Research On Simultaneous Measurement Of Temperature And Strain Of Petroleum Tank Based On BOTDR System

Distributed optical fiber sensing technology based on Brillouin scattering can simultaneously measure temperature and strain at any position of tens of kilometers or hundreds of kilometers of optical fiber,and has the characteristics of high precision,high spatial resolution,low cost and durability.It has become a research hotspot at home and abroad,and is widely used in digital health monitoring in highways,electric power,petrochemical and other industries.In practical applications,the Brillouin optical time domain reflectometry(BOTDR)has the advantage of single-ended measurement because it can continuously measure the temperature and strain before the break point after the fiber is broken,so it is useful in engineering applications.However,the traditional BOTDR system has a low signal-to-noise ratio due to the self-published Brillouin scattering signal,and a large number of averaging processes are required to improve the signal-to-noise ratio.This makes the system’s demodulation speed of temperature and strain information very slow,which affects its performance in some cases such as fire pre-warning,safety monitoring of hazardous chemical storage facilities,and other scenarios that have high requirements for system response speed.Aiming at the optimization of the demodulation algorithm of the BOTDR system and the temperature and strain monitoring of the oil storage tank,the following researches are carried out in this thesis:(1)On the basis of studying the temperature and strain sensing theory of Brillouin scattering effect,the characteristics of Brillouin scattering signal and the characteristics of Brillouin scattering spectrum,a BOTDR sensing system was designed and built.(2)On the basis of the traditional short-time Fourier transform BOTDR(STFT-BOTDR)system,the GPU-STFT parallel processing method is used to improve the slow processing speed of the CPU-STFT in the Brillouin scattering signal spectrum conversion process and the factors affecting the GPU-STFT algorithm are analyzed.Under the GTX 1080ti(12GB)graphics hardware,the STFT process with5.12 km fiber,8192 times averaged and 10240 points output was tested,and the results showed that the CPU is based on C language FFTW(the fastest FFT Algorithm on the CPU)The STFT algorithm has an efficiency improvement of about 21 times.Afterwards,based on the GPU-STFT algorithm to optimize the demodulation of the Brillouin scattering spectrum,the method of principal component analysis(PCA)pattern recognition was used to improve t the problem of slow demodulation of Brillouin Frequency Shift(BFS)from Brillouin scattering spectrum by traditional iterative fitting method,and many factors that affect the calculation efficiency of the PCA pattern recognition algorithm are analyzed,and finally the efficiency of the BFS extraction process is improved by about 60 times.comprehensive analysis of GPU-STFT+PCA pattern recognition compared to CPU-STFT+CFM fitting on the overall acceleration performance of the STFT process and the BFS extraction process.Under the condition of 5.12 km fiber,8192 times average,and the output is 10240 position points,the STFT process Shortened from 118.962 s to 5.557 s,the BFS extraction process was shortened from 19.632 s to 0.3263 s,and the overall STFT+BFS extraction process was shortened from 138.594 s to 5.905 s,achieving a performance improvement of about 23 times.(3)A preliminary study on the rapid demodulation of the BOTDR system in the temperature and strain monitoring of petroleum storage tanks was carried out.Considering that the storage tanks are easily deformed due to temperature changes or oil entering and exiting,seven types of optical fibers with different protection structures were selected and their strain shielding was tested.The results show that the domestic TZ03/A optical fiber strain shielding ability and Corning 900 micron tight-buffered optical fiber strain measurement ability are the best;Subsequently,the laying plan of the optical fiber was designed,and the stress concentration method was adopted to remove the torsion force of the internal structure of the temperature measuring optical fiber.Finally,the changes in the wall temperature and strain of the oil storage tank under the combined action of sunlight,ambient temperature and oil temperature were tested.Through comparison with the thermometer,the results show that the temperature demodulation result of the fast BOTDR system designed in this thesis is consistent with the change trend of the thermometer;the strain change is manifested as a larger strain in the high temperature region and a smaller distribution in the low temperature region.