This paper investigates nonlinear multi-mode interactions in subsea risers undergoing vortex-induced vibrations based on a computationally efficient reduced-order fluid-structure interaction model. Cross-flow responses as a result of a steady uniform current are considered. The geometrically nonlinear equations of riser motion are coupled with nonlinear wake oscillators which have been modified to capture the effect of initial curvatures of curved cylinder and to approximate the space-time varying hydrodynamic lift forces. The main objectives are to provide new insights into the vortex-induced vibration characteristics of risers under external and internal resonances and to distinguish nonlinear dynamic behaviors between curved catenary and straight toptensioned risers. The analyses of multi-mode contributions, lock-in regimes, response amplitudes, resonant nonlinear modes and curvatures are carried out and several interesting aspects are highlighted.
Nonlinear multi-mode interactions due to resonance excitations in mechanical continuous structures have been the subject of considerable research in the last 30 years (Nayfeh 2000). This is due to the fact that resonant multi-mode interactions are responsible for large-amplitude vibrations which potentially give rise to premature fatigue failures of structural systems. In subsea applications, a resonance mechanism of a cylindrical flexible structure subject to vortex-induced vibration (VIV) occurs when the vortex-shedding frequency locks on to one (or more) of the structural natural frequencies. This external resonance takes place in a certain range of the reduced flow velocity known as lock-in or synchronization region. Moreover, if system natural frequencies are closely spaced and commensurable in a nearly integer ratio, the vibrating structure may further experience a so-called internal resonance. The combination of external and internal resonances signifies a worst dynamic scenario which is often unavoidable for a long slender structure having a large number of degrees of freedom and natural frequencies. In this study, a robust reduced-order fluid-solid interaction model and numerical integration scheme, developed by the authors (Srinil et al., 2008), is considered in an attempt to investigate the multi-mode dynamic interactions of subsea risers undergoing VIV.
