Mooring cable for tension-moored floating body

Abstract

The present invention addresses the problem of providing a tension-mooring cable for a tension-moored floating body, with which it is possible to, without adjusting the lengths of the tension-mooring cables in each mooring cable bundle that moors and supports the tension-moored floating body, equalize tension generated in the tension-mooring cables and prevent occurrence of resonance with the frequency of waves on the ocean. The problem is solved by a mooring cable that is for a tension-moored floating body, and that links a connection part 5b formed on the tension-moored floating body and a sea bottom-mooring part 9 fixed to a sea bottom 103. The tension-mooring cable is configured such that tension is generated in a tension-mooring cable 7 due to buoyancy generated in a tension-moored floating body 5 when the tension-mooring cable 7 is linked to the tension-moored floating body 5, and the tension-moored floating body 5 can be held in a tension-moored state. The tension-mooring cable 7 is configured such that a low rigidity mooring cable 7b, having a low extensional rigidity which is the product of the modulus of longitudinal elasticity and the cross-sectional area of a cable material, is linked to a high rigidity mooring cable 7a having a higher extensional rigidity than that of the low rigidity mooring cable 7b, by a linking tool.

Description

Mooring rope for tension mooring float

[0001] The present invention relates to mooring lines for tension-moored floats, and more specifically to mooring lines for tension-moored floats that can equalize the tension generated in each mooring line without adjusting the length of each mooring line in each mooring line bundle that moor and support the tension-moored float, and that prevent resonance due to the wave period on the ocean and the vibration period caused by wind turbines.

[0002] Traditionally, wind power generation facilities have often been installed on land. However, on land, there are obstacles that block the wind, such as forests and buildings, which make turbulence more likely to occur and wind power can be unstable. This has led to the proposal of installing wind power generation facilities offshore. Offshore, there are no obstacles that block the wind, such as forests and buildings, so turbulence is less likely to occur compared to onshore, and stable, large wind power is often generated. Therefore, if wind power generation facilities are installed offshore, it is thought that wind power generation can be performed more stably than on land, and large amounts of electricity can be supplied.

[0003] When installing wind power generation facilities offshore, a structure is required to hold the power generation facilities (wind turbines and generators). Such structures include bottom-fixed structures, which are fixed to the seabed and extend offshore, where the wind turbines and generators are mounted, and floating structures, which are moored to the seabed with mooring lines and on which the wind turbines and generators are mounted. Floating structures have the advantage of being able to be installed in deeper waters than bottom-fixed structures. Deep waters are generally offshore, which means that wind conditions are favorable and wind power generation can be carried out stably. In particular, floating structures are advantageous in Japan, where the surrounding seas are deeper than in other countries.

[0004] Floating structures include catenary mooring structures in which a float is loosely connected to a mooring facility on the seabed by mooring lines to maintain its position. There are also tension leg platforms (TLPs), as described in Patent Document 1, in which the float is forcibly pulled down by the mooring lines to partially submerge it, and the buoyancy of the float generates tension in the mooring lines to maintain its position. Because tension leg platforms maintain the position of the float using the tension of the mooring lines, they are advantageous in that they are easier to miniaturize than catenary mooring structures and also provide greater stability due to the tension in the mooring lines.

[0005] Japanese Patent Application Laid-Open No. 2023-124020

[0006] A tension-moored float is supported by multiple mooring bundles, each consisting of multiple mooring lines, attached to each of the columns that make up the float. The mooring lines in each mooring bundle may be made of steel wire cable, which has high tensile rigidity and is resistant to stretching. In each mooring bundle, it is necessary to generate equal tension in each mooring line. If the tension is not equal, some mooring lines will generate no tension or only weak tension, while other mooring lines will generate excessively strong tension, resulting in uneven loads on the mooring lines, which could damage the mooring lines and supporting structures and pose durability issues. To equalize the tension in each mooring line, it is necessary to adjust the length of each mooring line using large winch equipment, etc. However, using large winch equipment, etc., increases the weight of the equipment installed on the float, necessitating an increase in the size of the float itself and resulting in increased manufacturing costs.

[0007] One possible solution to eliminate the need for equipment to adjust the length of mooring lines is to measure the depth to the top of the pile foundation, which secures the bottom of the mooring line to the seabed, on-site in advance, and determine the distance from the mooring line attachment point on the floating body, where the top of the mooring line is secured, to the top of the pile foundation as the required length of the mooring line. The mooring line is prefabricated on land to the required length, with the top end secured to the mooring line connection point on the floating body and the bottom end secured to the top of the pile foundation. However, the length of the mooring line is subject to various errors, such as measurement error in measuring the depth to the top of the pile foundation, manufacturing error in the mooring line, and thermal expansion and contraction due to temperature changes. Therefore, prefabricating the mooring lines to the required length does not ensure that the tension generated in each mooring line is uniform.

[0008] On the other hand, it is possible to make the mooring ropes using a material with low tensile rigidity (softness), such as polyester rope, and absorb the error by the elongation of the rope itself. However, if the mooring ropes are made using a material with low rigidity, the spring constant k of the mooring ropes will be small, and the natural frequencies of the vertical and roll-pitch systems specific to tension-moored floaters will be low, which could result in vertical resonance due to the vibration period of the wind turbines and roll-pitch resonance due to the wave frequency on the ocean.

[0009] If resonance occurs between the wave period on the ocean and the vibration period caused by the wind turbine, there is a risk that the mooring lines and supporting structures will be damaged, and durability will also be an issue.

[0010] Therefore, the object of the present invention is to provide a mooring rope for a tension-moored float that can equalize the tension generated in each mooring rope without adjusting the length of each mooring rope of each mooring rope bundle that supports the tension-moored float, and that prevents resonance with the wave period on the ocean and the vibration period caused by the wind turbine. Further objects of the present invention will become clear from the following description.

[0011] The above problems are solved by the following inventions.

[0012] 1. A mooring line for a tension-moored float that connects a connection part formed on a tension-moored float that supports a wind power generation facility offshore to a seabed mooring part fixed to the seabed, wherein the connection part and the seabed mooring part are connected by the tension mooring line, so that tension is generated in the tension mooring line by the buoyancy generated in the tension-moored float, and the tension-moored float can be maintained in a tension-moored state, the tension mooring line is made up of a low-rigidity mooring line with a low tensile rigidity calculated by multiplying the longitudinal elastic modulus of the line material by the cross-sectional area, and a high-rigidity mooring line with a higher tensile rigidity than the low-rigidity mooring line, connected via a connector, the line material of the low-rigidity mooring line is a resin rope, and the high-rigidity mooring line is a steel wire cable made up of a plurality of steel wires bundled together, 1. The tension mooring rope for a tension mooring float is characterized in that, when the tensile rigidity of the resin rope is taken as 1, the tensile rigidity of the steel wire cable is 2 to 5 times the tensile rigidity of the resin rope. 2. A mooring line for a tension-moored float that connects a connection part formed on a tension-moored float that supports a wind power generation facility offshore to a seabed mooring part fixed to the seabed, wherein the connection part and the seabed mooring part are connected by the tension mooring line, so that tension is generated in the tension mooring line by the buoyancy generated in the tension-moored float, allowing the tension-moored float to be maintained in a tension-moored state, wherein the tension mooring line is configured by connecting a low-rigidity mooring line and a high-rigidity mooring line that has higher tensile rigidity than the low-rigidity mooring line via a connector, wherein the low-rigidity mooring line is a resin rope, and the high-rigidity mooring line is a steel wire cable formed by bundling a plurality of steel wires in parallel, and wherein 50% or more of the total length of the tension mooring line is the high-rigidity mooring line. 3. The mooring rope for a tension moored floating body described in 1 or 2, characterized in that the resin rope is a polyester rope made by preparing multiple sub-ropes each made by twisting together multiple polyester wires and bundling the multiple sub-ropes in parallel.4. The mooring rope for a tension-moored float according to paragraph 1 or 2 above, characterized in that the tension-moored float comprises at least three hollow pillar-like columns extending vertically and arranged to form a triangle on a plane, three upper beams connecting the three columns above sea level and three lower beams connecting the columns below sea level, and the wind power generation facility is supported on one of the columns.

[0013] According to the present invention, since a portion of each mooring rope in each mooring rope bundle that supports and moored a tension-moored float is made of a material with low elongation rigidity, the tension generated in each mooring rope can be equalized by stretching the low-rigidity portion without adjusting the length of each mooring rope, preventing damage to the mooring ropes and support structures due to uneven loads on each mooring rope and achieving high durability. Furthermore, since the length of each mooring rope does not need to be adjusted, there is no need to use large winch equipment, etc., which reduces the weight of the equipment installed on the float, allows the float itself to be made smaller, and reduces manufacturing costs.

[0014] Furthermore, according to the present invention, the spring constant of the mooring ropes is larger and the natural frequency is higher than when the entire mooring rope is made of a material with low tensile rigidity, which prevents resonance with the wave periods on the ocean and the vibration periods caused by the wind turbines. This prevents damage to the mooring ropes and supporting structures due to resonance and achieves high durability.

[0015] Fig. 1 is a front view of an offshore wind power generation facility equipped with a tension-moored float using a mooring line for a tension-moored float according to an embodiment of the present invention; Fig. 2 is a perspective view of the tension-moored float of Fig. 1, where (A) is a perspective view of the tension-moored float from one side, and (B) is a perspective view of the tension-moored float as seen from the other side; Fig. 3 is a side view of the mooring line for the tension-moored float of Fig. 1; Fig. 4 is a cross-sectional view of the mooring line (high-rigidity mooring line) for the tension-moored float of Fig. 1; Fig. 5 is a graph showing the relationship between the length and combined elongation of the low-rigidity mooring line in the mooring line for the tension-moored float of Fig. 1; Fig. 6 is a graph showing the relationship between the length and combined spring constant of the low-rigidity mooring line in the mooring line for the tension-moored float of Fig. 1;

[0016] Preferred embodiments of the present invention will now be described.

[0017] 1 is a front view showing an offshore wind power generation facility 1 equipped with a tension-moored float for which a tension-moored float mooring line according to an embodiment of the present invention is used. As shown in FIG. 1 , the offshore wind power generation facility 1 is configured to include a wind power generation facility 3, a tension leg platform (TLP) 5 (hereinafter sometimes simply referred to as the float) on which the wind power generation facility 3 is mounted, tension mooring lines 7 whose upper ends are fixed to the tension mooring float 5, and seabed mooring parts 9 whose lower ends are fixed to the seabed.

[0018] The mooring line for the tension mooring float in this embodiment is a tension mooring line 7 that forcibly pulls down the tension mooring float 5 carrying the wind power generation equipment 3, causing part of the float to submerge, and holds the tension mooring float 5 in a fixed position by the tension generated by the buoyancy of the tension mooring float 5.

[0019] [Wind Power Generation Facility] The wind power generation facility 3 is a power generation facility that converts wind power into electric power, and as shown in Fig. 1, includes a tower 31, a nacelle 33, a boss 35, and blades 37. In this embodiment, the wind power rotates the blades 37 and the boss 35, and this rotational force rotates the rotor of the generator to which the boss 35 is connected, thereby converting the wind power into electric power.

[0020] The tower 31 is a support pillar that supports the entire wind power generation facility 3, and is a columnar structure that extends vertically. The outer shape of the tower 31 is preferably cylindrical or conical with a diameter that widens downward, but is not particularly limited as long as it has the strength to support the entire wind power generation facility 3. The tower 31 is supported by having its lower end connected to a flange portion 5a of a column 51 that is provided on the upper surface of the portion of the tension-moored float 5 that is exposed above the sea surface 101.

[0021] The nacelle 33 is a hollow structure with a built-in generator, is provided at the upper end of the tower 31, and is spindle-shaped with its axial direction horizontal. The nacelle 33 and the upper end of the tower 31 are connected by a rotation mechanism that enables the nacelle 33 to rotate around a vertical axis. The axial direction of the boss 35 of the nacelle 33 is oriented in the direction of the strongest wind pressure. Inside the nacelle 33 are a power transmission shaft that transmits the rotational force of the boss 35, a speed increaser (gearbox, etc.) connected to this power transmission shaft, a brake that stops the power transmission shaft in an emergency (such as a typhoon) or during inspection, and a generator whose rotor is connected to the power transmission shaft.

[0022] The boss 35 is a cylindrical member that supports the blades 37 and is coaxially connected to the tip of the power transmission shaft. The boss 35 is rotatable around a horizontal axis and is located at the horizontal tip of the nacelle 33.

[0023] The blades 37 are the blades of the wind turbine that convert wind power into rotational power, and a plurality of blades 37 protrude from the outer periphery of the boss 35 in the radial direction of the boss 35. In this embodiment, three blades 37 protrude at equal intervals in the circumferential direction of the boss 35. When the blades 37 receive wind, they rotate together with the boss 35 around the boss 35. The rotation of the boss 35 rotates the rotor of the generator, generating electricity. The electricity generated by the generator is transmitted to land via an undersea power transmission cable (not shown).

[0024] [Tension-moored float] Figure 2 is a perspective view showing the tension-moored float of Figure 1, Figure 2(A) is a perspective view of the tension-moored float seen from one side, and Figure 2(B) is a perspective view of the tension-moored float seen from the other side.

[0025] The tension-moored float 5 is a float that supports the wind power generation facility 3. As shown in Figure 1, the tension-moored float 5 floats on the ocean, with the wind power generation facility 3 mounted on the portion above the sea surface 101. The tension-moored float 5 has a sealed hollow portion inside, which gives the specific gravity of the entire exterior of the float 5 less than 1, allowing it to float on the ocean.

[0026] As shown in Figure 2, the tension-moored float 5 has at least three hollow pillar-like columns extending in the vertical direction and arranged to form a substantially equilateral triangle in a plan view. As shown in Figures 1 and 2, columns 52 and 53 are connected above sea level by an upper beam (bracing) 54 and below sea level 101 by a lower beam (pontoon) 57. Similarly, columns 51 and 52 are connected by an upper beam (bracing) 55 and a lower beam (pontoon) 59, and columns 51 and 53 are connected by an upper beam (bracing) 56 and a lower beam (pontoon) 58. In the illustrated example, column 51 has a flange portion 5a and is configured to support the wind power generation facility 3.

[0027] In the illustrated example, the columns 51, 52, and 53 are hexagonal columns, and are shown to have a deformed hexagonal shape with a protruding portion in plan view. The shape of the columns is not limited to this, and may be a regular hexagon, a pentagon, or a circle in plan view.

[0028] In the illustrated example, the upper beams 54, 55, 56 have a square prism shape, but are not limited to this and may be cylindrical or formed in a truss structure.

[0029] In this embodiment, the tension-moored float 5 may be provided with a ballast tank (not shown) inside each of the three columns 51, 52, and 53. The buoyancy and draft of the tension-moored float 5 can be adjusted by injecting and discharging ballast water into the ballast tank. When the tension-moored float 5 is installed, reducing the buoyancy can facilitate the attachment of the tension mooring lines 7.

[0030] 1 also shows an example in which the tension-moored float 5 is provided with nine connection points 5b, three sets of three on each of the columns 51, 52, and 53. The upper ends of the tension mooring lines 7 are connected to the connection points 5b, respectively. The lower ends of the tension mooring lines 7 are connected to the seabed mooring points 9 on the seabed side, thereby connecting the tension-moored float 5 to the multiple seabed mooring points 9, which are mooring facilities on the seabed 103.

[0031] As a result, the buoyancy of the tension-moored float 5 generates tension in the tension mooring lines 7, which holds the tension-moored float 5 in a fixed position. In other words, when the tension-moored float 5 is entirely submerged in the sea, the buoyancy acting in the opposite direction to gravity (upward) is greater than the gravity acting downward, so the tension-moored float 5 tries to rise to a position where the buoyancy and gravity are balanced. This is because buoyancy is generated in proportion to the volume of the portion of the tension-moored float 5 that is submerged below the sea surface. The tension mooring lines 7 hold the tension-moored float 5 at a depth where the buoyancy of the tension-moored float 5 becomes greater than gravity.

[0032] In this state, the rising of the tension-moored float 5 is prevented by the tension mooring lines 7, and so the forced buoyancy of the tension-moored float 5, which is a buoyancy force greater than gravity, is applied to the tension mooring lines 7. Tension is generated in the tension mooring lines 7 to which the forced buoyancy force is applied, resulting in a tension-moored state. In the tension-moored state, the tension generated in the tension mooring lines 7 holds the tension-moored float 5 in a fixed position.

[0033] In this embodiment, even if the sea level rises and falls (the seabed depth changes) due to tides, tension within a predetermined range is generated in the tension mooring ropes 7 of the tension-moored float 5, and the tension-moored state is maintained.

[0034] [Submarine Mooring Section] In this embodiment, the submarine mooring section 9 is the top of a pile foundation driven into the seabed 103. The submarine mooring section 9 is a mooring facility on the seabed 103, and holds the tension-moored float 5 in place via the tension mooring lines 7. The submarine mooring section 9 is equipped with a connecting section such as a hook or ring to which the lower end of the tension mooring line 7 is connected.

[0035] Other known mooring equipment can also be used for the seabed mooring unit 9, as long as it has a structure that prevents the tension applied by the tension mooring lines 7 from pulling it away from the seabed 103 when the position of the tension moored float 5 is maintained by forced buoyancy, and is corrosion-resistant so that it does not easily corrode in the sea. Specifically, gravity anchors, pile anchors, suction anchors, etc. can also be used. In this embodiment, the length of the tension mooring lines 7 is not adjusted, so any method that can reliably fix the position of the lower end of the tension mooring lines 7 will do, and is not particularly limited.

[0036] In this embodiment, nine seabed mooring units 9 are provided for one offshore wind power generation facility 1. The nine seabed mooring units 9 are grouped in groups of three, and are installed on the seabed vertically to each of the columns 51, 52, and 53 of the tension-moored float 5, for a total of three groups of seabed mooring units 9. In this embodiment, three connection units 5b are provided on each of the columns 51, 52, and 53, and the upper ends of the tension mooring lines 7 are fixed to the connection units 5b on the tension-moored float side. A total of three groups of seabed mooring units 9 are installed on the seabed vertically from the connection units 5b. A total of three groups of mooring lines, each consisting of three tension mooring lines 7, are fixed to the connection units 5b and the seabed mooring units 9. As a result, the upper end of the tension mooring line 7 is fixed to the connection part 5b provided on the tension mooring float 5, and the lower end of the tension mooring line 7 is fixed to the seabed mooring part 9, thereby mooring and holding the tension mooring float 5.

[0037] In this embodiment, the number of seabed mooring parts 9 and the number of tension mooring lines 7 may be set appropriately within a range that allows the pulling force applied from the tension mooring lines 7 to be distributed to an extent that the tension moored float 5 does not slip off the seabed 103 when held in place by forced buoyancy. For example, for one offshore wind power generation facility, six seabed mooring parts 9 and six tension mooring lines 7 may be used, and the tension moored float 5 may be moored and held by three sets of mooring lines, each consisting of two tension mooring lines 7.

[0038] [Tension mooring line] The tension mooring line 7 is a mooring line that connects the tension moored float 5 to the seabed mooring part 9, and uses the tension generated by the buoyancy of the tension moored float 5 to place the tension moored float 5 in a tension moored state and hold the tension moored float 5 in a fixed position.

[0039] Figure 3 is a side view of the mooring line for the tension-moored float shown in Figure 1. As shown in Figure 3, the tension mooring line 7 of this embodiment comprises a high-rigidity mooring line 7a made of a material with high tensile rigidity and a low-rigidity mooring line 7b made of a material with low tensile rigidity, connected in series by a connector 7c. The connector 7c is a device that connects the upper end of the high-rigidity mooring line 7a to the lower end of the low-rigidity mooring line 7b, and various types such as a hook-shaped or ring-shaped connector can be used, with no particular limitation on its configuration. The tension mooring line 7 has the strength to withstand the tension generated by the buoyancy of the tension-moored float 5, so that it will not yield or break, and is corrosion-resistant so that it will not easily corrode in the sea.

[0040] Figure 4 is a cross-sectional view of a mooring rope (high-rigidity mooring rope) for the tension-moored float of Figure 1. In this embodiment, the high-rigidity mooring rope 7a is a steel wire cable, as shown in Figure 4, in which multiple wires (galvanized steel wires) 71a are arranged in parallel, the outer peripheries of which are covered with a protective layer 70a. In this embodiment, the high-rigidity mooring rope 7a has, for example, approximately 200 to 400 wires, and a coating diameter of, for example, approximately 100 mm to 200 mm. In this embodiment, the illustrated example shows multiple wires bundled in parallel, but this is not limited to this, and the multiple wires do not have to be parallel as long as they are bundled.

[0041] The low-rigidity mooring rope 7b is preferably a resin rope. Examples of materials for the resin rope include polyamide and polyester. Examples of polyamide include nylon 6, nylon 66, nylon 11, and nylon 12. A polyester rope is preferably a resin rope. An example of a polyester rope is one in which multiple sub-ropes, each made of twisted wires (e.g., polyester filaments), are bundled in parallel, wrapped in a sand filter, and covered with a protective layer. An example of the protective layer is one made of woven or knitted polyester wire. In this embodiment, the low-rigidity mooring rope 7b has a diameter of, for example, approximately 280 mm.

[0042] In this embodiment, in the example of Figure 1, the low-rigidity mooring line 7b is attached to the connection part 5b on the floating body side, and the high-rigidity mooring line 7a is attached to the seabed mooring part 9 on the seabed side, but it is also possible to attach the low-rigidity mooring line 7b to the seabed mooring part 9 on the seabed side, and the high-rigidity mooring line 7a to the connection part 5b on the floating body side.

[0043] This tension mooring line 7 can both relax the length accuracy requirement during installation and avoid resonance of the tension-moored float 5. The length accuracy requirement during installation refers to the allowable range of length error, and relaxing the accuracy requirement means that the allowable range of error is wide. As mentioned above, in one set (three) of tension mooring lines 7, each tension mooring line 7 must be equally tensioned. If there is a large error in the length of each tension mooring line 7, the long tension mooring line 7 will not be tensioned or will only be tensioned weakly, and the short tension mooring line 7 will be tensioned excessively strongly, resulting in an unbalanced load on each tension mooring line 7 and risk of damage to the short tension mooring line 7 and the supporting structure due to the excessive tension, as well as durability issues. In the embodiment of the tension mooring rope 7, if the length error is within the allowable range, the low-rigidity mooring rope 7b of the short tension mooring rope 7 will stretch, so tension will also be generated in the long tension mooring rope 7, and excessively strong tension will not be generated in the short tension mooring rope 7.

[0044] As mentioned above, the resonance of the tension-moored float 5 occurs in the vertical direction due to vibration caused by the rated rotational speed of the wind turbine, and in the roll-pitch direction due to the wave period. The resonance with the wave period occurs because the spring constant k of the tension mooring rope 7 is small, and the natural frequency F of the system in the roll and pitch directions is ０This can occur when the frequency (Hz) is low. It can also occur in the vertical direction. In this embodiment, large wind turbines (10 MW or more) may be supported by TLPs. In this case, resonance between the vertical direction and the rotational period of the wind turbine, and resonance between the roll-pitch direction and the wave period become problems. The tension mooring line 7 of this embodiment is a hybrid mooring line of low-stiffness and high-stiffness mooring lines, in which the low-stiffness mooring line 7b and the high-stiffness mooring line 7a are connected via a connector 7c, regardless of the rotational period determined by the specifications of the wind turbine to be installed. Therefore, the tensile stiffness can be arbitrarily changed. Therefore, the axial stiffness of the mooring line in the axial direction, which is proportional to the tensile stiffness, can also be arbitrarily changed, and the natural period can be changed. As a result, resonance can be avoided. For example, wave periods of 4-5 seconds or more to approximately 20 seconds often occur frequently, and it is necessary to shift the natural period from this frequently occurring wave period. Because the tension mooring line of the present invention is a hybrid mooring line as described above, the stiffness can be arbitrarily changed, thereby changing the natural period. As a result, the natural period in the roll-pitch direction can be kept within 4 seconds or less, so that resonance with the wave period can be prevented.

[0045] Table 1 below shows the steady-state tension (4,900 kN (500 tons)) generated per tension mooring rope 7 of the embodiment, the total length (50 m), the tensile stiffness (661,500 kN) of the polyester rope (low-stiffness mooring rope 7b), and the tensile stiffness (2,142,000 kN) of the steel wire cable (high-stiffness mooring rope 7a). Note that the tensile stiffness is the product of the Young's modulus of the rope material and its cross-sectional area. Table 1 shows an example in which the high-stiffness mooring rope 7a has an tensile stiffness at least three times that of the low-stiffness mooring rope 7b. For the tension mooring rope 7 of the present invention, the tensile stiffness of the high-stiffness mooring rope 7a is preferably in the range of two to five times that of the low-stiffness mooring rope 7b. Forming a tension mooring rope 7 within this range allows for both relaxed installation accuracy requirements and avoidance of resonance due to wave periods and wind turbine vibrations.

[0046]

[0047] As described above, it has been explained that the ratio of the extension stiffness of the low-rigidity mooring lines 7b and the high-rigidity mooring lines 7a in the tension mooring lines 7 can both relax the installation accuracy requirements and avoid resonance with wave periods and wind turbine vibrations, but it is also possible to relax the installation accuracy requirements and avoid resonance with wave periods by changing the length ratio of the low-rigidity mooring lines 7b and the high-rigidity mooring lines 7a in the tension mooring lines 7. Below, embodiments using the length ratio of the low-rigidity mooring lines 7b and the high-rigidity mooring lines 7a will be described.

[0048] Figure 5 is a graph showing the relationship between the length of the low-rigidity mooring ropes and the combined elongation of the mooring ropes for the tension-moored float shown in Figure 1. The relationships shown in Figures 5 and 6 represent the case where the low-rigidity mooring ropes 7b and high-rigidity mooring ropes 7a shown in Table 1 are used. As shown in Figure 5, the horizontal axis represents the length of the low-rigidity mooring rope (polyester rope) 7b relative to the total length of 50 m, and the graph shows the elongation of the tension mooring rope 7 when an axial force of 4,900 kN is applied when the low-rigidity mooring ropes 7b and high-rigidity mooring ropes 7a are connected. Note that when the low-rigidity mooring rope 7b at the left end is 0 m, it means that all of the low-rigidity mooring ropes 7a are high-rigidity mooring ropes, while when the right end is 50 m, it means that all of the low-rigidity mooring ropes 7b are low-rigidity mooring ropes. Figure 5 shows that the elongation of the tension mooring rope 7 increases linearly as the proportion of low-rigidity mooring ropes 7b increases. For example, as in the example shown in the figure, if the total length of the tension mooring rope is 50 m and a stretch of 200 mm or more is required in the tension mooring rope 7 to relax the construction and installation accuracy, the length of the low-rigidity mooring rope 7b can be made 17 m or more to achieve relaxation of the construction and installation accuracy.

[0049] Figure 6 is a graph showing the relationship between the length of the low-stiffness mooring ropes and the combined spring constant of the mooring ropes for the tension-moored float shown in Figure 1. As shown in Figure 6, the horizontal axis represents the length of the low-stiffness mooring rope (polyester rope) 7b, and the graph shows the combined spring constant when the low-stiffness mooring rope 7b is connected to the high-stiffness mooring rope 7a. As shown in Figure 6, the combined spring constant increases nonlinearly as the proportion of the low-stiffness mooring rope 7b decreases and the proportion of the high-stiffness mooring rope 7a increases. For example, assuming a spring constant of 20,000 kN / m or more is required for the tension-moored float 5 to avoid resonance between the roll and pitch motions and the wave periods on the ocean, the length of the low-stiffness mooring rope 7b should be set to 25 m or less to avoid resonance between the wave periods and the roll and pitch motions.

[0050] As described above, based on consideration of the elongation and spring constant of the tension mooring rope 7, by setting the length of the low-rigidity mooring rope 7b in the range of 17m to 25m (34% to 50%) when the total length of the tension mooring rope 7 is 50m, it is possible to both relax the installation accuracy requirements and avoid resonance of the tension moored float 5.

[0051] In this embodiment, when the water depth is deep and the total length of the tension mooring rope 7 is longer, the natural frequency F ０ In this case, in order to avoid resonance with the wave period on the ocean, the proportion of the high-rigidity mooring ropes 7a may be increased when the total length of the tension mooring ropes 7 is 50 m. In this embodiment, the requirement for construction and installation accuracy due to the increase in the proportion of the high-rigidity mooring ropes 7a can be relaxed even if the proportion of the high-rigidity mooring ropes 7a is increased, because the high-rigidity mooring ropes 7a also elongate in proportion to the total length.

[0052] As described above, the deeper the water depth and the longer the total length of the tension mooring lines 7, the shorter the preferable length (and the lower the proportion) of the low-rigidity mooring lines 7b becomes, so the deeper the water depth, the longer the length (and the higher the proportion) of the high-rigidity mooring lines can be, and therefore the length of the low-rigidity mooring lines can be shortened. In this embodiment, if the total length of the tension mooring lines 7 is longer than 50 m, it is preferable that the length of the high-rigidity mooring lines 7a be 50% or more of the total length of the tension mooring lines 7.

[0053] On the other hand, the embodiments shown in Figures 5 and 6 show the ratio of high-rigidity mooring lines 7a to low-rigidity mooring lines 7b in the tension mooring lines 7 when the water depth (the distance from the seabed mooring part to the connection part of the float) is approximately 50m, but if the water depth becomes shallower, the ratio of high-rigidity mooring lines 7a can be reduced and the ratio of low-rigidity mooring lines 7b can be increased.

[0054] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the embodiments. It is obvious that a person skilled in the art would conceive of various modifications and improvements within the scope of the technical concept of the present invention, and these are also included in the present invention.

[0055] 1: Offshore wind power generation facility 3: Wind power generation facility 31: Tower 33: Nacelle 35: Boss 37: Blade 5: Tension mooring float 5a: Flange portion 5b: Connection portion 51, 52, 53: Column 54, 55, 56: Beam 57, 58, 59: Pontoon 7: Tension mooring line 7a: High-rigidity mooring line 70a: Protective layer 71a: Wire 7b: Low-rigidity mooring line 7c: Connector 9: Seabed mooring portion 101: Sea surface 103: Seabed

Claims

1. A mooring line for a tension-moored float that connects a connection part formed on a tension-moored float that supports a wind power generation facility offshore to a seabed mooring part fixed to the seabed, wherein the connection part and the seabed mooring part are connected by the tension mooring line, so that tension is generated in the tension mooring line by the buoyancy generated in the tension-moored float, and the tension-moored float can be maintained in a tension-moored state, the tension mooring line is connected via a connector to a low-rigidity mooring line having a low tensile rigidity calculated by multiplying the longitudinal elastic modulus of the rope material by its cross-sectional area, and a high-rigidity mooring line having a higher tensile rigidity than the low-rigidity mooring line, the rope material of the low-rigidity mooring line is a resin rope, and the high-rigidity mooring line is a steel wire cable formed by bundling a plurality of steel wires, The mooring rope for a tension-moored floating body is characterized in that, when the tensile rigidity of the resin rope is taken as 1, the tensile rigidity of the steel wire cable is 2 to 5 times the tensile rigidity of the resin rope.

2. A mooring line for a tension moored float that connects a connection part formed on a tension moored float that supports a wind power generation facility offshore to a seabed mooring part fixed to the seabed, wherein the connection part and the seabed mooring part are connected by the tension mooring line, so that tension is generated in the tension mooring line by the buoyancy generated in the tension moored float, and the tension moored float can be maintained in a tension moored state, the tension mooring line is configured to connect a low stiffness mooring line and a high stiffness mooring line having a higher tensile stiffness than the low stiffness mooring line via a connector, the low stiffness mooring line is a resin rope, and the high stiffness mooring line is a steel wire cable formed by bundling a plurality of steel wires in parallel, and the tension mooring line is characterized in that 50% or more of its total length is made of the high stiffness mooring line.

3. A mooring rope for a tension-moored floating body as described in claim 1 or 2, characterized in that the resin rope is a polyester rope made by preparing multiple sub-ropes each made by twisting together multiple polyester wires, and bundling the multiple sub-ropes in parallel.

4. The mooring line for a tension-moored float as described in claim 1 or 2, characterized in that the tension-moored float has at least three hollow, cylindrical columns extending vertically and arranged in a triangular shape on a plane, three upper beams connecting each of the three columns above sea level and three lower beams connecting each of the three columns below sea level, and the wind power generation equipment is supported on one of the columns.

Images