The necessity of natural gas pipeline leakage monitoring system
Since the 21st century, with the rapid development of China’s economy, the demand for natural gas consumption has increased rapidly, and the construction speed of oil and gas pipelines has been continuously accelerating. As of the end of 2019, the total length of China’s natural gas long-distance pipelines has reached 7.26 × 104km, forming a gas supply pattern of “West East Gas Transmission, North Gas Southward Migration, Sea Gas Landing, Backbone Interconnection, and Local Networking”. According to relevant plans, the total length of China’s oil and gas pipeline network will exceed 24 × 104km by 2025. Pipeline transportation is the most commonly used way of natural gas transportation, with the advantages of safety, reliability, low energy consumption, pollution-free, and basically unaffected by climate. Long distance natural gas pipelines often cross complex terrain areas such as rivers, mountains, and goaf areas, making them vulnerable to third-party damage leading to pipeline leaks. If exposed or leaked pipelines cannot be detected in a timely manner and corresponding measures cannot be taken, it will inevitably cause certain economic losses to the pipeline operation unit, and in severe cases, it may cause major accidents such as casualties. Timely and accurate detection of exposed or leaking natural gas pipelines is of great significance for ensuring the safe operation of pipelines and the safety of people’s lives and property along the route.
The leakage monitoring technology for long-distance natural gas transmission pipelines can be divided into two categories:
① Internal inspection methods based on magnetic flux, eddy current, camera, etc;
② External detection method based on physical parameters of pipeline pressure, temperature, flow rate, sound, and vibration. The idea of using a fiber optic temperature sensing system to detect natural gas pipeline leaks has been proposed, and a distributed fiber optic Raman scattering temperature measurement system has been developed. Based on the capture of temperature changes during pipeline leaks, monitoring of gas pipeline leaks has been achieved. The distributed fiber optic temperature monitoring method based on R-OTDR is used for pipeline safety monitoring.
The application of distributed fiber optic sensing technology in underground pipeline monitoring is being carried out. The advantages of Brillouin scattering time-domain analysis (BOTDA) technology include: dual end measurement with large dynamic range, short testing time and high accuracy, absolute temperature and strain can be measured, spatial resolution can reach 0.1 m, and long testing distance (up to 25 km). Research on distributed fiber optic natural gas pipeline leakage monitoring based on BOTDA fiber optic temperature measurement technology and on-site verification on natural gas pipelines.
Principles of Distributed Fiber Optic Technology
1. The principle of distributed fiber optic temperature measurement
“Distributed sensing” refers to using optical fibers as linear sensors to provide measurement information throughout the fiber optic process. Based on the analysis results of backscattered light generated by laser pulses propagating along the fiber optic, a single fiber optic can be used to replace thousands of single point sensors, which can save a lot of installation, calibration, and maintenance costs. When light propagates in optical fiber materials, Brillouin scattering occurs. Brillouin scattering is the scattering caused by the interaction between light waves and sound waves (generated by the Brownian motion of fiber material molecules) when propagating in optical fibers. The frequency of scattered light is shifted by Brillouin frequency relative to the incident light. Temperature and strain measurements can be achieved by measuring the frequency shift of the backscattered Brillouin light of pulsed light. There are two main types of temperature and strain measurement technologies based on Brillouin scattering principle: distributed fiber optic sensing technology based on optical time-domain reflection (BOTDR) and distributed fiber optic sensing technology based on optical time-domain analysis (BOTDA). To enhance signal strength, BOTDA distributed fiber optic sensing technology uses two opposing transmitted lights to enhance Brillouin scattering, resulting in higher temperature and strain measurement accuracy and larger measurement distance.
Principle of temperature reduction at leakage points
When a natural gas pipeline leaks, the gas overflows and expands in volume. According to the Joule Thomson effect, the local temperature around the leakage point of the pipeline will rapidly decrease, and a temperature gradient will form in the soil around the pipeline. When the temperature in the formation near the leakage point changes, the optical fiber laid near the pipeline can monitor this change in real time and transmit it to the monitoring system to determine the location of the leakage point. Designing experiments with different factors can establish corresponding relationships between different leakage conditions, leakage amounts, and temperature reductions, providing more valuable feedback data for long-distance natural gas pipeline leaks.
Distributed optical fibers based on BOTDA are temperature sensitive, and any increase or decrease in local temperature of the fiber can cause Brillouin frequency shift, and the temperature variable has a linear relationship with the amount of Brillouin frequency shift.
Principle of Leakage Point Temperature Positioning
When a laser pulse is inserted into a fiber at a certain angle, scattering occurs. The time for the laser pulse to propagate within the fiber can be calculated to locate the temperature change point:
Construction of fiber optic temperature measurement and monitoring system
Overview of Natural Gas Pipeline Monitoring Site
The total length of the natural gas pipeline project trunk line is about 715 km, with a diameter of 1422 mm, a design pressure of 12 MPa, and a design transmission capacity of 380 × 108 m3/a; The starting point of the branch line is the Changling distribution station of the main line, and the ending point is the Changchun distribution and cleaning station. The total length is 109 km, with a pipe diameter of 1016 mm, a design pressure of 10 MPa, and a design capacity of 114 × 108 m3/a. According to the design documents, the pipeline monitored this time is divided into two sections. In the high consequence area of the third level area, the monitoring range of the main line is 10 km downstream from the starting point of HC25 valve chamber, and the monitoring range of the branch line is 35 km downstream from the starting point of the transmission station.
Construction of Natural Gas Leakage Monitoring System
The temperature monitoring system consists of 2 fiber optic temperature measurement hosts, 1 server (including display), 1 switch, signal processing and analysis software, etc. The 2 fiber optic temperature measurement hosts are respectively arranged in the valve room and distribution station, and the server and switch are arranged in the distribution station. Using 2-core optical fibers from communication optical cables already laid in the same trench to form a measurement loop, real-time sensing of temperature changes around the optical fibers and transmitting them to the fiber optic temperature measurement host; The fiber optic temperature measurement host receives and processes optical signals along the pipeline for direct demodulation, and uploads the demodulated data to the server. After signal processing and analysis software calculation, if the temperature exceeds the alarm threshold, the temperature alarm and position will be displayed.
The installation and debugging of the temperature monitoring system will begin. Based on the coordinate data of the welding joints during the construction period and the construction data of communication optical cables, all cable lengths in the cable well and cable entry lengths will be deducted. The cable mileage will be aligned with the pipeline mileage to ensure the minimum error