Vortex-Induced Motion of Nonlinear Compliant Low Aspect Ratio Cylindrical Systems

摘要

Mooring systems employed for floating structures commonly result in nonlinear load-excursion characteristics. The nonlinear stiffness and the subsequent amplitude-dependent natural frequency influence the vortex-induced motion of the structure. The application of linear stiffness vortex-induced motion modelling and experimental data has been shown to produce significant uncertainties regarding the onset and vortex-induced response prediction of catenary moored cylindrical structures (Bjarke et al., 2003; Dijk et al., 2003). This study specifically investigated the 2-degrees-of-freedom vortex-induced motion of nonlinearly compliant elastically mounted rigid cylinders. Specifically considered was the 3rd-order polynomial hard-spring stiffness typical of catenary moorings. The linear and cubic compliance components were independently varied over the nonlinear compliance ratio range of 0 to 0.29. The study revealed that the analysis of nonlinear compliant structures, assuming the corresponding linear system response, is conservative with respect to mean inline structure displacement and the maximum inline and transverse oscillatory response. Relative to the nonlinear compliant system, the transverse and inline motions were overpredicted by adopting a linear restoring force model. The vortex-induced vibration initial lock-in conditions were also unaltered with increasing nonlinear stiffness. Conversely, the analysis of nonlinear compliant structures, assuming the corresponding linear system response, is not conservative with respect to the lock-out point and the maximum structure excursion (and consequently the maximum mooring line tension experienced) over the lock-in range. This has potential bearing on the way in which highly nonlinear compliant systems are modelled with regard to their vortex-induced motion response.