Monitoring Metal Loss Within Pipelines Using Fiber Bragg Grating Sensors Positioned on Repaired Sleeves

Pipeline networks are the most reliable and efficient means of transporting oil, gas, and fluids across extensive distances. Therefore, monitoring pipelines continuously and ensuring their correct operation is essential. In this work, we investigated the capability of Fiber Bragg grating (FBG) in monitoring localized and uniform metal loss produced inside pipelines repaired with composite sleeves. We performed numerical simulations using finite element method (FEM) to characterize the hoop strain of different wrapped pipeline structures, including those with glass fiber reinforced polymer (GFRP) and with carbon fiber reinforced polymer (CFRP) sleeves. Besides, we investigated the strain at various locations, including the pipe wall's outer surface and the sleeve's external surface. The numerical results demonstrate that the hoop strain produced by internal pressure can be transmitted to the top surface of the composite sleeve, and its magnitude increases under the presence of metal loss. Furthermore, we corroborated these numerical results by conducting experiment tests using two pipe sections repaired with 7 mm-GFRP and 6 mm-CFRP composite sleeves and an array of FBGs to monitor the hoop strain variation on the steel pipe and the sleeves in response to applied strain at areas affected by metal loss with various depth sizes. The experimental results show that the minimum detectable metal loss thickness are 0.832 mm using FBG on the steel pipe and 1.04 mm using FBG on 7 mm-GFRP sleeve, and 0.2 mm using FBG on the steel pipe and 0.429 mm using FBG on a 6 mm-CFRP sleeve, considering an internal pressure of ~1800 psi, and the case where the FBG is at the location of the metal loss defect. The obtained results prove that this non-destructive and non-intrusive approach harnessing FBG sensors placed on the composite sleeve effectively monitors the internal metal loss and ensures the continuous inspection of pipeline integrity.