A method of subsea cable health detection

Abstract

The application discloses a temperature-insensitive submarine cable health detection method based on forward stimulated Brillouin scattering under ocean current scouring, which measures the transverse pressure generated by seawater scouring on the surface of the optical fiber body wrapped outside the submarine cable to determine the strength of the submarine cable subjected to the ocean current scouring, and comprises the following steps: step one, laying the optical fiber on the submarine cable; step two, measuring the pressure of the ocean current scouring and collecting the forward Brillouin frequency shift change caused by the transverse pressure; and step three, processing the relationship between the forward Brillouin frequency shift change and the pressure to obtain the transverse pressure on the surface of the optical fiber, i.e. the pressure brought by the ocean current scouring on the submarine cable. The application can provide the ocean current velocity measurement result without temperature influence by using the frequency shift information of two FBS scattering peaks. The transverse pressure detection device has the advantage of distributed pressure measurement and can continuously measure the transverse pressure in a long distance range.

Description

Technical Field

[0001] This invention relates to submarine cable inspection, and more specifically to a method for health inspection of submarine cables under temperature-insensitive ocean current scouring based on forward stimulated Brillouin scattering. Background Technology

[0002] With the utilization of clean marine energy and the advancement of offshore wind power projects, submarine cables, as vital channels connecting the ocean and land, are playing an increasingly important role. Due to their long-term exposure to the complex and ever-changing marine environment, submarine cables are susceptible to corrosion and erosion from high-salinity seawater. This can lead to direct exposure and suspension of the cables, severely threatening their mechanical and electrical properties and affecting their reliable operation. To address this situation, establishing an online health status monitoring system for submarine cables is particularly important. Currently, monitoring methods for sections of submarine cables suspended due to burial depth and external forces such as ocean currents are limited, and existing methods are inefficient, costly, and time-consuming. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for detecting the health of submarine cables under temperature-insensitive ocean current scouring based on forward stimulated Brillouin scattering. The method determines the strength of the ocean current scouring effect on the submarine cable by measuring the lateral pressure generated by seawater scouring on the surface of the optical fiber body wrapped by the submarine cable.

[0004] To achieve the above objectives, the technical solution of the present invention is as follows:

[0005] A method for detecting the health of submarine cables, wherein the strength of the scouring effect of ocean currents on the submarine cable is determined by measuring the lateral pressure generated by seawater scouring on the surface of the optical fiber body wrapped around the submarine cable.

[0006] Furthermore, the method of determining the strength of the scouring effect of ocean currents on the submarine cable by measuring the lateral pressure generated by seawater scouring on the surface of the optical fiber body wrapped by the submarine cable includes:

[0007] Step 1: Lay the optical fiber on the submarine cable;

[0008] Step 2: Measure the pressure caused by ocean current scouring and collect the forward Brillouin frequency shift of the optical fiber caused by lateral pressure;

[0009] Step 3: Process the relationship between the forward Brillouin frequency shift and pressure to obtain the transverse pressure on the fiber surface, that is, the pressure brought by the ocean current scouring the submarine cable.

[0010] Furthermore, in step two, the forward Brillouin frequency shift of the optical fiber caused by the lateral pressure is acquired by using a lateral pressure detection device.

[0011] Further, step one includes:

[0012] While laying submarine cables, optical fibers are also laid, either by wrapping or laying them longitudinally on the outside of the polypropylene outer sheath of the submarine cable.

[0013] Furthermore, the optical fiber is a double-coated single-mode optical fiber.

[0014] Furthermore, the lateral pressure detection device includes a first distributed feedback laser, a first polarization controller, an electro-optic modulator, an erbium-doped fiber amplifier, an optical isolator, a first wavelength division multiplexer, a second distributed feedback laser, a second polarization controller, an analyzer, a photodetector, a data acquisition card, an arbitrary function generator, and a DC power supply.

[0015] The excitation light pulse from the first distributed feedback laser is polarized by the first polarization controller until the optical pulse intensity of the sinusoidal pulse segment modulated by the electro-optic modulator reaches the maximum value. After being amplified by the erbium-doped fiber amplifier, the pulse peak power is injected into the double-coated single-mode fiber through the optical isolator and the first wavelength division multiplexer to excite the transverse resonant acoustic field.

[0016] The probe light pulse output by the second distributed feedback laser is adjusted by the second polarization controller to reach the maximum polarization intensity. After passing through the first wavelength division multiplexer, it is injected into the double-coated single-mode fiber to read the transverse acoustic field signal. Then, it is output to the analyzer to demodulate the polarization modulation into intensity modulation. Finally, it is converted into photoelectric signal by the AC photodetector. The output photocurrent is sampled and recorded by the data acquisition card according to the trigger signal provided by the arbitrary function generator.

[0017] Furthermore, the intensity modulation and DC bias of the electro-optic modulator are controlled by an arbitrary function generator and a DC power supply, respectively. By changing the modulation frequency output by the arbitrary function generator, the transverse resonant acoustic wave field of optical fibers with different cladding diameters and the transverse resonant acoustic wave field of optical fibers with the same cladding diameter in different modes are excited, so as to realize distributed detection of transverse pressure and temperature insensitivity.

[0018] Furthermore, step three includes:

[0019] Cylindrical optical fiber with torsional radiation mode TR 0,m Sensitive to birefringence, changes in pressure can alter the birefringence of the optical fiber, leading to changes in TR. 0,m The induced FBS spectrum broadens, and the center frequency of its resonance peak is v m for:

[0020]

[0021] Where a is the fiber cladding radius, V T Let y be the transverse wave velocity.m For TR 0,m Eigenvalues ​​of the acoustic mode acoustic wave equation;

[0022] The equation for sound waves is:

[0023]

[0024] Where α is the transverse wave velocity V of the optical fiber. T With the longitudinal wave speed V L The ratio;

[0025] The transverse pressure on the fiber surface is considered as the axial strain ε a radial strain ε r The combined effect of the FBS spectrum resonance peaks shows a linear relationship with axial and radial strain, respectively. A certain TR 0,m The frequency shift dv corresponding to the acoustic mode is expressed as:

[0026]

[0027] in These are the frequency shift-axial strain coefficient and the frequency shift-radial strain coefficient, respectively; for a given TR 0,m The acoustic mode yields the frequency shift pressure coefficient C. v-P =dv / dP is:

[0028]

[0029] Considering axial strain and radial strain respectively, the frequency shift-axial strain coefficient and the frequency shift-radial strain coefficient are expressed as follows:

[0030]

[0031]

[0032] Among them, dV L / dε a and dV L / dε r The value is derived from the fiber material parameters, thus obtaining a certain TR. 0,m The relationship between the frequency shift of the acoustic mode and pressure, for different TRs 0,m Voice model, C v-P The values ​​are different; when temperature and pressure change simultaneously, both will cause changes in the center frequencies of the two different resonance peaks in the FBS spectrum; when m = i or j (i ≠ j), the center frequencies v of the i-th and j-th resonance peaks are different. i and v j They are represented as follows:

[0033] v i =v i_0 +Cv-T_i ·ΔT+C v-P_i ·ΔP (7)

[0034] v j =v j_0 +C v-T_j ·ΔT+C v-P_j ·ΔP (8)

[0035] Where v i_0 and v j_0 The original acoustic modes TR are respectively 0,i and TR 0,j The corresponding center frequency of the resonance peak; C v-T_i and C v-T_j They are TR 0,i and TR 0,j The corresponding frequency shift temperature coefficient, C v-P_i and C v-P_j They are TR 0,i and TR 0,j The corresponding frequency shift pressure coefficient; where ΔT and ΔP are the changes in temperature and pressure, respectively, i.e.:

[0036]

[0037] Where Δv i and Δv j TR 0,i and TR 0,j The resulting change in the center frequency of the FBS spectrum;

[0038] By utilizing the frequency shift information of two FBS scattering peaks, temperature-insensitive measurement of the lateral pressure on the fiber surface can be achieved. Then, by changing v... m Distributed measurement of transverse pressure is achieved by exciting acoustic modes in optical fibers with different cladding diameters.

[0039] Compared with the prior art, the advantages of this invention are as follows:

[0040] This invention employs a temperature-insensitive ocean current-driven health monitoring method for submarine cables based on forward stimulated Brillouin scattering (FSBS). The principle is that when ocean currents wash over the submarine cable and its outer test fiber, the cable and fiber are subjected to external lateral pressure. The axial and radial strains of the test fiber change with this external pressure. Since FBS is sensitive to changes in fiber strain, the method measures the change in the forward Brillouin frequency shift of fiber strain caused by external lateral pressure by actively exciting and detecting FBS signals in a double-coated single-mode fiber. This demodulates the magnitude of the lateral pressure on the fiber, thus determining the extent of ocean current scouring on the submarine cable. By changing the modulation frequency of the excitation light, the position of the corresponding fiber segment is obtained, thereby assessing the cable's health status. Compared with existing technologies, this method offers the following advantages:

[0041] 1. Temperature insensitive: By utilizing the frequency shift information of the two FBS scattering peaks, ocean current velocity measurement results can be provided without the influence of temperature.

[0042] 2. Real-time capability: It can acquire ocean current velocity information in real time and provide detection results promptly.

[0043] 3. The lateral pressure detection device of the present invention has the advantage of distributed pressure measurement, and can continuously measure the lateral pressure over a long distance. Attached Figure Description

[0044] Figure 1 A flowchart of a method for detecting the health of submarine cables under temperature-insensitive ocean current scouring based on forward stimulated Brillouin scattering, provided in an embodiment of the present invention;

[0045] Figure 2 This is a schematic diagram of the components of a transverse pressure detection device;

[0046] In the diagram: 1. First distributed feedback laser; 2. First polarization controller; 3. Electro-optic modulator; 4. Erbium-doped fiber amplifier; 5. Optical isolator; 6. First wavelength division multiplexer; 7. Double-coated single-mode fiber; 8. Second distributed feedback laser; 9. Second polarization controller; 10. Analyzer; 11. Photodetector; 12. Data acquisition card; 13. Arbitrary function generator; 14. DC power supply. Detailed Implementation

[0047] Example:

[0048] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0049] This embodiment provides a temperature-insensitive ocean current scouring method for submarine cables based on forward stimulated Brillouin scattering. By measuring the lateral pressure exerted by seawater scouring on the surface of the optical fiber encasing the submarine cable, the method determines the strength of the ocean current scouring effect on the cable. (See also...) Figure 1 As shown, the specific steps include the following:

[0050] Step 1: Lay the optical fiber on the submarine cable;

[0051] Specifically, optical fibers are wound or laid longitudinally on the surface of the submarine cable while laying the submarine cable, that is, on the outside of the polypropylene outer sheath of the submarine cable.

[0052] Thus, when ocean currents wash over the submarine cable and the outer test optical fiber, the cable and fiber are subjected to the same external lateral pressure.

[0053] Step 2: Measure the pressure caused by the ocean current scouring, and use a lateral pressure detection device to collect the changes in the forward Brillouin frequency shift of the optical fiber caused by the lateral pressure.

[0054] Submarine cables are subjected to pressure perpendicular to the surface of the fluid under the action of ocean currents. Changes in flow velocity will cause changes in pressure. Therefore, by measuring this pressure, the current ocean current velocity can be indirectly inferred. For this purpose, in this step, a transverse pressure detection device is used to actively excite and detect the forward stimulated Brillouin scattering signal in the optical fiber to measure the change in the forward Brillouin frequency shift of the optical fiber strain caused by external transverse pressure.

[0055] Step 3: Process the relationship between the forward Brillouin frequency shift and pressure to obtain the transverse pressure on the fiber surface, that is, the pressure brought by the ocean current scouring the submarine cable.

[0056] Thus, by processing the relationship between the forward Brillouin frequency shift and pressure, the magnitude of the lateral pressure on the optical fiber can be demodulated, the magnitude of the ocean current scouring effect on the submarine cable can be obtained, and the health status of the submarine cable can be judged.

[0057] In one specific embodiment, the optical fiber is a double-coated single-mode optical fiber, which can be regarded as multiple optical fibers with different cladding diameters connected together, and its outer coating material is thermoplastic polyester elastomer.

[0058] In one specific embodiment, such as Figure 2 As shown, the transverse pressure detection device includes a first distributed feedback laser 1, a first polarization controller 2, an electro-optic modulator 3, an erbium-doped fiber amplifier 4, an optical isolator 5, a first wavelength division multiplexer 6, a second distributed feedback laser 8, a second polarization controller 9, an analyzer 10, a photodetector 11, a data acquisition card 12, an arbitrary function generator 13, and a DC power supply 14.

[0059] The excitation pulse emitted by the first distributed feedback laser 1 is polarized by the first polarization controller 2 until the optical pulse intensity of the sinusoidal pulse segment modulated by the electro-optic modulator 3 reaches its maximum value. After pulse peak power amplification by the erbium-doped fiber amplifier 4, it is injected into the double-coated single-mode fiber 7 via the optical isolator 5 and the first wavelength division multiplexer 6 to excite the transverse resonant acoustic field. The probe pulse output by the second distributed feedback laser 8 is polarized by the second polarization controller 9 until its intensity reaches its maximum value. After passing through the first wavelength division multiplexer 6, it is injected into the double-coated single-mode fiber 7 to read the transverse acoustic field signal. The signal is then output to the analyzer 10 to demodulate the polarization modulation into intensity modulation, and then photoelectric conversion is performed by the AC photodetector 11. The output photocurrent is sampled and recorded by the data acquisition card 12 according to the trigger signal provided by the arbitrary function generator 13. Thus, since forward stimulated Brillouin scattering is sensitive to changes in fiber strain, the change in forward Brillouin frequency shift of fiber strain caused by external lateral pressure can be measured by actively exciting and detecting forward stimulated Brillouin scattering signals in the double-coated single-mode fiber 7. In this way, the magnitude of the lateral pressure on the fiber can be demodulated, and the magnitude of the scouring effect of ocean currents on the submarine cable can be obtained.

[0060] Furthermore, the intensity modulation and DC bias of the electro-optic modulator 3 are controlled by the arbitrary function generator 13 and the DC power supply 14, respectively. By changing the modulation frequency output by the arbitrary function generator 13, the transverse resonant acoustic wave field of optical fibers with different cladding diameters and the transverse resonant acoustic wave field of optical fibers with the same cladding diameter in different modes are excited, so as to realize the distributed detection of transverse pressure and temperature insensitivity.

[0061] In one specific embodiment, step three above includes:

[0062] Cylindrical optical fiber with torsional radiation mode TR 0,m Sensitive to birefringence, changes in pressure can alter the birefringence of the optical fiber, leading to changes in TR. 0,m The induced FBS spectrum broadens, and the center frequency of its resonance peak is v m for:

[0063]

[0064] Where a is the fiber cladding radius, V T Let y be the transverse wave velocity. m For TR 0,m The eigenvalues ​​of the acoustic mode acoustic wave equation are given by:

[0065]

[0066] Where α is the transverse wave velocity V of the optical fiber. T With the longitudinal wave speed V L The ratio;

[0067] The transverse pressure on the fiber surface is considered as the axial strain ε a radial strain ε r The combined effect of these factors means that the frequency shift of the FBS spectral resonance peak is linearly related to both axial and radial strain, respectively. Therefore, for a certain TR... 0,m The frequency shift dv corresponding to the acoustic mode is expressed as:

[0068]

[0069] in These are the frequency shift-axial strain coefficient and the frequency shift-radial strain coefficient, respectively; for a given TR 0,m The acoustic mode yields the frequency shift pressure coefficient C. v-P =dv / dP is:

[0070]

[0071] Considering axial strain and radial strain respectively, the frequency shift-axial strain coefficient and the frequency shift-radial strain coefficient are expressed as follows:

[0072]

[0073]

[0074] Where dV L / dε a and dV L / dε r The value is derived from the fiber material parameters, thus obtaining a certain TR. 0,m The relationship between the frequency shift of the acoustic mode and pressure, for different TRs 0,m Voice model, C v-P The values ​​are different; when temperature and pressure change simultaneously, both will cause changes in the center frequencies of the two different resonance peaks in the FBS spectrum; when m = i or j (i ≠ j), the center frequencies (v) of the i-th and j-th resonance peaks will change. i and v j They are represented as follows:

[0075] v i =v i_0 +C v-T_i ·ΔT+C v-P_i ·ΔP (7)

[0076] v j =v j_0 +C v-T_j ·ΔT+C v-P_j ·ΔP (8)

[0077] Where v i_0 and v j_0The original acoustic modes TR are respectively 0,i and TR 0,j The corresponding center frequency of the resonance peak; C v-T_i and C v-T_j They are TR 0,i and TR 0,j The corresponding frequency shift temperature coefficient, C v-P_i and C v-P_j They are TR 0,i and TR 0,j The corresponding frequency shift pressure coefficient. Where ΔT and ΔP represent the changes in temperature and pressure, respectively, i.e.:

[0078]

[0079] Where Δv i and Δv j TR 0,i and TR 0,j The resulting change in the center frequency of the FBS spectrum;

[0080] In this way, the frequency shift information of the two FBS scattering peaks can be used to measure the lateral pressure on the fiber surface while remaining temperature-insensitive, and then by changing v m Distributed measurement of transverse pressure is achieved by exciting acoustic modes in optical fibers with different cladding diameters.

[0081] In summary, the principle of this invention's temperature-insensitive ocean current-driven submarine cable health detection method based on forward stimulated Brillouin scattering (FSBS) is that when ocean currents wash over the submarine cable and its outer test optical fiber, the cable and fiber are subjected to external lateral pressure. The axial and radial strains of the test optical fiber change with the external pressure. Since FBS is sensitive to changes in fiber strain, the change in the forward Brillouin frequency shift of fiber strain caused by external lateral pressure is measured by actively exciting and detecting the FBS signal in a double-coated single-mode fiber. This demodulates the magnitude of the lateral pressure on the fiber, thus determining the extent of ocean current scouring on the submarine cable. By changing the modulation frequency of the excitation light, the position of the corresponding fiber segment is obtained, thereby determining the health status of the submarine cable. Compared with existing technologies, this method has the following advantages:

[0082] 1. Temperature insensitive: By utilizing the frequency shift information of the two FBS scattering peaks, ocean current velocity measurement results can be provided without the influence of temperature.

[0083] 2. Real-time capability: It can acquire ocean current velocity information in real time and provide detection results promptly.

[0084] 3. The lateral pressure detection device of the present invention has the advantage of distributed pressure measurement, and can continuously measure the lateral pressure over a long distance.

[0085] The above embodiments are merely illustrative of the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made based on the essence of the content of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method of subsea cable health detection, characterized in that, The application relates to a method for measuring the transverse pressure on the surface of an optical fiber body of a submarine cable, and judging the strength of the flow scouring effect on the submarine cable. Step one: laying the optical fiber on the submarine cable; Step two: measuring the pressure of the flow scouring effect and collecting the forward Brillouin frequency shift change caused by the transverse pressure; Step three: processing the relationship between the forward Brillouin frequency shift change and the pressure to obtain the transverse pressure on the surface of the optical fiber, i.e. the pressure caused by the flow scouring effect on the submarine cable. The torsion radiation mode TR in a cylindrical optical fiber 0,m is sensitive to birefringence, and a change in pressure causes a change in the birefringence of the fiber, resulting in a change in the TR 0,m induced FBS spectral broadening, the center frequency v m of the resonance peak is: where a is the fiber cladding radius, V T is the transverse wave velocity, y m is the TR 0,m eigenvalue of the acoustic mode acoustic wave equation; The acoustic wave equation is: wherein a is the ratio of the transverse wave speed V T to the longitudinal wave speed V L of the optical fiber. The transverse pressure on the fiber surface is considered as the axial strain ε a radial strain ε r The combined effect of the FBS spectrum resonance peaks shows a linear relationship with axial and radial strain, respectively. A certain TR 0,m The frequency shift dv corresponding to the acoustic mode is expressed as: where are the frequency shift-axial strain coefficient and the frequency shift-radial strain coefficient, respectively; for a certain TR 0,m acoustic mode, the frequency shift pressure coefficient C v-P = dv / dP is: The frequency shift-axial strain coefficient and the frequency shift-radial strain coefficient are respectively represented as: Among them, dV L / dε a and dV L / dε r The value is derived from the fiber material parameters, thus obtaining a certain TR. 0,m The relationship between the frequency shift of the acoustic mode and pressure, for different TRs 0,m Voice model, C v-P The values ​​are different; when temperature and pressure change simultaneously, both will cause changes in the center frequencies of the two different resonance peaks in the FBS spectrum; when m = i or j (i ≠ j), the center frequencies v of the i-th and j-th resonance peaks are different. i and v j They are represented as follows: v i = v i_0 + C v-T_i • ΔT + C v-P_i • ΔP (7) v j = v j_0 + C v-T_j • ΔT + C v-P_j • ΔP (8) where v i_0 and v j_0 are the acoustic mode TR 0,i and TR 0,j corresponding resonance peak center frequency; C v-T_i and C v-T_j are the TR 0,i and TR 0,j corresponding frequency shift temperature coefficient, C v-P_i and C v-P_j are the TR 0,i and TR 0,j corresponding frequency shift pressure coefficient; wherein ΔT and ΔP are the changes in temperature and pressure, respectively, i.e.: Where Δv i and Δv j TR 0,i and TR 0,j The resulting change in the center frequency of the FBS spectrum; The temperature-insensitive simultaneous measurement of the transverse pressure on the fiber surface is realized by using the frequency shift information of two FBS scattering peaks, and the v m The distributed measurement of the transverse pressure is realized by exciting the acoustic modes in the fiber with different cladding diameters.

2. The method of health monitoring of a subsea cable according to claim 1, characterized in that, In the step two, the transverse pressure detection device is used to collect the forward Brillouin frequency shift change caused by the transverse pressure.

3. The method of health monitoring of a subsea cable according to claim 1, characterized in that, The step one comprises: The optical fiber is laid on the submarine cable, and is wound or laid longitudinally on the outside of the polypropylene outer layer of the submarine cable.

4. A method of health monitoring of a subsea cable according to any one of claims 1 to 3, characterised in that, The optical fiber is a double-coated single-mode optical fiber.

5. The method of health monitoring of a subsea cable according to claim 2, wherein, The transverse pressure detection device comprises a first distributed feedback laser, a first polarization controller, an electro-optic modulator, an erbium-doped fiber amplifier, an optical isolator, a first wavelength division multiplexer, a second distributed feedback laser, a second polarization controller, a polarization analyzer, a photoelectric detector, a data acquisition card, an arbitrary function generator and a direct current power supply. The excitation light pulse output by the first distributed feedback laser passes through the first polarization controller to adjust the polarization state to the electro-optic modulator, and the optical pulse light intensity of the sinusoidal pulse segment output by the electro-optic modulator reaches the maximum value, and then the pulse peak power is amplified by the erbium-doped fiber amplifier, and then the optical isolator and the first wavelength division multiplexer are injected into the double-coated single-mode optical fiber to excite the transverse resonance acoustic wave field; The probe light pulse output by the second distributed feedback laser is adjusted by the second polarization controller to reach the maximum value, and then is injected into the double-coated single-mode optical fiber through the first wavelength division multiplexer to read the transverse acoustic wave field signal, and then is output to the polarization analyzer to demodulate the polarization modulation to intensity modulation, and then the photoelectric conversion is carried out by the alternating current photoelectric detector, and the output photoelectric current is sampled and recorded by the data acquisition card according to the trigger signal provided by the arbitrary function generator.

6. The method of health monitoring of a subsea cable according to claim 5, wherein, The intensity modulation and direct current bias of the electro-optic modulator are controlled by the arbitrary function generator and the direct current power supply respectively, and the transverse resonance acoustic wave field of the optical fiber with different cladding diameters and the transverse resonance acoustic wave field of the optical fiber with different modes of the same cladding diameter are excited by changing the modulation frequency output by the arbitrary function generator, so that the distributed detection of the transverse pressure and the temperature insensitivity are realized.

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