Internal Acoustic Inspection Approach for the Detection and Prevention of Pipeline Illegal Tapping

Summary Illegal tapping represents a persistent integrity and security challenge for liquid pipeline systems, often remaining undetected because unauthorized connections may not produce immediate leaks or measurable pressure drops. In this paper, we present an internal acoustic inspection methodology for the detection and prevention of product theft associated with illegal tapping in pipelines, based on wave propagation and resonance phenomena within the transported fluid. The proposed approach exploits pressure fluctuations and vibrational energy generated by controlled acoustic excitation to identify geometric discontinuities associated with unauthorized branches. Analytical modeling based on quarter-wavelength tube (QWT) theory is used to relate branch length to resonance frequency, while numerical simulations are performed to evaluate wave/branch interaction and spatial sensitivity along the main pipe. Laboratory-scale experiments are then used to validate the analytical and numerical predictions. The results show that when the excitation frequency matches the resonance condition of a lateral branch, measurable amplitude variations—either amplification or attenuation—occur in the acoustic signal propagating along the main pipe. These variations are strongly dependent on both excitation frequency and sensor position, leading to regions of high sensitivity and localized blind spots, consistent with modal acoustic behavior. Frequency-domain processing enables clear identification of resonance-induced effects that are not readily observed in the time domain. The method detected branch lengths on the order of 15–20 mm under controlled operating conditions in liquid pipelines, which are consistent with dimensions observed in real illegal tapping configurations. Owing to its internal, nonintrusive nature and compatibility with pigging operations, the proposed technique is well suited for application during commissioning and inspection campaigns and can be integrated with existing monitoring systems as a complementary detection tool. Because the method identifies the geometric presence of an unauthorized branch even in the absence of active product withdrawal, it may also support preventive detection by enabling identification of the theft apparatus before product extraction begins.