Research On Classification And Inversion Of Underwater Pipeline Defects Based On The Technology Of Dual-Frequency ACFM

Alternating current field measurement(ACFM)is an emerging electromagnetic non-destructive testing technique developed in recent years.Specific advantages include contactless,no need to remove surface coatings or paint coupling agent.Therefore,the technique has a good application prospect of rapid testing for subsea pipelines and structures.According to the testing requirements of classification and quantification of surface and subsurface defect,dual-frequency alternating current field measurement(DFACFM)technique and testing system are studied in the paper.Firstly,the ACFM finite element model of the two types of defect are established,the distribution of eddy current in workpiece induced by high and low frequency excitation current are analyzed.The different excitation parameters on the defect signals are analyzed and simulated,which including different frequency and lift-off height.And the optimal excitation frequency is determined for surface and subsurface defect testing by simulation,respectively.Secondly,the signal energy difference between two types of defect is employed to determine the classification criteria W and EH based on the determined excitation frequency.A classification approach is proposed to achieve defect classification accurately based on Wt and EHt which are classification thresholds estimated by defect experiment data.Finally,in order to define the quantification fitting equation of defect,the quantitative relation between characteristic values of signal and the defect size is discussed.In order to extract clean ACFM signal when scanning artificial defects on pipeline.A procedure for the enhancement of ACFM detection performance is proposed based on a Trade-Off(TO)algorithm.Optimal wavelet combination of mother wavelet and decomposition level based on the algorithm is determined to achieve a trade-off between the performance of denoising and the reservation of useful signal details.And the more reliable signal can be provided for the classification and quantification of defect based on the optimal wavelet denoising method.On the basis of DFACFM principle,the software and hardware modules such as signal processing box and data acquisition are optimum designed in this paper.Finally,the DFACFM defect rapid testing system is developed and the defect testing of Aluminum pipe is performed.The results show that the DFACFM system can classify surface and subsurface defect correctly as well as quantify the defect size.