This paper addresses the wake-induced instability of a pair of elastically mounted cylinders with one located in the other's wake. In particular, the effort is made to investigate the different instability mechanisms of the downstream cylinder in the near or far wake field of the upstream cylinder. It is found that the fluid-elastic instability of the downstream cylinder can be caused by either a Hopf or a stationary bifurcation, depending upon its position relative to the upstream cylinder.

INTRODUCTION

For a deepwater vertical riser cluster, one of the key design concerns is interference between the individual risers in strong ocean currents. The riser's lateral deflections are likely to be large, and the risers are prone to wake-induced clashing, with possible detrimental effects. In a series of papers published since 1993on TLP/SPAR riser clearance in currents with a relatively large initial spacing between these risers, Huse (1993, 1996) proposed a simple yet remarkably accurate mathematical model for estimating time-averaged drag loading on the downstream riser situated in the wake of an upstream one. Huse also made a number of important experimental observations. In one experiment carried out to investigate the interaction between vertical risers, it was observed that, in addition to the high-frequency, vortex-induced response of amplitudes up to one-half of the diameter, the downstream riser also had low-frequency, in-line oscillations of an apparently very irregular nature, with the peak-to-peak stroke of these oscillations measuring 30 to 40 diameters or more. In the last few years, other researchers have contributed to the work (Furnes, 2000; Li and Morrison, 2000). These recent research contributions focus upon developing structural models to quantify impact loading on and possible damages to the risers. Although relatively little work has been done on wake-induced, large-amplitude, low-frequency riser motions, there is a substantial amount of experimental

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