The transport of particles at low loading in near-horizontal pipes by intermittent flow

Abstract

The transport of sand particles at low concentration in horizontal and near horizontal pipes in air/water slug flow has been studied. Low concentration means less than 1 in 1000 by volume, a level of concentration of interest in the transport of sand by oil and gas in subsea flowlines (Stevenson & Thorpe, Proceedings of the Ninth International Conference on Multiphase Flow, Cannes, France, 1999) but much lower than the range of interest in all previous work on hydraulic conveying which may be thought to be a related field of study. The velocity of sands of sieving diameter of 1.1 and 0.51 mm have been measured for various gas and liquid flowrates and in liquids of viscosity in the range 1.0– 7.1 mPa s in two 12 m long pipes of internal diameter 40 and 70 mm . Video recordings have been analysed to determine the mechanisms involved in transport of particles in slug flow and are used to explain the observed decrease in particle propagation velocity with increased liquid viscosity. There being no totally satisfactory mechanistic model for intermittent flow in the literature, a semi-theoretical model for sand transport is not currently facilitated. Therefore the approach taken is that of dimensional analysis. Particle velocity, V P , is successfully correlated using dimensionless groups which include liquid viscosity, particle size and liquid and gas flow rates. The preferred correlation for sand in liquids of viscosity less than about 5 mPa s is V P j f =0.95 1+ j g j f − 1.38 j g j f +0.88 Fr f Re f Fr f d D 1.5 −0.180. All the parameters, except gravitational acceleration, have been varied in the experimental programme. It is found that particle velocity, V P is independent of pipe inclination in slug flow for pipe inclinations of up to three degrees. The above correlation is used to produce an equation that predicts the point of incipient-sand deposition. This equation can be used to estimate the maximum turndown of two-phase offshore oil production from satellite wells before sand will begin to deposit at the bottom of the pipe.