Internal Metal-Loss Detection in Bare and Repaired Pipelines Using FBG Sensors Under Multiphase Flow Conditions

Pipelines remain critical infrastructure in the oil and gas sector, and monitoring internal metal loss is essential for maintaining their integrity. Composite repairs are widely used, but they limit access to the steel surface and restrict the use of conventional inspection methods. This work develops an experimental framework to investigate strain transfer and defect detection in composite-repaired pipelines under multiphase flow conditions. Embedded and surface-mounted fiber Bragg grating (FBG) sensors were evaluated on bare and repaired pipes with metal-loss defects of 40%, 73%, and 83%, using flow-loop pressures of 50, 100, 200, and 300 psi. A differential strain-sensing approach was implemented to suppress common-mode effects and isolate defect-induced deformation. Embedded FBGs showed a linear increase in differential strain with pressure, with the sample containing 83% wall loss wrapped with glass fiber-reinforced polymer (GFRP) yielding {{\text {R}}^{{2}}} =0.97. Surface-mounted FBGs on carbon fiber-reinforced polymer (CFRP) detected severe defects, while GFRP wraps redistributed strain and reduced sensitivity. Bare pipes exhibited clear defect dependence, with minimum detectable wall-loss thresholds of about 30% at 50 psi, 26% at 100 psi, and 17% at 200 psi. The results show that composite stiffness and sensor placement govern strain-transfer efficiency, and that embedded sensors at the pipe–wrap interface offer the highest detectability under flow conditions. This study provides new experimental insight into FBG-based monitoring of composite-repaired pipelines and highlights the importance of sensor configuration when designing structural health monitoring strategies.