Density and Flow-Velocity Measurement Technology for Dredging Applications - Proof of Concept Study

Abstract

SummaryThe U.S. Army Corps of Engineers (USACoE) is searching for advanced technologies to provide physical property information about the dredge slurry traveling through the pipe in real-time, including density and the flow rate of the slurry. During the latter part of FY07, the USACoE funded a proof-ofconcept study at the Pacific Northwest National Laboratory (PNNL) in Richland, Washington, to investigate potential technical solutions to this problem by evaluating the viability of a patented acoustic method for measuring density and the advanced measurement approach employed using pulsecompression acoustics. A trip to Coos Bay, Oregon, was taken by PNNL staff in mid-August to observe the operation of the USACoE dredger ship Essayons, which dredges sand from the ocean floor and to acquire samples of dredge slurry material extracted from the Bay for use in this study.PNNL staff conducted a proof-of-concept evaluation study on these dredge slurry samples to determine the viability for eventual design and testing of an ultrasonic density sensor and flow meter. Two sets of trials were conducted in this study: 1) basic ultrasonic property measurements and signal transmission trials in through-transmission mode using both mono-frequency tone-burst and pulsecompression approaches, and 2) real-time density measurements. This technical letter report provides a summary of the work conducted and the results obtained, and documents the conclusions and recommendations from this proof-of-concept evaluation.The Essayons employs both a radioactive density sensor and a sonic flow meter; both commercially available devices. The current density sensor operates and performs satisfactorily, but the USACoE wants to replace these technologies, especially the density sensor, due to cost, maintenance and security protocols, and paperwork issues associated with their use. The operation of the current sonic flow meter can be problematic.Although the ultrasonic pulse-compression technique could not penetrate the slurry generated in the laboratory setup, the results are still inconclusive due to the large amount of air introduced into the slurry as a result of the mixing process required for particle suspension. The conditions seen in the laboratory may not be representative of the slurry conditions generated in a pipe from dredge material pulled from the sea floor during routine dredging operations. It is anticipated that a pair of transducers placed on the outside of the dredge piping may demonstrate successful penetration of the slurry and allow for observable waveforms to accurately measure wave speed and relative attenuation, which are linearly related to the density of the slurry. This would allow for an alternate indication of specific gravity to the radiation density measurement currently used, and provide a means for measurement of flow rate using through-transmission methods. Further study onboard a dredging ship is recommended.Data were obtained using the PNNL ultrasonic density sensor for sand slurries having a density of 1.17 ...