Underwater concrete structure void monitoring system and method

Abstract

The application discloses an underwater concrete structure void monitoring system. The system comprises a fixed fiber grating vibration sensing network permanently fixed on the surface of the measured underwater structure, a mobile underwater laser-acoustic excitation unit carried on an underwater robot, and a shore-based fiber grating demodulation and data processing center. The fixed fiber grating vibration sensing network comprises a plurality of fiber grating strings arranged according to a preset grid, and each grating node has a unique spatial coordinate. The mobile underwater laser-acoustic excitation unit is used for emitting high-energy pulsed laser to excite structure vibration. The shore-based fiber grating demodulation and data processing center is used for real-time demodulation of wavelength changes of each grating node, synchronous recording of the excitation time and position of the mobile underwater laser-acoustic excitation unit, identification of void defects through analysis of time-frequency characteristics and propagation characteristics of vibration signals. The application realizes tracking and quantitative diagnosis of the whole process from the initiation to the development of the void.

Description

Technical Field

[0001] This invention relates to the field of underwater engineering structure health monitoring and non-destructive testing technology, specifically to an underwater concrete structure void monitoring system and method. Background Technology

[0002] For large concrete structures such as underwater dam panels, lock walls, and underwater bridge piers, voids behind the concrete panels are one of the main hidden defects threatening their safe operation. The emergence and spread of this type of defect are long-term and insidious, making full-life-cycle tracking a pressing need in the field of structural health monitoring.

[0003] Existing methods for detecting voids in underwater concrete mainly fall into three categories: contact detection, acoustic telemetry, and fiber optic sensing.

[0004] Among them, contact detection technologies (such as manual tapping and listening, underwater ultrasonic contact scanning, etc.) are simple in principle, but have the following technical problems: First, they rely on divers or ROVs for contact operations, resulting in extremely low detection efficiency and high risk; Second, these methods are single "check-ups", and the sensor coupling state and tapping force are inconsistent each time, resulting in poor repeatability of detection results and making it impossible to achieve long-term tracking and comparison of the same voided area.

[0005] Acoustic telemetry technologies (such as conventional imaging sonar and multibeam echo sounding systems) can achieve non-contact large-area surveys, but they have the following technical problems: because their operating frequency is usually tens to hundreds of kilohertz, the resolution of the concrete interior and the interface behind it is insufficient, making it difficult to detect early micro-voids with a diameter of less than 0.5m, and even more difficult to quantitatively assess the depth and stiffness changes of the voids.

[0006] Fiber grating (FBG) sensing technology possesses unique advantages such as being passive, resistant to electromagnetic interference, allowing for wavelength division multiplexing of numerous measurement points on a single fiber, and maintaining zero-point drift over long periods. It has been proven to be an ideal sensing element for long-term strain / vibration monitoring of underwater structures. However, most existing underwater FBG monitoring systems are passive, capable only of capturing sudden structural vibrations caused by environmental loads such as earthquakes and water flow pulsations. The signals are weak, and the excitation sources are uncontrollable. Furthermore, they cannot actively and quantitatively extract structural modal parameters (such as local resonant frequencies and amplitudes) directly related to voiding defects, lacking a controllable, repeatable, and sufficiently powerful active excitation method. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention proposes an underwater concrete structure void monitoring system and method. This monitoring system can be deployed for extended periods in harsh underwater environments, is completely passive and interference-resistant, and can efficiently coordinate with mobile inspection excitation sources to achieve full-process tracking and quantitative diagnosis of voids from their inception to development.

[0008] To achieve the above objectives, the present invention provides an underwater concrete structure void monitoring system, which is characterized by: a fixed fiber optic grating vibration sensing network permanently fixed to the surface of the underwater structure being tested, a mobile underwater laser acoustic excitation unit mounted on an underwater robot, and a shore-based fiber optic grating demodulation and data processing center. The fixed fiber grating vibration sensing network includes multiple fiber grating strings arranged according to a preset grid, and each grating node has a unique spatial coordinate, used to sense the dynamic strain / vibration generated by the structure under excitation. The mobile underwater laser acoustic excitation unit is used to emit high-energy pulsed lasers at excitation points near the surface of the structure at preset coordinate positions during regular inspections. It generates strong pulsed sound waves through underwater breakdown and cavitation effects to excite structural vibration. The shore-based fiber optic grating demodulation and data processing center is connected to the fixed fiber optic grating vibration sensing network via a transmission optical cable. It is used to demodulate the wavelength changes of each grating node in real time, synchronously record the excitation time and position of the mobile underwater laser acoustic excitation unit, and identify void defects by analyzing the time-frequency characteristics and propagation characteristics of the vibration signal.

[0009] Furthermore, the mobile underwater laser acoustic excitation unit is integrated into a dedicated pressure-resistant cabin of the underwater robot.

[0010] Furthermore, the intensity of the pulsed acoustic wave is changed by adjusting the power supply voltage or drive current of the laser.

[0011] Furthermore, the method for identifying whether a signal has become vacant includes comparing the dominant frequency of the time-domain vibration signal with a reference spectral fingerprint. If the dominant frequency decreases and the amplitude increases to a set threshold, it is determined to be a suspected vacant signal.

[0012] Furthermore, methods for identifying whether a void has been removed include repeatedly exciting and inverting the wave velocity distribution of the panel; if the wave velocity decreases abnormally, a void defect is identified.

[0013] Furthermore, the method for identifying whether a void has been removed also includes using a comprehensive anomaly criterion fusion algorithm to determine whether a void defect exists; the comprehensive anomaly criterion fusion algorithm refers to using weighted evidence fusion to quantify the comprehensive anomaly index for each excitation point-sensor pair. Define the comprehensive anomaly index If the comprehensive abnormal index If the value exceeds the set threshold, it is marked as a void defect; The comprehensive anomaly index Expressed by the following formula In the formula, Represents a dimensionless exponent between 0 and 1. Indicates amplitude, Indicates decay time. A reference value representing group velocity. , , , Indicates the weight.

[0014] This invention also designs a method for monitoring voids in underwater concrete structures, applicable to the aforementioned underwater concrete structure void monitoring system, characterized by the following steps: S1) Deploy a fixed fiber optic grating vibration sensing network on the surface of the underwater structure and calibrate the coordinates of each sensor; S2) Deploy underwater robots regularly to integrate mobile underwater laser acoustic excitation units. The underwater robots navigate to each excitation point according to the planned path, emit high-energy pulsed lasers, and record the excitation time and position. When the fixed fiber optic grating vibration sensing network structure is intact, perform the first full-grid benchmark scan under water storage conditions to establish a benchmark spectrum fingerprint database. S3) The shore-based fiber optic grating demodulation and data processing center synchronously collects the time-domain vibration signals of all grating nodes; S4) The shore-based fiber optic grating demodulation and data processing center performs time-frequency analysis on the time-domain vibration signal, extracts time-frequency features, compares the extracted time-frequency features with the reference spectral fingerprint, and combines multipath acoustic wave propagation velocity tomography to comprehensively determine whether there is a void and generate a structural health cloud map.

[0015] Furthermore, in S1), the underwater installation process of the fixed fiber optic grating vibration sensing network includes the following steps: S11) Surface treatment of underwater structures; S12) Accurately mark the installation point of each sensor on the panel and mark the point; S13) Fix the sensor at the installation point; S14) The fiber optic cable leading out of the sensor is mechanically protected by a stainless steel armored hose, and the armored fiber optic cable is led along the panel to the predetermined junction point and connected to the oil-filled pressure-compensated underwater fiber optic junction box. S15) A multi-core trunk armored optical cable is led out from the underwater optical fiber junction box, along the dam surface to the top of the dam, and finally connected to the optical fiber distribution frame in the shore-based demodulation room.

[0016] Furthermore, in S4), the time-frequency analysis process executed by the shore-based fiber optic grating demodulation and data processing center is as follows: S41) Perform the same filtering and conversion process as when establishing the reference on all signals to obtain the dynamic strain signal under the current state; S42) For each excitation point-sensor pair, calculate the current principal resonance frequency. Amplitude and decay time ; S43) A weighted fusion algorithm is used to calculate the comprehensive anomaly index; the comprehensive anomaly index is calculated using the following formula. In the formula, A dimensionless exponent between 0 and 1. For amplitude, For decay time, As the baseline value for group velocity, ~ As weight; S44) Based on the comprehensive anomaly index The size determines the level of early warning.

[0017] Furthermore, in S44), if the comprehensive anomaly index A comprehensive abnormality index of <0.3 is considered normal; if the comprehensive abnormality index is 0.3≤ A value <0.5 indicates a slight abnormality, and closer monitoring is recommended for the next assessment; if the overall abnormality index is... A value ≥0.5 is considered a serious anomaly and is marked as a highly suspected void area.

[0018] The advantages of this invention are: 1. This invention achieves a leap from "one-time physical examination" to "life-long health monitoring"; This invention is the first to organically couple a mobile underwater laser-induced acoustic excitation unit with a permanently deployed fixed fiber optic grating vibration sensing network. The mobile inspection ROV provides standardized and repeatable acoustic excitation, while the fixed fiber optic grating vibration sensing network ensures that signals are received at exactly the same spatial location for each test. This solves the problems of inconsistent impact force and sensor coupling conditions caused by human factors in traditional contact testing. By comparing the wet spectral fingerprints of the same measurement point in different years, the complete evolution process of delamination defects from initiation to deterioration can be accurately tracked, truly achieving full life-cycle health monitoring. 2. The feasibility of implementing an underwater long-term passive monitoring network project using this invention; The receiver of this invention uses fiber optic grating sensors throughout, eliminating the need for any power supply or electronic components underwater. This fundamentally eliminates systemic failures such as cable corrosion, insulation degradation, electromagnetic interference, and node waterproofing failure that occur during long-term deployment of traditional piezoelectric hydrophones. A single optical fiber can connect dozens of measurement points, and by combining wavelength division multiplexing and space division multiplexing technologies, a high-density sensor network can be constructed on dam panels on a scale of hundreds of meters. Furthermore, the absolute wavelength encoding characteristics of the grating ensure the zero-point stability of the signal over a timescale of decades. 3. This invention achieves quantitative diagnosis through a deep integration of "active excitation + passive sensing"; This invention overcomes the fatal flaws of existing FBG monitoring systems, which can only passively receive environmental vibrations, have weak signals, and have uncontrollable excitation sources. By using a mobile underwater laser acoustic excitation unit to precisely excite broadband strong pulse sound waves covering hundreds to thousands of hertz at preset coordinates, the system generates bending vibration responses that can be distinguished by FBG. This allows the system to actively extract structural modal parameters directly related to the voiding defect—the main resonant frequency, amplitude, and decay time. For the first time, acoustic tomography is introduced as a supplementary criterion in the field of underwater FBG monitoring, thereby achieving quantitative assessment of the voiding range, voiding depth, and development trend. Its diagnostic capabilities far exceed those of conventional sonar. 4. This invention achieves system-level synergistic advantages in the "light-to-light" energy conversion chain; The complete energy conversion chain of this invention is as follows: electrical energy → pulsed laser → underwater cavitation acoustic energy → structural vibration energy → fiber grating wavelength shift → optical demodulation; wherein, the pulse energy, pulse width, and repetition frequency of the laser can be precisely controlled, the response of the FBG has a linear wavelength-strain relationship, and the input and output of the entire chain are precisely controllable optical quantities; this all-optical physical architecture of "using light to stimulate sound and using light to sense vibration" naturally possesses extremely strong anti-interference ability and signal repeatability, laying a physical foundation for establishing a long-term comparable structural health database; 5. This invention achieves system robustness and maintenance flexibility; This invention adopts an architecture that physically separates the excitation end and the receiving end: the permanently deployed underwater fixed fiber optic grating vibration sensing network is a passive device that requires no maintenance once installed; the mobile underwater laser acoustic excitation unit can be flexibly upgraded, replaced or repaired as needed without affecting the deployed sensing network; moreover, the failure of a single sensor does not affect the normal operation of the remaining nodes, and the overall robustness of the system is far higher than that of traditional solutions that rely on centralized electrical transducers. 6. This invention constructs a multi-criteria fusion diagnostic system, which significantly reduces the risk of environmental misjudgment; This invention constructs a four-dimensional criterion fusion system of "vibration modal frequency shift + abnormal amplitude increase + acoustic tomography + prolonged decay time", which fundamentally overcomes the problem that single vibration criteria are easily affected by environmental factors such as water flow and temperature.

[0019] The present invention relates to an underwater concrete structure void monitoring system and method, which for the first time utilizes a mobile pulsed laser to periodically provide standardized and highly repeatable acoustic excitation, and then uses a permanently deployed FBG array to capture the structural response. By comparing with wet-state reference fingerprints, long-term high-precision diagnosis of voids is achieved, realizing the whole process of voids from initiation to development and quantitative diagnosis. Attached Figure Description

[0020] Figure 1 This is a flowchart of the underwater concrete structure void monitoring method in this invention; Figure 2 This is a rear view of the integrated form of the mobile underwater laser acoustic excitation unit in this invention. Figure 3 This is a front view of the integrated form of the mobile underwater laser acoustic excitation unit in this invention; Figure 4 This is a top view of the integrated mobile underwater laser acoustic excitation unit in this invention. In the picture: 1. Control and data communication interface; 2. High-precision time synchronization fiber optic interface; 3. Main power supply interface; 4. Laser safety interlock interface; 5. Auxiliary sensor and status monitoring interface; 6. Optical window; 7. High-pressure gas nozzle; 8. Through-chamber connector; 9. Power management module; 10. Pump source and Q-switching module; 11. Cooling base; 12. Collimating and beam expander; 13. Focusing lens; 14. Anti-splash protection baffle. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] This invention designs an underwater concrete structure void monitoring system, including a fixed fiber optic grating (FBG) vibration sensing network permanently fixed to the surface of the underwater structure under test, a mobile underwater laser acoustic excitation unit mounted on an underwater robot (ROV), and a shore-based fiber optic grating demodulation and data processing center.

[0023] The fixed fiber optic grating vibration sensing network includes multiple fiber optic grating strings arranged according to a preset grid, and each grating node has a unique spatial coordinate, used to sense the dynamic strain / vibration generated by the structure under stress.

[0024] The vibration sensing mechanism of fiber optic gratings (FBGs) is as follows: FBG reflection wavelength shift. With dynamic strain The relationship is in The elastic-optic coefficient is used. By separating the effects of temperature through high-pass filtering, structural vibration can be obtained in real time.

[0025] The mobile underwater laser acoustic excitation unit is used to emit high-energy pulsed lasers at excitation points near the surface of the structure at preset coordinate positions during periodic inspections. It generates strong pulsed sound waves through underwater breakdown and cavitation effects to excite structural vibration.

[0026] Specifically, the mobile underwater laser acoustic excitation unit is integrated into a dedicated pressure-resistant cabin of the underwater robot (ROV). The dedicated pressure-resistant cabin is connected to the underwater robot (ROV) via a metal clamp or frame and is horizontally and stably installed on the belly of the working-class underwater robot (ROV).

[0027] The shore-based fiber optic grating demodulation and data processing center is connected to the fixed fiber optic grating vibration sensing network via a transmission optical cable. It is used to demodulate the wavelength changes of each grating node in real time, synchronously record the excitation time and position of the mobile underwater laser acoustic excitation unit, and identify void defects by analyzing the time-frequency characteristics and propagation characteristics of the vibration signal.

[0028] like Figure 2 The image shown is a rear view of the integrated mobile underwater laser acoustic excitation unit. Figure 3 The image shown is a front view of the integrated form of the mobile underwater laser acoustic excitation unit.

[0029] The mobile underwater laser acoustic excitation unit integrates a control and data communication interface 1, a high-precision time synchronization fiber optic interface 2, a main power supply interface 3, a laser safety interlock interface 4, and an auxiliary sensor and status monitoring interface 5, all located on the rear end.

[0030] The control and data communication interface 1 is used to transmit laser emission commands, status monitoring data, and system configuration parameters between the ROV control computer and the laser's internal MCU controller. The high-precision time synchronization fiber optic interface 2 is used to directly connect the dedicated time synchronization fiber optic cable in the ROV umbilical cable to the shore-based demodulator, employing IEEE 1588 PTP (Precise Time Protocol) or White Rabbit technology to achieve nanosecond-level precise synchronization between the ROV's local clock and the shore-based demodulator's clock. The main power supply interface 3 is used to supply high-voltage DC power from the ROV umbilical cable to the laser. The laser safety interlock interface 4 is used to connect external safety sensors (such as proximity switches and door status switches) deployed on the ROV frame, ensuring that high-energy pulses are emitted only under safe conditions to prevent accidental injury to divers or damage to equipment. The auxiliary sensor and status monitoring interface 5 is used to connect external monitoring sensors to perceive key status parameters of the pressure chamber and its surrounding environment in real time.

[0031] The mobile underwater laser acoustic excitation unit integrates an optical window 6 and a high-pressure gas nozzle 7 located on the front end.

[0032] The optical window 6 is made of sapphire glass, which has high light transmittance and is hard and pressure resistant. The high-pressure gas nozzle 7 is pointed in front of the window to blow away air bubbles or impurities adhering to the window.

[0033] like Figure 4 The diagram shown is a top-view structural diagram of the integrated mobile underwater laser-induced acoustic excitation unit. The integrated mobile underwater laser-induced acoustic excitation unit includes a power supply and communication management compartment, a laser generation compartment, and an optical path shaping module. Specifically, it includes a through-compartment connector 8, a power management module 9, a pump source and Q-switching module 10, a cooling base 11, a collimating beam expander 12, a focusing lens 13, and a splash-proof protection baffle 14.

[0034] The trans-cabin connector 8 penetrates the rear cover and is responsible for introducing the power and fiber optic signals provided by the ROV umbilical cable into the cabin, ensuring internal and external isolation. The power management module 9 is located in the power supply and communication management cabin and is used to connect the watertight connector to distribute and stabilize voltage for all internal equipment. The pump source and Q-switching module 10 is located in the laser generation cabin and is used to emit a green high-energy pulsed laser with a wavelength of 532nm. The cooling base 11 is located in the laser generation cabin and uses a high-efficiency heat-conducting plate attached tightly to the bottom of the laser, directly connected to the metal cabin wall, to transfer heat to the external water environment. The collimating beam expander 12 is located in the optical path shaping module. The laser first passes through here, changing from a divergent state to a parallel beam with a slightly larger diameter, which is beneficial for focusing. The focusing lens 13 is placed at the front exit of the cabin and focuses the parallel light from the reflector in the water to form a focal point with extremely high energy density. The focal point is located 5 to 10 centimeters in front of the external window. The anti-backsplash protection baffle 14 is located between the focusing lens 13 and the optical window 6, with a small hole in the center that allows only the main beam to pass through, effectively blocking the particles sputtered back from the focal point from contaminating the lens.

[0035] The working principle of the mobile underwater laser acoustic excitation unit follows a clear energy conversion chain: electrical energy → light energy → acoustic energy.

[0036] First, the laser converts the input electrical energy into a high-energy pulsed laser output; the output laser pulse energy... With pulse width Determines the peak power of the laser .

[0037] The high-energy pulsed laser penetrates the water at the underwater excitation point, forming a plasma shock wave and cavitation. The cavitation generates a secondary pressure pulse upon collapse, the intensity of which is positively correlated with the laser energy. If the peak sound pressure level of the shock wave... With laser pulse energy Approximately satisfy If the relationship is such that the shock wave is the sound source that excites the structure under test, then the shock wave is the sound source that excites the structure under test.

[0038] The intensity of the shock wave is changed by adjusting the power supply voltage or drive current of the laser. This energy conversion and control mechanism allows the intensity of the underwater sound source of this invention to be flexibly adjusted according to factors such as the thickness and distance of the structure being measured, thereby ensuring the signal-to-noise ratio of the detection signal and the defect identification capability.

[0039] This invention constructs a four-dimensional criterion fusion system of "vibration modal frequency shift + abnormal amplitude increase + acoustic tomography + prolonged decay time".

[0040] First dimension and second dimension: The void region can be considered as a thin plate with elastic support, and its fundamental frequency of excited bending vibration is... f With the radius of the void a and plate thickness h It approximately satisfies the following formula: In the formula, f The fundamental frequency of bending vibration in the delamination region. h Indicates the thickness of the thin plate. a Indicates the radius of the void. E represents the Young's modulus of an elastic sheet. Indicates the density of an elastic thin plate. is the Poisson's ratio for an elastic thin plate.

[0041] The loss of stability leads to a decrease in local equivalent stiffness, which manifests as a significant shift in the main frequency of the signal towards the lower frequency range and a marked increase in amplitude.

[0042] Specifically, the method for determining whether a signal has detached is as follows: the dominant frequency of the time-domain vibration signal is compared with the reference spectrum fingerprint. If the dominant frequency decreases and the amplitude increases to a set threshold, it is determined to be a suspected detachment.

[0043] Preferably, the main frequency offset rate The threshold is set at 15%~20%; The amplitude increase rate The threshold is set to 2.0 to 3.0 times; in, f A and A' are the currently measured principal resonant frequency and amplitude of the vacuolation region, respectively. and These are the reference principal resonance frequency and reference amplitude, respectively, in the wet reference fingerprint database.

[0044] More preferably, a tiered threshold can be used: when >15% and When the value is >2.0, it is judged as a suspected void; when >20% and When the value is >3.0, it is considered highly likely to be a void.

[0045] Based on the quantitative derivation of damage mechanics, for an equivalent circular thin plate with thickness h and void radius a, the fundamental frequency formula of the equivalent circular thin plate is: In the formula, This represents the fundamental frequency of bending vibration of an equivalent circular thin plate. This represents the first eigenvalue of a simply supported circular plate. =3.196, h This indicates the thickness of an equivalent circular thin plate. a Indicates the radius of the void. E represents the Young's modulus of an equivalent circular thin plate. This represents the density of an equivalent circular thin plate. This represents the Poisson's ratio of an equivalent circular thin plate.

[0046] When a void occurs, the panel that was originally in close contact with the dam body suddenly loses its constraint, and the equivalent support condition changes from approximately rigid contact to approximately simply supported boundary. At this time, not only will the eigenvalue λ in the frequency formula change abruptly, but the area of ​​the effective vibration region will also change abruptly from the minimum value at contact to the area of ​​the void region, resulting in a sharp drop in frequency.

[0047] Third dimension: Acoustic tomography is used to supplement the criteria. Specifically, excited strong pulsed acoustic waves propagate between fiber optic grating vibration sensors, and the propagation path is measured. Time difference of arrival Calculate group velocity Voiding leads to an abnormal decrease in wave velocity. Therefore, by repeatedly exciting and inverting the wave velocity distribution of the panel, if the wave velocity decreases abnormally, a voiding defect is identified. This is consistent with the results of voiding defect identification analysis by comparing the dominant frequency of the time-domain vibration signal with the reference spectral fingerprint.

[0048] Fourth dimension: A comprehensive anomaly criterion fusion algorithm can also be used to determine whether a void defect exists. Specifically, the comprehensive anomaly criterion fusion algorithm refers to using weighted evidence fusion to quantify the comprehensive anomaly index for each "trigger point-sensor pair". Define the comprehensive anomaly index If the comprehensive abnormal index If the value exceeds the set threshold, it is marked as a void defect.

[0049] Preferably, the comprehensive anomaly index The critical value is set to 0.5.

[0050] The comprehensive anomaly index Expressed by the following formula In the formula, Represents a dimensionless exponent between 0 and 1. Indicates amplitude, Indicates decay time. Represents group velocity; values ​​with subscript 0 are baseline values. , , , The weights are determined through calibration experiments.

[0051] The recommended values ​​for each weight coefficient in the formula are: =0.4、 =0.3、 =0.2、 =0.1; when When the value is greater than 0.5, the corresponding "excitation point-sensor pair" is determined. j,i The area in question is a void defect.

[0052] More preferably, a multi-level determination is adopted: <0.3 is considered normal; 0.3≤ <0.5, considered a minor abnormality; If the value is ≥0.5, it is considered a highly suspected void area.

[0053] The four-dimensional criterion fusion system of "vibration mode frequency shift + abnormal amplitude increase + prolonged decay time + acoustic tomography" fundamentally overcomes the problem that single vibration criteria are easily affected by environmental factors such as water flow and temperature.

[0054] Specifically, water flow pulsations or temperature changes may cause an overall shift in the vibration frequency of concrete panels. However, this environmentally induced shift is global. Under differential comparison with a fixed FBG sensor network, the relative frequency shift between intact and voided areas remains stable. This invention relies on the relative change at the same measurement point over different times, rather than the absolute frequency value. Furthermore, environmental factors typically do not simultaneously cause a significant increase in amplitude and a significant extension in decay time. The weighted evidence fusion algorithm of this invention can effectively eliminate false alarms caused by a single parameter anomaly. In addition, the introduced acoustic tomography supplementary criterion provides cross-verification from the physical dimension of guided wave propagation velocity, further reducing the risk of misjudgment to a significantly low level.

[0055] This embodiment takes the long-term monitoring of underwater panel voids in a dam as an example, combined with the attached... Figure 1 The process is described in detail. This invention designs a method for monitoring voids in underwater concrete structures, applicable to underwater concrete structure void monitoring systems, and includes the following steps: S1) A fixed fiber optic grating vibration sensing network is deployed on the surface of the underwater structure, and the coordinates of each sensor are calibrated.

[0056] Specifically, this embodiment uses an apodized fiber grating with a center wavelength of 1550nm, packaged in a 316L stainless steel tubular form. The grating region is kept under slight prestress within the package to ensure a response to both compressive and tensile strains. The package has an outer diameter of 4mm and flange mounting bases at both ends. Its first-order resonant frequency is designed to be greater than 10kHz to ensure a flat response to structural bending vibration frequencies (typically hundreds to thousands of hertz).

[0057] Preferably, the fixed fiber optic grating vibration sensing network uses a 2m×2m orthogonal grid (the grid spacing can be adjusted according to accuracy requirements). Multiple rows of sensor strings are arranged horizontally along different elevations, and vertically along selected sections along the dam axis. Twenty different wavelength FBGs are written at equal intervals (2 meters) on each fiber string, and multiple points are connected in series on a single fiber using wavelength division multiplexing technology.

[0058] The underwater installation process of the fixed fiber optic grating vibration sensing network is as follows: S11) Surface treatment of the underwater structure. Specifically, high-pressure water guns and wire brushes are used to remove biofouling, loose layers and oil stains from the concrete surface until a solid base surface is exposed.

[0059] S12) Positioning and laying out.

[0060] Using the underwater laser projector or underwater total station mounted on the ROV, accurately mark the installation point of each sensor on the panel and mark the point.

[0061] S13) Sensor fixed.

[0062] Press the sensor flange base firmly onto the concrete surface using underwater-grade high-strength epoxy resin putty, temporarily securing it with sandbags or mechanical clamps until the putty reaches its designed curing strength (approximately 24 hours at room temperature). This putty must possess a coefficient of thermal expansion compatible with concrete and exhibit good long-term water resistance.

[0063] S14) Fiber optic protection and splicing.

[0064] Each sensor's fiber optic pigtail is mechanically protected by a stainless steel armored flexible hose. The armored pigtails of each grating string are led along the panel to a predetermined junction point and connected to an oil-filled pressure-compensated underwater fiber optic junction box. Fiber optic splicing and coiling are completed inside the junction box, which is filled with insulating silicone oil and connected to an external bladder-type pressure compensator to ensure that the pressure inside the box is always slightly higher than the external water pressure, thus preventing water leakage in principle.

[0065] S15) A multi-core trunk armored optical cable is led out from the underwater optical fiber junction box, along the dam surface to the top of the dam, and finally connected to the optical fiber distribution frame in the shore-based demodulation room.

[0066] After the fixed fiber optic grating vibration sensor network is installed, an ROV carrying an underwater total station or a high-precision acoustic positioning system measures the actual spatial coordinates of each FBG sensor one by one. The measurement accuracy is controlled within ±5cm and the data is entered into the system database.

[0067] S2) Regularly deploy underwater robots (ROVs) to integrate mobile underwater laser acoustic excitation units. The underwater robots navigate to each excitation point according to the planned path, emit high-energy pulsed lasers, and record the excitation time and position. When the fixed fiber optic grating vibration sensing network structure is intact, perform the first full-grid benchmark scan under water storage conditions to establish a benchmark spectral fingerprint database.

[0068] The mobile underwater laser acoustic excitation unit is the "active impact source" of the underwater concrete structure void monitoring system, and is integrated on a working-class ROV.

[0069] In this embodiment, the mobile underwater laser acoustic excitation unit operates according to the following typical parameters: In this embodiment, the underwater robot (ROV) provides a 400V DC bus voltage to the laser via an umbilical cable. The internal laser diode pump module operates at this voltage, driving a frequency-doubled Nd:YAG Q-switched laser with an output wavelength of 532nm, a single-pulse energy of 300mJ, and a pulse width of 8ns. The above parameters are specific values ​​used in this embodiment only.

[0070] The pulsed laser beam was focused in the water at a distance of 10 cm from the dam's panel surface using an optical system and penetrated the water. Based on The resulting plasma shock wave and cavitation collapse pulse together constitute a strong pulse sound source with sufficient energy to penetrate water and excite bending vibration modes of the concrete panel within the effective area, which can be distinguished by fiber optic grating sensors. Actual measurements show that under these parameters, a single excitation can obtain an effective vibration signal within a maximum radius of approximately 3 meters.

[0071] When it is necessary to inspect thicker or more attenuated structures, the operator can increase the power supply voltage or drive current of the laser through the ROV control system to increase the single pulse energy of the output laser, thereby obtaining a stronger underwater sound source.

[0072] Specifically, the requirements for the ROV platform are as follows: the ROV must have an operating water depth of at least 150 meters, be equipped with a high-precision integrated navigation system (fiber optic inertial navigation INS + Doppler log DVL + depth gauge + ultra-short baseline positioning USBL), be able to achieve centimeter-level dynamic positioning and attitude maintenance, and be connected to the surface support vessel or dam top control room via an umbilical cable, which includes a power supply cable, a communication optical fiber, and a dedicated time synchronization optical fiber.

[0073] The integration of the mobile underwater laser-induced acoustic excitation unit includes: mounting the laser system within a separate cylindrical 316L stainless steel pressure-resistant chamber, integrated into the bottom of the ROV. In this embodiment, the specific design of the pressure-resistant chamber is as follows: ① Cabin: The flat-head cylindrical cabin is sealed with double-layer O-rings. All external electrical and fiber optic interfaces use dry-plug watertight connectors with a nominal pressure resistance depth of 300 meters.

[0074] ② Optical window: A 50mm diameter optical window is opened at the front of the cabin, using sapphire glass as the light-transmitting material. The window glass and the metal base are sealed by metallization welding or a special O-ring, which can withstand the pressure of a water depth of 150 meters.

[0075] ③ Internal Laser: A diode-pumped, frequency-doubled Nd:YAG Q-switched pulsed laser with an output wavelength of 532 nm, a single pulse energy of 300 mJ, a pulse width of 8 ns, and a maximum repetition frequency of 10 Hz. The laser power supply and heat dissipation system are integrated within the pressure-resistant chamber, and heat dissipation is conducted to the external water environment through the chamber walls.

[0076] ④ Focusing Optical Path: After being deflected by a reflector, the laser beam passes through a short-focal-length water-immersion focusing lens, with the focal point designed 5-10 cm outside the outer surface of the optical window. During laser emission, a high-pressure gas protection nozzle creates a micro-airflow on the window surface to prevent air bubbles generated by excitation from adhering to the window and affecting subsequent measurements. The high-pressure gas nozzle is supplied by a high-pressure gas cylinder carried by the ROV, and is sprayed for 0.1 seconds before each excitation to remove air bubbles from the window. The laser uses heat dissipation through the cabin wall and can operate continuously for 30 minutes at a 10Hz repetition frequency, meeting the requirements of a single inspection. The focal point is designed 5-10 cm outside the window; at this distance, the energy conversion efficiency of the laser penetrating the water is highest, and the peak sound pressure of the generated shock wave can reach over 1 MPa.

[0077] In this embodiment, the ROV deployment and excitation are as follows: The ROV is equipped with a mobile underwater laser acoustic excitation unit, which performs a full-coverage scan according to a preset grid of 1m × 1m. At each excitation point... The ROV hovers and positions itself, the laser actuator emits a pulse, and the excitation moment is precisely recorded. and actual excitation coordinates ( ) .

[0078] During excitation, the ROV controller sends an optical synchronization pulse to the shore-based demodulator via the synchronization fiber within the umbilical cable; the rising edge of this pulse marks a precise time. In this embodiment, the shore-based demodulator synchronously acquires data from all FBG channels at a sampling rate of 5000 Hz, recording data from... Complete time-domain waveform data from 0.1 seconds to 0.5 seconds later, with the raw data for each channel being a one-dimensional wavelength offset sequence. .

[0079] Each "excitation point j - sensor k" performs a high-pass filter (50Hz cutoff) on the recorded signal to eliminate temperature drift, and then converts it into dynamic strain. A windowed Fast Fourier Transform (FFT) is applied to it to obtain the spectrum. Automatically extract the center frequency of the first-order resonance peak in the spectrum. Peak amplitude The signal decay time was calculated using the logarithmic decay method. .Will( , , This is stored in the database as a wet-state reference fingerprint of the "excitation-reception" relationship.

[0080] S3) The shore-based fiber optic grating demodulation and data processing center synchronously collects the time-domain vibration signals of all grating nodes.

[0081] The steps for synchronously acquiring the time-domain vibration signals of all grating nodes are as follows: S31) The ROV arrives at each excitation point sequentially according to the same path procedure used during the baseline scan.

[0082] S32) At each excitation point, the ROV uses USBL and optical feature point comparison for precise positioning to ensure that the actual excitation position deviates from the position during the reference scan by no more than 10 cm.

[0083] S33) Following the same steps as S2), emit a laser pulse once more, and synchronously trigger acquisition via optical fiber to record the time-domain response signals of all FBG channels. .

[0084] Preferably, regular health monitoring is required every two years. The procedure strictly replicates the baseline scan process.

[0085] S4) The shore-based fiber optic grating demodulation and data processing center performs time-frequency analysis on the time-domain vibration signal, extracts time-frequency features, compares the extracted time-frequency features with the reference spectral fingerprint, and combines multipath acoustic wave propagation velocity tomography to comprehensively determine whether there is a void and generate a structural health cloud map.

[0086] The time-frequency characteristics include the main resonance frequency, amplitude, and decay time.

[0087] After each inspection, the shore-based fiber optic grating demodulation and data processing center automatically executes the following analysis process: S41) Signal preprocessing: Perform the same filtering and conversion process as when establishing the reference on all signals to obtain the dynamic strain signal under the current state.

[0088] S42) Feature Extraction: For each "excitation point-sensor pair", the current main resonance frequency is also calculated. Amplitude and decay time .

[0089] S43) Calculation of Comprehensive Anomaly Index: To address the unreliability of a single criterion, a weighted fusion algorithm is used to calculate the comprehensive anomaly index.

[0090] The comprehensive anomaly index is calculated using the following formula. In the formula, A dimensionless exponent between 0 and 1. For amplitude, For decay time, The value is the group velocity, with the value marked with a subscript of 0 being the baseline value. ~ The weights were determined using the analytic hierarchy process (AHP) combined with indoor model experiments, and the recommended value is [value missing]. =0.4、 =0.3、 =0.2、 =0.1, with a weighted sum of 1. This embodiment is calibrated using 100 sets of experimental data on voids of different sizes, with a misjudgment rate of ≤5%. A comprehensive anomaly index can be set. If the value exceeds the set threshold, it is marked as a void defect.

[0091] S44) Multi-level diagnostic judgment: based on comprehensive abnormality index Size, perform hierarchical pre- Police, specifically: like <0.3 is considered normal; If 0.3≤ <0.5 indicates a minor abnormality; close monitoring is recommended for the next test. like A value ≥0.5 is considered a serious anomaly and is marked as a highly suspected void area.

[0092] Simultaneously, for the red area, supplementary criteria need to be employed: using multiple surrounding sensors for acoustic tomography to calculate the guided wave group velocity along each path. If the wave velocity in a certain area decreases by more than 10% compared to the baseline, it cross-validates with the modal analysis results, ultimately confirming the defect.

[0093] The health cloud map generation involves spatial kriging interpolation of the comprehensive anomaly index of all FBG nodes to generate a structural health cloud map covering the entire dam panel; and the structural health cloud map intuitively uses color to mark abnormal areas.

[0094] Preferably, a detailed monitoring report is then generated, including: basic information of this inspection (time, environmental parameters), a list of suspected defects, and a trend chart comparing each defect area with historical data. The list of suspected defects includes the center coordinates, equivalent area (through connected component analysis), and severity level of each defect area. The trend chart comparing each defect area with historical data is used to assess whether voiding is developing.

[0095] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A monitoring system for voids in underwater concrete structures, characterized in that: It includes a fixed fiber optic grating vibration sensing network permanently fixed to the surface of the underwater structure under test, a mobile underwater laser acoustic excitation unit mounted on an underwater robot, and a shore-based fiber optic grating demodulation and data processing center. The fixed fiber grating vibration sensing network includes multiple fiber grating strings arranged according to a preset grid, and each grating node has a unique spatial coordinate, used to sense the dynamic strain / vibration generated by the structure under excitation. The mobile underwater laser acoustic excitation unit is used to emit high-energy pulsed lasers at excitation points near the surface of the structure at preset coordinate positions during regular inspections. It generates strong pulsed sound waves through underwater breakdown and cavitation effects to excite structural vibration. The shore-based fiber optic grating demodulation and data processing center is connected to the fixed fiber optic grating vibration sensing network via a transmission optical cable. It is used to demodulate the wavelength changes of each grating node in real time, synchronously record the excitation time and position of the mobile underwater laser acoustic excitation unit, and identify void defects by analyzing the time-frequency characteristics and propagation characteristics of the vibration signal.

2. The underwater concrete structure void monitoring system according to claim 1, characterized in that: The mobile underwater laser acoustic excitation unit is integrated into the underwater robot's dedicated pressure-resistant cabin.

3. The underwater concrete structure void monitoring system according to claim 2, characterized in that: The intensity of the pulsed acoustic wave is changed by adjusting the power supply voltage or drive current of the laser.

4. The underwater concrete structure void monitoring system according to claim 1, characterized in that: Methods for identifying whether a signal has become vacant include comparing the dominant frequency of the time-domain vibration signal with a reference spectral fingerprint. If the dominant frequency decreases and the amplitude increases to a set threshold, it is determined to be a suspected vacant signal.

5. The underwater concrete structure void monitoring system according to claim 4, characterized in that: Methods for identifying whether a void has been removed also include repeatedly exciting and inverting the wave velocity distribution of the panel; if the wave velocity decreases abnormally, a void defect is determined.

6. The underwater concrete structure void monitoring system according to claim 5, characterized in that: The method for identifying whether a void has been detached also includes using a comprehensive anomaly criterion fusion algorithm to determine whether a void defect exists; the comprehensive anomaly criterion fusion algorithm refers to using weighted evidence fusion to quantify the comprehensive anomaly index for each excitation point-sensor pair. Define the comprehensive anomaly index If the comprehensive abnormal index If the value exceeds the set threshold, it is marked as a void defect; The comprehensive anomaly index Expressed by the following formula In the formula, Represents a dimensionless exponent between 0 and 1. Indicates amplitude, Indicates decay time. A reference value representing group velocity. , , , Indicates the weight.

7. A method for monitoring voids in underwater concrete structures, applicable to the underwater concrete structure void monitoring system as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1) Deploy a fixed fiber optic grating vibration sensing network on the surface of the underwater structure and calibrate the coordinates of each sensor; S2) Deploy underwater robots periodically to integrate mobile underwater laser acoustic excitation units. The underwater robots navigate to each excitation point according to the planned path, emit high-energy pulsed lasers, and record the excitation time and location. And when the fixed fiber optic grating vibration sensing network structure is intact, the first full-grid benchmark scan is carried out under water storage conditions to establish a benchmark spectral fingerprint database; S3) The shore-based fiber optic grating demodulation and data processing center synchronously collects the time-domain vibration signals of all grating nodes; S4) The shore-based fiber optic grating demodulation and data processing center performs time-frequency analysis on the time-domain vibration signal, extracts time-frequency features, compares the extracted time-frequency features with the reference spectral fingerprint, and combines multipath acoustic wave propagation velocity tomography to comprehensively determine whether there is a void and generate a structural health cloud map.

8. The method for monitoring voids in underwater concrete structures according to claim 7, characterized in that: In S1), the underwater installation process of the fixed fiber optic grating vibration sensing network includes the following steps. S11) Surface treatment of underwater structures; S12) Accurately mark the installation point of each sensor on the panel and mark the point; S13) Fix the sensor at the installation point; S14) The fiber optic cable leading out of the sensor is mechanically protected by a stainless steel armored hose, and the armored fiber optic cable is led along the panel to the predetermined junction point and connected to the oil-filled pressure-compensated underwater fiber optic junction box. S15) A multi-core trunk armored optical cable is led out from the underwater optical fiber junction box, along the dam surface to the top of the dam, and finally connected to the optical fiber distribution frame in the shore-based demodulation room.

9. The method for monitoring voids in underwater concrete structures according to claim 8, characterized in that: In S4), the time-frequency analysis process executed by the shore-based fiber optic grating demodulation and data processing center is as follows: S41) Perform the same filtering and conversion process as when establishing the reference on all signals to obtain the dynamic strain signal under the current state; S42) For each excitation point-sensor pair, calculate the current principal resonance frequency. Amplitude and decay time ; S43) A weighted fusion algorithm is used to calculate the comprehensive anomaly index; the comprehensive anomaly index is calculated using the following formula. In the formula, A dimensionless exponent between 0 and 1. For amplitude, For decay time, As the baseline value for group velocity, ~ As weight; S44) Based on the comprehensive anomaly index The size determines the level of early warning.

10. The method for monitoring voids in underwater concrete structures according to claim 9, wherein... The characteristic lies in: S44), if the comprehensive abnormal index A comprehensive abnormality index of <0.3 is considered normal; if the comprehensive abnormality index is 0.3≤ A value <0.5 indicates a slight abnormality, and closer monitoring is recommended for the next assessment; if the overall abnormality index is... A value ≥0.5 is considered a serious anomaly and is marked as a highly suspected void area.

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