ABSTRACT

Vortex-induced vibrations of a vertical tensioned riser under the periodic platform motion, which is simplified as a sinusoidal excitation at riser's top end, are simulated in this paper. The viv-FOAM-SJTU solver is used to build computational fluid dynamics (CFD) model. Three different excitation periods are employed to study the responses of riser. The relative in-line vibration displacement is increased by decreasing the excitation period, and the trajectory shapes are changed with the variation of axial positions and excitation periods of riser. The cross-flow vibrations present multi-modal vibration phenomenon with the reduction of excitation period. The wake field of riser indicates that the washing out of riser by shed vortices contributes to the multi-mode vibrations.

INTRODUCTION

Marine riser is one of the most important parts of the offshore oil and gas exploitation system. Vortex-induced vibrations (VIV) always occurs when the riser is exposed to current. Platform connected at the top of riser generates reciprocating motion with the influence of waves, currents and winds, which produces a relatively oscillatory flow between the water and the riser. The vortex shedding frequencies are changed with the variation of the relatively velocity between riser and current. The resonance phenomenon, called the "lock-in" phenomenon, will happen when the vortex shedding frequency close to natural frequencies of the riser.

Vortex-induced vibrations of marine risers under platform motion have gradually received more attention. A comprehensive mini review of recent investigations have been done by Liu et al (2020).

Park et al (2004). experimentally studied the effect of top-end excitation on transverse vibrations of a hanging riser, and measured in-line and transverse displacements. They found transverse structural wave propagation, which was related with vortex shedding frequency. Senga and Koterayama (2005) experimentally and numerically investigated the vibrations of a flexible riser with irregularly top-end excitation, and developed a new numerical scheme. The numerical results showed good agreement with experimental results except for the long period motion.

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