Optical fiber measurement device for monitoring bolt axial force and optical fiber measurement system for monitoring bolt axial force

Abstract

An optical fiber measurement device (100) for monitoring bolt axial force comprises: an optical fiber (23) which is for measuring the strain of a body being measured; a fiber mounting and fixing body (25) which includes a fiber mount nut (21S), which is obtained by altering the shape of the outer peripheral portion of a nut (21) of one among a plurality of sets of bolts (20) and nuts (21) and installing the optical fiber (23) thereon; and an optical fiber strain distribution measurement unit (24) which measures the distribution of the hoop strain of the fiber mount nut (21S) caused by the axial force of a bolt along the optical fiber (23) mounted on the outer peripheral surface of the fiber mount nut (21S). The hoop strain and the distribution of the hoop strain of the fiber mount nut (21S) are measured in advance by the optical fiber strain distribution measurement unit (24), and the optical fiber (23) is mounted at a prescribed position in the axial direction that corresponds to the value of the hoop strain or the distribution of the hoop strain as measured in advance to monitor the bolt axial force.

Description

Optical fiber measurement device for monitoring bolt axial force and optical fiber measurement system for monitoring bolt axial force

[0001] The present disclosure relates to an optical fiber measurement device for monitoring axial bolt tension and an optical fiber measurement system for monitoring axial bolt tension.

[0002] As the use of wind turbines for wind power generation expands, problems associated with long-term use are being identified. For example, in the operation of wind turbine facilities, 20 to 30 percent of wind turbines are destroyed over a 10-year period due to blades falling or flying off. Analysis of the causes of these destructions has revealed that a large proportion of these destructions are due to loosening of bolts used in the connections that secure the components of the wind turbine, such as the connection between the blades and the nacelle (the blade mounting body) and the connection between the nacelle and the tower (the nacelle support). In analyzing the causes of destruction, particular attention has been paid to loosening of bolts, which reduces the axial force applied to the bolts (see, for example, Non-Patent Documents 1 and 2).

[0003] Therefore, to prevent such bolt loosening (reduction in axial force) and breakage, and to detect such bolts early if they do occur, it is necessary to check the installation status of the bolts at the joints, for example by measuring the bolt axial force through inspection or testing. Furthermore, to improve the situation in which the bolts loosen as described above, periodic inspections are also necessary in addition to initial inspections of wind power generation equipment. In light of this situation, periodic inspection guidelines have recently been established regarding inspection methods for wind power generation equipment (see Non-Patent Document 3).

[0004] On the other hand, inspecting and checking for loose or broken bolts is very costly, posing a major problem that affects the profitability of wind power generation. The total number of bolts used in the joints of the wind turbines mentioned above can be as many as 50 or more per joint. Therefore, it is currently difficult to inspect all bolts in the joints, such as their axial tension, during periodic inspections. Access to wind power generation facilities, taking into account safety and economic efficiency, is an urgent issue, especially for offshore facilities.

[0005] Previously, there was a wireless technology for measuring bolt axial force (see, for example, Non-Patent Document 4), but this used a nut equipped with a special mechanism that dynamically deforms its shape, and was not compatible with ordinary nuts. Furthermore, there was no conventional method for managing the axial torque of bolts used in wind power generation facilities that could be used for the entire facility, and only methods that could be used for individual inspections (see Non-Patent Document 5).

[0006] "Kyoto Prefecture Taikoyama Wind Farm Units 1 and 3 Bolt Damage Investigation Report," May 29, 2014. [online], [Retrieved September 25, 2023], Internet <URL: https: / / www.meti.go.jp / shingikai / sankoshin / hoan_shohi / denryoku_anzen / newenergy_hatsuden_wg / pdf / 003_01_01.pdf>, "Chubu Electric Power Co., Inc. Omaezaki Wind Farm Unit 3 Blade Mounting Bolt Fracture (Final Report)," July 30, 2015. [online], [Retrieved September 25, 2023], Internet <URL: https: / / www.meti.go.jp / shingikai / sankoshin / hoan_shohi / denryoku_anzen / newenergy_hatsuden_wg / pdf / 007_04_00.pdf>, Japan Electric Association "Guidelines for Periodic Inspection of Wind Power Generation Facilities," edited by the Power Generation and Transformation Special Committee, JEAG5005-2017, pp.11, 15, 16, 20, 22, 24, 27, 31, April 1, 2017 (first edition), NRJ_LST product catalog, [online], [searched September 25, 2023], Internet <URL: https: / / www.nord-lock.com / globalassets / mediavalet / web-assets / downloads / brochure / 00297_sb_lst_4-pager-jp-web.pdf>, NejiLaw catalog, [online], [searched September 25, 2023], Internet <URL: https: / / www.nejilaw.com / special_smartNeji.html>, Technical Report, "Definition of the IEA Wind 15-Megawatt Offshore Reference Wind Turbine," IEA Wind TCP Task Force 37, pp. 25-28, March 2020.

[0007] Japanese Patent Application Laid-Open No. 2010-216877

[0008] As briefly explained above, conventional wind power generation facilities use 80 to 300 bolts per blade. Therefore, for a three-blade system, the number of bolts used is as high as 200 to 900. While technically, bolt axial force testing is required every six months, regulations for actual onshore wind power generation facilities stipulate that only 20% of the bolts be inspected annually. Furthermore, for offshore wind power generation facilities, the requirements become even stricter during bad weather due to the increased cost of offshore access (reducing inspection frequency).

[0009] Furthermore, regarding the accuracy of quantitative inspections, there has traditionally been a problem with the lack of precision in inspecting bolt looseness. For example, there is a method of measuring bolt axial tension using ultrasonic waves, but this method has a measurement error of 10% or more.

[0010] Furthermore, in qualitative inspections, currently the main method of inspection for loose bolts is visual inspection. Although it is possible to use drones for offshore access, the essential viewpoint of measurement remains visual inspection.

[0011] The present disclosure discloses technology related to the inspection and testing of wind power generation equipment to solve the problems described above, and aims to enable the measurement system used in the inspection, such as that for bolt axial force, to be able to perform its own maintenance, and to automatically inspect the installation status, including axial force, of all bolts used in the components that make up the wind power generation equipment, even from a long distance.

[0012] The optical fiber measurement system for monitoring bolt axial tension disclosed herein is an optical fiber measurement device for monitoring bolt axial tension, in which a fixed object is fixed with a fixture composed of multiple sets of bolts and nuts, and the bolt axial tension acting on the bolt is monitored using an optical fiber, comprising: an optical fiber used for measuring strain on the object to be measured; a fiber attachment fixture having a fiber attachment nut with the optical fiber attached, the shape of the nut being modified so that the optical fiber is attached to the outer periphery of at least one of the multiple sets of standard bolt and nut combinations; and an optical fiber strain distribution measurement unit that measures the hoop strain distribution in the axial direction of the fiber attachment nut caused by the axial tension of the bolt along the optical fiber attached to the outer periphery surface of the fiber attachment nut, wherein the optical fiber strain distribution measurement unit measures the hoop strain and hoop strain distribution along the axial direction of the fiber attachment nut in advance, and the optical fiber is attached at a predetermined axial position corresponding to the previously measured hoop strain value or the hoop strain distribution to monitor the bolt axial tension.

[0013] According to the optical fiber measurement system for monitoring bolt axial force disclosed herein, the measurement system itself can monitor itself and can automatically inspect the installation status, including axial force, of all bolts used in the components that make up wind power generation equipment, even from a long distance.

[0014] 6A ; FIG. 6B is a block diagram showing an example of an application of the optical fiber measurement system for monitoring axial bolt tension according to a first embodiment; FIG. 6C is a diagram for explaining the equipment configuration of the wind turbine of FIG. 1; FIG. 6D is a diagram for explaining details of a joint portion between a blade and a hub inside part C of FIG. 2A; FIG. 6E is a diagram showing an example of an optical fiber measurement device for monitoring axial bolt tension according to a first embodiment; FIG. 6F is a diagram showing an example of a nut used as a fiber mounting fixture of the optical fiber measurement device for monitoring axial bolt tension according to a first embodiment; FIG. 6G is a diagram showing another example of a nut used as a fiber mounting fixture of the optical fiber measurement device for monitoring axial bolt tension according to a first embodiment; FIG. 6H is a diagram showing another example of a nut used as a fiber mounting fixture of the optical fiber measurement device for monitoring axial bolt tension according to a first embodiment; FIG. 6H is a diagram showing an example of a fiber mounting nut used in the optical fiber measurement device for monitoring axial bolt tension according to a second embodiment; FIG. 6H is a block diagram showing an example of a configuration of an optical fiber measurement system for monitoring axial bolt tension according to a second embodiment; Figure 1 shows another example of a two-port module used in the optical fiber measurement system for monitoring axial bolt force according to embodiment 2. Figure 2 shows a diagram for explaining a method of connecting an optical fiber and a two-port module used in the optical fiber measurement system for monitoring axial bolt force according to embodiment 2. Figure 3 shows an example of connection between an optical fiber and a two-port module used in the optical fiber measurement system for monitoring axial bolt force according to embodiment 2. Figure 4 shows a diagram for explaining the relationship between a two-port module used in the optical fiber measurement system for monitoring axial bolt force according to embodiment 2 and measurement data corresponding to the installation position of the total optical fiber line. Figure 5 shows a diagram for explaining the installation state of optical fibers when measuring bolt axial force collectively in the optical fiber measurement system for monitoring axial bolt force according to embodiment 2.12. FIG. 13 is a diagram for explaining a data management method in the optical fiber measurement system for monitoring bolt axial force according to embodiment 2. FIG. 14 is a diagram for explaining a test apparatus for measuring the hoop strain distribution generated in a nut in order to evaluate the bolt axial force. FIG. 15 is a diagram showing the relationship between the fiber length (position from a reference point) of the optical fiber wound around the nut of the test apparatus and the hoop strain when the bolt axial force is changed, measured with the test apparatus of FIG. 12. FIG. 16 is an explanatory diagram of a common coordinate system for comparing the hoop strain values ​​measured with the test apparatus of FIG. 12 with analytical values ​​obtained from an FEM analysis model. FIG. 17 is a diagram comparing the hoop strain values ​​measured using the test apparatus of FIG. 12 with the hoop strain values ​​obtained from the analysis model. FIG. 18 is a diagram showing the measurement results of the relationship between the bolt axial force and strain when the axial position of the nut is changed, measured with the test apparatus of FIG. 12. FIG. 19 is a diagram showing an example of the measurement of the amount of strain change when the bolt axial force is increased, measured with the test apparatus of FIG. 12. FIG. 19 is a diagram showing an example of the measurement of the amount of strain change when the bolt axial force is decreased, measured with the test apparatus of FIG. 12. Fig. 13 is a diagram showing another example of measurement of the amount of strain change when the bolt axial force is reduced, measured using the testing device of Fig. 12. Fig. 14 is a diagram showing an example of the configuration of an optical fiber measurement system for monitoring the bolt axial force according to a third embodiment.

[0015] The present disclosure relates to a measurement system for monitoring bolt axial tension using optical fibers, which is involved in the maintenance and inspection of wind turbines, including those used for offshore wind power generation. The present disclosure will be described below with reference to the drawings, taking a representative embodiment as an example.

[0016] First Embodiment An application example of an optical fiber measurement device 100 for monitoring bolt axial tension according to a first embodiment will be described with reference to FIGS.

[0017] Figure 1 is a block diagram showing the overall configuration of an offshore wind power generation facility, which is an application example of the optical fiber measurement device 100 for monitoring bolt axial tension according to embodiment 1. In Figure 1, the wind power generation facility includes a floating offshore wind power site consisting of multiple wind turbines 1, a fixed structure 2 consisting of a combination of bolts and nuts (not shown) for fixing the blades of the wind turbines 1 to hubs, measurement optical fibers (not shown, but described in detail below) wound around the nuts to dynamically measure changes in the axial tension of the bolts (at least once per second), and a dynamic cable 5 (see the dotted curve in Figure 1) several hundred meters long, the majority of which is installed underwater, for transmitting measurement signals measured by the optical fiber via a floater connection part 3 and a buoy 4 to a measurement equipment station 9 installed on land. The measurement signal transmitted from the floating substation 7 through this dynamic cable 5 is then transmitted to the measuring instrument station 9 through a cable connection 6 which is a connection between a static cable laid on the seabed and the dynamic cable provided in the wind power generation facility, a static cable 8 (total length approximately 30 km) also laid on the seabed, etc. The depth of the ocean where the wind turbines 1, floating substations 7, etc. are located is usually in the range of 50 m to 200 m, but it is also expected that they will be installed at depths of 3,000 m in the future.

[0018] As will be explained in detail below, the measurement equipment station 9 has a measurement signal analysis system that measures Brillouin and Rayleigh backscattered light due to strain generated in the bolt based on the measurement signal of the bolt axial force measured by the optical fiber. This allows for the frequency shift of the optical fiber measurement signal due to strain generated in the bolt to be determined, thereby enabling the distribution measurement of the amount of change in temperature and strain along the optical fiber, even when the temperature and strain change simultaneously (see, for example, Patent Document 1).

[0019] Although the above description has been given using an example in which there are multiple wind turbines, the present invention is not limited to this and there may be only one wind turbine. Furthermore, the optical fiber may be an optical fiber embedded in a power transmission line that supplies power obtained from a wind power generation facility.

[0020] Signals related to power generation generated by the wind turbines 1 of the wind power generation facility are sent to a substation 10 via the floating body connection unit 3, dynamic cable 5, buoy 4, cable connection unit 6, floating substation 7, static cable 8, etc., and then transmitted to a grid 13 symbolized by a steel tower via an underground power transmission cable 11 (with a total length of several tens of kilometers) and a switchyard 12. Therefore, hereinafter, the dynamic cable 5 and static cable 8 may be collectively referred to as a power transmission line. In this case, the power transmission line incorporates optical fiber that combines bolt axial force measurement and communication functions.

[0021] Next, the main equipment configuration of the wind turbine 1 of the wind power generation facility to which the optical fiber measurement device for monitoring bolt axial tension of the first embodiment is applied will be described with reference to Figures 2A and 2B. Here, Figure 2A is a diagram for explaining the equipment configuration of the wind turbine of Figure 1, and Figure 2B is a diagram for explaining the details of the joint between the blade and the hub inside part C of Figure 2A.

[0022] As shown in Fig. 2A, a wind turbine 1 generally comprises four main components: blades 15 that rotate when exposed to wind, a hub 16 that connects the blades to a rotating shaft, a nacelle 17 that houses a gearbox, a generator, etc., connected to the hub 16 via the rotating shaft, and a tower 18 that is fixedly supported by the nacelle 17. As shown in Fig. 2B, the joint 22 between the blades 15 and the hub 16 (inside part C in Fig. 2A) is fixed using a fiber attachment fixture 25 (described in detail below) that is made up of a combination of bolts 20 and nuts 21S to which fibers, described in detail below, are attached.

[0023] In the optical fiber measurement device for monitoring bolt axial tension according to the first embodiment, the joint 22 between the blade 15 and the hub 16, shown as two black and white doughnut-shaped disks in FIG. 2B , is more specifically fastened and fixed using a plurality of sets of bolts 20 and fiber attachment nuts 21S, each set combining a bolt 20 and a fiber attachment nut 21S to which an optical fiber for strain sensing is attached. In this case, the fixture that fixes the joint 22 is a fiber attachment fixture 25. Hereinafter, the joint 22 will also be referred to as a fixed body 22. The term "joint 22" is not limited to the joint between the blade and the hub, but also refers to the joint between the nacelle and the tower.

[0024] Next, the main components of the optical fiber measurement device 100 for monitoring bolt axial tension according to the first embodiment will be described with reference to the conceptual diagram of FIG. 3 . In FIG. 3 , the fixed object 22 is fixed by the fiber mounting fixture 25. In this case, the fixed object 22 is fixed by applying an axial load, i.e., an axial force (also referred to as bolt axial force), to the bolt 20 (the bolt shaft is indicated by a dashed line) that constitutes the fiber mounting fixture 25 and the fiber mounting nut 21S, whose fiber mounting diameter (the diameter of the fiber-wrapped portion), i.e., the diameter of the axial center portion of the outer periphery, is approximately the same as the nut outer diameter but slightly smaller. The fiber mounting fixture 25 may also include combinations of standard bolts 20 and standard nuts 21, in addition to combinations of standard bolts 20 and standard nuts 21. For reference, a top view of the bolt 20 is shown above the bolt 20, and a bottom view of the fiber mounting nut 21S is shown below the fiber mounting nut 21S. The dotted circle in this bottom view indicates the fiber mounting diameter.

[0025] An optical fiber 23 is wound multiple times around the outer periphery of the fiber attachment nut 21S to measure the hoop strain generated in the nut due to the bolt axial force (see FIG. 4D described below). The signal measured by the wound optical fiber 23 is processed by a Rayleigh-type and Brillouin-type optical fiber strain distribution measurement unit 24 (the optical fiber strain distribution measurement unit 24 has a processor for calculations and a memory for data storage; details will be described below), and the strain distribution state is found as the strain at a predetermined position of the nut around which the optical fiber 23 is wound, thereby monitoring the axial force applied to the bolt.

[0026] Next, the structure of the nut used in the fiber mounting nut 21S used in the fiber mounting fixture 25 will be described below with reference to FIGS. 4A to 4D. First, FIG. 4A illustrates the structure of a nut 21a, which has a shape equivalent to a standardized product (e.g., an M30 nut) but has been modified (by forming a new groove, as described below) for use in mounting an optical fiber. The top view is shown at the top, and the front view is shown at the bottom. In the front view, the portion designated by the letter E indicates the nut body, and the portion designated by the letter G indicates a groove formed in the nut body for mounting an optical fiber. Only this groove portion differs in structure from a standardized nut. Furthermore, this groove is formed on the upper surface (surface B, which contacts the fixed body 22; see "B" in FIG. 3) of the two upper and lower surfaces of the nut in the axial direction of the nut. The dotted line in the top view indicates the bottom surface of the groove G. As shown in this figure, the outer shape (peripheral shape) of the bottom surface of the groove G is circular with a diameter Dg. As shown in the front view, this groove is formed near the top surface of the nut with a width of Lg and a depth of (De-Dg) / 2, where De is the size of the nut, Dr is the nominal diameter of the nut, and Lw is the thickness of the nut.

[0027] Next, the structure of another nut used as a fiber mounting nut will be described below with reference to Figures 4B to 4D. First, the nuts shown in Figures 4B and 4C will be described. Both nuts shown in Figures 4B and 4C have the same thickness (Lw) as the nut 21a shown in Figure 4A. A groove of the same or similar shape as that shown in Figure 4A is formed on the surface B that contacts the fixed body 22. The upper surface also has a flange portion (brim portion) F with a larger outer circumference than the nut with the standard-equivalent shape (Da, Db > De). The thickness of the flange portion is Lf, which is thinner than the thickness of the nut with the standard-equivalent shape (Lf < Lw). Nut 21b shown in Figure 4B has the same shape as the standard-equivalent nut except for the lower portion, which is thinner. Nut 21c shown in Figure 4C has a truncated cone-shaped lower portion. These nuts 21b and 21c have flange portions (brim portions), which are expected to suppress the occurrence of bolt shaft breakage and the like caused by hoop strain in these portions.

[0028] Next, a fiber attachment nut 21Sa, in which a fiber is wound around a nut 21a having a structure equivalent to that shown in FIG. 4A, will be described with reference to FIG. 4D. FIG. 4D is a conceptual diagram showing a fiber attachment nut 21Sa in which an optical fiber is wound once or multiple times around a spiral groove, 1 mm wide and approximately 0.3 mm deep, formed on the cylindrical outer surface of the nut, over almost the entire axial direction of the nut. A waterproof film is applied to the surface of the optical fiber. Using such a fiber attachment nut as a fiber attachment fixture allows accurate detection of the intersection point between positive and negative hoop strain values ​​when the bolt axial force is loosened (details will be described below). The optical fiber used for this application is selected to have high bending resistance and low splice loss.

[0029] Second Embodiment Next, a configuration example of a measurement system to which the optical fiber measurement system 101 for monitoring bolt axial force according to the second embodiment is applied will be described using the block diagram of Fig. 5. In this diagram, dotted arrows represent electrical signal lines, and solid arrows represent optical signal lines. First, the relationship between the components indicated by the arrow symbol a will be described.

[0030] The optical fiber measurement system for monitoring bolt axial force of the present disclosure comprises a monitoring control and management system 30, which is a computer having a processor and memory for monitoring, controlling, and maintaining wind power generation equipment; an optical fiber strain distribution measurement unit 24 that receives command signals from this monitoring control and management system 30 and emits signals to start measuring the axial force of bolts that are components of the wind turbine fixed body 2 (this signal includes a signal to command measuring the hoop strain distribution that occurs in a specified nut from among the nuts set to the above-mentioned special shape); an optical switch 31 that specifies the measurement locations of the axial force of the wind turbine fixed body 2 and switches and outputs a command signal for selection control when monitoring each of multiple wind turbines together with the signal from the optical fiber strain distribution measurement unit 24; a power line-embedded optical fiber 32 having an optical fiber embedded in the above-mentioned power line to which the signal from the optical switch 31 is input; and a signal from this power line-embedded optical fiber 32 that is input and transmits all of these locations to a monitoring optical fiber that is arranged in a continuous line in order to measure the specified measurement locations. The signal from this unicursal type monitoring optical fiber 33 is extended to a rotary joint 36 for connecting to the rotating blades, for example, via a base plate nacelle 35 (also called a bedplate nacelle 35), which is a device that connects fixed devices and rotating devices among the components of the wind turbine, using a flexible optical fiber cable 34 for optical communication between the wind turbine tower and the rotating bodies of the wind turbine such as the blades, and then connected to the axial force monitoring optical fiber 23 via a blade monitoring optical fiber 37. Here, the base plate nacelle 35 is a nacelle with a unique shape for improving the power generation efficiency of the wind turbine.

[0031] Next, the relationship between the components indicated by the arrow b in an example configuration of an optical fiber measurement system for monitoring bolt axial force according to embodiment 2 will be described. A signal due to hoop strain of a nut caused by axial force applied to a bolt, which is a component of the wind turbine fixed body 2, is detected by the axial force monitoring optical fiber 23 and transmitted in this order through the blade monitoring optical fiber 37, rotary joint 36, base plate nacelle 35, flexible optical fiber cable 34, uniaxial connection type monitoring optical fiber 33, and power line embedded optical fiber 32, before finally being sent via the optical switch 31 to the optical fiber strain distribution measurement unit 24. The optical fiber strain distribution measurement unit 24, which receives the signal due to the nut hoop strain, has a processor and memory, and calculates the hoop strain of the object fixed body (fixed body) to be monitored using the Brillouin method or the Rayleigh method, corresponding to each required measurement location. The result is stored in memory, and the required data signal is sent to the monitoring control and management system 30 in response to a command from the monitoring control and management system 30. The monitoring control and management system 30 also stores the data transmitted from the optical fiber strain distribution measurement unit, and monitors the bolt axial force based on the stored data.

[0032] An example of the above-mentioned continuous-line monitoring optical fiber 33 is described in detail below with reference to the drawings. As mentioned above, wind power generation facilities use more than 10 bolts per blade, and it is difficult to say that inspection of such a large number of bolts is being carried out adequately. Therefore, an improvement in the inspection method is necessary to improve this situation. To achieve this goal, an optical fiber measurement system that measures bolt axial tension all at once is described below with reference to Figures 6A to 11. The realization of such a system shortens the time required to inspect bolts, leading to the ability to inspect more bolts during regular inspections.

[0033] First, an example of hardware for realizing the optical fiber measurement system for monitoring bolt axial tension according to the second embodiment will be described with reference to Figures 6A to 8B. Figures 6A and 6B are diagrams for explaining the total optical fiber line, which is the first component of the main components of the hardware for realizing the optical fiber measurement system for monitoring bolt axial tension according to the second embodiment. Figures 7A, 7B, and 7C are diagrams for explaining the two-port module, which is the second component of the main components of the hardware. Figures 8A and 8B are diagrams showing a method of connecting the two main components and a typical connection example.

[0034] First, the first component, the optical fiber total line 53, will be described with reference to Figures 6A and 6B. Figure 6A is a cross-sectional view of a power cable 50 (also called a power transmission line 50) with a power transmission cable 51 disposed at the center. Figure 6B is an enlarged perspective view illustrating the detailed structure of the optical fiber total line 53 shown in Figure 6A, which will be described in detail below.

[0035] 6A, a plurality of optical fiber total lines 53 each containing a plurality of optical fibers 23 are installed on the outer periphery of a power cable 50, and are arranged in a form sandwiched between a large number of armor wires 52. Also, as shown in FIG. 6B, a plurality of the optical fibers 23 (optical fiber F1 and optical fiber F2 shown in this figure) are arranged on the outermost periphery of the optical fiber total line 53.

[0036] Next, the two-port module, which is the second component, will be described in order using Figures 7A, 7B, and 7C. First, Figure 7A is a conceptual diagram of the two-port module. The two ports Pa and Pb in Figure 7A are separately connected to monitoring targets such as nuts, blades, and power cables, and are distinguishable by connection IDs (also simply referred to as IDs, where ID is an abbreviation for identifier). Ports Pa and Pb are distinguishable by color, length, etc., and are optical fibers long enough to be spliced ​​together. After the optical fibers are installed, the initial values ​​are the results of Brillouin and Rayleigh measurements made at the factory.

[0037] Next, Fig. 7B shows a hoop strain measurement module using a nut, which is an example of a two-port module, and similarly has two different ports Pa and Pb. Fig. 7C shows a blade deformation measurement module, another example of a two-port module, which has ports on the pressure side (PS side) and suction side (SS side) of a wind turbine blade. These two ports are referred to as port Pp and port Ps, respectively.

[0038] In the optical fiber measurement system for monitoring bolt axial force shown in Figure 5, the above-mentioned optical fiber line and two-port module are connected to each other in the form shown in Figure 8A, and the optical fiber line and module are combined and used in the form shown in Figure 8B, making it possible to monitor on a module basis.

[0039] Figure 8A is a diagram for explaining a method of connecting multiple monitoring objects, such as nuts, blades, and power cables, to optical fiber when the modularized monitoring objects are measured and monitored using optical fiber, which is a monitoring method.

[0040] In FIG. 8A, the optical fiber 23 provided on the outer periphery of the total optical fiber line and taken out from the total optical fiber line and two ports to be monitored (for example, port Pa and port Pb) are connected at a connection point C. P and C Q and are combined with each other.

[0041] Next, a specific connection example will be described with reference to Fig. 8B. In Fig. 8B, an optical fiber (see optical fiber F2 shown in Fig. 6B) is taken out from the outer periphery of the total optical fiber line 53, and is connected and coupled by the connection method described above in Fig. 8A using one port of a hoop strain measurement module (module M1, see Fig. 7B), such as a fiber mounting nut (connection point C shown in Fig. 8B). FM8B). Furthermore, this module M1 is coupled to another point of the optical fiber F2 at another port of the module M1. Furthermore, the optical fiber F2 is coupled to the module M2 at another point different from the above two points. Next, another port of the module M2 is connected to one port of the module M3 (connection point C shown in FIG. 8B). MM In the same manner, the module M3 and the module M4 are connected, and further, the module M4 and the optical fiber F2 are connected at another point.

[0042] In this way, the optical path of the light becomes a "single-stroke connection" as shown by the thick solid line in Fig. 8B. By utilizing this single-stroke connection, it is possible to monitor a large number of modules to be monitored using a single optical fiber.

[0043] 9 shows an example of the measurement signal measured at each module, corresponding to the position of each module, when multiple modules are monitored using a single optical fiber connected in one stroke using the above-mentioned coupling method. In this figure, the measured Brillouin frequency, shown as a dotted curve, is shown corresponding to the positions (shown as thick solid lines) of each module represented by symbols M1 to M4 and the total optical fiber line (in this Figure 9, the total optical fiber line is abbreviated as "total line").

[0044] As shown in Figure 9, the measured Brillouin frequency has a step at each connection point. Then, based on the characteristics of the curves in each module, each module can be identified through correlation analysis. By storing and analyzing the measured Brillouin frequency over time using the method described above, it is possible to monitor the aging and other changes of multiple monitoring targets using a single optical fiber.

[0045] 10 is a diagram showing a specific example in which the above-mentioned single optical fiber connected in one stroke is attached to a number of nuts, which are hoop strain measurement modules. In this Fig. 10, within a rectangular frame D surrounded by a dashed line, a group of elliptical shapes connected by straight lines and straight line portions protruding from both ends are an example of the above-mentioned single-stroke type monitoring optical fiber 33 (see Fig. 5) in which the optical fiber 23 is used.

[0046] For the sake of convenience, an example has been shown in which the monitoring optical fiber 33 is configured in a one-stroke connection manner only on a number of nuts within the elliptical group, but this is not limited to this example, and the one-stroke connection type monitoring optical fiber 33 may be configured on almost all nuts.

[0047] 10 shows a case where only the fiber mounting fixture 25 indicated by the dotted rectangular frame is designated as the measurement target. In other words, fixtures other than those indicated by the dotted line are not designated as measurement targets, so in this case, it is sufficient to select only the relevant portion of the measured signal.

[0048] Next, one method of data management when managing each module using the above-mentioned one-stroke connection monitoring optical fiber will be described with reference to Fig. 11. Fig. 11 shows an example of the results of managing the physical quantities (e.g., hoop strain) of multiple modules (M1 to M4) to be managed as the amount of change from the initial state, and determining whether the managed modules are normal or abnormal based on the magnitude of that amount of change.

[0049] In Figure 11, the leftmost column shows fiber coordinates, such as the axial distance of a nut, assuming equidistant coordinates. The top row shows the names of the segments to be managed and the names of management items such as elapsed time. Here, Monitor 1 represents elapsed time 1, Monitor 2 represents elapsed time 2, and Monitor 3 represents elapsed time 3, with the larger the number, the longer the elapsed time.

[0050] 11 shows that all modules except for module 2 (M2) are normal regardless of the elapsed time, but module 2 is abnormal at the monitoring 2 stage (for example, the bolt axial tension is loose). Therefore, for module 2, maintenance work (for example, tightening the axial tension of the fixed body) was performed after elapsed time 2 of monitoring 2, and as a result, module 2 returned to normal at monitoring 3 (elapsed time 3). Note that, since the maintenance work for this module 2 is expected to involve replacement of bolts and nuts, etc., it is determined that it is necessary to reset the measurement position, i.e., reset the coordinate distance of the fiber coordinates.

[0051] Next, the configuration of a test device for measuring the hoop strain distribution generated in a nut to evaluate the axial force of a bolt, which is a component of the fixing body 2, implemented to determine the formation position of the groove in the fiber attachment nut shown in Figures 3 and 4D above will be described with reference to Figure 12.

[0052] FIG. 12 is a diagram showing the configuration of a test device for measuring the hoop strain distribution generated in a nut by a bolt axial force in order to evaluate the bolt axial force when an object (also called a fixed body) is fixed in the optical fiber measurement device for monitoring a bolt axial force according to embodiment 1.

[0053] As shown in Figure 12, this testing device uses an optical fiber strain distribution measuring instrument 40 that is capable of measurements using both the Brillouin method and the Rayleigh method. By using an optical fiber strain distribution measuring instrument of this type, it is possible to simultaneously measure the amount of change in temperature and strain even when the temperature and strain of the object being measured change simultaneously (see, for example, Patent Document 1). The optical fiber strain distribution measuring instrument 40 has a processor for calculations and a memory for storing measurement signals.

[0054] The reason for using such an optical fiber distribution measuring instrument equipped with both the Brillouin and Rayleigh methods is that when a static cable is laid deep underground as shown in Figure 1 above, it is possible that, in terms of measurement accuracy, it may be necessary to consider not only the deformation of the cable due to strain, but also the deformation of the cable due to temperature changes that affects the measurement signal.

[0055] The method for checking the corresponding part is to apply a temperature, deformation, or vibration signal to the part, and check the strain generated in the optical fiber at that time from the signal measured by the optical fiber strain distribution measuring instrument 40. In other words, after the fiber-mounted fixture is installed on the fixture, the position of the part can be checked at any time.

[0056] As shown in Figure 12, the test equipment includes an ultrasonic axial force meter 41 for confirming and verifying the bolt axial force, an ultrasonic transducer 42 for transmitting the ultrasonic signal from the ultrasonic axial force meter 41 with as little attenuation as possible, a bolt tensioner 43 for generating a high axial force in the bolt with high precision, a nut with an optical fiber sensor 44 (here, a fiber with a length of approximately 10 m was used), and a temperature-compensating nut 45 (a nut with an optical fiber for temperature compensation wrapped around its periphery, similar to the optical fiber sensor nut 44). Note that during the test, bolts 20 and standard-shaped nuts 21, which are components of the fixed body 2 and have a nominal diameter equivalent to that used in actual wind turbine applications, are used simultaneously. In this case, as in Figure 3, the surface in contact with the fixed body 22 in Figure 12 is referred to as Surface B (see "B" in Figure 3), and the surface of the nut opposite Surface B is referred to as Surface A.

[0057] Next, the measurement results obtained using this testing device will be explained in order below using Figure 13 and Figures 15 to 19. Figure 13 shows the hoop strain distribution detected by an optical fiber wound around a nut of this testing device, with the bolt axial force as a parameter. The horizontal axis represents the distance (unit: m) from a specified plane along the optical fiber, and the vertical axis represents the hoop strain value (unit: με). In this test, the bolt axial force was varied from 0 to 300 kN (shown as a test condition parameter in Figure 13).

[0058] As shown in Figure 13, when the axial force conditions are the same, it can be seen that the strain value on side B is greater than the strain value on side A. Furthermore, the greater the axial force, the smaller the amount of strain change tends to be. In other words, it can be seen that the strain generated in the nut is not linear with respect to the axial force. However, it can be seen that the intersection point of the positive and negative strain values ​​(approximately 10 m in fiber length) does not change even when the axial force changes. In this test device, a total of 72 turns of optical fiber are wound around the optical fiber sensor nut 44 (each turn is approximately 141 mm long, and the resolution of the measuring device is set to less than half of one turn).

[0059] Next, the hoop strain values ​​measured with this testing device were compared with the analytical values ​​obtained using a 3D FEM analysis model. First, the coordinate system used in both cases will be explained using Figure 14. In Figure 14, the axial distance of the nut is shown as coordinate Z (unit: mm). Here, the Z range for comparison was from 0 mm (corresponding to the position of surface B in Figure 3) to 20 mm (corresponding to the position of surface A in Figure 3). Additionally, a circumferential coordinate θ was defined for the nut circumferential angle to compare the two. In this comparison, the values ​​at θ = 0° (see the data indicated by the black circle in Figure 15 below) and θ = 180° (see the data indicated by the white circle in Figure 15 below) were compared.

[0060] Next, using Figure 15, we compare the hoop strain values ​​measured with this testing device with the hoop strain values ​​calculated from the FEM analysis 3D model. The horizontal axis represents the axial distance (Z, as described above, in mm). The vertical axis represents the hoop strain value (in με). Figure 15 shows the results of a comparison between two cases: an axial force of 300 kN and a nut circumferential angle of θ = 0° (see the solid line for the measured value) and θ = 180° (see the dotted line for the measured value).

[0061] Because the diameter of the optical fiber used in the test was 0.25 mm, it was possible to obtain the strain distribution for the first time when the spatial resolution of the nut hoop strain was 0.25 mm. The characteristics of this test are that, regardless of the axial force, the hoop strain is positive at the contact end of the nut (the nut expands), and negative at the free end of the nut (the nut contracts), and the position of the zero strain point between these two positions (approximately 14 mm in the axial direction in Figure 15) does not change.

[0062] In this disclosure, we have developed a method for monitoring bolt axial force by utilizing the characteristics of the positive and negative hoop strain distribution and the zero strain position. While the characteristics of a nut's hoop strain distribution vary depending on the nut size or standard, the characteristics (positive and negative hoop strain distribution and zero strain position) are believed to remain constant for the same nut size or shape. Therefore, values ​​at individual measurement locations can be measured when installing a bolt axial force strain distribution measuring device and monitored as a managed object. As described in Figure 18 below, when the bolt axial force decreases, i.e., when the bolt tightening force loosens, the hoop strain distribution changes significantly, and the zero point where the positive and negative strains intersect changes significantly compared to the previous example. The cause of this is unknown, but it is thought to be due to changes in frictional conditions.

[0063] Here, to estimate the strain at a specific location, the Z value is the result of spline interpolation of measurement data every 50 mm. The numerical analysis results are those obtained using a three-dimensional FEM model. As shown in Figure 15, the two results are in agreement in terms of order of magnitude.

[0064] 13 and 15, it was found that when a fixed body is constructed using a standard-shaped nut, which is a standard product such as the one shown in Fig. 4A, unevenness in the magnitude of strain occurs depending on the axial position of the nut, and as a result, there is a possibility that a specific position in the axial direction of the bolt or nut will be subjected to a load greater than the rated load, and that this location will be the side of the nut that comes into contact with the fixed body when fastened. Note that such unevenness in the magnitude of strain can also cause cracks in the bolt shaft, so it is necessary to check this by inspection, etc.

[0065] In other words, it is presumed that the above-described factors are the cause of bolt loosening, breakage, etc., and therefore, in order to improve this situation, we have also proposed the use of specially shaped nuts, whose outer peripheral shape is modified from the standard shape, as components of the fixed body in wind turbines. Specifically, we have proposed the specially shaped nuts shown in Figures 4B and 4C.

[0066] Incidentally, since the purpose of the present disclosure is to monitor loosening of bolt axial force in wind turbine equipment and the like by measuring hoop strain, next, in order to obtain the relationship between bolt axial force and hoop strain, the measurement results when the measurement position (here, the axial position of the nut) is changed are organized and shown in Figure 16.

[0067] Figure 16 shows the relationship between bolt axial force and hoop strain, particularly at the initial circumferential angular position of 0°. The results show spline interpolation of measurement data every 50 mm to estimate hoop strain at a specific location. Note that similar values ​​to those at the 0° position were also measured at the 180° angular position.

[0068] First, Figure 16 also shows that the hoop strain value varies significantly depending on the axial position of the nut, regardless of the bolt axial force. It also shows that the closer the position from the surface of the nut to the object to be fixed, i.e., the smaller the axial position of the nut, the greater the hoop strain value. In other words, it shows that measuring at such a position provides better measurement sensitivity. It also shows that the strain generated in the nut is not linear with respect to the bolt axial force, and that the greater the axial force, the smaller the change in hoop strain. Furthermore, as mentioned above, there was almost no difference in the measured values ​​due to differences in the circumferential measurement position.

[0069] Note that a positive hoop strain value (measured values ​​at axial positions of 2.02 m, 6.96 m, and 11.90 m in this figure) means that the outer periphery of the nut is expanding, and a negative hoop strain value (measured values ​​at axial position of 18.92 m in this figure) means that the outer periphery of the nut is shrinking.

[0070] Next, we investigated the relationship between the change in bolt axial force and the change in strain. First, for the case where the bolt axial force was increased, we measured the change in fiber length (measurement position) and the change in hoop strain when the change in bolt axial force was set to 50 kN. Furthermore, since it has been proven that detecting bolt loosening is important, we measured the change in fiber length (measurement position) and the change in hoop strain when the change in bolt axial force was set to 10 kN and 5 kN to confirm how small a change in bolt axial force would cause strain. These results are explained below using Figures 17 to 19.

[0071] First, Figure 17 shows the measurement results of the amount of change in hoop strain when the bolt axial force is increased. In this case, the value of the range of change in the bolt axial force when the amount of change in the decrease in the bolt axial force is set to a constant value of 50 kN is used as a parameter, and the change range of the parameter for the change in axial force is set to six levels, from 0-50 kN to 250-300 kN. In this case, it can be seen that the amount of change in strain at position B is greater than the amount of change in strain at position A. It can also be seen that the position where the positive and negative strain values ​​intersect is at a fiber length of approximately 10 m.

[0072] Next, the measurement results of strain change during a reduction in bolt axial force will be described using Figures 18 and 19. Figure 18 shows the distribution of hoop strain change detected by an optical fiber wound around the nut of this test device, with the range of bolt axial force change as a parameter when the reduction in bolt axial force is set to a constant value of 10 kN. In this test, the range of change in the parameter for axial force change was from 290-300 kN to 250-260 kN (also shown as the parameter for axial force change in Figure 18). Here, the vertical axis represents strain change in the range from -40 με to 40 με. Note that the horizontal axis in Figure 18 is set to the same as in Figure 13, so its explanation will be omitted here.

[0073] Figure 19 shows another example of the measurement of strain change during a decrease in bolt axial force, measured using this testing equipment. Figure 19 shows the distribution of hoop strain change detected by an optical fiber wound around the nut of this testing equipment, with the range of change in bolt axial force as a parameter when the decrease in bolt axial force is set to a constant value of 5 kN. In this test, the range of change in the parameter for axial force change was from 245-250 kN to 225-230 kN (also shown in Figure 19 as a parameter for axial force change). The horizontal and vertical axes in Figure 19 are set in the same way as in Figure 18, so their explanation is omitted here.

[0074] As shown in Figures 18 and 19, the change in hoop strain at position B (see plane B in Figure 12) when the axial force is decreased is smaller than the value at position A (see plane A in Figure 12). Figure 18 also shows that the crossover point between positive and negative strain is located near a fiber length of approximately 14 m, regardless of the axial force change parameter. Furthermore, this position is significantly different from the position when the axial force is increased. This suggests a difference in the frictional force generated between the nut and bolt in response to the axial force. Figures 18 and 19 also show that the change in hoop strain when the axial force is changed appears to correspond to the range (magnitude) of the axial force change. Based on the above, the detection accuracy of the axial force change is estimated to be approximately 5 kN to 10 kN.

[0075] Embodiment 3. In the first and second embodiments, a system was proposed that improves bolt loosening by using a fiber attachment nut as a component of the fixed body. The optical fiber measurement system 102 for monitoring bolt axial tension in this embodiment 3 can achieve more accurate and reliable monitoring and maintenance of bolt axial tension than existing maintenance and inspection methods. This system will be described below using Figure 20.

[0076] 20, a fixed body 22 is fixed by a bolt 20 and the fiber attachment nut 21S shown in Fig. 3. An optical fiber 23 for measuring hoop strain is wound around the outer periphery of this nut 21S, and a signal due to strain generated in this nut is detected by this optical fiber 23 and analyzed by an optical fiber strain distribution measurement unit 24 to determine the strain distribution and the like.

[0077] At this time, during the initial setting in which the object to be fixed is fixed with the bolt 20 and the fiber attachment nut 21S, data such as strain distribution detected by the optical fiber 23 and analyzed by the optical fiber strain distribution measurement unit 24 is transmitted to a computer 26 which has a processor and memory and processes and outputs input data, and after this data is calculated as necessary, it is stored and saved in a first database (hereinafter abbreviated as the first DB) as initial data such as strain distribution. Here, axial forces of various magnitudes commanded by the computer 26 are applied to the bolt 20 by a bolt tensioner or the like (not shown), and each piece of data when the axial force is changed as a parameter is organized as a function of the bolt axial force and saved in the first database.

[0078] In addition, the hoop strain values ​​at each axial position of the fiber mounting nut 21S measured by the optical fiber strain distribution measuring unit 24 are stored and saved in a second database (hereinafter abbreviated as the second DB) at intervals of several fixed periods from the initial setting commanded by the computer 26.

[0079] In this case, the plurality of fixed periods are determined based on existing maintenance standards as periods shorter than the periodic inspection periods specified therein, and are stored and saved in the second database. Data relating to the plurality of fixed periods is retrieved by the computer and used when measuring the hoop strain at each axial position of each nut.

[0080] Then, based on the hoop strain data at each axial position of each nut measured at multiple fixed intervals and stored in the second database, the hoop strain data for each nut shape at the time of regular inspection as stipulated in existing maintenance standards is separated and estimated values ​​at the time of regular inspection are calculated by computer 26, and these are stored and saved in the second database as maintenance data and are used as data to be referenced during regular inspection.

[0081] Furthermore, the hoop strain occurring in the fiber attachment nut 21S is measured by the optical fiber strain distribution measurement unit at predetermined time intervals, and the measured data is stored in the measurement unit and in a computer as measured data related to the elapsed time. The computer then compares the stored measured data with the initial data stored in the first database or the maintenance data stored in the second database, thereby creating a maintenance plan for the fixed body.

[0082] Although the present disclosure describes various exemplary embodiments and examples, the various features, aspects, and functions described in one or more embodiments are not limited to the application of a particular embodiment, but may be applied to the embodiments alone or in various combinations. Accordingly, countless variations not illustrated are contemplated within the scope of the technology disclosed herein. For example, variations include modifying, adding, or omitting at least one component, or extracting at least one component and combining it with components of other embodiments. For example, the above description assumes an optical fiber having one core, but this is not limiting; similar effects are also achieved in multi-core optical fibers having two or more cores.

[0083] 1 Wind turbine, 2 Fixed body, 3 Floating body connection part, 4 Buoy, 5 Dynamic cable, 6 Cable connection part, 7 Floating substation, 8 Static cable, 9 Measuring instrument station, 15 Blade, 16 Hub, 17 Nacelle, 18 Tower, 20 Bolt, 21, 21a, 21b, 21c Nut, 21S, 21Sa Fiber mounting nut, 22 Fixed body (joint), 23 Optical fiber (axial force monitoring optical fiber), 24 Optical fiber strain distribution measurement part, 25 Fiber mounting fixture, 30 Monitoring control and management system, 31 Optical switch, 32 Optical fiber built into power transmission line, 33 Optical fiber for one-stroke connection type monitoring, 34 Flexible optical fiber cable, 35 Baseplate nacelle, 36 Rotary joint, 37 Blade monitoring optical fiber, 40 Optical fiber strain distribution measurement instrument, 41 Ultrasonic axial force meter, 42 Ultrasonic transducer, 43 Bolt tensioner, 44 Nut with optical fiber sensor, 45 Temperature compensation nut, 50 Power cable, 51 Power transmission cable, 52 Armored wire, 53 Total optical fiber wire, 100 Optical fiber measurement device for monitoring bolt axial force, 101, 102 Optical fiber measurement system for monitoring bolt axial force, M1, M2, M3, M4 Module

Claims

1. In a bolt axial force monitoring optical fiber measuring device that fixes an object to be fixed with a fixing body composed of multiple sets of bolts and nuts, and monitors the bolt axial force generated in the bolts using an optical fiber, Optical fibers used for measuring the strain of an object under test, The shape of the nut is modified so that the optical fiber is attached to the outer circumference of the nut relating to at least one combination of the plurality of bolts and nuts, and a fiber mounting fixture having a fiber mounting nut with the optical fiber attached is provided. An optical fiber strain distribution measuring unit measures the hoop strain distribution of the fiber mounting nut caused by the axial force of the bolt, along the optical fiber mounted on the outer surface of the fiber mounting nut, Equipped with, An optical fiber measuring device for monitoring bolt axial force, characterized in that the hoop strain and hoop strain distribution of the fiber mounting nut are measured in advance by the optical fiber strain distribution measuring unit, and the optical fiber is mounted at a predetermined position in the axial direction of the fiber mounting nut according to the previously measured hoop strain value or hoop strain distribution, and the bolt axial force is monitored.

2. The optical fiber measuring device for monitoring bolt axial force according to claim 1, characterized in that the optical fiber is mounted at a predetermined axial position of the fiber mounting nut according to the hoop strain distribution measured in advance, the hoop strain distribution is measured by the optical fiber strain distribution measuring unit to determine the position where the positive and negative values ​​of the hoop strain distribution intersect, and the bolt axial force is monitored based on the amount of change at the position where the positive and negative values ​​of the strain distribution intersect.

3. The optical fiber measuring device for bolt axial force monitoring according to claim 1 or 2, characterized in that the shape of the nut on which the optical fiber is mounted in the fiber mounting nut is cylindrical, and the predetermined axial position on which the optical fiber is mounted in the fiber mounting nut is set on the side of the upper and lower axial surfaces of the fiber mounting nut that fixes the object to be fixed.

4. The optical fiber measuring device for bolt axial force monitoring according to claim 1 or 2, characterized in that the optical fiber is arranged and attached to the fixing body in a manner that measures the bolt axial force generated in the bolts of the fixing body, which is fixed with multiple sets of bolts and the fiber mounting nuts, all at once.

5. The optical fiber measuring device for bolt axial force monitoring according to claim 3, characterized in that the optical fiber is arranged and attached to the fixing body in a manner that measures the bolt axial force generated in the bolts of the fixing body, which is fixed by a plurality of sets of bolts and the fiber mounting nuts, all at once.

6. In a bolt axial force monitoring optical fiber measurement system that fixes an object to be fixed with a fixing body composed of multiple sets of bolts and nuts, and monitors the bolt axial force generated in the bolts using optical fibers, Optical fibers used for measuring the strain of an object under test, The shape of the nut is modified so that the optical fiber is attached to the outer circumference of the nut relating to at least one combination of the plurality of bolts and nuts, and a fiber mounting fixture having a fiber mounting nut with the optical fiber attached is provided. An optical fiber strain distribution measuring unit measures the axial hoop strain distribution of the fiber mounting nut caused by the axial force of the bolt, along the optical fiber mounted on the outer surface of the fiber mounting nut, A total optical fiber cable is arranged on the outer surface portion of a power cable that transmits electricity, and has two or more optical fibers inside, A two-port module that can be identified by ID and can store physical quantities measured using the Rayleigh method and the Brillouin method with optical fibers provided in the optical fiber network, Equipped with, The optical fibers of the optical fiber bundle are taken out and connected to two different locations on the two-port module, the hoop strain and hoop strain distribution of the fiber mounting nut are measured in advance by the optical fiber strain distribution measurement unit, and the measured data is stored in the two-port module. A bolt axial force monitoring optical fiber measurement system characterized by mounting the optical fiber at a predetermined axial position of the fiber mounting nut according to the hoop strain value or hoop strain distribution stored in the two-port module, thereby monitoring the bolt axial force.

7. A bolt axial force monitoring structure module that fastens the fixed object by combining the fiber mounting nut with each of the bolts of the fixed object which is composed of multiple sets of bolts and nuts, and by applying bolt axial force, A computer having a processor and memory, which processes input data and outputs the result, In the initial setup when the fixed object is fixed, the system includes a bolt tensioner that can set the bolt axial force of the bolt axial force monitoring structure module to a predetermined value, and a first database that stores the measured data as initial data, which is used to measure the hoop strain at each axial position of each fiber mounting nut in relation to the bolt axial force when the bolt axial force is changed within a predetermined range as a parameter, using the optical fiber strain distribution measurement unit. A second database stores the values ​​of hoop strain at various axial positions of the fiber mounting nut over multiple fixed periods, measured for a period shorter than the periodic inspection period specified in the existing maintenance standards, after a certain period has elapsed since the initial setup, and stored as maintenance data. Equipped with, The optical fiber measurement system for monitoring bolt axial force according to claim 6, characterized in that, at predetermined elapsed time intervals, the hoop strain generated in the nut is measured by the optical fiber strain distribution measurement unit and stored in the optical fiber strain distribution measurement unit and the computer as measured data related to the elapsed time, and the maximum axial force for each fiber mounting nut obtained from the measured data stored by the computer is compared with the maximum axial force for each corresponding fiber mounting nut from the initial data stored in the first database, or the maximum axial force for each corresponding fiber mounting nut obtained from maintenance data after a certain period of time stored in the second database, to monitor the fixed body.

8. The optical fiber measurement system for bolt axial force monitoring according to claim 6 or 7, characterized in that the fixed object is a component of a floating offshore wind power site installed on the sea, and the optical fiber strain distribution measurement unit is installed at a remote location at a distance of approximately 10 km or more from the floating offshore wind power site.