Riser Vortex-induced Vibration Research Articles

Coupling Effects of a Top-Hinged Buoyancy Can on the Vortex-Induced Vibration of a Riser Model in Currents and Waves

In order to investigate the effects of the top-end dynamic boundary of risers caused by floater motions on their vortex-induced vibration (VIV) characteristics, a combined model comprising a buoyancy can with a relatively simple structural form and a riser is taken as the research object in the present study. The aspect ratios of the buoyancy can and the riser model are 5.37 and 250, respectively. A set of experimental devices is designed to support the VIV test of the riser with a dynamic boundary stimulating the vortex-induced motion (VIM) of the buoyancy can under different uniform flow and regular wave conditions. Several data processing methods are applied in the model test, i.e., mode superposition, Euler angle conversion, band pass filter, fast Fourier transform, and wavelet transform. Based on the testing results, the effect of low-frequency VIM on the high-frequency VIV of the riser is discussed in relation to a single current, a single wave, and a combined wave and current. It is found that the coupling effect of VIM on the riser VIV presents certain orthogonal features at low current velocities. The effect of the cross-flow VIM component on VIV is far more prominent than that of its counterpart, the in-line VIM, with increasing flow velocity. The VIM in the combined wave–current condition significantly enhances the modulation of vibration amplitude and frequency, resulting in larger fluctuation peaks of vibration response and further increasing the risk of VIV fatigue.

Physics-based Data-informed Prediction of Vertical, Catenary, and Stepped Riser Vortex-induced Vibrations

Physics-based Data-informed Prediction of Vertical, Catenary, and Stepped Riser Vortex-induced Vibrations

Application of regularization method with sparsity prior in riser vortex-induced vibration reconstruction

In the experimental investigation or in-situ monitoring of the vortex-induced vibration (VIV) of offshore risers, it is often necessary to reconstruct the full-field structural response based on discrete observations. Conventionally, this can be done using the modal decomposition method. However, the method can be invalid with fewer measurements than unknown modes and is unstable when the data contains measurement noises. This study presents a novel sparse modal decomposition method based on the compressed sensing theory and the Compressive Sampling Matching Pursuit (CoSaMP) algorithm. It formulates the reconstruction problem in a regularization framework with a prior related to the sparsity of the modal coordinates, which differs from the classic method that seeks a least-square solution. Verification tests demonstrate that our method outperforms the classic method regarding accuracy and stability, especially when the measurements are noisy and insufficient, highlighting its potential applications in engineering.

Numerical simulations of 2-DOF vortex-induced vibration of a circular cylinder in two and three dimensions: A comparison study

Finding a reliable and efficient numerical method is of great importance for the safety design of a offshore riser whose model can be simplified as vortex-induced vibration (VIV) of an elastically-mounted circular cylinder. In the current study, two-degree-of-freedom (2-DOF) VIV responses of a circular cylinder with a small mass ratio m*=2.6 and small mass damping parameter (m*+1)ζ=0.013 is numerically investigated by two- and three-dimensional method. The simulations by using the URANS (Unsteady Reynolds Averaged Navier-Stokes) in combination with the SST (Shear Stress Transport) k−ω turbulence model are performed at subcritical Reynolds numbers (Re ranges from 1×103 to 1.5×104). From the overall results, both the 2D and 3D simulations can obtain relatively accurate statistical results including VIV response amplitudes and frequency values. The main differences between the 2D and 3D simulations lie on the three-dimensional effects that exist in the supper upper branch and the flow transition condition. However, the 2D numerical simulations can save hundreds of times of the computational resources compared with a 3D simulation, hence is more suitable for engineering VIV prediction under such conditions. The comparison of simulation and experimental results in this study provides research support for the selection of appropriate simulation methods (2D or 3D) for 2-DOF VIV of an offshore riser in research and engineering.

Experimental investigation on vortex-induced vibration of a flexible pipe in combined uniform and oscillatory flow

Vortex-induced vibrations (VIV) induced by ocean currents can cause severe fatigue damage to marine risers. The background current and oscillatory flow caused by the motion of the floating platform form a combined flow field, which complicates the VIV of risers. In this paper, a series of model tests were conducted in a towing tank to investigate the VIV characteristics of a flexible pipe in combined uniform and oscillatory flow. Effects of oscillating amplitude, oscillating period and uniform flow velocity on VIV of the pipe were investigated and discussed based on the experimental results. VIV fatigue damage of the pipe was also calculated. Intermittent vibration, hysteresis and mode transition were observed in combined flow. Large oscillating amplitude and short oscillating period tend to enlarge the fatigue damage. But when the uniform flow velocity is too high, small oscillating amplitude and long oscillating period tend to enlarge the fatigue damage. Critical short period and combined ratio threshold are discovered in the VIV of the pipe in combined flow. VIV characteristics in different types of flows are also compared. VIV in combined flow may cause severe fatigue damage, which should be given enough attention.

Coupling effect of time-varying axial tension and combined unsteady flow on vortex-induced vibration of flexible risers

The surge and heave motions of the floating platform have a non-negligible effect on vortex-induced vibration (VIV) of marine risers, which generates time-varying axial tension of risers, i.e., parametric excitation, and unsteady flow and complicates the prediction of VIV. A semi-empirical model is improved to simulate the VIV of flexible risers with parametric excitation under combined unsteady flow. Both the flow velocity and the stiffness matrix are updated with time. Fifty-five cases are simulated to investigate the separate and coupling effects of parametric excitation and combined unsteady flow on VIV. The phenomena such as sum and difference frequencies, Mathieu-type resonance, intermittent vibration and frequency fluctuation are observed. The effects of parameters such as excitation amplitude dT, excitation frequency fp, combined ratio r and oscillation period P on VIV are discussed in detail. Some interesting conclusions are drawn: VIV may be significantly suppressed when transition-low-frequency parametric excitation and short-period combined unsteady flow act together. Larger r makes the effect of combined unsteady flow more significant. VIV is mainly dominated by the parametric excitation when fp is twice the natural frequency of the riser corresponding to the dominant mode.

Investigation on Numerical Simulation of VIV of Deep-Sea Flexible Risers

The vortex-induced vibration (VIV) of flexible risers is a complex fluid–structure interaction (FSI) phenomenon. In this study, we conducted a numerical simulation method based on the slicing method to study the vortex-induced vibration (VIV) of deep-sea flexible risers with different slenderness ratios and uniform flow velocities. The method combines the finite element model of the riser structure with the two-dimensional flow field slices solved by the Fluent solver. The fluid–structure interaction was realized by a self-compiled UDF program and the overset mesh technique. The numerical results were validated by comparing them with experimental data. The VIV characteristics of the riser, such as the vibration track, vibration mode, vibration frequency and wake vortex shedding mode, were analyzed. The article reveals the nonlinear dynamic features of flexible riser vibration, such as multi-frequency vibration, wide-frequency vibration and multi-modal vibration. The article also provides insights into the fluid–structure interaction mechanism of VIV of deep-sea flexible risers.

Experimental investigation on vortex-induced vibration of a flexible pipe with single/bi-frequency parametric excitation

Vortex-induced vibration (VIV) of marine risers is a complicated fluid-structure interaction problem in ocean engineering. The parametric excitation caused by the heaving of the platform complicates the VIV of risers. In this paper, a series of model tests were conducted in a towing tank to investigate the VIV characteristics of a flexible pipe with single/bi-frequency parametric excitation. The linear motor was connected to the end of the pipe through a spring, and the movement of the linear motor provided various parametric excitations for the pipe. With parametric excitation, additional frequency components such as excitation frequency, the multiples of excitation frequency and the sum and difference frequencies are excited. Parametric excitation has more significant effect on VIV in IL direction than that in CF direction. With the increase of excitation amplitude, the maximum RMS strain in CF direction decreases while that in IL direction increases in most cases. The phase plane plot and motion trajectory of the flexible pipe becomes less regular with parametric excitation. When bi-frequency parametric excitation is applied, more types of additional frequencies such as sum and difference frequencies between the two excitation frequencies and other frequencies can be observed in the response, and the parametric excitation component with larger weight has more significant effect on the VIV of the flexible pipe.

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.

Internal flow effect on the cross-flow vortex-induced vibration of marine risers with different support methods

The present work investigates the effect of internal flow on the vortex-induced vibration (VIV) characteristics of a marine riser under different boundary conditions. A numerical model of the riser structure is established by considering the complex flow fields. The developed model is solved by the finite element theory and Newmark method, and it is verified by the experimental data. The effects of internal flow velocity and density on the vortex-induced dynamic responses with different support methods are studied. The results demonstrate that among different support methods, internal flow effect (IFE) has the greatest influence on the vortex-induced response of the riser with hinged-cantilevered support. In the case of hinged-cantilevered support method, the increment of the internal flow velocity and internal flow density has a negative influence on the vortex-induced vibration of the riser. In the case of fixed at both ends support method, the vibration amplitude and mode of riser decrease with the increment of flow density in riser. For the hinged-hinged support condition, the increment of the internal flow velocity slightly increases the amplitude of the riser and the vibration amplitude of riser decreases with the increase of density. This research may provide an efficient method for the VIV prediction, optimization design and operation control of the marine risers in practical applications.

Internal flow effect with non-uniform temperature environment on VIV of marine risers under external shear current

ABSTRACT Vortex-induced vibration (VIV) of marine risers under external currents may cause serious fatigue damage, which is a challenging issue especially when the internal flow is included simultaneously. The temperature gradient between the conveying fluid and the surrounding seawater makes the internal flow effect more complicated. In this paper, the effect of internal flow with a non-uniform temperature environment is added to the riser’s dynamic governing equation, and the semi-empirical model is adopted to simulate VIV. A large number of cases are designed to analyse the dynamic response and fatigue damage of the riser when the internal flow and non-uniform temperature environment are individually considered. Then, the combined effect of the internal flow and temperature environment is compared with the simple superposition of their individual effects. The increase in the internal flow temperature could trigger mode jump and enlarge the fatigue damage.

Vortex-induced vibration of a flexible fluid-conveying riser due to vessel motion

As vessel motion and internal flow can exert impact on vortex-induced vibration (VIV) of flexible risers, it is of great significance to take these two factors into consideration while VIV is investigated. Therefore, crossflow (CF) VIV of a flexible fluid-conveying riser is studied when vessel motion is considered. The governing equation is firstly discretized based on finite element method and solved by Newmark-β method. Then, validation is conducted by comparing with experimental data. Subsequently, CF VIV response of a flexible riser is explored with consideration of both vessel motion and internal flow. The results show that regardless of low and high Keulegan-Carpenter (KC) numbers, vessel motion can have a notable effect on top tension and CF VIV characteristics. Variations of CF VIV amplitude, vibration frequency as well as top tension frequency are affected by both vessel motion and internal flow. With the increase of internal flow velocity, CF VIV amplitude is changed while mode jump occurs concomitant with frequency transition. Besides, both vessel motion and internal flow velocity can have an impact on energy allocation along the flexible fluid-conveying riser. Noteworthy is that although both vessel motion and internal flow can exert an impact on top tension and VIV response, the effect induced by vessel motion still plays a dominated role.

Application of a modified wake oscillator model to vortex-induced vibration of a free-hanging riser subjected to vessel motion

A modified wake oscillator model for modelling vortex-induced vibrations (VIVs) of a free-hanging riser subjected to vessel motion is presented. Different from the conventional wake oscillators which have typically utilized in the prediction of steady flow VIVs , the new model can be applied to simulate VIVs introduced by the vessel motion where the flows around the riser are not known in advance. This has been achieved by introducing an equivalent flow based on the relative flow velocity and coupling the wake oscillator with the acceleration of the riser in the lift direction. The modified wake oscillator model is tuned against the experiments of riser VIVs subjected to steady flows and a set of empirical parameters is obtained. The model is then applied to simulate the VIVs of a large-scale free-hanging water intake riser (WIR) subjected to different vessel motions and the simulated results are compared to those of experiments. The comparison shows that the proposed model is able to predict response frequency in good agreement with the experiment, and generates satisfactory prediction with respect to the response amplitude for three test cases out of five that have been studied in the present paper.

Numerical investigation of Vortex-Induced Vibration response for a full-scale riser with staggered buoyancy modules

Vortex-Induced Vibration (VIV) caused by ocean current is one of the main sources of the long-term fatigue damage of marine riser. Buoyancy module is an essential equipment for deep-water riser system, which can reduce the top tension remand. Based on the hydrodynamic force decomposition model and forced vibration test data, a time domain prediction model of VIV response for top-tensioned riser with buoyancy modules is established in this paper. Firstly, the numerical model is verified with the experimental measurements of an 11.48 m riser with buoyancy modules under uniform flow. Then, a 610 m riser is chosen as the research object, and five staggered buoyancy module configurations with different coverage ratios are simulated, to reveal the VIV response characteristics of full-scale riser with buoyancy modules. Some interesting phenomena are reasonably explained with VIV mechanism analysis. In terms of frequency response, displacement and modal order, the influence of coverage ratio on riser's VIV is analyzed. Finally, the optimal coverage of buoyancy modules for VIV suppression is discussed, which is of great significance in engineering application.

Cross-flow vortex-induced vibration of a flexible fluid-conveying riser undergoing external oscillatory flow

Cross-flow (CF) vortex-induced vibration (VIV) of a flexible riser considering both internal flow and external oscillatory flow is numerically investigated with consideration of combining the structural model with semi-empirical hydrodynamic force model by using Finite Element Method. The accuracy of the applied model is firstly examined by comparing the numerical results with the experimental data, which proves that the model can reproduce typical characteristics of CF VIV of a flexible riser undergoing external oscillatory flow. Then CF VIV of a flexible fluid-conveying riser subjected to external oscillatory flow is studied while the non-dimensional internal flow velocity and density ratio between internal and external flows are changed. The results show that regardless of the non-dimensional internal flow velocity and density ratio, typical VIV features of a flexible riser, such as intermittent VIV, amplitude modulation, hysteresis, mode and frequency transitions as well as standing and travelling wave responses, can be captured with variation of external oscillatory flow velocity. Moreover, VIV developing process, including building-up, lock-in and dying-out, can be detected for CF VIV. With the increase of the non-dimensional internal flow velocity and density ratio, high mode response can be effortlessly triggered for CF VIV, which is accompanied with occurrence of new vibrating frequencies. In addition, the vibrating frequency of CF VIV decreases while the non-dimensional internal flow velocity and density ratio are increased.

Time domain simulation of marine riser vortex-induced vibrations in three-dimensional currents

One of the major uncertainties related to present Vortex-Induced Vibrations (VIV) prediction practice is related to the simplification of real multi-directional current profiles into equivalent uni-directional ones. This study addresses the correlation between a recently updated time domain model and data from VIV experiments including both two- and three-dimensional (2D and 3D) currents. The test riser and current profile were modeled in three different ways, including 2D & 3D current profiles and linear & nonlinear models, as a basis for discussing the results from various perspectives. As a first step, VIV simulations with a nonlinear riser model including actual current profiles were carried out, demonstrating a good correlation with test data for 2D current and low velocity 3D current. However, simulated VIV displacements were somewhat underestimated for the high velocity 3D current profiles. With reference to literature noting possible drag reduction for 3D current cases, drag coefficient sensitivity studies were carried out, demonstrating better correlation for 3D current cases at high velocities. Then, the VIV fatigue damages obtained by the three alternative structural and current modeling procedures were compared to each other in a case study. The locations of the maximum fatigue damages differed between methods, and it was demonstrated that prediction using uni-directional current does not always lead to the highest fatigue damage. In conclusion, it was demonstrated that the time domain VIV model can describe the 3D current VIV, and by doing that, it was found that present VIV prediction practice can be improved.

Vortex-induced vibration of risers with staggered buoyancy modules of small aspect ratio

Vortex-induced vibration (VIV) is one of the critical design considerations for the offshore riser and pipeline system. Significant research effort has been devoted to better understanding and predicting riser VIV. Compared to the bare uniform riser configuration, the attached staggered buoyancy modules of a small aspect ratio will extensively alter riser hydrodynamic characters, and hence the riser dynamic response. In the current study, experiments were performed on forced inline (IL)-crossflow (CF) combined vibration of a rigid cylinder with staggered modules (module to bare cylinder diameter and length ratio of 2.5 and 1.0.) as well as self-induced vibration of two flexible models with 100% and 50% buoyancy module coverage ratios. From the rigid model experiment, positive regions for lift coefficient in the phase of velocity (Clv) for both riser and buoyancy module induced vibration are observed. The result indicates potential power-in excitation from both riser and buoyancy modules, which is later confirmed by the bi-frequency and bi-modal vibration from the flexible cylinder experiment. Furthermore, several other phenomena have been reported, including low reduced frequency for buoyancy module induced motion, the phase between CF and IL trajectory, and buoyancy module VIV suppression ability, emphasizing the connection between the rigid and flexible cylinder experiment.

Surface roughness effects on a tensioned riser vortex-induced vibration in the uniform current

Due to the long-term operation of marine riser, marine organisms will adsorb on its surface, thus affect its surface roughness. The surface roughness of the riser will affect the wake flow field of the riser, and then affect the dynamic characteristics of the vortex-induced vibration (VIV). The study of the influence of surface roughness on the characteristics of a rough riser VIV is helpful to improve our understanding of the mechanism of a rough riser VIV and ensure the safety of the riser during its service life. Based on the methods of computational fluid dynamics (CFD) and computational structure dynamics (CSD), this study presents the model considering tension variation with the structure vibration and the modified model of rough wall velocity gradient to construct the numerical calculation program of a rough riser VIV to study effects of various key parameters (surface roughness, inflow velocities) on the dynamic characteristics, vibration response, wake vortex shedding patterns and vibration trajectories of a rough riser. The results show that in the range of Reynolds number Re = 2.515 × 104∼1.258×105 and reduced velocity U* = 1.35–6.74, compared with a smooth riser, the streamwise vibration amplitude of a rough riser decreases, and with the increase of inflow velocity and surface roughness, the streamwise vibration amplitude of a rough riser decreases more and more obviously. In the range of reduced velocity U* = 2.70–5.39, the transverse vibration amplitude of a rough riser increases slightly. At a higher reduced velocity (U* = 6.74), the transverse vibration amplitude of the rough riser decreases. Larger surface roughness will enhance the multi-frequency and broadband vibration characteristics of the riser in the streamwise direction. With the increase of surface roughness, the multi-frequency vibration characteristics decrease gradually, but the broadband vibration characteristics increase gradually in the transverse direction. Surface roughness can suppress 2T and 2P wake vortex shedding pattern and enhance 2C wake vortex shedding pattern. This study will provide a profound understanding to guide the practical marine riser VIV research with the consideration of surface roughness.

Prediction of deepwater riser VIV with an improved time domain model including non-linear structural behavior

In this paper, vortex-induced vibrations (VIV) of deepwater risers including the VIV-induced tension fluctuations were simulated using a time domain VIV model. Data from a large-scale riser experiment was applied for correlation studies. As a first step, a new combination of in-line and cross-flow synchronization models with empirical parameters was proposed for better in-line response prediction. The performance of the improved load model was investigated based on a linear riser model. The results showed that the in-line response and its synchronization pattern were improved compared to the previous load model. Then, the improved load model was combined with a non-linear structural riser model to explore the effect of tension fluctuations. As observed in the tests, simulations showed that the tension fluctuations influenced the stability of VIV and possibly decreased the resulting vibration amplitudes. Lastly, the VIV prediction performance of the load model was evaluated with respect to frequency contents, stress levels, and fatigue damage. From the results, it was found that the improved load model in combination with the non-linear structural model reflected the response characteristics observed in the tests.

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.