Experimental study on distributed optical-fiber cable for high-pressure buried natural gas pipeline leakage monitoring

At present, fiber-optic cable monitoring technology uses an fiber-optic cable located at 300 mm above a buried natural gas pipeline to collect gas leakage information. However, the change in soil temperature caused by natural gas leakage is not tested and verified. This prevents the installment of an fiber-optic cable for monitoring gas leakage in the field. To improve the sensitivity of fiber-optic cable leakage monitoring and reduce the failure rate, the laying mode of fiber-optic cable should be verified and analyzed. In this study, Peng-Robinson (PR) real gas state equation, a Raman optical time-domain reflectometer (ROTDR), and finite element method (FEM) were combined to simulate the gas leakage of buried pipelines. The trend of soil temperature around the leak hole and laying mode of fiber-optic cable was studied. It can be concluded that the higher the pressure of pipeline and leakage hole, the greater the temperature change at 100 mm directly above the pipeline. On this basis, a field test was carried out to study the leakage of a buried high-pressure natural gas pipeline. The experimental results show that the gas leakage can be detected by an fiber-optic cable located at 100 mm above the pipeline, and it is difficult to detect the change in soil temperature when the fiber-optic cable is located 200 mm or farther. The experimental results are consistent with the simulation results. This indicates that the simulation method is correct and feasible. Thus, the fiber-optic cable needs to be placed 100 mm or closer to the buried pipeline to monitor the temperature change. The result of the test and the simulation result are coincident approximately. The conclusion is: evenly lay 4 optical cables within 100 mm around the pipe, and each cable can monitor the temperature change in the 90 degrees range above the leakage hole. This method can accurately monitor the leakage of the whole pipe section. The study results can guide the laying plan of fiber-optic cables and construction of natural gas pipelines and prevent accidents.