs 25 also serve as spikes that protrude into the seabed and are effective in resisting the horizontal force applied to the anchor vessel body 4 from the tether 5. In this way, by providing reinforcing members such as ribs 25 on the outer surface of the vessel 17 and eliminating protrusions on the inner surface of the vessel 17 that is surrounded by the vessel outer wall 20, the vessel inner wall 21, and the bottom 19, it becomes easier to recover heavy objects such as soil and sand that have been thrown into the vessel 17, which must be carried out when the floating platform 2 that supports the tower 1 is removed from the installation sea area.

[0035] (Fishing Reef Effect) Figure 7 is a diagram illustrating the fishing reef effect of the container 17. It is known that oysters, seaweed, and the like attach to the floating body 3 during operation, and that this serves as a fishing reef. Furthermore, in the case of the container 17 placed on the seabed, the surface of the heavy material, such as sediment, that is poured into the container 17 eventually becomes roughly flush with the seabed, providing a habitat for benthic organisms. Furthermore, the opening in the cavity 36 effectively exchanges seawater with the cavity 36, maintaining the water quality of the cavity 36 in the container's inner wall 21. In this way, the cavity 36 becomes a valuable space where various fish, including small fish, can take refuge from storms and strong water currents.

[0036] (Main Parts of Floating Body 3) Next, the structure of the floating body 3 will be described below with reference to Figures 8 and 9. Figure 8 is a side view of the floating body 3. Figure 9 is a perspective view showing the main parts of the floating body 3. For convenience of explanation, the arms 6, the first brace 13, the second brace 14, and the grips 15 extending perpendicular to the paper surface are omitted from Figure 8.

[0037] As shown in Figures 8 and 9, the float 3 is placed on the ocean while being moored by a tension mooring method, and is configured to support the tower 1. The float 3 includes a central pillar 12, a base 11, a plurality of arms 6, a plurality of grips 15, a plurality of guides 16, a stage 7, a plurality of first braces 13, a plurality of second braces 14, a plurality of tether fixing devices 8, and a plurality of tether winches 9.

[0038] The central pillar 12 is configured to support the tower 1. The tower 1 is fixed to the upper surface of the central pillar 12 by fastening means such as bolts and nuts. The central pillar 12 is disposed on a base 11. A tether storage space 10 for storing a tether 5 is also mounted on the base 11. A plurality of arms 6 (four in this example) extend radially from the base 11. In this regard, if the float 3 does not have a base 11, the four arms 6 may extend radially from the central pillar 12. Each arm 6 may extend parallel to a horizontal direction perpendicular to the height direction of the float 3 (extension direction of the tower 1). As shown in FIG. 8 , the upper surface of each arm 6 may be flush.

[0039] Each of the multiple (four in this example) grips 15 is connected to the tip of a corresponding one of the four arms 6. The axial direction A2 of each grip 15 is approximately perpendicular to or perpendicular to the axial direction A1 of the corresponding arm 6 and the axial direction of the central pillar 12 (see FIG. 8). Similarly, the axial direction A1 of each arm 6 is approximately perpendicular to or perpendicular to the axial direction of the central pillar 12. Each of the multiple (eight in this example) guides 16 is disposed on a corresponding one of the four grips 15. As shown in FIG. 2, two guides 16 are disposed on each grip 15 in this example. Each guide 16 is configured to contact a corresponding one of the multiple tethers 5 and to change the extension direction of the corresponding tether 5. Each tether 5 is bent by the guide 16.

[0040] The stage 7 is fixed to the central pillar 12. In particular, the stage 7 is arranged between the tower 1 and the base 11 in the height direction of the floating body 3. The stage 7 is fixed to the central pillar 12 with the central pillar 12 inserted into a through-hole 72 formed in the stage 7. A plurality of (four in this example) first braces 13 are connected to corresponding ones of the four arms 6 and the stage 7. Note that the first braces 13 may also be connected to corresponding ones of the four arms 6 and the central pillar 12. Each first brace 13 extends obliquely with respect to the height direction of the floating body 3.

[0041] Each of the multiple (four in this example) second braces 14 is disposed between two adjacent arms 6. One end of each second brace 14 is connected to one of the two adjacent arms 6, and the other end of each second brace 14 is connected to the other of the two adjacent arms 6. Each second brace 14 may extend parallel to the horizontal direction of the floating body 3.

[0042] A plurality of (eight in this example) tether fixing devices 8 are arranged on the stage 7 (see FIG. 2). Each tether fixing device 8 is configured to fix a corresponding one of the eight tethers 5. A plurality of (eight in this example) tether winding machines 9 are arranged on the stage 7 (see FIG. 2). Each tether winding machine 9 is configured to wind up and pay out a corresponding one of the eight tethers 5.

[0043] (Key points in the design of the float 3) The following points must be considered in order to reduce the manufacturing costs of the float 3: 1) Reduce the maximum tension applied to the tether 5 during operation. 2) Reduce the amount of ballast water carried during operation to suppress the weight of the steel material of the float 3.

[0044] (Reducing the maximum tension applied to the tether during operation) When the floater 3 is floating at the second waterline 27 in the absence of wind, waves, or swells, part of the buoyancy can be canceled by loading ballast water, and the tension applied to the tether 5 can be adjusted to the initial tension assumed at the time of design. On the other hand, when strong winds blow and a wind power generation tower 1 is loaded, a strong horizontal force is applied to the top of the tower 1 due to wind pressure. This force generates a large moment on the floater 3 floating on the water surface, as shown in the following formula: Note that L is the distance from the floater 3 to the top of the tower 1, and Fn is the wind pressure. M = L × Fn Here, M is the moment. L is the distance from the fulcrum to the force point. Fn is the component of the force that is perpendicular to the line connecting the fulcrum and the force point.

[0045] This moment must be supported by the tension generated in the tether 5. In this case, applying the above formula, L can be read as the distance between the tethers 5 and Fn as the tension applied to the tether 5, and therefore it can be seen that increasing the distance L between the tethers 5 is effective in reducing the tension Fn. For this reason, in this embodiment, the arms 6 are extended as long as their strength allows.

[0046] Furthermore, in this embodiment, one container 17 is designed to be pulled by two tethers 5, so by arranging guides 16 corresponding to each tether 5 at the same distance from the center of base 11, it is possible to withstand tension up to the upper limit of the strength of the two tethers 5. Grips 15 are provided at the tips of arms 6 to accommodate the placement of two guides 16.

[0047] On the other hand, the tension applied to the tether 5 also increases due to an increase in buoyancy caused by a temporary rise in water level due to waves or swells. Therefore, minimizing the horizontal cross-sectional area of ​​the floater 3 at the height of the second waterline 27 (hereinafter referred to as the waterline area) to suppress the increase in buoyancy caused by a rise in water level is important in suppressing the maximum value of tension in the tether 5. In this embodiment, it is necessary to minimize the diameters of the central pillar 12 and the first brace 13 within the range possible in terms of strength. The first brace 13 is installed diagonally toward the central pillar 12 and is gathered near the central pillar 12 around the second waterline 27, making it possible to suppress motions such as pitching (vertical shaking) and rolling (horizontal shaking) that occur in the floater 3 due to waves and swells.

[0048] (Reducing the amount of ballast water carried during operation to reduce the steel weight of the floater 3) If the amount of ballast water can be reduced, the weight of the steel used to construct the tanks that carry it can be reduced, contributing to lower manufacturing costs. Even if the outer walls of the compartments that carry ballast water during operation are removed to place them outside the floater 3, this does not affect the buoyancy generated on the floater 3, so it is desirable to reduce the number of compartments that carry ballast water as much as possible. Note that when reducing the amount of ballast water, it is desirable to consider the following two points: 1) When floating at the first waterline 28 during towing, the floater must be able to float stably without ballast water. 2) The displacement between the first waterline 28 and the second waterline 27 must be minimized.

[0049] (Things to consider in order to float stably without ballast water while towing) "Floating without ballast water" means that the weight and buoyancy of the float 3 are equal at the first waterline 28. For a steel float 3, the weight of the float 3 is roughly the same as the weight of the steel, so it is necessary to adjust the dimensions of each part of the float 3 so that the buoyancy is the same as the weight estimated in advance. For this reason, the volume (displacement) of the float 3 below the first waterline 28 is restricted.

[0050] Furthermore, "floating stably" means ensuring sufficient stability even when the float 3 is subjected to rocking due to waves and swells or an overturning moment due to wind while being towed. To achieve this, it is necessary to extend the arms 6 as far as possible and place the grips 15 at their tips to ensure a waterline area at a position away from the central pillar 12. Extending the arms 6 as far as possible while being constrained by the volume below the first waterline 28 of the float 3, i.e., the buoyancy, means that the arms 6 will have a thin, long, rod-like shape. This results in insufficient strength, and therefore requires a first brace 13 to support the arms 6 vertically and a second brace 14 to support them horizontally. However, because the second brace 14 is also subject to the aforementioned buoyancy constraints, it is preferable to make it as thin as possible.

[0051] Furthermore, it is preferable that the first brace 13 be as thin as possible, in accordance with the requirement of "minimizing the displacement between the first waterline 28 and the second waterline 27." However, when the arms 6, the first brace 13, and the second brace 14 are thinned, it is desirable to evaluate their strength and confirm that they have sufficient strength to prevent damage during towing and operation. Furthermore, "floating stably without ballast water during towing" means that the floating platform 2 can be towed independently to the installation area using only multiple tugboats, without using a large support vessel, which is effective in reducing the installation costs of the floating platform 2.

[0052] (Effect of the container 17 on stability during towing) As shown in Figure 10, the container 17 during towing is fixed directly below the arms 6 and the grips 15. If the floater 3 tilts significantly during towing, the grips 15 may come completely above the water surface, and at the same time, the grips 15 on the opposite side of the central pillar 12 may become completely submerged. In such a case, part of the container 17, which is normally completely submerged, will be exposed to the water surface. At this time, the top surface of the container 17 will have a waterline area. If the floater 3 is tilted further to lift the container 17, a large restoring force proportional to the waterline area of ​​the container 17 will be generated, and the container 17 will hardly lift at all. Therefore, to tilt the floater 3 further, it is necessary to submerge the part with a large waterline area, such as the base 11 below the central pillar 12, making it less likely for the floater 3 to tilt. In other words, if the float 3 tilts significantly due to motion caused by waves or swells while being towed, the container 17 will generate a greater than normal restoring force, which is expected to prevent the float 3 from capsizing.

[0053] (Axisymmetrical nature of the float 3) In the process of sinking the float 3 from the first waterline 28 to the second waterline 27, the horizontal position of the center of gravity of the increased displacement must be aligned with the horizontal position of the center of gravity when the float 3 is floating at the first waterline 28. If the two do not match, it will be necessary to load ballast water to correct the difference. In this embodiment, the central pillar 12 and base 11 are disposed directly below the tower 1. The axes of the central pillar 12 and base 11 may coincide with the axis of the tower 1. Furthermore, some of the components constituting the float 3 may be disposed so as to be axisymmetrical with respect to the axis of the tower 1.

[0054] (Minimizing the displacement from the first waterline 28 to the second waterline 27) Since the float 3 floats at the first waterline 28 without ballast water, in order to sink it down to the second waterline 27, it is necessary to load ballast water of a weight equivalent to at least the increase in displacement during that period minus the initial tension applied to the tether 5. Specifically, the following relationship holds: (Weight of ballast water) = (Displacement from the first waterline 28 to the second waterline 27) + (Weight of the anchor vessel body 4) - (Total initial tension of the tether 5)

[0055] From the above formula, it can be seen that it is important to minimize the "displacement from the first waterline 28 to the second waterline 27." Specifically, the following components must be reviewed. However, as mentioned above, the review must be limited to the range that provides sufficient strength.

[0056] 1) Freeboard of the base 11: In order to ensure that the base 11 has sufficient stability to quickly rise to the surface even if it is submerged for a short time due to waves or rolling during towing, it is necessary to ensure that its volume is at a certain height above the waterline. This height is called the freeboard.

[0057] 2) Diameter of the central pillar 12: In principle, the upper part of the central pillar 12 must have the same strength as the lower end of the tower 1, since it receives a large bending moment from the tower 1. However, there is almost no bending moment at the bottom of the central pillar 12. For this reason, the central pillar 12 can also be made into a triangular pyramid shape, thick at the top and thin at the bottom. However, this is more difficult to manufacture than a cylindrical shape, so care must be taken when evaluating the manufacturing costs.

[0058] 3) Diameter of arm 6: Because the arm 6 is long, simply narrowing it slightly will significantly reduce the displacement and the waterline area at the first waterline 28. In addition, since there is a possibility that a large compressive force will be applied by the tether 5 and the first brace 13 during operation, it is necessary to determine the diameter of the arm 6 after evaluating the buckling strength.

[0059] 4) Diameter of the first brace 13: Because there is a risk of buckling if a compressive force is applied, it is necessary to determine the diameter of the first brace 13 after sufficient strength evaluation. Note that if ballast water is loaded into the grip 15 during operation to offset buoyancy, the possibility of compressive force being applied to the first brace 13 can be significantly reduced.

[0060] 5) Diameter of second brace 14: Most of the second brace 14 is already submerged at the first waterline 28, and when changing the diameter of the second brace 14, it is necessary to pay attention to changes in the first waterline 28. Note that the forces acting on the second brace 14 are due to wind pressure as well as drift forces caused by currents, waves, and swells, and the load is not as large as that on the first brace 13.

[0061] 6) Diameter of the Grip 15: The grip 15 is important for ensuring stability at the first waterline 28, so when reducing the diameter of the grip 15, it is necessary to consider the effect on stability.

[0062] Up to this point, we have discussed the points to be considered regarding the overall configuration of the float 3. Next, we will describe matters specific to each individual member that constitutes the float 3, as necessary.

[0063] (Base 11) The base 11 secures buoyancy to support most of the weight of the tower 1 and central pillar 12 at the first waterline 28, and also has a tether storage space 10 mounted on its upper part. In addition, passing the arms 6, which are subject to a strong force, through the inside of the base 11 is advantageous in terms of strength design, as it makes it possible to offset part of the force between the two arms 6 arranged in opposing positions, and is effective in reducing the weight of steel materials.

[0064] (Arms 6) In this embodiment, four arms 6 extend in all directions from the base 11. A grip 15 is disposed at the end of each arm 6. During operation, when the extension direction of the tension acting on the tether 5 is changed by a guide 16 attached to the grip 15, a force compressing the arm 6 is generated, and at the same time, a force pushing the tip of the arm 6 downward is also generated. This downward force is transmitted to the first brace 13, which supports the arm 6 in the vertical direction, thereby supporting the arm 6. Furthermore, at this time, a bending moment is generated at the connection between the arm 6 and the first brace 13. Because this bending moment occurs repeatedly due to fluctuations in wind pressure, etc., fatigue cracks are likely to occur on the upper surface of the connection between the arm 6 and the first brace 13. Therefore, it is preferable to consider fatigue strength at the design stage. In addition, the plate thickness of the arm 6 and the connection position between the arm 6 and the first brace 13 may be reviewed in some cases. In principle, similar consideration is also required for the connection position between the arm 6 and the second brace 14. It is also possible to make part or all of the arm 6 a void section, thereby canceling part of the force pushing the tip of the arm 6 downward.

[0065] (Grip 15) In this embodiment, the grip 15 is attached to the tip of the arm 6. A guide 16 for bending the tether 5 is installed on the grip 15. As described above, the grip 15 ensures stability when the floater 3 floats at the first waterline 28 during towing. On the other hand, when the floater 3 floats at the second waterline 27 during operation, if the entire volume of the grip 15 becomes buoyant, the buoyancy will greatly exceed the downward force generated by bending the tether 5, and instead, excessive load will be placed on the arm 6 and the first brace 13. Therefore, when the floater 3 floats at the second waterline 27 during operation, it is advantageous in terms of strength design of the floater 3 to load ballast water into the grip 15 and use it as a ballast tank, which is effective in reducing the weight of the steel material of the floater 3.

[0066] (Guide 16) In this embodiment, the tether fixing device 8 that fixes the tether 5 is installed on the stage 7 attached to the central pillar 12. The extension direction of the tether 5, which extends straight from the tether fixing part 23 of the container 17, changes at the guide 16, and extends obliquely upward toward the tether fixing device 8 arranged on the stage 7. As the extension direction of the tether 5 changes, the guide 16 receives horizontal and vertical forces proportional to the tension. When the floating body 3 is subjected to wind pressure, for example, a lateral force may also act on the tether 5, and the guide 16 also receives this lateral force. The guide 16 may be provided with a mechanism to prevent the tether 5 from coming off the guide 16.

[0067] (Stage 7) The stage 7 is firmly fixed to the central pillar 12 at a position above the arm 6 and higher than the second waterline 27. Installing the stage 7 at a high position is effective in reducing the number of times the stage 7 is exposed to green waves during severe storms. On the other hand, when the wind power generation tower 1 is mounted on the central pillar 12, the position of the stage 7 can be determined from the perspective of ensuring clearance between the tips of the blades 40 and the stage 7. It is preferable to design the stage 7 on the assumption that it will be exposed to green waves. For example, if deep beams are provided on the stage 7, it is preferable to form as large a lightening hole as possible in the stage 7. It is preferable to minimize the area on which the deck of the stage 7 is laid. Furthermore, if a deck must be laid, a water-permeable structure such as grating may be provided on the stage 7.

[0068] (Tether fixing device 8) The tether fixing device 8 is installed on the stage 7 at a position adjacent to the tether hoist 9. The tether fixing device 8 is subjected to tension acting on the tether 5. For this reason, the tether fixing device 8 may be firmly fixed to the stage 7. Because the stage 7 is attached to the central pillar 12, if the tether fixing device 8 is installed at a position away from the central pillar 12, a large moment will be generated at the fixed position between the stage 7 and the central pillar 12. Therefore, it is preferable to install the tether fixing device 8 as close to the central pillar 12 as possible. The distance between two adjacent tether fixing devices 8 arranged on the stage 7 may be shorter than the distance between two adjacent guides 16. In this case, the two tethers 5 extending from one grip 15 to the stage 7 do not have to be parallel to each other.

[0069] (Tether hoist 9) A tether hoist 9 is installed on the stage 7. The tether hoist 9 enables the tether 5 to be let out when the anchor vessel body 4 is lowered to the seabed, the initial tension of the tether 5 during operation to be adjusted, and the tether 5 to be wound up when the floating platform 2 supporting the tower 1 is removed. When adjusting the initial tension of the tether 5, the tether 5 may be wound up slightly using the tether hoist 9 to release the load on the tether fixing device 8.

[0070] (Tether storage space 10) The tether storage space 10 is provided on the base 11. The tether 5 is stored in the tether storage space 10 when the float 3 is launched and towed. When the float 3 is in operation, the tether 5 is hardly stored in the tether storage space 10. Therefore, a water-permeable structure such as a lattice may be adopted for the side wall of the tether storage space 10. In this case, the drift force acting on the float 3 is reduced, and therefore it is possible to reduce the tension acting on the tether 5.

[0071] (Storage position of the anchor vessel body during launching and towing) Next, the storage position of the anchor vessel body 4 during launching and towing will be described below with reference to Figure 10. Figure 10 is a diagram showing the storage position of the anchor vessel body 4 during launching and towing. When the floater 3 is assembled and launched at a shipyard or the like, the anchor vessel body 4 is suspended by the tether 5 directly below the floater 3, and the floater 3 and the anchor vessel body 4 are launched in this state with their relative positions fixed so as not to move. At this time, the floater 3 floats at the first waterline 28. Furthermore, most of the tether 5 is stored in the tether storage space 10. When the floater 3 is floating at the first waterline 28, the moment of inertia caused mainly by the waterline plane of the arms 6 and the grips 15 is large, so that the floater 3 can ensure sufficient stability against pitching during towing. This allows the floating body 3 to be towed by several tugboats without the need for large barges or support ships, which is effective in reducing the installation costs of the floating platform 2.

[0072] Next, a method for installing the floating platform 2 according to this embodiment will be described below with reference to Fig. 11. Fig. 11 is a diagram for explaining the method for installing the floating platform 2 according to this embodiment. Fig. 11 shows each procedure (steps S1 to S4) from launching and towing the floating platform 2 to installing the floating platform 2. In step S1, an anchor vessel body 4 equipped with a plurality of containers 17 is placed directly below the float 3, and the float 3 is towed to a planned installation site while floating at the first waterline 28.

[0073] (Step S2: Installation of anchor vessel body) When the floating body 3 arrives at the planned installation location, the anchor vessel body 4 is lowered to the seabed by letting out multiple (eight in this example) tethers 5 into the sea. Note that it is preferable to investigate the condition of the seabed in advance and decide the planned installation location of the anchor vessel body 4 so that the vessel 17 of the anchor vessel body 4 does not run aground on large rocks.

[0074] (Step S3: Loading heavy objects into each container) Heavy objects are loaded from the ship 32 into the storage spaces S of the four containers 17 installed on the seabed using the heavy object loading device 35. The heavy objects are, for example, soil, sand, gravel, crushed stone, concrete pieces, etc. Specifically, as shown in FIG. 12 , after the heavy object loading hose 30 is lowered into the sea from the ship 32, the heavy object loading hose 30 is guided to a predetermined position using three propulsion devices 31 attached to the heavy object loading hose 30. Specifically, when the tip of the heavy object loading hose 30 reaches the storage space S of the container 17, the heavy object is loaded into the storage space S from the heavy object loading hose 30.

[0075] (Step S4: Installation of the floating platform) After the heavy load has been placed into the vessel 17 of the anchor vessel body 4, the float 3 is submerged to the second waterline 27 by winding up the tether 5 with the tether winch 9. If tension greater than the planned initial tension occurs in the tether 5 at this time, ballast water is loaded onto the float 3 and the tension of the tether 5 is adjusted so that the tension of the tether 5 reaches a predetermined value. Thereafter, each of the multiple tethers 5 is fixed to the float 3 using the multiple tether fixing devices 8. In this way, the floating platform 2 is installed offshore using a tension mooring method.

[0076] (Releasing Force of Float During Installation) Figure 8 shows the first waterline 28 and the second waterline 27 of the float 3. During the process of installing the float 3 by retracting the tether 5, the arms 6 and grips 15 of the float 3 are submerged below the water surface, causing the waterline area to decrease rapidly and the float 3 to lose its restoring force. However, in this state, the tension in the tether 5 is sufficiently large, so there is no risk of the float 3 capsizing. It is preferable not to load ballast water onto the float 3 until the tension in the tether 5 is sufficiently large.

[0077] (Heavy Load Drop-Off Device 35) FIG. 12 shows the heavy load drop-off device 35. To drop heavy loads such as soil and sand into a specific location on the seabed, it is preferable to use a means for accurately maintaining the tip of the heavy load drop-off hose 30 at the target position. Amidst disturbances such as wind, waves, and currents, it is difficult to quickly guide the tip of the heavy load drop-off hose 30 to the target position simply by controlling the position of the ship 32, as in the conventional method. Therefore, by providing a propulsion device 31 at the tip of the heavy load drop-off hose 30, it becomes possible to guide the tip of the heavy load drop-off hose 30 to the target position. Furthermore, it is preferable to provide multiple (three in this example) propulsion devices 31 on the heavy load drop-off hose 30 to control the shape (posture) of the entire heavy load drop-off hose 30 in the water. In this case, it is possible to more reliably drop heavy loads from the heavy load drop-off hose 30 into the storage space S of the container 17.

[0078] (Removal of the Floating Platform) When the floating platform 2 supporting the tower 1 is to be removed from the sea area where it is installed, the floating platform 2 can be removed without using a large support vessel, as was the case during installation, by performing the steps (steps S1 to S4) shown in Fig. 11 in reverse. In this case, it is necessary to recover heavy objects such as sediment stored in the container 17 by sucking seawater using the heavy object injection hose 30. A water jet spraying device or the like may be attached to the tip of the heavy object injection hose 30 in order to turn the surface of the heavy objects such as sediment deposited in the storage space S of the container 17 into a slurry.

[0079] As described above, the floating platform 2 supporting the tower 1 is a tension-moored floating platform comprising a float 3, an anchor vessel 4, and multiple tethers 5 connecting the float 3 and the anchor vessel 4. The anchor vessel 4 is not a conventional large concrete structure but is composed of multiple containers 17 for storing heavy loads such as inexpensive soil and sand. Because the floating platform 2 is self-supporting during towing, it can be towed by a tugboat without using a large dedicated ship or barge. Furthermore, installation of the floating platform 2 can be achieved without using a large support vessel by using a tether winch 9 mounted on the float 3 to pay out the tether 5 to lower the anchor vessel 4 into the sea, and then loading heavy loads such as soil and sand into each container 17 from a ship 32 equipped with a heavy load loading device 35. In this way, the manufacturing and installation costs of the floating platform 2 can be reduced. Furthermore, tension mooring can significantly suppress the motion of the float 3 during operation of the floating platform 2. Furthermore, the anchor vessel body 4, which is expected to have a fish reef effect, can realize a floating platform 2 that is also effective in revitalizing the fishing industry.

[0080] Although the embodiments of the present disclosure have been described above, the technical scope of the present invention should not be construed as being limited by the description of the present embodiments. The present embodiments are merely examples, and it will be understood by those skilled in the art that various modifications of the embodiments are possible within the scope of the invention described in the claims. The technical scope of the present invention should be determined based on the scope of the invention described in the claims and its equivalents.

[0081] This application appropriately incorporates the contents disclosed in Japanese Patent Application No. 2023-151768 filed on September 19, 2023.

Claims

1. A floating platform (2) comprising: a float (3) configured to support a tower (1) and arranged on the water by tension mooring; an anchor vessel body (4) comprising a plurality of vessels (17); and a plurality of tethers (5) connecting the float (3) and the anchor vessel body (4), wherein each of the plurality of vessels (17) is fixed to at least one of the plurality of tethers (5) and is installed on the bottom of the water, and each of the plurality of vessels (17) comprises: a bottom (19); a vessel outer wall (20) arranged to surround the outer periphery of the bottom (19); and a vessel inner wall (21) arranged to face the vessel outer wall (20), wherein a storage space (S) is formed by the bottom (19), the vessel outer wall (20), and the vessel inner wall (21), and the storage space (S) is used to store heavy objects including at least one of soil, sand, gravel, crushed stone, and concrete pieces. Floating platform (2).

2. The floating platform (2) according to claim 1, wherein the bottom (19) has an opening (190), the vessel inner wall (21) is arranged to surround the inner circumference of the bottom (19), and each of the plurality of vessels (17) further comprises a tether support member (22) formed on the vessel inner wall (21) and fixed to the at least one tether (5).

3. The floating platform (2) according to claim 2, wherein the container inner wall (21) has a cavity (36) that communicates with the opening and is open at the top.

4. A floating platform (2) as claimed in any one of claims 1 to 3, wherein an inner surface (26) of the container inner wall (21) facing the container outer wall is inclined with respect to the height direction of the container (17), and the distance between the container inner wall and the container outer wall in a horizontal direction perpendicular to the height direction gradually becomes smaller as it approaches the bottom.

5. The floating platform (2) of claim 2 or 3, wherein the tether support member (22) comprises: a first tether support member (22a) formed on the inner wall of the vessel and fixed to one of the plurality of tethers (5); and a second tether support member (22b) formed on the inner wall of the vessel and fixed to another one of the plurality of tethers (5).

6. The floating platform (2) according to any one of claims 1 to 5, wherein the floating body (3) comprises: a central column (12) configured to support the tower (1); a plurality of arms (6) extending radially from the central column (12) or a vicinity of the central column (12); a stage (7) fixed to the central column (12); a plurality of first braces (13) each connected to a corresponding one of the plurality of arms and the stage (7) or the central column; a plurality of second braces (14) each connected to two adjacent arms of the plurality of arms; a plurality of tether fixing devices (8) configured to fix a corresponding one of the plurality of tethers; and a plurality of tether winches (9) configured to wind up a corresponding one of the plurality of tethers.

7. The floating platform (2) according to claim 6, wherein the floating body (3) further comprises: a plurality of grips (15) each connected to an end of one of the plurality of arms, the axial direction of each of the plurality of grips being approximately perpendicular to the axial direction of the corresponding arm; and a plurality of guides (16) each arranged on a corresponding one of the plurality of grips (15), each of the plurality of guides being configured to contact a corresponding one of the plurality of tethers and to change the extension direction of the corresponding one tether; and wherein the tether fixing device (8) and the tether hoist (9) are arranged on the stage.

8. A floating platform (2) according to any one of claims 1 to 7, wherein the tower (1) is fitted with a nacelle (42) and blades (40), and the floating platform is an offshore wind power platform.

9. An anchor vessel body (4) configured to support a tower (1) and connected via a plurality of tethers (5) to a floating body (3) placed on the water by tension mooring, the anchor vessel body (4) comprising a plurality of vessels (17), each of which is fixed to at least one of the plurality of tethers (5) and installed on the bottom of the water, each of which comprises: a bottom (19); a vessel outer wall (20) arranged to surround the outer periphery of the bottom (19); and a vessel inner wall (21) arranged opposite the vessel outer wall (20), the bottom (19), the vessel outer wall (20), and the vessel inner wall (21) forming a storage space (S), and the storage space (S) storing a heavy object including at least one of soil, sand, gravel, crushed stone, and concrete pieces.

10. A float (3) arranged on water by tension mooring and configured to support a tower (1), the float (3) comprising: a central pillar (12) arranged to support the tower (1); a plurality of arms (6) extending radially from the central pillar (12) or near the central pillar (12); a plurality of grips (15) each connected to an end of one of the plurality of arms, the axial direction of each of the plurality of grips being approximately perpendicular to the axial direction of the corresponding arm; a stage (7) fixed to the central pillar (12); a plurality of first braces (13) each connected to a corresponding one of the plurality of arms and to the stage (7) or the central pillar; a plurality of second braces (14) each connected to two adjacent arms of the plurality of arms; and a plurality of tether fixing devices (8) configured to fix a corresponding one of a plurality of tethers.

11. A method for installing the floating platform (2) according to claim 1 on water by tension mooring, comprising the steps of: towing the floating body (3) to a planned installation site while the anchor vessel body (4) having the plurality of vessels (17) is placed directly below the floating body (3) and the floating body (3) is floating at a first waterline (28); placing the plurality of vessels (17) on the bottom of the water at the planned installation site by paying out the plurality of tethers (5) into the water; pouring the heavy load including at least one of soil, sand, gravel, crushed stone and concrete pieces into the accommodation space of each of the plurality of vessels (17) placed on the bottom of the water; reeling in the plurality of tethers (5); and sinking the floating body (3) to a second waterline (27) deeper than the first waterline (28). An installation method comprising: a step of adjusting tension of the plurality of tethers (5) so that the tension of the plurality of tethers (5) becomes a predetermined value; and a step of fixing each of the plurality of tethers (5) to the floating body (3).

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