Study On Detecting The Depth Of Underwater Pipeline By Electromagnetic Method And The Location Of Flame Retardant

With the continuous development of oil and gas pipeline in China, the proportion of crossing pipeline for long-distance oil and gas pipeline is growing, and has become an important part of it. So it is necessary to detect the buried depth and the coating state of the crossing pipeline to make sure whether it is safe. As the widely application of horizontal directional drilling, the crossing pipeline is buried more and more deeply, and the existing detecting technology can not meet the detecting requirement, making the crossing pipeline detection a new problem and challenge for the pipeline inspection researcher and worker. In order to solve these problems, combining the theory with experiment, the crossing pipeline detecting technology based on electromagnetic was studied and achieved good results in the paper. The details were summarized as follows:(1) By analyzing the regularity of detecting signal changed with the buried depth under different detecting frequency, the dynamic method for calculating the buried depth of crossing pipeline was ameliorated:by using the exponential function, logistic function and lognormal function, the average value of magnetic signal acquired at different height over the calibration points were fitted to get the highest fitting degree function; then, the acquired signal of each test points was processed by using the function to get buried depth. Compared with the existing calculation method, the new dynamic method for calculating the buried depth of crossing pipeline had a larger detecting range and higher accuracy.(2) By analyzing the influence on the detecting range and accuracy caused by the difference of the way of cable connected to the pipeline and the detecting frequency, the principle of how to select the frequency and cable connected loop in order to improve the detecting accuracy of the buried depth was proposed:during the buried depth detecting, the "cable-pipeline loop" should be used to enhance the signal strength around the crossing pipeline; and different frequency should be used when detecting different buried depth crossing pipeline:when the buried depth was less than10m, the1kHz,2kHz,8kHz or33kHz should be used; when the buried depth was between10m and20m, the1kHz or512Hz should be used; and when the buried depth was between20m and40m, the512Hz should be used; more over, when detecting the big diameter pipeline, the poor quality anticorrosion coating pipeline or the soil resistivity around the pipeline was high, the lower frequency should be used.(3) By analyzing the magnetic signal attenuation regular near the coating defect, the method of locating the coating defects of crossing pipeline was proposed:during the coating defects locating, the "cable-pipeline-earth loop" and close interval signal acquisition should be used, and by drawing the magnetic signal distributing curve along the pipeline using the data acquired by1or2m interval, and find out the abnormal attenuation related to the coating defect, the coating defect can be located.(4) The new dynamic method for calculating the buried depth of crossing pipeline and coating defect locating method proposed in this paper were used on-site test, the application results showed that the buried depth and coating defect location obtained by the new method were according with the excavation result, and could meet the testing requirements perfectly, had good engineering application effects.