Distributed stress monitoring system and monitoring method suitable for marine mooring cables

Abstract

The application discloses a distributed stress monitoring system and method suitable for a marine mooring cable, and belongs to the field of stress monitoring. The system comprises a stress monitoring cable embedded in the neutral axis position of the mooring cable and a signal demodulation module electrically connected with the stress monitoring cable through an end sealing connection assembly. The stress monitoring cable comprises a transmission optical fiber, a plurality of groups of sensing-compensation double grating groups and a composite packaging layer, the sensing-compensation double grating groups are adhered or engraved on the transmission optical fiber at intervals, the interval between two adjacent groups of sensing-compensation double grating groups is 10-50 cm, and the length of the stress monitoring cable is 1-15 km. The distributed stress monitoring system and method suitable for the marine mooring cable have the advantages of a kilometer-level super-long monitoring distance, a 10 cm-level high spatial resolution, high matching with the mechanical properties of the cable, precise temperature compensation, strong adaptability to long-term marine environment, and the like, and can realize high-precision distributed stress and strain monitoring of the cable in the whole life cycle.

Description

Technical Field

[0001] This invention relates to the field of stress monitoring technology, and in particular to a distributed stress monitoring system and method applicable to marine mooring cables. Background Technology

[0002] The safe and stable operation of large floating offshore platforms (such as FPSOs and semi-submersible platforms) heavily relies on the reliable connection between multiple large mooring cables and the seabed or seafloor foundation. As the core load-bearing carrier between the platform and the seabed foundation, the mooring cables must operate for extended periods in extremely complex marine environments. They must withstand continuous erosion from corrosive media such as high salinity, high humidity, and strong corrosion, as well as the repeated effects of dynamic loads from periodic waves, currents, and wind, while simultaneously bearing the weight of the platform itself and the transmission of operational loads. Under these harsh operating conditions, mooring cables are prone to fatigue damage, corrosion, and aging. If these issues are not monitored and warned of in a timely manner, failure can directly threaten the platform's main structure, upper-level production facilities, and personnel safety, leading to major safety accidents and substantial economic losses. Therefore, achieving accurate online stress and strain monitoring of large offshore mooring cables throughout their entire lifespan, and capturing their strain distribution characteristics and early damage signals in real time along their entire length, has become one of the key technical challenges urgently needing to be addressed in the field of marine engineering structural health monitoring.

[0003] Currently, strain monitoring technologies for mooring cables are mainly divided into two categories, but their technical characteristics are significantly mismatched with the monitoring needs of large marine mooring cables, making it difficult to meet engineering application requirements: 1. Resistance Strain Gauges and Strain Scale Sensors: These sensors were widely used strain measurement devices in the past, but their deployment and working principles have inherent limitations, making them unsuitable for monitoring large mooring cables. Specifically, on the one hand, due to installation limitations, these sensors are usually only attached to the outer surface of the cable or near the anchoring end, with sparse and fixed deployment points. This results in the acquisition of strain data from only a small number of discrete measurement points, failing to comprehensively reflect the continuous distributed strain state along the entire length of the long-distance mooring cable. Furthermore, they are difficult to capture early failure characteristics such as local stress concentration and minor damage, leading to a serious lack of completeness and representativeness of the monitoring data. On the other hand, these sensors rely on electrical signals for the transmission and conversion of strain information. However, harsh conditions in the marine environment, such as high salt spray and seawater immersion, as well as unavoidable electromagnetic interference during long-distance transmission, severely damage the stability and accuracy of the electrical signals. This not only increases measurement errors but also accelerates the aging and damage of the sensors themselves, resulting in extremely poor long-term reliability. They are prone to problems such as measurement data drift, signal interruption, or even complete failure, making them unable to support the continuous monitoring needs of the mooring cable throughout its entire life cycle.

[0004] 2. Traditional Fiber Bragg Grating (FBG) Strain Monitoring Technology: Although conventional FBG sensors possess high strain sensitivity and electromagnetic interference resistance, demonstrating certain technical advantages in short-distance structural monitoring, they still face significant technical bottlenecks in the kilometer-scale, high-resolution monitoring scenarios of large marine mooring cables. Specifically, firstly, in terms of monitoring range and spatial resolution, these sensors are typically installed using single-point deployment or a small number of series connections, limiting their effective monitoring length to the hundreds of meters. Furthermore, the spacing between sensor points is generally greater than 1m-3m, which is severely mismatched with the actual length of large mooring cables (several kilometers) and the need for fine-scale monitoring of key local areas (such as terminal anchorages and joint transition sections). This makes it impossible to achieve high spatial resolution distributed monitoring across the entire length of kilometer-scale mooring cables, and makes it difficult to accurately identify early damage signals such as localized stress concentrations. Secondly, in terms of packaging structure and mechanical compatibility, the packaging forms of conventional FBG sensors (such as metal packaging or single polymer packaging) fail to fully consider the complex twisted structure and dynamic load-bearing characteristics of large mooring cables, resulting in insufficient mechanical matching between the packaging layer and the cable body. The periodic stretching and bending deformation of the cable during service will cause significant interface stress concentration between the sensor packaging layer and the cable load-bearing rope bundle. Under long-term repeated action, it is easy to cause problems such as interface debonding and fatigue cracking of the packaging layer, which in turn leads to strain transmission distortion, measurement data drift, and ultimately causes the monitoring system to fail, making it unable to meet the long-term stable monitoring requirements in extreme marine environments. Summary of the Invention

[0005] The purpose of this invention is to provide a distributed stress monitoring system and method suitable for marine mooring cables, thereby solving the aforementioned technical problems.

[0006] To achieve the above objectives, the present invention provides a distributed stress monitoring system suitable for marine mooring cables, comprising a stress monitoring cable pre-embedded in the neutral axis position inside the mooring cable and a signal demodulation module electrically connected to the stress monitoring cable via an end-sealing connection assembly. The stress monitoring cable includes a transmission optical fiber and multiple sets of sensing-compensation dual grating groups spaced apart and adhered or inscribed on the transmission optical fiber, and a composite encapsulation layer wrapped around the multiple sets of sensing-compensation dual grating groups. The spacing between two adjacent sets of sensing-compensation dual grating groups is 10cm-50cm, and the length of the stress monitoring cable is 1km-15km.

[0007] Preferably, the sensing-compensation dual-grating group comprises sensing gratings and temperature compensation gratings arranged at intervals, encapsulated within a quartz glass tube. Within the same sensing-compensation dual-grating group, both the sensing gratings and temperature compensation gratings are weakly reflective gratings with a reflectivity of 0.1%-0.5%. The center wavelength difference between the sensing gratings and the temperature compensation gratings is 5nm-10nm, and the center wavelengths of all sensing-compensation dual-grating groups are located in the 1520nm-1560nm band. The center wavelength interval between adjacent sensing-compensation dual-grating groups is 0.5nm-2nm. This achieves accurate differentiation and identification of each sensing-compensation dual-grating group through a composite coding method combining wavelength division multiplexing and time differentiation. Wavelength division multiplexing divides the 1520nm-1560nm band into multiple independent wavelength channels, each wavelength... Each channel corresponds to one or more sets of sensing-compensation dual gratings. Utilizing the independent transmission characteristics of different wavelength optical signals in the transmission optical fiber, isolated transmission of grating signals within different wavelength channels is achieved. Time differentiation is realized through the frequency-sweeping light source or pulse signal of the signal demodulation module. Based on the distance difference between the sensing-compensation dual gratings at different positions and the signal demodulation module, a distinguishable time difference is formed in the time it takes for the reflected light from each grating to return to the signal demodulation module. This time difference feature is captured by time-domain segmented demodulation, enabling signal differentiation between sensing-compensation dual gratings at different positions within the same wavelength channel. Through composite coding of wavelength division multiplexing and time differentiation, cascaded identification of multiple sets of sensing-compensation dual gratings is achieved on a single transmission optical fiber, thereby realizing distributed stress monitoring with a spatial resolution on the order of 10cm over a long distance of 1km-15km.

[0008] Preferably, the composite encapsulation layer is composed of a resin matrix reinforced with a mixture of carbon fiber and glass fiber, wherein the volume fraction of carbon fiber is 30%-50%, the volume fraction of glass fiber is 20%-40%, and the remainder is the resin matrix, which is epoxy resin or vinyl ester resin; the tensile strength of the composite encapsulation layer is not less than 800 MPa, the elastic modulus is not less than 40 GPa, and the elongation at break is not less than 2%, thereby matching the mechanical properties of the load-bearing rope bundle of the mooring cable.

[0009] Preferably, the end sealing connection assembly includes a fixed flange connected to the end anchor of the mooring cable by bolts, and an optical fiber connector passing through the center of the fixed flange and electrically connected at both ends to the stress monitoring cable and the signal demodulation module, respectively. The side of the fixed flange facing away from the mooring cable is also fixedly connected to the signal demodulation module via a sealing interface.

[0010] Preferably, the signal demodulation module includes a light source unit, a multiplexing and demultiplexing unit, a photoelectric conversion unit, a data acquisition and processing unit, and a temperature-strain decoupling unit. The light source unit inputs the optical signal into the stress monitoring cable via the multiplexing and demultiplexing unit. The optical signal reflected by the stress monitoring cable is transmitted to the data acquisition and processing unit after passing through the multiplexing and demultiplexing unit and the photoelectric conversion unit. The center wavelength information of each grating is extracted. The data acquisition and processing unit is connected to the temperature-strain decoupling unit and is used to obtain the strain value after eliminating the interference of temperature changes in the marine environment on strain measurement using a preset temperature-strain relationship.

[0011] Preferably, the spacing between two adjacent sets of sensing-compensation dual gratings is 10cm-20cm; The length of the stress monitoring cable is 5km-8km.

[0012] Preferably, the composite encapsulation layer is a cylindrical shape with a diameter of 2mm-5mm.

[0013] A monitoring method for a distributed stress monitoring system applicable to marine mooring cables includes the following steps: S1. Stress monitoring cable installation: During the stranding process of mooring cable production, the pre-fabricated stress monitoring cable is placed inside the neutral axis of the load-bearing rope bundle of the mooring cable, and completes the stranding operation together with the load-bearing rope bundle; after the load-bearing rope bundle is stranded and formed, a cable sheath is formed on its outside through extrusion or coating process. S2. System Initialization and Parameter Calibration: Secure the fixed flange of the end sealing connection assembly to the anchor at the end of the mooring cable using bolts to ensure the mechanical fixation reliability of the stress monitoring cable lead-out end; connect the transmission optical fiber of the stress monitoring cable to the multiplexing and demultiplexing unit of the signal demodulation module through the optical fiber connector, ensuring that the insertion loss is ≤0.3dB and the return loss is ≥60dB; start the signal demodulation module, debug the light source unit, multiplexing and demultiplexing unit, and photoelectric conversion unit, calibrate the strain sensitivity coefficient and temperature sensitivity coefficient, and verify the sealing performance of the sealing joint to ensure that the sealing pressure is ≥10MPa, eliminate the risk of seawater leakage, and complete the overall system initialization; S3. Optical Signal Transmission and Reflection Acquisition: The light source unit of the signal demodulation module outputs broadband or swept-frequency optical signals, which are then distributed to the corresponding wavelength channels by the multiplexing and demultiplexing unit and input into the transmission optical fiber of the stress monitoring cable through the end-sealed connection component. When the optical signal propagates in the transmission optical fiber, it is reflected sequentially by each sensing-compensation dual grating group deployed along the line. The reflected optical signals of different dual grating groups are distinguished by wavelength-time composite encoding. The reflected optical signal is transmitted back through the transmission optical fiber and sequentially passes through the end-sealed connection component and the multiplexing and demultiplexing unit to complete signal separation. Then, the optical signal is converted into an electrical signal by the photoelectric conversion unit and transmitted to the data acquisition and processing unit. S4. Temperature-Strain Decoupling Calculation: The data acquisition and processing unit extracts the center wavelength offset of the sensing grating and temperature compensation grating in each sensing-compensation dual grating group and transmits it to the temperature-strain decoupling unit; the temperature-strain decoupling unit obtains the strain value after eliminating the interference of temperature changes in the marine environment on strain measurement based on the preset temperature-strain relationship.

[0014] Preferably, in step S1, a hot-melt process is used to form the cable sheath, so that the cable sheath and the composite encapsulation layer surface of the stress monitoring cable are fused together. The bonding strength between the two is not less than 5MPa, which avoids relative slippage or interface debonding due to vibration and bending during long-term service. This achieves an integrated structure of the stress monitoring cable and the mooring cable, and the stress monitoring cable is continuously laid along the length of the mooring cable, covering a range of 1km-15km of the cable.

[0015] Preferably, the preset temperature-strain relationship expression in step S4 is as follows: ; ; In the formula, and These represent the center wavelength offsets of the sensing grating and the temperature compensation grating, respectively. Indicates the strain sensitivity coefficient; Indicates the temperature sensitivity coefficient; Indicates the amount of temperature change; This represents the strain value after eliminating the interference of temperature changes in the marine environment on strain measurements; The strain values ​​obtained after eliminating the interference of temperature changes in the marine environment on strain measurements are obtained. .

[0016] Therefore, the distributed stress monitoring system and method applicable to marine mooring cables described above have the following beneficial effects: 1. Ultra-long monitoring distance and high spatial resolution: By deploying dual grating groups with equal spacing along the transmission fiber axis through "sensing-compensation", combined with wavelength division multiplexing and time division multiplexing composite coding method, the effective monitoring length of a single array reaches 1km-15km, and the spacing between adjacent dual grating groups is 10cm-50cm, achieving a spatial resolution on the order of 10cm, which is far superior to the hundred-meter-level length and meter-level resolution of traditional FBG solutions. It can fully cover the entire length of large mooring cables and capture local details. 2. Composite encapsulation structure highly compatible with mooring cables: The composite encapsulation layer is made of resin-based composite material reinforced with a mixture of carbon fiber and glass fiber. Its tensile strength is ≥800MPa, elastic modulus is ≥40GPa, and elongation at break is ≥2%. It is highly compatible with the mechanical properties of the cable load-bearing rope bundle, which not only ensures accurate strain transmission, but also avoids interface stress concentration and fatigue damage caused by stiffness differences, significantly improving the mechanical compatibility and reliability of the sensor array in long-term marine environment service. 3. Structured temperature compensation and high-precision decoupling algorithm: Each grating group is equipped with a sensing grating and a temperature compensation grating encapsulated in a low elastic modulus quartz glass tube (isolates strain and only responds to temperature). Combined with a decoupling algorithm based on the center wavelength offset of the dual gratings, the interference of temperature changes on strain measurement can be effectively eliminated, realizing high-precision pure strain measurement under complex ocean temperature fields. 4. Pre-embedded installation and long-term marine environment adaptability: The monitoring array is pre-embedded in the internal neutral axis position during the cable production process. The composite encapsulation layer and the cable sheath are combined by a hot-melt process (bonding strength ≥5MPa), which can avoid environmental damage such as seawater erosion, ultraviolet radiation and mechanical impact. With the end double O-ring seal (sealing pressure ≥10MPa) and reliable fixing structure, the long-term stability and service life of the monitoring system are greatly improved, which can meet the monitoring needs of the entire life cycle of the cable.

[0017] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the distributed stress monitoring system for marine mooring cables described in this invention. Figure 2 This is an axial cross-sectional view of the stress monitoring cable of the distributed stress monitoring system for marine mooring cables described in this invention. Figure 3 This is a radial sectional view showing the layout of the distributed stress monitoring system for marine mooring cables described in this invention. Figure 4 This is a radial cross-sectional view of the stress monitoring cable of the distributed stress monitoring system for marine mooring cables described in this invention.

[0019] Figure Labels 1. Stress monitoring cable; 11. Sensing grating; 12. Temperature compensation grating; 13. Transmission fiber optic cable; 2. Mooring cable; 21. Load-bearing rope bundle; 22. Cable sheath; 3. End sealing connection assembly; 31. Fiber optic connector; 32. Sealing interface; 33. Fixing flange; 4. Composite encapsulation layer; 5. Quartz glass tube. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are merely illustrative of the embodiments of the present invention and are not intended to limit the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of this application. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout.

[0021] It should be noted that the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion, such as a process, method, system, product, or server that includes a series of steps or units, not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such process, method, product, or device.

[0022] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0023] like Figures 1-4 As shown, a distributed stress monitoring system suitable for marine mooring cables includes a stress monitoring cable 1 embedded in the neutral axis position inside the mooring cable 2 and a signal demodulation module electrically connected to the stress monitoring cable 1 via an end-sealing connection component 3. The stress monitoring cable 1 includes a transmission optical fiber 13 and multiple sets of sensing-compensation dual grating groups that are spacedly adhered to or inscribed on the transmission optical fiber 13, as well as a composite encapsulation layer 4 that wraps around the multiple sets of sensing-compensation dual grating groups. The spacing between two adjacent sets of sensing-compensation dual grating groups is 10cm-50cm, and the length of the stress monitoring cable 1 is 1km-15km.

[0024] The sensing-compensation dual-grating group includes sensing gratings 11 and temperature compensation gratings 12 arranged at intervals, encapsulated within a quartz glass tube 5. Within the same sensing-compensation dual-grating group, both sensing gratings 11 and temperature compensation gratings 12 are weakly reflective gratings with a reflectivity of 0.1%-0.5%. The center wavelength difference between sensing gratings 11 and temperature compensation gratings 12 is 5nm-10nm, and the center wavelengths of all sensing-compensation dual-grating groups are located in the 1520nm-1560nm band. The center wavelength interval of sensing gratings 11 in adjacent sensing-compensation dual-grating groups is 0.5nm-2nm. This achieves accurate differentiation and identification of each sensing-compensation dual-grating group through a composite coding method combining wavelength division multiplexing (WDM) and time differentiation. WDM divides the 1520nm-1560nm band into multiple independent wavelength channels. Each wavelength channel corresponds to one or more sets of sensing-compensation dual gratings. Utilizing the independent transmission characteristics of different wavelength optical signals in the transmission fiber 13, isolated transmission of grating signals within different wavelength channels is achieved. Time differentiation is realized through the frequency sweep light source or pulse signal of the signal demodulation module. Based on the distance difference between the sensing-compensation dual gratings at different positions and the signal demodulation module, a distinguishable time difference is formed in the time it takes for the reflected light from each grating to return to the signal demodulation module. This time difference feature is captured by time-domain segmented demodulation, enabling signal differentiation of sensing-compensation dual gratings at different positions within the same wavelength channel. Through composite coding of wavelength division multiplexing and time differentiation, the cascaded identification of multiple sets of sensing-compensation dual gratings is achieved on a single transmission fiber 13, thereby realizing distributed stress monitoring with a spatial resolution on the order of 10 cm over a long distance of 1km-15km.

[0025] During its service at sea, the mooring cable 2 undergoes periodic stretching and bending under the combined effects of waves, currents, and wind loads. Axial strain changes near its internal neutral axis are transmitted to the stress monitoring cable 1 via the load-bearing rope bundle 21. The composite encapsulation layer 4 effectively transmits external strain to the sensing grating 11 on the transmission optical fiber 13, causing its center wavelength to shift with changes in strain and temperature. Simultaneously, the temperature compensation grating 121, encapsulated within the quartz glass tube 5, experiences minimal axial strain and only responds to temperature changes; its center wavelength shift reflects the amount of temperature change. Therefore, by measuring the center wavelength shift of the sensing grating 11 and the temperature compensation grating 12 within the same dual-grating group using the signal demodulation module, the strain and temperature changes at the measuring point can be calculated using the temperature-strain decoupling formula, enabling distributed monitoring of the stress-strain state along the entire length of the mooring cable 2.

[0026] The composite encapsulation layer 4 is composed of a resin matrix reinforced with a mixture of carbon fiber and glass fiber, wherein the volume fraction of carbon fiber is 30%-50%, the volume fraction of glass fiber is 20%-40%, and the remainder is the resin matrix, which is epoxy resin or vinyl ester resin. The tensile strength of the composite encapsulation layer 4 is not less than 800 MPa, the elastic modulus is not less than 40 GPa, and the elongation at break is not less than 2%, thus matching the mechanical properties of the load-bearing rope bundle 21 of the mooring cable 2.

[0027] The end sealing connection assembly 3 includes a fixed flange 33 bolted to the end anchor of the mooring cable 2, and an optical fiber connector 31 passing through the center of the fixed flange 33 and electrically connected at both ends to the stress monitoring cable 1 and the signal demodulation module, respectively. The side of the fixed flange 33 facing away from the mooring cable 2 is also fixedly connected to the signal demodulation module via a sealing interface 32. The sealing interface 32 adopts a double O-ring sealing structure to ensure no leakage under external pressure exceeding 10 MPa. The optical fiber connector 31 adopts an FC / APC type structure with an insertion loss of no more than 0.3 dB and a return loss of no less than 60 dB.

[0028] The signal demodulation module includes a light source unit, a multiplexing and demultiplexing unit, a photoelectric conversion unit, a data acquisition and processing unit, and a temperature-strain decoupling unit. The light source unit inputs the optical signal into the stress monitoring cable 1 via the multiplexing and demultiplexing unit. The optical signal reflected by the stress monitoring cable 1 is transmitted to the data acquisition and processing unit after passing through the multiplexing and demultiplexing unit and the photoelectric conversion unit. The center wavelength information of each grating is extracted. The data acquisition and processing unit is connected to the temperature-strain decoupling unit and is used to obtain the strain value after eliminating the interference of temperature changes in the marine environment on strain measurement using a preset temperature-strain relationship.

[0029] The spacing between two adjacent sets of sensor-compensation dual gratings is 10cm-20cm; the length of stress monitoring cable 1 is 5km-8km.

[0030] The composite encapsulation layer 4 is a cylindrical shape with a diameter of 2mm-5mm.

[0031] A monitoring method for a distributed stress monitoring system applicable to marine mooring cables includes the following steps: S1. Deployment of stress monitoring cable 1: During the production and stranding process of mooring cable 2, the prefabricated stress monitoring cable 1 is placed inside the neutral axis position of the load-bearing rope bundle 21 of mooring cable 2, and the stranding operation is completed together with the load-bearing rope bundle 21; after the load-bearing rope bundle 21 is stranded and formed, a cable sheath 22 is formed on its outside through extrusion or coating process. S2. System Initialization and Parameter Calibration: Secure the fixed flange 33 of the end sealing connection assembly 3 to the anchor at the end of the mooring cable 2 with bolts to ensure the mechanical fixation reliability of the stress monitoring cable 1 lead-out end; connect the transmission optical fiber 13 of the stress monitoring cable 1 to the multiplexing and demultiplexing unit of the signal demodulation module through the optical fiber connector 31, ensuring that the insertion loss is ≤0.3dB and the return loss is ≥60dB; start the signal demodulation module, debug the light source unit, multiplexing and demultiplexing unit, and photoelectric conversion unit, calibrate the strain sensitivity coefficient and temperature sensitivity coefficient, and verify the sealing performance of the sealing joint to ensure that the sealing pressure is ≥10MPa, eliminate the risk of seawater leakage, and complete the overall system initialization; S3. Optical Signal Transmission and Reflection Acquisition: The light source unit of the signal demodulation module outputs broadband or swept-frequency optical signals, which are then distributed to the corresponding wavelength channels by the multiplexing and demultiplexing unit and input into the transmission optical fiber 13 of the stress monitoring cable 1 through the end-sealed connection component 3. When the optical signal propagates in the transmission optical fiber 13, it is reflected sequentially by each sensing-compensation dual grating group laid along the line. The reflected optical signals of different dual grating groups are distinguished by wavelength-time composite encoding. The reflected optical signal is transmitted back in reverse through the transmission optical fiber 13 and sequentially passes through the end-sealed connection component 3 and the multiplexing and demultiplexing unit to complete signal separation. Then, the optical signal is converted into an electrical signal by the photoelectric conversion unit and transmitted to the data acquisition and processing unit. S4. Temperature-Strain Decoupling Calculation: The data acquisition and processing unit extracts the center wavelength offset of the sensing grating 11 and temperature compensation grating 12 in each sensing-compensation dual grating group and transmits it to the temperature-strain decoupling unit; the temperature-strain decoupling unit obtains the strain value after eliminating the interference of temperature changes in the marine environment on strain measurement based on the preset temperature-strain relationship.

[0032] In step S1, a cable sheath 22 is formed by a hot-melt process, which fuses the cable sheath 22 with the surface of the composite encapsulation layer 4 of the stress monitoring cable 1. The bonding strength between the two is not less than 5MPa, which avoids relative slippage or interface debonding due to vibration and bending during long-term service. This achieves an integrated structure between the stress monitoring cable 1 and the mooring cable 2, and the stress monitoring cable 1 is continuously laid along the length of the mooring cable 2, covering a range of 1km-15km of the cable length.

[0033] The preset temperature-strain relationship expression mentioned in step S4 is as follows: ; ; In the formula, and These represent the center wavelength offsets of the sensing grating 11 and the temperature compensation grating 12, respectively. Indicates the strain sensitivity coefficient; Indicates the temperature sensitivity coefficient; Indicates the amount of temperature change; This represents the strain value after eliminating the interference of temperature changes in the marine environment on strain measurements; The strain values ​​obtained after eliminating the interference of temperature changes in the marine environment on strain measurements are obtained. .

[0034] In this embodiment, the data acquisition and processing unit integrates strain data and temperature change data from all monitoring points along the entire length of the mooring cable 2 to form strain distribution maps and temperature distribution maps along the entire length of the cable. Combined with preset cable safety strain thresholds and fatigue damage assessment models, the system performs real-time assessment of the cable's structural health status and identifies early failure characteristics such as local stress concentration and fatigue damage. When strain data in a certain area exceeds the safety threshold or an abnormal change occurs, the system automatically triggers an early warning signal, providing data support for the full life cycle management, maintenance decisions, and safety risk prevention and control of the mooring cable 2.

[0035] Specifically, the structural health status assessment methods are as follows: First, the theoretical basis is established: the center wavelength of all dual-grating groups is located in the 1520nm-1560nm band, the center wavelength difference between sensing grating 11 and temperature compensation grating (12) in the same group is 5nm-10nm, and the center wavelength interval between adjacent sensing grating groups is 0.5nm-2nm; the core indicator of structural health assessment (stress concentration factor) Health Index All are based on strain data. ,and Wavelength offset of the sensing grating and temperature-compensated grating wavelength offset Derived through the decoupling formula: It can be seen that by dividing the wavelength band (wavelength division multiplexing) and different center wavelengths, signal isolation and precise positioning can be achieved in different monitoring sections (10cm-20cm), ensuring the maximum strain value within the entire monitoring range of the mooring cable. The average strain value of all measuring points within the full length monitoring range of the mooring cable It can be mapped to the specific location of the cable.

[0036] Then adjust the stress concentration factor. : ; In the formula, Indicates the first The maximum wavelength offset (nm) of the sensing grating within the monitoring segment, which is divided into monitoring segments by center wavelength intervals of 0.5nm-2nm; Indicates the first The average wavelength shift (nm) of the sensing gratings within the monitoring segment is calculated from the center wavelength shift of all sensing gratings within that segment. The arithmetic mean is obtained; Indicates the first The average wavelength offset (nm) of the temperature compensation grating within the monitoring section is 5nm-10nm different from the wavelength of the sensing grating, allowing the signal to be independently separated.

[0037] Then, a health level assessment is performed (supplementary wavelength measurement accuracy requirements): Judgment prerequisite: Wavelength measurement accuracy must be ≤0.01nm (matching center wavelength interval resolution of 0.5nm-2nm) to ensure , The measurement error is ≤0.1%, thus ensuring The calculation accuracy meets the requirements for health level classification; Threshold correlation: Determining the safe strain threshold through wavelength offset calibration under known strain. —For example, applying force to cables The corresponding strain, the measurement sensor grating , combined Reverse verification , This indicates when the cable is subjected to allowable strain. At that time, the center wavelength offset corresponding to the sensing grating.

[0038] Then signal separation is performed: based on wavelength division multiplexing logic, the 1520nm-1560nm band is divided into several channels (e.g., one channel for every 20nm), and each channel corresponds to 50-100 sets of dual gratings (since the wavelength interval between adjacent sets of sensing gratings is 0.5nm-2nm, a single channel can accommodate 40-80 sets), thus achieving signal isolation within the same channel; Next, wavelength extraction is performed: the time difference of reflected light is captured by time differentiation, and the wavelength of each dual grating group is accurately extracted. and Positioning to the first based on channel and time difference Monitoring section; Calculate the health index again According to the revised The formula calculates the stress concentration factor, combined with The corresponding wavelength offset threshold is used to obtain the health index. : ; in, ; In the formula, Indicates the safety strain threshold The corresponding allowable stress concentration factor; Indicates the safety strain threshold The corresponding maximum safe strain threshold; Indicates the safety strain threshold The corresponding average safe strain threshold; Finally, the spectrum is generated: based on the center wavelength identifier of each monitoring segment (e.g., channel 1 corresponds to 1520-1540nm, and the wavelength within the channel is...). The grating group corresponds to a center wavelength of 1520 ± 0.5 nm. (nm), mark the wavelength and physical location of the warning segment in the "Health Index Distribution Map".

[0039] The fatigue life prediction method is as follows: First, a weak reflection grating (0.1%-0.5%) is selected as the dual grating, and its center wavelength is precisely distributed in the 1520nm-1560nm band to ensure that the wavelength shift signal under dynamic load can be captured at high frequency. Then, the theoretical foundation was established: the core of fatigue life prediction is the cumulative damage degree. Dependence on strain amplitude , This represents the minimum strain value within the entire monitoring range of the mooring cable, and the temperature compensation grating... Used to eliminate temperature fluctuations Interference; the narrow bandwidth characteristics of the weak reflection grating (matching center wavelength interval 0.5nm-2nm) enable the signal demodulation module to quickly distinguish wavelength shifts under different cycles, and realize high-frequency statistics of strain cycles (such as 1-10Hz cycles caused by wave loads).

[0040] Constructing strain amplitude Direct mapping relationship with wavelength shift: ; In the formula, and These represent the maximum wavelength offset (nm) and minimum wavelength offset (nm) of the sensing grating within a strain cycle (e.g., at the peak of the wave). Then, calibrate the wavelength based on the SN curve: ; In the formula, This indicates the fatigue life of the cable under corresponding strain cycles; and The fitting coefficients of the SN fatigue curves are represented. Indicates the elastic modulus; Among them, coefficient , The calibration needs to be combined with the wavelength characteristics of the grating: through fatigue testing of cable materials, different levels of strain are applied, and the corresponding strain cycles of the sensing grating are measured. and Derivation of strain amplitude and stress amplitude ,and And then fit to obtain and ; elastic modulus Matching: The composite encapsulation layer has an elastic modulus ≥ 40 GPa, matching the cable's load-bearing rope bundle, and is calibrated. Simultaneously, the grating wavelength offset corresponding to the composite encapsulation layer needs to be measured to ensure... Consistent with the wavelength-strain mapping relationship.

[0041] Then, high-frequency wavelengths are acquired: based on the fast frequency sweeping characteristics of the frequency sweeping light source (the frequency sweeping range covers 1520nm-1560nm), the center wavelength of each sensing grating is acquired at a sampling rate of 100Hz to capture wavelength fluctuations under dynamic loads. Cyclic statistics: Strain cycles are identified by wavelength fluctuations (e.g., when the wavelength offset rises from the minimum to the maximum and then returns to the minimum, it is determined as one cycle), and the statistics for each cycle are calculated. and and calculate and ; Grading: by Classification by grade (e.g., 1 grade for every 50 MPa), corresponding to the wavelength offset difference range for different grades (e.g.) correspond ), count the number of cycles for each level ; Lifetime calculation: Substitute into the SN curve fitting formula to calculate the lifespan. Fatigue life corresponding to each monitoring point and cumulative damage : ; ; In the formula, and They represent the first The maximum and minimum wavelength shifts of the sensing grating at each monitoring point within a certain strain cycle; Indicates the first The number of strain cycles that each monitoring point has withstood; Then, combined with the design life Get remaining lifespan : ; when In 2010, fatigue risk sections were located by identifying the center wavelength of the grating.

[0042] The abnormal operating condition early warning methods are as follows: First, the key wavelength is designed: the spacing between adjacent dual grating groups is 10cm-50cm, and precise positioning is achieved through wavelength-time composite coding. The fiber optic connector insertion loss is ≤0.3dB and the return loss is ≥60dB to ensure distortion-free wavelength signal transmission. Both the static threshold and dynamic mutation of the anomaly warning are based on real-time strain. ,Depend on It can be seen that the rapid response of high-fidelity transmission of wavelength signals is the key to accurate early warning. Indicates the first At each monitoring point, the sensing grating at time... wavelength offset, Indicates the first At each monitoring point, the temperature compensation grating at time... Wavelength offset; Meanwhile, the uniqueness of the center wavelength (interval between adjacent groups is 0.5nm-2nm) enables precise location of anomalies; and the real-time monitoring of wavelength shift enables rapid response to abnormal operating conditions (response time ≤0.1s, time difference resolution for matching time differentiation).

[0043] Specifically, the static threshold warning expression is as follows: ; In the formula, Indicates the safety factor; definition ,when Timely triggering of warnings; Example: If ,but A general warning is triggered when the real-time wavelength deviation of the sensing grating is ≥7.4nm.

[0044] The dynamic mutation early warning expression is as follows: ; In the formula, Indicates the adjacent sampling time ( The wavelength offset difference (nm) of the sensing grating; Wavelength change rate threshold: ,like ,but A dynamic warning is triggered when the wavelength change rate is ≥0.06 pm / s.

[0045] The wavelength warning method is as follows: During system initialization, based on... , and Calculate the wavelength change rate threshold for different warning levels. The data is then stored in the data acquisition and processing unit; each sensor grating is acquired at a sampling rate of 0.1s. and To ensure that the signal is distortion-free (based on the wavelength transmission characteristics with a return loss of ≥60dB). Dual logic judgment: Static judgment: Comparison and Satisfy for 3 seconds If so, the corresponding level of warning will be triggered; Dynamic determination: Calculate the wavelength change rate of adjacent sampling points. If the wavelength change rate of adjacent sampling points reaches or exceeds the preset dynamic change threshold for two consecutive cycles, a change warning will be triggered. Location and Output: Anomalies are located by “wavelength identifier + time difference” (e.g., a grating with a center wavelength of 1530.5nm, a return time difference of 0.01ms, corresponding to a cable location of 1.5km). When outputting warning information, the grating wavelength parameters and physical location are attached.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A distributed stress monitoring system suitable for marine mooring cables, characterized in that: The system includes a stress monitoring cable embedded in the neutral axis of the mooring cable and a signal demodulation module electrically connected to the stress monitoring cable via an end-sealing connection assembly. The stress monitoring cable includes a transmission optical fiber and multiple sets of sensing-compensation dual gratings spaced apart or inscribed on the transmission optical fiber, as well as a composite encapsulation layer wrapped around the multiple sets of sensing-compensation dual gratings. The spacing between two adjacent sets of sensing-compensation dual gratings is 10cm-50cm, and the length of the stress monitoring cable is 1km-15km.

2. The distributed stress monitoring system for marine mooring cables according to claim 1, characterized in that: The sensing-compensation dual-grating assembly comprises a sensing grating and a temperature compensation grating arranged at intervals, encapsulated within a quartz glass tube. Within the same sensing-compensation dual-grating assembly, both the sensing grating and the temperature compensation grating utilize weakly reflective gratings with a reflectivity of 0.1%-0.5%. The center wavelength difference between the sensing grating and the temperature compensation grating is 5nm-10nm, and the center wavelengths of all sensing-compensation dual-grating assemblies are located in the 1520nm-1560nm band. The center wavelength interval between adjacent sensing-compensation dual-grating assemblies is 0.5nm-2nm. This achieves accurate differentiation and identification of each sensing-compensation dual-grating assembly through a composite coding method combining wavelength division multiplexing (WDM) and time differentiation. WDM divides the 1520nm-1560nm band into multiple independent wavelength channels, each wavelength channel... Corresponding to one or more sets of sensing-compensation dual grating groups, the independent transmission characteristics of different wavelength optical signals in the transmission optical fiber are utilized to achieve isolated transmission of grating signals in different wavelength channels. Time differentiation is achieved through the frequency sweep light source or pulse signal of the signal demodulation module. Based on the distance difference between the sensing-compensation dual grating groups at different positions and the signal demodulation module, the time when the reflected light from each grating returns to the signal demodulation module forms a distinguishable time difference. This time difference feature is captured by time-domain segmented demodulation, realizing the signal differentiation of sensing-compensation dual grating groups at different positions in the same wavelength channel. Through the composite coding of wavelength division multiplexing and time differentiation, the serial identification of multiple sets of sensing-compensation dual grating groups on a single transmission optical fiber is realized, thereby achieving distributed stress monitoring with a spatial resolution on the order of 10cm over a long distance of 1km-15km.

3. The distributed stress monitoring system for marine mooring cables according to claim 2, characterized in that: The composite encapsulation layer is composed of a resin matrix reinforced with a mixture of carbon fiber and glass fiber. The volume fraction of carbon fiber is 30%-50%, the volume fraction of glass fiber is 20%-40%, and the remainder is the resin matrix, which is epoxy resin or vinyl ester resin. The tensile strength of the composite encapsulation layer is not less than 800 MPa, the elastic modulus is not less than 40 GPa, and the elongation at break is not less than 2%, thus matching the mechanical properties of the load-bearing rope bundle of the mooring cable.

4. The distributed stress monitoring system for marine mooring cables according to claim 3, characterized in that: The end sealing connection assembly includes a fixed flange that is bolted to the end anchor of the mooring cable and an optical fiber connector that passes through the center of the fixed flange and is electrically connected at both ends to the stress monitoring cable and the signal demodulation module, respectively. The side of the fixed flange away from the mooring cable is also fixedly connected to the signal demodulation module via a sealing interface.

5. The distributed stress monitoring system for marine mooring cables according to claim 4, characterized in that: The signal demodulation module includes a light source unit, a multiplexing and demultiplexing unit, a photoelectric conversion unit, a data acquisition and processing unit, and a temperature-strain decoupling unit. The light source unit inputs the optical signal into the stress monitoring cable via the multiplexing and demultiplexing unit. The optical signal reflected by the stress monitoring cable is transmitted to the data acquisition and processing unit after passing through the multiplexing and demultiplexing unit and the photoelectric conversion unit. The center wavelength information of each grating is extracted. The data acquisition and processing unit is connected to the temperature-strain decoupling unit to obtain the strain value after eliminating the interference of temperature changes in the marine environment on strain measurement using a preset temperature-strain relationship.

6. The distributed stress monitoring system for marine mooring cables according to claim 5, characterized in that: The spacing between two adjacent sets of sensor-compensation dual gratings is 10cm-20cm; The length of the stress monitoring cable is 5km-8km.

7. The distributed stress monitoring system for marine mooring cables according to claim 5, characterized in that: The composite encapsulation layer is a cylindrical shape with a diameter of 2mm-5mm.

8. The monitoring method for a distributed stress monitoring system applicable to marine mooring cables as described in any one of claims 5-7, characterized in that: Includes the following steps: S1. Stress monitoring cable installation: During the stranding process of mooring cable production, the pre-fabricated stress monitoring cable is placed inside the neutral axis of the load-bearing rope bundle of the mooring cable, and completes the stranding operation together with the load-bearing rope bundle; after the load-bearing rope bundle is stranded and formed, a cable sheath is formed on its outside through extrusion or coating process. S2. System initialization and parameter calibration: Secure the fixed flange of the end sealing connection assembly to the anchor at the end of the mooring cable with bolts to ensure the mechanical fixation reliability of the stress monitoring cable lead-out end; Connect the transmission optical fiber of the stress monitoring cable to the multiplexing and demultiplexing unit of the signal demodulation module through the optical fiber connector, and ensure that the insertion loss is ≤0.3dB and the return loss is ≥60dB. Start the signal demodulation module, debug the light source unit, multiplexing and demultiplexing unit, and photoelectric conversion unit, calibrate the strain sensitivity coefficient and temperature sensitivity coefficient, and at the same time verify the sealing performance of the sealing joint to ensure that the sealing pressure is ≥10MPa, eliminate the risk of seawater leakage, and complete the overall system initialization. S3. Optical Signal Transmission and Reflection Acquisition: The light source unit of the signal demodulation module outputs broadband or swept-frequency optical signals, which are then distributed to the corresponding wavelength channels by the multiplexing and demultiplexing unit and input into the transmission optical fiber of the stress monitoring cable through the end-sealed connection component. When the optical signal propagates in the transmission optical fiber, it is reflected sequentially by each sensing-compensation dual grating group deployed along the line. The reflected optical signals of different dual grating groups are distinguished by wavelength-time composite encoding. The reflected optical signal is transmitted back through the transmission optical fiber and sequentially passes through the end-sealed connection component and the multiplexing and demultiplexing unit to complete signal separation. Then, the optical signal is converted into an electrical signal by the photoelectric conversion unit and transmitted to the data acquisition and processing unit. S4. Temperature-Strain Decoupling Calculation: The data acquisition and processing unit extracts the center wavelength offset between the sensing grating and the temperature compensation grating in each sensing-compensation dual grating group and transmits it to the temperature-strain decoupling unit. The temperature-strain decoupling unit obtains the strain value after eliminating the interference of temperature changes in the marine environment on strain measurement based on the preset temperature-strain relationship.

9. The monitoring method of the distributed stress monitoring system for marine mooring cables according to claim 8, characterized in that: In step S1, a cable sheath is formed using a hot-melt process, which fuses the cable sheath with the surface of the composite encapsulation layer of the stress monitoring cable. The bonding strength between the two is not less than 5MPa, which avoids relative slippage or interface debonding due to vibration and bending during long-term service. This achieves an integrated structure between the stress monitoring cable and the mooring cable, and the stress monitoring cable is continuously laid along the length of the mooring cable, covering a range of 1km-15km of the cable.

10. The monitoring method for a distributed stress monitoring system applicable to marine mooring cables according to claim 8, characterized in that: The preset temperature-strain relationship expression mentioned in step S4 is as follows: ； ； In the formula, and These represent the center wavelength offsets of the sensing grating and the temperature compensation grating, respectively. Indicates the strain sensitivity coefficient; Indicates the temperature sensitivity coefficient; Indicates the amount of temperature change; This represents the strain value after eliminating the interference of temperature changes in the marine environment on strain measurements; The strain values ​​obtained after eliminating the interference of temperature changes in the marine environment on strain measurements are obtained. .

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