# Application of FBG Based Sensor in Pipeline Safety Monitoring
## 5 Experimental Results
### Corrosion Detection Results  
Prior to a pipeline corrosion experiment, a numerical simulation using SAP2000 software was conducted to verify the pipeline corrosion monitoring theory based on hoop strain measurement. There are some basic assumptions in this numerical model: (1) the pipeline model was modeled by a shell element; (2) the material is isotropic; (3) each part with different thickness is connected to another by a rigid connection; (4) the end restraint is selected as a hinge; and (5) the elasticity modulus is \(210\times 10^{9}\) Pa.  
The deformation nephogram generated by the SAP2000 software is shown in Figure 12 and the wall thicknesses are marked by dimensions. To be more explicit, the scaling of nephogram is taken as 2000, and the place circled in red is magnified five times. The numerical results are extracted and displayed in Figure 13. Additionally, the linear equation between the hoop strain and the wall thickness of the numerical model is established. The numerical analytical results show a linear variation of the hoop strain with a regression coefficient over 0.999, which demonstrates that the hoop strain is in inverse proportion to the wall thickness of pipeline strictly. It also can be seen that the pipeline wall extends uniformly and the circumferential deformations of the pipeline decrease according to the rise of the wall thicknesses. The numerical results proved that the corrosion monitoring theory based on hoop strain measurement is valid.  
Figure 11: Hoop strain responses induced by leakage point S1.  
It is difficult to achieve corrosion dates from the actual pipeline model within a short time since corrosion is a long-term process. Therefore, in order to investigate the performance of a FBG strain hoop sensor in the pipeline corrosion detection, an experiment was conducted on a PVC pipeline model since the PVC material is easy to be machined. This model (shown in Figure 14) consists of seven segments with the same outer diameter of 250 mm but different wall thicknesses (e.g., 7.8 mm, 6.0 mm, 5.1 mm, 4.4 mm, 4.2 mm, 3.8 mm and 3.4 mm). The different wall thicknesses represent different uniform corrosion levels. FBG strain hoop sensors were mounted on the segments with wall thicknesses 5.1 mm, 4.4 mm, 4.2 mm, 3.8 mm and 3.4 mm, respectively. These sensors were connected to an interrogator (Micron Optics SM130) that sent data via the Ethernet to a computer. An air pump was used to provide a pressurized condition to simulate the working environment of a practical pipeline. The demodulation instrument collected data from the FBG strain hoop sensors on the PVC pipeline model with the internal pressure being loaded from 0 kPa to 50 kPa.  
Figure 12: The deformation Nephogram generated by SAP2000 software.  
Figure 13: Hoop strain extracted from numerical results.  
The hoop strain variation measured by five FBG strain hoop sensors is shown in Figure 15. Because PVC is not an absolute linear elastic material, leading to the hoop strain variation under the internal pressure is not extremely linearized. Therefore, the regression coefficient of the linear variations of the FBG strain hoop sensor is 0.975, less than 0.999. Nevertheless, hoop strain variation and wall thickness of the pipeline still present as being an inverse proportion relationship. The hoop strains measured by the FBG strain hoop sensors give the sensitivity 1767 \(\upmu\varepsilon/\)mm for the PVC pipeline model, proving that the FBG strain hoop sensors are sensitive to the hoop strain variations caused by corrosion. Therefore, considering the good performance of the FBG strain hoop sensor in the experimental results, the hoop strain measured by the FBG strain hoop sensor can reflect the corrosion degree accurately.