Study On Damage Identification Of Offshore Wind Turbine Support Structures Based On Vibration Characteristics

The offshore platform structure is the base of offshore production and life.Compared with the onshore structure,its long-term service marine environment is extremely complex,which is subject to the interaction of corrosion,fatigue,collision,earthquake and adverse environmental loads.Structures are extremely vulnerable to various forms of damage,resulting in a decline in the bearing capacity of the structure,and even more serious will lead to the failure of the platform.It will not only cause huge economic losses and casualties,but also have adverse social and political effects.Therefore,in order to avoid damage accidents of offshore platforms and ensure the safe operation of offshore platforms,it is particularly important to develop an effective health detection and safety monitoring system for offshore platforms.Damage identification and detection technology is the core issue of structural health detection and safety monitoring.In recent years,structural damage identification methods based on vibration testing have made great progress in aerospace,civil engineering,mechanical engineering and marine engineering.According to the different research objectives,it can be divided into four levels: 1.Determine whether the structure is damaged;2.Determine the location of the damage;3.Identify the degree of damage;4.Predict the residual life of the structure.Generally speaking,the first three levels belong to the research field of damage identification,and the higher the target level,the more difficult it is.According to the different data used,damage analysis methods can be divided into three categories: methods based on time domain data,methods based on time-frequency domain data and methods based on modal domain data.1.Based on the traditional modal strain energy index,a comprehensive modal energy index is proposed for the modal domain data.The finite element model is established,the numerical simulation is carried out,and the basic modal parameters of the structure are calculated.In order to test the accuracy,we also consider:(1)damage detection of low-order modes;(2)damage detection with noise pollution;(3)damage detection with modeling errors,and the influence of three factors on the structure.The experimental results show that the proposed comprehensive modal energy indexmethod has the advantages of high accuracy and robustness in damage identification and assessment.2.For the time-frequency domain data,the non-linear identification method based on frequency domain coherence analysis is studied and applied to structural damage identification.The global damage factor and local damage factor are proposed,and two structural damage indicators are constructed.In order to verify the proposed damage index function for structural damage identification,a single-column offshore wind power support structure test model was developed.Considering the noise in the measurement environment and the non-linearity of the model structure itself,the relative damage function is used to eliminate its influence.Based on the coherence function theory,the overall damage factor is constructed.The local damage factor is constructed for the coherence function between adjacent nodes.The analysis results show that the overall damage factor method can directly express the damage degree of the structure,and has high sensitivity.By comparing the relative damage functions between the adjacent nodes,it is found that the local damage factor near the simulated damage node is the largest,which indicates that the local damage factor method can accurately point out the location of the damage of the structure.The feasibility of damage location and quantification based on the two damage factors theory is verified.3.Aiming at the method of time-domain data,the applicability of time-domain coherence analysis in non-linear damage identification is studied.By introducing slip time window technology,local time coherence function and peak coherence function are proposed,and two structural damage indices are constructed.In order to identify structural damage in time domain,the experimental model and simulated damage conditions are the same as those in frequency domain.The influence factors of time-domain coherence function are studied respectively:(1)the influence of signal amplitude on time-domain coherence;(2)the influence of phase difference on time-domain coherence;(3)the influence of noise on time-domain coherence;(4)the influence of structural damage on time-domain coherence.The local temporal coherence function and peak coherence function are proposed,which not only inherit the advantages of the temporal coherence function,but also improve the noiserobustness and damage sensitivity,and establish the structural damage factor A and B.By analyzing the response signals before and after structural damage,and calculating the structural damage factor A,the value less than 0 indicates structural damage,and the smaller the structural damage factor value,the greater the damage degree,which proves the feasibility of structural damage factor A for identifying the occurrence of damage and the relative degree of damage.The structural damage is calculated by the peak coherence function between adjacent nodes.Factor B can accurately locate the location of the damage.It shows that the two structural damage indicators can accurately quantify and locate the damage.