Current-induced oscillations and instabilities of a multi-tube flexible riser

Abstract

Experiments have been conducted on a five cylinder cluster, of similar geometry to that of a multi-tube riser, in steady cross-flow. In the majority of experiments four of the five cylinders were sensibly rigid, whereas the fifth, one of the peripheral ones, was flexibly mounted. The experiments were conducted in a wind tunnel, with the cluster mounted on a rotative arrangement, so that the flexible cylinder could be at any desired azimuthal orientation vis-à-vis the flow vector. Motion of the flexible cylinder was measured by imbedded accelerometers. In addition to the vibration measurements, hot wire anemometers were used to measure periodicities in the flow in and around the cluster, when all the cylinders were rigid. Both the vibration and hot wire signals were analysed by FIT techniques. For certain orientations of the current, violent fluidelastic instabilities were found to occur. At all current orientations, the flexible cylinder responded to Strouhal-type periodicities in the flow, but for some orientations the amplitude of motion was much larger than for others.