Acoustic Characteristics of Liquid Hydrocarbon Releases From Buried Pipelines: An Experimental Evaluation

Characterizing the acoustic energy associated with small pipeline leaks is of particular interest to pipeline operators who are considering deployment of acoustic based External Leak Detection (ELD) systems along their pipelines. Small leaks are defined here as product leaks having release rates and/or release volumes that fall below the detection threshold currently associated with conventional or traditional leak detection technologies, including but not limited to Computation Pipeline Monitoring (CPM) systems. Characterization of such acoustic energy could be used to predict and evaluate the performance of acoustic based ELD systems in a variety of candidate deployment locations. It could also be used to optimize system performance of existing or future deployed acoustic based ELD systems. This study focuses on investigating the transmission of acoustic energy caused by pressurized fluid releases through two different soil mediums (a dry soil and a saturated soil). Specifically, signal attenuation and frequency content as a function of sensor location from the release source were investigated. To accomplish this, geophones were placed within a large soil filled tank to listen passively to controlled releases of hydrocarbon liquids from a buried pipe segment. These releases were driven through circular shaped orifices ranging in diameter from 0.79 to 4 mm and by pressures ranging from 50 to 500 psi. Signal attenuation was observed in both the longitudinal and radial directions however the effect was more significant in the radial direction. This does not necessarily imply that anisotropic effects exist, but rather a possible explanation is that the acoustic waves traveling along the pipe walls (i.e. in the longitudinal direction) are less attenuated and can therefore carry the acoustic energy further in that direction. In addition, it was found that the dominant bandwidth of the leak signals (which is approximately 600 Hz but it can be as high as 1200 Hz) is inversely proportional to orifice diameter and proportional to the release pressure. Also, the dominant frequency was found to be slightly higher in the saturated soil environment. This study provides insights into expected acoustics characteristics of small liquid leaks, which can help in the selection and placement of appropriate acoustic based ELD systems.