Dynamics of a confined pipe aspirating fluid and concurrently subjected to external axial flow: An experimental investigation

Abstract

The system under consideration comprises a flexible pipe and a rigid outer tube; the pipe and tube are concentric and vertically cantilevered from their top ends in a large water-filled cylindrical tank. The space between the pipe and tube forms an annulus around the upper portion of the pipe. The working fluid is water. The fluid enters the annulus from its top end with flow velocity Uo and is discharged from its bottom end; the fluid exits from the tank by flowing upwards in the pipe with velocity Ui.This flow configuration represents an idealized model of one of the modes of operation of ‘salt-mined caverns’, which are large underground cavities created to store hydrocarbons, such as natural gas and oil, in large quantities. At sufficiently high flow velocities the central pipe, referred to as the ‘brine-string’, vibrates, and may impact on the rigid cemented casing around it; sometimes this impact results in damage or breakage of the brine-string. This paper presents an experimental investigation of this system. Numerous experiments were performed in a laboratory-scale apparatus for several flow velocity ratios Uo∕Ui, ranging from 0.04 to 1.00. In all these experiments the pipe loses stability via first-mode flutter at a sufficiently high flow velocity Ui. For low flow velocity ratios, Uo∕Ui = 0.04 - 0.07, an interesting dynamical behaviour is observed, in which the rms amplitude versus Ui curves show two sharp increases, as the flow velocity is increased, with a plateau in-between. For higher flow velocity ratios, Uo∕Ui = 0.10 - 1.00, the pipe becomes unstable at relatively lower flow velocities, and the plateau in the rms amplitude disappears.