The Reserch Of Fiber Optic Long Distance Pipeline Leak Detection Technology Based On Double Mach-Zehnder Interferometric

As the rapid development of the oil and gas industry and consumption, long distanceoil and gas pipeline which is an important means of transporting oil and gas resourcesplayed a more and more importantly strategic position in the national economy, at thesame time, people pay more and more attention to pipeline’s security issues. In recentyears, China’s oil and gas transportation pipeline nets were seriously damaged by theartificial invasions. In order to ensure the safety of the long distance oil and gas pipelines,it is necessary to study the technology which can monitor and pre-warning the damage ofartificial invasions along the long distance pipeline. This will bring great social andeconomic benefits.Distributed optical fiber sensing system which combines the distributed optical fibersensing and optical fiber interference technology is based on the principle of phasedetection, so its detection of high sensitivity has been recognized by almost people. But itshigh precision locating technology should be developed further.Firstly, this paper established a seismic wave model caused by the outside destructionactivities. According to the longitudinal wave (P wave) can be sensed by optical fibersensor in the four seismic wave, so optical sensing model is proposed. The output signalexpression of Mach-Zehnder interferometer is analyzed when output signal’s phasechanges caused by the outside destruction activities, and the basic Mach-Zehnderinterferometer was improved, and constructs the double Mach-Zehnder institutions ofdistributed optical fiber sensing system. After deducing the principle of its position, theconclusion is gotten that the time delay of two output interference signal is the key of thesystem location technology.Because the influence of single-mode optical fiber birefringence, the optical fibersystem produce a seriously polarization degradation phenomenon, and then build a ellipticpolarized model of single-mode optical fiber. Through simulation the polarizationdegradation influence on this optical fiber sensing system make the conclusion that thepolarization degradation will seriously affect optical fiber sensing system’s location precision.According to the polarization degradation’s influence on the system, feedback controlscheme is used to modulate system polarization, and design a polarization feedbackcontrol system based on double Mach-Zehnder interferometer. According to the theoreticalanalysis, this system can used to eliminate the polarization decline. Finally, the conclusionthat this feedback control system can be used to modulate the visibility of the outputinterference streak from interferometer and to make the visibility of the output interferencestreak always close to 1 is make.LabVIEW system simulation platform is developed, and its modulation anddemodulation and localization module are simulated by this software. The simulationresults show that the distributed optical fiber sensing system based on doubleMach-Zehnder interferometer has the function to pre-warning and positioning thedestruction behavior along pipeline.