Method for analyzing vortex-induced vibration characteristics of production riser under coupling of internal and external flow

Abstract

The application discloses a method for analyzing vortex-induced vibration characteristics of a production riser under the coupling action of internal and external flows, and establishes a numerical analysis model of the production riser under the coupling action of internal and external flows to determine the vortex-induced vibration characteristics of the riser containing two-phase flow. The model is analyzed and solved by using the finite element method and the Newmark-beta method, and the vortex-induced vibration response law of the production riser under the coupling action of internal and external flows is summarized. The method for analyzing the vortex-induced vibration characteristics of the production riser under the coupling action of external sea currents and internal multiphase flow provides effective theoretical guidance for the safety of the multiphase flow production riser.

Description

Technical Field

[0001] This invention relates to the field of oil and gas development, specifically to a method for analyzing the vortex-induced vibration characteristics of production risers under the coupling effect of external ocean currents and internal multiphase flows. Background Technology

[0002] Deepwater oil and gas risers are crucial components of oil and gas development systems, serving as vital structures connecting the seabed and production platforms. Under the influence of incoming currents, risers generate alternating wake vortices, leading to vortex-induced vibration (VEM). Prolonged vibration can reduce riser strength and increase the risk of failure. Therefore, the VEM response of marine risers has received continuous attention from researchers. Extensive studies have been conducted on the VEM response of marine risers under external environmental loads, particularly for flexible risers with high aspect ratios. These studies primarily analyze the modes, frequencies, and standing wave and traveling wave responses of VEM, laying the foundation for a deeper understanding of the VEM mechanism and response laws.

[0003] Compared with traditional risers containing pure liquid flow, gas-liquid two-phase flow in the riser will cause a decrease in the riser's natural frequency and an increase in the vibration amplitude and frequency. An increase in the flow rate and the two-phase mixing density in the riser will cause a simultaneous decrease in the riser's natural frequency and vibration response frequency, while its modal response remains unchanged. As the air intake ratio increases, the riser's natural frequency will also decrease, but the vibration frequency will increase, thereby inducing the riser to generate higher-order modes of vibration.

[0004] Most current studies on riser vortex-induced vibration response only consider the influence of external marine environmental conditions or the fluid inside the riser. Research considering the coupling effect of internal and external flows is scarce, and the understanding of the riser vortex-induced vibration response characteristics and mechanisms under the coupling effect of internal multiphase flow dynamic phase change and external marine environmental load parameter changes during oil and gas extraction remains unclear. Summary of the Invention

[0005] The purpose of this invention is to provide a method for analyzing the vortex-induced vibration characteristics of mining risers under the coupling effect of internal and external flows, and to provide effective theoretical guidance for the safety of multiphase flow mining risers.

[0006] To achieve the above objectives, an analysis method for the vortex-induced vibration characteristics of mining risers under the coupling effect of internal and external flows is proposed. This method includes a numerical analysis model of the mining riser considering the coupling effect of internal and external flows, and an analysis method for the multiphase flow model inside the riser. These methods include mechanical model analysis, fluid dynamics model analysis, wake oscillator model analysis, and two-phase flow model analysis within the riser. The finite element method and the Newmark-β method are used to solve the model. The specific solution steps are shown below:

[0007] S1: Simulate the vortex-induced vibration process of the deep-sea riser containing two-phase flow based on parameters such as the magnitude of the external ocean current velocity, riser length, riser inner and outer diameters, and internal flow velocity.

[0008] S2: Analyze the forces on the mining riser unit and the fluid unit section inside the pipe, establish a mechanical model of the mining riser considering the coupling effect of internal and external flows, and on this basis, establish a two-phase flow model considering the influence of the flow pattern inside the pipe on the dynamic response of the riser, and calculate the mass and velocity distribution of the fluid inside the pipe;

[0009] S3: Combine the calculated fluid mass and velocity distribution inside the pipe with the mining riser model, and use the Newmark-β method based on the boundary conditions of the model to solve the mining riser mechanical model analysis method considering the coupling effect of internal and external flows;

[0010] S4: Based on the calculated dynamic response of the riser considering the coupling effect of internal and external flows, the intensity and frequency of the vortex-induced vibration of the riser were simulated and analyzed, and the response law of the vortex-induced vibration of the mining riser under the coupling effect of internal and external flows was summarized.

[0011] Furthermore, the two-phase flow inside the two-phase flow riser in S1 can generate dynamic excitation on the mining riser. When the two-phase flow is transported in the mining riser, the mining riser is further simplified into an internal transport multiphase flow riser with simple support at both ends. The riser is placed in the transversely flowing ocean current and undergoes large deformation and bending. At the same time, as the seawater bypasses the cylindrical pipe, it will generate vortex-induced force due to vortex shedding, which will act on the mining riser, thereby causing the riser to generate vortex-induced vibration.

[0012] Furthermore, in S2, during the derivation of the basic equations for phase flow, the two phases are treated as single-phase flows and the interphase interactions are taken into account. Then, the equations for each phase are combined. The specific steps are as follows:

[0013] Based on the force analysis diagram of the fluid micro-element segment, the force balance equation in the x-direction of the multiphase flow micro-element segment inside the pipe can be obtained:

[0014]

[0015] Force balance equations in the x-direction of a small segment of the riser:

[0016]

[0017] Shear force Q and bending moment M can be expressed as:

[0018]

[0019] Solving equations (1), (2), (3), and (4) simultaneously, we can obtain the differential equation of motion of the deep-water riser in the downstream direction (x direction) for two-phase flow as follows:

[0020]

[0021] Similarly, the differential equation of motion in the crossflow direction (y-direction) of the mining riser can be obtained:

[0022]

[0023] Where: EI—bending stiffness (N·m) 2 x—displacement in the downstream direction (m); y—displacement in the cross direction (m); z—axial position of the riser (m); T—section tension (N); c—structural damping; F x (z,t) — Downstream marine environmental load (N);

[0024] F y (z,t) — Cross-current marine environmental load (N); v l v g —The velocity of gas and liquid (m / s); m r m l With m g —Mass per unit length of riser, mass of liquid and gas per unit length of riser (kg);

[0025] In the axial direction, the riser needs to consider its own weight and the weight of the fluid inside, therefore the effective tension of the riser cross-section varies along the pipe. Besides the top tension and its own buoyancy, the upper riser also bears the weight of the lower riser; therefore, the tension on the cross-section is T = T(z), expressed as:

[0026]

[0027] In the formula: T top —Top tension (N); ρ w —Seawater density (kg / m³) 3 D0—Outer diameter of riser (m).

[0028] Furthermore, the boundary conditions in S3 are: The vibration control matrix equation of the mining riser in the global coordinate system was calculated using the Newmark-β method.

[0029] Where [M], [C], and [K] are the mass matrix, damping matrix, and stiffness matrix, respectively; {u} represents the riser's acceleration, velocity, and displacement vectors, respectively; {f} represents the external fluid force load.

[0030] Furthermore, in S4, during the vibration solution of the mining riser, the computation time step is kept consistent, and the wake oscillator model is coupled with the riser control equation for iterative calculation. The displacement time history response of the mining riser in the downstream and cross-flow directions can be obtained respectively. By performing a fast Fourier transform on the displacement time history curve, the frequency response of the mining riser in the downstream and cross-flow directions can be obtained.

[0031] Furthermore, in S4, the undamped free vibration of the suspended riser was calculated using differential control equations to obtain the natural frequency of the model. A comparative analysis of the vortex-induced vibration characteristics under gas-liquid two-phase flow and pure liquid flow, the comparative analysis of the vortex-induced vibration characteristics under different displacements, the comparative analysis of the vortex-induced vibration characteristics under different two-phase mixing densities, and the comparative analysis of the vortex-induced vibration characteristics under different intake ratios was summarized.

[0032] Based on the above analysis method, a dynamic analysis model of the riser system under the coupling effect of internal and external flows was established to analyze the vortex-induced vibration characteristics of the riser containing gas-liquid two-phase flow. The effects of factors such as displacement, mixed fluid density, and inlet ratio on vortex-induced vibration were studied. The present invention has the following conclusions:

[0033] (1) The method for analyzing the vortex-induced vibration characteristics of a mining riser under the coupling effect of internal and external flow provided by this invention shows that the liquid two-phase flow will cause the natural frequency of the riser to decrease. Under the same external excitation, this results in a larger vibration amplitude, a larger vibration frequency, and a higher-order modal response. Therefore, in practical engineering problems, the influence of the gas-liquid two-phase flow inside the pipe on the vortex-induced vibration of the riser should be the focus.

[0034] (2) The method for analyzing the vortex-induced vibration characteristics of a mining riser under the coupling effect of internal and external flows provided by this invention shows that as the fluid discharge and fluid mixing density inside the pipe increase, the axial tension of the riser decreases, the natural frequency decreases, and the vibration amplitude increases. Due to the frequency locking phenomenon at this time, the vibration frequency shows a trend of decreasing synchronously with the natural frequency, while the mode shape remains unchanged.

[0035] (3) The method for analyzing the vortex-induced vibration characteristics of the mining riser under the coupling effect of internal and external flow provided by the present invention shows that the increase in the air intake ratio leads to a decrease in riser stiffness and an increase in vibration amplitude. As the gas content in the pipe increases, the two-phase flow in the pipe will induce a higher-order mode of vibration in the riser, and both the vibration frequency and mode shape will increase. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the model solution process of this invention;

[0037] Figure 2 This is a schematic diagram of the simplified physical model of the present invention;

[0038] Figure 3 This is a force diagram of a micro-element segment of the riser pipe according to the present invention;

[0039] Figure 4 This is a force diagram of a fluid micro-element segment according to the present invention;

[0040] Figure 5 This is a schematic diagram of the motion of a certain cross section of the riser pipe for mining under ocean currents at a certain moment according to the present invention;

[0041] Figure 6 This is a schematic diagram of the fluid dynamics outside the mining riser of the present invention;

[0042] Figure 7 This is a schematic diagram of the physical model for mass conservation of the fluid unit in this invention;

[0043] Figure 8 This is a comparison diagram of the motion trajectories at the midpoint of the vertical pipe for gas-liquid two-phase flow and pure liquid flow according to the present invention;

[0044] Figure 9 This is a comparison diagram of the motion trajectory at the midpoint of the riser under different displacements according to the present invention;

[0045] Figure 10 This is a comparison diagram of the motion trajectory at the midpoint of the riser under different two-phase mixing densities according to the present invention;

[0046] Figure 11 This is a comparison diagram of the movement trajectory at the midpoint of the riser under different air intake ratios according to the present invention; Detailed Implementation

[0047] To better understand the purpose, structure, and function of this invention, the following detailed description, in conjunction with the accompanying drawings, provides an analysis method for the vortex-induced vibration characteristics of a mining riser under the coupling effect of internal and external flows.

[0048] In the description of this invention, it should be noted that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0049] like Figure 1-4As shown, in a marine environment, the riser is subjected to both its own weight and the effects of external ocean currents and internal multiphase flows. External ocean currents alternately release vortices on the riser surface, generating vortex-induced vibrations. The mass, density, and other parameters of the internal multiphase flow change over time, and the two-phase flow within the riser can exert dynamic excitation on it. Analysis methods include mechanical modeling, fluid dynamics modeling, wake oscillator modeling, and two-phase flow modeling. The finite element method and the Newmark-β method are used to solve the model, and the specific solution steps are shown below:

[0050] S1: Simulate the vortex-induced vibration process of the deep-sea riser containing two-phase flow based on parameters such as the magnitude of the external ocean current velocity, riser length, riser inner and outer diameters, and internal flow velocity.

[0051] S2: Analyze the forces on the mining riser unit and the fluid unit section inside the pipe, establish a mechanical model of the mining riser considering the coupling effect of internal and external flows, and on this basis, establish a two-phase flow model considering the influence of the flow pattern inside the pipe on the dynamic response of the riser, and calculate the mass and velocity distribution of the fluid inside the pipe;

[0052] S3: Combine the calculated fluid mass and velocity distribution inside the pipe with the mining riser model, and use the Newmark-β method based on the boundary conditions of the model to solve the mining riser mechanical model analysis method considering the coupling effect of internal and external flows;

[0053] S4: Based on the calculated dynamic response of the riser considering the coupling effect of internal and external flows, the intensity and frequency of the vortex-induced vibration of the riser were simulated and analyzed, and the response law of the vortex-induced vibration of the mining riser under the coupling effect of internal and external flows was summarized.

[0054] The two-phase flow inside the two-phase flow riser in S1 can generate dynamic excitation on the mining riser. When the two-phase flow is transported in the mining riser, the mining riser is further simplified into an internal transport multiphase flow riser with simple support at both ends. The riser is placed in the transversely flowing ocean current and undergoes large deformation and bending. At the same time, as the seawater bypasses the cylindrical pipe, it will generate vortex-induced force due to vortex shedding, which will act on the mining riser, thereby causing the riser to generate vortex-induced vibration.

[0055] In S2, during the derivation of the basic equations for phase flow, the two phases are treated as single-phase flows and the interphase interactions are taken into account. Then, the equations for each phase are combined. The specific steps are as follows:

[0056] Based on the force analysis diagram of the fluid micro-element segment, the force balance equation in the x-direction of the multiphase flow micro-element segment inside the pipe can be obtained:

[0057]

[0058] Force balance equations in the x-direction of a small segment of the riser:

[0059]

[0060] Shear force Q and bending moment M can be expressed as:

[0061]

[0062] Solving equations (1), (2), (3), and (4) simultaneously, we can obtain the differential equation of motion of the deep-water riser in the downstream direction (x direction) for two-phase flow as follows:

[0063]

[0064] Similarly, the differential equation of motion in the crossflow direction (y-direction) of the mining riser can be obtained:

[0065]

[0066] Where: EI—bending stiffness (N·m) 2 x—displacement in the downstream direction (m); y—displacement in the cross direction (m); z—axial position of the riser (m); T—section tension (N); c—structural damping; F x (z,t) — Downstream marine environmental load (N);

[0067] F y (z,t) — Cross-current marine environmental load (N); v l v g —The velocity of gas and liquid (m / s); m r m l With m g —Mass per unit length of riser, mass of liquid and gas per unit length of riser (kg);

[0068] In the axial direction, the riser needs to consider its own weight and the weight of the fluid inside, therefore the effective tension of the riser cross-section varies along the pipe. Besides the top tension and its own buoyancy, the upper riser also bears the weight of the lower riser; therefore, the tension on the cross-section is T = T(z), expressed as:

[0069]

[0070] In the formula: T top —Top tension (N); ρ w —Seawater density (kg / m³) 3 D0—Outer diameter of riser (m).

[0071] Furthermore, the boundary conditions in S3 are: The vibration control matrix equation of the mining riser in the global coordinate system was calculated using the Newmark-β method.

[0072] Where [M], [C], and [K] are the mass matrix, damping matrix, and stiffness matrix, respectively; {u} represents the riser's acceleration, velocity, and displacement vectors, respectively; {f} represents the external fluid force load.

[0073] The classical Morison equations are used to discuss the fluid forces acting on the riser. Assume the external ocean current velocity at a certain depth within the riser is U. c A schematic diagram of the cross-section of the riser under the ocean current at a certain moment is shown below. Figure 5 As shown.

[0074] Assume the relative velocity between the ocean current and the drilling riser is V. r Based on the relative velocities of the fluid in the downstream and cross-flow directions within the riser, the relative velocity of the ocean current relative to the riser can be obtained. From this relative velocity, the steady-state drag force and lift acting on the riser can be calculated. The drag force is directed along the direction of the relative fluid velocity, and the lift force is perpendicular to the drag force direction. A schematic diagram of the external fluid forces acting on the riser is shown below. Figure 6 As shown.

[0075] according to Figure 6 We can obtain:

[0076]

[0077] Based on the force diagram of the riser and the Morrison equation, the steady-state drag force and lift acting on the riser can be obtained:

[0078] The steady-state drag force acting on the mining riser is:

[0079]

[0080] The steady-state lift force acting on the mining riser is:

[0081]

[0082] Based on the force analysis, the fluid force components acting on the riser in the x and y directions can be obtained as follows:

[0083]

[0084] The pulsating drag force F acting on the mining riser D 'and pulsating lift F L 'for:

[0085]

[0086] Among them, C D With C LThese are the pulsating drag force coefficient and the pulsating lift force coefficient, respectively.

[0087] Therefore, combining equations (12) and (13) with equation (9), the external fluid force acting on the mining riser can be obtained as follows:

[0088]

[0089] Because the outer cross-section of the mining riser is circular, the steady-state lift coefficient is... The value is usually taken as 0. Steady-state drag coefficient. Let the value be 1.2, and substitute it into equation (14). Also, assume... Ignoring the influence of higher-order terms, the fluid force expression can be simplified to:

[0090]

[0091] In the formula, the pulsating drag force coefficient C D With the pulsating lift coefficient C L It is obtained through the variables of the wake oscillator.

[0092] In the differential equation of riser motion, m g m l With ν g ν l The flow parameters are the most important physical parameters for solving the riser motion equations considering the gas and liquid distribution within the pipe, and their values ​​depend on the multiphase flow distribution within the pipe. During riser operation, the fluid inside the pipe returns to the sea surface along the riser. As the temperature rises and the pressure inside the pipe gradually decreases, the gas gradually expands and bursts, increasing the gas content and changing the flow pattern. This paper establishes a numerical model for calculating the flow parameters to address the gas-liquid two-phase flow problem.

[0093] Gas phase mass conservation physical model such as Figure 7 As shown. According to the law of conservation of mass, the change in mass of the control unit is the input mass of the control unit minus the output mass. For the gas phase, the input mass of the control unit is:

[0094] ρ g ν g E g Adt+q g The output quality of the dtdz(21) control unit is:

[0095]

[0096] The change in internal mass caused by the change in gas porosity is as follows:

[0097]

[0098] Therefore, the gas phase continuity equation inside the pipe can be expressed as:

[0099]

[0100] The continuity equation for the liquid phase inside the extraction riser is:

[0101]

[0102] Where: ρ—density of the gas / liquid phase (kg / m³) 3 );

[0103] ν—velocity of the gas / liquid phase (m / s);

[0104] E g —Gas porosity ( / );

[0105] E l —Liquid holdup; ( / )

[0106] A—Cross-sectional area inside the pipe (m²) 2 );

[0107] B m —Local volume coefficient of liquid inside the pipe;

[0108] R ms —Local solution gas-liquid ratio;

[0109] ρ gs —Gas density under standard conditions (kg / m³) 3 );

[0110] q g —Gas production ( / ).

[0111] According to the law of conservation of momentum, the rate of change of an object's momentum with time is equal to the sum of the external forces applied to the object. Therefore, the following momentum equation can be obtained:

[0112]

[0113] For gases, the momentum equation can be expressed as:

[0114]

[0115] For liquids, the momentum equation can be expressed as:

[0116]

[0117] In the control unit,

[0118] E g +E l =1 (29)

[0119]

[0120] Therefore, the total momentum equation can be written as:

[0121]

[0122] During mining, the pressure inside the pipe changes with water depth, fluid mass, and velocity. The pressure drop inside the pipe can be expressed as:

[0123]

[0124] The masses of the gas and liquid phases inside the tube can be expressed as follows:

[0125]

[0126] In the formula: f x — is the coefficient of flow friction ( / );

[0127] D0—Outer diameter of the mining riser (m);

[0128] D i —Inner diameter of the mining riser (m);

[0129] The main modes of gas-liquid two-phase flow are bubble, slug, agitation and annular flow

[36] , which are expressed as follows:

[0130] Bubble flow:

[0131] ν sg ≤0.429×ν sl +0.357×ν oo (34)

[0132] Slug flow:

[0133] ν sg >0.429×ν sl +0.357×ν oo (36)

[0134] Agitated flow:

[0135] Circular flow:

[0136]

[0137] In the formula: ν sg —Surface velocity of the gas (m / s);

[0138] ν sl —Surface velocity of the liquid (m / s);

[0139] νoo —The limiting upward velocity of the bubble (m / s);

[0140] σ—Surface tension (N / m).

[0141] In S4, during the vibration solution of the mining riser, the computation time step is kept consistent. The wake oscillator model is coupled with the riser control equation for iterative calculation. The displacement time history response of the mining riser in the longitudinal and transverse directions can be obtained respectively. By performing a fast Fourier transform on the displacement time history curve, the frequency response of the mining riser in the longitudinal and transverse directions can be obtained.

[0142] The undamped free vibration of the suspended riser was calculated using differential control equations in S4, and the natural frequency of the model was obtained. The comparative analysis of vortex-induced vibration characteristics under gas-liquid two-phase flow and pure liquid flow, the comparative analysis of vortex-induced vibration characteristics under different displacements, the comparative analysis of vortex-induced vibration characteristics under different two-phase mixing densities, and the comparative analysis of vortex-induced vibration characteristics under different intake ratios were summarized.

[0143] like Figure 8-11 As shown in the comparative analysis of vortex-induced vibration characteristics under gas-liquid two-phase flow and pure liquid flow, it can be seen that, compared with pure liquid flow, under the same external excitation, the gas-liquid two-phase flow inside the pipe has a stronger excitation effect on the riser, resulting in a larger vibration amplitude, a higher vibration frequency, and a higher-order modal response. Therefore, in practical engineering problems, the influence of gas-liquid two-phase flow inside the pipe on the vortex-induced vibration of the riser should be given special attention.

[0144] Comparative analysis of vortex-induced vibration characteristics under different displacements reveals that, under the same external excitation, the increase in the gas-liquid two-phase mixing velocity inside the pipe significantly affects the riser stiffness with increasing displacement. This leads to a decrease in effective cross-sectional tension, a larger vibration amplitude, and a lower vibration frequency, while the modal response remains unchanged. In practical engineering, while ensuring production output, appropriately reducing the fluid displacement inside the pipe can help reduce vortex-induced vibration of the riser.

[0145] Comparative analysis of eddy-induced vibration characteristics under different two-phase mixing densities shows that, under the same external excitation, as the two-phase mixing density increases, the vibration amplitude of the riser increases, the vibration response frequency decreases, and the mode shape remains unchanged.

[0146] Comparative analysis of vortex-induced vibration characteristics under different air intake ratios shows that, under the same external excitation, as the air intake ratio increases, the natural frequency of the riser decreases, while the vibration response frequency and vibration amplitude both increase. Furthermore, the increase in air intake ratio will excite higher-order mode shapes of the riser.

[0147] The present invention leads to the following conclusions:

[0148] By establishing a dynamic analysis model of a riser system under the coupling effect of internal and external flows, the vortex-induced vibration characteristics of a riser containing gas-liquid two-phase flow were analyzed. The effects of factors such as displacement, mixed fluid density, and inlet ratio on vortex-induced vibration were studied. It can be seen that gas-liquid two-phase flow will cause a decrease in the natural frequency of the riser. Under the same external excitation, it will result in a larger vibration amplitude, a higher vibration frequency, and a higher order modal response. Therefore, in practical engineering problems, the influence of gas-liquid two-phase flow inside the pipe on the vortex-induced vibration of the riser should be given special attention.

[0149] As the fluid discharge and mixing density inside the pipe increase, the axial tension of the riser decreases, the natural frequency decreases, and the vibration amplitude increases. Due to frequency locking, the vibration frequency decreases synchronously with the natural frequency, while the mode shape remains unchanged.

[0150] An increase in the intake ratio leads to a decrease in riser stiffness and an increase in vibration amplitude. As the gas content inside the pipe increases, the two-phase flow inside the pipe will induce a higher-order mode of vibration in the riser, with both the vibration frequency and mode shape increasing.

[0151] It is understood that, as described through some embodiments, various changes or equivalent substitutions can be made to these features and embodiments by those skilled in the art without departing from the spirit and scope of the invention. Furthermore, under the teachings of this invention, modifications can be made to these features and embodiments to adapt to specific circumstances and materials without departing from the spirit and scope of the invention. Therefore, this invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of this invention.

Claims

1. A method for analyzing the vortex-induced vibration characteristics of a mining riser under the coupling effect of internal and external flows, characterized in that, Numerical analysis models and multiphase flow analysis methods for mining risers considering the coupling effect of internal and external flows are proposed. These include mechanical model analysis methods, fluid dynamic model analysis methods, wake oscillator model analysis methods, and two-phase flow model analysis methods within the riser. The finite element method and Newmark-β method are used to solve the models. The specific solution steps are shown below: S1: Simulate the vortex-induced vibration process of a deep-sea riser containing two-phase flow based on the magnitude of the external ocean current velocity, riser length, riser inner and outer diameters, and internal flow velocity. S2: Analyze the forces on the mining riser unit and the fluid unit section inside the pipe, establish a mechanical model of the mining riser considering the coupling effect of internal and external flows, and on this basis, establish a two-phase flow model considering the influence of the flow pattern inside the pipe on the dynamic response of the riser, and calculate the mass and velocity distribution of the fluid inside the pipe; S3: Combine the calculated fluid mass and velocity distribution inside the pipe with the mining riser model, and use the Newmark-β method based on the boundary conditions of the model to solve the mining riser mechanical model analysis method considering the coupling effect of internal and external flows; S4: Based on the calculated dynamic response of the riser considering the coupling effect of internal and external flows, the strength and frequency of the vortex-induced vibration of the riser are simulated and analyzed to obtain the vortex-induced vibration response law of the mining riser under the coupling effect of internal and external flows. In the derivation of the basic equations for phase flow in S2, the two phases are treated as single-phase flows and the interphase interactions are taken into account. Then, the equations for each phase are combined. The specific steps are as follows: Based on the force analysis diagram of the fluid micro-element segment, the force balance equation in the x-direction of the multiphase flow micro-element segment inside the pipe is obtained as follows: The force balance equation in the x-direction of the micro-segment of the mining riser is as follows: The shear force Q and bending moment M are: Combining the above equations, we obtain the differential equation for the downstream (x-direction) motion of a deep-water riser in two-phase flow: The differential equation of motion in the crossflow direction (y-direction) of the extraction riser is obtained as follows: Where: EI—bending stiffness (N·m) 2 x—displacement in the downstream direction (m); y—displacement in the cross direction (m); z—axial position of the riser (m); T—section tension (N); c—structural damping; F x (z,t) — Downstream marine environmental load (N); F y (z,t) — Cross-current marine environmental load (N); v l v g —The velocity of gas and liquid (m / s); m r m l With m g —Mass per unit length of riser, mass of liquid and gas per unit length of riser (kg); In the axial direction, the effective tension of the riser section varies along the pipe due to the riser's own weight and the weight of the fluid inside. Besides the tension at the top and its own buoyancy, the upper riser also bears the weight of the lower riser. Therefore, the tension on the section is T = T(z), expressed as: In the formula: T top —Top tension (N); ρ w —Seawater density (kg / m³) 3 D0—Outer diameter of riser (m).

2. The method for analyzing the vortex-induced vibration characteristics of a mining riser under the coupling effect of internal and external flows as described in claim 1, characterized in that, The two-phase flow inside the two-phase flow riser in S1 can generate dynamic excitation on the mining riser. When the two-phase flow is transported in the mining riser, the mining riser is further simplified into an internal transport multiphase flow riser with simple support at both ends. The riser is placed in the transversely flowing ocean current and undergoes large deformation and bending. At the same time, as the seawater bypasses the cylindrical pipe, it will generate vortex-induced force due to vortex shedding, which will act on the mining riser, thereby causing the riser to generate vortex-induced vibration.

3. The method for analyzing the vortex-induced vibration characteristics of a mining riser under the coupling effect of internal and external flows as described in claim 1, characterized in that, The boundary conditions in S3 are: The vibration control matrix equation of the mining riser in the global coordinate system was calculated using the Newmark-β method. Where [M], [C], and [K] are the mass matrix, damping matrix, and stiffness matrix, respectively; {u} represents the riser's acceleration, velocity, and displacement vectors, respectively; {f} represents the external fluid force load.

4. The method for analyzing the vortex-induced vibration characteristics of a mining riser under the coupling effect of internal and external flows as described in claim 1, characterized in that, In the process of solving the vibration of the mining riser in S4, the calculation time step is kept consistent, and the wake oscillator model is coupled with the riser control equation for iterative calculation to obtain the displacement time history response of the mining riser in the downstream and cross-flow directions, respectively. By performing a fast Fourier transform on the displacement time history curve, the frequency response of the mining riser in the downstream and cross-flow directions is obtained.

5. The method for analyzing the vortex-induced vibration characteristics of a mining riser under the coupling effect of internal and external flows according to claim 1, characterized in that, In S4, the undamped free vibration of the suspended riser is calculated using differential control equations to obtain the natural frequency of the model. The comparative analysis of vortex-induced vibration characteristics under gas-liquid two-phase flow and pure liquid flow, the comparative analysis of vortex-induced vibration characteristics under different displacements, the comparative analysis of vortex-induced vibration characteristics under different two-phase mixing densities, and the comparative analysis of vortex-induced vibration characteristics under different intake ratios are summarized.

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