A method for local fatigue life analysis of a dynamic flexible riser including a riser accessory

Abstract

This invention provides a method for analyzing the local fatigue life of a dynamic flexible riser including riser accessories, comprising: Step 1, establishing two equivalent models for the flexible riser; Step 2, outputting the amplitude response operator curves of the float under different wave incidence angles; Step 3, obtaining the curves of mechanical parameters changing with the riser position; Step 4, obtaining the friction parameters of each layer of the flexible riser; Step 5, performing local stress analysis at four locations on the flexible riser; Step 6, performing initialization solution; Step 7, determining the fatigue hotspot locations and load magnitudes for each of the four riser accessory regions; Step 8, correcting the stress amplitude of the two stress-life curves; and Step 9, calculating the damage degree and predicting the fatigue life using the rainflow counting method. This invention is the first to propose an integrated multimodal synergistic fatigue damage analysis method for riser-accessories, thereby realizing different fatigue assessment methods for different hotspot locations on the flexible riser.

Description

Technical Field

[0001] This invention belongs to the field of fatigue analysis of dynamic flexible risers, and particularly relates to a method for local fatigue life analysis of dynamic flexible risers including riser accessories. Background Technology

[0002] In the development of offshore oil resources in my country, flexible riser systems are widely used as connecting conduits between floating or static structures and seabed equipment. Flexible risers are nonlinear dynamic systems composed of multiple layers of irregularly shaped materials. Under the alternating loads of ocean currents and waves, flexible risers experience continuous motion and fluctuations, leading to dynamic fatigue failure, a significant reason for their shortened service life.

[0003] (1) Currently, patented methods for fatigue analysis of flexible risers only focus on the tension, torsion, and bending of the riser structure itself under the action of ocean currents, that is, simply analyzing the fatigue life of the flexible riser body. However, in actual service, flexible risers are equipped with accessories such as buoyancy blocks, counterweights, bending reinforcements, and seabed fixing devices. These additional accessories have a significant impact on the fatigue life analysis of the flexible riser itself.

[0004] In the rigid-flexible interface area where the flexible riser meets the float, a bending reinforcement made of polymer material needs to be arranged to alleviate the fatigue load on the flexible riser.

[0005] In the drooping and floating areas of the flexible riser, counterweights and buoyancy blocks need to be arranged and clamped onto the flexible riser to keep it in a drooping or floating state.

[0006] A seabed fixing device needs to be installed in the mud contact area of ​​the flexible riser. It uses a composite material traction rope to clamp the flexible riser, which fixes the dynamic and static intersection of the flexible riser and restricts the range of movement of the flexible riser at this point.

[0007] (2) When fluid flows through a cylindrical structure, under specific flow velocity conditions, periodically alternating vortices will form in the wake region of the cylinder. The main flow-induced vibration problem faced by flexible risers in marine environments stems from vortex-induced vibration. When ocean currents flow through the riser, periodically alternating vortices will be generated at its tail, forming alternating fluid pressure.

[0008] Currently, all patents analyzing the fatigue life of flexible risers focus on fatigue damage caused by ocean current forces alone, but have never analyzed fatigue damage caused by the simultaneous combination of ocean current forces and vortex-induced vibration loads.

[0009] In summary, there is an urgent need for a fatigue life analysis method that combines dynamic flexible risers with their accessories and fully considers the synchronous influence of ocean current forces and vortex-induced vibrations. This method would provide a theoretical basis for evaluating the actual fatigue load state of the overall flexible riser system during service, thereby improving the long-term reliability and safety level of deep-sea flexible risers. Summary of the Invention

[0010] Purpose of the invention: The present invention aims to solve the following technical problems:

[0011] (1) This invention mainly focuses on the fatigue life analysis method of flexible risers and their accessory systems, filling the gap in the current method that only provides fatigue analysis of the riser itself, but lacks a fatigue life analysis method for the entire riser system.

[0012] (2) This invention combines conventional ocean current force and vortex-induced vibration to generate reciprocating periodic stress, and analyzes the mechanical performance of riser under synchronous action, which makes up for the shortcomings of the single hydrodynamic analysis of ocean current force in existing patents, and makes the mechanical analysis of flexible riser closer to the actual situation.

[0013] This invention provides a method for analyzing the local fatigue life of a dynamic flexible riser including riser accessories, comprising the following steps:

[0014] Step 1: First, establish two equivalent models for the flexible riser: a layered equivalent model and an overall equivalent model.

[0015] Step 2: Obtain the geometry, mass, center of mass, and moments of inertia in three directions of the floating body as input conditions for the geometric model. Output the amplitude response operator curves (RAOs) of the floating body under different wave incidence angles through hydrodynamic analysis.

[0016] Step 3: Based on the overall equivalent model of the flexible riser, add a system model of bending reinforcement, buoyancy block, counterweight, and seabed fixing device, and perform a global hydrodynamic simulation analysis of the riser system. Output the tension, bending moment, and curvature of the riser at the four accessory positions of bending reinforcement, buoyancy block, counterweight, and seabed fixing device to obtain the curves of the mechanical parameters changing with the position of the riser.

[0017] Step 4: Obtain the friction parameters of each layer of the flexible riser; the stress-strain curve of the polymer material used for the bending reinforcement, and fit it using Yeoh's third-order constitutive equation; density; dimensions, bending, torsion, and axial stiffness of the bending reinforcement; dimensions, density, bending, torsion, and axial stiffness of the buoyancy block and counterweight; weight and mooring rope length of the seabed fixing device;

[0018] Step 5: Combining the output information from Step 1, Step 3, and Step 4, perform local stress analysis at four locations of the flexible riser: the location of the bending reinforcement, the location of the buoyancy block, the location of the counterweight block, and the location of the seabed fixing device.

[0019] Step 6: Using the fluid volumetric method multiphase flow model VOF and the turbulent kinetic energy-dissipation rate viscous model k-epsilon viscous model, define the gas-liquid mixture material, define the baseline values ​​and initial conditions, set boundary conditions to perform numerical wave generation for open channel flow, set the solver and termination conditions, and perform initialization solution.

[0020] Step 7: Integrate the local stress parameters of the four locations in Step 5 with the high-cycle cyclic stress caused by vortex-induced vibration in Step 6 to form the fatigue hot spot locations and load magnitudes of the four riser accessory areas.

[0021] Step 8: Obtain the stress life curves of each layer of material in the flexible riser and the stress life curve of the polymer material used for the bending reinforcement. Then, use the Gerber correction model to correct the stress amplitude of the two stress life curves.

[0022] Step 9: Based on the linear cumulative damage theory, fatigue stress analysis is performed on the fatigue hotspot locations of the four riser accessory areas to obtain the tension stress time history curve and bending stress time history curve. The damage degree is calculated by the rainflow counting method and the fatigue life is predicted. For the hotspot area of ​​the bending reinforcement, fatigue analysis needs to be performed on the bending reinforcement and the flexible riser together.

[0023] Step 1 includes: performing simulation and analytical calculations of radial load, torsional load, and axial load on the skeleton layer and the pressure-resistant armor layer of the flexible riser irregular structure, respectively. After adding the sealing layer and outer covering layer of the regular structure, the overall calculation results are fitted and compared with the calculation results of the layered equivalent model, and error analysis is performed to form a layered equivalent model that can guide the actual situation.

[0024] In step 1, the calculation process for the radial load parameters includes:

[0025] Equivalent radial elastic modulus of the skeleton layer and the compressive armor layer for:

[0026] ,

[0027] Where E is the elastic modulus of the metal material of the skeleton layer and the pressure-resistant armor layer, t is the layer thickness, L is the total length of a single buckle, and c is the overlap length of a single buckle.

[0028] The calculation process for torsional load parameters includes:

[0029] Equivalent torsional elastic modulus of the skeleton layer and the compressive armor layer for:

[0030] ,

[0031] Where A is the cross-sectional area of ​​each layer;

[0032] In step 1, the calculation process for the axial load parameters includes:

[0033] Equivalent Poisson's ratio of the skeleton layer and the compressive armor layer under axial load for:

[0034] ,

[0035] in Poisson's ratio for the metallic materials of the skeleton layer and the pressure-resistant armor layer.

[0036] In step 1, the overall radial, torsional, and axial loads are calculated on the layered equivalent model. The calculation results are then fitted with the calculation results of the overall equivalent model, and error analysis is performed to output an overall equivalent model of the flexible riser with accurate mechanical properties.

[0037] By adjusting the Poisson's ratio, radial elastic modulus, torsional elastic modulus, and density parameters in the finite element simulation parameters of the overall equivalent model, the strain at each point on the overall equivalent model is the same as the strain at each point on the layered equivalent model after the same tensile and torsional loads are applied. At this time, the adjusted parameter set is the most reasonable overall equivalent model.

[0038] The layered equivalent model is used as the input condition for local stress analysis, while the overall equivalent model is used as the input condition for environmental load calculation.

[0039] Step 2 includes: acquiring and monitoring ocean current parameters in the current sea area, including current speed and direction; wave parameters, including wave height, period, and wave direction; seabed parameters, including seabed type and soil density; and seawater parameters, including seawater density, as input conditions for environmental load calculation.

[0040] The forced vibration of a floating body under wave action is given by the frequency domain equation. describe: ,

[0041] Where M is the mass matrix of the floating body; For the additional mass matrix; The potential flow damping matrix; This is the linearized viscous damping matrix; To recover the stiffness matrix in still water; Additional stiffness matrix provided for mooring or connection systems; The complex amplitude vector of the six-degree-of-freedom motion at the center of gravity of the buoyancy; Angular frequency; The angle of incidence of the wave; It is the imaginary unit.

[0042] Step 3 includes: the model of the bending reinforcement is a hollow conical tube; the seabed fixing device includes a counterweight, a clamp and a mooring rope.

[0043] Step 5 includes:

[0044] Step 5.1, Analysis of the location of bending reinforcement;

[0045] A joint simulation mechanical model of the bending stiffener and the flexible riser with layered equivalent model is established. The bending stiffener and the flexible riser are initially in a non-contact state. Nonlinear static calculations are performed on the flexible riser and bending stiffener assembly in the state.

[0046] The flexible riser is simplified into anisotropic beam elements, and its constitutive relation is obtained through layered integration:

[0047] ,

[0048] in:

[0049] ,

[0050] , , Let be the elastic modulus, cross-sectional area, and moment of inertia of the i-th layer material, respectively. is the distance from the neutral axis of the i-th layer to the global neutral axis; This is the equivalent axial tensile stiffness; Equivalent bending stiffness; It is the axial force; For bending moment; For axial strain; For curvature;

[0051] The bending stiffener is a segmented variable cross-section beam model. The moment of inertia of the cross-section varies along the arc length coordinate s. The high-polymer polyurethane material used in the bending stiffener is fitted using Yeoh's third-order constitutive equation.

[0052] ,

[0053] in, Strain energy per unit reference volume; , , These are material constants; For deformation tensors;

[0054] Step 5.2, Buoyancy block position analysis;

[0055] A joint simulation mechanical model of the buoyancy block and the flexible riser is established. At the same time, a radial clamping force is applied to the buoyancy block and transmitted to the flexible riser, and a vertical upward buoyancy force is applied to the flexible riser. The buoyancy block and the flexible riser are in frictional contact. Nonlinear static calculations are performed on the flexible riser body under the current state.

[0056] Buoyancy blocks are usually installed on the outside of risers in a spaced-out manner. The buoyancy section equivalent continuity method is used to convert the geometric and mass properties of the buoyancy block section to the riser unit.

[0057] Overall equivalent buoyancy section outer diameter for:

[0058] ,

[0059] in, The outer diameter of the bare tube; The outer diameter of the buoyancy block; The axial length of a single buoyancy block; The installation spacing of the buoyancy blocks;

[0060] Overall equivalent buoyancy for:

[0061] ,

[0062] in, The density of seawater; It is the acceleration due to gravity; This refers to the buoyancy per unit length of the bare tube.

[0063] The buoyancy block applies a radial clamping force to the riser through bolt pre-tightening, and the elastic contact pressure of the buoyancy block... for:

[0064] ,

[0065] in, The axial contact length of the buoyancy block; The lateral distance from a point on the contact surface of the buoyancy block to the center line of the contact; For clamping force;

[0066] Step 5.3, Counterweight position analysis;

[0067] A joint simulation mechanical model is established for the layered equivalent model of the counterweight and the flexible riser. At the same time, a radial clamping force is applied to the counterweight and transmitted to the flexible riser, and a vertical downward counterweight force is applied to the flexible riser. The counterweight and the flexible riser are in frictional contact. Nonlinear static calculations are performed on the flexible riser body under the current state.

[0068] The counterweight applies radial clamping force to the riser through bolt pre-tightening, and the elastic contact pressure of the counterweight... for:

[0069] ,

[0070] in, This refers to the axial contact length of the counterweight. It is the lateral distance from a point on the contact surface of the counterweight to the center line of the contact;

[0071] Step 5.4, Analysis of the location of the seabed fixing device;

[0072] A joint simulation mechanical model of the seabed fixing device and the flexible riser layer equivalent model is established. A radial clamping force is applied to the flexible riser and a tension load is applied to the riser after it is displaced to a certain position. Nonlinear static calculations are performed on the flexible riser body in the current state.

[0073] The clamps in the seabed fixing device apply radial clamping force to the riser through bolt pre-tightening, and the elastic contact pressure of the seabed fixing device... for:

[0074] ,

[0075] in, The axial contact length of the clamp in the seabed fixing device; The lateral distance from a point on the contact surface of the clamp in the bed fixing device to the contact center line;

[0076] Rope connection point ( , ) and counterweight anchor point ( The catenary equation is satisfied between ,0)

[0077] ,

[0078] ,

[0079] in, This represents the horizontal tension component of the rope. The mass per unit length of the rope; It is the acceleration due to gravity; The horizontal coordinates of the connection point between the tether and the hose; The vertical coordinates of the connection point between the tether and the hose; The horizontal coordinates of the counterweight anchor point; This represents the vertical height of the tether at the corresponding horizontal position x. is the characteristic length of the catenary.

[0080] Step 6 includes:

[0081] Extract the water flow and pressure parameters at each point on the riser boundary and couple them with the overall equivalent model of the riser body in step 1 to obtain the high-cycle cyclic stress caused by the continuous vibration displacement of the flexible riser under wave flow.

[0082] Volumetric Multiphase Flow (VOF) Model: Defines a volume fraction variable Within the computing unit This indicates that the unit is filled with water; This indicates that the cell is filled with gas; This indicates that the cell contains a free surface, and the density within each cell is... Dynamic viscosity Determined by the phase fraction mixing:

[0083] ,

[0084] ,

[0085] in The density of water, The density of the gas, The dynamic viscosity of water, The dynamic viscosity of the gas;

[0086] Numerical wave generation for open channel flow employs a velocity inlet method, which specifies the velocity of water particles and the wave rise at the inlet boundary:

[0087] ,

[0088] Where H is the wave height and ω is the wave angular frequency. For wave number, Let z represent the water depth and z represent the position of the wave surface. Indicates time, For the point located at coordinates (x, z) at time... The instantaneous velocity in the horizontal direction, Represents the horizontal coordinate.

[0089] Step 8 includes:

[0090] The stress-life curve of a metallic material is represented as follows:

[0091] ,

[0092] Where N is the number of failure cycles, Let m be the stress range, and m be the negative reciprocal slope of the stress-life curve. It is the intercept constant related to material, detail construction, and survival probability;

[0093] The stress-life curve of a polymer material is represented as follows:

[0094] ,

[0095] in is the fatigue strength coefficient, and b is the fatigue strength index. This represents the number of loop iterations.

[0096] The Gerber modified model corrects the equivalent stress amplitude after a given stress cycle:

[0097] ,

[0098] in This is the equivalent stress amplitude. Stress amplitude: , Mean stress: , This represents the ultimate tensile strength of the material.

[0099] Step 9 includes:

[0100] Within a cyclic block containing k different stress amplitudes, the total damage D is:

[0101] ,

[0102] in Equivalent stress amplitude Number of times it appears This refers to the number of cycles required for structural fatigue failure under constant amplitude alternating stress corresponding to this stress range, i.e., when D=1, fatigue damage occurs in the flexible riser or bending reinforcement.

[0103] The predicted fatigue life is:

[0104] ,

[0105] in For the predicted fatigue life, The current stress cycle time;

[0106] Step 9.1, Bending reinforcement area;

[0107] The bending reinforcement area requires joint fatigue calculation of the bending reinforcement and the flexible riser to obtain the fatigue life of the bending reinforcement body and the flexible riser respectively. The fatigue life time of the first failure is selected from the fatigue life of the bending reinforcement body and the flexible riser to predict the fatigue life of the interface between the floating platform and the flexible riser.

[0108] Step 9.2, area of ​​buoyancy blocks, counterweights, and seabed fixing devices;

[0109] Fatigue calculations need to be performed on the flexible riser body in the areas of buoyancy block, counterweight block, and seabed fixing device to obtain the fatigue life of the first layer of the flexible riser to fail in the areas of buoyancy block, counterweight block, and seabed fixing device.

[0110] The fatigue life of the overall flexible riser system is obtained by combining steps 9.1 and 9.2.

[0111] Beneficial effects: (1) This invention proposes for the first time a multimodal synergistic fatigue damage analysis method for the integrated riser and accessories. The bending reinforcement, buoyancy block, counterweight, seabed fixing device and flexible riser are modeled and analyzed as a combination of mechanical properties, and the bending reinforcement provides flexible fatigue protection for the main body of the riser; the influence of the local stiffness change caused by the local bearing effect of buoyancy, counterweight, and seabed fixing device is analyzed. Thus, different fatigue assessment methods are realized for different hot spots on the flexible riser.

[0112] (2) This invention aims to overcome the limitations of traditional single-field simulation analysis and plans to construct a fully coupled, nonlinear unified analysis method. It includes hydrodynamic analysis of the hull and piping system, flow field analysis, dynamic analysis, and fatigue analysis, and integrates nonlinear factors such as vortex-induced vibration parameters and outer sheath wear. It realizes a precise and complete flexible riser analysis system. Attached Figure Description

[0113] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, and the advantages of the present invention in the above and / or other aspects will become clearer.

[0114] Figure 1 This is a diagram of a flexible riser accessory system.

[0115] Figure 2 This is a diagram of the method architecture of the present invention.

[0116] Figure 3 This is a schematic diagram of the overall model of the skeleton layer.

[0117] Figure 4 This is a cross-sectional view of the skeleton layer.

[0118] Figure 5 This is a schematic diagram of the overall model of the pressure-resistant armor layer.

[0119] Figure 6 This is a cross-sectional view of the compression armor.

[0120] Figure 7 This is a schematic diagram of a hierarchical equivalent model.

[0121] Figure 8 This is a schematic diagram of the equivalent radial elastic modulus of the skeleton layer and the compressive armor layer.

[0122] Figure 9 This is a schematic diagram of the equivalent torsional elastic modulus of the skeleton layer and the compressive armor layer.

[0123] Figure 10 This is a schematic diagram of the equivalent Poisson's ratio under axial load.

[0124] Figure 11 This is a schematic diagram of the overall equivalent model of the flexible riser.

[0125] Figure 12 This is a model diagram of a bending reinforcement component.

[0126] Figure 13 It is a model diagram of the buoyancy block and the counterweight block.

[0127] Figure 14 This is a model diagram of a seabed anchorage device.

[0128] Figure 15 This is a schematic diagram of radial load calculation for the equivalent model.

[0129] Figure 16 This is a schematic diagram of axial load calculation for the equivalent model.

[0130] Figure 17 This is a schematic diagram of the input of the floating body's geometric parameters.

[0131] Figure 18 This is a schematic diagram of the input of the buoyancy moment of inertia.

[0132] Figure 19 This is a diagram showing the tension distribution along the flexible riser.

[0133] Figure 20 This is a diagram showing the distribution of curvature along the flexible riser.

[0134] Figure 21 This is a schematic diagram of the stress-strain curve of a polymer material.

[0135] Figure 22 This is a schematic diagram of the curve fitted to Yeoh's third-order constitutive equation.

[0136] Figure 23 This is a simulation calculation diagram of a bending reinforcement component.

[0137] Figure 24 This is a simulation calculation diagram of a flexible riser.

[0138] Figure 25 This is a schematic diagram of the joint simulation calculation of the buoyancy block or counterweight block and the flexible riser.

[0139] Figure 26 This is a schematic diagram of the joint simulation calculation of the seabed fixing device and the flexible riser.

[0140] Figure 27 This is a time history graph of the amplitude of the flexible riser.

[0141] Figure 28 This is a diagram showing the vibration pattern of a flexible riser.

[0142] Figure 29 This is the fitted stress-life curve of the polymer material.

[0143] Figure 30 It is a time history curve of axial tensile stress in a flexible riser.

[0144] Figure 31 It is a time history curve of the bending curvature of a flexible riser. Detailed Implementation

[0145] This embodiment provides a method for analyzing the local fatigue life of a dynamic flexible riser including riser accessories, comprising the following steps:

[0146] Step 1: First, establish two equivalent models for the flexible riser: a layered equivalent model and an overall equivalent model.

[0147] Simulation and analytical calculations of radial load, torsional load, and axial load were performed on the skeleton layer and pressure-resistant armor layer of the flexible riser irregular structure. After adding the sealing layer and outer covering layer of the regular structure, the overall calculation results were fitted and compared with the calculation results of the layered equivalent model, and error analysis was performed to form a layered equivalent model that can guide the actual situation.

[0148] The overall model and cross-sectional view of the skeleton layer and the compressive armor layer are as follows: Figure 3 , Figure 4 , Figure 5 , Figure 6 As shown. The hierarchical equivalent model is as follows. Figure 7 As shown.

[0149] The calculation process for radial load parameters is as follows:

[0150] Equivalent radial elastic modulus of the skeleton layer and the compressive armor layer for:

[0151] ,

[0152] Where E is the elastic modulus of the two metal layers, t is the layer thickness, L is the total length of a single buckle, and c is the overlap length of a single buckle, as detailed below. Figure 8 As shown.

[0153] The calculation process for torsional load parameters is as follows:

[0154] Equivalent torsional elastic modulus of the skeleton layer and the compressive armor layer for:

[0155] ,

[0156] Where A represents the cross-sectional area of ​​each layer, as shown in the figure below. Figure 9 As shown.

[0157] The calculation process for axial load parameters is as follows:

[0158] Its equivalent Poisson's ratio under axial load for:

[0159] ,

[0160] in The Poisson's ratio for two layers of metal materials, specifically as follows: Figure 10 As shown.

[0161] The overall radial, torsional, and axial loads are calculated on the layered equivalent model. The calculation results are then fitted with the overall equivalent model calculation results, and error analysis is performed to output an overall equivalent model of the flexible riser with accurate mechanical properties. Figure 11 As shown.

[0162] By adjusting the Poisson's ratio, radial elastic modulus, torsional elastic modulus, and density parameters in the finite element simulation parameters of the overall equivalent model, it is ensured that the strain at each point on the overall equivalent model is the same as the strain at each point on the layered equivalent model after applying the same tensile and torsional loads. The adjusted parameter set is then considered the most reasonable overall equivalent model.

[0163] The layered equivalent model is used as the input condition for local stress analysis, while the overall equivalent model is used as the input condition for environmental load calculation.

[0164] Step 2: Obtain the geometry, mass, center of mass, and moments of inertia in three directions of the floating body as input conditions for the geometric model. Output the RAOs (Response Amplitude Operator Curve) curves of the floating body under different wave incidence angles through hydrodynamic analysis.

[0165] Acquire and monitor ocean current parameters in the current sea area: current speed and direction; wave parameters: wave height, period, and wave direction; seabed parameters: seabed type and soil density; and seawater parameters: seawater density, as input conditions for environmental load calculation.

[0166] The forced vibration of a floating body under wave action can be expressed by the frequency domain equation. describe: ,

[0167] Where M is the mass matrix of the floating body (6×6), which is assembled from the total mass m, the coordinates of the center of mass and the moments of inertia Ixx, Iyy, Izz; An additional mass matrix is ​​added to reflect the inertial effects of the fluid around the buoy; The potential flow damping matrix originates from the energy dissipation of radiated waves. This is the linearized viscous damping matrix; The stiffness matrix for still water recovery is non-zero only for heave, roll, and pitch, and is determined by the waterline surface area and the relative position of the center of buoyancy and the center of gravity. Additional stiffness matrix provided for mooring / connection systems; It is the complex amplitude vector of six degrees of freedom motion at the center of gravity of the floating body, which includes sway, roll, heave, pitch, pitch and yaw.

[0168] Step 3: Based on the overall equivalent model of the flexible riser, add bending reinforcement, buoyancy block, counterweight block and seabed fixing device system model, perform global hydrodynamic simulation analysis of the riser system, output the tension, bending moment and curvature on the riser at the four accessory positions, and obtain the curves of mechanical parameters changing with the riser position.

[0169] Model of bending stiffener as follows Figure 12 As shown, it is a hollow conical tube, and its detailed dimensions, density, bending stiffness, axial stiffness, and torsional stiffness need to be input in the hydrodynamic analysis.

[0170] The models of the buoyancy block and the counterweight are as follows: Figure 13 As shown, both of these are two-part clamp structures, and their detailed dimensions, downward pressure on the flexible riser, density, bending stiffness, axial stiffness, and torsional stiffness need to be input in the hydrodynamic analysis.

[0171] Model of seabed anchorage device, such as Figure 14 As shown, it consists of a counterweight, a clamp, and a rope. The hydrodynamic analysis requires inputting the downward pressure of the clamp on the flexible riser, the weight of the counterweight, and the length of the rope.

[0172] Step 4: Obtain the friction parameters of each layer of the flexible riser; the stress-strain curve of the polymer material used for the bending reinforcement, and fit it using the Yeoh third-order constitutive equation; density, dimensions of the bending reinforcement, bending, torsion, and axial stiffness; dimensions, density, bending, torsion, and axial stiffness of the buoyancy block and counterweight; weight and mooring rope length of the seabed fixing device.

[0173] Step 5: Combining the output information from Steps 1, 3, and 4, perform local stress analysis at four locations of the flexible riser: the bending reinforcement location, the buoyancy block location, the counterweight location, and the seabed fixing device location.

[0174] Step 5.1, Analysis of the location of bending reinforcement;

[0175] A joint simulation mechanical model of the bending stiffener and the flexible riser with layered equivalent model is established. The bending stiffener and the flexible riser are initially in a non-contact state. Nonlinear static calculations are performed on the flexible riser and bending stiffener assembly in the state.

[0176] The flexible riser is simplified into anisotropic beam elements, and its constitutive relation is obtained through layered integration: ,

[0177] in: ,

[0178] , , Let be the elastic modulus, cross-sectional area, and moment of inertia of the i-th layer material; is the distance from the neutral axis of the i-th layer to the global neutral axis; This is the equivalent axial tensile stiffness; Equivalent bending stiffness;

[0179] The bending stiffener is a segmented variable cross-section beam model, whose moment of inertia varies along the arc length coordinate s. The high-polymer polyurethane material used is fitted with the Yeoh third-order constitutive equation: ,

[0180] in, Strain energy per unit reference volume; , , These are material constants; It is a deformation tensor.

[0181] Step 5.2, Buoyancy block position analysis;

[0182] A joint simulation mechanical model of the buoyancy block and the flexible riser is established. At the same time, a radial clamping force is applied to the buoyancy block and transmitted to the flexible riser, and a vertical upward buoyancy force is applied to the flexible riser. The buoyancy block and the flexible riser are in frictional contact. Nonlinear static calculations are performed on the flexible riser body under the current state.

[0183] Buoyancy blocks are usually installed on the outside of risers in a spaced-out manner. This invention adopts the buoyancy segment equivalent continuity method to convert the geometric and mass properties of the buoyancy block segments into equivalent riser units.

[0184] Overall equivalent buoyancy section outer diameter for:

[0185] ,

[0186] in, The outer diameter of the equivalent buoyancy section; The outer diameter of the bare tube; The outer diameter of the buoyancy block; The axial length of a single buoyancy block; This refers to the installation spacing of the buoyancy blocks.

[0187] Overall equivalent buoyancy for:

[0188] ,

[0189] in, The density of seawater; It is the acceleration due to gravity; This represents the buoyancy per unit length of the bare tube.

[0190] The buoyancy block applies a radial clamping force to the riser through bolt pre-tightening, and its elastic contact pressure for:

[0191] ,

[0192] in, The axial contact length of the buoyancy block; It is the lateral distance from a point on the contact surface of the buoyancy block to the contact center line; This refers to the clamping force.

[0193] Step 5.3, Counterweight position analysis;

[0194] A joint simulation mechanical model is established for the layered equivalent model of the counterweight and the flexible riser. At the same time, a radial clamping force is applied to the counterweight and transmitted to the flexible riser, and a vertical downward counterweight force is applied to the flexible riser. The counterweight and the flexible riser are in frictional contact. Nonlinear static calculations are performed on the flexible riser body under the current state.

[0195] The counterweight applies radial clamping force to the riser through bolt pre-tightening, and its elastic contact pressure for:

[0196] ,

[0197] in, This refers to the axial contact length of the counterweight. It is the lateral distance from a point on the contact surface of the counterweight to the contact center line; This refers to the clamping force.

[0198] Step 5.4, Analysis of the location of the seabed fixing device;

[0199] A joint simulation mechanical model of the seabed fixing device and the flexible riser layer equivalent model is established. A radial clamping force is applied to the flexible riser and a tension load is applied to the riser after it is displaced to a certain position. Nonlinear static calculations are performed on the flexible riser body in the current state.

[0200] The clamps in the seabed fixing device apply radial clamping force to the riser through bolt pre-tightening, and their elastic contact pressure for:

[0201] ,

[0202] in, The axial contact length of the clamp in the seabed fixing device; The lateral distance from a point on the contact surface of the clamp in the bed fixing device to the contact center line; For clamping force;

[0203] Rope connection point ( , ) and counterweight anchor point ( The catenary equation is satisfied between ,0)

[0204] ,

[0205] ,

[0206] in, This represents the horizontal tension component of the rope. The mass per unit length of the rope; This is the acceleration due to gravity.

[0207] Step 6: Using the fluid volumetric method multiphase flow model VOF and the k-epsilon viscous model, define the gas-liquid mixture material, define the baseline values ​​and initial conditions, set boundary conditions to perform numerical wave generation for open channel flow, set the solver and termination conditions, and perform initialization solution.

[0208] Extract the water flow and pressure parameters at each point on the riser boundary and couple them with the overall equivalent model of the riser body in step 1 to obtain the high-cycle cyclic stress caused by the continuous vibration displacement of the flexible riser under wave flow.

[0209] Volumetric Multiphase Flow (VOF) Model: Defines a volume fraction variable Within the computing unit This indicates that the unit is filled with water (liquid phase). This indicates that the unit is filled with gas (gas phase). This indicates that the cell contains a free surface, and the density within each cell is... Dynamic viscosity Determined by the phase fraction mixing:

[0210] ,

[0211] ,

[0212] in The density of water, The density of the gas, The dynamic viscosity of water, Let be the dynamic viscosity of the gas.

[0213] Numerical wave generation for open channel flow employs a velocity inlet method, which specifies the velocity of water particles and the wave rise at the inlet boundary:

[0214] ,

[0215] Where H is the wave height and ω is the wave angular frequency. For wave number, Let z represent the water depth and z represent the position of the wave surface. Indicates time, Let be a point located at coordinates (x, z), at time... The instantaneous velocity in the horizontal direction, This represents the horizontal coordinate of the point.

[0216] Step 7: Integrate the local stress parameters of the four locations in Step 5 with the high-cycle cyclic stress caused by vortex-induced vibration in Step 6 to form the fatigue hot spot locations and load magnitudes of the four riser accessory areas.

[0217] Step 8: Obtain the stress life curves of each layer of material in the flexible riser and the stress life curve of the polymer material used for the bending reinforcement. Then, use the Gerber correction model to correct the stress amplitude of the two stress life curves.

[0218] The stress-life curve of a metallic material is represented as follows:

[0219] ,

[0220] Where N is the number of failure cycles, Let m be the stress range, and m be the negative reciprocal slope of the stress-life curve. It is the intercept constant related to material, detail construction, and survival probability.

[0221] The stress-life curve of a polymer material is represented as follows:

[0222] ,

[0223] in is the fatigue strength coefficient, and b is the fatigue strength index. This represents the number of iterations.

[0224] The Gerber modified model corrects the equivalent stress amplitude after a given stress cycle:

[0225] ,

[0226] in This is the equivalent stress amplitude. Stress amplitude: , Mean stress: , The ultimate tensile strength of the material;

[0227] Step 9: Based on the linear cumulative damage theory, fatigue stress analysis is performed on the fatigue hot spots of the four riser accessory areas to obtain the tension stress time history curve and bending stress time history curve. The damage degree is calculated by the rainflow counting method and the fatigue life is predicted. Among them, the bending reinforcement hot spot area needs to be analyzed by combining the bending reinforcement with the flexible riser.

[0228] Within a cyclic block containing k different stress amplitudes, the total damage D is:

[0229] ,

[0230] in Equivalent stress amplitude Number of times it appears This refers to the number of cycles required for structural fatigue failure under constant amplitude alternating stress corresponding to this stress range, i.e., when D=1, fatigue damage occurs in the flexible riser or bending reinforcement.

[0231] The predicted fatigue life is:

[0232] ,

[0233] in For the predicted fatigue life, The current stress cycle time;

[0234] Step 9.1, Bending reinforcement area;

[0235] The bending reinforcement area requires joint fatigue calculation of the bending reinforcement and the flexible riser to obtain the fatigue life of the bending reinforcement body and the flexible riser respectively. The fatigue life time of the first failure is selected from the fatigue life of the bending reinforcement body and the flexible riser to predict the fatigue life of the rigid-flexible interface between the floating platform and the flexible riser.

[0236] Step 9.2, area of ​​buoyancy blocks, counterweights, and seabed fixing devices;

[0237] For the buoyancy block, counterweight block, and seabed fixing device areas, only fatigue calculations need to be performed on the flexible riser body to obtain the fatigue life of the first failure layer of the flexible riser in the buoyancy block, counterweight block, and seabed fixing device areas respectively.

[0238] Combining steps 9.1 and 9.2 above, the fatigue life of the overall flexible riser system is obtained.

[0239] In one specific embodiment of the present invention, such as Figure 1 , Figure 2 As shown, a method for analyzing the local fatigue life of a dynamic flexible riser including riser accessories is provided, comprising the following steps:

[0240] Step 1: First, establish two equivalent models for the flexible riser: a layered equivalent model and an overall equivalent model.

[0241] Simulation and analytical calculations of radial load, torsional load, and axial load were performed on the skeleton layer and the pressure-resistant armor layer of the flexible riser irregular structure. The calculation results of the irregular body were fitted and compared with the calculation results of the equivalent model, and error analysis was performed to form a layered equivalent model that can guide the actual practice.

[0242] The overall axial, radial and torsional loads of the layered equivalent model are calculated. The calculation results are then fitted with the calculation results of the overall equivalent model and error analysis is performed to output a flexible riser overall equivalent model with accurate mechanical properties.

[0243] The layered equivalent model is used as the input condition for local stress analysis, while the overall equivalent model is used as the input condition for environmental load calculation. For example... Figure 15 , Figure 16 As shown.

[0244] Step 2: Obtain the geometry, mass, center of mass, and moments of inertia in three directions of the floating body as input conditions for the geometric model. Output the RAOs curves of the floating body under different wave incidence angles through hydrodynamic analysis.

[0245] Acquire and monitor ocean current parameters in the current sea area: current speed and direction; wave parameters: wave height, period, and direction; seabed parameters: seabed type and soil density; and seawater parameters: seawater density, as input conditions for environmental load calculations. Figure 17 , Figure 18 As shown.

[0246] Figure 17 In this context, details of ceometry refers to the geometric details; name is the name of the geometry; attachedassembly path is the file save location; environment constants are environmental parameters; water depth is the water depth; water density is the water density; gravitational acceleration is the gravitational acceleration; water size X is the lateral length of the water body; and water size Y is the longitudinal length of the water body.

[0247] Figure 18 In this context, "details of point mass" refers to the details of the center of mass; "name" is the name of the center of mass; "activity" refers to the activity of the center of mass; "point mass properties" refers to the characteristics of the center of mass; "mass definition" is the definition of the center of mass; "Z" is the depth of the center of mass; "inertia properties" are the inertia properties; "Kxx", "Kyy", and "Kzz" are the positions of the center of mass in the whole; and "Ixy", "Ixz", and "Iyz" are the moments of inertia of the center of mass in the three directions.

[0248] Step 3: Based on the overall equivalent model of the flexible riser, add system models for bending reinforcements, buoyancy blocks, counterweights (dimensions, density, axial stiffness, bending stiffness, torsional stiffness), and seabed fixing devices (total weight, mooring rope length) to perform a global hydrodynamic simulation analysis of the riser system. The focus is on outputting the tension, bending moment, and curvature on the riser at these four accessory locations, obtaining curves showing the variation of these mechanical parameters with the riser's position. For example... Figure 19 , Figure 20 As shown.

[0249] Step 4: Obtain the friction parameters of each layer of the flexible riser; the stress-strain curve of the polymer material used for the bending reinforcement, and fit it using the Yeoh third-order constitutive equation; density, dimensions of the bending reinforcement, bending, torsion, and axial stiffness; dimensions, density, bending, torsion, and axial stiffness of the buoyancy block and counterweight; weight and mooring rope length of the seabed fixing device. Figure 21, Figure 22 As shown.

[0250] Step 5: Combining the output information from Steps 1, 3, and 4, perform local stress analysis at four locations of the flexible riser (bending reinforcement location, buoyancy block location, counterweight location, and seabed fixing device location).

[0251] Step 5.1, Analysis of the location of bending reinforcement;

[0252] A joint simulation mechanical model of the bending stiffener and the flexible riser, with the bending stiffener and the flexible riser initially in a non-contact state, is established. Nonlinear static calculations are then performed on the combined flexible riser and bending stiffener in this state. Figure 23 , Figure 24 As shown.

[0253] Step 5.2, Buoyancy block position analysis;

[0254] A joint simulation mechanical model is established for the buoyancy block and the flexible riser, which are equivalent to layers. A radial clamping force is applied to the buoyancy block and transmitted to the flexible riser, while an upward vertical buoyancy force is applied to the flexible riser. The buoyancy block and the flexible riser are in frictional contact. Nonlinear static calculations are performed on the flexible riser body under this state.

[0255] Step 5.3, Counterweight position analysis;

[0256] A joint simulation mechanical model is established, employing a layered equivalent model of the counterweight and the flexible riser. A radial clamping force is applied to the counterweight and transmitted to the flexible riser, while a vertically downward counterweight force is applied to the flexible riser. The counterweight and flexible riser are in frictional contact. Nonlinear static calculations are performed on the flexible riser under this condition. Figure 25 As shown.

[0257] Step 5.4, Analysis of the location of the seabed fixing device;

[0258] A joint simulation mechanical model was established, combining a layered equivalent model of the seabed fixing device and the flexible riser. A radial clamping force was applied to the flexible riser, along with a tensile load after it displaced to a certain position. Nonlinear static calculations were then performed on the flexible riser under this condition. Figure 26 As shown.

[0259] Step 6: Use the volumetric fluid flow (VOF) model and the k-epsilon viscosity model (k-epsilon model) to perform the multiphase flow model. (Model). Define the gas-liquid mixture material, define the baseline values ​​and initial conditions, set boundary conditions to perform numerical wave generation for open channel flow, set the solver and termination conditions, and perform initialization solution;

[0260] The flow and pressure parameters at various points along the riser boundary are extracted and coupled with the overall equivalent model of the riser body for calculation, yielding the high-cycle cyclic stress caused by the continuous vibration displacement of the flexible riser under wave flow. For example... Figure 27 , Figure 28 As shown.

[0261] Figure 27 The horizontal axis represents the time of water flow, and the vertical axis represents the swing amplitude of the flexible riser.

[0262] Figure 28 The horizontal axis represents the distance of the flexible riser's lateral swing, and the vertical axis represents the distance of the flexible riser's longitudinal swing.

[0263] Step 7: Integrate the local stress parameters of the four locations in Step 5 with the high-cycle cyclic stress caused by vortex-induced vibration in Step 6 to form the fatigue hot spot locations and load magnitudes of the four riser accessory areas.

[0264] Step 8: Obtain the stress-life curves of each layer of the flexible riser material, and the stress-life curve of the polymer material used for the bending reinforcement. Then, use the Gerber correction model to correct the stress amplitude of these two stress-life curves. For example... Figure 29 As shown.

[0265] Step 9: Based on the linear cumulative damage theory, fatigue stress analysis is performed on the four fatigue hotspot locations to obtain the axial tensile stress time history curve and the bending curvature time history curve. The damage degree is calculated using the rainflow counting method, and the fatigue life is predicted. For the hotspot area of ​​the bending reinforcement, fatigue analysis needs to be performed jointly on the bending reinforcement and the flexible riser. For example... Figure 30 , Figure 31 As shown.

[0266] Step 9.1, Bending reinforcement area;

[0267] In this area, joint fatigue calculations are required for the bending stiffener and the flexible riser to determine their respective fatigue lives. The fatigue life of the bending stiffener and the flexible riser that fails first will be selected to estimate the fatigue life at the rigid-flexible interface between the floating platform and the flexible riser.

[0268] Step 9.2, area of ​​buoyancy blocks, counterweights, and seabed fixing devices;

[0269] In this area, only fatigue calculations need to be performed on the flexible riser body to obtain the fatigue life of the first layer to fail in the flexible riser in each of the three areas.

[0270] Combining steps 9.1 and 9.2 above, the fatigue life of the overall flexible riser system is obtained.

[0271] This invention provides a method for analyzing the local fatigue life of a dynamic flexible riser including riser accessories. Many methods and approaches exist for implementing this technical solution; the above description is merely a preferred embodiment of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications should also be considered within the scope of protection of this invention. All components not explicitly stated in this embodiment can be implemented using existing technologies.

Claims

1. A method of localised fatigue life analysis of a dynamically flexible riser comprising a riser accessory, characterised in that, Includes the following steps: Step 1: First, establish two equivalent models for the flexible riser: a layered equivalent model and an overall equivalent model. Step 2: Obtain the geometry, mass, center of mass, and moments of inertia in three directions of the floating body as input conditions for the geometric model. Output the amplitude response operator curves (RAOs) of the floating body under different wave incidence angles through hydrodynamic analysis. Step 3: Based on the overall equivalent model of the flexible riser, add a system model of bending reinforcement, buoyancy block, counterweight, and seabed fixing device, and perform a global hydrodynamic simulation analysis of the riser system. Output the tension, bending moment, and curvature of the riser at the four accessory positions of bending reinforcement, buoyancy block, counterweight, and seabed fixing device to obtain the curves of the mechanical parameters changing with the position of the riser. Step 4: Obtain the friction parameters of each layer of the flexible riser; the stress-strain curve of the polymer material used for the bending reinforcement, and fit it using the Yeoh third-order constitutive equation; density, dimensions of the bending reinforcement, bending, torsion, and axial stiffness; dimensions, density, bending, torsion, and axial stiffness of the buoyancy block and counterweight; weight and mooring rope length of the seabed fixing device. Step 5: Combining the output information from Step 1, Step 3, and Step 4, perform local stress analysis at four locations of the flexible riser: the location of the bending reinforcement, the location of the buoyancy block, the location of the counterweight block, and the location of the seabed fixing device. Step 6: Using the fluid volumetric method multiphase flow model VOF and the turbulent kinetic energy-dissipation rate viscous model k-epsilon viscous model, define the gas-liquid mixture material, define the baseline values ​​and initial conditions, set boundary conditions to perform numerical wave generation for open channel flow, set the solver and termination conditions, and perform initialization solution. Step 7: Integrate the local stress parameters of the four locations in Step 5 with the high-cycle cyclic stress caused by vortex-induced vibration in Step 6 to form the fatigue hot spot locations and load magnitudes of the four riser accessory areas. Step 8: Obtain the stress life curves of each layer of material in the flexible riser and the stress life curve of the polymer material used for the bending reinforcement. Then, use the Gerber correction model to correct the stress amplitude of the two stress life curves. Step 9: Based on the linear cumulative damage theory, fatigue stress analysis is performed on the fatigue hotspot locations of the four riser accessory areas to obtain the tension stress time history curve and bending stress time history curve. The damage degree is calculated by the rainflow counting method and the fatigue life is predicted. For the hotspot area of ​​the bending reinforcement, fatigue analysis needs to be performed on the bending reinforcement and the flexible riser together.

2. The method of claim 1, wherein, Step 1 includes: performing simulation and analytical calculations of radial load, torsional load, and axial load on the skeleton layer and the pressure-resistant armor layer of the flexible riser irregular structure, respectively. After adding the sealing layer and outer covering layer of the regular structure, the overall calculation results are fitted and compared with the calculation results of the layered equivalent model, and error analysis is performed to form a layered equivalent model that can guide the actual situation.

3. The method of claim 2, wherein, In step 1, the calculation process for the radial load parameters includes: Equivalent radial elastic modulus of skeleton layer, pressure-resistant armor layer Is: ， Where E is the elastic modulus of the metal material of the skeleton layer and the pressure-resistant armor layer, t is the layer thickness, L is the total length of a single buckle, and c is the overlap length of a single buckle. The calculation process for torsional load parameters includes: Equivalent torsional elastic modulus of the skeleton layer, the pressure-resistant armor layer Is: ， Where A is the cross-sectional area of ​​each layer; In step 1, the calculation process for the axial load parameters includes: Equivalent Poisson's ratio of the skeleton layer and the compressive armor layer under axial load for: ， in Poisson's ratio for the metallic materials of the skeleton layer and the pressure-resistant armor layer.

4. The method according to claim 3, characterized in that, In step 1, the overall radial, torsional, and axial loads are calculated on the layered equivalent model. The calculation results are then fitted with the calculation results of the overall equivalent model, and error analysis is performed to output an overall equivalent model of the flexible riser with accurate mechanical properties. By adjusting the Poisson's ratio, radial elastic modulus, torsional elastic modulus, and density parameters in the finite element simulation parameters of the overall equivalent model, the strain at each point on the overall equivalent model is the same as the strain at each point on the layered equivalent model after the same tensile and torsional loads are applied. At this time, the adjusted parameter set is the most reasonable overall equivalent model. The layered equivalent model is used as the input condition for local stress analysis, while the overall equivalent model is used as the input condition for environmental load calculation.

5. The method according to claim 4, characterized in that, Step 2 includes: acquiring and monitoring ocean current parameters in the current sea area, including current speed and direction; wave parameters, including wave height, period, and wave direction; seabed parameters, including seabed type and soil density; and seawater parameters, including seawater density, as input conditions for environmental load calculation. The forced vibration of a floating body under wave action is given by the frequency domain equation. describe: , Where M is the mass matrix of the floating body; For the additional mass matrix; The potential flow damping matrix; This is the linearized viscous damping matrix; To recover the stiffness matrix in still water; Additional stiffness matrix provided for mooring or connection systems; The complex amplitude vector of the six-degree-of-freedom motion at the center of gravity of the buoyancy; Angular frequency; The angle of incidence of the wave; It is the imaginary unit.

6. The method according to claim 5, characterized in that, Step 3 includes: the model of the bending reinforcement is a hollow conical tube; the seabed fixing device includes a counterweight, a clamp and a mooring rope.

7. The method according to claim 6, characterized in that, Step 5 includes: Step 5.1, Analysis of the location of bending reinforcement; A joint simulation mechanical model of the bending stiffener and the flexible riser with layered equivalent model is established. The bending stiffener and the flexible riser are initially in a non-contact state. Nonlinear static calculations are performed on the flexible riser and bending stiffener assembly in the state. The flexible riser is simplified into anisotropic beam elements, and its constitutive relation is obtained through layered integration: ， in: ， , , Let be the elastic modulus, cross-sectional area, and moment of inertia of the i-th layer material, respectively. is the distance from the neutral axis of the i-th layer to the global neutral axis; This is the equivalent axial tensile stiffness; Equivalent bending stiffness; It is the axial force; For bending moment; For axial strain; For curvature; The bending stiffener is a segmented variable cross-section beam model. The moment of inertia of the cross-section varies along the arc length coordinate s. The high-polymer polyurethane material used in the bending stiffener is fitted using Yeoh's third-order constitutive equation. ， in, Strain energy per unit reference volume; , , These are material constants; For deformation tensors; Step 5.2, Buoyancy block position analysis; A joint simulation mechanical model of the buoyancy block and the flexible riser is established. At the same time, a radial clamping force is applied to the buoyancy block and transmitted to the flexible riser, and a vertical upward buoyancy force is applied to the flexible riser. The buoyancy block and the flexible riser are in frictional contact. Nonlinear static calculations are performed on the flexible riser body under the current state. Buoyancy blocks are usually installed on the outside of risers in a spaced-out manner. The buoyancy section equivalent continuity method is used to convert the geometric and mass properties of the buoyancy block section to the riser unit. Overall equivalent buoyancy section outer diameter for: ， in, The outer diameter of the bare tube; The outer diameter of the buoyancy block; The axial length of a single buoyancy block; The installation spacing of the buoyancy blocks; Overall equivalent buoyancy for: ， in, The density of seawater; It is the acceleration due to gravity; This refers to the buoyancy per unit length of the bare tube. The buoyancy block applies a radial clamping force to the riser through bolt pre-tightening, and the elastic contact pressure of the buoyancy block... for: ， in, The axial contact length of the buoyancy block; The lateral distance from a point on the contact surface of the buoyancy block to the center line of the contact; For clamping force; Step 5.3, Counterweight position analysis; A joint simulation mechanical model is established for the layered equivalent model of the counterweight and the flexible riser. At the same time, a radial clamping force is applied to the counterweight and transmitted to the flexible riser, and a vertical downward counterweight force is applied to the flexible riser. The counterweight and the flexible riser are in frictional contact. Nonlinear static calculations are performed on the flexible riser body under the current state. The counterweight applies radial clamping force to the riser through bolt pre-tightening, and the elastic contact pressure of the counterweight... for: ， in, This refers to the axial contact length of the counterweight. It is the lateral distance from a point on the contact surface of the counterweight to the center line of the contact; Step 5.4, Analysis of the location of the seabed fixing device; A joint simulation mechanical model of the seabed fixing device and the flexible riser layer equivalent model is established. A radial clamping force is applied to the flexible riser and a tension load is applied to the riser after it is displaced to a certain position. Nonlinear static calculations are performed on the flexible riser body in the current state. The clamps in the seabed fixing device apply radial clamping force to the riser through bolt pre-tightening, and the elastic contact pressure of the seabed fixing device... for: ， in, The axial contact length of the clamp in the seabed fixing device; The lateral distance from a point on the contact surface of the clamp in the bed fixing device to the contact center line; Rope connection point ( , ) and counterweight anchor point ( The catenary equation is satisfied between ,0) ， ， in, This represents the horizontal tension component of the rope. The mass per unit length of the rope; It is the acceleration due to gravity; The horizontal coordinates of the connection point between the tether and the hose; The vertical coordinates of the connection point between the tether and the hose; The horizontal coordinates of the counterweight anchor point; This represents the vertical height of the tether at the corresponding horizontal position x. is the characteristic length of the catenary.

8. The method according to claim 7, characterized in that, Step 6 includes: Extract the water flow and pressure parameters at each point on the riser boundary and couple them with the overall equivalent model of the riser body in step 1 to obtain the high-cycle cyclic stress caused by the continuous vibration displacement of the flexible riser under wave flow. Volumetric Multiphase Flow (VOF) Model: Defines a volume fraction variable Within the computing unit This indicates that the unit is filled with water; This indicates that the cell is filled with gas; This indicates that the cell contains a free surface, and the density within each cell is... Dynamic viscosity Determined by the phase fraction mixing: ， ， in The density of water, The density of the gas, The dynamic viscosity of water, The dynamic viscosity of the gas; Numerical wave generation for open channel flow employs a velocity inlet method, which specifies the velocity of water particles and the wave rise at the inlet boundary: ， Where H is the wave height and ω is the wave angular frequency. For wave number, Let z represent the water depth and z represent the position of the wave surface. Indicates time, For the point located at coordinates (x, z) at time... The instantaneous velocity in the horizontal direction, Represents the horizontal coordinate.

9. The method according to claim 8, characterized in that, Step 8 includes: The stress-life curve of a metallic material is represented as follows: ， Where N is the number of failure cycles, Let m be the stress range, and m be the negative reciprocal slope of the stress-life curve. It is the intercept constant related to material, detail construction, and survival probability; The stress-life curve of a polymer material is represented as follows: ， in is the fatigue strength coefficient, and b is the fatigue strength index. This represents the number of loop iterations. The Gerber modified model corrects the equivalent stress amplitude after a given stress cycle: ， in This is the equivalent stress amplitude. Stress amplitude: , Mean stress: , This represents the ultimate tensile strength of the material.

10. The method according to claim 9, characterized in that, Step 9 includes: Within a cyclic block containing k different stress amplitudes, the total damage D is: ， in Equivalent stress amplitude Number of times it appears This refers to the number of cycles required for structural fatigue failure under constant amplitude alternating stress corresponding to this stress range, i.e., when D=1, fatigue damage occurs in the flexible riser or bending reinforcement. The predicted fatigue life is: ， in For the predicted fatigue life, The current stress cycle time; Step 9.1, Bending reinforcement area; The bending reinforcement area requires joint fatigue calculation of the bending reinforcement and the flexible riser to obtain the fatigue life of the bending reinforcement body and the flexible riser respectively. The fatigue life time of the first failure is selected from the fatigue life of the bending reinforcement body and the flexible riser to predict the fatigue life of the interface between the floating platform and the flexible riser. Step 9.2, area of ​​buoyancy blocks, counterweights, and seabed fixing devices; Fatigue calculations need to be performed on the flexible riser body in the areas of buoyancy block, counterweight block, and seabed fixing device to obtain the fatigue life of the first layer of the flexible riser to fail in the areas of buoyancy block, counterweight block, and seabed fixing device. The fatigue life of the overall flexible riser system is obtained by combining steps 9.1 and 9.2.

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