CN110296885B

CN110296885B - Mechanical fault monitoring method for photoelectric composite submarine cable

Info

Publication number: CN110296885B
Application number: CN201910193537.7A
Authority: CN
Inventors: 赵丽娟; 徐志钮; 胡志伟; 李永倩
Original assignee: North China Electric Power University
Current assignee: North China Electric Power University
Priority date: 2019-03-14
Filing date: 2019-03-14
Publication date: 2022-05-03
Anticipated expiration: 2039-03-14
Also published as: CN110296885A

Abstract

The invention discloses a mechanical fault monitoring method for a photoelectric composite submarine cable, belonging to the technical field of fault monitoring. The method is to use 110kV YJQ41 multiplied by 300mm²The submarine cable is a research object, a structure dynamics finite element model is established based on ANSYS/LS-DYNA explicit dynamics analysis, three typical fault conditions of the submarine cable such as stretching, torsion and anchoring are modeled and simulated, the strain distribution along the submarine cable composite optical fiber and the corresponding relation between the strain and stress of the relevant structure in the submarine cable and the optical fiber strain in the fault development process are obtained, the functional relation between the cable core and the armored strain and the optical fiber strain in the stretching and torsion states and the evaluation indexes of the damage state of the submarine cable and the optical fiber strain in the anchoring state are established, and the submarine cable fault state fault evaluation method can be used for evaluating the mechanical fault state fault of the submarine cableAnd identifying the type, and preliminarily judging the development degree of the corresponding fault by combining the established functional relation and the damage evaluation index.

Description

Mechanical fault monitoring method for photoelectric composite submarine cable

Technical Field

The invention belongs to the technical field of fault monitoring, and particularly relates to a mechanical fault monitoring method for a photoelectric composite submarine cable.

Background

The photoelectric composite submarine cable is high in manufacturing cost, complex in structure and severe in operating environment, monitoring and fault diagnosis of the operating state of the photoelectric composite submarine cable are the premise of follow-up fault prevention and maintenance, and the photoelectric composite submarine cable is very important for ensuring safe and stable operation of the submarine cable. Random and diverse mechanical stresses in the actual installation and operation of the submarine cable pose a great threat to the submarine cable. The reasons for causing mechanical failure mainly include external force damage caused by fishing gear and ship anchor in human activities, damage in laying process and damage caused by submarine topography environment, and the submarine cable can be bent under the action of external force and can also be partially twisted and stretched or even broken.

Due to the special working environment of the submarine cable, a plurality of traditional land cable monitoring modes are difficult to be directly applied to submarine cable monitoring. The development of the distributed optical fiber sensing technology provides opportunities for state monitoring of the submarine cable, the submarine cable operation and typical fault states such as anchoring, twisting and stretching can be reflected as changes of strain and stress of each structure of the submarine cable, if the main strain and stress can be further linked with the strain of the composite optical fiber in the submarine cable, the submarine cable operation state can be expected to be monitored according to the corresponding relation between the characteristic quantity of the optical fiber scattering signal and the strain. Up to now, a submarine cable state monitoring system based on distributed optical fiber sensing can basically and effectively monitor strain information of a composite optical fiber, but due to the lack of diagnostic characteristic quantity and evaluation standard for reflecting the state of the submarine cable, the problem of poor information of big data is caused. In order to establish fault threshold information of the submarine cable, Nishimoto T, Tayama H and the like respectively adopt optical fiber sensors to detect mechanical extrusion and armor abrasion which are manually applied on the submarine cable of 66kV and 6.6kV, so that fault point positioning is realized; the model number of Wang Guozhei is YJQF 41-26/353 multiplied by 120mm based on submarine cable mechanical test standard pair²Three-core composite cable sample inletCoiling, stretching and tension bending tests are carried out, the structural change of the submarine cable is researched through the tests, and the optical fiber performance is tested; the middle-sea oil research institute adopts navy anchors of different weights to carry out a drop anchor impact test of the photoelectric composite cable, analyzes the deformation of an electric unit and an optical unit in the submarine cable, and provides the combined action that the submarine cable is changed into local depression and overall subsidence in the anchoring process. The entity tests are mainly limited to qualitative analysis, and the biggest problem is that related strain and stress data in the test process are difficult to extract. In contrast, the defects of high difficulty, high cost, few research cases and difficult information acquisition of the entity experiment can be effectively overcome by adopting a numerical calculation method to establish and solve the submarine cable finite element model, and the submarine cable problem is more and more generally researched by adopting a modeling means. Zhang Xu et al adopts finite element software to construct a typical single inner armor photoelectric composite submarine cable simulation model, compares simulation and physical test results, and verifies the reliability of a modeling research tensile result; the deformation of the submarine optical cable and the stress change of armor are subjected to simulation analysis through a finite element method by the Anseria, but optical fibers in the submarine optical cable are not involved; knapp. rh performed nonlinear analysis on armored cables with non-uniform tensile and torsional mechanical properties using numerical simulations and achieved good agreement with experimental results. In summary, the existing research on submarine cable modeling simulation mainly includes analyzing submarine cable deformation and internal physical quantity in a certain fault state independently, lacks systematicness, does not form an identification index of a fault type, and does not relate to quantitative research on the relation between the development degree of a certain fault and optical fiber strain and establishment of diagnosis characteristic quantity.

In 2013, based on a BOTDR technology, online monitoring on a submarine cable in the east China sea is successfully achieved, a large amount of Brillouin frequency shift data changing along with the position and time of the submarine cable is obtained, but fault diagnosis of the submarine cable subjected to mechanical stress based on measurement data cannot be effectively achieved according to the current research situation, and the function of a system cannot be fully exerted.

Disclosure of Invention

The invention aims to provide a mechanical fault of a photoelectric composite submarine cableThe monitoring method is characterized in that the voltage is 110kV YJQ41 multiplied by 300mm²The submarine cable is a research object, a structural dynamics finite element model is established based on ANSYS/LS-DYNA explicit dynamics analysis, three typical fault conditions of the submarine cable such as stretching, twisting and anchoring are modeled and simulated, strain distribution of the submarine cable along the composite optical fiber under the fault conditions, corresponding relations between strain and stress of related structural components in the submarine cable and optical fiber strain in the fault development process are obtained, and functional relations between cable core and armor strain and stress and optical fiber strain under the stretching and twisting states and evaluation indexes of the submarine cable damage state and optical fiber strain under the anchoring state are established, wherein the submarine cable mechanical fault state monitoring method comprises the following steps:

submarine cable structure and state monitoring system

1.1. submarine cable state monitoring system based on BOTDR

At 110kV YJQ41 multiplied by 300mm²A, B islands in a certain sea area in the east of China, which are connected by photoelectric composite submarine cables, are used for electric energy transmission, communication operation of electric power system channels and transmission of real-time operation information of power grid dispatching automation; therefore, a three-dimensional comprehensive monitoring system of the 110kV photoelectric composite submarine cable is built, wherein a Brillouin optical time domain scattering (BOTDR) -based 110kV submarine cable state monitoring subsystem monitors strain and temperature distribution of the submarine cable in real time by adopting a distributed optical fiber measuring instrument; the sea cable state monitoring subsystem is provided with 6 channels, and 1/2, 3/4 and 5/6 of the channels are respectively applied to monitoring of east, middle and west three-phase sea cables; the BOTDR testing equipment is placed on the B island side, laser pulses are incident into the submarine cable composite single-mode fiber after being connected through a common cable, the laser is transmitted in the fiber to generate Brillouin scattering light, the Brillouin scattering light is reversely transmitted to a transmitting end, and the frequency shift and the intensity of Brillouin scattering light signals are detected at the transmitting end through a Data Acquisition Unit (DAU) and a Data Processing Unit (DPU) to obtain strain and temperature information of the fiber;

1.2, monitoring effectiveness of submarine cable strain by Brillouin frequency shift

The frequency shift, the intensity, the temperature and the strain of the Brillouin scattering have linear relations; for a conventional single mode optical fiber, the Brillouin frequency shift v_BWith temperature T and strain epsilonThe expression is as follows

ν_B(T)＝ν_B0+1.158×10^-4ΔT (1)

ν_B(ε)＝ν_B0+5.6ε (2)

In the formula, v_B0Is an initial temperature T₀And the Brillouin frequency shift is in GHz when no strain exists; Δ T ═ T-T₀Is relative to T₀The temperature variation of (a);

2, finite element modeling of submarine cables

2.1 submarine Cable geometric modeling

From three angles of the quality of grid subdivision, the possibility of initial penetration and the influence of each structural component in the submarine cable on a simulation result, the practical submarine cable is simplified, a conductor shielding layer, an insulation shielding layer and a brass band layer which are relatively thin in thickness and have little influence on a model in the submarine cable are omitted, an inner and outer tegument layer are simplified into a shell structure, and the finally established submarine cable model comprises a copper cable core, a crosslinked polyethylene insulating layer, a lead alloy sheath, a high-density polyethylene sheath, a polyethylene terephthalate (PET) filling strip, an optical unit, an inner and outer tegument layer and an armor layer, wherein the optical unit comprises an optical fiber, a steel pipe and a polypropylene sheath.

In the geometric modeling of the submarine cable, modeling of three stranded structure components of armor, PET filler strips and optical units is the most critical, and an accurate stranded structure model is crucial to truly expressing the structural mechanical characteristics of the submarine cable, so that a single stranded body is constructed by depending on an APDL language specific to ANSYS, and then the single stranded body is copied under a cylindrical coordinate system according to the number of stranded roots to obtain a stranded layer;

2.2 selection of cell Material and mesh Subdivision

For an inner and outer tegument layer with a thin structure and small contribution in the submarine cable, selecting a SHELL163 thin-SHELL unit to model the inner and outer tegument layer, and solving the Belytschko-Tsay algorithm of the simple SHELL unit by default, wherein the Belytschko-Tsay algorithm is efficient and stable; the explicit body unit SOLID164 is selected from other parts of the submarine cable, and the submarine cable is solved by adopting a default single-point integration algorithm, so that the single-point integration has a good effect on a large deformation unit under the condition of effectively controlling the hourglass mode, and the solving efficiency can be theoretically improved by 8 times compared with the full integration algorithm;

the materials of each part of the submarine cable can be divided into two types according to the physical characteristics: one type is a metal material, which comprises a cable core, a lead alloy sheath, an armor and a steel pipe in an optical unit, wherein the metal material has typical elastic-plastic characteristics, a classic BKIN (nonlinear mechanical fastening) model is adopted as a material model of the metal material, and the model adopts a piecewise linear function to express the constitutive relation of the material. The elastic modulus of other non-metal structural component materials is much smaller than that of metal materials, the simulation process is always in an elastic stage, a linear elastic model Isotropic is adopted for research, and for the ship anchor, a Rigid material model Rigid is adopted for the ship anchor because the deformation of the anchor before and after the anchor is smashed can be ignored;

the mesh generation mode can influence the initial penetration of the model, the minimum unit size proportion and the calculation convergence, and mesh generation is carried out on each structural component of the submarine cable independently in a user-defined sweeping mode on the premise of comprehensively considering mesh generation density and calculation efficiency;

2.3 contact definition and sandglass control

The automatic contact is selected as a contact type for modeling, any surface of a model is allowed to be in contact with other surfaces including the automatic contact, the method is very suitable for the situation that the contact surface cannot be predicted when the mechanical fault of the submarine cable is modeled, and a widely-used single-side contact algorithm is adopted to solve the self-contact or large deformation problem; in summary, the submarine cable modeling contact is defined as an automatic single-sided contact;

the hourglass mode is a state which is stable in mathematics but cannot be realized physically, the overall hourglass energy of a calculation model in a simulation process must be controlled to be less than 10% of the overall internal energy, and a simulation result is effective; reducing their respective hourglass energies by increasing the mesh split quality and applying an hourglass control algorithm to the streamer components where the hourglass energy is larger; after many attempts, the ratio of the hourglass energy to the internal energy of the simulation whole process is controlled to be below 2%;

3, load application in case of submarine Cable failure

3.1 tensile failure load application

Selecting a typical tensile load, applying the typical tensile load to the axial direction of the submarine cable model, wherein the tensile rate and the tensile load action time of the submarine cable are respectively 5 per mill and 0.1s, and stretching at a constant speed; applying fixed constraint to all nodes on one end face of the model, cutting out a small section of submarine cable section with the length of the other end as a component, setting the material type of the component as a rigid body, and applying rigid body displacement-time load to the component to further stretch the submarine cable;

3.2 torsional fault load application

Similar to the tensile load application method, the submarine cable is twisted by applying a fixed constraint to one end node of the submarine cable, applying a rigid body rotation-time load to the rigid body part of the assembly, twisting at a constant speed, controlling the direction and angle of the submarine cable twist by setting the positive and negative values and the maximum value of the rigid body rotation load, and controlling the speed of the twist by changing the time of the load application. The difference of the twisting direction is embodied as the rigid body rotation direction is the same as and opposite to the stranding direction of the submarine cable. According to the actual winding method of the submarine cable, the torsion angle range of 0-20 degrees under the typical torsion condition is adopted, the torsion speeds are respectively +/-4 degrees/s, +/-6 degrees/s and +/-8 degrees/s, the positive torsion speed indicates that the rigid body torsion direction is the same as the stranding direction of the submarine cable, and the negative torsion speed indicates that the direction is opposite.

3.3 Anchor-to-pound fault load application

Selecting 660kg Hall anchors which are most representative as modeling objects according to the size of a ship above a water area where a submarine cable is located and the anchor matching condition, and simplifying the ship anchors on the premise of ensuring that the weight of the anchors and the projection area of the bottom are not changed; the method comprises the steps of obtaining the maximum falling speed of a ship anchor at the moment of contacting the submarine cable according to a falling speed calculation method of the ship anchor, controlling the falling speed of the ship anchor to be smaller than or equal to the maximum speed so as to meet the actual situation, controlling the collision area of the ship anchor and the submarine cable to be located in the middle area of a simulation section during modeling, applying fixed constraint to nodes at two ends of the submarine cable, and applying rigid body displacement-time load vertically downwards to the ship anchor part. Because the impact process of the anchor cable is extremely short, the whole impact process is considered to be uniform. Taking a typical anchor-tamping fault condition, wherein the anchor-tamping depth is 8cm, the impact time is set to be 0-0.036 s according to the calculated speed of the ship anchor, the load application time is slightly 0.04s to avoid abnormal convergence of a model caused by sudden unloading of a structure, the solving time is set to be 10s long enough to obtain a stable state after the actual ship anchor stops sinking, and the ship anchor is kept at the deepest impact position in the rest time; because the influence of soil penetration resistance is added in the calculation of the ship anchor kinetic energy, the resistance of the soft water saturated soil wrapping the submarine cable to the anchor cable at the impact moment can be ignored relatively;

finite element simulation results and analysis

4.1, tensile simulation results analysis

The strain distribution rule of the axial optical fibers of the submarine cable in the stretching process is similar to other cylinders with similar structures in the submarine cable in the axial stretching process, the most important cable core component in the submarine cable is taken as a representative for analysis, the strain of a cable core in the stretching process is distributed along the axial direction of the submarine cable, the strains of the optical fibers and the cable core in the axial direction of the submarine cable are increased along with the increase of the stretching degree, the strain of the cable core is not changed along with the change of the axial distance and is in direct proportion to the stretching degree, the strain amount of a main stretching area applied by the optical fibers along the submarine cable on rigid body displacement load is maximum, the strain amount of the optical fibers is smaller as the farther the optical fiber is from the main stretching area of the submarine cable, and the characteristic is taken as the criterion of the submarine cable in the stretching state for fault location; taking average strain of the optical fiber and the cable core in a region with relatively obvious optical fiber strain change within the range of 2-4.5 m in the axial direction of the submarine cable, and establishing a relation between the cable core strain and the optical fiber strain;

therefore, the cable core strain and the optical fiber strain are approximately linear, meanwhile, for the cylinder structure of the cable core in the submarine cable and the stranded structure of the optical fiber, the excess length of the optical fiber is gradually exhausted along with the accumulated strain during actual use, and under the condition that the excess length of the optical fiber is not considered during modeling, the initial strains of the optical fiber and the cable core are zero, and accordingly, the data that the cable core strain and the optical fiber strain are approximately linear are fitted, and the coefficient R is determined²0.9877, standard error RMSE 1.237X 10^-4Formula (3);

ε_c＝1.204ε_f (3)

in the formula, epsilon_cIndicating strain of cable core,. epsilon_fIndicating strain in optical fibre(ii) a The stretching degree and the state of the submarine cable can be judged based on the formula (3) and the optical fiber strain;

4.2 torsion simulation results analysis

4.2.1, the twisting direction is opposite to the twisting direction of the submarine cable

(1) Optical fiber

The strain distribution of the axial optical fibers of the submarine cable at different torsion angles is increased along with the increase of the torsion degree of the submarine cable, the strain of the optical fibers at the load application end and the restraint end of the submarine cable is slightly higher than that of the middle section under the influence of the end effect, and the strain of the middle area is basically equal; the relation between the average fiber strain and the torsion angle in the middle area of the simulation section shows that the visible fiber strain is in direct proportion to the torsion angle;

(2) cable core

Because the outer layer unit of the cable core is stressed maximally during twisting, the strain and stress of the cable core refer to the strain and stress of the outermost layer of the cable core; the relation between the cable core strain and the stress and the optical fiber strain in the submarine cable torsion state can be known, the cable core strain in the torsion state is always in direct proportion to the optical fiber strain, the fitting result is as shown in formula (4), and the fitting R is²＝0.9999， RMSE＝1.04×10^-5；

ε_c＝4.16ε_f (4)

The cable core material is changed from elastic deformation to plastic deformation along with the increase of the torsion angle, the cable core stress is in direct proportion to the optical fiber strain in the elastic stage of the material, after the yield point is exceeded, the cable core stress and the optical fiber strain meet the linear relation, and the result of fitting by adopting a piecewise linear function is as shown in the formula (5); first segment R of fitting function ²1, RMSE 0.1171; second segment R²The result is not influenced by the torsional speed, 1 and RMSE 0.5608;

in the formula, σ_cThe unit is the cable core stress and is 0.1 MPa.

(3) Armor

Under different torsional speeds, the relationship curve of the armor strain and the stress and the optical fiber strain can be knownThe strain and the stress of the armor are approximately proportional to the strain of the optical fiber, because when the twisting direction is opposite to the twisting direction of the submarine cable, the optical fiber and the armor twisting structure are in a synchronous discrete state in the twisting process, the strain and the stress of the optical fiber and the armor are only generated by twisting, and the tension of the submarine cable caused by twisting can be ignored, so that the strain and the stress of the armor cannot be suddenly changed due to the superposition of twisting and stretching and are always linear with the strain of the optical fiber; in the set 20-degree reverse torsion angle range, the armor is always in an elastic stage, the maximum stress borne by the armor does not reach the yield point of a material, and the torsion speed basically has no influence on the result; the fitting results are shown in the formulas (6) and (7), and the formula (6) fits R²＝0.9993， RMSE＝7.572×10^-6(ii) a Fitting of formula (7) R²＝0.9990，RMSE＝20.53，

ε_t＝1.15ε_f (6)

σ_t＝2.593×10⁶

ε

_f 0≤ε_f≤8.28×10^-4 (7)

In the formula, epsilon_tIs the armor strain; sigma_tThe unit is 0.1MPa for the sheathing stress.

4.2.2, the twisting direction is the same as the twisting direction of the submarine cable,

in this state, the stress and strain of the cylinder such as the cable core are still generated by torsion and are irrelevant to the torsion direction; when the torsion degree is small, the strain and stress of the stranded structures such as the optical fiber, the armor and the like are mainly generated by torsion, and the change of the strain and the stress at the moment is not large; as the degree of torsion increases, its strain will start to stress due to both torsion and tension, with its value increasing significantly;

(1) optical fiber

According to the strain distribution of the optical fiber in the axial direction of the submarine cable at different torsion angles, the visible optical fiber strain is increased along with the increase of the torsion degree of the submarine cable, and the optical fiber strain values in the axial direction of the submarine cable are basically equal, so that a curve of the average strain of the optical fiber along the submarine cable along the torsion angle is made; it can be seen that when the torsion degree is not large, the optical fiber strain and the torsion angle form a linear relation, the increase rate of the optical fiber strain is accelerated along with the deepening of the torsion degree, and finally the optical fiber strain is increased back to the linear increase and can be divided into three sections;

(2) cable core

According to the relation between the cable core strain and stress and the optical fiber strain under different twisting speeds, for a cylindrical cable core structure, the change rule of the strain and stress along with a twisting angle is irrelevant to the twisting direction, and the cable core strain and stress under a twisting state respectively form linear and piecewise linear function relations with the twisting angle; the cable core strain and the relation between the stress and the optical fiber strain are respectively fitted, the fitting results are shown as a formula (8) and a formula (9), and the fitting R of the formula (8)²0.9999, 0.9951 and 0.9996, respectively, and an RMSE of 1.458X 10, respectively^-6、2.318×10^-5And 1.185X 10^-5(ii) a Fitting of formula (9) R²0.9999, 0.9949 and 0.9997, respectively, RMSE 1.746, 11.79 and 5.542, respectively;

(3) armor

According to the relation between the armor strain and the stress and the optical fiber strain under different twisting speeds, the armor strain and the optical fiber strain can be divided into two stages in the twisting process, the armor strain is in direct proportion to the optical fiber strain when the twisting degree is not large, and the strain is only generated by twisting; when the twisting degree is further increased, the strain is generated by twisting and stretching, and the armor strain is further increased along with the increase of the strain of the optical fiber; the relation between the armor stress and the optical fiber strain is divided into three stages, the change rule of the armor stress and the optical fiber strain is similar to that of the first two stages, and the armor is in an elastic stage at the moment; when the armor stress is further increased to the yield point of the material along with the increase of the torsion degree, the armor generates plastic deformation, and the relation between the stress and the optical fiber strain enters a third stage; the torsion speed has no influence on the result basically; the results of fitting using the piecewise function are shown in equations (10) and (11); fitting of formula (10) R²0.9994 and 0.9998, respectively, and an RMSE of 4.434X 10, respectively^-7And 1.274X 10^-5(ii) a Fitting of formula (11) R²0.9994, 0.9994 and 0.9996 respectively, RMSE 4.496, 23.39 and 25.03 respectively, and the first-stage functions of equations (10) and (11) all have a high degree of linear fit, which is consistent with the laws reflected by equations (6) and (7),

the results of 4.2.1 knots are combined to show that under the same torsion angle, the change degree of the strain and the stress of the internal optical fiber and armor stranded structure is obviously greater than that of the submarine cable when the torsion direction is opposite to the stranding direction when the submarine cable is the same as the stranding direction; predicting the strain and stress of the cable core and the armor according to the established relation between the strain and stress of the cable core and the armor and the strain of the optical fiber under the twisted state of the submarine cable, equations (4) to (11)) and the actually measured strain of the optical fiber of the BOTDR system, and further judging the degree of the twisting fault of the submarine cable;

4.3 analysis of Anchor-smashing simulation result

(1) General analysis of Anchor-smashing process

The submarine cable only has local deformation in the early stage of anchoring failure and then has overall deformation, and the whole process can be divided into a local deformation stage of an armor layer, a submarine cable sinking stage and a submarine cable rebounding stage; the time interval of only armoured local deformation of the submarine cable is 0 to t₁(t₁0.003s), the time interval during which the total subsidence occurs is t₁～t₂(t₂0.0036s), followed by t₂～t₃(t₃0.004s) appears slightly kick-backing, the submarine cable of whole process sinks the displacement volume and is 7.3cm, and ship anchor displacement volume is 8cm, and at the stage of sinking of submarine cable, the armor passes through to surrender and reinforce the stage from the elasticity stage, and single armor steel wire self deformation on the anchor cable contact surface can be ignored this moment, and along with sinking of ship anchor takes place to sink in step, explains that the armor dress is to a certain extent to the seaThe internal structure of the cable has a protective effect, but the degree of protection is limited.

(2) Optical fiber

Under the anchoring state, the change rule of the strain distribution of the axial optical fiber of the submarine cable along with time can be known from the relation between the average value of the strain of the optical fiber in the middle main impact area of the simulation section of the submarine cable and the time and the displacement of the ship anchor, the strain of the axial optical fiber of the submarine cable is integrally increased along with the increase of anchoring depth, the distribution of the strain is symmetrical, the optical fiber at the middle anchoring point is reduced, the strain of the optical fiber is increased towards two sides near the anchoring point, the bending degree of the submarine cable is reduced after a certain distance from the anchoring point, and the strain of the optical fiber tends to be reduced; the characteristic of the optical fiber strain distribution can be used as a basis for judging the anchor failure of the submarine cable according to the optical fiber strain.

(3) Armor

The plastic strain is permanent strain generated in the deformation process, the permanent strain is taken as a material damage mark in engineering, the plastic strain of the armor reflects the damage state of the submarine cable to a great extent, the armor stress and the plastic strain at the central point and the positions of 5cm, 10cm and 15cm on two sides of the central point are extracted by taking the anchor falling central point as a reference, and the relationship between the armor average stress, the plastic strain and the optical fiber strain and time is drawn into a curve for comparison;

according to the constitutive relation of elastic-plastic materials in engineering, the state of the armor in the whole anchoring process is divided into the following time periods: in the OA time period, the armor is in an elastic stage, the elastic limit of the material is reached at the point A, and the middle part of the OA section is provided with a fluctuation interval with basically unchanged stress, because the submarine cable is transited from a local deformation stage of the armor to a sinking stage in the fluctuation interval; in the AB time period, after the point A, the armor stress is greater than the yield strength of the material, the armor is subjected to plastic deformation and enters a yield stage, the armor strain is still increased in the time period without increasing the stress, the stress-time curve shows sawtooth-shaped fluctuation and seems to lose the capacity of resisting deformation, and the lowest point of the AB section on the stress curve is called as a yield point; in the BC time period, the submarine cable continues to sink along with the ship anchor, the armor is transited from the yield stage to the strengthening stage, the stress of the armor is firstly reduced and then increased along with the time, the magnitude of the armor stress in the stage is always smaller than the yield strength of the material of the armor, and the plastic strain is kept unchanged; in the CD time period, the armor is still in a strengthening stage, the stress of the armor continuously increases along with the time, which shows that the stress must be increased to increase the strain of the armor, the armor recovers the capability of resisting deformation, which is also the main characteristic of the strengthening stage, after the C point, the strength of the armor stress exceeds the yield strength of the material again, and the plastic strain value of the armor begins to increase again; in the DE time period, due to the unloading of the displacement load of the rigid body of the ship anchor, the sinking stage of the submarine cable is ended and enters the rebound stage, the armor still retains the plastic strain but the stress is reduced due to a small amount of rebound;

(4) analysis of main characteristic moment of submarine cable under anchor failure

The sea cable anchoring mainly comprises three processes: the process I is a local deformation stage of the submarine cable at the initial stage of collision, and the time is 0-0.003 s; the process II is a submarine cable sinking stage, and the time is 0.003 s-0.036 s; the process III is a rebound stage of the submarine cable, and the time is 0.036 s-0.04 s; process II is subdivided into four stages: the first stage is the second half of the OA stage, from 0.003s to 0.0045s, the submarine cable initially sinks in the stage, from the beginning of the submarine cable sinking to the point A, the armor does not have plastic deformation in the elastic stage, the armor has birdcage-shaped deformation, the submarine cable maintains a normal structure in the second stage, from 0.0045s to 0.012s, the armor birdcage-shaped deformation further develops, the armor has plastic strain in the yield stage, and the submarine cable internal structure still basically keeps normal under the protection of the armor; the third stage is a BC stage, from 0.012s to 0.03s, the sinking amount of the submarine cable is further increased in the stage, during the sinking process of the submarine cable, the armor firstly undergoes a small-segment process from the birdcage-shaped bulge to the gradual recovery of deformation, the armor stress is reduced in the process, then the stress is increased again in the strengthening stage, the plastic strain is kept unchanged, the extrusion of the stranded layer of the submarine cable to the internal structure in the stage is obviously increased, the armor protection tends to be extreme, and therefore, the stage needs to be highly noticed; the fourth stage is a CD section, from 0.03s to 0.036s, the armor cannot maintain a normal twisted structure at the stage, the effective protection on the internal structure of the submarine cable is lost, the cable core section in the submarine cable deforms, and the normal use of the submarine cable is endangered, so that the submarine cable is considered to be damaged at the stage; based on the analysis in the past, fault identification and feature extraction can be carried out.

The invention has the beneficial effect that the invention uses 110kV YJQ41 multiplied by 300mm²The submarine cable is a research object, a structure dynamics finite element model is established based on ANSYS/LS-DYNA explicit dynamics analysis, modeling simulation is carried out on three typical fault conditions of submarine cable stretching, twisting and anchoring, strain distribution along the composite optical fiber under the fault conditions and corresponding relations between strain and stress of related structural components in the submarine cable and optical fiber strain in the fault development process are obtained, and functional relations between cable core and armor strain and stress and optical fiber strain under the stretching and twisting states are established, and evaluation indexes of the damage state of the submarine cable and the optical fiber strain in the anchoring state are provided, based on the analysis, an optical fiber strain distribution characteristic mode under different mechanical faults of the submarine cable and a submarine cable mechanical fault identification method based on the size and the spatial distribution of the optical fiber strain are provided, and reference is provided for state monitoring of the submarine cable mechanical fault.

Drawings

FIG. 1 is a comparison of submarine cable routing and strain data.

Fig. 2 is an hourglass energy/internal energy-time curve.

FIG. 3 is a graph of fiber to core strain distributions during stretching, where (a) fiber strain distribution under a tensile failure and (b) core strain distribution under a tensile failure.

FIG. 4 is a graph showing the relationship between cable core strain and optical fiber strain in the stretched state of the submarine cable.

FIG. 5 is the fiber strain at the twisted state of the submarine cable, wherein (a) the axial fiber strain distribution of the submarine cable at different twist angles, and (b) the fiber strain-twist angle curves.

FIG. 6 is a relationship between cable core strain/stress and optical fiber strain under a twisted state of a submarine cable, wherein (a) the cable core strain-optical fiber strain curve and (b) the cable core stress-optical fiber strain curve.

FIG. 7 is a graph of armor strain/stress versus fiber strain for a twisted submarine cable, wherein (a) is the armor strain-fiber strain curve and (b) is the armor stress-fiber strain curve.

FIG. 8 is the fiber strain at the twisted state of the submarine cable, wherein (a) the submarine cable axial fiber strain distribution at different twist angles, and (b) the fiber strain-twist angle curves.

FIG. 9 is a relationship between cable core strain/stress and optical fiber strain under a twisted state of a submarine cable, wherein (a) the cable core strain-optical fiber strain curve and (b) the cable core stress-optical fiber strain curve.

FIG. 10 is a graph of armor strain/stress versus fiber strain for a twisted submarine cable, wherein (a) is the armor strain-fiber strain curve and (b) is the armor stress-fiber strain curve.

Fig. 11 is a graph of displacement of a ship anchor and displacement of a central point of a submarine cable versus time under the anchor smashing fault.

Fig. 12 shows the optical fiber strain in the anchored state of the submarine cable, wherein (a) the axial optical fiber strain distribution of the submarine cable at different times, and (b) the optical fiber strain-time/ship anchor displacement curve.

Figure 13 is a graph of armor stress/plastic strain versus time/ship anchor displacement.

Detailed Description

The invention provides a mechanical fault monitoring method for a photoelectric composite submarine cable, which is built and finished in 2012 by adopting 110kV YJQ41 multiplied by 300mm to ensure the safe and stable operation of the submarine cable²A, B islands in a certain sea area in the east of China, which are connected by photoelectric composite submarine cables, are used for electric energy transmission, communication operation of electric power system channels and transmission of real-time operation information of power grid dispatching automation; the invention uses 110kV YJQ41 multiplied by 300mm²The submarine cable is a research object, a structural dynamics finite element model is established based on ANSYS/LS-DYNA explicit dynamics analysis, three typical fault conditions of the submarine cable such as stretching, twisting and anchoring are modeled and simulated, strain distribution of the submarine cable along the composite optical fiber under the fault conditions, corresponding relations between strain and stress of related structural components in the submarine cable and optical fiber strain in the fault development process are obtained, and functional relations between cable core and armor strain and stress and optical fiber strain under the stretching and twisting states and evaluation indexes of the submarine cable damage state and optical fiber strain under the anchoring state are established, wherein the submarine cable mechanical fault state monitoring method comprises the following steps:

submarine cable structure and state monitoring system

1.1. submarine cable state monitoring system based on BOTDR

The Brillouin optical time domain scattering (BOTDR) -based 110kV submarine cable state monitoring subsystem monitors strain and temperature distribution of a submarine cable in real time by adopting a distributed optical fiber measuring instrument; the sea cable state monitoring subsystem has 6 channels, and 1/2, 3/4 and 5/6 of the channels are respectively applied to the monitoring of the east, middle and west three-phase sea cables. The BOTDR testing equipment is placed on the B island side, laser pulses are incident into the submarine cable composite single-mode fiber after being connected through a common cable, the laser is transmitted in the fiber to generate Brillouin scattering light, the Brillouin scattering light is reversely transmitted to a transmitting end, and the frequency shift and the intensity of Brillouin scattering light signals are detected at the transmitting end through a Data Acquisition Unit (DAU) and a Data Processing Unit (DPU) to obtain strain and temperature information of the fiber; to date, this system has accumulated a large amount of in-field submarine cable strain data.

The frequency shift, the intensity, the temperature and the strain of the Brillouin scattering have linear relations; for a conventional single mode optical fiber, the Brillouin frequency shift v_BThe expression for T and strain ε is as follows

ν_B(T)＝ν_B0+1.158×10^-4ΔT (1)

ν_B(ε)＝ν_B0+5.6ε (2)

at present, the most reliable characteristic quantity is Brillouin frequency shift, the Brillouin frequency shift of a line to be measured under the same condition is continuously measured, a standard curve is obtained after averaging, and the difference value of a subsequent measured curve and the standard curve is used as a measurement curve of the line to be measured. Although a joint measurement of temperature and strain can be achieved hereby, both fiber temperature and strain lead to a brillouin shift change, from which temperature and strain cannot be distinguished alone. Considering that the temperature of the submarine cable generally rises or falls in a whole form, and the strain is fluctuated and fluctuated with the position due to the influence of terrain and local external force and is locally changed, the temperature and the strain can be distinguished to a certain extent according to the characteristic.

The optical fiber compounded in the submarine cable is loose, which causes doubt on whether the strain of the optical fiber can effectively reflect the strain of the submarine cable, and the point is explained according to actual data. The brillouin shift can be converted to strain according to the strain sensitivity coefficient 0.0482 ± 0.001 MHz/. mu. epsilon.brillouin, as shown in fig. 1. The strain distribution of the east sea cable is shown in the implementation, the sea cable routing is indicated by the dash-dot-dash line.

From the submarine cable routing data, islands a and B are very steep near the coast. As can be seen from fig. 1, the strain values of islands a and B near 100m of landing point are large, which should be due to the gravity of the sea cable. There is a steep slope near the a island 270m where the altitude changes rapidly from-2 m to-20 m, and correspondingly there is also a peak in strain. There is a shoal at a distance a of 1900m, corresponding to an altitude of-10.2 m, and correspondingly the strain there has a small valley. The reason for this is that the shoal is smooth and the weight of the sea cable does not easily cause large strains. However, the topography on both sides of the shoal is steeper, with correspondingly greater strain. From the above analysis, the submarine cable routing is closely related to the strain. The above results show that although the composite optical fiber in the submarine cable is loose, the strain condition of the submarine cable can be effectively reflected according to the Brillouin frequency shift of the optical fiber. This provides support for the validity of the results of subsequent modeling analysis.

2, finite element modeling of submarine cables

2.1 submarine Cable geometric modeling

The geometric modeling directly determines the quality of mesh subdivision, further influences the solving efficiency and the accuracy of results, the submarine cable has a complex structure, and a geometric model which is beneficial to finite element subdivision and calculation is established on the premise of trying to restore the real structure of the submarine cable. The method simplifies the actual submarine cable from the quality of grid subdivision, the possibility of initial penetration and the influence of each structural component in the submarine cable on a simulation result, omits a conductor shielding layer, an insulation shielding layer and a brass band layer which are relatively thin in the submarine cable and have little influence on a model, simplifies an inner and outer tegument layer into a shell structure, and finally establishes a submarine cable model which comprises a copper cable core, a crosslinked polyethylene insulating layer, a lead alloy sheath, a high-density polyethylene sheath, a polyethylene terephthalate (PET) filling strip, an optical unit, the inner and outer teguments and an armor layer, wherein the optical unit comprises an optical fiber, a steel pipe and a polypropylene sheath.

In the geometric modeling of the submarine cable, modeling of three stranded structure components of armor, PET filler strips and optical units is the most critical, and an accurate stranded structure model is crucial to truly expressing the structural mechanical characteristics of the submarine cable, so that a single stranded body is constructed by depending on an APDL language specific to ANSYS, and then the single stranded body is copied under a cylindrical coordinate system according to the number of stranded roots to obtain a stranded layer.

2.2 selection of cell Material and mesh Subdivision

2.3 contact definition and sandglass control

the hourglass mode is a state which is stable in mathematics but cannot be realized physically, the overall hourglass energy of a calculation model in a simulation process must be controlled to be less than 10% of the overall internal energy, and a simulation result is effective; reducing their respective hourglass energies by increasing the mesh split quality and applying an hourglass control algorithm to the streamer components where the hourglass energy is larger; after many attempts, the ratio of the hourglass energy to the internal energy of the simulation whole process is controlled to be below 2%; FIG. 2 is a diagram showing the relationship between sand leakage energy and internal energy in the process of anchoring a sea cable. The simulation results of the mechanical modeling are reliable from an energy point of view.

3, load application in case of submarine Cable failure

3.1 tensile failure load application

Selecting a typical tensile load, applying the typical tensile load to the axial direction of the submarine cable model, wherein the tensile rate and the tensile load action time of the submarine cable are respectively 5 per mill and 0.1s, and stretching at a constant speed; applying fixed constraint to all nodes on one end face of the model, cutting a small section of submarine cable section with the length of the other end as a component, setting the material type of the component as a rigid body, and applying rigid body displacement-time load to the component to further stretch the submarine cable;

3.2 torsional fault load application

3.3 Anchor-to-pound fault load application

finite element simulation results and analysis

4.1, tensile simulation results analysis

The strain distribution rule of the axial optical fibers of the submarine cable in the stretching process is similar to that of other cylindrical (ring) bodies with similar structures in the submarine cable in the axial stretching process, the most important cable core components in the submarine cable are taken as representatives to analyze, the strain of a cable core in the stretching process is distributed along the axial direction of the submarine cable, the strains of the optical fibers and the cable core in the axial direction of the submarine cable are increased along with the increase of the stretching degree, the strain of the cable core is not changed along with the change of the axial distance and is in direct proportion to the stretching degree, the strain quantity of a main stretching area applied by the rigid body displacement load of the optical fibers along the line is maximum, the strain quantity of the optical fibers is smaller as the criterion of the submarine cable in the stretching state, and the fault location is carried out by taking the characteristic as the criterion of the submarine cable in the stretching state; taking the average strain of the optical fiber and the cable core in the area with relatively obvious optical fiber strain change within the range of 2-4.5 m in the axial direction of the submarine cable in the graph 3, and establishing the relation between the cable core strain and the optical fiber strain (as shown in the graph 4);

as can be seen from FIG. 4, it is seen that the cable core strain and the optical fiber strain are approximately linear, which accords with the theoretical analysis of the tensile mechanical properties of the submarine cable in the literature, and meanwhile, for the cylinder structure of the cable core in the submarine cable and the twisted structure of the optical fiber, the conclusion that the twisted structure wound outside the cylinder is approximately proportional to the strain of the cylinder in the isometric deformation is obtained in the linear tensile test, and the conclusion further verifies the reliability of the relationship between the cable core and the optical fiber strain obtained here. Meanwhile, as the excess fiber length is gradually exhausted along with the accumulated strain in actual use, under the condition that the excess fiber length is not considered in modeling, the initial strains of the optical fiber and the cable core are zero, and accordingly, the data that the cable core strain and the optical fiber strain are approximately linear (as shown in figure 4) is fitted, and the coefficient R is determined²0.9877, standard error RMSE 1.237X 10^-4Formula (3);

ε_c＝1.204ε_f (3)

in the formula, epsilon_cIndicating strain of cable core,. epsilon_fRepresenting the fiber strain; the stretching degree and the state of the submarine cable can be judged based on the formula (3) and the optical fiber strain;

4.2 torsion simulation results analysis

(1) The strain distribution of the optical fiber in the axial direction of the submarine cable at different torsion angles (as shown in fig. 5 (a)) of the optical fiber increases along with the increase of the torsion degree of the submarine cable, the optical fiber strains at the load application end and the restraint end of the submarine cable are slightly influenced by the end effect higher than the middle section, and the strains in the middle area are basically equal; from the relationship between the mean fiber strain and the twist angle in the middle region of the simulated section (as shown in fig. 5 (b)), it can be seen that the fiber strain is proportional to the twist angle;

(2) cable core

Because the outer layer unit of the cable core is stressed maximally during twisting, the strain and stress of the cable core refer to the strain and stress of the outermost layer of the cable core; from the relationship between cable core strain and stress and optical fiber strain in the twisted state of the submarine cable (as shown in fig. 6 (a)), it can be known that the cable core strain is always in direct proportion to the optical fiber strain in the twisted state, the fitting result is as shown in formula (4), and the fitting R is as shown in formula (4)²＝0.9999，RMSE＝1.04×10^-5；

ε_c＝4.16ε_f (4)

As can be seen from fig. 6(b), the cable core material is changed from elastic deformation to plastic deformation along with the increase of the torsion angle, the cable core stress is in direct proportion to the optical fiber strain in the elastic stage of the material, and after the yield point is exceeded, the cable core stress and the optical fiber strain satisfy a linear relationship, and the result of fitting by adopting a piecewise linear function is as shown in formula (5); first segment R of fitting function ²1, RMSE 0.1171; second segment R²The torsional speed has no influence on the result (the curve in the figure is a fitted curve);

in the formula, σ_cThe unit is the cable core stress and is 0.1 MPa.

(3) Armor

Under different twisting speeds, from a relation curve (shown in fig. 7) of the armor strain and the stress and the optical fiber strain, it can be known that the armor strain and the stress are approximately in direct proportion to the optical fiber strain from fig. 7, because when the twisting direction is opposite to the stranding direction of the submarine cable, the optical fiber and the armor stranded structure are in a synchronous discrete state in the twisting process, the optical fiber strain and the armor strain and stress are generated only by twisting, and the submarine cable stretching caused by twisting is negligible, so the armor strain and stress cannot be suddenly changed due to the superposition of twisting and stretching and are always linear with the optical fiber strain; from FIG. 7(b)One step, the armor is always in an elastic stage within the set 20-degree reverse torsion angle range, the maximum stress borne by the armor does not reach the yield point of the material, and the torsion speed basically has no influence on the result; the fitting results are shown in the formulas (6) and (7), and the formula (6) fits R²＝0.9993，RMSE＝7.572×10^-6(ii) a Fitting of formula (7) R²0.9990, RMSE 20.53. (the curve in the figure is a fitting curve)

ε_t＝1.15ε_f (6)

σ_t＝2.593×10⁶

ε

_f 0≤ε_f≤8.28×10^-4 (7)

in this state, the stress and strain of the cylindrical (ring) body such as the cable core are still generated by torsion and are irrelevant to the torsion direction; when the torsion degree is small, the strain and stress of the stranded structures such as the optical fiber, the armor and the like are mainly generated by torsion, and the change of the strain and the stress at the moment is not large; as the degree of torsion increases, its strain will start to stress due to both torsion and tension, with its value increasing significantly;

(1) optical fiber

The strain distribution of the optical fiber in the axial direction of the submarine cable at different torsion angles is shown in fig. 8(a), and it can be seen that the optical fiber strain increases with the increase of the torsion degree of the submarine cable, and the optical fiber strain values in the axial direction of the submarine cable are substantially equal. The average strain along the fiber is plotted as a function of the twist angle as shown in FIG. 8 (b). It can be seen that when the torsion degree is small, the optical fiber strain and the torsion angle form a linear relationship, the increase rate of the optical fiber strain is accelerated along with the deepening of the torsion degree, and finally the optical fiber strain increases back to the linear increase and can be divided into three sections.

(2) Cable core

The relationship between the strain and stress of the cable core and the strain of the optical fiber is shown in figure 9 according to different twisting speeds. For the cylindrical cable core structure, the change rule of the strain and the stress along with the torsion angle is independent of the torsion direction, and fig. 5(b) and 6 show that the cable is in a torsion stateThe core strain and stress respectively form linear and piecewise linear function relations with the torsion angle; according to the three-segment relation between the fiber strain and the torsion angle in fig. 8(b), the three-segment function can be adopted to respectively fit the relation between the cable core strain and the fiber strain in fig. 9, the fitting results are shown as the formula (8) and the formula (9), and the fitting R of the formula (8)²0.9999, 0.9951 and 0.9996, respectively, and an RMSE of 1.458X 10, respectively^-6、2.318×10^-5And 1.185X 10^-5(ii) a Fitting of formula (9) R²0.9999, 0.9949 and 0.9997, respectively, RMSE 1.746, 11.79 and 5.542, respectively;

(3) armor

As can be seen from fig. 10(a), the armor strain and the optical fiber strain can be divided into two stages in the twisting process according to the relationship between the armor strain and the stress and the optical fiber strain at different twisting speeds (as shown in fig. 10), the armor strain and the optical fiber strain are in direct proportion when the twisting degree is not large, and the strain is generated only by twisting; when the twisting degree is further increased, the strain is generated by twisting and stretching, and the armor strain is further increased along with the increase of the strain of the optical fiber; as can be seen from fig. 10(b), the relationship between the armor stress and the optical fiber strain is divided into three stages, the first two stages are similar to the change rule of the strain, and the armor is in the elastic stage; when the armor stress is further increased to the yield point of the material along with the increase of the torsion degree, the armor generates plastic deformation, and the relation between the stress and the optical fiber strain enters a third stage; the torsion speed has no influence on the result basically; as shown in fig. 10, the fitting using the piecewise function results in the equations (10) and (11); fitting of formula (10) R²0.9994 and 0.9998, respectively, and an RMSE of 4.434X 10, respectively^-7And 1.274X 10^-5(ii) a Fitting of formula (11) R²0.9994, 0.9994 and 0.9996, respectively, and RMSE 4.496, 23.39 and 25.03, respectively, (fitting curves are shown inFig. 10. ) The first-segment functions of the formulas (10) and (11) have high linear fitting degree, which is consistent with the rules reflected by the formulas (6) and (7),

4.3 analysis of Anchor-smashing simulation result

(1) General analysis of Anchor-smashing process

The displacement of the ship anchor and the displacement of the central point of the submarine cable in the main impact area in the anchoring process change along with time as shown in the graph of fig. 11. In the early stage of the anchoring failure, the submarine cable only deforms locally and then deforms integrally, and the whole process can be divided into a local deformation stage of the armor layer (the sinking of the submarine cable can be ignored in the period), a submarine cable sinking stage (measured by displacement of the central point of the submarine cable) and a submarine cable rebounding stage; the prior art describes that a naval anchor carries out a drop anchor impact test on a photoelectric composite cable, analyzes the deformation process of an armor and a cable core of the submarine cable in the anchoring process to obtain the deformation of the submarine cable, and simultaneously comprises the results of local indentation of the armor and total sinking bending deformation of the submarine cable. As further shown in FIG. 11, the time interval during which only the sheath is locally deformed in the submarine cable is 0 to t₁(t₁0.003s), time to total settlingInterval t₁～t₂(t₂0.0036s), followed by t₂～t₃(t₃0.004s) appears slightly kick-backing, whole process submarine cable sinks the displacement volume and is 7.3cm, ship anchor displacement volume is 8cm, at the stage of sinking of submarine cable, the armor passes through to surrender and reinforce the stage (see the analysis in 4.3(3) section) from the elasticity stage, single armor steel wire self deformation on the anchor cable contact surface can neglected at this moment, and along with sinking of ship anchor takes place synchronous sinking, it is basically parallel with cable core central point displacement curve to show in figure 11 that this stage ship anchor displacement curve is sunk to the cable core, the armor sinks to take place the extrusion to lead to cable core deformation after reaching the protection limit. The degree of armor deformation near the anchor cable contact area obtained by the submarine cable after the literature dissection anchor drop impact test is consistent with the degree of deformation of the inner cable core; the armor is shown to have a protective effect on the internal structure of the submarine cable to a certain extent, but the degree of protection is limited.

(2) Optical fiber

In the anchoring state, the change rule of the strain distribution of the axial optical fiber of the submarine cable along with time (as shown in fig. 12(a)) is the relation between the average value of the strain of the optical fiber in the middle main impact area of the submarine cable simulation section and the time and the displacement of the ship anchor (as shown in fig. 12 (b)). As can be seen from fig. 12(a)), the whole axial optical fiber strain of the submarine cable increases with the increase of the anchor hitting depth and the distribution thereof has symmetry, the optical fiber strain at the middle anchor drop point becomes smaller, the optical fiber strain increases toward both sides in the vicinity of the anchor drop point, the bending degree of the submarine cable decreases after a certain distance from the anchor drop point, and the optical fiber strain tends to decrease; the characteristic of the optical fiber strain distribution can be used as a basis for judging the anchor failure of the submarine cable according to the optical fiber strain.

(3) Armor

The plastic strain is a permanent strain generated in the deformation process, the permanent strain is taken as a material damage mark in engineering, the plastic strain of the armor reflects the damage state of the submarine cable to a great extent, the armor stress and the plastic strain at the central point and the positions of 5cm, 10cm and 15cm on the two sides of the central point are extracted by taking the anchor falling central point as a reference, and the relationship between the armor average stress, the plastic strain and the optical fiber strain and the time is drawn into a curve together in the graph 13 for comparison;

as shown in fig. 13, according to the constitutive relation of the engineering elastic-plastic material, the state of the armor in the whole anchoring process is divided into the following time periods: in the OA time period, the armor is in an elastic stage, the elastic limit of the material is reached at the point A, and the middle part of the OA section is provided with a fluctuation interval with basically unchanged stress, because the submarine cable is transited from a local deformation stage of the armor to a sinking stage in the fluctuation interval; in the AB time period, after the point A, the armor stress is greater than the yield strength of the material, the armor is subjected to plastic deformation and enters a yield stage, the armor strain is still increased in the time period without increasing the stress, the stress-time curve shows sawtooth-shaped fluctuation and seems to lose the capacity of resisting deformation, and the lowest point of the AB section on the stress curve is called as a yield point; in the BC time period, the submarine cable continues to sink along with the ship anchor, the armor is transited from the yield stage to the strengthening stage, the stress of the armor is firstly reduced and then increased along with the time, the magnitude of the armor stress in the stage is always smaller than the yield strength of the material of the armor, and the plastic strain is kept unchanged; in the CD time period, the armor is still in a strengthening stage, the stress of the armor continuously increases along with the time, which shows that the stress must be increased to increase the strain of the armor, the armor recovers the capability of resisting deformation, which is also the main characteristic of the strengthening stage, after the C point, the strength of the armor stress exceeds the yield strength of the material again, and the plastic strain value of the armor begins to increase again; in the DE time period, due to the unloading of the displacement load of the rigid body of the ship anchor, the sinking stage of the submarine cable is ended and enters the rebound stage, the armor still retains the plastic strain but the stress is reduced due to a small amount of rebound;

The sea cable anchoring mainly comprises three processes: the process I is a local deformation stage of the submarine cable at the initial stage of collision, and the time is 0-0.003 s; the process II is a submarine cable sinking stage, and the time is 0.003 s-0.036 s; the process III is the rebound stage of the submarine cable, and the time is 0.036 s-0.04 s. Process II is subdivided (as can be seen from fig. 13) into four stages. The first stage is the second half of the OA stage, from 0.003s to 0.0045s, the submarine cable is initially sunk in the first stage, from the beginning of sinking of the submarine cable to the point A, the armor is in the elastic stage and does not generate plastic deformation, the armor generates birdcage-shaped deformation, the inner part of the submarine cable maintains a normal structure (shown as stage II-1 in the table 1), as shown as stage I in the table 1, and the submarine cable is in a normal state before the first stage; the second stage is an AB stage, from 0.0045s to 0.012s, the armor birdcage shape deformation further develops, plastic strain appears in the yield stage, the internal structure of the submarine cable is still basically normal under the protection of the armor, and the stage is defined as a light attention stage (as shown in the stage II-2 in the table 1). The third stage is a BC stage, from 0.012s to 0.03s, the sinking amount of the submarine cable is further increased in the stage, during the sinking process of the submarine cable, the armor firstly undergoes a small-section process from the birdcage-shaped bulge to the gradual recovery of deformation, the armor stress is reduced in the process, then the stress is increased again in the strengthening stage, the plastic strain is kept unchanged, the extrusion of the stranded layer of the submarine cable on the internal structure in the stage is obviously increased, the protection of the armor tends to the limit, and therefore high attention is required in the stage (as shown in stage II-3 in Table 1); the fourth stage is a CD stage, from 0.03s to 0.036s, the armor cannot maintain a normal stranded structure at the stage, the effective protection on the internal structure of the submarine cable is lost, the section of the cable core inside the submarine cable is deformed, and the normal use of the submarine cable is endangered, so that the submarine cable is considered to be damaged at the stage (as shown in stages II-4 and III in Table 1).

(5) Sea cable anchor smashing state monitoring index

Based on the analysis of the anchoring process, a submarine cable damage state evaluation table corresponding to different stages of anchoring is established, and corresponding characteristic moments for distinguishing different damage states of the submarine cable in table 1 are respectively 0.0045s, 0.012s and 0.03 s.

Table 1 submarine cable damage state evaluation table under anchor failure

According to the relationship between the optical fiber strain and the time in the anchoring process shown in fig. 12(b), the optical fiber strain corresponding to the anchoring main characteristic moment can be obtained: when t is 0.0045s, the optical fiber should beBecomes 4.25X 10^-3(ii) a When t is 0.012s, the optical fiber strain is 25.4 × 10^-3(ii) a Fiber strain at t 0.03s 46.3 × 10^-3. Therefore, the evaluation index of the damage state of the submarine cable anchorage based on the optical fiber strain is established and is shown in table 2.

TABLE 2 submarine cable anchoring damage state evaluation index based on optical fiber strain

4.4 Fault identification and feature extraction

From the foregoing analysis, it can be seen that the submarine cable has different mechanical failure types, and the strain amplitude and spatial distribution of the optical fiber inside the submarine cable have different characteristics. The deformation states of the submarine cable and the strain characteristics of the optical fiber under three typical failures of submarine cable stretching, twisting and anchoring are shown in table 3.

TABLE 3 characterisation of mechanical faults in submarine cables

Optical fiber strain data can be obtained based on actually measured optical fiber Brillouin frequency shift data and a 1.3-section method, and the type and development degree of the fault of the submarine cable can be further judged by combining a table 4 by means of a submarine cable part strain/stress and optical fiber strain relation model (tables 4, formulas (3) -11, fig. 13 and tables 3) established by the method, so that a basis is provided for finding the mechanical fault of the submarine cable in time and taking necessary measures.

5, conclusion

The invention adopts a finite element modeling method to carry out simulation analysis on three typical fault conditions of stretching, twisting and anchoring which may be suffered by submarine cable operation, and systematically studies the strain distribution of optical fibers along the line and the corresponding change relation with the strain/stress of cable cores and armor parts. The main conclusions are as follows:

(1) under three typical mechanical failure states, the amplitude and the spatial distribution of the strain of the optical fiber along the submarine cable have different characteristics, and the failure type of the submarine cable can be identified according to the different characteristics.

(2) Under the stretching state of the submarine cable, the strain of the cable core is approximately proportional to the strain of the optical fiber. Under the twisting state, when the twisting direction is opposite to the twisting direction, the submarine cable bulges, the cable core strain is in direct proportion to the optical fiber strain, and the cable core stress and the optical fiber strain meet the piecewise linear function relationship; the armor strain and stress are approximately proportional to the fiber strain. When the twisting direction is the same as the twisting direction, the sea cable is bundled, the relation between the cable core strain and the optical fiber strain can be represented by a three-section function, the first section and the last section are linear, and the middle section meets the second-order polynomial relation. The armor strain at the early stage of torsion is in direct proportion to the optical fiber strain, and then the armor strain is increased in a quadratic polynomial form along with the optical fiber strain; the relation between the armor stress and the optical fiber strain can be represented by a three-section function, the first two sections are consistent with the strain rule, and the third section of the armor stress and the optical fiber strain are approximately in a linear relation. The establishment of the relation function can provide reference for judging the fault degree of the submarine cable based on the optical fiber strain.

(3) The anchoring damage state of the submarine cable can be divided into four stages, and the submarine cable state can be judged by combining the actually measured optical fiber strain and the evaluation index of the anchoring damage degree based on the optical fiber strain established by the invention.

Claims

1. A mechanical fault monitoring method for a photoelectric composite submarine cable is characterized by comprising the following steps of 110kV YJQ41 multiplied by 300mm²The submarine cable is a research object, a structure dynamics finite element model is established based on ANSYS/LS-DYNA explicit dynamics analysis, three typical fault conditions of stretching, twisting and anchoring of the submarine cable are modeled and simulated, strain distribution of the composite optical fiber along the submarine cable under the fault conditions and corresponding relations between strain and stress of related structural components in the submarine cable and optical fiber strain in the fault development process are obtained, and functional relations between cable core and armor strain and stress and optical fiber strain in the stretching and twisting states are establishedThe state monitoring method of the mechanical fault of the submarine cable comprises the following steps: 2. finite element modeling of submarine cable

2.1 submarine Cable geometric modeling

From three angles of the quality of grid subdivision, the possibility of initial penetration and the influence of each structural component in the submarine cable on a simulation result, the practical submarine cable is simplified, a conductor shielding layer, an insulation shielding layer and a brass band layer which are relatively thin in thickness and have little influence on a model in the submarine cable are omitted, an inner and outer tegument layer are simplified into a shell structure, and the finally established submarine cable model comprises a copper cable core, a crosslinked polyethylene insulating layer, a lead alloy sheath, a high-density polyethylene sheath, a polyethylene terephthalate (PET) filling strip layer, an optical unit, the inner and outer teguments and an armor layer, wherein the optical unit comprises an optical fiber, a steel pipe and a polypropylene sheath;

2.2 selection of cell Material and mesh Subdivision

the materials of each part of the submarine cable can be divided into two types according to the physical characteristics: one type is a metal material, which comprises a cable core, a lead alloy sheath, an armor and a steel pipe in an optical unit, wherein the metal material has typical elastic-plastic characteristics, a classic BKIN (nonlinear mechanical fastening) model is adopted as a material model of the metal material, and the model adopts a piecewise linear function to express the constitutive relation of the material; the elastic modulus of other non-metal structural component materials is much smaller than that of metal materials, the simulation process is always in an elastic stage, a linear elastic model Isotropic is adopted for research, and for the ship anchor, a Rigid material model Rigid is adopted for the ship anchor because the deformation of the anchor before and after the anchor is smashed can be ignored;

2.3 contact definition and sandglass control

the hourglass mode is a state which is stable in mathematics but cannot be realized physically, the overall hourglass energy of a calculation model in a simulation process must be controlled to be less than 10% of the overall internal energy, and a simulation result is effective; reducing their respective hourglass energies by increasing the mesh split quality and applying an hourglass control algorithm to the streamer components where the hourglass energy is larger; after a plurality of attempts, the ratio of the hourglass energy to the internal energy in the whole simulation process is controlled to be below 2%;

3. load application for submarine cable fault conditions

3.1 tensile failure load application

According to the mechanical test standard of the submarine cable proposed by the international large power grid committee, selecting a typical tensile load to be applied to the axial direction of a submarine cable model, wherein the tensile rate and the tensile load action time of the submarine cable are respectively 5 per mill and 0.1s, and stretching at a constant speed; applying fixed constraint to all nodes on one end face of the model, cutting out a small section of submarine cable section with the length of the other end as a component, setting the material type of the component as a rigid body, and applying rigid body displacement-time load to the component to further stretch the submarine cable;

3.2 torsional fault load application

Similar to the method for applying the tensile load, the submarine cable is twisted by applying fixed constraint on one end node of the submarine cable, applying rigid body rotation-time load on the rigid body part of the assembly, twisting at a constant speed, controlling the twisting direction and angle of the submarine cable by setting the positive value and the negative value and the maximum value of the rigid body rotation load, and controlling the twisting speed by changing the time for applying the load; the difference of the twisting directions is that the rotation direction of the rigid body is the same as or opposite to the twisting direction of the submarine cable, according to the actual winding method of the submarine cable, the twisting angle range of 0-20 degrees under the typical twisting condition is taken, the twisting speeds are respectively +/-4 degrees/s, +/-6 degrees/s and +/-8 degrees/s, the positive twisting speed represents that the rigid body twisting direction is the same as the twisting direction of the submarine cable, and the negative twisting speed represents that the directions are opposite;

3.3 Anchor-to-pound fault load application

Selecting 660kg Hall anchors which are most representative as modeling objects according to the size of a ship above a water area where a submarine cable is located and the anchor matching condition, and simplifying the ship anchors on the premise of ensuring that the weight of the anchors and the projection area of the bottom are not changed; obtaining the maximum falling speed of the ship anchor at the moment of contacting the submarine cable according to a falling speed calculation method of the ship anchor, controlling the falling speed of the ship anchor to be less than or equal to the maximum speed so as to accord with the actual situation, controlling the collision area of the ship anchor and the submarine cable to be positioned in the middle area of the simulation section during modeling, applying fixed constraint on nodes at two ends of the submarine cable, and applying vertical downward rigid body displacement-time load only on the ship anchor part; the whole impact process is considered to be uniform speed because the impact process of the anchor cable is extremely short; taking a typical anchor-tamping fault condition, wherein the anchor-tamping depth is 8cm, setting the impact time to be 0-0.036 s according to the ship anchor calculation speed, in order to avoid abnormal convergence of a model caused by sudden unloading of a structure, slightly prolonging the load application time to 0.04s, setting the solving time to be 10s long enough to obtain the stable state of the actual ship anchor after stopping sinking, and keeping the ship anchor at the deepest impact position in the residual time; because the influence of soil penetration resistance is added in the calculation of the ship anchor kinetic energy, the resistance of the soft water saturated soil wrapping the submarine cable to the anchor cable at the impact moment can be ignored relatively, and the modeling of the soil around the submarine cable is omitted;

4. finite element simulation results and analysis

4.1, tensile simulation results analysis

For cylinders with similar structures in the submarine cable, the cylinders are similar in the axial stretching process, analysis is carried out by taking the most important cable core part in the submarine cable as a representative, the strains of the optical fiber and the cable core in the axial direction of the submarine cable are increased along with the increase of the stretching degree, the strain of the cable core is not changed along with the change of the axial distance and is in direct proportion to the stretching degree, the strain quantity of a main stretching area applied by the optical fiber along the line on rigid body displacement load is maximum, and the strain quantity of the optical fiber is smaller as the farther the distance from the main stretching area of the submarine cable is, so that the strain distribution characteristic of the optical fiber is taken as a criterion of the submarine cable in the stretching state for fault location; taking average strain of the optical fiber and the cable core in a region with relatively obvious optical fiber strain change within the range of 2-4.5 m in the axial direction of the submarine cable, and establishing a relation between the cable core strain and the optical fiber strain; the method can obtain that the strain of the cable core and the strain of the optical fiber are approximately in a linear relation, meanwhile, because the excess length of the optical fiber is gradually exhausted along with the accumulated strain during actual use, under the condition that the excess length of the optical fiber is not considered during modeling, the initial strains of the optical fiber and the cable core are zero, the strain data of the cable core and the strain data of the optical fiber are fitted according to the initial strains, and the coefficient R is determined²0.9877, standard error RMSE 1.237X 10^-4Formula (3);

ε_c＝1.204ε_f (3)

in the formula, epsilon_cIndicating strain of cable core,. epsilon_fRepresenting the fiber strain; the stretching degree and the state of the submarine cable can be judged based on the formula (3) and the optical fiber strain; it is characterized in that the preparation method is characterized in that,

1. submarine cable structure and state monitoring system

1.1 submarine cable state monitoring system based on BOTDR

At 110kV YJQ41 multiplied by 300mm²A, B islands in a certain sea area in the east of China, which are connected by photoelectric composite submarine cables, are used for electric energy transmission, communication operation of electric power system channels and transmission of real-time operation information of power grid dispatching automation; therefore, a three-dimensional comprehensive monitoring system of 110kV photoelectric composite submarine cable is builtThe 110kV submarine cable state monitoring subsystem based on Brillouin optical time domain scattering (BOTDR) monitors strain and temperature distribution of a submarine cable composite optical fiber in real time by adopting a distributed optical fiber measuring instrument; the sea cable state monitoring subsystem is provided with 6 channels, and 1/2, 3/4 and 5/6 of the channels are respectively applied to monitoring of east, middle and west three-phase sea cables; the BOTDR testing equipment is placed on the B island side, laser pulses are incident into the submarine cable composite single-mode fiber after being connected through a common cable, the laser is transmitted in the fiber to generate Brillouin scattering light, the Brillouin scattering light is reversely transmitted to a transmitting end, and the frequency shift and the intensity of Brillouin scattering light signals are detected at the transmitting end through a Data Acquisition Unit (DAU) and a Data Processing Unit (DPU) to obtain strain and temperature information of the fiber;

ν_B(T)＝ν_B0+1.158×10^-4ΔT (1)

ν_B(ε)＝ν_B0+5.6ε (2)

4.2 torsion simulation results analysis

(1) Optical fiber

The strain of the axial optical fiber of the submarine cable is integrally increased along with the increase of the torsion degree of the submarine cable, the strain distribution of the optical fiber is slightly higher than that of the middle section at the load application end and the restraint end under the influence of the end effect, and the strains in the middle area are basically equal; the optical fiber strain and the torsion angle are in direct proportion according to the relation between the average optical fiber strain and the torsion angle in the middle area of the simulation section;

(2) cable core

Because the outer layer unit of the cable core is stressed most when twisted,the strain and the stress of the cable core refer to the strain and the stress of the outermost layer of the cable core; the relation between the cable core strain and the stress and the optical fiber strain under the submarine cable torsion state can be known, the cable core strain under the torsion state is always in direct proportion to the optical fiber strain, the fitting result is as shown in formula (4), and the fitting R is²＝0.9999，RMSE＝1.04×10^-5；

ε_c＝4.16ε_f (4)

The cable core material is changed from elastic deformation to plastic deformation along with the increase of the torsion angle, the cable core stress is in direct proportion to the optical fiber strain in the elastic stage of the material, after the yield point is exceeded, the cable core stress and the optical fiber strain meet the linear relation, and the result of fitting by adopting a piecewise linear function is as shown in the formula (5); first segment R of fitting function²1, RMSE 0.1171; second segment R²The result is not influenced by the torsional speed, 1 and RMSE 0.5608;

in the formula, σ_cThe unit is the cable core stress and is 0.1 MPa;

(3) armor

According to the relation curve of the armor strain and stress and the optical fiber strain, the armor strain and stress are approximately in direct proportion to the optical fiber strain at different twisting speeds, because when the twisting direction is opposite to the stranding direction of the submarine cable, the optical fiber and the armor stranding structure are in a synchronous discrete state in the twisting process, the optical fiber strain and the armor strain and stress are only generated by twisting, and the submarine cable stretching caused by twisting can be ignored, so that the armor strain and stress can not be suddenly changed due to the superposition of twisting and stretching and are always linear with the optical fiber strain; in the set 20-degree reverse torsion angle range, the armor is always in an elastic stage, the maximum stress borne by the armor does not reach the yield point of a material, and the torsion speed basically has no influence on the result; fitting results of armor strain and relation between stress and optical fiber strain are shown as formulas (6) and (7), and fitting R of formula (6)²＝0.9993，RMSE＝7.572×10^-6(ii) a Fitting of formula (7) R²＝0.9990，RMSE＝20.53，

ε_t＝1.15ε_f (6)

σ_t＝2.593×10⁶ε_f 0≤ε_f≤8.28×10^-4 (7)

In the formula, epsilon_tIs the armor strain; sigma_tThe unit is 0.1Mpa for armor stress;

4.2.2, the twisting direction is the same as the twisting direction of the submarine cable

In this state, the stress and strain of the cylinder such as the cable core are still generated by torsion and are irrelevant to the torsion direction; when the degree of torsion is not large, the strain and stress of stranded structures such as optical fibers, armors and the like are mainly generated by torsion, and the change of the strain and the stress at the moment is not large; as the degree of torsion increases, the strain and stress thereof begin to be caused by both torsion and tension, and the values thereof increase significantly;

(1) optical fiber

According to the strain distribution of the optical fiber in the axial direction of the submarine cable at different torsion angles, the fact that the optical fiber strain is increased along with the increase of the torsion degree of the submarine cable can be known, the optical fiber strain values in the axial direction of the submarine cable are basically equal, therefore, a curve that the average strain along the optical fiber changes along with the torsion angle is made, it can be seen that when the torsion degree is not large, the optical fiber strain and the torsion angle form a linear relation, the rate of the optical fiber strain increase is accelerated along with the deepening of the torsion degree, and finally the optical fiber strain increase returns to the linear increase and can be divided into three sections;

(2) cable core

For the cylindrical cable core structure, the change rule of the strain and the stress along with the torsion angle is irrelevant to the torsion direction, the linear and piecewise linear function relation with the torsion angle is always respectively formed, three-piecewise function fitting is carried out on the relation between the fiber strain and the torsion angle and the relation between the cable core strain and the stress and the fiber strain, the fitting result is shown as a formula (8) and a formula (9), and the fitting R of the formula (8) is shown as a formula (8)²0.9999, 0.9951 and 0.9996, respectively, and an RMSE of 1.458X 10, respectively^-6、2.318×10^-5And 1.185X 10^-5(ii) a Fitting of formula (9) R²0.9999, 0.9949 and 0.9997, respectively, RMSE 1.746, 11.79 and 5.542, respectively;

(3) armor

According to the relation between the armor strain and the stress and the optical fiber strain under different twisting speeds, the armor strain and the optical fiber strain can be divided into two stages in the twisting process, the armor strain is in direct proportion to the optical fiber strain when the twisting degree is not large, and the strain is only generated by twisting; when the twisting degree is further increased, the strain is generated by twisting and stretching, and the armor strain is further increased along with the increase of the strain of the optical fiber; the relation between the armor stress and the optical fiber strain is divided into three stages, the change rule of the armor stress and the optical fiber strain is similar to that of the first two stages, and the armor is in an elastic stage at the moment; when the armor stress is further increased to the yield point of the material along with the increase of the torsion degree, the armor generates plastic deformation, and the relation between the stress and the optical fiber strain enters a third stage; the torsion speed has no influence on the result basically; fitting results of the relations between the armor strain and the stress and the optical fiber strain by adopting a piecewise function are shown in the formulas (10) and (11); fitting of formula (10) R²0.9994 and 0.9998, respectively, and an RMSE of 4.434X 10, respectively^-7And 1.274X 10^-5(ii) a Fitting of formula (11) R²0.9994, 0.9994 and 0.9996 respectively, RMSE 4.496, 23.39 and 25.03 respectively, and the first-stage functions of the formulas (10) and (11) have higher linear fitting degree which is consistent with the rules reflected by the formulas (6) and (7);

the results of 4.2.1 knots are combined to show that under the same torsion angle, the change degree of the strain and the stress of the internal optical fiber and armor stranded structure is obviously greater than that of the submarine cable when the torsion direction is opposite to the stranding direction when the submarine cable is the same as the stranding direction; according to the established relation (expression (4) to expression (11)) between the cable core and the armor strain and the stress and the optical fiber strain in the submarine cable torsion state and the optical fiber strain actually measured by the BOTDR system, the strain and the stress of the cable core and the armor can be predicted, and the torsion fault degree of the submarine cable can be further judged;

4.3 analysis of Anchor-smashing simulation result

(1) General analysis of Anchor-smashing process

The submarine cable only has local deformation in the early stage of anchor failure and then has overall deformation, and the whole process can be divided into: the method comprises the following steps of (1) performing local deformation stage, submarine cable sinking stage and submarine cable rebounding stage on an armor layer; the time interval of only armoured local deformation of the submarine cable is 0-t₁，t₁0.003s, the time interval during which the total subsidence occurs is t₁～t₂，t₂0.0036s, then at t₂～t₃，t₃Slight resilience appears in the time of 0.004s, the sinking displacement of the submarine cable in the whole process is 7.3cm, the displacement of the ship anchor is 8cm, the armor is transited from the elastic stage to the yielding and strengthening stage in the sinking stage of the submarine cable, the deformation of a single armor steel wire on the contact surface of the anchor cable can be ignored at the moment, the armor sinks synchronously along with the sinking of the ship anchor, the armor extrudes the cable core after reaching the protection limit to cause the deformation of the cable core, and the armor has a protection effect on the internal structure of the submarine cable to a certain extent, but the protection degree is limited;

(2) optical fiber

Under the anchoring state, the relation between the strain distribution of the submarine cable axial optical fiber and time shows that the strain of the submarine cable axial optical fiber is integrally increased along with the increase of anchoring depth and the distribution of the strain is symmetrical, the optical fiber at the middle anchoring point is reduced, the strain of the optical fiber is increased towards two sides near the anchoring point, the bending degree of the submarine cable is reduced after a certain distance from the anchoring point, and the strain of the optical fiber tends to be reduced; the characteristic of the optical fiber strain distribution can be used as a basis for judging the anchor failure of the submarine cable according to the optical fiber strain;

(3) armor

The sea cable anchoring mainly comprises three processes: the process I is a local deformation stage of the submarine cable at the initial stage of collision, and the time is 0-0.003 s; the process II is a submarine cable sinking stage, and the time is 0.003 s-0.036 s; the process III is a rebound stage of the submarine cable, and the time is 0.036 s-0.04 s; process II is subdivided into four stages: the first stage is the second half of the OA stage, from 0.003s to 0.0045s, the submarine cable initially sinks in the stage, from the beginning of the submarine cable sinking to the point A, the armor does not have plastic deformation in the elastic stage, the armor has birdcage-shaped deformation, the submarine cable maintains a normal structure in the second stage, from 0.0045s to 0.012s, the armor birdcage-shaped deformation further develops, the armor has plastic strain in the yield stage, and the submarine cable internal structure still basically keeps normal under the protection of the armor; the third stage is a BC stage, from 0.012s to 0.03s, the sinking amount of the submarine cable is further increased in the stage, during the sinking process of the submarine cable, the armor firstly undergoes a small-segment process from the birdcage-shaped bulge to the gradual recovery of deformation, the armor stress is reduced in the process, then the stress is increased again in the strengthening stage, the plastic strain is kept unchanged, the extrusion of the stranded layer of the submarine cable to the internal structure in the stage is obviously increased, the armor protection tends to be extreme, and therefore, the stage needs to be highly noticed; the fourth stage is a CD section, from 0.03s to 0.036s, the armor cannot maintain a normal twisted structure at the stage, the effective protection on the internal structure of the submarine cable is lost, the cable core section in the submarine cable deforms, and the normal use of the submarine cable is endangered, so that the submarine cable is considered to be damaged at the stage; based on the analysis in the past, feature extraction and fault identification can be carried out.