Sea pipe riser fatigue damage evaluation method

Abstract

The invention relates to the technical field of marine oil and gas resource exploitation, and provides a sea pipe riser fatigue damage evaluation method. Comprising the steps of 1, obtaining model parameters and sea condition data; step 2, establishing a sea pipe riser finite element model; step 3, determining a stand pipe constraint form, and establishing a coupling relationship among the structural members; 4, factors sensitive to the influence on the service life of the riser are obtained, and calculated loading factors are determined; 5, calculating the strength of the riser, determining the position of a dangerous point, and checking the combined stress strength; step 6, according to the stress result of the riser and the S-N curve, calculating the residual fatigue life of the riser based on the determination of a correction factor, a Brown-Miller critical plane algorithm, a strength factor method, a Miner linear cumulative damage theory and a rain flow counting method; according to the method, the most easily-broken position of the riser and the pre-estimation of the service life of the position can be effectively solved when the sea pipe riser works in an actual sea area.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore oil and gas resource exploitation, and provides a method for evaluating the fatigue damage of a submarine pipeline riser. Background Art

[0002] As a connection channel between the sea surface and the seabed, the submarine pipeline riser is one of the important components of the offshore oil and gas exploitation system. Compared with other offshore structures, the marine environment where the submarine pipeline riser is located is more complex and harsh. The inside of the pipeline is corroded by flowing media such as high-temperature and high-pressure oil or natural gas, and the outside is also affected by natural environments such as sea wind, sea waves, ocean currents, sea ice and earthquakes. In such a complex and changeable harsh environment, the submarine pipeline riser is easily damaged, leading to various unimaginable accidents, such as oil and gas leakage, fire or even explosion, causing casualties, economic losses and environmental pollution, and even destroying the entire marine environmental ecosystem, with extremely serious consequences.

[0003] In recent years, damage and failure problems of submarine pipeline risers have occurred frequently. From the perspective of stress failure analysis, the failure forms of risers are mainly attributed to two factors: fatigue limit failure and sudden load failure. At present, there is a lack of technologies for damage, detection, analysis and evaluation of submarine pipeline risers, and further improvement is needed in theory and engineering applications. Many scholars at home and abroad have carried out some frontier research work. Although there are relevant reports on fatigue damage events caused by wave, wind and current loads and vortex-induced vibration of risers, a systematic safety evaluation system has not been established to evaluate the safety stability and predicted life of in-service risers. In view of China's oil and gas development strategy, it is of crucial importance to reasonably and accurately identify the damage mechanism of risers under complex loads and make a reasonable safety evaluation. Due to the complex marine environment where the riser is located and the difficulty in predicting the accuracy of the fatigue life prediction model of the load, the design code recommends using a larger fatigue safety factor to ensure the safety of the riser structure during its service life. The domestic research on the vibration response prediction of risers and the vibration fatigue damage of risers is relatively less, and some practical theories and calculation methods for riser fatigue life prediction and reliability evaluation are given. The fatigue life prediction methods for submarine pipeline risers are all fatigue life prediction methods based on the S-N curve. It includes a simplified method for predicting the vibration fatigue damage of risers in uniform incoming flow, which uses the maximum value of the riser vibration response to calculate the corresponding stress range. The disadvantage is that the long-term distribution of the alternating stress range caused by riser vibration is not considered, resulting in a large deviation in the predicted results of riser vibration fatigue damage. The empirical orthogonal function (EOF) method is used to simplify and process a large amount of measured flow velocity data, and then the designed flow velocity distribution along the riser length direction is depicted. On this basis, the vibration fatigue damage of the riser is predicted. The vibration fatigue damage of the riser is studied through experiments. An in-depth analysis of the structural integrity and safety reliability assessment of submarine pipeline risers is carried out, including the failure forms, causes and fault analysis of risers, the static and dynamic strength assessment and sensitivity analysis of risers, and the research on riser fatigue assessment and reliability analysis methods. The research finds that for a single-layer riser, the magnitudes of the vibration fatigue damage of the riser in the flow direction and the vibration fatigue damage of the riser in the transverse direction are quite the same. However, in the current industrial community, the vibration fatigue damage of the riser in the flow direction is often ignored, which should attract the attention of riser designers. The main purpose of marine engineering risk assessment is not only to make the structure meet the requirements of the code, but more importantly, through the risk assessment of marine engineering, the ability of the structure to resist risks should be improved as much as possible to ensure the safe operation of marine structures during the design life. Summary of the Invention

[0004] The object of the present invention is to propose a method for evaluating the fatigue damage of a submarine pipeline riser in view of the deficiencies of the prior art, and solve the following technical problems: Due to the special structure of the submarine pipeline riser and the complexity of the working environment loads, there are some problems in the analysis of riser failure principle, the establishment of riser mechanical model, the data acquisition and dynamic simulation of risers, and the research on fatigue protection. Specifically, they include: In terms of the numerical simulation of risers, the multi-layer riser structure has particularities. During the transportation of oil and gas products, the outer pipe bears a large tensile load to balance the axial pressure from the inner high-temperature and high-pressure pipe. For risers in complex environmental conditions, coupled with the particularity of this multi-layer pipe structure, the stress distribution becomes more complex, requiring designers to calculate and analyze more accurately; In terms of the influencing factors of risers, since the working environment of risers is relatively complex, they will be affected by various factors and thus subjected to various loads. When analyzing complex multi-layer risers, it is necessary to identify as comprehensively as possible the influencing factors on the risers and consider the loads borne by the risers, so as to simulate the riser stress more accurately; In terms of the analysis and evaluation of riser damage, there are currently no mature standards and methods for reference. It is necessary to analyze and calculate the fatigue damage of the riser under complex load stresses. According to the calculation results, a guiding method for riser damage evaluation should be given, and the analysis and evaluation method should be refined, which requires more in-depth research work.

[0005] To achieve the above object, the present invention adopts the following technical solutions: A method for evaluating the fatigue damage of a submarine pipeline riser, comprising the following steps: Step 1, obtain model parameters and sea condition data; Step 2, establish a finite element model of the submarine pipeline riser; Step 3, determine the constraint form of the riser and establish the coupling relationship between each structural member; Step 4, obtain the factors sensitive to the riser life and determine the loading factors for calculation; Step 5, perform strength calculation on the riser, determine the position of the dangerous point, and conduct a combined stress strength check; Step 6, based on the riser stress results and the S-N curve, and based on the determination of the correction factor, the Brown-Miller critical plane algorithm, the strength factor method, the Miner linear cumulative damage theory, and the rain flow counting method, deduce the remaining fatigue life of the riser.

[0006] Further, step 2 includes: based on the model parameters and sea conditions, establish a finite element model of the submarine pipeline riser and perform mesh division on the model.

[0007] Further, the model is modeled using solid elements, the elbows are refined in mesh, and after applying constraints and loading conditions, the calculation is submitted.

[0008] Further, step two also includes conducting a comparative analysis on the straight pipe section based on the vertical management theory analysis model, and obtaining the maximum bending stress on the riser from the stress calculation formula of a circular pipe in material mechanics.

[0009] Further, step three includes selecting an elastic boundary to reflect the interaction between the pipeline and the soil mass, using a linearly elastic foundation beam to reflect the force and displacement relationship between the pipeline and the soil mass, and using different stiffnesses of the elastic foundation beam to replace different soil qualities to study the influence of soil characteristics on the vibration characteristics of the pipeline.

[0010] Further, step three also includes establishing a rigid connection at the joint of the inner and outer pipes, keeping the relative positions between the inner and outer connection positions unchanged during the analysis process, establishing a distributed coupling between the riser constraint position and the riser, and coupling the two surfaces at one point.

[0011] Further, the sensitive factors include the influence of the pipe clamp position on the riser strength, the influence of the underwater pile position on the riser strength, the influence of the wave height on the riser strength, the influence of the flow velocity on the riser strength, the influence of the inner pipe pressure on the riser strength, the influence of the submarine pipeline drift on the riser strength, the influence of vortex-induced vibration on the riser strength, and the influence of corrosion on the riser strength.

[0012] Further, step four also includes the fatigue damage analysis of the riser. The fatigue damage analysis of the riser focuses on considering the influence of the riser constraint position, the marine environment, the submarine pipeline drift, and corrosion on the riser strength and service life.

[0013] Further, step five includes conducting strength calculations on the riser, obtaining the strength calculation results of the riser, and determining the position of the dangerous point.

[0014] Further, the fatigue damage algorithm for the submarine pipeline riser selects the Brown-Miller critical plane algorithm based on the maximum principal strain. The infinite life design calculation of the riser should be based on FOS, and the fatigue life calculation of the submarine pipeline riser should use the Miner linear cumulative damage theory and the rain flow counting method.

[0015] The beneficial effects of the present invention are as follows: The present invention aims at typical factors such as the environmental load of the submarine pipeline riser, the riser pipe clamp constraint position, the underwater pile constraint position, and the inner pipe pressure, establishes the influence rules of the changes of different typical factors on the stress numerical distribution and the maximum value position of the riser, through the analysis and research of typical finite element models, and in combination with the simulation analysis results of the riser, details a fatigue damage evaluation method and steps for the submarine pipeline riser, which can effectively solve the most vulnerable fracture position of the submarine pipeline riser during actual sea area operations and the prediction of the service life of this position.

[0016] By conducting damage analysis and stress evaluation on the in-service riser of the actual subsea water injection pipeline, comparing with the results of conventional simulation analysis methods, verifying the accuracy of damage analysis and stress evaluation for the in-service riser, achieving a positioning accuracy of the damage area of the riser ≥ 85%, and further determining the feasibility and superiority of the established method. Description of the Drawings

[0017] Figure 1 It is a flowchart of a method for evaluating the fatigue damage of a subsea pipeline riser of the present invention; Figure 2 It is a schematic diagram of the global model of the subsea pipeline riser system in the method for evaluating the fatigue damage of a subsea pipeline riser of the present invention; Figure 3 It is a schematic diagram of the finite element model of the subsea pipeline riser in the method for evaluating the fatigue damage of a subsea pipeline riser of the present invention; Figure 4 It is a schematic diagram of the constraint and load distribution loading of the subsea pipeline riser in the method for evaluating the fatigue damage of a subsea pipeline riser of the present invention; Figure 5 It is a schematic diagram of the process of the present invention. Detailed Embodiments

[0018] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0019] Embodiment 1

[0020] As Figure 1 shown, a method for evaluating the fatigue damage of a subsea pipeline riser includes the following steps: S1: Based on the model parameters and sea conditions, establish a finite element model of the subsea pipeline riser, perform mesh division on the model. Since the pipeline deformation is large, and the end cap has a rounding effect on the pipeline and is subjected to large forces, the overall model should be modeled using solid elements, and the mesh should be refined at the elbow. After applying constraints and loading conditions, submit the calculation.

[0021] Based on the theoretical analysis model of the riser, conduct a comparative analysis on the straight pipe section. The maximum bending stress of the riser is obtained from the stress calculation formula of the circular pipe in material mechanics. Based on the comparison and analysis of the theoretical calculation method and the calculation results of the finite element analysis software, further verify the accuracy of the model; By comparing the calculation results of the equivalent riser with the calculation results of the double-layer pipe, the calculation results of the double-layer pipe are closer to the actual detected stress value, that is, the double-layer pipe modeling is more accurate.

[0022] S2: Determine the constraint form of the riser. Select an elastic boundary to reflect the interaction between the pipeline and the soil. Use a linearly elastic foundation beam to represent the force and displacement relationship between the pipeline and the soil. Replace the different soil properties with different stiffnesses of the elastic foundation beam to study the influence of soil properties on the vibration characteristics of the pipeline. The elastic foundation beam boundary has more advantages than the relatively simple fixed constraint. Equivalent the two ends of the cross shoulder to elastic supports, which more realistically reflects the constraint of the soil and enables the research results to be better applied to engineering practice.

[0023] Meanwhile, establish the coupling relationship between each structural component. Couple the connections of multiple layers of pipes at the same point, that is, establish a rigid connection at the connection between the inner and outer pipes. This area becomes a rigid body, and the relative positions between the inner and outer connection positions remain unchanged during the analysis process. Establish a distributed coupling between the riser constraint position and the riser. Couple the two surfaces at one point, that is, establish a constraint relationship between each node in this area and the reference point. Perform a weighted average on the motion of each node in this area so that the resultant force and resultant moment received in this area are equivalent to the force and moment at the reference point. The coupling form of the underwater pile is the same as that of the pipe clamp.

[0024] At the connection between the outer pipe and the inner pipe, establish a coupling relationship between the inner and outer pipes. Couple the connections of the inner and outer pipes at the same point, that is, establish a rigid connection at the connection between the inner and outer pipes. This area becomes a rigid body, and the relative positions between the inner and outer connection positions remain unchanged during the analysis process.

[0025] S3: Conduct risk identification for the submarine pipeline riser. The purpose of risk identification for the submarine pipeline riser is to find out as many potential risk factors as possible that affect the operation of the riser. Only by correctly and comprehensively identifying the potential risk factors of the submarine pipeline riser can appropriate, timely, and effective measures be proposed for the normal, safe, and reliable operation of the submarine pipeline riser. On the one hand, judge through people's subjective perceptual knowledge and long-term work experience summary; on the other hand, analyze relevant objective data and risk accident statistics, collect and summarize, and list various possible risk factors of the riser.

[0026] For the determined risk factors of the submarine pipeline riser, study the structural sensitivity of the riser under multiple factors. Analyze the typical factors of the submarine pipeline riser and explore the influence characteristics of the changes of these typical factors on the stress of the riser through finite element model analysis, which is convenient for detecting, analyzing, and protecting the key area positions of the riser.

[0027] Study the sensitivity of the structural parameters of the riser under multiple factors and determine the loading factors during the fatigue analysis and calculation of the riser, specifically including the influence of the pipe clamp position on the strength of the riser, the influence of the underwater pile position on the strength of the riser, the influence of the wave height on the strength of the riser, the influence of the flow velocity on the strength of the riser, the influence of the inner pipe pressure on the strength of the riser, the influence of the submarine pipeline drift on the strength of the riser, the influence of vortex-induced vibration on the strength of the riser, and the influence of corrosion on the strength of the riser.

[0028] According to the analysis of the influence law of the stress of the riser by the position constraint of the pipe clamp and the position constraint of the underwater pile, the position where the maximum stress occurs is located at the position of the lowermost pipe clamp, followed by the position of the underwater pile at the elbow end. In terms of constraint protection, the safety of the riser mainly focuses on the lowermost pipe clamp and the position of the underwater pile at the elbow end. The lowermost pipe clamp and the position of the underwater pile at the elbow end should be moved and adjusted regularly to avoid long-term stress concentration, which may lead to the failure of the riser. Moreover, the lower the position of the lowermost pipe clamp and the closer the position of the underwater pile at the elbow end is to the elbow, the better the strength performance of the riser. If the suspended length of the elbow is too large, the stress concentration position will appear at the elbow. The influence of the middle suspended length on the stress magnitude and position of the submarine pipeline is relatively small.

[0029] For wave loads, there are mainly four commonly used theories in engineering, namely: linear (Airy) wave theory, Stokes wave theory, cosine wave theory, and solitary wave theory. In deep water areas, the main factors affecting the properties of waves are wave steepness h / L and relative water depth H / L; in shallow water areas, the main influencing factor is relative wave height h / H. It is found that according to the analysis of the influence law of wave height and flow velocity on the stress of the riser, the position where the maximum stress occurs is located at the position of the lowermost pipe clamp, and both the wave height and the flow velocity have a linear influence on the stress. The greater the wave height, the greater the stress on the riser; the flow velocity has a greater impact on the stress and life of the riser. The greater the flow velocity, the greater the stress on the riser.

[0030] In addition to the environmental loads of wind, wave and current, the environmental loads also include pressure loads. Considering the internal fluid pressure, it is found that the positions of the pipe clamps on the outer pipe, the positions of the underwater piles, the wave height and the flow velocity have no influence on the stress of the inner pipe.

[0031] The influence of the drift distance of the submarine pipeline on the stress of the inner pipe shows a linear law. The greater the drift distance of the submarine pipeline, the greater the stress on the riser, and the position of the maximum stress on the outer pipe is located at the position of the lowermost pipe clamp.

[0032] Assume that the riser is vertically supported on the platform, has a uniform circular cross-section, and assume that the wave and the flow are in the same direction. The vortex-induced vibration of the riser is regarded as a simple harmonic motion. It is found that the influence of vortex-induced vibration on the riser is very small, so the influence of vortex-induced vibration on the riser can be ignored in strength calculation.

[0033] Subsea risers are located in a seawater environment. Their interior is subject to internal corrosion from the flowing medium, while their exterior is subject to external corrosion from environmental influences. Therefore, corrosion is one of the risk factors for subsea risers that requires increased consideration. The main contributing factors include: the corrosiveness of the medium inside the pipe, anti-corrosion measures, cathodic protection, the quality of the outer protective layer, the corrosiveness of seawater, and stress corrosion. These factors must be fully considered to avoid corrosion damage to subsea risers. Studies have found that riser corrosion has a significant impact on the life of the riser. When the same thickness of corrosion is applied to the inner and outer pipes, fatigue damage occurs first in the outer pipe after cyclic loading. Therefore, greater attention should be paid to the protection of the outer pipe.

[0034] Based on the above conclusions, the fatigue damage analysis of risers should focus on the influence of riser restraint positions (clamp positions and underwater pile positions), marine environment (wave height and current velocity), pipeline drift, and corrosion on riser strength and life. Other factors have little effect on riser strength. The mechanical model of the pipeline riser can also solve the stress and strain results of each layer of the riser under different load forms. S4: Based on the above analysis, a strength calculation is performed on the riser to obtain the strength calculation results of the riser and determine the location of the dangerous point. Due to the complex working environment of the riser, it will be affected by many factors and, in turn, by various loads, including: the pull of the upper node of the platform, the support force of the seabed soil at the lower end, the pressure, impact force, buoyancy of the seawater, and its own gravity. Due to the complex stress conditions, the axial stress is relatively large. For general pipelines, the circumferential stress is the largest, the axial stress is second, and the radial stress is the smallest. Therefore, the strength design is generally based on the Trisca criterion, ignoring the effect of axial stress on pipeline yield and adopting the circumferential stress criterion. The stress conditions of the offshore pipeline riser are complex, so a combined stress strength check is required. When performing fatigue strength checks or estimating life under unstable variable amplitude stress, a certain cumulative damage theory must be used. The equivalent stress method is relatively convenient for fatigue strength checks under variable amplitude stress. The equivalent stress method uses the power function equation of the structural SN curve and performs fatigue strength checks based on the principle of equal fatigue damage. S5: Based on the riser material properties, the riser's SN curve is determined using the material's elastic modulus and tensile strength, and corrections are made to the riser's wall thickness. The fatigue properties of metal components under load are influenced by numerous factors. In addition to the influence of their inherent geometric properties, differences in stress amplitude, mean stress, and ultimate stress throughout the load history also lead to variations in the ultimate fatigue life. Therefore, the SN curve used for fatigue damage calculations must be based on extensive testing or statistical data. Based on the riser stress results and the material's SN curve, and based on relevant theory, the riser's remaining fatigue life is inferred. Specifically, the correction factor should be set based on factors such as loading conditions, temperature, surface smoothness, and environmental factors.

[0035] The fatigue damage algorithm for the submarine pipeline riser selects the Brown-Miller critical plane algorithm based on the maximum principal strain. The infinite-life design calculation of the riser should be carried out based on FOS (Factor of Safety), and the fatigue life calculation of the submarine pipeline riser should use the Miner linear cumulative damage theory and the rain-flow counting method.

[0036] The fatigue damage of a structure under alternating stress is a cumulative process. Each cycle of alternating stress will cause a certain amount of fatigue damage and consume a certain amount of life. If the structure is under variable-amplitude alternating stress, by accumulating the fatigue damage caused by stress cycles of different amplitudes according to appropriate principles, the total damage degree can be obtained. The most commonly used fatigue damage accumulation model in ocean engineering structures is the Miner linear cumulative damage theory. However, the Miner linear cumulative damage theory is based on the following assumptions: (1) Each load cycle during the specimen loading process consumes a certain component of the specimen's effective life; (2) The fatigue damage of the specimen is proportional to the work it absorbs, and this work is proportional to the ratio of the number of stress cycles and the number of failure cycles reached at this stress value; (3) The total damage amount when the specimen reaches failure is a constant; (4) The damage is independent of the order of load application; (5) When the sum of all damage components generated by multi-cycle stress is 1, the specimen fails.

[0037] Example 2

[0038] A method for evaluating the fatigue damage of a submarine pipeline riser: I. Method Overview The method for evaluating the fatigue damage of a submarine pipeline riser is mainly based on the S-N curve method and the Paris fatigue crack propagation calculation method. This method measures the S-N curve through experiments to express the anti-fatigue performance of the structure. Specifically, if the structure undergoes fatigue failure after cycling N times under a cyclic load with a constant stress amplitude, then the fatigue life of this structure under an alternating stress range S is considered to be N. This method can effectively handle the effects of mean stress, stress amplitude variation, multiaxial stress, and stress concentration, and comprehensively evaluate the fatigue life of the submarine pipeline riser.

[0039] II. Main Steps 1. Determine the evaluation object: Clearly define the part of the submarine pipeline riser to be evaluated and its corresponding fatigue damage characteristics.

[0040] 2. Obtain the S-N curve: Obtain the S-N curve of the submarine pipeline riser through experimental methods, that is, the curve describing the relationship between the number of cycles and the fatigue life of the structure under a cyclic load with a constant stress amplitude.

[0041] 3. Determine the load conditions: Based on the actual working conditions, determine the range of alternating stresses to which the subsea pipeline riser is subjected, including average stress, stress amplitude, etc.

[0042] 4. Calculate fatigue life: Based on the SN curve and given load conditions, the fatigue life of the submarine riser is calculated using the Miner linear cumulative damage criterion.

[0043] 5. Fatigue damage assessment: Based on the calculated fatigue life and actual usage, the fatigue damage level of the submarine pipeline riser is assessed to determine whether repair or replacement is necessary.

[0044] 6. Develop maintenance measures: Based on the assessment results, develop appropriate maintenance measures, such as regular inspection, repair, or replacement, to ensure the safe operation of the submarine riser.

[0045] 3. Precautions 1. When conducting fatigue damage assessment on submarine pipeline risers, various complex factors should be fully considered, such as the marine environment, pipeline materials, and manufacturing process.

[0046] 2. Since the SN curve is measured by experimental methods, the test conditions and methods should be fully understood before evaluation to ensure the accuracy of the evaluation results.

[0047] 3. In practical applications, appropriate evaluation methods and steps should be selected according to specific circumstances, and the evaluation methods should be continuously optimized and improved.

[0048] 4. To ensure the safe operation of subsea pipeline risers, a comprehensive maintenance management system should be established, with daily inspections and regular testing strengthened to promptly detect and address potential fatigue damage.

[0049] Example 3

[0050] For a submarine pipeline near the platform, the outer pipe was Φ377×13mm, and the inner pipe was Φ273×13mm, both made of 16Mn. Clamps were installed at 0.5m above water, 4m above water, 2m below water, and 6m below water. Submerged piles were installed at 6m, 13m, and 30m in the horizontal section, and the pipeline was sunk into the mud at 36m. The operating water depth was 15m, and horizontal wave loads were applied, with a design wave height of 6.0m and a period of 8.1s. The surface current velocity was 1.02m / s, the mid-section velocity was 0.91m / s, and the bottom current velocity was 0.87m / s. The inner pipe fluid pressure was 4MPa, the design wind speed under normal operating conditions was 30.8m / s, and the corrosion loss was 1mm. An evaluation and analysis was conducted using a patented method for evaluating fatigue damage in submarine pipeline risers.

[0051] The riser model is loaded according to the above loads to obtain the stress nephogram of the outer pipe. The maximum stress is 128.6 MPa, which occurs at the position of the lowest pipe clamp of the outer pipe. Based on the above calculation results, with the help of Fe-Safe fatigue analysis software, relevant parameters are set. The endurance limit of the material is set according to the software default value of 10e 7 settings, that is to say, when the life of the material exceeds 10e 7 cycles, it is considered an infinite life state. The fatigue analysis of the riser is carried out according to the above calculation steps, and the fatigue calculation results are obtained. Under this working condition, the outer pipe undergoes fatigue failure at the position of the lowest pipe clamp when the number of load cycles reaches about 1.656 million times, and the fatigue life distribution nephogram. The lowest pipe clamp is located 6 m underwater, and the length of the pipe clamp model is 300 mm, so the position of the maximum stress is about 6.15 m underwater.

[0052] Use an ROV to carry an underwater non-contact magnetic testing (MTM) device to move uniformly directly above the subsea pipeline along the seabed. A non-contact scanning magnetometer is used to detect the on-site stress of the subsea pipeline near the platform end and the riser of this subsea pipeline.

[0053] Under the excitation of environmental loads, the pipeline undergoes complex vibrations, and the stress state at the key points is complex. The maximum stress value is not allowed to exceed the yield limit of the pipeline material. The material of the riser is 16Mn, and its yield limit is 343 MPa. According to the on-site test results, the stress intensity at the key points of the riser at the platform end is about 120 MPa. The outer pipe of the riser at the platform end is integrally fractured, and the fracture position is about 5.5 m underwater, at the middle pipe section position between the third and fourth pipe clamps, and the fracture is not at the weld.

[0054] Based on the above finite element analysis and on-site test data, the maximum stress generated by the riser and its occurrence position are compared. The maximum stress calculated by the finite element model is relatively close to the maximum stress measured on-site, the calculated life, and the position where the riser is prone to stress concentration and fracture. This shows that the calculation method and theory are feasible and applicable to the calculation of subsea pipeline risers.

[0055] By comparing the measured damage stress values of the subsea pipeline riser with the stress values calculated by the finite element model, it can be seen that the calculation results of the finite element model are slightly greater than the measured results. According to the service life evaluation results of the subsea pipeline riser obtained by this subsea pipeline riser damage evaluation method, it is slightly longer than the actual service life of the subsea pipeline riser. And the positioning accuracy of the stress concentration point position is ≥85%, meeting the requirements of on-site application.

[0056] Therefore, the fatigue damage evaluation method for a subsea pipeline riser described in the patent is safe and feasible.

[0057] The above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution of the present invention and its inventive concept, making equivalent substitutions or changes should be covered within the protection scope of the present invention.

Claims

1. A method for evaluating the fatigue damage of a subsea pipeline riser, which determines the vulnerable positions of the subsea pipeline riser and estimates the remaining life of the vulnerable positions through the calculation of the remaining life of the riser, is characterized in that It includes the following steps: Step 1: Obtain model parameters and sea condition data; Step 2: Establish a finite element model of the submarine pipeline riser; Step 3: Determine the riser constraint form and establish the coupling relationship between various structural components; Step 4: Obtain the factors sensitive to the riser life and determine the loading factors for calculation; Step 5: Conduct strength calculation on the riser, determine the location of the dangerous point, and conduct combined stress strength check; Step 6: Based on the riser stress results and S-N curve, calculate the remaining fatigue life of the riser based on the Brown-Miller critical plane algorithm, strength factor method, Miner linear cumulative damage theory or rain flow counting method.

2. The method for evaluating fatigue damage of a submarine pipeline riser according to claim 1, wherein, Step 2 includes: Based on the model parameters and sea conditions, establish a finite element model of the submarine pipeline riser and conduct mesh division on the model.

3. The method for evaluating fatigue damage of a submarine pipeline riser according to claim 2, wherein The model is modeled using solid elements, and the mesh is refined at the elbow. After applying constraints and loading conditions, submit for calculation.

4. The method for evaluating fatigue damage of a submarine pipeline riser according to claim 3, characterized in that Step 2 also includes conducting a comparative analysis on the straight pipe section based on the theoretical analysis model of the riser, and obtaining the maximum bending stress of the riser from the circular pipe stress calculation formula in material mechanics.

5. A method for evaluating fatigue damage of a submarine pipeline riser according to claim 4, characterized in that Step 3 includes selecting an elastic boundary to reflect the interaction between the pipeline and the soil, using a linear elastic foundation beam to reflect the force and displacement relationship between the pipeline and the soil, and using different stiffnesses of the elastic foundation beam to replace different soil qualities to study the influence of soil characteristics on the vibration characteristics of the pipeline.

6. The method for evaluating fatigue damage of a submarine pipeline riser according to claim 5, wherein Step 3 also includes establishing a rigid connection at the connection between the inner and outer pipes, keeping the relative position between the inner and outer connection positions unchanged during the analysis process, establishing a distributed coupling between the riser constraint position and the riser, and coupling the two surfaces at a point.

7. The method for evaluating fatigue damage of a submarine pipeline riser according to claim 6, wherein The sensitive factors include the influence of the pipe clamp position on the riser strength, the influence of the underwater pile position on the riser strength, the influence of the wave height on the riser strength, the influence of the flow velocity on the riser strength, the influence of the inner pipe pressure on the riser strength, the influence of the submarine pipeline drift on the riser strength, the influence of vortex-induced vibration on the riser strength, and the influence of corrosion on the riser strength.

8. A method for evaluating fatigue damage of a submarine pipeline riser according to claim 7, characterized in that, Step 4 also includes riser fatigue damage analysis, and the riser fatigue damage analysis focuses on considering the influence of the riser constraint position, marine environment, submarine pipeline drift, and corrosion on the riser strength and life.

9. A method for evaluating the fatigue damage of a submarine pipeline riser according to claim 8, characterized in that, Step 5 includes conducting strength calculation on the riser, obtaining the strength calculation results of the riser, and determining the location of the dangerous point.

10. A method for evaluating fatigue damage of a submarine pipeline riser according to claim 9, characterized in that, The fatigue damage algorithm of the submarine pipeline riser selects the Brown-Miller critical plane algorithm based on the maximum principal strain. The infinite life design calculation of the riser should be based on FOS. The fatigue life calculation of the submarine pipeline riser should use the Miner linear cumulative damage theory and the rain flow counting method.

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