Vortex induced vibration of a marine riser has been treated as a stochastic problem because of its strong non-linearity and difficulty of deterministic calculation. In this research the stability of vortex induced vibration of a flexible riser is studied experimentally. A flexible riser model was forced to oscillate at its top end in sinusoidal mode and irregularly. Each experiment was done four times in order to confirm the stability of the phenomenon. From these experiments we concluded that VIV of the riser forced to oscillate in sinusoidal mode at its top end is very stable, but it is not as stable in the case of irregular oscillation.
In ocean activities, marine risers play one of the most important roles. We can easily find their usages such as transporting the natural resources from the seabed and scientific drilling. The dynamics of a marine riser is very complex due to its flexibility and the external forces such as changeable currents and waves. Vortex induced vibration (VIV) is one reason that makes the dynamics of marine riser so complex. VIV has been treated as a stochastic problem because of its strong non-linearity. Many researchers have studied VIV of marine risers, among the most recent: Yamamoto et al. (2004), Fontaine et al. (2005) and Wang et al. (2005).
When we discuss VIV or compare experimental results with numerical ones, the stability of VIV is a very important factor. We can easily imagine that the main motion of marine riser would be same when the same irregular oscillation is generated at its top end. However, because of its non-linearity, we do not know whether the same amplitude and period of VIV occurs when the same irregular oscillation is generated at its top end.