Method for identifying damage of pile foundation in marine environment based on distributed strain monitoring of optical fiber

Abstract

The application discloses a kind of marine environment pile foundation damage identification method based on optical fiber distributed strain monitoring, belong to structural health monitoring and ocean engineering technical field, comprising: along the axial uniform distribution of distributed fiber optic sensor in pile foundation, for strain monitoring;Strain response data of pile foundation under the action of wave load along pile length is collected by distributed fiber optic demodulator;Modal characteristic parameters of pile foundation structure are identified by random subspace identification method algorithm, and modal characteristic parameters include modal frequency and modal shape;Modal strain energy is calculated based on modal shape, and modal strain energy distribution diagram is obtained;Damage area is identified, whether it is judged to exceed the set threshold by comparing the change of modal strain energy in monitoring state and reference state to determine damage, and damage identification result is obtained.The application has the advantages of high spatial resolution, high identification accuracy, good environmental adaptability, accurate positioning, high automation and wide applicability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of structural health monitoring and ocean engineering, and particularly relates to a marine environment pile foundation damage identification method based on optical fiber distributed strain monitoring. BACKGROUND

[0002] With the continuous development of marine resources, the construction of large marine structures such as offshore wind power, cross-sea bridges, deep-sea platforms, etc. is increasing. As an important bearing foundation, offshore pile foundation structures bear the key vertical and horizontal load transmission function, and their structural safety is directly related to the stability and service life of the entire engineering system.

[0003] During long-term service, offshore pile foundation structures are often subjected to the coupling action of multiple complex dynamic loads such as wave impact, tidal current, wind load, earthquake, etc., resulting in structural degradation problems such as cracks, fatigue damage, corrosion, etc. inside the pile foundation. Once the damage develops to a certain extent, it may cause structural stiffness degradation, load capacity reduction or even failure, which seriously threatens the safe operation of offshore facilities. Therefore, establishing a high-precision and practical pile foundation damage identification technology is one of the key research directions in the field of ocean engineering.

[0004] The commonly used pile foundation health monitoring and damage identification methods mainly include:

[0005] ① Acceleration and displacement response method: acceleration or displacement meters are arranged at the top of the pile or the pile body to extract structural dynamic response characteristic parameters for modal identification and damage estimation. However, this method has poor sensitivity to local damage, and it is difficult to obtain the full-length response of the pile foundation, resulting in monitoring blind areas.

[0006] ② Non-destructive testing methods such as ultrasonic waves and X-rays: mainly applied to local area detection, with high precision, but limited detection range, difficult to be deployed on a large scale in underwater environment, and long cycle and high cost.

[0007] ③ Point-type optical fiber sensing technology (such as FBG): optical fiber Bragg grating sensors are arranged at key positions of the pile foundation to monitor strain or temperature changes, but the spatial distribution is discrete, making it difficult to form a continuous strain field, and cannot meet the global modal analysis requirements.

[0008] The above methods generally have the following shortcomings: spatial distribution is discontinuous, making it difficult to achieve full-length damage identification; the response to local micro-damage is limited, and the identification accuracy is not high; real-time performance and environmental adaptability are poor, making it difficult to meet the long-term monitoring needs in complex marine environments.

[0009] In recent years, with the development of distributed optical fiber sensing technology, especially the distributed optical fiber strain monitoring system based on the principle of Brillouin scattering and Raman scattering, the full-length and continuous strain information of the structure can be obtained, which has the advantages of high resolution, strong anti-electromagnetic interference, and is suitable for harsh environment, etc., which provides a new technical path for the health monitoring of marine pile foundation. On the other hand, the application of modal strain energy analysis method in structural damage identification is gradually increasing. This method can effectively reveal the modal response changes caused by local stiffness changes by calculating and comparing the strain energy distribution of the structure under different modes, and has high damage identification sensitivity and positioning ability. However, there is still a lack of real-time damage identification technology for pile foundation structure in complex marine environment by deeply integrating distributed optical fiber strain monitoring and modal strain energy analysis. Therefore, it is urgent to develop a new type of pile health monitoring and damage evaluation method which integrates continuous strain collection, high-precision modal identification and damage sensitive discrimination, so as to improve the automation and intelligent level of marine structure safety monitoring. SUMMARY

[0010] In view of the deficiencies in the background art, the purpose of the present application is to provide a marine environment pile damage identification method based on optical fiber distributed strain monitoring. The technical scheme adopted is:

[0011] S1) uniformly distribute distributed optical fiber sensors along the axial direction of the pile foundation for strain monitoring;

[0012] S2) collect the strain response data of the pile foundation along the pile length under the action of wave impact load by using a distributed optical fiber demodulator;

[0013] S3) apply the random subspace identification method to identify the modal characteristic parameters of the pile foundation structure, including modal frequency and modal shape;

[0014] S4) calculate the modal strain energy based on the modal shape and obtain the modal strain energy distribution diagram;

[0015] S5) identify the damage area, compare the change of modal strain energy in the monitoring state and the reference state, judge whether it exceeds the set threshold to determine the damage, and obtain the damage identification result.

[0016] Preferably, in step S2), the strain response data of the pile foundation along the pile length under the action of wave impact load is collected by using a distributed optical fiber demodulator, and the specific process is as follows:

[0017] A pile foundation structure is established in a wave environment, distributed optical fiber sensors are arranged in the axial direction of the pile foundation, strain response data of the pile foundation under excitation is continuously collected by using a distributed optical fiber demodulator, and the strain response data is formed into a strain response matrix in time sequence form, and the calculation formula is as follows:

[0018]

[0019] wherein, x denotes the position of the fiber optic sensor laid on the pile foundation, t denotes the data acquisition time, denotes the matrix.

[0020] Preferably, in the step S3), the modal characteristic parameters of the pile foundation are identified by using the random subspace identification method algorithm, and the modal characteristic parameters include modal frequency and modal shape, and the specific steps are as follows:

[0021] By processing the strain response matrix in the frequency domain or the time domain, the modal characteristic parameters of the pile foundation are identified by using the random subspace identification method algorithm, including the modal frequency and the modal shape, and the specific identification steps are as follows:

[0022] ①Pretreatment of strain response matrix: the sequence in the strain response matrix is de-trended, band-pass filtered and segmented window function processed to enhance the spectral resolution, and further standardized;

[0023] ②Construction of response matrix: the multi-channel distributed pretreated strain response matrix is constructed into a response matrix , each row represents a sampling channel, and each column represents a sampling point at different times, and the calculation formula is as follows:

[0024]

[0025] In the formula, denotes the strain of the pile foundation at position , time , is the number of channels, is the transposition operation;

[0026] ③Random subspace identification method algorithm: according to the response matrix, a state space expression form of a discrete linear time-invariant system is output by using the random subspace identification method algorithm, and the formula of the state space expression form is:

[0027]

[0028] wherein, is the system state vector; is the system output vector; is the state transition matrix, which determines the dynamic behavior of the system; is the observation matrix, which projects the state variable into the observable space; and are white noise;

[0029] ④Construction of Hankel matrix: by The Hankel matrix is formed by arranging the time-delayed nested matrix, and the specific formula is as follows:

[0030] ⑤ Singular value decomposition: the Hankel matrix is decomposed into different parts by singular value decomposition, and the model order is constructed by retaining the first main singular values, removing some noise and unimportant information, and extracting the main vibration modal characteristics. The formula is as follows:

[0031]

[0032] where, is the left singular vector, representing the modal space; is the singular value diagonal matrix; the main singular value is the main modal intensity; is the time-domain dynamic response matrix; is the transpose operation;

[0033] ⑥ State space parameter solution: the state space parameters include state matrix and observation matrix , the state matrix and observation matrix are extracted, which are used to derive the modal frequency and modal shape;

[0034] The modal frequency is obtained from the eigenvalue of the state matrix ;

[0035] The modal shape is obtained from the corresponding column of the observation matrix , and the formula is as follows:

[0036]

[0037] In the formula, represents the th modal shape, represents the observation matrix, represents the th column of the observation matrix , represents the construction of the observation matrix, represents the correlation vector;

[0038] where, the specific process of extracting the state matrix and the observation matrix is as follows:

[0039] The construction of the observation matrix is obtained by singular value decomposition, and the formula is as follows:

[0040]

[0041] where is the preserved singular vector, is the square root of the diagonal matrix of singular values, is the dimension, is the number of preserved principal singular values;

[0042] Extracting state matrix : Extracting state transition matrix from constructed observation matrix by projection method;

[0043] First, the constructed observation matrix is blocked by time step, and the formula is as follows:

[0044]

[0045] Further, the following formula is obtained:

[0046]

[0047] where, represents the last row is removed; represents the first row is removed; is the pseudo-inverse;

[0048] Extracting observation matrix : The observation matrix is the first row of the constructed observation matrix , and the formula is as follows:

[0049]

[0050] The observation matrix maps the state vector to the output , which represents the response of each mode to each measuring point, i.e. the modal shape ;

[0051] After extracting the state matrix and the observation matrix , further solve the modal frequency and the damping ratio :

[0052] Solve the eigenvalue of the state matrix , and the formula is as follows:

[0053]

[0054] wherein denotes the state matrix of eigenvalues, is the eigenvector;

[0055] the modal frequency and damping ratio of the first mode are calculated, and the formulas are as follows:

[0056]

[0057] wherein, denotes the modal frequency of the first mode, denotes the phase angle of the eigenvalue, denotes the sampling time step, is the phase angle of the eigenvalue, is the modal logarithm of the eigenvalue.

[0058] Preferably, the modal strain energy is calculated based on the modal shape, and the modal strain energy distribution map is obtained, and the specific steps are as follows:

[0059] Based on the known modal shape of the pile foundation, the pile foundation is discretely processed according to the segmentation, and is divided into a plurality of units. According to the structure dynamics theory, the modal strain energy of each unit of the structure is calculated, and the modal strain energy of each unit under a certain mode is calculated , and the formula is as follows:

[0060]

[0061] wherein, is the modal strain energy of the first unit under the first mode; is the modal shape vector of the first mode at the unit ; is the stiffness matrix of the first unit; is the transposition operation;

[0062]

[0063] wherein, is the elastic modulus, is the sectional inertia moment of the pile foundation, is the unit length of the pile foundation;

[0064] Further, the total strain energy of the first mode of the whole pile foundation is calculated, and the formula is as follows:

[0065]

[0066] In the formula For the Unit in the The strain energy under the first mode, The total number of units divided for the pile foundation structure;

[0067] Furthermore, the modal strain energy can be simplified into an integral form by continuous strain field calculation, as follows:

[0068]

[0069] in, For the Unit in the Modal strain energy under the first mode; is the strain distribution obtained; For the The volume of the unit;

[0070] By quantitatively analyzing the modal strain energy of each unit of the pile foundation under different modes, its distribution along the pile length direction is visualized as a modal strain energy distribution diagram.

[0071] Preferably, in step S5), the damaged area is identified, the changes in the modal strain energy in the monitoring state and the reference state are compared, and it is determined whether the damage is determined to be greater than a set threshold. The specific steps are as follows:

[0072] Comparing the modal strain energy of the pile foundation in the reference state and the current state, the strain energy change rate is obtained, which is as follows:

[0073]

[0074] in, The first Modal strain energy of the element; is the modal strain energy in the current state; is the damage change rate, including >0.05 is mild damage; >0.10 is moderate damage; >0.20 is severe injury;

[0075] In order to eliminate the influence of the total energy difference of the structure, the normalization index is defined The formula is:

[0076]

[0077] Set a threshold, if the strain energy rate of change of a certain area exceeds the threshold, it is determined that the area may be damaged, and the robustness of the judgment can be further enhanced by combining the change trend of multiple modes;

[0078] Output the damage identification result of the pile foundation, including: ① damage section position; ② damage rate ; ③ visualization atlas; ④ automatically generate warning or maintenance suggestion.

[0079] Compared with the prior art, the present application proposes a marine environment pile foundation damage identification method based on optical fiber distributed strain monitoring, which has the following advantages:

[0080] ① High spatial resolution, full-length monitoring is realized

[0081] By arranging distributed optical fiber sensors along the longitudinal direction of the pile foundation, continuous strain monitoring in the full-length range of the pile foundation structure can be realized, which greatly improves the monitoring coverage and resolution accuracy compared with traditional point sensors, and can identify millimeter-level crack-induced small stiffness changes;

[0082] ② Fusion modal strain energy theory, high identification accuracy

[0083] The introduction of modal strain energy theory can effectively capture the modal energy changes caused by local damage, realize quantitative analysis and accurate positioning of damage position and degree. This technology has high identification sensitivity for early damage or hidden cracks, and can early warning of structure degradation;

[0084] ③ Good environmental adaptability, suitable for harsh marine environment

[0085] The distributed optical fiber sensor used has the characteristics of anti-electromagnetic interference, corrosion resistance, high humidity and high pressure resistance, and can work stably in seawater immersion, wave washing, high salt fog and other complex marine environments for a long time, which is significantly better than traditional electronic sensor systems;

[0086] ④ High damage positioning accuracy and strong fault resolution capability

[0087] Combined with modal identification and high-density sampling arrangement of optical fiber, the damage positioning accuracy of the method of the present application can be within 0.5 meters, which can effectively identify fine cracks, interface peeling and other local damage, and avoid the "monitoring blind area" existing in traditional methods;

[0088] ⑤ High degree of automation, suitable for long-term structure health monitoring

[0089] The system can realize data linkage with the shore-based monitoring center, cooperate with automatic acquisition, data preprocessing and identification algorithm, have all-weather online monitoring and remote diagnosis capability, and support intelligent management of the whole life cycle of offshore pile foundation;

[0090] ⑥Wide applicability and good engineering promotion value

[0091] The method of the present invention is not only applicable to offshore wind power foundation piles, but can also be extended to various pile foundation structure health monitoring scenarios such as offshore oil platform jackets, cross-sea bridge foundations, deep-sea floating platform anchor piles, etc., and has broad engineering application prospects and economic value. BRIEF DESCRIPTION OF THE DRAWINGS

[0092] Figure 1 This is a flow chart of the method for identifying pile foundation damage in a marine environment based on optical fiber distributed strain monitoring according to the present invention;

[0093] Figure 2 This is a layout diagram of the distributed optical fiber sensor of the present invention. DETAILED DESCRIPTION

[0094] The following will be combined with the drawings in the embodiments of this application to further clearly and completely describe the technical solutions in the embodiments of this application. It should be noted that the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without making any creative work shall fall within the scope of protection of this application.

[0095] In order to make the invention objectives, technical solutions and advantages of this application clearer, the embodiments of this application are further described in detail in conjunction with the drawings in the specification: In order to more clearly understand the above-mentioned objectives, features and advantages of the present invention, the advantages of the present invention will be further illustrated by comparing the embodiments in conjunction with the drawings and specific implementation methods.

[0096] The present invention proposes a method for identifying pile foundation damage in marine environment based on optical fiber distributed strain monitoring. Figure 1 As shown, the steps of the method are described in detail:

[0097] S1) Distributed optical fiber sensors are evenly distributed along the axial direction of the pile foundation for strain monitoring;

[0098] Specifically, in step S1), the distributed optical fiber sensors are evenly arranged along the axial direction of the pile foundation, and the specific process for strain monitoring is as follows:

[0099] Distributed optical fiber sensors are evenly arranged along the longitudinal axis of offshore pile foundation structures (such as steel pipe piles, cast-in-place piles, etc.). Figure 2 The optical fiber can be fixed in the following two ways:

[0100] ①Embedded layout: The optical fiber is pre-buried on the surface or inside the structure before concrete forming; ②Applied layout: The optical fiber is adhered to the surface of the pile foundation using structural adhesive or industrial adhesive, and a protective coating or protective shell is provided.

[0101] The fiber type preferably uses Brillouin scattering type optical fiber (BOTDA or BOTDR), the sampling interval is recommended to be no more than 20-50 cm, and the resolution is better than 1με, which can meet the requirements of high-precision strain field acquisition.

[0102] S2) collecting strain response data of the pile foundation along the pile length under the action of wave impact load by a distributed optical fiber demodulator;

[0103] Specifically, in the step S2), the strain response data of the pile foundation along the pile length under the action of wave impact load is collected by a distributed optical fiber demodulator, and the specific process is as follows:

[0104] A pile foundation structure is established in a natural or artificial wave environment, a distributed optical fiber sensor (such as BOTDA) is arranged in the axial direction of the pile foundation, strain response data of the pile foundation under actual working conditions or test excitation is continuously collected by a distributed optical fiber demodulator (i.e. the change of the optical signal in the optical fiber due to the strain of the pile foundation is converted into electrical signal data that can be analyzed and processed), and the strain response data is formed into a strain response matrix in time sequence form, and the calculation formula is as follows:

[0105]

[0106] wherein, represents the position of the optical fiber sensor arranged on the pile foundation, represents the data collection time, represents the matrix;

[0107] Wherein, when the distributed optical fiber demodulator continuously collects, ① the sampling frequency is recommended to be ≥50Hz, covering the first to third order modes; ② the sampling interval is recommended to be set at every 0.25-0.5m, and the spatial resolution is sufficient to capture the mode shape distribution (the distribution of the deformation of the pile foundation vibration, i.e. to know where the deformation of the pile foundation is large and where the deformation is small).

[0108] S3) applying a random subspace identification method algorithm to identify the modal characteristic parameters of the pile foundation structure, including modal frequency and modal shape;

[0109] Specifically, in the step S3), the random subspace identification method algorithm is applied to identify the modal characteristic parameters of the pile foundation structure, including modal frequency and modal shape, and the specific steps are as follows:

[0110] Pile foundation as a space bar or flexible column structure, there are different vibration modes when vibrating, called modal, its modal presents the characteristics of low frequency, discrete and compliance, the first order modal is sensitive to the overall stiffness of the pile foundation, and is an important reference to judge whether the pile foundation is damaged or not. The modal characteristic parameters of the pile foundation, including modal frequency (i.e. natural frequency of free vibration of the structure) and modal shape (i.e. strain distribution of the structure when vibrating at a certain order), are extracted by processing the strain response matrix in frequency domain or time domain. The modal characteristic parameter identification adopts stochastic subspace identification (SSI) algorithm, which does not need to know the input of excitation, has strong noise resistance, and is suitable for identifying the modal characteristic parameters of the pile foundation under environmental excitation (such as wave, wind). The specific identification steps are as follows:

[0111] ①Pretreatment of strain response data: the sequence in the strain response matrix is de-trended (remove the overall drift trend, i.e. the original strain response sequence may have an overall drift trend, and the de-trend operation is to remove the overall drift trend), band-pass filtered (only the target modal frequency range is reserved, such as 0.5~10Hz), and segmented window function processed (the strain response data is divided into small segments, and processed with a specific window function, such as Hamming, Welch) to enhance the spectral resolution, and further standardized (i.e. the data is normalized to improve stability);

[0112] ②Construction of response matrix: the multi-channel distributed pretreated strain response matrix is constructed into a response matrix , each row represents a sampling channel (i.e. the position of the pile foundation), and each column represents the sampling point at different times, and the calculation formula is as follows:

[0113]

[0114] In the formula, represents the strain of the pile foundation at position , time , is the number of channels, is the transpose operation;

[0115] ③Stochastic subspace identification algorithm: according to the response matrix, the stochastic subspace identification (SSI) algorithm outputs a state space expression form of a discrete linear time-invariant system, and the formula of the state space expression form is as follows:

[0116]

[0117] Among them, is the system state vector (describes the internal state of the system, which is unobservable); is the system output vector (i.e. the strain response data obtained by the distributed optical fiber sensor); is the state transition matrix, which determines the system dynamic behavior (how the system state changes over time, and is related to modal frequencies and damping); is the observation matrix, which projects the state variables into the observable space (i.e., modal shapes); and is the white noise (representing process and measurement noise);

[0118] ④ Constructing the Hankel matrix: by arranging the response matrices at different times in a certain time delay nested arrangement to form the Hankel matrix, the specific formula is as follows:

[0119] ⑤ Singular Value Decomposition (SVD): singular value decomposition is performed on the Hankel matrix, which is decomposed into different parts, and the model order is constructed by retaining the first main singular values, removing some noise and unimportant information, and extracting the main vibration modal characteristics, the formula is as follows:

[0120]

[0121] where, is the left singular vector, representing the modal space; is the singular value diagonal matrix, and the main singular value represents the main modal intensity; is the time-domain dynamic response matrix; is the transpose operation;

[0122] ⑥ State space parameter solving: the state space parameters include the state matrix and the observation matrix , the state matrix and the observation matrix are extracted, which are used to derive the modal frequency and modal shape;

[0123] Modal frequency is obtained from the eigenvalue of the state matrix ;

[0124] Modal shape is the relative response value of distributed strain along the axial direction of the pile foundation, which can be normalized and plotted for comparison, and the modal shape is obtained from the corresponding column of the observation matrix , the formula is as follows:

[0125]

[0126] where represents the th modal shape, represents the observation matrix, Represents the observation matrix No. List, Indicates the construction of the observation matrix, represents the correlation vector;

[0127] Among them, the extracted state matrix With the observation matrix The specific process is as follows:

[0128] The observation matrix is ​​constructed by the results obtained from the singular value decomposition , the formula is as follows:

[0129]

[0130] In the formula is the retained singular vector (corresponding to the main singular vector part), is the square root of the singular value diagonal matrix (take the main singular value diagonal matrix part), For the dimension, is the number of main singular values ​​retained;

[0131] Extract state matrix :Use the projection method to construct the observation matrix Extract the state transition matrix ;

[0132] First construct the observation matrix Divide into blocks by time steps, the formula is as follows:

[0133]

[0134] We further obtain the following formula:

[0135]

[0136] in, Indicates to remove the last OK; Indicates to remove the OK; For pseudo-rebellion;

[0137] Extract observation matrix : Observation matrix Is to construct the observation matrix Before The formula is as follows:

[0138]

[0139] Observation matrix The state vector Mapping to output , which represents the response of each mode to each measuring point, that is, the modal vibration shape ;

[0140] After extracting the state matrix and the observation matrix Then, we further solve the modal frequency and damping ratio :

[0141] Solving the state matrix The eigenvalue of , the formula is as follows:

[0142]

[0143] In the formula Represents the state matrix The eigenvalues ​​of is the eigenvector;

[0144] Calculate the Modal frequency of the mode and damping ratio , the formula is as follows:

[0145]

[0146] in, Indicates the The first modal frequency, represents the phase angle of the eigenvalue, is the sampling time step, is the eigenvalue phase angle, is the eigenvalue modulo the logarithm.

[0147] S4) calculating the modal strain energy based on the modal vibration shape and obtaining a modal strain energy distribution diagram;

[0148] Specifically, in step S4), the modal strain energy is calculated based on the modal vibration shape, and the modal strain energy distribution diagram is obtained. The specific steps are as follows:

[0149] Based on the known modal vibration shape of the pile foundation, the pile foundation is divided into several units according to the segmented discretization process. According to the structural dynamics theory, the modal strain energy of each unit of the structure is calculated, and the modal strain energy of each unit in a certain mode is calculated. , the formula is as follows:

[0150]

[0151] in, For the Unit in the Modal strain energy under the first mode; For the The first mode in the unit The modal formation vector at ; For the The stiffness matrix of each element; is the transpose operation;

[0152]

[0153] in, is the elastic modulus, is the moment of inertia of the pile foundation section, is the length of the pile foundation unit;

[0154] Furthermore, the pile foundation structure as a whole Total strain energy of the first mode The formula is:

[0155]

[0156] In the formula For the Unit in the The modal strain energy under the first mode, The total number of units divided for the pile foundation structure;

[0157] In practical applications, especially in combination with optical fiber strain monitoring, modal strain can be simplified into an integral form through the continuous strain field, as shown below:

[0158] in, For the Unit in the Modal strain energy under the first mode; is the strain distribution obtained; For the The volume of the unit;

[0159] By quantitatively analyzing the modal strain energy of each unit of the pile foundation under different modes, its distribution along the pile length direction is visualized as a modal strain energy distribution diagram.

[0160] S5) Identify the damaged area, compare the changes in modal strain energy between the monitoring state and the reference state, and determine whether it exceeds the set threshold to determine damage;

[0161] Specifically, in step S5), the damaged area is identified, the changes in the modal strain energy in the monitoring state and the reference state are compared, and it is determined whether the damage is determined to be greater than the set threshold. The specific steps are as follows:

[0162] Comparing the modal strain energy of the pile foundation in the baseline state (undamaged) and the current state, the strain energy change rate is obtained, which is as follows:

[0163]

[0164] in, The first Modal strain energy of the element; is the modal strain energy in the current state (possibly with damage); is the damage change rate, including >0.05 is mild damage; >0.10 is moderate damage; >0.20 is severe injury;

[0165] In order to eliminate the influence of the total energy difference of the structure, the normalization index is defined The formula is:

[0166]

[0167] It is used to enhance the sensitivity of discrimination and is suitable for use in cases where the modal variation range is small or the damage is subtle. A threshold is set (e.g., 5% to 15%). If the strain energy change rate of a certain area exceeds the threshold, it is judged as a possible damage area. The robustness of the judgment can be further enhanced by combining the change trends of multiple modes.

[0168] Output damage identification results of pile foundation structure, including: ① location of damaged section (such as fiber optic measurement point number range); ② damage change rate ( ΔE ); ③ Visualization maps (such as damage heat maps and strain energy distribution maps); ④ Automatic generation of early warnings or maintenance recommendations; the results can be used for structural maintenance plan formulation, emergency maintenance judgment and structural safety assessment.

[0169] Although the preferred embodiments of the present application have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present application.

[0170] Obviously, those skilled in the art may make various changes and modifications to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application is intended to include these modifications and variations.

Claims

1. A method for identifying pile foundation damage in a marine environment based on optical fiber distributed strain monitoring, characterized in that: include: S1) Distributed optical fiber sensors are evenly distributed along the axial direction of the pile foundation for strain monitoring; S2) Using a distributed optical fiber interrogator to collect strain response data along the length of the pile under wave impact loads; S3) applying a random subspace identification algorithm to identify modal characteristic parameters of the pile foundation structure, the modal characteristic parameters including modal frequency and modal vibration shape; ① Preprocessing the strain response matrix: Detrending, bandpass filtering, and piecewise window function processing are performed on the sequence in the strain response matrix to enhance the spectral resolution and further standardize the process; ②Construct response matrix: construct the strain response matrix after multi-channel distributed preprocessing into a response matrix ; ③ Random subspace identification algorithm: Based on the response matrix, the random subspace identification algorithm is used to output the state space representation of a discrete linear time-invariant system; ④Construct Hankel matrix: By transforming the response matrix at different moments The Hankel matrix is ​​formed by nesting and arranging them at a certain delay; ⑤ Singular value decomposition: Perform singular value decomposition on the Hankel matrix and decompose the Hankel matrix into different parts. The main singular values ​​are used to construct the model order, remove some noise and unimportant information, and extract the vibration mode characteristics; ⑥ State space parameter solution: State space parameters include state matrix and the observation matrix , extract the state matrix and the observation matrix , used to derive modal frequencies and modal vibration shapes; S4) calculating the modal strain energy based on the modal vibration shape and obtaining a modal strain energy distribution diagram; S5) Identify the damaged area, and determine whether the damage exceeds the set threshold by comparing the changes in modal strain energy between the monitoring state and the reference state, and obtain the damage identification result.

2. The method for identifying pile foundation damage in a marine environment based on optical fiber distributed strain monitoring according to claim 1, characterized in that: In step S2), the strain response data of the pile foundation along the length of the pile under the action of wave impact load is collected by a distributed optical fiber demodulator. The specific process is as follows: A pile foundation structure is established in a wave environment. Distributed optical fiber sensors are arranged in the axial direction of the pile foundation. The strain response data of the pile foundation under excitation are continuously collected by a distributed optical fiber demodulator. The strain response data are then converted into a strain response matrix in the form of a time series. The calculation formula is as follows: ； in, Indicates the location of the fiber optic sensor on the pile foundation. Indicates the data collection time. Represents a matrix.

3. The method for identifying pile foundation damage in a marine environment based on optical fiber distributed strain monitoring according to claim 2, characterized in that: In step S3), the modal characteristic parameters of the pile foundation are identified by applying a random subspace identification algorithm. The modal characteristic parameters include modal frequency and modal vibration shape. The specific steps are as follows: By processing the strain response matrix in the frequency domain or time domain, the random subspace identification algorithm is used to identify the modal characteristic parameters of the pile foundation, including the modal frequency and modal vibration shape. The specific identification steps are as follows: ① Preprocessing the strain response matrix: Detrending, bandpass filtering, and piecewise window function processing are performed on the sequence in the strain response matrix to enhance the spectral resolution and further standardize the process; ②Construct response matrix: construct the strain response matrix after multi-channel distributed preprocessing into a response matrix , each row represents a sampling channel, each column represents a sampling point at different times, and the calculation formula is as follows: ； In the formula Indicates the location ,time The pile foundation strain response matrix at time , is the number of channels, is the transpose operation; ③ Random subspace identification algorithm: Based on the response matrix, the random subspace identification algorithm is used to output a state space expression of a discrete linear time-invariant system. The state space expression is: ； in, is the system state vector; is the system output vector; is the state transfer matrix, which determines the dynamic behavior of the system; is the observation matrix, which projects the state variables into the observable space; and is white noise; ④Construct Hankel matrix: By transforming the response matrix at different moments The Hankel matrix is ​​formed by nesting and arranging with a certain delay. The specific formula is as follows: ； ⑤ Singular value decomposition: Perform singular value decomposition on the Hankel matrix and decompose the Hankel matrix into different parts. The main singular values ​​are used to construct the model order, remove some noise and unimportant information, and extract the vibration modal characteristics. The formula is as follows: ； in, is the left singular vector, representing the modal space; is a diagonal matrix of singular values; the main singular value is the main mode intensity; is the time domain dynamic response matrix, is the transpose operation; ⑥ State space parameter solution: State space parameters include state matrix and the observation matrix , extract the state matrix and the observation matrix , used to derive modal frequencies and modal vibration shapes; Modal frequency From the state matrix The eigenvalue of to obtain; Mode shape From the observation matrix The corresponding column is obtained as follows: ； In the formula Indicates the The first mode shape, represents the observation matrix, Represents the observation matrix No. List, Indicates the construction of the observation matrix, represents the correlation vector; Among them, the extracted state matrix With the observation matrix The specific process is as follows: The observation matrix is ​​constructed by the results obtained from the singular value decomposition , the formula is as follows: ; In the formula are the retained singular vectors, is the square root of the singular value diagonal matrix, For the dimension, is the number of main singular values ​​retained; Extract state matrix :Use the projection method to construct the observation matrix Extract the state transition matrix ; First construct the observation matrix Divide into blocks by time steps, the formula is as follows: ； We further obtain the following formula: ； in, Indicates removing the last OK; Indicates to remove the OK; For pseudo-rebellion; Extract observation matrix : Observation matrix Is to construct the observation matrix Before The formula is as follows: ； Observation matrix The state vector Mapping to output , which represents the response of each mode to each measuring point, that is, the modal vibration shape ; After extracting the state matrix and the observation matrix Then, we further solve the modal frequency and damping ratio : Solving the state matrix The eigenvalue of , the formula is as follows: ; In the formula Represents the state matrix The eigenvalues ​​of is the eigenvector; Calculate the modal frequency of the i-th mode and damping ratio , the formula is as follows: ; in, Indicates the The first modal frequency, represents the sampling time step, is the eigenvalue phase angle, is the eigenvalue modulo the logarithm.

4. The method for identifying pile foundation damage in a marine environment based on optical fiber distributed strain monitoring according to claim 1, characterized in that: In step S4), the modal strain energy is calculated based on the modal vibration shape, and the modal strain energy distribution diagram is obtained. The specific steps are as follows: Based on the known modal vibration shape of the pile foundation, the pile foundation is divided into several units according to the segmented discretization process. According to the structural dynamics theory, the modal strain energy of each unit of the structure is calculated, and the modal strain energy of each unit in a certain mode is calculated. , the formula is as follows: ； in, For the Unit in the Modal strain energy under the first mode; For the The first mode in the unit The modal formation vector at ; For the The stiffness matrix of each element; is the transpose operation; ; in, is the elastic modulus, is the moment of inertia of the pile foundation section, is the length of the pile foundation unit; Furthermore, the pile foundation Total strain energy of the first mode The formula is: ； In the formula For the Unit in the The modal strain energy under the first mode, The total number of units divided for the pile foundation structure; Furthermore, the modal strain energy can be simplified into an integral form by continuous strain field calculation, as follows: ； in, For the Unit in the Modal strain energy under the first mode; is the strain distribution obtained; For the The volume of the unit; By quantitatively analyzing the modal strain energy of each unit of the pile foundation under different modes, its distribution along the pile length direction is visualized as a modal strain energy distribution diagram.

5. The method for identifying pile foundation damage in a marine environment based on optical fiber distributed strain monitoring according to claim 1, characterized in that: In step S5), the damaged area is identified, and the damage is determined by comparing the changes in the modal strain energy in the monitoring state with the reference state to determine whether it exceeds the set threshold. The specific steps are as follows: Comparing the modal strain energy of the pile foundation in the reference state and the current state, the strain energy change rate is obtained, which is as follows: ; in, The first Modal strain energy of the element; is the modal strain energy in the current state; is the damage change rate, including >0.05 is mild damage; >0.10 is moderate damage; >0.20 is severe injury; In order to eliminate the influence of the total energy difference of the structure, the normalization index is defined The formula is: ； A threshold is set. If the strain energy change rate of a certain area exceeds the threshold, it is determined to be a possible damage area. The change trends of multiple modes can be further combined to enhance the robustness of the judgment. Output the damage identification results of the pile foundation, including: ① location of the damaged section; ② damage change rate ; ③ Visual map; ④ Automatically generate warnings or maintenance suggestions.

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