Mooring cable for taut mooring floating body

Abstract

To provide a mooring cable for a taut mooring floating body having resonance with ocean wave period prevented by allowing tension generated in each taut mooring cable to be even without adjusting length of each taut mooring cable of each mooring cable bundle mooring and supporting the taut mooring floating body.SOLUTION: A mooring cable for a taut mooring floating body for coupling a connection part 5b formed on the taut mooring floating body and a sea bottom mooring part 9 fixed to the sea bottom 103, has tension generated in a taut mooring cable 7 by buoyancy generated in the taut mooring floating body 5 by being coupled by the taut mooring cable 7, in which the taut mooring floating body 5 is composed to be capable of being retained in the taut mooring state, and the taut mooring cable 7 has a low rigidity mooring cable 7b with low stretching rigidity which is modulus of longitudinal elasticity and cross section area of the mooring material multiplied, and a high rigidity mooring cable 7a with high elongation rigidity compared to the low rigidity mooring cable 7b coupled through a coupling tool.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a mooring cable for a tension moored floating body. Specifically, without adjusting the length of each mooring cable in each mooring cable bundle that moors and supports the tension moored floating body, the tension generated in each mooring cable can be made uniform, and the present invention also relates to a mooring cable for a tension moored floating body in which resonance due to the wave period on the ocean and the vibration period generated from the windmill is prevented.

Background Art

[0002] Conventionally, many wind power generation facilities have been installed on land. However, on land, there are obstacles that block the wind, such as mountains, forests, and buildings, which easily generate turbulent airflows and the wind power may not be stable. Therefore, it has been proposed to install wind power generation facilities on the ocean. On the ocean, there are no obstacles that block the wind, such as mountains, forests, and buildings, and turbulent airflows are less likely to occur compared to land, and large and stable wind power often occurs. Therefore, if a wind power generation facility is installed on the ocean, it is considered that wind power generation can be performed more stably than on land and a large amount of electric power can be supplied.

[0003] When installing a wind power generation facility on the ocean, a structure for holding the power generation facility (windmill and generator) is required. Such structures include a fixed-bottom type that is fixed to the seabed and reaches the ocean surface, and on which the windmill and generator are mounted on the ocean surface, and a floating body type that consists of a floating body moored to the seabed by mooring cables, and on which the windmill and generator are mounted on this floating body. The floating body type has the advantage that it can be installed in deeper sea areas than the fixed-bottom type. Since deep sea areas are generally off-shore sea areas, the wind conditions are good, which is advantageous in that stable wind power generation can be performed. In particular, in Japan, since the surrounding sea areas are deeper compared to other countries, the floating body type is advantageous.

[0004] As a floating body type structure, there is a catenary mooring type structure in which a mooring facility on the seabed and a floating body are gently connected by mooring cables to maintain the position. In addition, like in Patent Document 1, there is also a Tension Leg Platform (TLP) that forcibly pulls down a floating body with mooring cables to submerge a part of it, generates tension in the mooring cables by the buoyancy of the floating body, and performs position holding. Since the tension leg platform holds the position of the floating body with the tension of the mooring cables, it is easier to miniaturize the floating body compared to the catenary mooring type, and it is also advantageous in that high stability can be obtained due to the tension state of the mooring cables.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The tension leg platform is mooring-supported by providing a plurality of sets of mooring cable bundles each composed of a plurality of mooring cables to each column constituting the floating body. Each mooring cable of each mooring cable bundle may use a steel wire cable made of a material with high elongation rigidity and difficult to elongate. In each mooring cable bundle of one column, it is necessary for each mooring cable to generate tension evenly. If the tension is not even, there will be a state where no tension or only weak tension is generated in a certain mooring cable, and an excessively strong tension will be generated in other mooring cables, resulting in uneven loading on each mooring cable. There is a risk that the mooring cable or support structure with uneven loading will be damaged, and problems will also occur in durability. In order to make the tension generated in each mooring cable even, it is necessary to adjust the length of each mooring cable using large winch equipment or the like. However, using large winch equipment or the like will increase the weight of the installations on the floating body, and the floating body itself has to be enlarged, which will also lead to an increase in manufacturing costs.

[0007] As a configuration that eliminates the need for equipment to adjust the length of the mooring cable, it is conceivable to measure in advance on-site the depth from the lower end of the mooring cable to the top of the pile foundation fixed to the seabed, and to grasp in advance the distance from the mooring cable attachment part that fixes the upper end of the mooring cable on the floating body side to the top of the pile foundation as the required length of the mooring cable. The mooring cable is manufactured on land to the required length, the upper end is fixed by the connection part of the mooring cable on the floating body side, and the lower end is fixed by the top of the pile foundation. However, the length of the mooring cable includes various errors such as measurement errors in measuring the depth to the top of the pile foundation, manufacturing errors in manufacturing the mooring cable, thermal expansion and contraction due to temperature changes, etc. Therefore, by manufacturing the mooring cable in advance to the required length, it is not possible to equalize the tension generated in each mooring cable.

[0008] On the other hand, it is conceivable to manufacture the mooring cable using a material with low elongation rigidity (soft) such as a polyester rope, and absorb the error by the elongation of the rope itself. However, when manufacturing the mooring cable using a material with low rigidity, the spring constant k of the mooring cable becomes small, the natural vibration frequencies of the system in the vertical and roll-pitch directions peculiar to the tension moored floating body become low, and there is a risk of vertical resonance due to the vibration period of the windmill and roll-pitch direction resonance due to the ocean wave period on the ocean.

[0009] When resonance occurs between the ocean wave period and the vibration period of the windmill, there is a risk of damage to the mooring cable and the support structure, and problems also arise in durability.

[0010] Therefore, an object of the present invention is to provide a mooring cable for a tension moored floating body that can equalize the tension generated in each mooring cable without adjusting the length of each mooring cable in each mooring cable bundle that moors and supports the tension moored floating body, and also prevents resonance with the ocean wave period and the vibration period of the windmill. Further, other objects of the present invention will become apparent from the following description.

Means for Solving the Problems

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

[0012] (Claim 1) A mooring cable for a tension mooring floating body that connects between a connection part formed on a tension mooring floating body that supports an offshore wind power generation facility and a subsea mooring part fixed to the seabed, wherein the connection part and the subsea mooring part are connected by the tension mooring cable, and due to the buoyancy generated in the tension mooring floating body, tension is generated in the tension mooring cable, and the tension mooring floating body is configured to be able to be held in a tension mooring state, the tension mooring cable is connected via a coupler between a low-rigidity mooring cable having a low elongation rigidity obtained by multiplying the longitudinal elastic modulus of the cable material by the cross-sectional area and a high-rigidity mooring cable having a higher elongation rigidity than the low-rigidity mooring cable, the cable material of the low-rigidity mooring cable is a resin rope, and the high-rigidity mooring cable is a steel wire cable formed by bundling a plurality of steel strands, the tension mooring cable is characterized in that when the elongation rigidity of the resin rope is set to 1, the elongation rigidity of the steel wire cable is 2 to 5 times the elongation rigidity of the resin rope. A mooring cable for a tension mooring floating body. (Claim 2) A mooring cable for a tension mooring floating body that connects between a connection part formed on a tension mooring floating body that supports an offshore wind power generation facility and a subsea mooring part fixed to the seabed, wherein the connection part and the subsea mooring part are connected by the tension mooring cable, and due to the buoyancy generated in the tension mooring floating body, tension is generated in the tension mooring cable, and the tension mooring floating body is configured to be able to be held in a tension mooring state, the tension mooring cable is connected via a coupler between a low-rigidity mooring cable and a high-rigidity mooring cable having a higher elongation rigidity than the low-rigidity mooring cable, the low-rigidity mooring cable is a resin rope, and the high-rigidity mooring cable is a steel wire cable formed by bundling a plurality of steel strands in parallel, the tension mooring cable is characterized in that 50% or more of the total length is the high-rigidity mooring cable. A mooring cable for a tension mooring floating body. (Claim 3) The mooring rope made of resin is a polyester rope characterized in that a plurality of sub-ropes each formed by twisting a plurality of polyester strands are prepared, and the plurality of sub-ropes are bundled in parallel, which is the mooring cable for a tension mooring floating body according to claim 1 or 2. (Claim 4) The tension mooring floating body includes at least three vertically extending hollow columnar columns arranged in a triangular shape on a plane, and three upper beams connected above the sea surface and three lower beams connected below the sea surface between each of the three columns, and the wind power generation facility is supported by one of the columns, which is the mooring cable for a tension mooring floating body according to claim 1 or 2.

Advantages of the Invention

[0013] According to the present invention, since a part of each mooring cable in each mooring cable bundle for mooring and supporting a tension mooring floating body uses a material with low elongation rigidity, without adjusting the length of each mooring cable, the tension generated in each mooring cable can be equalized by the elongation of the low-rigidity part, preventing damage to the mooring cable and the support structure due to uneven load on each mooring cable, and realizing high durability. Also, according to the present invention, since it is not necessary to adjust the length of each mooring cable, it is not necessary to use large winch equipment, etc., the weight of the installations on the floating body can be reduced, the floating body itself can be miniaturized, and the manufacturing cost can also be reduced.

[0014] Furthermore, according to the present invention, compared with the case where the entire length of each mooring cable uses only a material with low elongation rigidity, the spring constant of the mooring cable is large and the natural frequency is high, so resonance with the wave period on the ocean and the vibration period of the windmill is prevented, damage to the mooring cable and the support structure due to resonance is prevented, and high durability can be realized.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, preferred embodiments of the present invention will be described.

[0017] FIG. 1 is a front view showing an offshore wind power generation facility 1 including a tension moored floating body in which the mooring cable for the tension moored floating body according to an embodiment of the present invention is used. As shown in FIG. 1, the offshore wind power generation facility 1 includes a wind power generation facility 3, a tension moored floating body (Tension Leg Platform (TLP)) 5 on which the wind power generation facility 3 is mounted (hereinafter, simply referred to as a floating body in some cases), a tension mooring cable 7 having an upper end fixed to the tension moored floating body 5, and a subsea mooring part 9 to which the lower end of the tension mooring cable 7 is fixed to the seabed.

[0018] The mooring cable for the tension moored floating body in the present embodiment is a tension mooring cable 7 that forcibly pulls down the tension moored floating body 5 on which the wind power generation facility 3 is mounted to partially submerge it, and holds the tension moored floating body 5 in a fixed position by the tension generated by the buoyancy of the tension moored floating body 5.

[0019] 〔Wind power generation facility〕 The wind power generation facility 3 is a power generation facility that converts wind power into electricity. As shown in FIG. 1, it includes a tower 31, a nacelle 33, a boss 35, and blades 37. In this embodiment, in the wind power generation facility 3, the blades 37 and the boss 35 are rotated by the wind power, and the rotor of the generator to which the boss 35 is connected is rotated by this rotational force, thereby converting the wind power into electricity.

[0020] The tower 31 is a support column that supports the entire wind power generation facility 3, and is a columnar structure extending in the vertical direction. The outer shape of the tower 31 is preferably a cylindrical shape or a conical shape with a diameter expanding downward, but it is not particularly limited as long as the strength to support the entire wind power generation facility 3 can be obtained. The lower end of the tower 31 is connected and supported to a flange portion 5a provided on the upper surface of a portion of the tension mooring floating body 5 that is exposed above the sea surface 101.

[0021] The nacelle 33 is a hollow structure incorporating a generator, is provided at the upper end of the tower 31, and has a spindle shape with its axial direction being 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 about a vertical axis. The nacelle 33 is oriented in the direction of the axial direction of the boss 35 in the direction where the wind pressure is the strongest. Inside the nacelle 33, there are provided a power transmission shaft for transmitting the rotational force of the boss 35, a speed increaser (such as a gearbox) connected to this power transmission shaft, a brake for stopping the power transmission shaft during an emergency (such as a typhoon) or during inspection, and a generator with a rotor 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 about the horizontal direction as the central axis, and is located at the horizontal tip of the nacelle 33.

[0023] The blade 37 is a blade of a windmill that converts wind power into rotational force, and a plurality of blades project radially from the outer circumference of the boss 35 in the radial direction of the boss 35. In this embodiment, three blades 37 project at equal intervals in the circumferential direction of the boss 35. When the blade 37 receives wind, it rotates together with the boss 35 about the boss 35. When the boss 35 rotates, the rotor of the generator rotates, and power generation is performed. The electric power generated by the generator is transmitted to land via a submarine power transmission cable (not shown).

[0024] 〔Tension mooring floating body〕 FIG. 2 is a perspective view showing the tension mooring floating body of FIG. 1. FIG. 2(A) is a perspective view of the tension mooring floating body seen from one side, and FIG. 2(B) is a perspective view of the tension mooring floating body seen from the other side.

[0025] The tension mooring floating body 5 is a floating body that supports the wind power generation facility 3. As shown in FIG. 1, the tension mooring floating body 5 is floating on the ocean, and the wind power generation facility 3 is mounted on the part above the sea surface 101. The tension mooring floating body 5 has a sealed hollow portion inside, so that the specific gravity of the whole outer shape is less than 1, and it floats on the ocean.

[0026] Further, as shown in FIG. 2, the tension mooring floating body 5 includes at least three vertically extending hollow columnar columns arranged in a substantially equilateral triangle shape when viewed from a plane. Also, as shown in FIGS. 1 and 2, between the column 52 and the column 53, above the sea surface, they are connected by an upper beam (bracing) 54, and below the sea surface 101, they are connected by a lower beam (pontoon) 57. Similarly, between the column 51 and the column 52, they are connected by an upper beam (bracing) 55 and a lower beam (pontoon) 59, and between the column 51 and the column 53, they are connected by an upper beam (bracing) 56 and a lower beam (pontoon) 58. In the illustrated example, the column 51 is provided with a flange portion 5a and is configured to support the wind power generation facility 3.

[0027] In the illustrated example, columns 51, 52, and 53 are hexagonal columns and are shown as having a deformed hexagonal shape with a part protruding on a plane. The shape of the column is not limited to this, and it may be a regular hexagonal shape, a pentagonal shape, or a circular shape on a plane.

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

[0029] In the present embodiment, the tension mooring floating body 5 may be provided with ballast tanks (not shown) inside each of the three columns 51, 52, and 53. By injecting and discharging ballast water into and from the ballast tanks, the buoyancy and draft of the tension mooring floating body 5 can be adjusted. When installing the tension mooring floating body 5, by reducing the buoyancy, the attachment of the tension mooring cable 7 can be facilitated.

[0030] Also, FIG. 1 shows an example in which the tension mooring floating body 5 is provided with a total of nine connection parts 5b, which are three sets of three locations each from the respective columns 51, 52, and 53. The upper ends of the tension mooring cables 7 are connected to these connection parts 5b respectively. The lower ends of these tension mooring cables 7 are connected to the subsea mooring parts 9 on the seabed side, whereby the tension mooring floating body 5 is connected to a plurality of subsea mooring parts 9, which are mooring facilities on the seabed 103.

[0031] As a result, due to the buoyancy of the tension mooring floating body 5, a tension is generated in the tension mooring cable 7, and as a result, the tension mooring floating body 5 is held in a fixed position. That is, when the entire tension mooring floating body 5 is submerged in the sea, the buoyancy acting in the opposite direction (upward) of gravity is greater than the gravity acting downward, so it tries to float up to a position where the buoyancy and gravity are balanced. This is because the buoyancy is generated in proportion to the volume of the part of the tension mooring floating body 5 submerged below the sea surface. The tension mooring cable 7 holds the tension mooring floating body 5 at a depth where the buoyancy of the tension mooring floating body 5 becomes greater than the gravity.

[0032] In this state, since the upward floating of the tension moored floating body 5 is blocked by the tension mooring cable 7, the forced buoyancy of the tension moored floating body 5, which is the buoyancy greater than the gravity, is applied to the tension mooring cable 7. Tension is generated in the tension mooring cable 7 to which the forced buoyancy is applied, and a tension mooring state is achieved. In the tension mooring state, the tension moored floating body 5 is held in a fixed position by the tension generated in the tension mooring cable 7.

[0033] In this embodiment, even if there is a rise and fall of the sea surface (change in the seabed depth) due to the tide, a tension within a predetermined range is generated in the tension mooring cable 7, and the tension mooring state is maintained for the tension moored floating body 5.

[0034] 〔Subsea Mooring Part〕 In this embodiment, the subsea mooring part 9 is the top of the pile foundation driven into the seabed 103. The subsea mooring part 9 is a mooring facility on the seabed 103 that holds the tension moored floating body 5 in a fixed position via the tension mooring cable 7. The subsea mooring part 9 is provided with a connecting part such as a hook or a ring to which the lower end of the tension mooring cable 7 is connected.

[0035] As long as the subsea mooring part 9 has a structure that does not come out of the seabed 103 due to the tensile force applied from the tension mooring cable 7 when holding the position of the tension moored floating body 5 with forced buoyancy and has corrosion resistance that does not easily corrode in the sea, other known mooring facilities can also be used. Specifically, a gravity anchor, a pile anchor, a suction anchor, etc. can also be used. In this embodiment, since the length of the tension mooring cable 7 is not adjusted, any method that can reliably fix the position of the lower end of the tension mooring cable 7 is acceptable, and it is not particularly limited.

[0036] In this embodiment, for one offshore wind power generation facility 1, nine subsea mooring parts 9 are provided. The nine subsea mooring parts 9 are grouped into three sets with three parts in each set being close to each other, and are installed on the seabed in the vertical direction of each of the columns 51, 52, and 53 of the tension moored floating body 5, and a total of three sets of subsea mooring parts 9 are provided. In this embodiment, three connection parts 5b are provided on each of the columns 51, 52, and 53. The upper end of the tension mooring cable 7 is fixed to the connection part 5b on the tension mooring floating body side, and the same number of seabed mooring parts 9 as the connection part 5b are installed from the connection part 5b to the seabed in the vertical direction. Three sets of mooring cable bundles each consisting of three tension mooring cables 7 are fixed to the connection part 5b and the seabed mooring part 9. As a result, the upper end of the tension mooring cable 7 is fixed to the connection part 5b provided on the tension mooring floating body 5, the lower end of the tension mooring cable 7 is fixed to the seabed mooring part 9, and the tension mooring floating body 5 is moored and held.

[0037] In this embodiment, the number of seabed mooring parts 9 and the number of tension mooring cables 7 may be appropriately set within a range in which the tensile force applied from the tension mooring cables 7 can be dispersed to such an extent that the tension mooring floating body 5 is not pulled out from the seabed 103 when the tension mooring floating body 5 is held in a fixed position by the forced buoyancy. For example, for one offshore wind power generation facility, six seabed mooring parts 9 and six tension mooring cables 7 may be used, and the tension mooring floating body 5 may be moored and held by three sets of mooring cable bundles each consisting of two tension mooring cables 7.

[0038] 〔Tension Mooring Cable〕 The tension mooring cable 7 is a mooring cable that connects the tension mooring floating body 5 and the seabed mooring part 9. The tension mooring floating body 5 is kept in a tension mooring state by the tension generated by the buoyancy of the tension mooring floating body 5, and the tension mooring floating body 5 is held in a fixed position.

[0039] FIG. 3 is a side view of the mooring cable for the tension mooring floating body of FIG. 1. As shown in FIG. 3, the tension mooring cable 7 of this embodiment is formed by connecting a high-rigidity mooring cable 7a made of a material with high elongation rigidity and a low-rigidity mooring cable 7b made of a material with low elongation rigidity in series by a coupler 7c. The coupler 7c is a device that connects the upper end of the high-rigidity mooring cable 7a and the lower end of the low-rigidity mooring cable 7b, and various types such as hook-shaped and ring-shaped ones can be used, and its configuration is not particularly limited. The tension mooring cable 7 has a strength that does not yield or break under the tension generated by the buoyancy of the tension mooring floating body 5 and corrosion resistance that does not easily corrode in the sea.

[0040] FIG. 4 is a cross-sectional view of the mooring cable (high-rigidity mooring cable) for the tension mooring floating body of FIG. 1. In this embodiment, as shown in FIG. 4, the high-rigidity mooring cable 7a is a steel wire cable, and the steel wire cable is composed of a plurality of strands (zinc-plated steel wires) 71a arranged in parallel, and the outer periphery of these strands is covered by a protective layer 70a. In this embodiment, the high-rigidity mooring cable 7a has, for example, about 200 to 400 strands and a coating diameter of, for example, about 100 to 200 mm. In this embodiment, in the illustrated example, an example in which a plurality of strands are bundled in parallel is shown, but it is not limited thereto, and as long as a plurality of strands are bundled, they do not have to be in parallel.

[0041] The low-rigidity mooring cable 7b is preferably a resin rope. Examples of the material of the resin rope include polyamide, polyester, etc. Examples of polyamide include nylon 6, nylon 66, nylon 11, nylon 12, etc. As the resin rope, a polyester rope is preferred. The polyester rope can be exemplified by bundling a plurality of sub-ropes in which strands (for example, linear bodies made of polyester) are twisted in parallel, winding a sand filter, and covering the outside with a protective layer. The protective layer can be exemplified by being woven or knitted with polyester threads. In this embodiment, the low-rigidity mooring cable 7b has a diameter of, for example, about 280 mm.

[0042] In this embodiment, in the example of FIG. 1, the low-rigidity mooring cable 7b is attached to the connection part 5b on the floating body side, and the high-rigidity mooring cable 7a is attached to the seabed mooring part 9 on the seabed side, but the low-rigidity mooring cable 7b may be attached to the seabed mooring part 9 on the seabed side, and the high-rigidity mooring cable 7a may be attached to the connection part 5b on the floating body side.

[0043] This tension mooring cable 7 can achieve both relaxation of the length accuracy requirement during installation and avoidance of resonance of the tension mooring floating body 5. The length accuracy requirement during installation refers to the allowable range of length error. Relaxing the accuracy requirement means that the allowable range of error is wider. As described above, in one set (three pieces) of the tension mooring cables 7, it is necessary for the tension to be evenly generated in each tension mooring cable 7. If there is a large error in the length of each tension mooring cable 7, the tension may not be generated or only a weak tension may be generated in the long tension mooring cable 7, while an excessively strong tension may be generated in the short tension mooring cable 7, resulting in uneven loading on each tension mooring cable 7. There is a risk that the short tension mooring cable 7 with excessive tension and the support structure may be damaged, and problems may also occur in terms of durability. In the tension mooring cable 7 of the embodiment, if the length error is within the allowable range, the low-rigidity mooring cable 7b of the short tension mooring cable 7 will stretch, so that tension will also be generated in the long tension mooring cable 7, and an excessively strong tension will not be generated in the short tension mooring cable 7.

[0044] As described above, the resonance of the tension moored floating body 5 occurs in the vertical direction due to the vibration caused by the rated rotational speed of the wind turbine, and in the roll-pitch direction, it occurs due to the wave period. Regarding the resonance with the wave period, it may occur when the spring constant k of the tension mooring cable 7 is small and the natural vibration frequency F0 (Hz) of the system in the roll and pitch directions is low. It may also occur similarly in the vertical direction. In this embodiment, a large wind turbine (10 MW or more) may be supported by a TLP. In this case, the resonance between the vertical direction and the rotation period of the wind turbine, and the resonance between the roll-pitch direction and the wave period become problems. In the tension mooring cable 7 of the present embodiment, regardless of the rotation period of the windmill to be installed, since the low-rigidity mooring cable 7b and the high-rigidity mooring cable 7a are connected via the coupler 7c, it is a hybrid mooring cable of a low-rigidity mooring cable and a high-rigidity mooring cable. Therefore, the elongation rigidity can be arbitrarily changed. As a result, the axial rigidity of the mooring cable, which is in a proportional relationship with the elongation rigidity, can also be arbitrarily changed, and the natural period can be changed. Consequently, resonance can be avoided. For example, the wave period often has a high appearance frequency in the range of 4 to 5 seconds or more to about 20 seconds. It is necessary to shift the wave period with a high appearance frequency and the natural period. Since the tension mooring cable of the present invention is a hybrid mooring cable as described above, as a result of being able to arbitrarily change the rigidity, the natural period can be changed. As a result, the natural period in the roll-pitch direction can be made to be within a period of 4 seconds or less, so that resonance with the wave period can be prevented from occurring.

[0045] Regarding the tension mooring cable 7 of the embodiment, the steady tension (4,900 kN (500 tons)) generated per one, the total length (50 m), the elongation rigidity (661,500 kN) of the polyester rope (low-rigidity mooring cable 7b), and the elongation rigidity (2,142,000 kN) of the steel wire cable (high-rigidity mooring cable 7a) are shown in Table 1 below. The elongation rigidity is the product of the longitudinal elastic modulus (Young's modulus) of the cable material and the cross-sectional area. In Table 1, an example is shown in which the high-rigidity mooring cable 7a has an elongation rigidity more than three times that of the low-rigidity mooring cable 7b. In the tension mooring cable 7 of the present invention, when the elongation rigidity of the low-rigidity mooring cable 7b is set to 1, the elongation rigidity of the high-rigidity mooring cable 7a is preferably in the range of 2 to 5 times. By forming the tension mooring cable 7 within such a range, it is possible to achieve both relaxation of the installation accuracy requirements and avoidance of resonance due to wave periods and vibrations of the windmill.

[0046]

Table 1

[0047] As described above, it has been explained that by the ratio of the elongation rigidity between the low-rigidity mooring cable 7b and the high-rigidity mooring cable 7a in the tension mooring cable 7, it is possible to achieve both relaxation of the installation accuracy requirements and avoidance of resonance caused by wave periods and vibrations of the windmill. However, by the ratio of the lengths of the low-rigidity mooring cable 7b and the high-rigidity mooring cable 7a in the tension mooring cable 7, it is also possible to achieve both relaxation of the construction installation accuracy and avoidance of resonance with the wave period. Hereinafter, embodiments based on the ratio of the lengths of the low-rigidity mooring cable 7b and the high-rigidity mooring cable 7a will be described.

[0048] FIG. 5 is a graph showing the relationship between the length of the low-rigidity mooring cable and the combined elongation in the mooring cable for the tension mooring floating body of FIG. 1. The relationships shown in FIGS. 5 and 6 show the case where the low-rigidity mooring cable 7b and the high-rigidity mooring cable 7a shown in Table 1 are used. As shown in FIG. 5, the length of the low-rigidity mooring cable (polyester rope) 7b with respect to the total length of 50 m is taken on the horizontal axis, and the elongation of the tension mooring cable 7 when an axial force of 4,900 kN acts when the low-rigidity mooring cable 7b and the high-rigidity mooring cable 7a are connected is shown. Note that when the length of the low-rigidity mooring cable 7b at the left end is 0 m, it means that all are high-rigidity mooring cables 7a, and conversely, the 50 m at the right end are all low-rigidity mooring cables 7b. From FIG. 5, the greater the proportion of the low-rigidity mooring cable 7b, the greater the elongation of the tension mooring cable 7 linearly. For example, as in the illustrated example, when the total length of the tension mooring cable is 50 m and an elongation of 200 mm or more is required in the tension mooring cable 7 for relaxation of the construction installation accuracy, the length of the low-rigidity mooring cable 7b can be made 17 m or more to achieve relaxation of the construction installation accuracy.

[0049] FIG. 6 is a graph showing the relationship between the length of the low-rigidity mooring cable and the combined spring constant in the mooring cable for the tension mooring floating body of FIG. 1. As shown in FIG. 6, the length of the low-rigidity mooring cable (polyester rope) 7b is taken on the horizontal axis, and the result of the combined spring constant when the low-rigidity mooring cable 7b and the high-rigidity mooring cable 7a are connected is shown. As shown in Fig. 6, the greater the proportion of the low-rigidity mooring cable 7b and the smaller the proportion of the high-rigidity mooring cable 7a, the more non-linearly the synthesized spring constant increases. For example, as in the illustrated example, if a spring constant of 20,000 kN / m or more is required for the tension mooring floating body 5 to avoid resonance with the roll and pitch motions of the ocean wave period, the length of the low-rigidity mooring cable 7b can be set to 25 m or less to avoid resonance between the wave period and the roll-pitch motion.

[0050] As described above, considering the elongation and spring constant of the tension mooring cable 7, when the total length of the tension mooring cable 7 is 50 m, by setting the length of the low-rigidity mooring cable 7b in the range of 17 m to 25 m (34% - 50%), it is possible to achieve both relaxation of the installation accuracy requirements and avoidance of resonance of the tension mooring floating body 5.

[0051] In this embodiment, when the water depth is deep and the total length of the tension mooring cable 7 becomes longer, the natural vibration frequency F0 (Hz) of the tension mooring floating body 5 becomes lower. In this case, in order to avoid resonance with the ocean wave period, the proportion of the high-rigidity mooring cable 7a when the total length of the tension mooring cable 7 is 50 m may be increased. Regarding the requirements for construction and installation accuracy due to the increase in the proportion of the high-rigidity mooring cable 7a in this embodiment, since the high-rigidity mooring cable 7a also elongates in proportion to the total length, even if the proportion of the high-rigidity mooring cable 7a is increased, the requirements for construction and installation accuracy can be relaxed.

[0052] From the above, the deeper the water depth and the longer the total length of the tension mooring cable 7, the shorter (and the lower the ratio) the preferable length of the low-rigidity mooring cable 7b becomes. Therefore, the proportion of the high-rigidity mooring cable can be increased in length (and the ratio) as the water depth increases, so that the length of the low-rigidity mooring cable can be shortened.

[0053] On the other hand, in the embodiments shown in Figs. 5 and 6, the proportions of the high-rigidity mooring cable 7a and the low-rigidity mooring cable 7b in the tension mooring cable 7 when the water depth (distance from the seabed mooring part to the connection part of the floating body) is about 50 m are shown. When the water depth becomes shallower, the proportion of the high-rigidity mooring cable 7a can be decreased and the proportion of the low-rigidity mooring cable 7b can be increased.

[0054] The present invention has been described with reference to the embodiments, but the present invention is not limited to the embodiments. It is natural for those skilled in the art to conceive various modifications and improvements within the scope of the technical idea of the present invention, and these are also included in the present invention.

Explanation of Reference Numerals

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

Claims

1. A mooring cable for a tension mooring floating body that connects between a connection part formed on a tension mooring floating body that supports an offshore wind power generation facility and a subsea mooring part fixed to the seabed, wherein the connection part and the subsea mooring part are connected by the tension mooring cable, so that due to the buoyancy generated in the tension mooring floating body, tension is generated in the tension mooring cable, and the tension mooring floating body is configured to be able to be held in a tension mooring state, the tension mooring cable is connected via a coupler between a low-rigidity mooring cable having a low elongation rigidity obtained by multiplying the longitudinal elastic modulus of the cable material by the cross-sectional area and a high-rigidity mooring cable having a higher elongation rigidity than the low-rigidity mooring cable, the cable material of the low-rigidity mooring cable is a resin rope, and the high-rigidity mooring cable is a steel wire cable formed by bundling a plurality of steel strands, the tension mooring cable is characterized in that when the elongation rigidity of the resin rope is set to 1, the elongation rigidity of the steel wire cable is 2 to 5 times the elongation rigidity of the resin rope. A mooring cable for a tension mooring floating body.

2. A mooring cable for a tension mooring floating body that connects between a connection part formed on a tension mooring floating body that supports an offshore wind power generation facility and a subsea mooring part fixed to the seabed, wherein the connection part and the subsea mooring part are connected by the tension mooring cable, so that due to the buoyancy generated in the tension mooring floating body, tension is generated in the tension mooring cable, and the tension mooring floating body is configured to be able to be held in a tension mooring state, the tension mooring cable is connected via a coupler between a low-rigidity mooring cable and a high-rigidity mooring cable having a higher elongation rigidity than the low-rigidity mooring cable, the low-rigidity mooring cable is a resin rope, and the high-rigidity mooring cable is a steel wire cable formed by bundling a plurality of steel strands in parallel, the tension mooring cable is characterized in that 50% or more of the total length is the high-rigidity mooring cable. A mooring cable for a tension mooring floating body.

3. The polyester rope according to claim 1 or 2, wherein the resin rope is a polyester rope formed by preparing a plurality of sub-ropes formed by twisting a plurality of polyester strands and bundling the plurality of sub-ropes in parallel. A mooring cable for a tension mooring floating body.

4. The tension mooring floating body includes at least three vertically extending hollow columnar columns arranged in a triangular shape on a plane, and includes three upper beams connected above the sea surface and three lower beams connected below the sea surface between the three columns respectively. The mooring cable for the tension mooring floating body according to claim 1 or 2, characterized in that the wind power generation facility is supported by one of the columns.

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