Deepwater riser-free drilling string safety analysis method considering vortex-induced vibration

Abstract

The invention discloses a safety analysis method for a deepwater riser-free drilling string considering vortex-induced vibration, which comprises the following steps of: S1, simulating the deepwater riser-free drilling string according to related parameters, and simulating the vibration condition of the drilling string in the actual drilling process; s2, establishing a seawater section drill string mechanical model, and calculating dynamic characteristics of the seawater section drill string under different influence factors; s3, establishing a mud line boundary, and taking the sea water section drill string parameters as initial parameters of a stratum section drill string; s4, calculating and considering the dynamic characteristics of the drill string of the stratum section under the drill string-well wall contact and drill bit-stratum coupling effects; s5, analyzing the drill string according to the dynamic characteristics; and S6, predicting the safety condition of the drill column. According to the method, the effects of the marine environment load and the stratum are considered at the same time, the overall dynamic characteristics of the riser-free drilling string are obtained, and theoretical guidance is provided for guaranteeing the safety of the riser-free drilling string in the later period.

Description

Technical Field

[0001] The invention relates to the technical field of deepwater drilling, and in particular to a deepwater riser-less drilling drill string safety analysis method taking vortex-induced vibration into consideration. Background Art

[0002] With the increasing consumption of oil and natural gas resources, deepwater oil and gas has become a growth point for global oil and gas reserves and production. In the 21st century, major global oil and gas discoveries are mainly concentrated in the ocean. In the past 10 years, about 65% to 77% of the world's new oil and gas reserves come from the ocean, and more than 40% of the world's annual new recoverable reserves come from the deep sea. At present, marine drilling mainly uses riser drilling. With the development of deep-sea oil and gas fields, riser drilling has serious defects. The load and space resources of the drilling ship are limited. The riser is heavy and occupies a large space, which greatly increases the mechanical load of the drilling ship. Riser-free drilling technology abandons the riser system that must be used in conventional marine drilling. The drilling platform no longer bears the interactive loads on the riser and large buoyancy blocks caused by deep-sea waves, currents, etc. The load reduction on the platform can reduce the requirements of the mooring system accordingly, and the reliability of the mooring system can also be guaranteed, extending the limit of deep-water operations. It is now adaptable to deepwater and ultra-deepwater drilling, and can be used in deep-water drilling operations in the ocean above 3,000 meters.

[0003] Existing patented technologies mainly focus on drilling fluid recovery technology for riserless drilling and control devices for drill string safety, and there is no systematic consideration of the drill string safety analysis method for riserless drilling under the coupling of ocean wind and wave currents, drill string-well wall contact, and drill bit-rock interaction. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a deepwater riserless drilling drill string safety analysis method considering vortex-induced vibration, comprising the following steps: S1. Simulating the deepwater riserless drilling drill string according to relevant parameters to simulate the vibration of the drill string during the actual drilling process; S2. Establishing a mechanical model of the seawater section drill string, and calculating the dynamic characteristics of the seawater section drill string under different influencing factors; S3. Establishing a mudline boundary, and using the seawater section drill string parameters as the initial parameters of the formation section drill string; S4. Calculating the dynamic characteristics of the formation section drill string considering the drill string-well wall contact and drill bit-formation coupling; S5. Analyzing the drill string according to the dynamic characteristics; S6. Predicting the safety of the drill string.

[0005] Furthermore, the relevant parameters in step S1 specifically include: external current velocity, drill string length, drill string inner and outer diameters, drilling fluid density, and formation.

[0006] Furthermore, the seawater section drill string mechanics model in step S2 specifically includes: a drill string mechanics sub-model, a wake oscillator sub-model, a marine environmental load sub-model, and a riser-free drilling drill string sub-model for the formation section.

[0007] Furthermore, the S2 step specifically includes the following sub-steps: S21. Establishing a drill string mechanics sub-model, a wake oscillator sub-model, a marine environmental load sub-model and a riser-free drilling drill string sub-model for the formation section respectively; S22. Using the drill string mechanics sub-model to perform force analysis and calculation on the drill string and the drilling fluid in the pipe, using the wake oscillator sub-model to analyze and calculate the vortex shedding of the flexible pipe, using the marine environmental load sub-model to calculate the current velocity and current force respectively, and using the riser-free drilling drill string sub-model for the formation section to perform discrete analysis and calculation on the formation section drill string.

[0008] Furthermore, the force analysis and calculation of the drill string and the drilling fluid in the pipe using the drill string mechanics sub-model is specifically as follows:

[0009] The force analysis of the drill string and the drilling fluid in the pipe is performed respectively to obtain the force balance equation of the drilling fluid in the drill string in the y direction and the force balance equation of the drill string in the y direction. Combined with the calculation formula of shear force Q and bending moment M, the differential equation of the motion of the drilling fluid in the drill string downstream direction y and the differential equation of the motion of the drilling fluid in the cross-flow direction z of the drill string are obtained;

[0010] The force balance equation of the drilling fluid microelement in the drill string in the y direction is:

[0011]

[0012] The force balance equation of the drill string microelement segment in the y direction is:

[0013]

[0014] The calculation formulas of the shear force Q and the bending moment M are:

[0015] The differential equation of motion of the drilling fluid in the drill string downstream in the direction y is:

[0016]

[0017] The differential equation of motion of the drilling fluid in the cross-flow direction z of the drill string is:

[0018]

[0019] In the above formula, EI is the bending stiffness, N·m 2 ; y is the displacement in the downstream direction, m; z is the displacement in the cross-stream direction, m; x is the axial position of the drill string, m; T is the cross-sectional tension, N; c is the structural damping; F y(x, t) is the downstream ocean environmental load, N; F z (x, t) is the cross-current ocean environmental load, N; v1 is the velocity of the liquid, m / s; m r 、m l They are respectively the mass of the standpipe per unit length and the liquid in the standpipe per unit length, kg.

[0020] Furthermore, the calculation formula for analyzing and calculating the vortex shedding of the flexible pipe string using the wake oscillator sub-model is specifically:

[0021] In the formula, q y and q z are the dimensionless wake array variables in the downstream and crossstream directions, and are the first-order derivatives of the dimensionless wake array variables with respect to time in the downstream and cross-stream directions, and are the first-order derivatives of the dimensionless wake array variables with respect to time in the downstream and cross-stream directions, ω s is the vortex shedding frequency, ε y , ε z , A y , A z are dimensionless constants, is the acceleration of the drill string downstream, is the acceleration of the drill string in the cross flow direction;

[0022] Among them, the vortex shedding frequency ω s Determined by the external ocean current velocity, the calculation formula is:

[0023] Where Uc is the ocean current velocity, m / s, Streamwise velocity, S t is the Tesla number; D0 is the outer diameter of the drill string, m.

[0024] Furthermore, the mudline boundary established in the step S3 specifically includes: ocean load boundary, mudline boundary, formation section drill string boundary; the seawater section drill string parameters specifically include: axial tension, torque, and torsion angle.

[0025] Furthermore, the calculation formula of the ocean load boundary is:

[0026] The calculation formula of the mudline boundary is: In the above formula, v y is the displacement in the downstream direction, m; v zis the cross-flow displacement, m; H is the water depth of the seawater section, m; t is the time, s; x is the axial direction; u and v are both displacements; h is the mudline wellhead height; T is the axial tension; M0 is the torque.

[0027] The present invention provides a deepwater riser-less drilling drill string safety analysis method considering vortex-induced vibration, which has the following beneficial effects:

[0028] The present invention takes into account the marine environmental load and the effect of the formation at the same time, and uses the finite element method to establish a coupling model of ocean current load, drill string-well wall contact and drill bit-rock interaction, and analyzes the bending moment, stress and vortex-induced vibration characteristics of the deepwater riserless drilling drill string in the seawater section. It also analyzes the trajectory of the drill string in the formation section, contact force and other dynamic characteristics, and obtains the overall dynamic characteristics of the riserless drilling drill string, providing theoretical guidance for ensuring the safety of the riserless drilling drill string in the later stage. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0030] Figure 1 A flow chart of the method provided by the present invention;

[0031] Figure 2 A basic principle diagram of a riser-free drilling drill string provided by the present invention;

[0032] Figure 3 A simplified physical model of the seawater section drill string provided by the present invention;

[0033] Figure 4 A schematic diagram of the force analysis of the drilling fluid micro-element segment provided by the present invention;

[0034] Figure 5 A schematic diagram of the force analysis of a drill string micro-element segment provided by the present invention;

[0035] Figure 6 A schematic diagram of the fluid force at a certain interface at a certain moment provided by the present invention;

[0036] Figure 7 A schematic diagram of a double-node beam unit model of a drill string provided by the present invention;

[0037] Figure 8 Schematic diagram of the contact between the drill string and the well wall provided by the present invention. DETAILED DESCRIPTION

[0038] It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention.

[0039] The following is a detailed description of the implementation method of the present invention in conjunction with the accompanying drawings. Only some embodiments are described, not all embodiments. For the purpose of clarity, representations and descriptions that are not related to the present invention are omitted in the drawings and descriptions.

[0040] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below. Obviously, the implementation cases described are part of the embodiments of the present invention, not all of the embodiments, and cannot be understood as limiting the scope of the implementation of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the protection scope of the present invention.

[0041] During deepwater riserless drilling, the drill string is directly exposed to the marine environment. Under the influence of wind and ocean currents, the drill string will vibrate and deform. The overall stress condition of the drill string is particularly complex. Based on the actual situation, an ocean load model is established. The Ekman drift theory is used to calculate the ocean current velocity at any depth underwater, and the Morison equation is used to calculate the fluid force of the ocean current on the drill string. The drill string is subjected to vortex-induced vibration in seawater, and a wake oscillator model is established. The wake oscillator coupling term is discussed through the improved van der pol nonlinear equation. The seawater section drill string is connected to the formation section drill string through the mudline boundary. In the formation section, a double-node beam unit is used to perform discrete analysis on the drill string, and the Lagrange equation is used to establish a dynamic model of the drill string in the formation section. In the actual drilling process, the drill string is in constant contact with the well wall and the formation. Therefore, at the boundary of the drill string in the formation section, the drill string-well wall effect and the drill bit-formation effect are considered, such as Figure 2 shown.

[0042] like Figure 1 As shown, the present invention provides a method for safety analysis of deepwater riserless drilling drill string considering vortex-induced vibration, comprising the following steps: S1. Simulating the deepwater riserless drilling drill string according to relevant parameters to simulate the vibration of the drill string during actual drilling; S2. Establishing a mechanical model of the seawater section drill string, and calculating the dynamic characteristics of the seawater section drill string under different influencing factors; S3. Establishing a mudline boundary, and using the seawater section drill string parameters as the initial parameters of the formation section drill string; S4. Calculating the dynamic characteristics of the formation section drill string considering the contact between the drill string and the well wall and the coupling between the drill bit and the formation; S5. Analyzing the entire drill string according to the displacement, vibration and other dynamic characteristics of the seawater section and the formation section drill string; S6. Predicting the safety of the drill string by calculating the maximum equivalent stress and bending moment of the drill string.

[0043] The relevant parameters in step S1 specifically include: external current velocity, drill string length, drill string inner and outer diameters, drilling fluid density, and formation.

[0044] like Figure 3 As shown, the drill string mechanics model of the seawater section in step S2 specifically includes: a drill string mechanics sub-model, a wake oscillator sub-model, a marine environmental load sub-model, and a riser-free drilling drill string sub-model for the formation section.

[0045] Step S2 specifically includes the following sub-steps: S21. Establish a drill string mechanics sub-model, a wake oscillator sub-model, a marine environmental load sub-model and a riser-free drilling drill string sub-model for the formation section respectively; S22. Use the drill string mechanics sub-model to perform force analysis and calculation on the drill string and the drilling fluid in the pipe, use the wake oscillator sub-model to analyze and calculate the vortex shedding of the flexible pipe, use the marine environmental load sub-model to calculate the current velocity and current force respectively, and use the riser-free drilling drill string sub-model for the formation section to perform discrete analysis and calculation on the drill string in the formation section.

[0046] The drill string mechanics sub-model is used to perform force analysis and calculation on the drill string and the drilling fluid in the pipe as follows:

[0047] The stress analysis of the drill string and the drilling fluid in the pipe is carried out separately, such as Figure 4 , Figure 5 As shown, the force balance equation of the drilling fluid micro-element section in the drill string in the y direction and the force balance equation of the drill string micro-element section in the y direction are obtained, and combined with the calculation formula of the shear force Q and the bending moment M, the differential equation of the motion of the drilling fluid in the drill string downstream direction y and the differential equation of the motion of the drilling fluid in the drill string cross-flow direction z are obtained;

[0048] The force balance equation of the drilling fluid microelement in the drill string in the y direction is:

[0049]

[0050] The force balance equation of the drill string microelement segment in the y direction is:

[0051]

[0052] The calculation formulas of the shear force Q and the bending moment M are:

[0053] The differential equation of motion of the drilling fluid in the drill string downstream in the direction y is:

[0054]

[0055] The differential equation of motion of the drilling fluid in the cross-flow direction z of the drill string is:

[0056]

[0057] In the axial direction, the drill string without a riser is not only subject to the top tension and its own buoyancy, but also to the gravity of the lower drill string. Therefore, the tension T on the drill string section is T (z), which can be expressed as:

[0058]

[0059] In the above formula, EI is the bending stiffness, N·m 2 ; y is the displacement in the downstream direction, m; z is the displacement in the cross-stream direction, m; x is the axial position of the drill string, m; T is the cross-sectional tension, N; c is the structural damping; F y (x, t) is the downstream ocean environmental load, N; F z (x, t) is the cross-current ocean environmental load, N; v1 is the velocity of the liquid, m / s; m r 、m l are the mass of the standpipe per unit length and the liquid in the standpipe per unit length, kg; T top is the top tension, N; ρ w is the density of seawater, kg / m 3 ; D0 is the outer diameter of the drill string, m; WOB is the bit pressure, N; T2 is the lower end tension, N.

[0060] Wake oscillator sub-model: The seawater section drill string has a large aspect ratio, so it is considered as a flexible pipe string. The flexible pipe string uses the wake oscillator model to describe the vortex shedding characteristics of the ocean current on the pipe string. The wake oscillator coupling term is discussed based on the van der pol nonlinear equation, and the result is more accurate and reliable. The calculation formula for analyzing and calculating the vortex shedding of the flexible pipe string using the wake oscillator sub-model is as follows:

[0061]

[0062] In the formula, q y and q z are the dimensionless wake array variables in the downstream and crossstream directions, and are the first-order derivatives of the dimensionless wake array variables with respect to time in the downstream and cross-stream directions, and are the first-order derivatives of the dimensionless wake array variables with respect to time in the downstream and cross-stream directions, ω s is the vortex shedding frequency, ε y , ε z , A y , A z are dimensionless constants, is the acceleration of the drill string downstream, is the acceleration of the drill string in the cross flow direction;

[0063] Among them, the vortex shedding frequency ω s Determined by the external ocean current velocity, the calculation formula is: Where Uc is the ocean current velocity, m / s, Streamwise velocity, S t is the Tesla number; D0 is the outer diameter of the drill string, m.

[0064] Marine environmental load model:

[0065] 1) Calculation of ocean current velocity:

[0066] The current velocity at a certain location and time in the deep sea can be obtained by vector superposition of the wind current velocity and the surface tidal velocity, expressed as: vr c =Vr cw +Vr d ; The velocity of deep-sea currents is generally analyzed using the Ekman drift theory, which first uses the sea surface wind speed to determine the friction depth.

[0067] Then use the Ekman drift theory to get the velocity value at any depth underwater: Where h is the distance from the seabed, m; H is the water depth, m.

[0068] 2) Calculation of ocean current force:

[0069] like Figure 6 As shown, By analyzing the force of the drill string without a riser and combining it with the Morison equation, the fluid force component in the y direction of the drill string can be obtained: The pulsating drag force F acting on the drill string D ':

[0070]

[0071] Combining the above formulas and ignoring the influence of higher-order terms, the expression of ocean current force is obtained as follows: Where, vc is the ocean current velocity, m / s; is the drag force coefficient; is the lift coefficient.

[0072] Drill string model for drilling without riser in formation section: double-node beam element is used to perform discrete analysis on the drill string in formation section, such as Figure 7 As shown, each drill string node has six degrees of freedom, namely three xyz translations, two lateral rotation angles and one torsion angle.

[0073] The movement of the drill string in the formation is represented by the displacement of the beam element nodes: {U i} e =[x i ,y i,z i ,θ xi ,θ yi ,θ zi ,x j ,y j ,z j ,θ xj ,θ yj ,θ zj ]; The Lagrange equation controlling the motion of the drill string can be expressed as: In the formula, T is kinetic energy; V is potential energy; F i is the external force applied to the node.

[0074] According to the above two formulas, the total kinetic energy expression of the beam element can be deduced as follows:

[0075]

[0076] In the formula, are the translation speeds on the X, Y, and Z axes, m / s; θ x ,θ y ,θ z are the rotation angles of the unit section around the X, Y, and Z axes, rad; are the angular velocities of the unit section around the X, Y, and Z axes, rad / s; e is the section eccentricity, m; Ω is the actual rotation speed of the drill string in the formation, rad / s; ρ is the density of the drill string, kg / m 3 ; l e is the length of the beam element, m; A is the cross-sectional area of ​​the beam element, m 2 ;I x is the polar moment of inertia of the beam element section, m 4 ;I yz is the moment of inertia of the beam element section, m 4 .

[0077] The total potential energy of the beam element is expressed as: Where, E is the elastic modulus of the drill string, Pa; G is the shear modulus of the drill string, Pa.

[0078] The nodes of the drill string beam unit are subject to centrifugal force and gravity, where the gravity components of the beam unit in x, y, and z can be expressed as: q x =qcosα;q y =qsinα;q z =0; where q is the equivalent gravity of the unit drill string, N / m; α is the angle between the axis of the beam unit and the vertical direction. Therefore, the equivalent nodal force of the gravity vector of the drill string beam unit can be:

[0079]

[0080] In the cross section of the drill string in the formation, the center of gravity and the centroid are not completely coincident. The centrifugal force only exists when the drill string rotates. Therefore, the centrifugal force generated by the rotation of the beam element in x, y, and z can be expressed as: f x =0; f y =eqΩ 2 cosβ;f z =eqΩ 2 sinβ; where β is the centroid phase angle, rad.

[0081] Similarly, the centrifugal force vector of the beam unit node can be obtained, and its equivalent force can be expressed as:

[0082]

[0083] Substituting the above formula into the equation, we can get: Where [M], [C], and [K] are the mass matrix, damping matrix, and stiffness matrix, respectively; {U} are generalized acceleration, velocity, and displacement vectors respectively; {F} is the external force vector.

[0084] The mudline boundary established in step S3 specifically includes: ocean load boundary, mudline boundary, and formation section drill string boundary; the seawater section drill string parameters specifically include: axial tension, torque, and torsion angle.

[0085] The calculation formula for the ocean load boundary is:

[0086] One end of the drill string for riserless drilling is in the seawater, and the other end is in the formation environment. There is a submarine wellhead near the mud line. The seawater section of the drill string is affected by the ocean load, which will produce lateral displacement and vortex-induced vibration at the upper part. Although the presence of the submarine wellhead will restrict the displacement of the drill string, the force and torque of the upper end of the drill string will also be transmitted to the formation section drill string, changing the dynamic characteristics of the formation section drill string. Therefore, in the present invention, the axial tension, torque and torsion angle of the drill string are used to connect the upper and lower ends of the drill string.

[0087] The calculation formula of mudline boundary is: In the above formula, v y is the displacement in the downstream direction, m; v z is the cross-flow displacement, m; H is the water depth of the seawater section, m; t is the time, s; x is the axial direction; u and v are both displacements; h is the mudline wellhead height; T is the axial tension; M0 is the torque.

[0088] Drill string boundary of formation section:

[0089] 1) Drill string-well wall effect:

[0090] In the actual drilling process, the rotary table constrains the movement of the drill string, and the drill string is subjected to torque and tension. The drill bit is subjected to reverse torque and excitation force from the formation rock, and also constrains the lateral displacement of the drill string. In addition, the well wall also constrains the radial displacement of the drill string, such as Figure 8 As shown in Figure 1, when the drill string collides with the wellbore, it will be subjected to a normal force, a tangential friction force, and an additional friction torque. When the elastic deformation of the wellbore is restored, the drill string will rebound and the node will be free again. N It can be expressed as: Where D w is the wellbore diameter, m; D0 is the drill string outer diameter, m; v r is the radial velocity of the drill string, m / s; u r is the radial displacement of the drill string, m; k h is the wellbore stiffness, N / m; C h is the damping coefficient, N·s / m.

[0091] Tangential friction force F f and friction torque T f It can be expressed as: F f =μ(v s )F N ; T f =(d0 / 2)μ(v s )F N ; In the formula, μ(v s ) is the friction coefficient when the drill string contacts the well wall, which can be calculated by the static-dynamic exponential decay model: In the formula, μ k is the coefficient of kinetic friction; μ s is the static friction coefficient; d e is the attenuation coefficient; v s is the slip rate.

[0092] 2) Drill bit-stratum interaction:

[0093] As the drill bit collides with the formation rock, the drilling pressure F WOB The rate of change over time t is: F WOB (t)=WOB+

[0094] W f sin(ω f t); where WOB is the bit pressure applied during drilling, N; W f is the dynamic drilling pressure value, N; ω f is the drilling pressure fluctuation coefficient, which is related to the drill bit type and the drill string speed: ω f =n b Ω; where n b is the drill bit coefficient, PDC drill bit n b =1, roller drill bit nb =3, Ω is the drill string rotation speed, rad / s.

[0095] Friction torque T on the drill bit bit It is expressed as: Where D bit is the drill bit diameter, m; μ is the formation friction coefficient.

[0096] The present invention takes into account the marine environmental load and the effect of the formation at the same time, and uses the finite element method to establish a coupling model of ocean current load, drill string-well wall contact and drill bit-rock interaction, and analyzes the bending moment, stress and vortex-induced vibration characteristics of the deepwater riserless drilling drill string in the seawater section. It also analyzes the trajectory of the drill string in the formation section, contact force and other dynamic characteristics, and obtains the overall dynamic characteristics of the riserless drilling drill string, providing theoretical guidance for ensuring the safety of the riserless drilling drill string in the later stage.

[0097] The above is only a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the form disclosed herein, and should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications and environments, and can be modified within the scope of the concept described herein through the above teachings or the technology or knowledge of the relevant field. The changes and modifications made by those skilled in the art shall not deviate from the spirit and scope of the present invention, and shall be within the scope of protection of the claims attached to the present invention.

Claims

1. A deepwater riserless drilling drill string safety analysis method considering vortex-induced vibration, characterized in that: The following steps are involved: S1. Simulate the drill string of deepwater riserless drilling according to relevant parameters to simulate the vibration of the drill string during the actual drilling process; S2. Establish a mechanical model of the seawater section drill string and calculate the dynamic characteristics of the seawater section drill string under different influencing factors; S3. Establish the mudline boundary and use the drill string parameters of the seawater section as the initial parameters of the drill string of the formation section; S4. Calculate the dynamic characteristics of the drill string in the formation section considering the contact between the drill string and the wellbore wall and the coupling between the drill bit and the formation; S5. Analyze the drill string according to its dynamic characteristics; S6. Predict the safety of the drill string.

2. The deepwater riserless drilling drill string safety analysis method considering vortex-induced vibration according to claim 1 is characterized in that: The relevant parameters in step S1 specifically include: external current velocity, drill string length, drill string inner and outer diameters, drilling fluid density, and formation.

3. The deepwater riserless drilling drill string safety analysis method considering vortex-induced vibration according to claim 1 is characterized in that: The seawater section drill string mechanics model in step S2 specifically includes: a drill string mechanics sub-model, a wake oscillator sub-model, a marine environmental load sub-model, and a riser-free drilling drill string sub-model for the formation section.

4. The method for safety analysis of deepwater riserless drilling considering vortex-induced vibration according to claim 3 is characterized in that: The S2 step specifically includes the following sub-steps: S21. Establish a drill string mechanics sub-model, a wake oscillator sub-model, a marine environment load sub-model, and a drill string sub-model for formation section riser-free drilling respectively; S22. Use the drill string mechanics sub-model to analyze and calculate the forces on the drill string and the drilling fluid in the pipe, use the wake oscillator sub-model to analyze and calculate the vortex shedding of the flexible pipe string, use the marine environmental load sub-model to calculate the current velocity and current force respectively, and use the drill string sub-model for drilling without a riser in the formation section to perform discrete analysis and calculation of the drill string in the formation section.

5. The method for safety analysis of deepwater riserless drilling considering vortex-induced vibration according to claim 4 is characterized in that: The use of the drill string mechanics sub-model to perform force analysis and calculation on the drill string and the drilling fluid in the pipe is specifically as follows: The force analysis of the drill string and the drilling fluid in the pipe is performed respectively to obtain the force balance equation of the drilling fluid in the drill string in the y direction and the force balance equation of the drill string in the y direction. Combined with the calculation formula of shear force Q and bending moment M, the differential equation of the motion of the drilling fluid in the drill string downstream direction y and the differential equation of the motion of the drilling fluid in the cross-flow direction z of the drill string are obtained; The force balance equation of the drilling fluid microelement in the drill string in the y direction is: The force balance equation of the drill string microelement segment in the y direction is: The calculation formulas of the shear force Q and the bending moment M are: The differential equation of motion of the drilling fluid in the drill string downstream in the direction y is: The differential equation of motion of the drilling fluid in the cross-flow direction z of the drill string is: In the above formula, EI is the bending stiffness, N·m 2 ; y is the displacement in the downstream direction, m; z is the displacement in the cross-stream direction, m; x is the axial position of the drill string, m; T is the cross-sectional tension, N; c is the structural damping; F y (x, t) is the downstream ocean environmental load, N; F z (x, t) is the cross-flow ocean environmental load, N; v1 is the velocity of the liquid, m / s; m r 、m l They are respectively the mass of the standpipe per unit length and the liquid in the standpipe per unit length, kg.

6. The method for safety analysis of deepwater riserless drilling considering vortex-induced vibration according to claim 4 is characterized in that: The calculation formula for analyzing and calculating the vortex shedding of the flexible pipe string using the wake oscillator sub-model is specifically: In the formula, q y and q z are the dimensionless wake array variables in the downstream and crossstream directions, and are the first-order derivatives of the dimensionless wake array variables with respect to time in the downstream and cross-stream directions, and are the first-order derivatives of the dimensionless wake array variables with respect to time in the downstream and cross-stream directions, ω s is the vortex shedding frequency, ε y , ε z , A y , A z are dimensionless constants, is the acceleration of the drill string downstream, is the acceleration of the drill string in the cross flow direction; Among them, the vortex shedding frequency ω s Determined by the external ocean current velocity, the calculation formula is: Where Uc is the ocean current velocity, m / s, Streamwise velocity, S t is the Tesla number, D0 is the outer diameter of the drill string, m.

7. The method for safety analysis of deepwater riserless drilling considering vortex-induced vibration according to claim 1, characterized in that: The mudline boundary established in the step S3 specifically includes: ocean load boundary, mudline boundary, and formation section drill string boundary; the seawater section drill string parameters specifically include: axial tension, torque, and torsion angle.

8. The method for safety analysis of deepwater riserless drilling considering vortex-induced vibration according to claim 7 is characterized in that: The calculation formula for the ocean load boundary is: The calculation formula of the mudline boundary is: In the above formula, v y is the displacement in the downstream direction, m; v z is the cross-flow displacement, m; H is the water depth of the seawater section, m; t is the time, s; x is the axial direction; u and v are both displacements; h is the mudline wellhead height; T is the axial tension; M0 is the torque.

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