By using fiber optic sensors to measure pipe wall thickness, gas pipeline workers can track and monitor the rate of pipeline corrosion. Connecting the sensors to networked control instruments allows workers to perform the work remotely. Accurately tracking changes in corrosion rates is a significant challenge for natural gas pipeline operators. If the measured corrosion rate exceeds 2 mils per year, operators are required to take action to mitigate corrosion or carry out repairs. While many proven techniques, such as corrosion sampling tubes and resistance probes, can be used to measure corrosion, the vast majority of these techniques measure the corrosion of the gas, not the changes in the pipe wall. Because these methods can only indirectly extrapolate the corrosion rate of a pipeline, the accuracy of the measurement is affected by a variety of factors. To address this problem, Fiber Optic Systems Technology (FOX-TEK) has developed a solution that combines a highly sensitive FT fiber optic wall thickness sensor with networked monitoring instruments and a satellite or battery-powered modem. This system can accurately track pipeline corrosion remotely from anywhere. The system uses FT sensors and connected monitors to measure the minute-by-minute changes in wall thickness caused by corrosion. This system is non-invasive and can be used to measure the outer surface of the pipe. Under constant pressure and temperature, the change in stress on the outer surface of a pipe or pressure vessel is inversely proportional to the change in pipe wall thickness. By comparing the measured stress changes with data recorded by the FT sensor monitor and the device's geometry, the company can transform the raw data from the sensors into useful parameters including temperature, pressure bending strain, and the degree of pipe wall thinning. Operating Principle The practical principle of the sensor is low-coherence optical interference. The sensor is made using conventional single-mode optical fiber; the small diameter and flexibility of the fiber allow the sensor to be packaged into form factors suitable for monitoring a variety of problems, including corrosion and pipe bending. While other custom instrument and meter lengths are used for special projects, FT coil sensors can be used in most applications and are capable of measuring total sensor displacement exceeding ±15 mm. This dynamic range, combined with the sensor measurement length, defines the stress range; a 10 m test length sensor has a stress range of ±1500 με. During the project planning phase, the measurement length and sensor structure need to be selected to meet specific project objectives. Calculating pipe wall loss based on pipe corrosion typically requires stress information from three FT sensors. One sensor, located at the point of interest, derives stress information based on internal pressure, pipe wall thickness, and surface temperature. Two other sensors provide compensation signals, enabling the system to separate signal components based on differences in operating temperature and pressure. Once compensation is complete, the stress measured in the area of ​​interest is inversely proportional to the remaining wall thickness (Equation 1): where ε = stress; P = internal pressure; D = pipe diameter; v = Poisson ratio; E = Young coefficient; t = wall thickness. Because internal corrosion is an extremely slow process, data collection exceeding 30 days is sometimes necessary to separate signals related to wall thickness loss from background signals. One of the two currently available FT sensors is used to demodulate the sensor's optical signal. The FT 3405 is primarily used for continuous monitoring applications (Figure 1A) and is AC-powered, while the FT 3410 is battery-powered and more compact, suitable for outdoor, phased monitoring applications (Figure 1B). The FT 3405's programmability allows the company to create an automated scan schedule for all connected FT sensors. All data collected from the sensors is stored in permanent memory. These features help reduce working hours, ensure a continuous and stable data flow, and capture more data. Multiple 3405s can be interconnected and connected to a single data transmission system, allowing the company to transmit data directly to remote offices daily, reducing the number of accesses required. Operating Sensors Due to the increasing potential for corrosion to spread, a natural gas pipeline operator in the southeastern United States planned to use corrosion sampling tubes to track changes in two pipelines traversing a low-lying region. Although a single corrosion sampling tube indicated that the wall loss rate was within acceptable limits, the operator wanted to determine the true wall loss rate through direct measurement. The two pipelines were uncovered (Figure 2), so all procedures were conducted openly, and FOX-TEK used ultrasonic scanning at several locations (Figure 3) to quickly assess the current condition of the pipeline walls. [align=center] [/align] Operators installed additional corrosion sampling tube ports, and FOX-TEK personnel installed 24 FT sensors at three locations on the two pipelines (Figure 4). Fifteen coil FT sensors were installed at the bottom or 6 o'clock position, four FT sensors were installed at 12 o'clock as a pressure reference, and five dedicated FT temperature sensors were also installed. After epoxy treatment, the fiber optic cable is guided through a PVC conduit and routed into a housing (Figure 5). All information collected from the FT sensor system along the pipeline (Figure 6) is analyzed using the company's Data Management and Analysis Tool (DMAT) program. DMAT is designed to work in conjunction with FOX-TEK equipment to simulate real-world processes and generate reports. Due to the nature of the system, pressure fluctuations within the pipeline are not uniform. To account for these pressure fluctuations, the raw data is averaged every 2 hours. To further eliminate data discrepancies, analysts aggregate and analyze the ratio of wall thickness stress to pressure reference stress and its trends every seven days. To simplify data representation, data is calculated on average monthly. The pressure reference sensor and wall thickness sensor are governed by mechanical and thermal stresses. The DMAT's built-in modem isolates the raw stress signals containing pressure/temperature correlations; only those signals indicating wall thickness variations are plotted. Figure 7 shows typical data obtained from a single sensor. When DMAT was used to eliminate the coupling between overall stress, pressure, and temperature, FOX-TEK analysts obtained a graph as shown in Figure 8, which displays the current wall thickness trend through analysis of one of the wall thickness sensors. Figure 9 shows monthly data obtained from FT sensors at three locations. Table 1 shows short-term (past two months), three-month, and long-term (since monitoring began) trends. Values ​​indicated by "xxx" are not statistically significant. The data collected over four months best matches the current average wall loss rate through the sensor to approximately 0.0012 inches per year. Given the system's sensitivity of 0.002 inches, this corrosion rate is below the system's measurable sensitivity. Applications and Benefits Beyond the technological advantages, monitoring corrosion using the FOX-TEK FT sensor system offers cost-recovery benefits, including: • Faster corrosion data acquisition from hard-to-reach locations; • More reliable data through direct measurement of areas of concern on the pipeline; • Better inspection logging by maintaining reliable, real-time data; • Reduced labor costs associated with inspections; • Extended uptime through end-of-life assessments; • Greater reliability and lower risk. The FOX-TEK system has a wide range of applications in corrosion monitoring, capable of monitoring corrosion in various types of pipelines, and is particularly suitable for tracking typical pipe wall losses. Utilizing the "cellular" capabilities of the FT sensor monitors and remote power supply, FOX-TEK can remotely monitor pipelines. This reduces setup costs while ensuring consistency of pipeline-related information. By using fiber-optic sensors to measure wall thickness, gas pipeline operators can track and monitor how quickly pipelines are corroding. Linking the sensors to networked monitoring instrumentation allows them to do this remotely.