Vibration Of Riser Research Articles

Axial-transverse coupled vortex-induced vibration characteristics of composite riser under gas-liquid two-phase internal flow

In this paper, the dynamic behavior of axial (AX) and transverse coupled vortex-induced vibration (VIV) of composite riser under gas-liquid two-phase internal flow is studied. Firstly, based on Hamilton's principle, considering the structural characteristics of composite materials, gas-liquid two-phase internal flow, AX and transverse nonlinear coupling vibration and nonlinear external excitation, a more realistic VIV prediction model of composite riser is proposed. Compared with the existing experimental results, the validity of the model in this paper is verified, and it is found that the VIV prediction of the riser is more accurate when considering the AX and transverse coupling effects. Then, the effects of current velocity, liquid phase velocity, gas volume fraction and fiber orientation angle on the coupled VIV characteristics of gas-liquid mixed transport composite riser are studied. The results show that with the increase of current velocity, gas volume fraction and liquid phase velocity, the in-line (IL) vibration of the riser changes modally before the cross-flow (CF), but the fiber orientation angle has the opposite effect on this phenomenon. At the same time, when the two directions are in two different vibration modes, the vibration frequency in IL direction is obviously larger than that in CF direction. In addition, the riser AX vibration is almost unaffected by the parameters, but its vibration frequency is much larger than the other two directions, and the vibration center has shifted. We also find that the specific liquid phase velocity value will cause the chaotic vibration of the riser in the CF and IL directions, and the vibration displacements of the composite riser are almost equal in both directions.

Numerical study of the vortex-induced vibration of a riser taking into account the variation of the tension

ABSTRACT Previous studies of the marine riser VIV have primarily considered the constant values of the tension, but limited work has been done to consider the variations of the tension with riser vibrations. In addition, previous studies have mainly focused on the effect of the inflow velocity profile on a riser VIV, while a study of a riser VIV at a wide range of velocities is still lacking. This study presents a model that can take into account the variation of the tension with riser vibrations. The VIV characteristics of the riser were investigated based on the fluid-structure coupling methods. It is shown that the model that takes into account the variation of the tension is able to accurately capture the bimodal character of the amplitude-ratio curve and the vortex shedding pattern, and to calculate the vibration frequencies accurately.

Prediction model for multidirectional vortex-induced vibrations of catenary riser in convex/concave and perpendicular flows

This study presents an advanced numerical prediction model based on nonlinear wake oscillators for the investigation of multidirectional vortex-induced vibration (VIV) responses of a long catenary riser depending on the structural curved configuration versus the incoming flow direction. By considering a uniform free-stream flow aligned with the initial curvature plane of the catenary riser in a convex or concave shape, the normal flow velocity component is nonlinearly sheared owing to the spanwise variation of inclination angles. This is different from the perpendicular flow case in which the normal flow velocity is spatially constant. To capture the influence of flow direction on the three-dimensionally coupled cross-flow and in-line VIV, distributed wake oscillators are introduced and applied to simulate the fluctuating hydrodynamic lift and drag forces that account for the important effects of cylinder inclination, variable vortex excitation frequencies, global–local​ displacement relationships, and relative flow-cylinder velocities. The amplified mean drag force and the axial force depending on the tangential flow velocity are also considered. These empirical hydrodynamic effects are nonlinearly coupled with the equations of riser three-dimensional motion, and the overall governing equations are numerically solved to assess the multimode, multi-frequency and multi-degree-of-freedom responses. Validation of the model is carried out for VIV of flexible straight and curved cylinders based on experimental results in the literature. Parametric investigations are performed by varying the incoming flow velocities in perpendicular, convex and concave configurations for a long catenary riser with a low mass-damping ratio. In-plane (horizontal and vertical) and out-of-plane VIV features are presented and discussed in terms of space–time varying amplitudes, drag-amplified shape reconfigurations, oscillation frequencies, lock-in occurrences, dual resonances, orbital motion trajectories, fluid–structure energy transfer, and multimodal contributions. Overall prediction results highlight the influence of the cylinder geometric configuration versus the incoming flow orientation on multidirectional VIV characteristics that should be recognised and incorporated into practical analysis tools.

Experimental Study on Vortex-Induced Vibration of Tension Leg and Riser for Full Depth Mooring Tension Leg Platform

According to the geometric parameters of the tension leg platform, the test model was made with a scale ratio of 1:61. The model was used to conduct the full-depth simulation test of uniform flow and wave current combination in the test pool. The model test results showed that when the reduced speed was between 5.5 and 8.5, and the lateral motion response of the platform was the most significant. In the interval of the reduced speed, the response frequency of transverse vortex-induced motion was close to the natural transverse frequency of the platform, and resonance occurred. The amplitude of surge motion increased with the increase of reduced speed. Due to the pull of the floating body, the tension of tension legs and risers increased with the flow rate but did not increase significantly in the floating body lock zone. The mooring tension had a certain limiting effect on the floating body sway. The displacement modes of tension legs and risers were greatly affected by the flow velocity. With the flow velocity increasing, the mode order increased. In addition, the increase in tension caused by the large displacement of the floating body had a certain impact on the displacement amplitude. The wave could reduce the sway of the floating body and strengthen the surge. Therefore, under the combined action of wave and current, the tension amplitude of the tension leg and riser was increased compared with that under the uniform flow. The conclusions obtained in this paper could be used for reference in the engineering design of tension legs and 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.

Test Study on Vortex-Induced Vibration of Deep-Sea Riser under Bidirectional Shear Flow

A model test was carried out to reveal the vortex-induced vibration characteristics of a deep-sea riser under bidirectional shear flow. Bandpass filtering and modal analysis were used to process the test strain data, and the amplitude and frequency response characteristics of the vortex-induced vibration of the riser in the bidirectional shear flow field were obtained. The results of the test data analysis show that the dominant frequency of the vortex-induced vibration of the riser model under bidirectional shear flow is locked in the natural frequency of the riser and does not increase with the increase in flow velocity, that the average resistance coefficient of the riser model has little change under different flow velocities because of the distribution characteristics of the “bidirectional shear” flow field, that there is an extreme value of the shear force in the middle of the riser model, and that the Strouhal number in the transverse direction of the vortex-induced vibration under bidirectional shear flow is less than the recommended value of the current vortex-induced vibration prediction software. The obtained results provide basic data for the prediction of vortex-induced vibration and research into the fatigue analysis method of a riser under an internal wave flow field.

Experimental exploration of unpowered propellers for vortex-induced vibration suppression of flexible riser

Passive control of the vortex-induced vibrations (VIVs) of flexible risers has been typically studied to alleviate their VIV responses by means of breaking the spanwise correlation, modifying the structural characteristic, or disturbing the flow field. However, this paper proposed using innovative suppression devices—unpowered propellers—to mitigate VIVs and output energy through passive rotation under flowing fluids. The effectiveness of three-, four-, and six-blade propellers for VIV suppression was experimentally examined in the Reynolds number range of 2600–10,400. Seven identical propellers were installed uniformly along a riser facing the incoming flow for VIV suppression. The response parameters of the riser models, including the displacement, frequency, trajectory, and fatigue damage, were investigated. Moreover, a comprehensive evaluation of the unpowered propellers, determined by the suppression efficiency and the cost, was also conducted. The experimental results showed that all three types of propellers significantly decreased the VIV amplitudes and inhibited high-order frequency components. Increasing the number of blades effectively enhanced the amplitude suppression of the propellers, whereas it had almost no effect on the vibration frequencies of the controlled risers. It is worth noting that the three-blade propellers with the best comprehensive evaluation may not be a reasonable choice in high-velocity water.

Experimental investigation on the effectiveness of serrated helical strakes in suppressing vortex-induced vibrations for a flexible riser

The effectiveness of an unconventional configuration of serrated helical strakes (SSs) in alleviating the vortex-induced vibrations (VIVs) of flexible risers was evaluated experimentally in the Reynolds number range of 2600–10,400. Five SSs with varying serration densities and lengths were designed and compared with conventional strakes (CSs) to highlight the unparalleled features of this new geometry. The VIV parameters of the bare and straked risers, including the displacement, frequency, fatigue damage, and suppression efficiency, were systematically investigated. The comprehensive evaluation of the helical strakes was also made considering the factors of amplitude suppression, frequency inhibition, drag force, and cost control. The experimental results confirmed that all the helical strakes were capable of suppressing the VIV responses of flexible riser models. With identical strake areas, the SSs were more effective in inhibiting the VIV amplitude and fatigue damage than the CSs; whereas, the serrated geometry had few effects in improving the reduction of VIV frequency. Moreover, serration density is a critical factor in determining the suppression performance of SSs under the same strake heights and areas. An increase in the strake area enhanced VIV suppression performance; however, excess strakes deteriorated the comprehensive evaluation index due to the increased cost.

Experimental investigation on flow-induced vibration control of flexible risers fitted with new configuration of splitter plates

Experimental investigations within the Reynolds number range of 2600–9100 were performed to examine the effectiveness of serrated splitter plates in controlling the flow-induced vibration (FIV) of flexible risers. Conventional splitter plates were arranged for contrast to highlight the unparalleled features of the new splitter plates. Bending strains of FIV responses were measured and converted into critical parameters of displacement, dominant frequency, oscillation orbit, suppression efficiency, and fatigue damage for analysis. Test results illustrate that for flexible risers with conventional or serrated splitter plates, a galloping phenomenon occurs at a low reduced velocity, and standing wave characteristics invariably dominate their FIV responses. As the reduced velocity varies increasingly, the conventional splitter plate triggers a continuous monotonous increase in the FIV amplitudes of controlled risers. However, owing to the phase difference in the interaction of the shear layers between proximal plates and distal plates, the FIV amplitudes of the risers fitted with serrated splitter plates start to stay constant virtually and then distinctly decline with appearing chaotic oscillation trajectories. Moreover, the control performance of serrated splitter plates with uniformly distributed serrations in alleviating FIV amplitudes is optimal at high reduced velocities; whereas excessively dense or sparse serrations can reduce FIV inhibition.

Three-dimensional nonlinear vortex-induced vibrations of top-tension risers considering platform motion

A novel three-dimensional (3-D) dynamic model considering nonlinearity of tension is proposed to explore in-line (IL), cross-flow (CF) and axial (AX) vortex-induced vibration (VIV) responses of riser under platform motion. The interaction between riser and external flow is simulated by the van der Pol equation, and the nonlinear equations of motion are established by Hamilton's principle and the Galerkin method, and solved by Runge-Kutta method. The influences of heave and swing motions of platform on VIV responses of the riser are investigated. It is found that for large velocity of external flow, the axial displacement of riser results in an increase in IL excitation mode and also an increase in IL oscillating displacement. When the riser is excited by swing motion of platform, with the increase of excitation amplitude, the maximum response amplitudes will also increase in all the three directions, and the amplitudes in high frequency domain increase more obviously. Combined effect of heave and swing motions of the platform results in the increase of VIV amplitudes in IL and AX directions, and combined action of forced and parametric excitations has little effect on response amplitude of riser in CF direction.

Galloping of two-dimensional section of a marine riser attached with fairing under different inflow angles

Fairing is one of the most effective and popular devices for suppressing vortex-induced vibration (VIV) of marine risers. However, when the fairing is stuck and cannot rotate due to biological contamination and other reasons, it may cause serious galloping oscillation and endanger the safety of the riser. In this paper, a reliable two-dimensional fluid-structure interaction (FSI) model with two degrees of freedom (DOF) was established based on computational fluid dynamics (CFD) and finite element method (FEM). The steady flow fields and galloping characteristics of a faired riser under different inflow attack angles were studied by numerical method. The results showed that when the structure is stationary, there is a critical attack angle αc ∈ (25°, 30°), which divides the vortex shedding state and the variation law of the hydrodynamic coefficient. This is because when the inflow attack angle α > αc, a reattachment of the shear layer occurs on one side of the fairing. In the 2 DOF FSI analysis, typical cross-flow (CF) direction galloping was observed in the region of α < αc. When α > αc, the oscillation was relatively weak, and the amplitudes in the in-line (IL) and CF directions were similar. When the α was near αc, the vibration was smallest. According to the structure of the wake vortex, the stable reattachment of the separated shear layer generated on one side of the fairing affected by the inflow attack angle is not easy to cause galloping. Only when the periodic reattachment occurs, it will trigger a strong galloping.

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.

Adaptive fuzzy backstepping control of flexible marine riser with uncertain parameters and input saturation constraint

To suppress the vibration of a flexible riser system with input saturation and uncertain parameters under the influence of unknown external disturbances, an adaptive fuzzy backstepping boundary control algorithm was designed. First, the backstepping method and Lyapunov function are used to analyze the subsystem and design the virtual control law. On this basis, the adaptive fuzzy system was designed to approximate the unknown nonlinear term in the design process, and the smooth hyperbolic tangent function and the auxiliary system based on Nussbaum function were introduced to limit control input. The stability and uniform boundedness of the closed-loop control system are proved by the Lyapunov stability theory without simplifying or discretizing the infinite dimensional dynamic model. Finally, the effectiveness of the proposed control algorithm is verified by comparing the control effect with proportional–integral–derivative control and the previous adaptive backstepping control by MATLAB simulation. The simulation results show that the proposed control algorithm can overcome the uncertainty of system parameters and effectively suppress the riser vibration under input constraints, and its control effect is better than proportional–integral–derivative control and adaptive backstepping control.

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.

Dynamic response of five-riser group subjected to vortex-induced vibration in a cylindrical arrangement configuration

The vortex-induced vibration of the cylindrical arrangement five-riser group was studied in the wave-current water flume. The fluid was divided into column fluid, inter-column fluid, and extra-column effective fluid, and then the dynamic response rule of the five-riser group under multiple fluids and the mechanism of the fluid inside the virtual cylinder were explored. In the experiment, the distance between the adjacent risers was 4.0 D. The results show that the interference effect has a weaker influence on the dominant frequency and displacement response of the upstream risers. The vibration response of the symmetrical double riser in the middle stream maintains symmetry, and the vibration response of the symmetrical double riser downstream is asymmetrical. Second-order mode weights of the five risers and the isolated riser overlap. As the reduced velocity increases in the low reduced velocity range, the interference effect hinders the conversion process from the first-order mode to the second-order mode. The interference degree of the five risers is quantified by the interference ratio, and it is found that the adjacent interference in the cross-flow direction mainly promotes the riser vibration, and mainly suppresses the vibration of the riser in the in-line direction.

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.

Experimental investigation of the vortex-induced motion of tension leg platform system with suppression system under current

The vortex-induced motion (VIM) of a tension leg platform (TLP) and vortex-induced vibration (VIV) of tension legs and risers occur only under the action of water flow. The VIM results in a high amplitude motion in the cross-flow (CF) direction, and the VIV leads to fatigue failure of the tension legs and risers. A three-dimensional (3D) physical model investigation was conducted on the overall dynamic response of the platform and acceleration at different positions of the tension legs in the in-line (IL) and CF directions. Different lengths of the splitter plate behind the TLP columns were observed, and the suppression effect of the VIM at six flow velocities was investigated. The experimental results indicated the obvious VIM phenomenon of the platform without the splitter plate, and multi-frequency time-varying coupling characteristics were observed when the heading angle was 45°. Furthermore, the results revealed that the VIM phenomenon of the platform was evidently suppressed, in amplitude and frequency, and the force coefficients are reduced significantly, after the splitter plates were arranged behind each column of the platform. However, the splitter plate slightly affected the VIV of the tension legs.

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.

Nonlinear vibration model and response characteristic of drilling risers in deep-sea under soft suspension evacuation condition

In order to provide theoretical reference for the design and operation of soft suspension riser, firstly, a three-dimensional nonlinear vibration model of risers is established using the Hamilton variation and virtual work principles. In the model, three main nonlinear factors were taken into account: longitudinal-lateral (LG/LT) coupling vibration, vortex induced vibration (VIV) and large deformation. The finite element method (FEM) is used to realize the numerical solution of the nonlinear vibration model. Secondly, the correctness and effectiveness of the nonlinear vibration model is verified by the experimental measured data and ANSYS software. The progressiveness of the model is verified by comparing with the uncoupled model and hard suspension, and it demonstrate that, the proposed model can reflect the high nonlinearity of the riser system and describe the nonlinear vibration of the real riser more accurately. The natural frequency of the nonlinear model was much higher and the modal response more complex. The nonlinear coupling effects increase the vibration amplitude in the cross flow (CF) direction and the in-line (IL) direction. Thirdly, the model was used to analyze the effects of structure parameters, environmental parameters and operation parameters on the response characteristics of soft suspension evacuation riser. Some important phenomena have been found: reasonable allocation rate of buoyancy block can effectively restrain the vibration of riser. The optimal allocation rate of buoyancy block is related to the riser structure and operation conditions, which can be determined using proposed nonlinear model. Meanwhile, it is necessary to evacuate along the flow direction as much as possible, with an evacuation speed of lower than 1 m/s.