Riser stability analysis method and system coupled with multiple working conditions, sea loads and corrosion

Abstract

This application relates to the field of offshore oil and gas drilling engineering technology, and discloses a method and system for analyzing the stability of risers under multiple operating conditions coupled with corrosion. This method calculates wave, current, ice, and wind loads under wind-ice flow and wind-wave flow conditions, and combines this with calculations based on deflector parameters derived from corrosion rate and time. The multi-condition loads are integrated as external inputs, and corrosion parameters are used as geometric boundary conditions into the riser mechanical model, forming a load-structure coupled analysis model. Based on this model, the riser stress is calculated using the fourth strength theory, and the finite difference method is used to solve for the lateral displacement, bending moment, and resultant stress. Finally, the stability is verified using the allowable stress method. A corresponding system exists. Using this application, the coupled analysis of multi-condition loads and corrosion effects is achieved, improving the accuracy of riser stability assessment.

Description

Technical Field

[0001] This application relates to the field of marine oil and gas drilling engineering technology, specifically a method and system for analyzing the stability of risers coupled with multi-condition marine load and corrosion. Background Technology

[0002] In offshore oil and gas drilling operations, the riser system is a key piece of equipment connecting the subsea wellhead to the offshore drilling platform, and its stability is directly related to operational safety.

[0003] Existing methods for riser stability analysis have significant limitations: First, these methods typically focus on single marine environmental conditions, failing to comprehensively consider the combined effects of multiple extreme conditions, resulting in incomplete load calculations. Second, during long-term service, the performance degradation of critical components such as the deflector in the riser system under seawater corrosion is not adequately considered, leading to discrepancies between the analysis results and actual conditions. Furthermore, current technologies lack effective means to couple multi-condition environmental loads with long-term corrosion effects, making it difficult to accurately predict the actual mechanical response of the riser in complex marine environments. These shortcomings prevent traditional analysis methods from meeting the demands of high-precision stability assessments, posing potential risks to offshore drilling operations. Summary of the Invention

[0004] The purpose of this application is to provide a method and system for analyzing the stability of risers under multi-condition marine load and corrosion coupling, so as to solve the technical problems mentioned in the background art.

[0005] To achieve the above objectives, this application discloses the following technical solutions:

[0006] In a first aspect, this application discloses a method for analyzing the stability of risers coupled with multi-condition marine loads and corrosion, the method comprising:

[0007] Calculate marine environmental loads under multiple operating conditions: calculate wave loads, ocean current loads, ice loads, and sea wind loads under wind-ice-current and wind-wave-current operating conditions respectively;

[0008] Calculate the corrosion parameters of the slant: Calculate the parameters of the slant after corrosion based on the corrosion rate and corrosion time. The parameters of the slant after corrosion include the tilt angle of the slant after corrosion and the remaining strength at the opening position of the slant.

[0009] Analytical model construction: The multi-condition marine environmental loads are used as external load inputs, and the parameters of the corroded siphon are used as geometric boundary conditions. They are integrated into the riser mechanical model to form a riser analysis model that couples load and structure.

[0010] Analysis of riser stability: Based on the riser analysis model and combined with the fourth strength theory, the axial stress, bending stress, circumferential stress and radial stress of the riser are calculated, and the finite difference method is used for numerical solution to obtain the lateral displacement, bending moment and resultant stress of the riser.

[0011] Stability verification: Based on the lateral displacement, bending moment and resultant stress, the stability of the riser is verified using the allowable stress method.

[0012] Optionally, the calculation of multi-condition marine environmental loads includes:

[0013] Wave loads and current loads are calculated based on the Morrison equations, wherein the wave loads include wave forces and wave inertial forces, and the current loads include current resistance.

[0014] The ice load is calculated based on the Lamé solution, wherein the ice load includes static ice force;

[0015] The sea wind load is calculated based on the wind pressure formula, the wind pressure height variation coefficient, and the wind vibration coefficient.

[0016] Optionally, the calculation of multi-condition marine environmental loads further includes:

[0017] Under the aforementioned wind and ice flow conditions, the ocean current velocity at different depths is calculated based on the shallow water current velocity formula, and the ocean current force is calculated in combination with the drag coefficient.

[0018] Under the aforementioned wind, wave, and current conditions, the horizontal velocity and acceleration of wave water particles are calculated based on Airy wave theory, and the wave force is calculated by combining the drag coefficient and the inertial force coefficient.

[0019] Optionally, the calculation of the oblique device corrosion parameters includes:

[0020] The amount of thickness reduction of the directional tube wall is determined based on the corrosion rate and the corrosion time.

[0021] Based on the aforementioned thickness reduction, the tilt angle of the oblique device after corrosion is derived through geometric relationships;

[0022] Based on the aforementioned thickness reduction, the remaining load-bearing capacity at the window opening location of the diagonal device is calculated.

[0023] Optionally, the analysis of the riser stability includes:

[0024] Based on the axial force balance relationship of the riser, the axial stress of each section of the riser is calculated.

[0025] Based on the moment balance relationship of the riser segment, the bending stress of the riser is calculated.

[0026] Based on the Lamé solution of elasticity for a cylinder subjected to uniformly distributed internal and external pressures, calculate the circumferential and radial stresses of the riser.

[0027] Based on the axial stress, bending stress, circumferential stress, and radial stress, the combined stress of the riser is calculated using the fourth strength theory.

[0028] Optionally, the numerical solution using the finite difference method includes:

[0029] The control equations of the riser analysis model are discretized.

[0030] Solving the discretized system of linear algebraic equations yields the lateral displacement, rotation angle, bending moment, and shear force at each node of the riser.

[0031] Optionally, the stability check includes:

[0032] Based on the lateral displacement, the bending moment, and the combined stress, the strength of the riser is checked using the allowable stress method, wherein the allowable stress for strength is determined based on the yield strength of the steel.

[0033] Based on the lateral displacement, the bending moment, and the resultant stress, the stability of the riser is checked using the allowable stress method, wherein the allowable stress for stability is determined based on the overall stability coefficient.

[0034] Optionally, the verification of the riser strength using the allowable stress method includes:

[0035] Calculate the axial stress strength of the riser pipe under axial force and bidirectional bending moment:

[0036] The axial stress intensity is compared with the allowable strength stress, wherein the allowable strength stress is taken as 1 / 1.6 of the yield strength of the steel.

[0037] Optionally, the method of verifying the stability of the riser using the allowable stress method includes:

[0038] Calculate the bending stress of the riser pipe under the combined action of axial force and bending moment;

[0039] The overall stability coefficient is calculated based on the slenderness ratio of the circular tube component, and then the allowable stress for stability is determined.

[0040] By comparing the bending stress with the allowable stress for stability, it can be determined whether the stability of the riser meets the requirements.

[0041] Secondly, this application discloses a system for analyzing the stability of risers using the multi-condition marine load and corrosion coupling method described above. The system includes:

[0042] The load calculation module is configured to calculate wave load, ocean current load, ice load and sea wind load under wind-ice-current conditions and wind-wave-current conditions, respectively.

[0043] The corrosion parameter calculation module is configured to calculate the post-corrosion deflector parameters based on the corrosion rate and corrosion time. The post-corrosion deflector parameters include the deflector tilt angle and the remaining strength at the deflector window position after corrosion.

[0044] The analysis model construction module is configured to take the multi-condition marine environmental load as the external load input and the corrosion-induced oblique device parameters as the geometric boundary conditions, and integrate them together into the riser mechanical model to form a riser analysis model that couples load and structure.

[0045] The stability analysis module is configured to calculate the axial stress, bending stress, circumferential stress and radial stress of the riser based on the riser analysis model and in combination with the fourth strength theory, and to obtain the lateral displacement, bending moment and resultant stress of the riser by numerical solution using the finite difference method.

[0046] The stability verification module is configured to perform stability verification of the riser pipe using the allowable stress method based on the lateral displacement, the bending moment, and the resultant stress.

[0047] Beneficial Effects: This application presents a method and system for analyzing the stability of risers under multiple marine load and corrosion coupling conditions. It comprehensively considers the combined effects of various extreme conditions such as wind-ice flow and wind-wave flow. Furthermore, by solving the riser analysis model that couples load and structure, it improves the accuracy and reliability of riser stability analysis in complex marine environments. Compared with traditional methods, this invention solves the technical biases caused by single-condition analysis and neglecting the influence of corrosion, providing a more scientific basis for safety assessment in marine drilling operations. Attached Figure Description

[0048] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 A flowchart illustrating the riser stability analysis method under multi-condition marine load and corrosion coupling provided in this application embodiment;

[0050] Figure 2 This is a system block diagram of a riser stability analysis system coupled with multi-condition marine load and corrosion, provided for an embodiment of this application. Detailed Implementation

[0051] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application. Secondly, in this document, the term "comprising" is intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements, but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus.

[0052] To facilitate understanding of the technical solutions provided in the embodiments of this application, the background technology involved in the embodiments of this application will be described below.

[0053] In offshore oil and gas drilling operations, the riser system, as the core hub connecting the subsea wellhead and the offshore drilling platform, undertakes critical functions such as guiding the drill string, transporting drilling fluid, and isolating seawater. Its structural stability directly determines the continuity and safety of drilling operations and is an important foundation for ensuring the smooth progress of offshore oil and gas resource development. However, the marine environment is extremely complex and harsh. Natural loads such as wind, waves, currents, and ice constantly act on the riser structure, while the strong corrosiveness of seawater will erode the key components of the riser system over a long period of time. This makes riser stability analysis face multiple technical challenges, and existing riser stability analysis methods have many obvious limitations and are no longer suitable for the high-precision assessment requirements of complex marine environments.

[0054] Firstly, at the level of marine environmental load analysis, existing methods generally adopt a single-condition analysis mode. In actual marine operation scenarios, extreme environments often appear in combination. For example, cold sea areas often face wind-ice-current conditions caused by the combined effects of wind, ice, and current, while temperate sea areas frequently encounter wind-wave-current conditions caused by the superposition of wind, waves, and current. Under these conditions, wave loads, ocean current loads, ice loads, and sea wind loads interact and couple with each other, jointly constituting the complex stress environment of the riser. However, traditional analysis methods only calculate loads for a single condition, such as calculating wave loads or ocean current loads separately, completely ignoring the superposition effect of multiple extreme conditions. This results in incomplete and one-sided load calculation results, failing to fully reflect the actual stress state of the riser and laying the groundwork for potential errors in subsequent stability assessments. This deficiency also directly leads to the inability of traditional methods to achieve comprehensive coverage of riser loads, which contrasts sharply with the multi-condition marine environmental load calculation technical solution in this application.

[0055] Secondly, regarding the performance considerations of key components, existing technologies severely neglect the impact of corrosion on the riser system during long-term service. The directional drilling rig, a core component of the riser system, is continuously corroded when immersed in highly saline and corrosive seawater. This corrosion manifests as a gradual thinning of the pipe wall, leading to a shift in the directional drilling rig's tilt angle and a decrease in the load-bearing capacity of its opening positions. Traditional stability analyses rely solely on the initial design parameters of the directional drilling rig, failing to incorporate this corrosion-induced performance degradation into the analysis. This results in component parameters in the analysis model that are significantly inconsistent with actual service conditions, ultimately leading to a substantial deviation between the analysis results and the actual working conditions of the riser, thus failing to provide a reliable reference for equipment maintenance and safety assessment. This deficiency is precisely why this application specifically includes a corrosion parameter calculation module to accurately solve for the key parameters of the directional drilling rig after corrosion.

[0056] Finally, at the level of analytical methodology, existing technologies lack effective means to couple and analyze multi-condition environmental loads with long-term corrosion effects. The actual mechanical response of the riser is the result of the combined effects of environmental loads and structural corrosion, with a complex interrelationship between the two: corrosion-induced component performance degradation alters the structural mechanical properties of the riser, thus affecting its ability to withstand environmental loads; while loads under different conditions accelerate the corrosion process, creating a vicious cycle. Traditional analytical methods separate environmental load calculations from structural corrosion assessments, performing independent analyses, which fails to accurately capture this coupling effect. This makes it difficult to accurately predict key mechanical indicators such as axial stress and bending stress of the riser in complex marine environments, and also fails to effectively determine whether parameters such as lateral displacement and bending moment meet safety requirements. This imperfection in the analytical system directly results in low assessment accuracy of traditional methods, failing to meet the urgent need for high-precision assessment of riser stability in current offshore oil and gas drilling operations, and bringing potential risks such as equipment failure, operation interruption, and even personnel casualties to offshore drilling operations.

[0057] In summary, the existing methods for analyzing the stability of risers have shortcomings in the three core dimensions of operating condition coverage, corrosion consideration, and coupled analysis, making them difficult to adapt to the actual application needs in complex marine environments. There is an urgent need for a new analysis method and system that can comprehensively solve the above problems, which provides a clear practical basis and application scenario for the technical solution proposed in this application.

[0058] This embodiment provides a method for analyzing the stability of risers under coupled marine loads and corrosion under multiple operating conditions in the first aspect, such as... Figure 1 As shown, the method includes the following steps in sequence:

[0059] Step 1 - Calculate marine environmental loads under multiple operating conditions: Calculate wave loads, ocean current loads, ice loads, and sea wind loads under wind-ice-current and wind-wave-current operating conditions respectively.

[0060] In practice, the wind-ice-current condition corresponds to high-latitude cold sea areas (such as the Arctic Ocean and the winter operation area of ​​the Bohai Sea), while the wind-wave-current condition corresponds to temperate / tropical sea areas (such as the South China Sea and the Gulf of Mexico). Wave loads are collected using an S4 wave height meter to obtain wave height and period data, ocean current loads are obtained using an acoustic Doppler current profiler (ADCP) to obtain depth-velocity distribution, ice loads are collected using a piezoelectric ice pressure gauge to obtain extrusion pressure, and sea wind loads are collected using a three-cup anemometer on the top of the platform combined with altitude data.

[0061] Step 2 - Calculate the corrosion parameters of the directional compass: Calculate the parameters of the directional compass after corrosion based on the corrosion rate and corrosion time. The parameters of the directional compass after corrosion include the tilt angle of the directional compass after corrosion and the remaining strength at the opening position of the directional compass.

[0062] In practical implementation, the material of the deflector is 13Cr stainless steel or super duplex steel. The corrosion rate is determined by a combination of corrosion plate measurement and electrochemical workstation monitoring (conversion using the Tafel curve method). The corrosion time is obtained from the equipment service records of the platform operation and maintenance system.

[0063] Step 3 - Constructing the analysis model: The multi-condition marine environmental loads are used as external load inputs, and the parameters of the corroded siphon are used as geometric boundary conditions. These are integrated into the riser mechanical model to form a riser analysis model that couples load and structure.

[0064] In practical implementation, the riser mechanical model is constructed based on the Euler-Bernoulli beam theory (satisfying the assumption that the riser length-to-diameter ratio is >20). Wave load is a periodic distributed load, ice load is an impact concentrated load, ocean current load is a steady-state distributed load, and sea wind load is a dynamic distributed load. The input form is defined according to the load type.

[0065] Step 4 - Analyze the stability of the riser: Based on the riser analysis model and combined with the fourth strength theory, calculate the axial stress, bending stress, circumferential stress and radial stress of the riser, and use the finite difference method to solve numerically to obtain the lateral displacement, bending moment and resultant stress of the riser.

[0066] In practical implementation, the finite difference method requires discretizing the riser pipe along its length into... Each micro-element segment ( For example, a 1000m riser pipe Each infinitesimal segment (10m in length) establishes an independent mechanical equilibrium equation.

[0067] Step 5 - Perform stability check: Based on lateral displacement, bending moment and resultant stress, the allowable stress method is used to complete the stability check of the riser.

[0068] In practical implementation, the allowable stress method reference parameter is the mechanical properties of the riser material (such as the yield strength of API 5LX80 steel). The verification covers all nodes along the entire length, with a focus on stress concentration areas such as joints and diagonal openings.

[0069] Based on the above, this embodiment addresses three major shortcomings of traditional analysis by establishing a complete chain of multi-condition load, corrosion parameters, coupled model, stability analysis, and verification: First, the multi-condition load covers typical extreme environments in cold / temperate seas, and real data is collected using specialized equipment to avoid missing key loads in a single condition, laying a comprehensive load foundation for analysis; second, the corrosion parameters of the directional drilling rig quantify material compatibility and service degradation, reducing the deviation between the model and the actual state; third, the coupled model integrates load and structure based on Euler-Bernoulli beam theory, capturing the cyclical effect of load accelerating corrosion and corrosion reducing strength, and combining the fourth strength theory and the finite difference method to make the resultant stress calculation more accurate; finally, the allowable stress method is used for verification, improving the accuracy of stability analysis, avoiding safety risks caused by traditional biases, and providing a reliable assessment basis for drilling operations.

[0070] As an optional implementation method of this embodiment, calculating the marine environmental load under multiple operating conditions includes:

[0071] Wave loads and current loads are calculated based on the Morrison equations, where wave loads include wave force and wave inertial force, and current loads include current resistance.

[0072] Ice loads are calculated based on the Lamé solution, where ice loads include static ice forces;

[0073] The sea wind load is calculated based on the wind pressure formula, the wind pressure height variation coefficient, and the wind vibration coefficient.

[0074] In practical application, the Morrison equation is applicable to ( The diameter of the riser pipe is [missing information]. For small-scale structures (where the wavelength is 1), the wave drag force formula is: In the formula The drag coefficient (derived from the Reynolds number) Sure: hour , hour , hour ); (This refers to the density of seawater). The velocity of water particles in the wave. (This refers to the dynamic viscosity of seawater). This refers to the length of the riser pipe that is submerged.

[0075] The formula for wave inertial force is: In the formula (This is the inertia coefficient, where 1.2 is taken for smooth surfaces and 1.5 for rough surfaces.) This refers to the volume of water drained by the riser pipe.

[0076] The formula for ocean current resistance is: In the formula This is the ocean current drag coefficient (refer to API RP 2A standard, 0.7 for circular cross-sections). The average maximum ocean current velocity over many years in the operating area (monitored by ADCP over a long period, such as 1.5-2.0 m / s in the South China Sea).

[0077] Secondly, Lamé's solution applies to elastic body contact, and the static ice force formula is derived from contact stress: In the formula The compressive strength of ice (obtained through field ice sample tests; 3-5 MPa for multi-year ice in the Arctic, and 1-2 MPa for one-year ice). (where the contact area is) The contact length between the riser and the ice layer (e.g., 0.5-1.0m in winter in the Bohai Sea) needs to be considered, taking into account the difference in elastic modulus between ice and steel (9GPa for ice, 206GPa for steel) on the contact stress.

[0078] In addition, the wind pressure formula is: In the formula The average wind speed is 10 minutes (collected by a platform anemometer, 30-40 m / s under typhoon conditions).

[0079] Wind pressure height variation coefficient: In the formula For platform height (e.g., 50m) ).

[0080] Wind vibration coefficient: In the formula The natural frequency of the riser (calculated using structural dynamics, 0.5-2.0Hz, e.g.) hour ).

[0081] Final sea wind load: In the formula This refers to the windward area of ​​the riser pipe.

[0082] Based on the above, this embodiment improves calculation accuracy by employing dedicated calculation theories and parameter calibrations for different load matching methods: Firstly, wave / current loads are determined using the Morrison equation, with Re segmentation... Surface condition determination First, it avoids the bias caused by traditional fixed coefficients and reduces the error after model calibration; second, it uses the Lamé solution for ice loads, combined with the strength of on-site ice samples, overcoming the shortcomings of traditional empirical formulas that ignore elastic interactions, and improving the accuracy of static ice force calculations in the scene; third, it integrates three coefficients for sea wind loads. Coverage height effect, It covers dynamic effects, avoids large deviations in traditional single wind speed calculations, and improves the accuracy of typhoon conditions; ultimately, it provides high-quality load input for the coupled model, reducing analysis errors from the source.

[0083] Based on the aforementioned calculation of multi-condition marine environmental loads, as a further optional implementation method of this embodiment, the calculation of multi-condition marine environmental loads also includes:

[0084] Under wind and ice flow conditions, the ocean current velocity at different depths is calculated based on the shallow water current velocity formula, and the ocean current force is calculated in combination with the drag coefficient.

[0085] Under wind, wave and current conditions, the horizontal velocity and acceleration of wave water particles are calculated based on Airy wave theory, and the wave force is calculated by combining the drag coefficient and inertia coefficient.

[0086] In practical implementation, the wind-ice-flow condition corresponds to shallow water areas (water depth). Ocean current velocity has a nonlinear distribution along depth, as shown in the formula: In the formula For depth velocity at point ( At that time, the sea surface (At the time, it was underwater) For sea surface speed (ADCP monitoring). The water depth (obtained using a depth sounder). The current velocity profile index is used (e.g., 0.2-0.3 for semi-diurnal tides, 0.3-0.4 for diurnal tides); the ocean current force is calculated in depth segments (every 5m segment), and the integral yields the ocean current force as follows: .

[0087] Secondly, the Airy wave theory is applicable to In deep waters, horizontal speed: Horizontal acceleration: In the formula Wave height (collected by wave buoy). (where wavelength is) (T is the wave period) (where angular frequency is the frequency) (wave number) For time, Horizontal coordinates; Wave force dynamic adjustment coefficient: hour , , hour , According to time step ( Calculate the instantaneous force to obtain the time history curve: .

[0088] Based on the above, this embodiment provides differentiated calculations for two working conditions to improve scenario adaptability: First, the wind-ice-current working condition uses shallow water current formulas, breaking through the traditional uniform flow assumption. The velocity profile index is calibrated according to tidal type, and piecewise integration is used to improve the accuracy of current force calculation, adapting to shallow water environments. Second, the wind-wave-current working condition uses Airy wave theory to accurately capture the spatiotemporal dynamic characteristics of deep water waves. Dynamically adjusting coefficients avoids the decoupling of fixed coefficients from reality and improves the matching degree of time history curves. The two schemes respectively cover the core sea areas of cold-zone shallow water and temperate-zone deep water, solving the problem of insufficient accuracy of traditional general methods in specific sea areas and providing accurate load input for the coupled model.

[0089] As an optional implementation method of this embodiment, the calculation of the corrosion parameters of the directional device includes:

[0090] The amount of thickness reduction of the directional tube wall is determined based on the corrosion rate and corrosion time.

[0091] Based on the thickness reduction, the tilt angle of the inclined plate after corrosion is derived through geometric relationships;

[0092] Based on the thickness reduction, the remaining load-bearing capacity at the window opening location of the diagonal device is calculated.

[0093] In practical implementation, the formula for calculating the thickness reduction is as follows: In the formula The corrosion rate is expressed in mm / a. The corrosion time (obtained from operation and maintenance records, such as 5 years).

[0094] The thickness after corrosion is: , The initial thickness is obtained from the factory report, such as 20mm.

[0095] Secondly, the inclinometer is initially a conical segment with an initial inclination angle of: In the formula The outer diameter of the larger end (e.g., 200mm). The outer diameter of the smaller end (e.g., 150mm). The length of the tapered section (e.g., 500mm); due to eccentric installation and asymmetrical corrosion, the amount of thinning is differentiated. (Large end side) (Small end side, measured with a mounting plate, e.g., 1.2mm, 1.0mm), outer diameter after corrosion: , Inclination angle after corrosion: .

[0096] In addition, the initial cross-sectional area is: In the formula, The outer diameter of the window opening, such as 180mm; Inner diameter, e.g., 120mm; area after corrosion (thinning of the inner wall). (Monitored by an internal wall probe) Residual strength (including stress concentration from corrosion pits): In the formula The yield strength of the material (e.g., 480 MPa for 13Cr steel). Corrosion damage coefficient ( Use 0.7 if necessary, otherwise 0.9 (determined by endoscopy). , hour .

[0097] Based on the above, this embodiment solves the problem of accurate calculation of corrosion parameters through quantitative formulas: First, the tilt angle considers asymmetric corrosion, capturing the attitude deviation caused by installation eccentricity and avoiding the misjudgment of no change under the traditional uniform corrosion assumption; second, the residual strength integrates cross-sectional loss and stress concentration. The coefficient quantifies the impact of pitting corrosion (local stress amplified by 1.5-2.0 times), avoiding the overestimation of some strengths caused by the traditional method of only calculating the area; ultimately, the structural parameters of the coupled model truly reflect the corrosion state, improving the accuracy of the analysis from the structural dimension and providing reliable data for verification.

[0098] As an optional implementation method of this embodiment, the analysis of the stability of the riser includes:

[0099] Based on the axial force balance relationship of the riser, the axial stress of each section of the riser is calculated.

[0100] Based on the moment balance relationship of the riser segment, the bending stress of the riser is calculated.

[0101] Based on the Lamé solution of elasticity for a cylinder subjected to uniformly distributed internal and external pressures, calculate the circumferential and radial stresses of the riser.

[0102] The combined stress of the riser is calculated using the fourth strength theory based on axial stress, bending stress, circumferential stress, and radial stress.

[0103] In practical implementation, the axial force consists of the riser's own weight, the drilling fluid buoyancy, and the axial component of the environmental load. The riser's own weight can be directly output by engineering calculation software using the riser material density (e.g., steel density 7850 kg / m³), outer diameter (e.g., 508 mm), inner diameter (e.g., 445 mm), and total length (e.g., 1000 m), combined with the gravity formula. The drilling fluid buoyancy, based on Archimedes' principle, is calculated using the drilling fluid density (e.g., 1200 kg / m³). 3 The axial component of the environmental load is obtained by calculating the volume of the inner hole of the riser pipe; the axial component of the environmental load is extracted from the previously calculated wave load and ice load according to the principle of force decomposition; the axial stress is obtained by dividing the axial force by the cross-sectional area of ​​the corresponding segment; the cross-sectional area is determined by the initial design parameters such as the outer diameter and inner diameter of the riser pipe.

[0104] Secondly, the moment balance and force balance relationship of the micro-segment of the riser follow the basic principles of mechanics of materials. The bending moment is solved by discretizing the transverse load (such as the transverse load of waves accounting for 85%-90% of the total wave force) using the finite difference method. The bending stress is obtained by the ratio of the bending moment to the section modulus. The section modulus is directly calculated from the outer diameter and inner diameter of the riser using the formula for the section modulus of a circular pipe in the engineering manual.

[0105] Furthermore, the inner wall of the riser is subjected to drilling fluid pressure (directly obtained from drilling operation parameters, such as 25 MPa for deep well operations), while the outer wall is subjected to seawater pressure (based on seawater density of 1025 kg / m³). 3 The depth is calculated based on the static pressure formula (e.g., 10.06 MPa for a depth of 1000m), combined with Lamé's solution, to calculate the stress formula as follows:

[0106]

[0107] In the formula, The inner diameter of the riser pipe (obtained from the initial design parameters, such as 445mm / 2=222.5mm). This refers to the outer diameter of the riser pipe (e.g., 508mm / 2=254mm). To calculate the distance from a point to the center of a circle, we usually take... Calculate the circumferential stress on the outer wall.

[0108] Furthermore, considering the scenario of multi-directional stress interaction in the riser, the resultant stress is calculated as follows:

[0109]

[0110] In the formula, For axial stress, For bending stress, For circumferential stress, For radial stress (compressive stress is negative, such as the radial stress of the outer wall, which is approximately -10 MPa).

[0111] Based on the above, this embodiment achieves a balance between accuracy and efficiency through targeted stress calculations: axial stress and bending stress rely on mature mechanical principles and engineering tools to avoid increasing the difficulty of obtaining them, while ensuring parameter accuracy; the Lamé solution formula is adapted to the internal and external pressure scenarios of the riser, accurately reflecting the wall thickness distribution characteristics of circumferential and radial stresses, breaking through the limitations of traditional simplified calculations; the fourth strength theory's combined stress formula integrates four-dimensional stresses, comprehensively capturing the stress interaction effects under multi-load coupling, avoiding safety misjudgments caused by single stress assessments, and providing comprehensive and realistic stress data support for subsequent stability verification.

[0112] Based on the aforementioned stability analysis of the riser, as a further optional implementation method of this embodiment, the numerical solution is performed using the finite difference method, including:

[0113] Discretize the control equations of the riser analysis model;

[0114] Solving the discretized system of linear algebraic equations yields the lateral displacement, rotation angle, bending moment, and shear force at each node of the riser.

[0115] In practical implementation, the lateral vibration control equation of the riser is constructed based on the Euler-Bernoulli beam theory. Among the core parameters of the equation, the bending stiffness is determined by the elastic modulus of the steel (e.g., 206 GPa) and the moment of inertia of the cross section (calculated from the outer and inner diameters of the riser). The mass per unit length includes the mass of the riser itself (calculated from the material density and cross-sectional area) and the additional mass of the drilling fluid (estimated at 1.0 times the density of seawater). The lateral load is the discretized load calculated earlier. The spatial fourth derivative is discretized using the central difference scheme, discretizing the riser along its length into J nodes (e.g., J=100 for a 1000m riser, with a node spacing of 10m). The fourth derivative of the i-th node is discretized according to the central difference rule. The temporal second derivative is discretized using the Newmark-β method. In the Newmark-β method, the acceleration weight coefficient is 0.25 and the velocity weight coefficient is 0.5 to achieve unconditional stability. The discretized formula follows the time step iteration logic of this method.

[0116] Secondly, the discretized system of equations takes the following form: In the formula The mass matrix is ​​in diagonal form, with the diagonal elements being the product of the mass per unit length of the corresponding node and the distance between the nodes. The Rayleigh damping matrix is ​​obtained by linearly combining the mass matrix and the stiffness matrix, and the combination coefficients are determined by the first two natural frequencies of the riser (obtained through structural dynamics calculations). The stiffness matrix is ​​derived discretized by considering the boundary conditions of the top platform constraint (hinged support) and bottom wellhead constraint (fixed) of the riser. Let be the lateral displacement vector. The i-th element in the vector corresponds to the lateral displacement value (unit: m) of the i-th node after the riser is discretized. It is one of the core unknowns of the equation system. Let be the lateral velocity vector, where the i-th element corresponds to the lateral velocity of the i-th node (unit: m / s). This is a lateral acceleration vector, where the i-th element corresponds to the lateral acceleration of the i-th node (unit: m / s²). 2 ); The load vector is obtained by distributing the lateral loads according to the node positions. The Newton-Raphson iterative method is used to solve the equations, with the convergence condition set as the displacement residual being less than 10. -6 m; After obtaining the lateral displacement of each node, the rotation angle is calculated through the central difference relationship of the displacements of adjacent nodes, the bending moment is calculated through the second-order difference relationship between the bending stiffness and the node displacement, and the shear force is calculated through the third-order difference relationship between the bending stiffness and the node displacement.

[0117] Based on the above, this embodiment achieves accurate and efficient numerical solutions by adapting mature discretization methods to the characteristics of the riser: the combination of central difference and Newmark-β method balances spatial discretization accuracy and temporal iteration stability, and reduces implementation difficulty by relying on existing numerical calculation theory; the linear algebraic equations specifically integrate the mass, damping, and stiffness characteristics of the riser, especially the inclusion of boundary conditions, and the explicit definition of lateral displacement, velocity, and acceleration vectors, so that the equations can truly map the actual constraint state and dynamic response of the riser, avoiding the adaptation deviation of general models; the iterative solution and subsequent mechanical parameter derivation are logically coherent, and continuous distribution data of lateral displacement, rotation angle, bending moment, and shear force can be obtained completely, providing detailed mechanical response support down to the node for stability verification, while convergence accuracy control ensures data reliability.

[0118] As an optional implementation method of this embodiment, stability verification includes:

[0119] The strength of the riser is checked based on lateral displacement, bending moment and resultant stress, where the allowable stress is determined based on the yield strength of the steel.

[0120] The stability of the riser is checked using the allowable stress method based on lateral displacement, bending moment and resultant stress, where the allowable stress for stability is determined based on the overall stability coefficient.

[0121] In practice, the yield strength of steel is obtained through a tensile test (e.g., API 5LX80 steel yield strength ≥ 552 MPa, with ≥ 3 test samples per group, and the average value is taken). The allowable stress is obtained by dividing the yield strength of the steel by the strength safety factor, which is taken as 1.6 according to the API RP2R1 standard. (e.g., 552MPa / 1.6≈345MPa). The check covers all nodes along the entire length of the riser, with a focus on stress concentration areas such as joints and angled valve openings. The check criterion is that the fourth strength theory resultant stress ≤ allowable strength stress. If the resultant stress exceeds the limit, the local structural condition (e.g., corrosion thinning, load concentration) needs to be evaluated.

[0122] Secondly, the overall stability coefficient is related to the slenderness ratio of the riser pipe, where the slenderness ratio is: In the formula The length coefficient is 0.8 (taken in conjunction with the constraint state of the top hinged support and bottom fixed support of the riser). To calculate the segment length (e.g., 100m). The radius of inertia of the cross section (calculated from the moment of inertia and area of ​​the cross section, such as...) , The moment of inertia of the circular cross-section of the riser pipe is calculated from the outer diameter and inner diameter using the formula for the moment of inertia of a circular pipe. (This refers to the cross-sectional area of ​​the riser pipe, calculated from its outer and inner diameters). Overall stability coefficient. Selected according to relevant offshore oil engineering design specifications: hour , hour The formula for calculating the allowable stress for stability is as follows: The verification criterion is that the sum of axial compressive stress and bending stress is less than or equal to the allowable stress for stability. Axial compressive stress is calculated by dividing the axial force by the cross-sectional area, and bending stress is calculated by dividing the maximum bending moment by the section modulus.

[0123] Based on the above, this embodiment achieves precise dual verification of strength and stability through a standardized and adapted allowable stress system: the strength verification relies on material test data and industry safety factors to ensure that the assessment benchmark meets engineering standards, and focuses on key areas to capture the risk of local stress concentration; in the stability verification, the slenderness ratio calculation fits the actual constraint state of the riser, the overall stability coefficient selection follows industry standards, and the allowable stress formula for stability establishes a correlation between strength and stability assessment, avoiding the omission of instability risks caused by traditional fragmented verification; the dual-dimensional verification forms a complete safety assessment link, covering both the risk of material strength failure and the risk of overall structural instability, providing a comprehensive basis for judging the safety of risers for offshore drilling operations.

[0124] Based on the aforementioned stability check, as a further optional implementation method of this embodiment, the strength check of the riser pipe using the allowable stress method includes:

[0125] Calculate the axial stress strength of the riser pipe under axial force and bidirectional bending moment:

[0126] Compare the axial stress intensity with the allowable strength stress, where the allowable strength stress is taken as 1 / 1.6 of the yield strength of the steel.

[0127] In practical implementation, the multi-directional effects of waves and ocean currents on the riser will generate in-plane bending moments during actual stress on the riser. out-of-plane bending moment (All results were obtained from numerical solutions using the finite difference method, such as...) , The axial stress intensity needs to integrate the axial force and the bidirectional bending moment. The calculation formula is as follows: In the formula It is an axial force. , The coefficient for plastic development of the cross section (for circular tube sections, refer to GB50017-2017 "Standard for Design of Steel Structures" and take 1.05). , The in-plane and out-of-plane section modulus (symmetric for circular tubes), .

[0128] Secondly, the yield strength of the steel is determined through a tensile test (e.g., API 5LX80 steel has a yield strength of 552 MPa, therefore the allowable stress is 552 MPa / 1.6 ≈ 345 MPa). During verification, the calculated axial stress strength is compared with the allowable stress. If the strength meets the requirements, then the strength is sufficient. If the axial stress strength in local areas such as the window of the deflector exceeds the standard, it is necessary to combine the previously calculated corrosion parameters to assess whether the strength decreases due to corrosion thinning, or to optimize the structure by increasing the local wall thickness.

[0129] Based on the above, this embodiment improves the comprehensiveness of the strength verification of the riser by integrating stress intensity calculation with bidirectional bending moment: the axial stress intensity formula covers both in-plane and out-of-plane bending moments, avoiding the strength assessment deviation caused by traditional methods that only consider unidirectional bending moment; the introduction of the section plastic development coefficient fits the actual stress characteristics of the circular pipe, ensuring the engineering adaptability of the stress calculation; the allowable stress is adopted with a fixed ratio clearly defined in the specifications, avoiding inconsistencies in the verification benchmark caused by ambiguous safety factor values; at the same time, the traceability logic when local exceedances occur can be directly linked to the previous calculation results such as corrosion parameters, forming a closed loop of problem location and optimization direction, improving the engineering practicality of the technical solution.

[0130] Based on the aforementioned stability verification, as another further optional implementation method of this embodiment, verifying the stability of the riser using the allowable stress method includes:

[0131] Calculate the bending stress of the riser pipe under the combined action of axial force and bending moment;

[0132] The overall stability coefficient is calculated based on the slenderness ratio of the circular tube component, and then the allowable stress for stability is determined.

[0133] By comparing the bending stress with the allowable stress for stability, it can be determined whether the stability of the riser meets the requirements.

[0134] In practice, axial force amplifies the bending moment (second-order effect), which needs to be included in the bending stress calculation. The bending stress calculation formula is as follows: In the formula The maximum bending moment of the riser pipe is obtained by numerically solving it using the finite difference method. For section modulus, The Euler critical force (calculated using Euler's formula) ,in For bending stiffness, (Effective length).

[0135] Secondly, because the riser pipe has a large length-to-diameter ratio (usually >20) and a slenderness ratio often exceeding 200, the overall stability coefficient is corrected according to the following formula: The allowable stress for stability is obtained by multiplying the overall stability coefficient and the allowable strength stress. For example, when the allowable strength stress is 345 MPa, the allowable stress for stability is approximately 0.237 × 345 ≈ 81.8 MPa.

[0136] Furthermore, during the comparison, the bending stress containing second-order effects is compared with the allowable stress for stability, such as... This indicates a risk of instability, which can be addressed by increasing the thickness of the riser wall (reducing the cross-sectional radius of inertia and slenderness ratio) and optimizing the constraints of the top platform (reducing the length coefficient). Improvements can be made through methods such as […]; if the bending stress is less than or equal to the allowable stress for stability, then the stability requirement is met.

[0137] Based on the above, this embodiment improves the pertinence of stability verification by correcting for second-order effects and large slenderness ratio coefficients: the bending stress formula containing second-order effects accurately quantifies the amplification effect of axial force on bending moment, avoiding the underestimation of instability risk caused by neglecting this effect in traditional methods; the Euler critical force calculation relies on mature formulas to ensure the reliability of basic data; the overall stability coefficient correction formula is adapted to the scenario of large slenderness ratio of risers, breaking through the applicability limitations of traditional small slenderness ratio formulas, and accurately reflecting the structural instability resistance when the slenderness ratio exceeds 200; the verification results are directly related to the optimization scheme, and specific measures such as wall thickness adjustment and constraint optimization can be quickly determined according to the instability risk, so that the stability verification extends from risk assessment to solution, improving the operability of the project.

[0138] This embodiment provides a system for analyzing the stability of risers using the multi-condition marine load and corrosion coupling method described above, as a second aspect. Figure 2 As shown, the system includes:

[0139] The load calculation module is configured to calculate wave load, ocean current load, ice load and sea wind load under wind-ice-current conditions and wind-wave-current conditions, respectively.

[0140] The corrosion parameter calculation module is configured to calculate the post-corrosion deflector parameters based on the corrosion rate and corrosion time. The post-corrosion deflector parameters include the deflector tilt angle and the remaining strength at the deflector window position after corrosion.

[0141] The analysis model building module is configured to take the multi-condition marine environmental loads as external load inputs and the corrosion-induced oblique parameters as geometric boundary conditions, and integrate them into the riser mechanical model to form a riser analysis model that couples load and structure.

[0142] The stability analysis module is configured to calculate the axial stress, bending stress, circumferential stress and radial stress of the riser based on the riser analysis model and combined with the fourth strength theory. The finite difference method is used for numerical solution to obtain the lateral displacement, bending moment and resultant stress of the riser.

[0143] The stability verification module is configured to perform stability verification of the riser pipe based on lateral displacement, bending moment, and resultant stress using the allowable stress method.

[0144] It should be noted that this system corresponds to the aforementioned method for analyzing the stability of risers coupled with multi-condition marine load and corrosion. Therefore, for the parts not described in detail in this system (including but not limited to specific implementation techniques and effects), please refer to the relevant records in the aforementioned method for analyzing the stability of risers coupled with multi-condition marine load and corrosion. This text will not elaborate on these details here.

[0145] In the embodiments provided in this application, it should be understood that the embodiments described herein can be implemented in hardware, software, firmware, middleware, code, or any suitable combination thereof. For hardware implementation, the processor may be implemented in one or more of the following: application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field-programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, other electronic units designed to implement the functions described herein, or combinations thereof. For software implementation, some or all of the processes of the embodiments may be performed by a computer program instructing the associated hardware. During implementation, the program may be stored in a computer-readable storage medium or transmitted as one or more instructions or code on a computer-readable storage medium. Computer-readable storage media include computer storage media and communication media, wherein communication media include any medium that facilitates the transmission of a computer program from one place to another. Storage media may be any available medium accessible to a computer. Computer-readable storage media may include, but are not limited to, RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage media or other magnetic storage devices, or any other medium capable of carrying or storing desired program code having the form of instructions or data structures and accessible to a computer.

[0146] Finally, it should be noted that the above description is only a preferred embodiment of this application and is not intended to limit this application. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for analyzing the stability of risers under coupled marine loads and corrosion under multiple operating conditions, characterized in that, The method includes: Calculate marine environmental loads under multiple operating conditions: calculate wave loads, ocean current loads, ice loads, and sea wind loads under wind-ice-current and wind-wave-current operating conditions respectively; Calculate the corrosion parameters of the slant: Calculate the parameters of the slant after corrosion based on the corrosion rate and corrosion time. The parameters of the slant after corrosion include the tilt angle of the slant after corrosion and the remaining strength at the opening position of the slant. Analytical model construction: The multi-condition marine environmental loads are used as external load inputs, and the parameters of the corroded siphon are used as geometric boundary conditions. They are integrated into the riser mechanical model to form a riser analysis model that couples load and structure. Analysis of riser stability: Based on the riser analysis model and combined with the fourth strength theory, the axial stress, bending stress, circumferential stress and radial stress of the riser are calculated, and the finite difference method is used for numerical solution to obtain the lateral displacement, bending moment and resultant stress of the riser. Stability verification: Based on the lateral displacement, bending moment and resultant stress, the stability of the riser is verified using the allowable stress method.

2. The method for analyzing the stability of risers under coupled marine loads and corrosion under multiple operating conditions as described in claim 1, characterized in that, The calculation of multi-condition marine environmental loads includes: Wave loads and current loads are calculated based on the Morrison equations, wherein the wave loads include wave forces and wave inertial forces, and the current loads include current resistance. The ice load is calculated based on the Lamé solution, wherein the ice load includes static ice force; The sea wind load is calculated based on the wind pressure formula, the wind pressure height variation coefficient, and the wind vibration coefficient.

3. The method for analyzing the stability of risers under coupled marine loads and corrosion under multiple operating conditions as described in claim 2, characterized in that, The calculation of multi-condition marine environmental loads also includes: Under the aforementioned wind and ice flow conditions, the ocean current velocity at different depths is calculated based on the shallow water current velocity formula, and the ocean current force is calculated in combination with the drag coefficient. Under the aforementioned wind, wave, and current conditions, the horizontal velocity and acceleration of wave water particles are calculated based on Airy wave theory, and the wave force is calculated by combining the drag coefficient and the inertial force coefficient.

4. The method for analyzing the stability of risers under coupled marine loads and corrosion under multiple operating conditions as described in claim 1, characterized in that, The calculated corrosion parameters for the oblique device include: The amount of thickness reduction of the directional tube wall is determined based on the corrosion rate and the corrosion time. Based on the aforementioned thickness reduction, the tilt angle of the oblique device after corrosion is derived through geometric relationships; Based on the aforementioned thickness reduction, the remaining load-bearing capacity at the window opening location of the diagonal device is calculated.

5. The method for analyzing the stability of risers under coupled marine loads and corrosion under multiple operating conditions according to claim 1, characterized in that, The analysis of the riser stability includes: Based on the axial force balance relationship of the riser, the axial stress of each section of the riser is calculated. Based on the moment balance relationship of the riser segment, the bending stress of the riser is calculated. Based on the Lamé solution of elasticity for a cylinder subjected to uniformly distributed internal and external pressures, calculate the circumferential and radial stresses of the riser. Based on the axial stress, bending stress, circumferential stress, and radial stress, the combined stress of the riser is calculated using the fourth strength theory.

6. The method for analyzing the stability of risers under coupled marine loads and corrosion under multiple operating conditions as described in claim 5, characterized in that, The numerical solution using the finite difference method includes: The control equations of the riser analysis model are discretized. Solving the discretized system of linear algebraic equations yields the lateral displacement, rotation angle, bending moment, and shear force at each node of the riser.

7. The method for analyzing the stability of risers under coupled marine loads and corrosion under multiple operating conditions as described in claim 1, characterized in that, The stability check includes: Based on the lateral displacement, the bending moment, and the combined stress, the strength of the riser is checked using the allowable stress method, wherein the allowable stress for strength is determined based on the yield strength of the steel. Based on the lateral displacement, the bending moment, and the resultant stress, the stability of the riser is checked using the allowable stress method, wherein the allowable stress for stability is determined based on the overall stability coefficient.

8. The method for analyzing the stability of risers under coupled marine loads and corrosion under multiple operating conditions as described in claim 7, characterized in that, The method of verifying the strength of the riser pipe using the allowable stress method includes: Calculate the axial stress strength of the riser pipe under axial force and bidirectional bending moment: The axial stress intensity is compared with the allowable strength stress, wherein the allowable strength stress is taken as 1 / 1.6 of the yield strength of the steel.

9. The method for analyzing the stability of risers under coupled marine loads and corrosion under multiple operating conditions as described in claim 7, characterized in that, The method of verifying the stability of the riser pipe using the allowable stress method includes: Calculate the bending stress of the riser pipe under the combined action of axial force and bending moment; The overall stability coefficient is calculated based on the slenderness ratio of the circular tube component, and then the allowable stress for stability is determined. By comparing the bending stress with the allowable stress for stability, it can be determined whether the stability of the riser meets the requirements.

10. A system for analyzing the stability of risers under multi-condition marine load and corrosion coupling as described in any one of claims 1-9, characterized in that, The system includes: The load calculation module is configured to calculate wave load, ocean current load, ice load and sea wind load under wind-ice-current conditions and wind-wave-current conditions, respectively. The corrosion parameter calculation module is configured to calculate the post-corrosion deflector parameters based on the corrosion rate and corrosion time. The post-corrosion deflector parameters include the deflector tilt angle and the remaining strength at the deflector window position after corrosion. The analysis model construction module is configured to take the multi-condition marine environmental load as the external load input and the corrosion-induced oblique device parameters as the geometric boundary conditions, and integrate them together into the riser mechanical model to form a riser analysis model that couples load and structure. The stability analysis module is configured to calculate the axial stress, bending stress, circumferential stress and radial stress of the riser based on the riser analysis model and in combination with the fourth strength theory, and to obtain the lateral displacement, bending moment and resultant stress of the riser by numerical solution using the finite difference method. The stability verification module is configured to perform stability verification of the riser pipe using the allowable stress method based on the lateral displacement, the bending moment, and the resultant stress.

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