a b s t r a c tA time-domain approach for predicting dynamic characteristics and vortex-induced vibration (VIV) response of deepwater risers is proposed based on finite element simulation combined with a hydrodynamic model. Effects of axially varying structural parameters, i.e. the effective tension and bending stiffness, on the dynamic characteristics and VIV of slender riser are examined using the presented approach. Our numerical results indicate that axially varying structural parameters can efficiently change the modal wave length as well as the modal displacement. The vibration amplitude of riser VIV response is influenced by the complex effect of factors involving the axial tension, bending stiffness and modal wave length. Generally speaking, for lower modes the response amplitude is larger at the axial position where the tension is smaller, whereas for higher modes the response amplitude is larger at the axial position where the bending stiffness is lower. Moreover, vibration wave velocity increases with increasing modal wave length along the riser length.

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Multimode vortex-induced vibration (VIV) of slender risers, respectively, in stepped and shear flows is explored by finite element simulations. Taking account of the interactionbetween fluid and structure, a hydrodynamic model is proposed and embedded into the finite element simulation so as to carry out dynamic response of multimode VIV in timedomain. Multimode VIV in both stepped and shear flow fields is examined. In the case of stepped flow, a semi-empirical formula of modal weight is given. In the case of shear flow, modal excitation region can be determined based on modal energy, and participating modes approximately distribute in scattering groups.

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