Strength and ultimate bending resistance mechanical evaluation method for underwater wellhead connector

Abstract

The invention discloses a strength and ultimate bending resistance mechanical evaluation method for an underwater wellhead connector. The strength and ultimate bending resistance mechanical evaluation method comprises the following steps: establishing a finite element analysis model of the underwater wellhead connector; corresponding material plasticity attributes and material elasticity attributes are given to all parts contained in the finite element analysis model; the working condition of the underwater wellhead connector is selected, and if the selected working condition is the service working condition, the strength of the underwater wellhead connector is evaluated; if the combined load working condition is selected, the bending resistance of the underwater wellhead connector is evaluated; according to the evaluation method designed by the invention, various real load conditions borne by the underwater wellhead connector in service are considered, the plastic performance of the connector material is fully exerted, the performance evaluation of the underwater wellhead connector is realized, the current situation that the performance analysis of the underwater wellhead connector in China is incomplete is made up, and the method is suitable for popularization and application. And the safety and reliability of an underwater oil-gas production system in China are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of underwater wellhead system research, and particularly to a mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector. Background Art

[0002] With the gradual depletion of onshore oil resources and the gradual discovery of rich offshore oil and gas reserves, the development and utilization of deep-water oil and gas fields have become the main development trend of the future oil industry. The underwater wellhead system has become an indispensable core equipment in offshore oil and gas development due to its advantages of avoiding harsh offshore environmental conditions, flexible later maintenance, and good economy. The underwater wellhead connector is one of the core components of the underwater wellhead system, mainly used to connect the underwater wellhead to equipment such as the underwater Christmas tree and blowout preventer, and at the same time has a sealing function to prevent oil and gas leakage, as well as functions such as bearing internal and external loads and maintaining the production safety of the equipment. Once the wellhead connector fails, serious accident consequences will occur. Therefore, its reliability is of great significance for ensuring the safe development of offshore oil and gas.

[0003] At present, most of the research on wellhead connectors focuses on evaluating the sealing performance of wellhead connectors on the basis of considering material elastoplasticity, and analyzing the strength of wellhead connectors within the elastic range of materials, and determining and evaluating the strength of wellhead connectors by using the relationship between stress data and yield strength. Such strength analysis methods have two major defects: First, only the influence of hydrostatic pressure load is considered, and the coupling effects of multiple loads such as gravity and bending moment are not taken into account; Second, the judgment criterion based on elastic theory leads to the potential of material plastic deformation not being fully exploited, resulting in distortion of the structural bearing capacity evaluation. However, during actual service, the connector needs to bear the coupling action of multiple loads such as axial tension, internal and external pressure difference, and bending moment. Existing research lacks systematic research on key issues such as the service state analysis of the connector under multi-field coupling loads and the evaluation of the ultimate bending resistance. This limitation of theoretical research directly leads to significant deviations in the bending performance evaluation in engineering practice, which not only affects the prediction accuracy of the remaining life of the equipment, but also poses a potential threat to the safe production of deep-water oil and gas fields.

[0004] Therefore, there is an urgent need for a mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector to solve the above technical problems. Summary of the Invention

[0005] The present invention aims to solve the above technical problems, that is, to solve the lack of systematic research on key issues such as the service state analysis of the connector under multi-field coupling loads and the evaluation of the ultimate bending resistance in existing research, which directly leads to significant deviations in the bending performance evaluation in engineering practice, not only affecting the prediction accuracy of the remaining life of the equipment, but also posing a potential threat to the safe production of deep-water oil and gas fields.

[0006] For this purpose, the present invention provides a mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector, comprising the following steps:

[0007] Establish a finite element analysis model of the underwater wellhead connector;

[0008] Assign corresponding material plasticity properties and material elasticity properties to each component included in the finite element analysis model;

[0009] Select the working condition of the underwater wellhead connector. If the selected condition is the service condition, perform a strength evaluation on the underwater wellhead connector; if the selected condition is the combined load condition, perform a bending resistance evaluation on the underwater wellhead connector;

[0010] Among them, the step of "performing a bending resistance evaluation on the underwater wellhead connector" includes:

[0011] Apply combined loads, set corresponding boundary conditions and material parameters to the finite element analysis model of the underwater wellhead connector, wherein the combined loads include actual gravity, actual oil and gas pressure, actual top axial force and initial bending moment;

[0012] Use finite element software to perform finite element analysis and calculation on the finite element analysis model according to the elastic-plastic stress analysis method, and judge whether the finite element analysis model converges according to the analysis and calculation results;

[0013] If the finite element analysis model converges, increase the current initial bending moment by a preset difference to form a new initial bending moment and apply it to the finite element analysis model, and repeat the finite element analysis calculation and judge whether the finite element analysis model converges until it is judged that the finite element analysis model does not converge and stop;

[0014] Output the analysis and calculation results corresponding to each converged bending moment value, and evaluate the analysis and calculation results according to the evaluation and acceptance criteria corresponding to the elastic-plastic stress analysis method to obtain the ultimate bending moment value.

[0015] In the specific implementation manner of the above mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector, the step of "performing a bending resistance evaluation on the underwater wellhead connector" further includes:

[0016] If it is judged that the finite element analysis model does not converge after the first finite element analysis calculation, decrease the initial bending moment value by a set difference to form a new initial bending moment value and apply it to the finite element analysis model, then re-perform the finite element analysis calculation, and judge whether the finite element analysis model converges according to the analysis and calculation results;

[0017] If the first judgment indicates that the finite element analysis model converges, stop reducing the initial bending moment value. If it does not converge, repeat the above process until the first judgment shows that the finite element analysis model converges.

[0018] In the specific implementation of the above mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector, the step of "evaluating the strength of the underwater wellhead connector" includes:

[0019] Apply service loads, set corresponding boundary conditions and material parameters to the finite element analysis model of the underwater wellhead connector. Among them, the service loads include the actual service gravity and the actual service oil and gas pressure, and both the actual service gravity and the actual service oil and gas pressure are values obtained by multiplying the original gravity and the original oil and gas pressure by their corresponding load coefficients respectively;

[0020] Use finite element software to perform finite element analysis and calculation on the finite element analysis model according to the elastoplastic stress analysis method, and judge whether the finite element analysis model converges according to the analysis and calculation results;

[0021] If the finite element analysis model converges, output the corresponding analysis and calculation results, and evaluate the analysis and calculation results according to the evaluation and acceptance criteria corresponding to the elastoplastic stress analysis method and output the evaluation results.

[0022] In the specific implementation of the above mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector, the step of "evaluating the strength of the underwater wellhead connector" also includes:

[0023] If the finite element analysis model does not converge, adjust one or both of the application method of the service load and the boundary conditions, then re-apply them to the finite element analysis model and perform finite element analysis and calculation, and judge whether the finite element analysis model converges according to the analysis and calculation results;

[0024] If the finite element analysis model does not converge, repeat the above process until the finite element analysis model converges.

[0025] In the specific implementation of the above mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector, the step of "adjusting the application method of the service load" specifically includes:

[0026] Apply the actual service gravity and the actual service oil and gas pressure to the finite element analysis model successively according to the set number of loading times.

[0027] In the specific implementation manner of the above mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector, the categories of the combined load conditions include normal conditions, extreme conditions, and survival conditions. The category of the combined load condition is determined on the premise of selecting the combined load condition, and the corresponding original oil and gas pressure and original top axial force are selected according to the determined category, and then the corresponding bending resistance evaluation is performed.

[0028] In the specific implementation manner of the above mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector, before the step of evaluating the bending resistance, a load coefficient range corresponding to the elastic-plastic stress analysis method is selected according to the ASME standard and the API standard, and a specific load coefficient is selected according to the category of the combined load condition. The actual gravity, actual oil and gas pressure, and actual top axial force are all values obtained by multiplying the corresponding original gravity, original oil and gas pressure, and original top axial force by the corresponding load coefficient.

[0029] In the specific implementation manner of the above mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector, before assigning the corresponding material plasticity attributes to each component, a material plasticity attribute acquisition step is performed. This acquisition step specifically includes:

[0030] Establish a stress-strain curve model according to the ASME BPVC VIII 2 standard and the strain hardening characteristics of the stress-strain curve;

[0031] The yield strength, ultimate tensile strength, elastic modulus, and true stress for evaluating the true strain of each component material are respectively input into the stress-strain curve model to obtain the material plasticity attributes corresponding to the component material.

[0032] In the specific implementation manner of the above mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector, the finite element analysis model includes a refined model of the underwater wellhead connector and a simplified model of the underwater wellhead and the production tree body. The refined model of the underwater wellhead connector adopts a locking ring structure.

[0033] Compared with the prior art, the beneficial effects of the present invention are:

[0034] The present invention is used to evaluate the maximum bending resistance and the strength under service conditions of an underwater wellhead connector. This method is based on a refined model of the underwater wellhead connector, taking into account the actual load conditions such as gravity, bending moment, axial load, and oil and gas pressure load borne by the underwater wellhead connector during service, and giving full play to the plastic properties of the connector material. Combining with international standards such as ASME and API, it realizes the performance evaluation of the underwater wellhead connector, makes up for the imperfect status of the performance analysis of domestic underwater wellhead connectors, improves the safety and reliability of China's underwater oil and gas production system, and avoids the failure of the underwater wellhead connector during service. Brief Description of the Drawings

[0035] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings, in which:

[0036] Figure 1 is a flowchart of the mechanical evaluation method for the strength and ultimate bending resistance of the underwater wellhead connector provided by the present invention;

[0037] Figure 2 is a schematic structural diagram of the finite element analysis model;

[0038] Figure 3 is a stress change diagram borne by the underwater wellhead connector;

[0039] Figure 4 is a strain diagram borne by the underwater wellhead connector;

[0040] Figure 5 is an equivalent stress nephogram of the locking ring;

[0041] Figure 6a is a first principal stress diagram at the location with relatively large local strain;

[0042] Figure 6b is a second principal stress diagram at the location with relatively large local strain;

[0043] Figure 6c is a third principal stress diagram at the location with relatively large local strain;

[0044] Figure 7 is a finite element nephogram of the contact pressure of the VX steel ring;

[0045] Figure 8 is a data diagram of the contact pressure of the sealing strip at the weak sealing location extracted.

[0046] List of Reference Numerals:

[0047] 1. VX steel ring; 2. Locking piston; 3. Unlocking piston; 4. Secondary unlocking piston; 5. Nut; 6. Bolt; 7. Christmas tree body; 8. Outer body; 9. Actuating ring; 10. Locking ring; 11. Lower body; 12. Wellhead. Detailed implementation manners

[0048] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Apparently, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0049] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "upper", "lower", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the system or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention. In addition, the use of the terms "first", "second", etc. to limit the components is only for the convenience of distinguishing the above components. Without further statement, the above terms have no special meaning and should not be construed as indicating or implying relative importance.

[0050] In the description of the present invention, it should be noted that unless otherwise clearly specified and limited, the terms "installation", "setting", "connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0051] The present invention relates to the technical field of underwater wellhead system research, and particularly to a mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector. The purpose is to solve the problem that the existing research lacks systematic research on key issues such as the service state analysis of the connector under multi-field coupling loads and the evaluation of the ultimate bending resistance, which directly leads to significant deviations in the bending performance evaluation in engineering practice, not only affecting the prediction accuracy of the remaining life of the equipment, but also posing a potential threat to the safe production of deepwater oil and gas fields. For this purpose, the present invention provides a mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector. This method is based on a refined model of the underwater wellhead connector, considers the actual load conditions such as gravity, bending moment, axial load, oil and gas pressure load, etc. that the underwater wellhead connector bears during service, and gives full play to the plastic properties of the connector material. Combining international standards such as ASME and API, it realizes the performance evaluation of the underwater wellhead connector, makes up for the imperfect status of the performance analysis of domestic underwater wellhead connectors, improves the safety and reliability of China's underwater oil and gas production system, and avoids the failure of the underwater wellhead connector during service.

[0052] Next, in combination with the accompanying drawings, a mechanical evaluation method for the strength and ultimate bending resistance of the underwater wellhead connector provided by the embodiments of the present invention will be described in detail.

[0053] Referring to Figure 1 , the present invention provides a mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector, including the following steps:

[0054] S1, establishing a finite element analysis model of the underwater wellhead connector;

[0055] S2, assigning corresponding material plasticity properties and material elasticity properties to each component included in the finite element analysis model;

[0056] S3, selecting the working conditions of the underwater wellhead connector;

[0057] S4, if the selected working condition is the service condition, then performing a strength evaluation on the underwater wellhead connector;

[0058] S5, if the selected working condition is the combined load condition, then performing a bending resistance evaluation on the underwater wellhead connector;

[0059] Among them, step S5 of "performing a bending resistance evaluation on the underwater wellhead connector" includes:

[0060] Applying a combined load and setting corresponding boundary conditions and material parameters to the finite element analysis model of the underwater wellhead connector, where the combined load includes the actual gravity, actual oil and gas pressure, actual top axial force, and initial bending moment;

[0061] Performing finite element analysis and calculation on the finite element analysis model by using finite element software according to the elastic-plastic stress analysis method, and judging whether the finite element analysis model converges according to the analysis and calculation results. The finite element software can be Abaqus software;

[0062] If the finite element analysis model converges, then increasing the current initial bending moment by a preset difference to form a new initial bending moment and applying it to the finite element analysis model, repeating the finite element analysis calculation and judging whether the finite element analysis model converges until it is judged that the finite element analysis model does not converge and stops;

[0063] Outputting the analysis and calculation results corresponding to each converged bending moment value, and evaluating the analysis and calculation results according to the evaluation and acceptance criteria corresponding to the elastic-plastic stress analysis method to obtain the ultimate bending moment value. The elastic-plastic stress analysis method is selected according to the ASME standard and API standard.

[0064] In the above step S1, the finite element analysis model includes a refined model of the underwater wellhead connector and a simplified model of the underwater wellhead and the production tree body. When designing the underwater wellhead and production tree body model, only the part connected to the underwater wellhead connector needs to be established, and the complex parts on the model are simplified. The underwater wellhead connector established in the present invention adopts a locking ring structure. The locking piston moves downward to drive the excitation ring downward, causing the locking ring to contract radially towards the axis. The teeth on the locking ring tightly fit with the teeth of the wellhead, applying an upward pulling force to the wellhead. At the same time, the bolts fix the tree body so that it cannot move, thereby tightly connecting the underwater wellhead and the production tree body. Specifically, as Figure 2 shown, the finite element analysis model includes a VX steel ring 1, a locking piston 2, an unlocking piston 3, a secondary unlocking piston 43, a nut 5, a bolt 6, a production tree body 7, an outer body 8, an excitation ring 9, a locking ring 10, a lower body 11, and a wellhead 12.

[0065] It should be noted that the structure of the underwater wellhead connector described above is only an example, and the present application is not specifically limited. The evaluation method of the present application is also applicable to other underwater wellhead connector structures.

[0066] In the above embodiment, before assigning the corresponding material plasticity properties to each component, a material plasticity property acquisition step is performed. Specifically, this acquisition step includes:

[0067] Establish a stress-strain curve model according to the ASME BPVC VIII 2 standard and the strain hardening characteristics of the stress-strain curve;

[0068] Input the yield strength, ultimate tensile strength, elastic modulus, and true stress for evaluating the true strain corresponding to each component material into the stress-strain curve model to obtain the material plasticity property corresponding to the component material.

[0069] Specifically, the stress-strain curve model is specifically:

[0070]

[0071] K = 1.5R 1.5 -0.5R 2.5 -R 3.5

[0072]

[0073] If:

[0074] r1 + r2 ≤ ε p

[0075] Then adopt:

[0076]

[0077] Otherwise, adopt:

[0078]

[0079] In the formula, R is the ratio of the yield strength to the ultimate tensile strength; σ ys - Yield strength, MPa; σ uts - Ultimate tensile strength, MPa; K is the material parameter of the stress-strain curve model; m1, m2 are the stress-strain curve fitting exponents; A1 is the curve fitting constant in the elastic region of the stress-strain curve; A2 is the curve fitting constant in the plastic region of the stress-strain curve; ε p - Stress-strain curve fitting parameter; ε ys - 0.2% engineering offset strain; ε1 is the true plastic strain in the micro-strain region of the stress-strain curve; ε2 is the true plastic strain in the macro-strain region of the stress-strain curve; σ t - The true stress for evaluating the true strain can be the membrane, membrane plus bending or membrane, membrane plus bending plus peak stress; H is the stress-strain curve fitting parameter; r1 is the true strain in the micro-strain region of the stress-strain curve; r2 is the true strain in the macro-strain region of the stress-strain curve; ε t - Total true strain; E y - Elastic modulus, MPa.

[0080] Regarding the value of the true stress for evaluating the true strain, exemplarily, the tensile strength of the material is determined according to the elastic parameters of the material. For example, if the tensile strength of the material is 725 MPa, then the value range of this stress is 0 - 725 MPa, and then the values are taken at equal intervals starting from zero until 725 MPa is reached. Under the condition that the yield strength, ultimate tensile strength, and elastic modulus remain unchanged, substituting these values into the stress-strain curve model respectively can obtain a total true strain. According to these total true strain values and the corresponding true stress values for evaluating the true strain, a material plastic curve is fitted, and this material plastic curve is the material plastic property.

[0081] In one embodiment, the step of "evaluating the bending resistance of the underwater wellhead connector" further includes:

[0082] If it is determined that the finite element analysis model does not converge after the first execution of the finite element analysis calculation, then the initial bending moment value is reduced by a set difference to form a new initial bending moment value and applied to the finite element analysis model, and then the finite element analysis calculation is performed again, and it is judged whether the finite element analysis model converges according to the analysis calculation results;

[0083] If the finite element analysis model converges in the first judgment, stop reducing the initial bending moment value. If it does not converge, repeat the process of "reducing the initial bending moment value by a set difference to form a new initial bending moment value and applying it to the finite element analysis model, then re-performing the finite element analysis calculation, and judging whether the finite element analysis model converges according to the analysis calculation results" until the finite element analysis model converges in the first judgment.

[0084] In one embodiment, the categories of the combined load conditions include normal conditions, extreme conditions, and survival conditions. On the premise of selecting the combined load conditions, determine the category of the combined load conditions, select the corresponding original oil and gas pressure and original top axial force according to the determined category, and then perform the corresponding bending resistance evaluation. The combined loads corresponding to the combined load conditions are shown in Table 1. The values of the original top axial force and the original oil and gas pressure in Table 1 are only examples, and the purpose is to reflect the differences among the three conditions.

[0085] Table 1 is the combined load distribution under the combined load conditions

[0086]

[0087]

[0088] In one embodiment, before the step of performing the bending resistance evaluation, select the load coefficient range corresponding to the elastic-plastic stress analysis method according to the ASME standard and the API standard, and select the corresponding specific load coefficient according to the category of the combined load conditions. As shown in Table 2, the actual gravity, the actual oil and gas pressure, and the actual top axial force are all the values obtained by multiplying the corresponding original gravity, the original oil and gas pressure, and the original top axial force by the corresponding load coefficient. The original gravity includes the gravity of the Christmas tree and the self-gravity of the subsea wellhead connector.

[0089] Table 2 is the load coefficient corresponding to different condition categories

[0090] Operating conditions Elastoplastic stress analysis load factor (without thermal load) Normal condition 2.4 Ultimate condition 2.0 Survival condition 1.6

[0091] In one embodiment, step S4 of "performing a strength evaluation on the subsea wellhead connector" includes:

[0092] Apply the service load and set the corresponding boundary conditions and material parameters to the finite element analysis model of the subsea wellhead connector. Among them, the service load includes the service actual gravity and the service actual oil and gas pressure, and both the service actual gravity and the service actual oil and gas pressure are the values obtained by multiplying the original gravity and the original oil and gas pressure by the corresponding load coefficients respectively; the load coefficient corresponding to the service condition is the same as the load coefficient corresponding to the normal condition under the combined load condition;

[0093] Use finite element software to perform finite element analysis and calculation on the finite element analysis model according to the elastoplastic stress analysis method, and judge whether the finite element analysis model converges based on the analysis and calculation results;

[0094] If the finite element analysis model converges, output the corresponding analysis and calculation results, and evaluate the analysis and calculation results according to the evaluation and acceptance criteria corresponding to the elastoplastic stress analysis method and output the evaluation results. If the output evaluation result meets the standard requirements, it indicates that the design of the underwater wellhead connector meets the service condition requirements. If the output evaluation result does not meet the standard requirements, it indicates that the design of the underwater wellhead connector does not meet the service condition requirements.

[0095] In the above embodiment, the step of "evaluating the strength of the underwater wellhead connector" further includes:

[0096] If the finite element analysis model does not converge, adjust one or both of the application method of the service load and the boundary conditions, then re-apply them to the finite element analysis model and perform finite element analysis and calculation, and judge whether the finite element analysis model converges according to the analysis and calculation results;

[0097] If the finite element analysis model does not converge, repeat the process of "adjusting one or both of the application method of the service load and the boundary conditions, then re-apply them to the finite element analysis model and perform finite element analysis and calculation, and judge whether the finite element analysis model converges" until the finite element analysis model converges.

[0098] Regarding the application method of the service load, exemplarily, the step of "adjusting the application method of the service load" specifically includes:

[0099] Apply the actual service gravity and the actual service oil and gas pressure to the finite element analysis model step by step according to the set number of loading times.

[0100] For example, if the set number of loading times is 10 times, that is, the actual service gravity and the actual service oil and gas pressure are divided into 10 parts and applied to the finite element analysis model step by step. If the model does not converge, the set number of loading times can also be adjusted and the finite element analysis and calculation can be performed again.

[0101] Regarding the adjustment method of the boundary conditions, exemplarily, the following several adjustments can be made.

[0102] First, increase the number of analysis steps to make the constraint conditions applied to the components take effect gradually;

[0103] Second, adjust the surface-to-surface contact setting and select a reasonable master and slave surface;

[0104] Third, increase the contact control conditions to stabilize the contact analysis and improve the convergence;

[0105] Fourth, reasonably set binding constraints on components to reduce the computational amount;

[0106] Fifth, set a reasonable load amplitude change curve to make the load application process as smooth as possible;

[0107] Sixth, load different types of loads in a step-by-step increasing manner, etc.

[0108] In the above embodiments, the material parameters refer to the density, Poisson's ratio, Young's modulus, yield strength, tensile strength, stress-strain values, etc. of the material. The material parameters can be obtained by referring to relevant standards, and the specific parameters of different materials are different.

[0109] In the above embodiments, according to the elastoplastic analysis method of API 17G, a plastic collapse method with an elastoplastic true stress-strain curve having strain hardening is selected. The evaluation and acceptance criteria of this method include global criteria, local criteria, and functional failures:

[0110] Global criteria: Under the premise of considering the load factor for the applied load, the analysis can reach convergence; the convergence of the model described above is considered to meet the global criteria.

[0111] Local criteria: Under the premise of considering the load factor, the places where the stress and strain are too large meet the local failure criteria; specifically, the local criteria include local failure inspection criteria and plastic collapse inspection criteria.

[0112] Functional failure: The connector does not have functional failures, such as seal leakage.

[0113] The local failure inspection criteria are:

[0114] When performing local failure analysis, for the selection of the load factor, it is mentioned in API 17G to refer to the coefficient of 1.28 in ASME VIIIDiv.3: KD-232. For the sake of conservatism and improving the analysis efficiency, the load factor used in the evaluation is the same as the load factor of API 17TR7. For the components with excessive local stress and strain, the local failure determination criteria are adopted for judgment, referring to the elastic-plastic criterion in ASME VIII 5.3.3.

[0115] ε peq +ε ef ≤ε L

[0116] Among them, ε peq is the equivalent plastic strain; ε ef is the forming strain, which is taken as 0 after heat treatment; ε L is the strain limit.

[0117] The strain limit is:

[0118]

[0119] Among them, δ1, δ2, and δ3 are the three principal stresses; δ e is the von Mises equivalent stress; ε lu , m2, α sl It can be obtained by referring to Table 5.7 of ASME VIII: α sl = 2.2.

[0120] The equivalent plastic strain ε peq <the strain limit ε L , then the local failure judgment criterion is satisfied and the structure is safe.

[0121] The plastic collapse inspection criteria are as follows:

[0122] According to the ASME VIII and API 17G standards for preventing plastic collapse, the elastoplastic stress analysis method is used for plastic collapse inspection. It is required that the model can converge and complete the solution considering the load factor; and there is no failure of the connector functionality.

[0123] Functional failures include sealing criteria

[0124] The sealing criteria are as follows:

[0125] When there is an internal working pressure in the container, the sealing criteria follow ISO 13628-7:2006. When the internal medium of the container is liquid, the sealing pressure needs to be greater than 1.2 times the working pressure; when the internal medium of the container is gas, the sealing pressure needs to be greater than 2 times the working pressure. When the load factor is considered in the analysis load, the sealing criteria consider the load factor on this basis.

[0126] The nut and bolt evaluation criteria are as follows:

[0127] In the strength analysis and maximum bending resistance evaluation of the subsea wellhead connector, the same analysis method is used for the analysis of nuts and bolts. The evaluation is carried out according to C.6.4.4-Structural Load Capacity Criteria for 2% Strain Method in API 17G. The acceptance criteria need to meet that the global membrane equivalent plastic strain is less than 2% and the local equivalent plastic strain is less than 10%. The load factor used in the normal conditions of the working condition, service condition, and combined load condition is 2.4, and the load factor used in the bending resistance evaluation is 1.6.

[0128] Taking the mechanical evaluation of the bending resistance of the subsea wellhead connector under the combined load condition as an example for specific description.

[0129] The results of the underwater wellhead connector model completed through finite element analysis software are as follows Figure 3 and Figure 4 shown Figure 3 is shown as the stress change diagram borne by the underwater wellhead connector Figure 4 is shown as the strain diagram borne by the underwater wellhead connector

[0130] Check whether local failure occurs in the places with relatively large local strains of each key component of the underwater wellhead connector. The examples are as follows

[0131] Taking the locking ring as an example, extract the equivalent stress nephogram of the locking ring and the triaxial stress diagram at the place with relatively large local strain under the condition that the end of the Christmas tree body bears a tensile force of 4448 kN and the internal pressure of the underwater wellhead connector is 34.5 MPa, as shown in Figure 5 and Figures 6a - 6c shown. According to Figure 5 and Figures 6a - 6c the result data shown, combined with the ASME VIII standard for inspection, the local failure inspection results are obtained, as shown in Table 3

[0132] Table 3 Local failure results table of the locking ring

[0133]

[0134] It can be seen from Table 3 that the equivalent plastic strain value of the locking ring is 0.0389, which is less than the strain limit value of 0.039. Among them, the strain limit is obtained through the above numerical calculation, and no local failure occurs in the locking ring

[0135] Conduct a sealing performance evaluation on the underwater wellhead connector, and check whether the width of the pressure band that meets the sealing pressure on the VX steel ring meets the requirements. The examples are as follows

[0136] Apply an internal working pressure of 5000 to the underwater wellhead connector and analyze it under the condition of applying an end tensile load to the top of the Christmas tree body to obtain the finite element nephogram of the contact pressure of the VX steel ring, as shown in Figure 7 shown. It can be seen from Figure 7 that under the action of an internal pressure of 5000 PSI and a tensile load borne by the top of the tree body, a uniform and continuous sealing band is formed at the left end of the upper sealing surface. From the middle position to the right end, the sealing band is stratified and the contact pressure drops, but all can well meet the sealing requirements; a uniform and continuous sealing band is formed at the right end of the lower sealing surface. From the middle position to the left end, there is an interruption until the leftmost side, and the contact pressure drops, but it can still better meet the sealing requirements. The weakest part of the seal appears on the tensile side of the upper sealing surface and the extrusion side of the lower sealing surface. Extract the contact pressure data of the sealing band at the weakest part of the seal, as shown in Figure 8 shown. It can be seen from Figure 8It can be seen that at an internal pressure of 5000 PSI, the sealing width of the upper sealing surface is 3.2 mm - 7.3 mm, and the sealing width of the lower sealing surface is 2.85 mm - 6.15 mm. Both its sealing pressure and the sealing band width meet the sealing requirements.

[0137] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the protection scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector, characterized in that, The steps include: Establish a finite element analysis model of the subsea wellhead connector; Assign corresponding material plastic properties and material elastic properties to each component included in the finite element analysis model; Select the working condition of the subsea wellhead connector. If the service condition is selected, perform a strength assessment on the subsea wellhead connector; If the combined load condition is selected, perform a bending resistance assessment on the subsea wellhead connector; Among them, the step of "performing a bending resistance assessment on the subsea wellhead connector" includes: Apply combined loads and set corresponding boundary conditions and material parameters to the finite element analysis model of the subsea wellhead connector, where the combined loads include actual gravity, actual oil and gas pressure, actual top axial force, and initial bending moment; Perform finite element analysis and calculation on the finite element analysis model by using finite element software according to the elastoplastic stress analysis method, and judge whether the finite element analysis model converges according to the analysis and calculation results; If the finite element analysis model converges, increase the current initial bending moment by a preset difference to form a new initial bending moment and apply it to the finite element analysis model, repeat the finite element analysis calculation and judge whether the finite element analysis model converges until it is judged that the finite element analysis model does not converge and stop; Output the analysis and calculation results corresponding to each converged bending moment value, and evaluate the analysis and calculation results according to the evaluation and acceptance criteria corresponding to the elastoplastic stress analysis method to obtain the ultimate bending moment value.

2. The mechanical evaluation method for the strength and ultimate bending resistance of the underwater wellhead connector according to claim 1, wherein The step of "performing a bending resistance assessment on the subsea wellhead connector" also includes: If it is judged that the finite element analysis model does not converge after the first finite element analysis calculation, reduce the initial bending moment value by a set difference to form a new initial bending moment value and apply it to the finite element analysis model, then perform finite element analysis calculation again, and judge whether the finite element analysis model converges according to the analysis and calculation results; If it is judged that the finite element analysis model converges for the first time, stop reducing the initial bending moment value. If it does not converge, repeat the above process until it is judged that the finite element analysis model converges for the first time.

3. The mechanical evaluation method for the strength and ultimate bending resistance of the underwater wellhead connector according to claim 1, characterized in that The step of "performing a strength assessment on the subsea wellhead connector" includes: Apply service loads and set corresponding boundary conditions and material parameters to the finite element analysis model of the subsea wellhead connector, where the service loads include service actual gravity and service actual oil and gas pressure; Perform finite element analysis and calculation on the finite element analysis model by using finite element software according to the elastoplastic stress analysis method, and judge whether the finite element analysis model converges according to the analysis and calculation results; If the finite element analysis model converges, output the corresponding analysis and calculation results, and evaluate the analysis and calculation results according to the evaluation and acceptance criteria corresponding to the elastoplastic stress analysis method and output the evaluation results.

4. The mechanical evaluation method for the strength and ultimate bending resistance of the underwater wellhead connector according to claim 3, characterized in that The step of "performing a strength assessment on the subsea wellhead connector" also includes: If the finite element analysis model does not converge, adjust one or both of the application method of the service load and the boundary conditions, then reapply them to the finite element analysis model and perform finite element analysis calculation, and judge whether the finite element analysis model converges according to the analysis and calculation results; If the finite element analysis model does not converge, repeat the above process until the finite element analysis model converges.

5. The mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector according to claim 4, characterized in that The steps of "adjusting the application method of service loads" specifically include: Loading the actual service gravity and the actual service oil and gas pressure onto the finite element analysis model successively according to the set number of loading times.

6. The mechanical evaluation method for the strength and ultimate bending resistance of the underwater wellhead connector according to claim 1, characterized in that, The categories of the combined load conditions include normal conditions, extreme conditions, and survival conditions. On the premise of selecting the combined load condition, determine the category of the combined load condition, select the corresponding original oil and gas pressure and original top axial force according to the determined category, and then perform the corresponding bending resistance evaluation.

7. The mechanical evaluation method for the strength and ultimate bending resistance of the underwater wellhead connector according to claim 6, characterized in that, Before performing the bending resistance evaluation step, select the load coefficient range corresponding to the elastic-plastic stress analysis method according to the ASME standard and API standard, and select the corresponding specific load coefficient according to the combined load condition category. The actual gravity, actual oil and gas pressure, and actual top axial force are all the values obtained by multiplying the corresponding original gravity, original oil and gas pressure, and original top axial force by the corresponding load coefficient.

8. The mechanical evaluation method for the strength and ultimate bending resistance of the underwater wellhead connector according to claim 1, characterized in that, Before assigning the corresponding material plasticity properties to each component, perform the material plasticity property acquisition step, which specifically includes: Establish a stress-strain curve model according to the ASME BPVC VIII 2 standard and the strain hardening characteristics of the stress-strain curve; Input the yield strength, ultimate tensile strength, elastic modulus, and true stress for evaluating the true strain of each component material into the stress-strain curve model respectively to obtain the material plasticity property corresponding to the component material.

9. The mechanical evaluation method for the strength and ultimate bending resistance of an underwater wellhead connector according to claim 1, characterized in that, The finite element analysis model includes a refined model of the underwater wellhead connector and a simplified model of the underwater wellhead and the production tree body. The refined model of the underwater wellhead connector adopts a locking ring structure.

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