In-line Directions Research Articles

Flow-induced vibration of tandem flexible cylinders with a larger-diameter upstream cylinder

A systematic experimental study is conducted for the flow-induced vibration (FIV) of tandem flexible cylinder pairs with diameter ratios (D,u/D,d) ranging from 1 to 2. The diameter of the downstream cylinder (DC), D,d, is fixed at 16 mm, while that of the upstream cylinder (UC), D,u, evenly varies from 16 mm to 32 mm. Cylinders with a total length of 2.7 m and an immersion depth of 1.2 m are allowed to freely oscillate in the cross-flow and in-line directions. Results are examined for Re = 1600–11,200 (based on D,d) and a fixed streamwise separation between the cylinder centers T =6D,d. It is shown that the FIV of the DC (DC-FIV) is strongly sensitive to D,u/D,d, and that increasing D,u/D,d significantly weakens the high-mode and high-frequency response of the DC. A deeper analysis of the dual-cylinder interaction reveals the perturbation mechanism of the UC on the DC-FIV, i.e., the UC forces the DC to oscillate following its dominant frequency and dominant mode through “vibration control”. A map containing eight response patterns is further provided in the D,u/D,d–Vr plane. The Strouhal number rule between DC-FIV and its reduced velocity, which is broken by the UC, is reconstructed with a variation factor.

Investigation on the flow-induced vibration of tandem cylinders with varying diameters at subcritical Reynolds numbers

The flow interference among multiple cylinders is highly complex, and investigating it holds substantial engineering relevance. This paper considering three different arrangements at subcritical Reynolds numbers, a series of experiments were conducted to study the flow-induced vibrations (FIV) of rigid cylinders in tandem with variable diameters. Both cylinders are free to respond in both in-line (IL) and cross-flow (CF) directions. Utilizing a dynamic data acquisition system to obtain micro-strain time history information. The reference cylinder diameter (D) of 30 mm was employed as the standard for the spacing ratio (S/D). Both cylinders were exposed to uniform water flow at varying S/D: 3.0, 4.0, 6.0, and 8.0. A parametric investigation was conducted to analyze the effects of the relative spacing of the cylinders on the vibration response of the system. The results indicate that vibration intensity escalates as flow velocity increases. For the same cylinders, unsteady wave-induced vibrations (WIV) behind the upstream cylinder hinder the appearance of the lower branch of the amplitude response. Furthermore, the S/D substantially influences the cylinder vibration response. In the arrangement where the small cylinder is located upstream, the downstream cylinder undergoes WIV under smaller S/D, and the shielding effect is inconspicuous. In contrast, when the larger cylinder is located upstream, the shielding effect intensifies, leading to synchronous vibrations of the upstream and downstream cylinders. The S/D appears to exert minimal influence on the vibration under these conditions. In summary, the interference between cylinders in tandem configurations significantly impacts the vibration characteristics of the cylinders.

Vortex-induced vibration prediction of an inclined flexible cylinder based on machine learning methods

Vortex-induced vibration (VIV) is a main cause of fatigue damage to slender flexible structures in offshore engineering, e.g., marine risers, mooring lines, submarine cables and pipelines. In this paper, attempts have been made to predict the VIV response of an inclined flexible cylinder commonly encountered in practice using different machine learning methods. Three different machine learning algorithms (i.e., BP neural network, support vector machine and Gaussian process regression) are employed to establish the prediction models. The data in our previous experimental tests (Xu et al., 2018) are adopted to establish the data set of the present research. The numbers of data samples for constructing the displacement and frequency prediction models in each direction are 10,500 and 500, respectively. The inclination angle, incoming flow velocity, axial tension and the position along the cylinder span are used as the input variables and the output variables are the root mean square values of the in-line and cross-flow displacements and the dominant oscillation frequencies. Compared to the previous studies, the effect of the axial tension and the large inclination cases are considered to establish more comprehensive prediction models. The results show that all the three methods can predict the displacement and frequency responses in the cross-flow and in-line directions with acceptable accuracy. After a detailed comparative analysis of the three machine learning methods, it is found that the BP neural network models tend to result in a certain degree of overfitting and some response characteristics predicted by the GPR models are not consistent with the conclusions from our previous experimental studies. Within the parameter space of the present research, the support vector machine is considered to be the most suitable method for predicting the VIV of the inclined flexible cylinder.

CFD Simulation of Vortex Induced Motion of a Caisson during Installation Process

This paper presents the CFD investigation of the vortex induced motion (VIM) of a caisson model during the installation process. The simulations were performed by solving the unsteady 3D Navier-Stokes equation. The Dynamic Fluid-Body Interaction (DFBI) tool is utilized to simulate the motions. Typical draft conditions of a caisson during installation process are considered. The present methodology is verified through the free decay and the VIM of a cylinder simulations in two-dimension. Free decay simulations show that the natural frequency decrease with the increasing draft of the caisson. Maximum response amplitudes occur in the range of 10<Vr <13 for the draft Dp=0.1m. The resonance occurs when the crossflow response frequency f y is very close to the natural frequency f n. The inline/crossflow response frequency increases as the draft increases at the same reduced velocity V r. The response frequency in the inline direction is double of that in the crossflow direction. Eight-shaped trajectories are observed. The maximum displacements in the crossflow direction increase as the drafts increase.

Comparative study of spread and vertical mooring configuration in floating collar net cage under irregular waves and current

Offshore aquaculture is one of the national strategic programs of the Ministry of Fishery and Ocean Affairs of Indonesia to increase national fish cultivation capacity. One solution is using a simple flexible floating collar net which is suitable for small to medium cage volume. In designing such floating net, its mooring line configuration is vital to maintain fixed position of structural part of the cage volume. The purpose of this study is to compare design aspect of two types of mooring systems, i.e., spread and vertical configuration, by investigating selected parameters: mooring line tension, collar offset, and volume reduction. The mooring configuration and floating collar net cage are analysed through a numerical model based on Morison-force equations, under environmental condition adopted from Kangean field East Java. Load cases from collinearly in-line and between-line directions under extreme waves and current condition are checked against relevant safety factor. The dynamics analysis using the time domain simulation is based on 900 seconds cut-off according to its maximum wave elevation profile to generate the hydrodynamics forces to the mooring systems.

Experimental investigation on vortex/wake-induced force of double unequal-diameter flexible cylinders in tandem

An experimental investigation on the vortex/wake-induced force of double unequal-diameter cylinders in tandem was performed. In the experiment, the upstream-to-downstream cylinder diameter ratio was 1.569, and the gap spacing ratio between the cylinder models ranged from 3 to 8. Under uniform flow with Reynolds number up to 11 328, the strains caused by the vortex/wake-induced vibrations were measured for the double flexible cylinders. Based on the measured strains, the displacements were reconstructed with the mode superposition method. Then, the vortex/wake-induced forces were obtained inversely. Subsequently, the time-varying excitation and added mass coefficients were identified by the forgetting factor least squares method. The results show that the wake-induced force coefficient and excitation coefficient of the downstream cylinder are smaller than those of the single cylinder, while the added mass coefficient becomes larger. In addition, the downstream cylinder under frequency capture can absorb a higher proportion of energy from the wake to excite larger vibration amplitudes, especially in the in-line direction.

Experimental investigation of the effect of rotation rate and current speed on the dynamic response of riserless rotating drill string

Dynamic responses of rotating riserless drill string were investigated by varying rotation rate and current speed. Rotation rate ranged from 20 to 180 r/min, current speed ranged from 0.1 to 0.8 m/s, corresponding to Reynolds number of 1600–12,800 and rotation Reynolds number of 134–1206. Results show that the vibration energy of the rotating drill string is concentrated in rotation frequency, and its frequency doublings are stable in time series; equilibrium deflections are consistent in cross-flow (CF) and in-line (IL) directions and trajectory shows an oval-shape in still water. IL dominant frequency of rotating drilling string coupling VIV decreases, while CF dominant frequency and overall section dominant frequency increase with rotation rate. Vibration frequency transitions from high to low and its bandwidth increases in high shear-rate flow. Equilibrium deflection and vibration amplitude are higher than those of non-rotating drill string when Re ≤ 2400 due to the strong coupling of rotation and current. Rotation reduces mode participation, vibration envelopes are deflected to one side with both traveling and standing waves coexisting. Vibration amplitude reaches a maximum due to multiple resonances. Therefore, rotation rate near natural frequency should be avoided, and the rotation interference effect at low reduced velocity should not be overlooked.

On symmetric and alternating-symmetric patterns in the wake of a cylinder undergoing vortex-induced vibrations in the inline or close to the inline direction

We provide a quantitative study of two types of vortex-shedding patterns that are observed within the first response range of a flexibly-mounted cylinder allowed to oscillate only in the direction of flow (the inline direction) or directions close to the inline, and undergoing Vortex-Induced Vibration (VIV). We had recently shown that within this range, two types of shedding patterns are observed: a symmetric shedding, where two vortices of the same size are shed from the two sides of the cylinder simultaneously, and an alternating-symmetric pattern, where two vortices are still shed simultaneously, but with different sizes, and their sizes alternate in each cycle of oscillations. Here, we use bubble image velocimetry (BIV) results to quantitatively study these two types of wakes. We introduce a novel method of representing the wakes that we refer to as the “Vortex Arm” representation, which makes it possible to observe the differences between the two types of shedding very clearly. We discuss the paths of these vortices in the wake and observe that while the two vortices in a symmetric shedding do not interact, the large and small vortices shed in an alternating-symmetric pattern interact, resulting in deviations from their straight downstream paths and even upstream movements of the smaller vortex. We show how the symmetric and alternating-symmetric patterns are influenced when the direction of oscillations makes an angle with respect to the direction of the incoming flow.

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 on vortex-induced vibrations of a flexible cylinder subjected to multi-directional flows

Vortex-induced vibrations (VIVs) of a flexible cylinder subjected to multi-directional flows have been studied based on a wake oscillator model. The multi-directional flow comprises two slabs of flows in different directions, with each slab having a uniform uni-directional profile. The dynamics of the flexible cylinder is described based on the linear Euler–Bernoulli beam theory, and a wake oscillator model is uniformly distributed along the cylinder to model the hydrodynamic force acting on it. The dynamics of the coupled system has been solved numerically using the finite element method, and simulations have been conducted with the cylinder subjected to multi-directional flows with different angles between the two slabs. A large number of different initial conditions have been applied, and more than one steady-state response has been captured. The steady-state responses exhibit two different patterns: one is characterized by two waves traveling in opposite directions, while the other is dominated by a single traveling wave. The cross-flow VIV primarily occurs in the local cross-flow direction, and a transition of its vibrating direction happens at the interface of the two flows. Such transition is not observed in the inline VIV, and significant vibrations at the double frequency appear in both local cross-flow and inline directions. Energy analysis shows that this transition is boosted by a specific energy transfer pattern between the structure and the flow, which excites the vibration of the cylinder in some directions while damps it in others.

The flow around a forced cylinder between two parallel walls

The vortex induced vibrations of a cylinder tightly confined in a narrow channel were numerically studied in two dimensions. Attached to a linear spring and subject to hydrodynamic forces and torques, the circular cylinder is free to move in both the cross-flow and in-line directions as well as to rotate about its axis. Low mass ratios and moderate Reynolds numbers were considered. The cases where boundaries, either physical or numerical, are far from the cylinder have deserved much attention, and share some of the phenomenology observed in the system here studied. As the upstream flow is increased, the tightly confined system shows an initial branch, followed by a synchronization or lock-in region that ends in a decoherence region. Nevertheless, within the lock-in region, no clear upper and lower branches appear when the cylinder is placed in a narrow channel, instead the system's behavior sensibly depends on the mass ratio. We managed to find a modified Strouhal parameter that collapses the range of excitation for the reduced masses considered and suggests a scaling for the system's natural frequency in the medium. Results offer a portrait of the motion, forces, and flow around the cylinder under such constrains.

Forced oscillation experiments for each vibration mode of closely spaced parallel pipes in steady flow

In the industrially widespread use of multiple bundled pipes, the effects of mutual interference, in addition to the vibrations generated in single columns, must be considered, and estimating their behavior is critical to the design. In this study, based on previous research that identified the vibration modes occurring in two bundled pipes placed in a steady flow, such as (1) translational motion in the crossflow direction, (2) approaching and moving away, and (3) torsional motion, we developed four types of oscillators that can emulate the oscillation patterns (mode (2) uses different mechanisms for in-line and crossflow direction oscillation.), conducted forced oscillation experiments, and measured fluid forces. The mechanism of generation of the behavior patterns, measured in the towing and vibration measurement experiments of the elastic body model, was examined from the viewpoint of fluid force. A database of added mass and linear damping coefficients with parameters of distance between the center of the column and flow direction, in addition to oscillation patterns, was compiled for the development of a future two-tube behavior estimation program.A comparison of the measured fluid force of two columns with that of a single column, obtained by forced oscillation experiments for three vibration modes, showed that two closely spaced columns may vibrate over a wider frequency band and have larger amplitudes than a single column because of the expanded region of negative linear damping coefficients. Based on the results of the fluid force measurements for each vibration mode, rotational motion is more likely to occur in the tandem configuration, whereas approaching and moving away motion is more likely to occur in the side-by-side configuration because of the interchanged positive and negative added mass coefficients and strength of the negative linear damping coefficient.

Experimental Study on Vortex-Induced Vibration of Steel Tubes in Transmission Towers at Various Inflow Conditions

With the extensive construction of ultra-high voltage (UHV) transmission lines, the fatigue damage of steel tube members caused by vortex-induced vibration (VIV) in tubular towers has received growing attention. Although some progress has been made in the research of the VIV of steel tubes in uniform flow, there is still relatively scarce research on the VIV of steel tubes at various inflow conditions. In this paper, a series of wind tunnel tests are conducted to investigate the VIV of the steel tube subjected to uniform, turbulent, and oblique flows. Three turbulence intensities (Iu = 5.9%, 9.7%, and 12.6%), and four yaw angles (α = 10°, 20°, 30°, and 40°) are considered. The results show that the VIV response of the steel tube in the in-line (IL) direction is negligible compared to that in the cross-flow (CF) direction. The displacement amplitude gradually decreases as the turbulence intensity increases, accompanied by a more unstable response. For the inclined steel tube, the VIV maximum amplitude almost remains constant when α ≤ 20°, while it sharply decreased in the case of α = 30° and α = 40°. Furthermore, it was found that the so-called independent principle is applicable for α ≤ 10°.

Numerical investigation of noise suppression and amplification in forced oscillations of single and tandem cylinders in high Reynolds number turbulent flows

The noise generated by a three-dimensional turbulent wake behind an oscillating single or tandem cylinder(s) is studied using detached eddy simulation and the Ffowcs Williams-Hawkings (FW-H) method. This hybrid methodology is validated using some experimental data of a turbulent flow past a single or tandem stationary cylinder(s) and some numerical data (obtained from a direct numerical simulation) of a laminar flow past tandem oscillating cylinders. For an oscillating cylinder at a fixed Reynolds number Re=120,000, the sound energy is suppressed compared to that of a stationary cylinder provided the value of oscillation frequency approaches 57% of the original vortex-shedding frequency of the stationary cylinder. This noise reduction arises from the structural oscillations suppressing the noise generated by the vortex shedding from the stationary cylinder—the additional source of noise stemming from the structural oscillations is too small to compensate for the mechanism responsible for the noise suppression. The noise generated by the cylinder motion includes contributions from the loading and thickness noise, whereas the noise generated by vortex shedding from the stationary cylinder only includes the loading noise. The flow past tandem stationary cylinders exhibits a larger number of spectral peaks in the sound pressure power spectrum and the peaks occur at a lower frequency compared to the flow past a single cylinder. The drag force fluctuations result in the propagation of the sound pressure in the in-line direction, whereas the lift force fluctuations result in the propagation of the sound pressure in the transverse direction. Five different scenarios of stationary and/or oscillating tandem cylinders are studied. The second and third invariants of the velocity gradient tensor are used to visualize the distinct vortex structures in the wake for the five different scenarios of tandem cylinders. A single cylinder oscillating in the transverse direction exhibits a larger sound pressure level (SPL) than the streamwise oscillation of the same cylinder. In contrast, the streamwise (in-line) oscillation of one or the other cylinder in a tandem array results in a larger SPL than the transverse oscillation of one or the other cylinder. Finally, the SPL is more sensitive to the oscillations of the rear (downstream) cylinder than the front (upstream) cylinder.

Effects of surface roughness on two-degree-of-freedom vortex-induced vibration of a circular cylinder in oscillatory flow

The vortex-induced vibration (VIV) response performances of a two-degree-of-freedom circular cylinder with different surface roughnesses were numerically investigated in this work. Four typical roughnesses values of 1 × 10−4, 5 × 10−3, 1 × 10−2, and 2 × 10−2 were examined for the range of the reduced velocity (Vr = 1–20). The VIV response characteristics including the vibration amplitude, vibration frequency, VIV trajectory, vortex shedding flowing pattern, and hydrodynamic force for different rough cylinders were systematically compared. The numerical results showed that the VIV response of roughness cylinder experiences five evolutions as Vr increases, including five typical X–Y trajectories: centrosymmetry, “M” or “W,” “dough twists” or “candy,” “∞,” and asymmetrical ∞, exhibiting three vortex shedding flow pattern: S mode, C + S mode, and SS mode (a pair of symmetric vortices along the x-axis) under certain conditions. The VIV response was sensitive to the roughness in regime IV and regime V. Furthermore, the normalized vibration frequency of the rough cylinder was an integer multiple of the oscillatory flow frequency, except for the Vr ranged from 6 to 7. The vibration frequency in in-line (IL) direction was consistent with the frequency of the oscillatory flow, which was immune to the change of roughness. Additionally, when roughness was equal to 2 × 10−2, the reduction of maximum vibration amplitude in the cross-flow direction reached 31.2%, comparing with smooth cylinder, whereas the maximum vibration amplitude in the IL direction increased only by 6.2%.

Effect of internal solitary wave on the dynamic response of a flexible riser

Dynamic responses can arise when a flexible riser encounters the internal solitary wave (ISW) in the ocean. It is, thus, necessary to explore its dynamical behaviors induced by ISW. First, the governing equations of the flexible riser, ISW, and hydrodynamic force are introduced and interpreted. The accordant validations are performed so as to examine the accuracy of the applied models. Then, the dynamic response of the flexible riser is studied with the increase of the nondimensional ISW amplitude. Representative characteristics, such as vortex-induced vibration (VIV), top tension, and stress along the flexible riser, are mainly analyzed and discussed. The results prove that with the propagation of ISW, VIV in both inline (IL) and crossflow (CF) directions can be incurred. The maximum values of IL and CF oscillation frequencies, top tension, inline deflection, as well as riser stress can be detected while the ISW trough reaches to the flexible riser. Furthermore, all these four dynamic characteristics show an increasing trend with the increase of the nondimensional ISW amplitude. Note that the maximum values of oscillation frequencies, inline deflection, and stress on the upper section of the flexible riser tend to be larger compared with those on the lower section. In addition, the maximum inline deflection, top tension, and stress along the flexible riser can be affected by VIV responses.

Hydrodynamic coefficients of a forced oscillating flexible pipe with energy competition model

The objective of this study is to reveal the characteristics of hydrodynamic force and coefficients undergoing vortex-induced vibration (VIV) of a flexible pipe in an oscillatory flow. The flexible pipe is forced to oscillate with different amplitudes and periods in the still water to simulate the equivalent oscillatory flow. Different from the traditional model, the hydrodynamic forces for the flexible pipe under the new energy competition force model in both in-line (IL) and cross-flow (CF) directions are divided into drag, excitation and added mass force. The identification methods of the hydrodynamic coefficients for the energy competition force model are then established. The detailed features of the hydrodynamic coefficients under the new force model in an oscillatory are illustrated. The distribution comparison of the two forces, i.e., drag force plays a role in energy dissipation and excitation force acts as energy input, explains the differences of the VIV response in magnitudes under different oscillatory flows. By summarizing the hydrodynamic coefficients, the drag and excitation coefficients are found to be inverse proportion to the Keulegan-Carpenter (KC) number. The independence of the added mass coefficients and KC number is revealed. On the basis of the observation, the empirical models for hydrodynamic coefficients are preliminarily proposed. The effects of the oscillatory flow on the hydrodynamics are established to be determined by the KC numbers under the same reduced velocity. The present investigations will provide a better guideline for developing flow-induced vibration prediction methods in future work.

Experimental investigation on nonlinear dynamic response of towing cable under vessel motion

Marine towing cable is a crucial equipment of the underwater towing system which can acquire the marine resources distribution in the sea. The marine towing cable will experience nonlinear dynamic problems due to the vortex shedding and vessel motion during the towing movement. In order to investigate the dynamic response characteristics of the towing cable during the working process, a set of experimental devices for towing cable is designed and a PPR (Polypropylene-Random) pipe with the aspect ratio of 150 is tested with coupled uniform towing and vessel motion. The characteristics of response amplitude and frequency under uniform towing are compared and analyzed comprehensively with different towing speeds and towing depths. The key characteristics of nonlinear dynamic response under vessel motion are then discussed. Some phenomena involving multi-modal participation, frequency transition and amplitude modulation are captured. The vessel motion will excite ωCT - ωT, ωCT + ωT, ωT frequency components in the CF (cross-flow) direction which may be caused by the parametric excitation, and wave frequency ωT dominates the IL (in-line) direction response. Some extreme motion conditions may enlarge the response displacement in the IL and CF direction. It is suggested that the vessel motion should be taken into account in the evaluation of the nonlinear dynamic analysis of marine towing cable.

Experimental study on dynamic responses of a tensioned flexible vertical cylinder under waves and combined current and waves

The two degree-of-freedom (2DOF) dynamic responses of a flexible vertical cylinder under waves only and combined current and waves in a finite water depth were experimentally studied. The cylinder has multi-frequency response in both in-line (IL) and crossflow (CF) directions under waves. The response frequencies in the IL direction are multiples of wave frequency. The CF response frequencies include the multiples of wave frequency and the natural frequency of the cylinder, indicating that the CF response is controlled by both the wave actions and the vortex shedding. When the natural frequency of the cylinder is a multiple of the wave frequency, the CF vibration has the largest amplitude. A top tension can suppress the CF response more than the IL response amplitude, and the suppression effect increases with the increase of the top tension. With the decrease of the water depth, the response amplitudes in both directions decrease. Under combined current and waves, the IL response amplitude increases with the increase of KC number and the primary frequency of IL response is still the wave frequency. For a current velocity where VIV occurs, the superimposition of waves on current reduces the CF response amplitude because the vortex shedding frequency moves away from the natural frequency. The multi-frequencies of CF response are related to both the natural frequency of the cylinder and the wave frequency. The trajectories of the cylinder are mainly dependent on the ratio of the natural frequency to the wave frequency under waves or combined current and waves. There can be 2–6 cycles in the CF direction for each IL cycle with different wave frequencies and the top tension.

Coupled cross-flow and in-line vibration characteristics of frequency-locking of marine composite riser subjected to gas-liquid multiphase internal flow

In this paper, the frequency locking characteristics of marine composite riser coupled with cross-flow (CF) and in-line (IL) under the combined action of internal and external ocean currents in gas-liquid multiphase transportation are studied. Based on Hamilton's variational principle, the coupled vibration control equations of composite marine riser in CF and IL directions were established considering the combined action of internal and external ocean currents in gas-liquid multiphase transport. The Van der Pol wake oscillator model was used to simulate the flow force of the ocean current on the CF and IL directions of the slender riser. Newmark-β and fourth-order Runge-Kutta coupling iterative method were used to solve the coupled dynamics equation. The effectiveness and accuracy of the proposed method were verified by comparing with the results in literature. Subsequently, the effects of liquid phase velocity, fiber orientation angle and top axial tension on the frequency-locking characteristics of marine riser were given. The research results show that the internal flow velocity, external ocean current velocity, fiber orientation angle and top axial tension have a great influence on the coupled vibration stability and frequency locking characteristics of the riser. In addition, compared with the uniform single internal flow, the critical value of the velocity of the gas-liquid multiphase mixed internal flow is larger; The frequency-locking interval of the riser “shifts to the right” with the increase of liquid flow velocity and fiber orientation angle, and “shifts to the left” with the increase of top axial tension. It is also found that the frequency locking range of the riser decreases with the increase of fiber orientation angle and top axial tension, and expands with the increase of liquid flow velocity.