| Vortex induced vibration(VIV) is one of the main factors which cause structure instability and fatigue damage of deep-sea riser. In order to enhance the riser's safety performance and prolong its service life, people have been committed to the research of VIV suppression measures. In this paper, based on commercial CFD software ANSYS15.0,numerical simulation is conducted to study the suppression measure of helical strakes, and perform a systematic analysis on the suppression effect and suppression mechanism of a long flexible riser and tandem double risers' VIV. The main research results are as follows:(1) A comprehensive review on the state of study and the research methods of riser VIV and its suppression measures in domestic and overseas abroad is made. And focus more on the analysis of the study state of helical strakes.(2) The vortex induced vibration principle is introduced briefly. And make a system introduction about related knowledge of CFD numerical simulation, such as fluid solid coupling simulation method, the choice of turbulence model and dynamic mesh technology,etc.(3) Fluid-structure interaction technique is applied to simulate VIV of flexible riser with length to diameter ratio(L/D) 482 under uniform flow. By comparison with the results of related references to verify the feasibility of this scheme. Secondly, riser with L/D=250 is simulated. The results show that: with L/D increasing, higher order modal appears in cross-flow and in-line vibration; vibration amplitude in cross-flow changes little; the in-line displacement and amplitude increase about three times, and the in-line displacement is over the cross-flow displacement response.(4) Through series of simulations on the smoothed riser and risers with different helical strakes under uniform flow with Re=7800 to study the effect of geometric parameters(height, pitch) on the suppress ability. In the given parameter combinations, P17.5D/H0.25 D with three columns strakes is the most effective in suppressing transverse amplitude, which is decreased more than 80%.(5) Simulations on VIV of flexible riser with L/D=482 under uniform flow and shear flow are carried. And through comparison of smoothed riser and risers with helical strakes,show that: for smoothed riser, under uniform flow the riser's displacement and amplitude in both in-line and cross-flow direction are larger. Finally the transverse vibration modal locksin second order, and vortex wake field discharge form is single; under shear flow the riser is more likely to be stimulated to higher modal. Besides, in the trailing vortex shedding different forms occur, such as 2P, 2S, P+S form. Additional helical strakes can effectively suppress the VIV of riser, reduce vibration modal and change the forms of vortex shedding.(6) Finally numerical simulation on the tandem double risers is carried out, study the suppression characteristics of tandem double risers' VIV with helical strakes. Through analysis find that: in cross-flow direction, the two risers are locking in second order modal,and vibrate with the same frequency in inverting form. In in-line direction, the two risers are vibrating in the same phase. And the in-line frequency is almost two times the cross-flow vibration frequency. With helical strakes fixed, the suppress efficiency is higher on upstream riser's VIV vibration, and has little effect on the displacement amplitude of downstream riser. But the strakes can effectively reduce the two-way vibration frequency of the double risers. |
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