Mooring systems utilised for floating structures typically introduce non-linear load-excursion behaviour. This non-linear compliance and the accompanying amplitude dependent natural frequency, influences the Vortex-Induced Vibration (VIV) response of the structure. The application of linear compliance VIV modelling and experimental data has been demonstrated to produce significant uncertainties regarding VIV onset and response prediction of catenary moored cylindrical structures (Bjarke et al. 2003; Dijk et al. 2003). The vortex-induced vibration issues associated with catenary moored cylindrical structures were investigated through non-linearly compliant elastically mounted rigid cylinder experiments. In particular, third order polynomial, hard spring stiffness, (typical of catenary moorings) was considered. The effect on transverse VIV lock-in and vibration amplitudes was examined using a single degree of freedom experimental rig. The experimental rig consisted of a moderately damped, elastically mounted rigid cylinder, restricted in all but the cross-flow direction through use of linear slide mechanism. The linear and cubic compliance components were independently varied over the non-linear compliance ratio of 0 to 0.3. All experimentation was conducted within the stable sub-critical Reynolds number range. The experimental data was compared to numerical results produced by the VIV modelling software package VisFlo. The program utilises a vortex-in-cell discrete vortex numerical method that was modified to allow the inclusion of varying degrees of structural non-linearity.

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