Current-Induced Fluidelastic Instabilities of a Multi-Tube Flexible Riser: Theoretical Results and Comparison With Experiments

Abstract

Previous experiments on a five-riser cluster in steady cross-flow have indicated that for certain orientations of the cluster the peripheral, or wing, risers can undergo violent flow-induced vibrations. It was shown that these vibrations were not due to vortex shedding; furthermore, it was suggested that they are due to a classical self-excited fluidelastic instability. In the present paper, a previously developed quasi-steady fluidelastic stability analysis for a group of circular cylinders in steady crossflow is modified to enable the stability of a flexible riser in a five-riser bundle to be analyzed. As input to the theoretical model, the static fluid force coefficients on a peripheral riser, and the manner in which they vary with displacement, are required. These were measured for a number of orientations of the cluster with respect to the free-stream current, using a wind tunnel model. Using this data in the analysis, the stability of a five-riser cluster was investigated. Instability is predicted to occur for the same orientations of the riser as obtained from the experimental results; however, the quantitative agreement between experimental and theoretical critical flow velocities for instability to occur is not as good. Theory also predicts the system to be stable for those orientations where no instability was obtained experimentally.