Vortex-induced Vibration Of Flexible Riser Research Articles

Vortex-induced vibration of flexible riser transporting high-speed spiral flow in deep-sea mining

According to Hamilton's principle and a mechanical decomposition model proposed for high-speed spiral flow, vibration governing equations of flexible riser transporting high-speed spiral flow are developed for vortex-induced vibration (VIV) analysis for lifting pipe system in deep-sea mining. The classic van der Pol oscillator is used to simulate the dynamic characteristics of VIV. The governing equations are then discretized and solved by the finite element method and the Runge-Kuta method. Furthermore, dynamic characteristics of VIV of the flexible riser transporting straight flow and spiral flow are investigated and the effects of the spiral flow incidence velocity and angle controlled by the jet device on VIV responses are evaluated. Also, the VIV dynamics considering different damping ratios are examined. The results show that the rotational angular velocity of the spiral flow would amplify the mean deflection of the in-line (IL) direction, and the incidence velocity has a significant nonlinear effect on the dominant frequency of the riser, which would induce the high-order dominant vibration modes when the incidence velocity is large sufficiently. However, the incidence angle has relatively less effects on the VIV responses of flexible riser. Moreover, the locked interval of VIV is less influenced by damping ratio below 0.1.

Study on suppressing the vortex-induced vibration of flexible riser in frequency domain

Vortex-induced vibration (VIV) may cause severe fatigue damage on deep-sea flexible risers. In many researches on active control of VIV, numerical simulation is widely used because of its suitability for parametric studies and lower cost compared to experiments. However, the existing numerical simulations rarely consider the change of lift during the active control of VIV due to the complexity of the control method. Moreover, the calculation time of numerical simulation is relatively long in the time domain. To solve these problems, the active control proposed in this paper is carried out in the frequency domain. A boundary control method considering the change of lift force is proposed through an active control bending moment is applied to the top of riser. Compared with the experimental and numerical results of the flexible riser model under shear flow, the effectiveness of the proposed method is verified. In addition, the effects of different shear currents and different controlled bending moments on structural fatigue damage are studied. The results demonstrated that the reduction of fatigue damage is smaller when the control bending moment is small. As the control bending moment increases, the reduction of fatigue damage increases. However, when the control bending moment exceeds the critical value, the fatigue damage no longer decreases. From the total power perspective, the control energy and the proportion of energy in the system increase with the growth of the control moment. It is difficult to directly obtain the optimal control bending moment although there is an optimal control bending moment. Trial calculations are used to obtain the optimal control bending moment in this paper. The greater the shear currents, the greater the required control bending moment.

LQR control on multimode vortex-induced vibration of flexible riser undergoing shear flow

Under the actions of ocean currents and/or waves, deep-sea flexible risers are often subject to vortex-induced vibration (VIV). The VIV can lead to severe fatigue and structural safety issues caused by oscillatory periodic stress and large-amplitude displacement. As flexible risers have natural modes with lower frequency and higher density, a multimode VIV is likely to occur in risers under the action of ocean currents, which is considered as shear flow. To decrease the response level of the VIV of the riser actively, a multimode control approach that uses a bending moment at the top end of the riser via an LQR optimal controller is developed in this study. The dynamic equations of a flexible riser including the control bending moment in shear flow are established both in the time and state-space domains. The LQR controllers are then designed to optimize the objective function, which indicates the minimum cost of the riser's VIV response and control input energy based on the Riccati equation of the closed-loop system under the assumption that the lift coefficient distribution is constant. Finally, the VIV responses of both the original and closed-loop systems under different flow velocities are examined through numerical simulations. The results demonstrate that the designed active control approaches can effectively reduce the riser displacement/angle by approximately 71%–89% compared with that of the original system. Further, for multimode control, the presented mode-weighted control is more effective than the mode-averaged control; the decrease in displacement is approximately 1.13 times than that of the mode-averaged control. Owing to the increase in flow velocity as more and higher-order modes are excited, the VIV response of the original system decreases slightly while the frequency response gradually increases. For the closed-loop system, the response becomes smaller and more complicated, and the efficiency of the controller becomes lower at a certain flow velocity.

Numerical prediction of vortex-induced vibration of flexible riser with thick strip method

We present numerical prediction results of vortex-induced vibration (VIV) of a long flexible tensioned riser subject to uniform currents. The VIV model of long length-to-diameter ratio is considered and ‘thick’ strip technique based on high-order spectral/hp element method is employed for computational simulation. The model parameter of the riser for the simulation is chosen according to the dimensional counterparts used in the experimental tests in Lehn (2003). The numerical results are displayed in terms of motion responses, hydrodynamic forces and wake patterns as well and compared and discussed with the available data in the literature.

Analysis on multi-frequency vortex-induced vibration and mode competition of flexible deep-ocean riser in sheared fluid fields

Multi-frequency vortex-induced vibration of flexible riser in lineally sheared fluid fields with different shearing parameters is explored by using the numerical approach. By combining the finite element method with a hydrodynamic model, the approach can consider mode competition based on modal energy and can carry out nonlinearly simultaneously dynamic response in time domain. Our analysis shows that multi-frequency VIV may occur both in non-uniform and uniform fluid fields. And, the behaviors of multi-frequency VIV are different from single-frequency VIV. Because several modes are involved and compete with each other, and consequently the determination of modal excitation region become more complicated. As the towing speed (or the shearing parameter) increases in sheared flow, the average RMS displacement does not regularly rise (or drop), but slightly fluctuates owing to changes of the participating mode and its excitation region. On the other hand, the average RMS stress gradually rises owing to higher-order modes being included. Moreover, it is found that the dominant frequency distributing along structural span significantly changes with the towing speed, and the length of the first dominant frequency gets smaller due to larger shearing parameter along with more intense competition between the participating modes.

Modeling of fluid–structure interaction for simulating vortex-induced vibration of flexible riser: finite difference method combined with wake oscillator model

This paper proposes a numerical simulation method for the dynamic motion of a flexible riser pipe undergoing vortex-induced vibration (VIV). The method is based on a finite difference scheme for solving nonlinear structural dynamics of the pipe and wake oscillator model for quantifying vortex-induced forces acting on the pipe, the combination of which can offer a very efficient and stable computation. To investigate the accuracy of the method, we performed simulations of the VIV of riser pipes under uniform flow and sheared flow conditions; and then compared obtained results with experiments of preceding works. We consequently confirmed that the present method can simulate a couple of important aspects of the VIV of the pipes: frequency, mode shape, and amplitude of displacement of cross-flow displacement.