Monitoring pipelines by detecting damage, leaks, and third-party intrusion is vital to protecting people and the environment. Guided wave acoustics has been widely applied for pipeline inspection. This technology propagates stimulated acoustic waves along the pipeline, guided by its boundaries. Detection and characterization of the acoustic response allow defect identification and localization. However, this technology is limited to single point sensing, narrow frequency detection, limited sensitivity, and high production costs. Fiber optic sensing is an attractive platform for distributed infrastructure monitoring. Acoustic/vibration fiber optic sensors have been developed but are limited in application due to complex fabrication, low sensitivity, and narrow operating frequency range. Therefore, there is a need for a sensitive and economical acoustic sensor system capable of multipoint analysis and operation over a broad frequency range.

NETL’s novel system addresses these limitations by integrating GAW and the optical fiber platform. The acoustic sensor is based on a single-mode/multimode/single-mode (SMS) multimodal interference fiber structure. These sensors, capable of detecting frequencies from 10 Hz up to 400 kHz, are easily fabricated and highly sensitive to ultrasonic waves (i.e., vibrations). Whenever the fiber experiences vibrations, the fiber undergoes tensile and compressive strains. This slight deformation of the optical fiber affects its optical parameters, visualized as a shift in its spectral pattern upon data acquisition and analysis. Several SMS acoustic sensors can be easily multiplexed and placed along a pipe for distributed monitoring.

A distributed feedback (DFB) laser powers the sensors, while a guided wave collar generates acoustic waves along the pipeline. Defects such as cracks or corrosion create reflective acoustic waves that alter the optical signals, enabling real-time defect detection and localization through advanced data analytics. Each acoustic sensor detects these acoustic waves and generates a specific optical signal. If the guided acoustic waves encounter a defect (i.e., crack or corrosion) within the pipe, they experience reflection dependent upon the size, location, shape, and properties of the defect. These reflected acoustic waves are detected by the highly sensitive fiber acoustic sensors. This results in the generation of an optical response distinct from that obtained in a flawless pipe. This optical signature, sent along the main optical fiber, contains information about the defect which can be evaluated via data analytics, permitting the identification and localization of any defect in real time.

The feasibility of the fiber optic/GAW fusion sensor system has been demonstrated using a 50-foot-long pipe mounted with the fiber sensors along its length and a guided wave collar system at one end. The system proved capable of detecting the acoustic emissions resulting from simulated third-party intrusion (i.e., pipe strikes) providing proof of concept for practical applications in monitoring pipeline health.