| Deep-sea oil exploration depends on offshore oil drilling platforms,and marine risers connect offshore platforms and seabed.Risers bear static loads including gravity and top tension,as well as dynamic loads induced by currents.In recent years,platforms gradually move to deep seas.Aspect ratios of risers grow rapidly with the development of ocean exploration.As a result,high-mode vibrations are more likely to be excited in risers.Fatigue damage caused by vortex-induced vibration can result in structural failure,which may lead to major accidents.Therefore,vortex-induced vibrations of deep-sea risers have become a major subject of ocean engineering.In practice,floating platforms are usually under winds,waves and currents,they may drive risers below them to reciprocate in waters,thus forming relative oscillatory flows between risers and water.Platform motions have key impact on vortex-induced vibrations of risers.However,there is still not much research on vortex-induced vibrations of risers under platform motions.Vortex-induced vibrations of a flexible riser under horizontal platform motions have been numerically investigated in the thesis.Strip theory is employed to reduced computation cost of flow fields.The SST k-ω turbulence model is adopted when solving RANS equations on each strip.And the small displacement Bernoulli–Euler bending beam theory is used to model the riser.Support excitation is used to simulate horizontal platform motions,i.e.,the top end of the flexible riser is forced to oscillate harmonically.Firstly,vortex-induced vibrations of flexible cylinder in an oscillatory flow are studied.Numerical results of cross-flow vibrations agree well with the test,thus validating our in-house solver viv-FOAM-SJTU’s capacity of simulating vortex-induced vibrations in oscillatory flows.The in-line vibrations are found to consist of three components,the low-frequency oscillation,the first-natural-frequency vibration during the riser reversal,and the second-natural-frequency vibration due to vortex shedding.Then vortex-induced vibrations of risers under single-direction platform motions are studied.Three kinds of trajectories are found along the span of the riser:”X”,”II”,and ”O”.The vertical riser is only excited at the top end,while the horizontal cylinder is stimulated at both ends,which can be equivalent to shear oscillatory flow and uniform oscillatory flow,separately.Low-order oscillations can be observed in the high-order modal weight of in-line vibrations of riser excited at the top end,which are caused by shear rate of the equivalent shear oscillatory flow.Vortex-induced vibrations of risers under multi-direction platform motions are further studied,including “∞” shaped and parabolic platform motions.Finally,vortex-induced vibrations of risers subject to both platform motion and currents are studied.Amplitude of vortex-induced vibrations are small when the riser is moving downstream due to the small relative current velocity,while large when the riser is moving upstream.Amplitudes are larger compared to top-end excitation only case while the period of large-amplitude vibrations doubles. |
