Method for predicting dynamic characteristics of drilling riser in ultra-deep water free standing mode

Abstract

The invention discloses a method for predicting dynamic characteristics of a drilling riser in an ultra-deep water free standing mode, which comprises the following steps of: establishing a two-dimensional coordinate system by taking a submarine blowout preventer as an original point, and establishing a fluid infinitesimal and riser infinitesimal mechanical equilibrium equation according to the Alembert principle; in consideration of environmental loads generated by waves, ocean currents and wind and buoyancy generated by a buoyancy barrel on the top of the riser, a classical bent beam theory is adopted to deduce a transverse differential equation of the riser in the ultra-deep water free standing mode, boundary conditions of a dynamic behavior prediction model of the riser in the ultra-deep water free standing mode are determined, and a finite element method is adopted to predict the dynamic behavior of the riser in the ultra-deep water free standing mode. An equation is solved by combining a Newmark-beta integral method, a dynamic behavior prediction model of the marine riser in the ultra-deep water free standing mode is finally established, and the dynamic characteristic behavior of the marine riser in the mode can be clearly predicted.

Description

Technical Field

[0001] This invention relates to the field of marine oil and gas resource exploration and development technology, and in particular to a method for predicting the dynamic characteristics of the riser in ultra-deepwater free-standing drilling mode. Background Technology

[0002] As offshore oil and gas resource development expands into deep-sea and ultra-deep-water areas, drilling risers are playing an increasingly important role in deep-water oilfield development. Drilling risers connect the drilling platform to the subsea blowout preventer, isolating the platform from seawater while creating a channel for oil and gas migration. In deep-water and ultra-deep-water drilling, the required length of the drilling riser is often very long. However, in the event of an impending typhoon or other severe weather event, urgently retrieving the entire drilling riser is both time-consuming and cumbersome. One solution to this problem is to install a buoyancy tank and a detachable assembly on the conventional drilling riser, allowing it to be disconnected near the sea surface. This allows the longer section of the riser to remain in a free-standing position in the seawater to withstand severe weather such as typhoons. However, in this position, the riser will undergo significant deformation and stress changes under external loads, making it highly susceptible to structural failure.

[0003] Currently, methods for predicting the dynamic characteristics of risers mainly target connected or suspended modes, while methods for predicting the dynamic characteristics of risers in a free-standing mode are almost nonexistent. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for predicting the dynamic characteristics of drilling risers in ultra-deepwater free-standing mode. Taking into account the effects of marine environmental loads, it clearly predicts the dynamic behavior of risers in ultra-deepwater free-standing mode, providing a theoretical basis for the safety assessment of drilling riser systems in ultra-deepwater free-standing mode.

[0005] This invention is achieved using the following technical solution: A method for predicting the dynamic characteristics of riser in ultra-deepwater free-standing drilling mode includes the following steps: Step S1: Simplify the riser system in the free-standing mode according to the actual working conditions. Establish a two-dimensional coordinate system with the subsea blowout preventer as the origin, establish the mechanical equilibrium equations of the fluid micro-element and the riser micro-element, and derive the calculation formula for the axial tension of the riser. Step S2: Based on the environmental loads generated by waves, currents and wind, as well as the buoyancy generated by the buoyancy bucket at the top of the riser, and derive the lateral differential equation of the riser, establish a predictive model for the dynamic behavior of the riser in the ultra-deepwater free-standing mode. Step S3: Based on the actual engineering conditions, determine the boundary conditions of the prediction model for the dynamic behavior of the riser in the ultra-deepwater free-standing mode, and solve the model using the finite element method combined with the Newmark-β integral method; Step S4: Using the ultra-deepwater free-standing mode riser dynamic behavior prediction model, simulate and analyze the dynamic behavior prediction of the target ocean free-standing mode riser.

[0006] Specifically, the derivation of the axial tension of the riser in step S1 includes the following sub-steps: Step S11: In the free-standing mode, the axial force of the riser pipe is mainly provided by the upward tension from the buoyancy tank, the upward buoyancy from the buoyancy block in the actual configuration, and its own weight. The micro-segment... z The strain at a location is expressed as: ; Axial stress is expressed as: ; Step S12: The axial damping force per unit length of the riser pipe is proportional to the velocity and opposite to the direction of motion, expressed as: ; Step S13: According to Newton's second law, establish the force equilibrium equation for the infinitesimal element as follows: ; Among them, longitudinal external force The main consideration is the tension of the lower tubular section. By combining the expression for the axial damping force of the riser pipe with the given length, we can obtain: ; Step S14: Based on the stress analysis of the riser element in the free-standing mode, the longitudinal tensile force on the riser mainly includes the weight of the lower pipe column itself and the weight of the LMRP suspended at the bottom. The axial tension is expressed as: .

[0007] Specifically, the calculation of environmental load in step S2 includes: The wind-sea current velocity vector is calculated based on measured or statistical wind speed values, and the expression is: ; in, This represents the surface wind and ocean current vector, in m / s. The measured wind speed is in m / s. Wind speed factor; Based on the average tidal range of spring, mid-spring, and neap tides , , The estimated surface tidal velocity vector is expressed as: or or ; in, The mean velocity vector at the sea surface during spring tides is expressed in m / s. The mean velocity vector at mid-tide sea surface is expressed in m / s. The mean velocity vector at the sea surface during neap tides is expressed in m / s. The surface velocity distribution of the ocean current is calculated based on the sum of the wind-driven current velocity and the tidal current velocity: ; The ocean current velocity is calculated by combining the wind-driven current velocity and the surface tidal current velocity, and then using an empirical formula. The expression is: ; in, This is the distance from the seabed, in meters (m). The depth is measured in meters (m).

[0008] Specifically, the derivation of the transverse differential equation of the riser in step S2 includes: Based on the equilibrium of forces, the formula for the resultant force in the horizontal direction is as follows: ; ; in, This represents the resultant force in the horizontal direction. The axial tension inside the riser pipe per unit length. The internal shear force per unit length of the riser pipe. This refers to the mass per unit length of the riser pipe, expressed in kg. It is the damping coefficient of the riser pipe; The external lateral load per unit length of the riser pipe; Based on the relationship between shear force and bending moment in mechanics of materials, and combined with the formula for the resultant force in the horizontal direction, the lateral bending control equation of the riser under lateral marine environmental loads in the free-standing mode is obtained as follows: ; The relationship between the shear force and bending moment of the riser pipe column is as follows: , , ; This represents the bending moment of the riser pipe column.

[0009] Specifically, in the boundary conditions of the model in step S3, the top of the free-standing riser is an unconstrained free boundary, and the heave motion of the buoyancy tank along the longitudinal direction is not considered. The bottom is connected to the subsea wellhead device through a flexible joint, which allows for a certain degree of rotation. The boundary condition expression is as follows: .

[0010] Specifically, step S3 includes model discretization and solving the dynamic response of the riser, wherein the model discretization includes the following sub-steps: Step SA1: Divide the model into several small units along the length of the riser pipe, and discretize the model using the Hermite cubic interpolation function to obtain its shape function: ; Step SA2: Discretize the differential equations of motion of the riser system to obtain the discretized dynamic equilibrium equations, expressed as: ; in, , , These are the mass matrix, damping matrix, and stiffness matrix, respectively. , , These are acceleration, velocity, and displacement, respectively. This is the external load vector; Step SA3: Calculate the element matrix and element mass matrix for each matrix. Represented as: ; Element damping matrix Represented as: ; Overall element stiffness Composed of a bending stiffness matrix and a geometric stiffness matrix, it is represented as: ; in, .

[0011] Specifically, the solution for the dynamic response of the riser includes the following sub-steps: Step SB1: Solve the dynamic response of the riser using time-domain analysis, and obtain the velocity, displacement, and acceleration based on the Newmark-β integral method. The relationship between them is represented as: ; ; ; Step SB2: Based on the obtained velocity, displacement, and acceleration The solution is obtained by combining the Newmark-β integral under the motion of the multi-degree-of-freedom system of the riser. Velocity and acceleration at any given moment.

[0012] Specifically, step SB2 includes: Under the motion of a multi-degree-of-freedom system with a riser, the Newmark-β integral in each step satisfies: ; Solving by combining the expressions of each matrix yields: ; ; ; Solving for the given information The formulas for calculating velocity and acceleration at time t are: ; ; in, , , , , , .

[0013] The beneficial effects of this invention are as follows: This invention establishes a two-dimensional coordinate system with the subsea blowout preventer as the origin. Based on d'Alembert's principle, it establishes the mechanical equilibrium equations for the fluid micro-element and the riser micro-element. Considering the environmental loads generated by waves, currents, and wind, as well as the buoyancy generated by the buoyancy tank at the top of the riser, it derives the transverse differential equations of the riser in the ultra-deepwater free-standing mode using classical bending beam theory. It then determines the boundary conditions of the predictive model for the dynamic behavior of the riser in the ultra-deepwater free-standing mode. Using the finite element method combined with the Newmark-β integral method, it solves the equations, ultimately establishing a predictive model for the dynamic behavior of the riser in the ultra-deepwater free-standing mode, which can clearly predict the dynamic characteristics of the riser in this mode. In severe sea conditions, the riser in the free-standing model is prone to structural failure. This invention, based on actual working conditions and environmental conditions, can clearly identify the specific locations where failure risks exist. Furthermore, by changing parameters (such as environmental load, buoyancy tank configuration, top tension, riser wall thickness, etc.), it obtains sensitivity analyses of the dynamic characteristics under corresponding parameters, providing a theoretical basis for the selection of engineering parameters. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0015] Figure 1 This is a diagram of the water-supporting pipe in the free-standing mode according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the force analysis of the water-shed pipe micro-element in the free standing mode in an embodiment of the present invention; Figure 3 This is a schematic diagram of the boundary conditions of the riser in the free-standing mode in an embodiment of the present invention; Figure 4 This is a schematic diagram of the force analysis of the riser unit in an embodiment of the present invention; Figure 5 This is a flowchart illustrating the solution process for the dynamic model of the riser in the free-standing mode in this embodiment of the invention. Figure 6 This is a schematic diagram of the dynamic characteristic distribution of the riser in the free-standing mode of this invention. Figure 1 ; Figure 7 This is a schematic diagram of the dynamic characteristic distribution of the riser in the free-standing mode of this invention. Figure 2 ; Figure 8 This is a schematic diagram of the dynamic characteristic distribution of the riser in the free-standing mode of this invention. Figure 3 . Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0017] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0018] The following is in conjunction with the appendix Figures 1 to 8 The following describes some embodiments of the present invention in detail. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0019] This invention proposes a method for predicting the dynamic characteristics of the riser in ultra-deepwater free-standing drilling mode. In a preferred embodiment, the method specifically includes the following steps: S1: Based on the actual engineering situation, the riser system in the free standing mode is reasonably simplified. A two-dimensional coordinate system is established with the subsea blowout preventer as the origin. According to D'Alembert's principle, the mechanical equilibrium equations of the fluid micro-element and the riser micro-element are established. On this basis, the calculation formula of the axial force of the riser is derived. S2: Considering the environmental loads generated by waves, currents and wind, as well as the buoyancy generated by the buoyancy bucket at the top of the riser, the transverse differential equation of the riser is derived using the classical bending beam theory, and a predictive model of the dynamic behavior of the riser in the ultra-deepwater free-standing mode is established. S3: Based on the actual engineering situation, determine the boundary conditions of the prediction model of the dynamic behavior of the riser in the ultra-deep water free-standing mode, and solve the equations by using the finite element method combined with the Newmark-β integral method. S4: Based on the prediction model of the dynamic behavior of the riser in the ultra-deepwater free-standing mode, simulate and analyze the prediction of the dynamic behavior of the target ocean free-standing mode riser.

[0020] In this embodiment, the configuration of the typhoon-resistant drilling riser is mostly similar to that of a conventional drilling riser. The difference lies in the need to install a release assembly near the sea surface of the riser, and a buoyancy tank capable of rapid inflation to provide tension to the lower riser. Some configurations also install a flexible joint above the buoyancy tank, which increases the freedom of movement of the near-sea surface buoyancy tank release assembly, facilitating rapid disconnection and connection of the upper riser. A typical schematic diagram of a typhoon-resistant drilling riser in free-standing mode is shown below. Figure 1 As shown, after the upper riser is disconnected at the near-sea surface assembly, the buoyancy tank needs to be quickly emptied and filled with air to provide buoyancy several times the total weight of the lower riser, thus ensuring that the lower riser is always under tension.

[0021] The freestanding riser is suspended upside down in the water, with its top tensioned by an upward buoyancy tank, ensuring that the riser does not experience axial compression. Simultaneously, the axial tension also helps reduce lateral deformation of the riser. Force analysis is as follows: Figure 2 .

[0022] This invention employs classical bending beam theory, considering that the slope, bending moment, shear force, and axial force of the infinitesimal segment exhibit first-order variations. .

[0023] According to the principle of force equilibrium, the net force in the horizontal direction of the infinitesimal segment is 0, therefore we can conclude that: ; ; In the formula, denoted as mass per unit length of the riser pipe (kg); c is the damping coefficient of the riser pipe; N is the axial tension inside the riser pipe per unit length (N); and f is the external lateral load on the riser pipe per unit length (N).

[0024] Ignoring the second-order terms in the above equation, we get: ; The sum of the bending moments about the zero point on the left side of the element is zero, therefore we can obtain: ; .

[0025] According to mechanics of materials, the relationship between shear force and bending moment in a tubular column is as follows: ; ; Substituting, we can obtain .

[0026] By considering the relationship between column shear force and bending moment, as well as the force balance, the lateral bending control equation for the riser under lateral marine environmental loads in the free-standing mode can be obtained as follows: .

[0027] In its free-standing mode, the riser is primarily subjected to the upward tension provided by the buoyancy tank, the upward buoyancy provided by the buoyancy block (in the actual configuration), and its own weight. The longitudinal force analysis of the riser is as follows: Figure 2 The strain of the infinitesimal element at position z is: ; Axial stress is expressed as: ; From the above formula, we can obtain: .

[0028] The axial damping force per unit length of the riser is proportional to the velocity and opposite to the direction of motion, and can be expressed as: .

[0029] According to Newton's second law, the force equilibrium equations for the infinitesimal element are as follows: ; In the above formula, the longitudinal external force The main consideration is the tension of the lower tubular section. Substituting, we get: .

[0030] according to Figure 2In the stress analysis, the longitudinal tensile force on the riser mainly includes the weight of the lower pipe string itself and the weight of the LMRP suspended at the bottom. Therefore, the axial tension is expressed as follows: .

[0031] In this embodiment, the method for calculating marine environmental load is as follows: For calculating shallow sea current velocities, the annual current velocity distribution in a certain sea area can be obtained from hydrological observation statistics, or it can be further calculated by fitting empirical formulas with statistical data. Specifically, this includes: Calculation of wind-driven ocean current velocity: Given measured or statistical wind speed values, the wind-driven ocean current velocity can be calculated using the following formula: ; In the formula, Surface wind current (m / s); The measured wind speed is (m / s). The wind speed factor ranges from 0.024 to 0.050, and is usually set to 0.030.

[0032] Calculation of maximum tidal current velocity: The tidal current velocity vector can be calculated based on the average tidal range of spring, mid-spring, and neap tides. , , Make an estimate: or or ; In the formula, The mean velocity vector at the sea surface during spring tide (m / s); The mean velocity vector at mid-tidal sea surface (m / s); This represents the mean velocity vector at the sea surface during neap tides (m / s).

[0033] Calculation of sea surface current velocity: Sea surface current velocity is considered to consist of two parts: wind-driven current velocity and tidal current velocity. The surface velocity distribution of the current can be obtained by adding the two together. ; In the formula, Let be the wind-driven ocean current velocity vector (m / s); The current velocity vector is (m / s).

[0034] Calculation of ocean current velocity at different depths: Due to friction between the seabed and water layers, the current velocity gradually decreases with depth. In shallow waters, the current velocity can be calculated by combining wind-driven current velocity and surface tidal current velocity using the following empirical formula: ; In the formula, The distance from the seabed (m); The depth of the seabed is in meters (m).

[0035] In this embodiment, as Figure 3 As shown, the top of the free-standing riser is an unconstrained free boundary, and the heave motion of the buoyancy tank along the longitudinal direction is not considered. The bottom is connected to the subsea wellhead device through a flexible joint, which allows for some rotation. Therefore, the boundary conditions can be expressed as follows: .

[0036] The dynamic model of the riser in a free-standing mode is discretized and solved using the finite element method. The riser is divided into a finite number of elements, and each element considers three degrees of freedom: longitudinal, lateral, and rotational displacements. The element displacement analysis is as follows: Figure 4 The displacement field within the element can be described using a cubic spline interpolation function. Furthermore, based on the governing equations of the riser, the element matrix can be derived using the principle of minimum potential energy.

[0037] Considering the axial displacement, lateral displacement, and rotation angle of the riser, the displacement matrix of the riser element can be obtained: .

[0038] The displacement field function of the riser pipe can be expressed as follows: Where N is a shape function, and: ; .

[0039] The stiffness matrix can be obtained according to the principle of minimum potential energy: ; .

[0040] Quality matrix: ; Damping matrix: .

[0041] The above formula represents local coordinates. The nodal displacement matrix under the global coordinate system The element displacement matrix is: ; Among the corners The element displacement vectors are identical in both the local and global coordinate systems. Therefore, the following transformation can be made: ; Therefore, we can conclude that: ;in: ; The matrix for transforming from unit coordinates to global coordinates can be represented as follows: ; Therefore, the matrix in the global coordinate system can be obtained: .

[0042] In this embodiment, the model is solved using the finite element method, combined with the Newmark-β integral method to solve the equations. The model is divided into several micro-elements along the length of the riser, and the Hermite cubic interpolation function is used to discretize the model, resulting in the following shape function: .

[0043] Discretizing the motion differential equations of the riser system yields the discretized dynamic equilibrium equations as follows: ; In the formula , , These are the mass matrix, damping matrix, and stiffness matrix, respectively. , , These are acceleration, velocity, and displacement, respectively. This is the external load vector.

[0044] Element mass matrix : ; Element damping matrix : ; Overall element stiffness It consists of the bending stiffness matrix and the geometric stiffness matrix: ; in: .

[0045] The dynamic response of the riser is solved using time-domain analysis, and the equations are derived using the Newmark-β method. Based on the Newmark-β integral method, the relationship between velocity, displacement, and acceleration *a* is obtained as follows: ; ; .

[0046] Solving for the given information The formulas for calculating velocity and acceleration at time t are: ; ; in , , , , , .

[0047] In this invention, the motion of the riser is a multi-degree-of-freedom system; therefore, each step of the Newmark-β integral satisfies the following equation: ; Substituting the solutions, we get: ; ; ; Solving the above equation yields Then, the velocity and acceleration are obtained, and the solution flowchart is as follows. Figure 5 As shown.

[0048] In one specific embodiment, the drilling riser system of a platform in the South China Sea and marine environmental parameters were selected to conduct a dynamic characteristic analysis of the free-standing riser. The free-standing suspended riser system detaches from the assembly near the sea surface to install a buoyancy tank; the remaining configuration is the same as a conventional drilling riser. However, the buoyancy tank needs a large diameter to provide sufficient buoyancy and is not suitable for installation in areas with high flow velocities. Therefore, a non-integral buoyancy tank is used, attached to the outside of the ordinary riser to provide buoyancy. For ease of comparison, the total length of the buoyancy tank is considered to be the length of two single risers. The riser configuration at the top of the blue buoyancy tank is shown in Table 1, with the top of the buoyancy tank 196m above the water surface. When the riser is detached using a free-standing suspended system, the riser is disconnected from the top of the buoyancy tank for detachment. In this case, the configuration from the filling valve to the upper flexible joint in the riser configuration is not considered; only the dynamic characteristics of the riser in the area from the LMRP to the buoyancy tank when it is freely standing in seawater are analyzed. The loads and related parameters on the drilling riser in free-standing mode are shown in Table 2.

[0049] Table 1 Configuration of Freestanding Ripple System Table 2 Other relevant parameters for freestanding riser pipes When severe weather occurs, the platform, carrying the severed riser pipe above the buoyancy tank, is suspended for evacuation. The buoyancy tank begins to inflate, providing tension to the lower riser pipe. Considering a buoyancy coefficient of 3G, which is three times the wet weight of the lower riser pipe, in a free-standing mode, the riser pipe begins to deform from the top, reaching maximum deformation in approximately 37 seconds before gradually stabilizing. The dynamic characteristics of the stable deformation pattern of the riser pipe after 200 seconds are analyzed as follows. Figure 6 , 7 8. In the free-standing mode, the displacement of the riser gradually increases from the bottom to the top, reaching a maximum displacement of 114m at the top; the rotation angle of the riser gradually increases from the top to the bottom, reaching 5.1° at the flexible joint at the bottom; the bending moment of the riser reaches a maximum of 24kN·m at the top, and also exhibits a relatively large extreme value at the bare single section near the bottom. Both the stress and tension of the riser show a gradual increasing trend from the bottom to the top, with the stress distribution exhibiting abrupt changes at the joint due to variations in cross-sectional configurations.

[0050] In the free-standing mode, the top of the riser is free. Under the load of ocean currents, the lower tubing undergoes lateral deformation as it is dragged. As the ocean current load gradually decreases along the depth direction, along with its own weight, the lateral displacement of the riser gradually decreases, resulting in a suspended mode. The bottom of the riser is connected to the LMRP via a flexible joint. At this location, the lateral displacement is fixed but rotation is possible. Therefore, the bottom connection point of the free-standing riser has the largest rotation angle, and this location is most prone to excessive rotation, leading to flexible joint failure and accidents. In the free-standing mode, the top of the riser's buoyancy tank is located 196m below the water surface, within the wind and current friction depth calculated according to Ekman drift theory (364m below the water surface). It bears significant marine environmental loads, especially given the large diameter of the buoyancy tank, resulting in a huge load at the top. Therefore, the bending moment increases sharply closer to the top. Furthermore, after the buoyancy tank provides three times the weight of the riser, the overall riser bears significant tension. Simultaneously, due to variations in the wall thickness of individual risers under different configurations, the location of the Mises stress at the individual riser cross-section exhibits abrupt changes, with the maximum stress occurring at the mid-section where the cross-section changes: at either the first blue or first purple single-piece joint from top to bottom, rather than at the top where maximum tension occurs. Therefore, using a thinner gray single-piece riser near the bottom can, to some extent, reduce the axial tension of the riser during connection operations, reducing the burden on the platform tensioner. However, for the free-standing mode, configuring a gray single-piece riser near the bottom, its thinner wall thickness leads to reduced strength, resulting in higher stress at this location, making it a potential point of initial strength failure.

[0051] Dynamic prediction of the riser system in the free-standing drilling mode in the target area reveals that the dynamic characteristic prediction method for the riser in ultra-deepwater free-standing drilling mode established in this invention can clearly identify the location of structural risks in the riser system under free-standing mode, providing guidance for the safety assessment of the riser in this mode. Furthermore, by changing parameters (such as environmental load, buoyancy tank configuration, top tension, riser wall thickness, etc.), sensitivity analysis of the dynamic characteristics under corresponding parameters can be obtained, providing a theoretical basis for the selection of engineering parameters. For the foregoing embodiments, in order to simplify the description, they are all described as a series of actions. However, those skilled in the art should understand that this application is not limited to the described order of actions, because according to this application, some steps can be performed in other orders or simultaneously. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions involved are not necessarily essential to this application.

[0052] The above embodiments describe the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Modifications and variations made by those skilled in the art without departing from the spirit and scope of the invention should be within the protection scope of the appended claims.

Claims

1. A method for predicting the dynamic characteristics of the riser in ultra-deepwater free-standing drilling mode, characterized in that, Includes the following steps: Step S1: Simplify the riser system in the free-standing mode according to the actual working conditions. Establish a two-dimensional coordinate system with the subsea blowout preventer as the origin, establish the mechanical equilibrium equations of the fluid micro-element and the riser micro-element, and derive the calculation formula for the axial tension of the riser. Step S2: Based on the environmental loads generated by waves, currents and wind, and the buoyancy generated by the buoyancy bucket at the top of the riser, and derive the lateral differential equation of the riser, establish a predictive model for the dynamic behavior of the riser in the ultra-deepwater free-standing mode. Step S3: Based on the actual engineering conditions, determine the boundary conditions of the prediction model for the dynamic behavior of the riser in the ultra-deep water free-standing mode, and solve the model using the finite element method combined with the Newmark-β integral method; Step S4: Using the ultra-deepwater free-standing mode riser dynamic behavior prediction model, simulate and analyze the dynamic behavior prediction of the target ocean free-standing mode riser.

2. The method for predicting the dynamic characteristics of the riser in ultra-deepwater free-standing drilling as described in claim 1, characterized in that, The derivation of the axial tension of the riser in step S1 specifically includes the following sub-steps: Step S11: In the free-standing mode, the axial force of the riser pipe is mainly provided by the upward tension from the buoyancy tank, the upward buoyancy from the buoyancy block in the actual configuration, and its own weight. The micro-segment... z The strain at a location is expressed as: ； Axial stress is expressed as: ； Step S12: The axial damping force per unit length of the riser pipe is proportional to the velocity and opposite to the direction of motion, expressed as: ； Step S13: According to Newton's second law, establish the force equilibrium equation for the infinitesimal element as follows: ； Among them, longitudinal external force The main consideration is the tension of the lower tubular section. By combining the expression for the axial damping force of the riser pipe with the given length, we can obtain: ； Step S14: Based on the stress analysis of the riser element in the free-standing mode, the longitudinal tensile force on the riser mainly includes the weight of the lower pipe column itself and the weight of the LMRP suspended at the bottom. The axial tension is expressed as: 。 3. The method for predicting the dynamic characteristics of the riser in ultra-deepwater free-standing drilling as described in claim 1, characterized in that, The calculation of environmental load in step S2 includes: The wind-sea current velocity vector is calculated based on measured or statistical wind speed values, and the expression is: ； in, This represents the surface wind and ocean current vector, in m / s. The measured wind speed is in m / s. Wind speed factor; Based on the average tidal range of spring, mid-spring, and neap tides , , The estimated surface tidal velocity vector is expressed as: or or ; in, The mean velocity vector at the sea surface during spring tides is expressed in m / s. The mean velocity vector at mid-tide sea surface is expressed in m / s. The mean velocity vector at the sea surface during neap tides is expressed in m / s. The surface velocity distribution of the ocean current is calculated based on the sum of the wind-driven current velocity and the tidal current velocity: ; The ocean current velocity is calculated by combining the wind-driven current velocity and the surface tidal current velocity, and then using an empirical formula. The expression is: ； in, This is the distance from the seabed, in meters (m). The depth is measured in meters (m).

4. The method for predicting the dynamic characteristics of the riser in ultra-deepwater free-standing drilling as described in claim 2, characterized in that, The derivation of the transverse differential equation of the riser in step S2 specifically includes: Based on the equilibrium of forces, the formula for the resultant force in the horizontal direction is as follows: ； ； in, This represents the resultant force in the horizontal direction. The axial tension inside the riser pipe per unit length. The internal shear force per unit length of the riser pipe. This refers to the mass per unit length of the riser pipe, expressed in kg. It is the damping coefficient of the riser pipe; The external lateral load per unit length of the riser pipe; Based on the relationship between shear force and bending moment in mechanics of materials, and combined with the formula for the resultant force in the horizontal direction, the lateral bending control equation of the riser under lateral marine environmental loads in the free-standing mode is obtained as follows: ； The relationship between the shear force and bending moment of the riser pipe column is as follows: , , ; This represents the bending moment of the riser pipe column.

5. The method for predicting the dynamic characteristics of the riser in ultra-deepwater free-standing drilling as described in claim 4, characterized in that, In the boundary conditions of the model in step S3, the top of the free-standing riser is an unconstrained free boundary, and the heave motion of the buoyancy tank along the longitudinal direction is not considered. The bottom is connected to the subsea wellhead device through a flexible joint, which allows for a certain degree of rotation. The boundary condition expression is as follows: 。 6. The method for predicting the dynamic characteristics of the riser in ultra-deepwater free-standing drilling as described in claim 5, characterized in that, Step S3 specifically includes model discretization and solving the dynamic response of the riser, wherein the model discretization includes the following sub-steps: Step SA1: Divide the model into several small units along the length of the riser pipe, and discretize the model using the Hermite cubic interpolation function to obtain its shape function: ； Step SA2: Discretize the differential equations of motion of the riser system to obtain the discretized dynamic equilibrium equations, expressed as: ; in, , , These are the mass matrix, damping matrix, and stiffness matrix, respectively. , , These are acceleration, velocity, and displacement, respectively. This is the external load vector; Step SA3: Calculate the element matrix and element mass matrix for each matrix. Represented as: ； Element damping matrix Represented as: ； Overall element stiffness Composed of a bending stiffness matrix and a geometric stiffness matrix, it is represented as: ； in, .

7. The method for predicting the dynamic characteristics of the riser in ultra-deepwater free-standing drilling as described in claim 6, characterized in that, The solution for the dynamic response of the riser includes the following sub-steps. Step SB1: Solve the dynamic response of the riser using time-domain analysis, and obtain the velocity, displacement, and acceleration based on the Newmark-β integral method. The relationship between them is represented as: ； ； ； Step SB2: Based on the obtained velocity, displacement, and acceleration The solution is obtained by combining the Newmark-β integral under the motion of the multi-degree-of-freedom system of the riser. Velocity and acceleration at any given moment.

8. The method for predicting the dynamic characteristics of the riser in ultra-deepwater free-standing drilling as described in claim 7, characterized in that, Step SB2 specifically includes: Under the motion of a multi-degree-of-freedom system with a riser, the Newmark-β integral in each step satisfies: ； Solving by combining the expressions of each matrix yields: ； ； ； Solving for the given information The formulas for calculating velocity and acceleration at time t are: ； ； in, , , , , , .

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