Unstable Transients Affecting Flow Assurance during Hydraulic Transportation of Granular Two-Phase Slurries

Abstract

Hydraulic two-phase transport applied in the dredging, mining, and deep-sea mining industries involves the transportation of sand, gravel, polymetallic nodules, or other particulate tailings as a solids phase and water as a liquid phase. Regardless of the type or size of the granular material, the slurry flow is always subject to transient behavior. Most transient behavior can be attributed to the centrifugal pump as variations in pump pressure and mixture velocity over time, but transients can also be caused by microscopic slurry mechanisms, specifically the amplification of density waves in a pipeline. Density wave amplification in horizontal pipelines at mixture velocities just above the deposition limit velocity was reported and researched in the 1990s. New experiments showing a density wave amplification in a system with combined vertical and horizontal pipelines and at mixture velocities far above the deposition limit suggest that another type of density wave amplification mechanism exists. The newly proposed density wave amplification mechanism is hypothesized to be caused by a change in average particle velocity as the slurry flows from a vertical pipe into a horizontal pipe. Density waves that grow too large cause system blockages or possibly a failure of the pump drive. This article considers centrifugal pump–induced transients and density wave amplification effects separately and how these effects influence each other. Three case studies showing density wave amplification are analyzed, one from the literature and two from new data sets. Furthermore, the causes of these transients are discussed, and where possible, solutions are proposed to avoid these undesirable instabilities.