A safety assessment method for deep-sea gently corrugated flexible riser

Abstract

The present invention relates to the technical field of marine engineering structure safety diagnosis, and in particular to a method for assessing the safety of a deep-sea, gently corrugated, flexible riser. The method comprises the following steps: establishing a riser model; obtaining an acceleration envelope diagram of the model; if the maximum monitored acceleration values ​​all fall within the corresponding acceleration envelope diagram, the riser is deemed safe; otherwise, the model is modified; and further monitoring and comparison are performed. If the actual linear acceleration data of the riser are all less than or equal to a threshold value, the riser is deemed to be in a safe operating condition; if the data are greater than the threshold value, the riser is deemed to be in a dangerous operating condition, triggering an early warning program and taking emergency response measures. The method model provided by the present invention is more accurate and comprehensive, and can ensure the long-term safe and efficient operation of the riser.

Description

Technical Field

[0001] The present invention relates to the technical field of safety diagnosis of marine engineering structures, and in particular to a safety assessment method for a deep-sea gently corrugated flexible riser. Background Art

[0002] As the pace of offshore oil and gas development moves into the deep sea, the technical challenges it brings are also endless. Flexible risers connect offshore platforms with seabed oil and gas resources to complete tasks such as extraction. Deep-sea working conditions are harsh. Under the action of various forms of loads such as wind, waves, and currents, the flexible risers with a gentle corrugated configuration will vibrate for a long time. If they are in dangerous vibration conditions for a long time, the life of the risers may be greatly reduced or even damaged. As the "throat" part of the offshore oil and gas development system, once the flexible riser is damaged, not only will the mining work be completely paralyzed, resulting in huge economic losses, but it will also cause irreparable impacts on the environment. Therefore, it is very important to pay attention to whether it is in a safe working state, conduct reliability assessments, and conduct timely inspections or emergency responses. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a safety assessment method for deep-sea gentle-waveform flexible risers. After using monitoring data to judge or correct the model, the corrected model is used to analyze the monitoring data and perform safety and reliability assessments, making the model more accurate and complete. The model is then reacted to the monitoring data to provide early warning, assessment and guidance for diagnostic decisions on the working status of the riser, providing a basis for triggering assessment, detection or emergency response, and ensuring the long-term safe and efficient operation of the riser.

[0004] The present invention is achieved through the following technical solutions:

[0005] A method for safety assessment of a deep-sea gently corrugated flexible riser comprises the following steps:

[0006] S1: Input the relevant parameters of deep-sea gentle corrugated flexible riser and establish a gentle corrugated flexible riser model in orcaflex software;

[0007] S2: Input the environmental parameters of the two working conditions into the orcaflex software respectively, so that the corresponding environmental parameters are applied to the gentle corrugated flexible riser model. The two working conditions here can be the one-year condition and the one-hundred-year condition respectively.

[0008] S3: Obtain the linear acceleration of multiple monitoring nodes of the slow-wave flexible riser model under two working environment parameters, and draw an acceleration envelope diagram of the slow-wave flexible riser model during the monitoring period of each monitoring node;

[0009] S4: Obtain the maximum value of the linear acceleration of each monitoring node during the monitoring period, and compare the maximum value with the acceleration envelope diagram of the corresponding monitoring node of the gentle-waveform flexible riser model under the once-a-year operating condition in step S3. If all the maximum values ​​fall within the corresponding acceleration envelope diagram, the actual deep-sea gentle-waveform flexible riser is considered safe, and there is no need to modify the gentle-waveform flexible riser model. The process directly jumps to step S6. If the maximum value of any monitoring node falls outside the corresponding acceleration envelope diagram, the process jumps to step S5 and modifies the gentle-waveform flexible riser model.

[0010] S5: Check whether the environmental parameters input in step S2 are the environmental parameters of the actual sea area where the deep-sea gently corrugated flexible riser is located, and determine whether the input environmental parameters are reasonable. If it is determined that the input environmental parameters are incorrect or unreasonable, re-enter the corresponding environmental parameters and repeat steps S2-S5 until the maximum values ​​of the linear accelerations of all monitoring nodes fall within the corresponding acceleration envelope graph. This completes the correction of the gently corrugated flexible riser model and jumps to step S6.

[0011] S6: Export the linear acceleration results of all monitoring nodes of the slow-wave flexible riser model during the monitoring period under the once-a-year operating condition, take the maximum value of the linear acceleration as the threshold of the linear acceleration, monitor the linear acceleration data of each monitoring node of the actual deep-sea slow-wave flexible riser in real time and compare it with the threshold. If the linear acceleration data of the actual deep-sea slow-wave flexible riser are all less than or equal to the threshold, it means that the actual deep-sea slow-wave flexible riser is in a safe working condition. If the linear acceleration data of the actual deep-sea slow-wave flexible riser is greater than the threshold, it means that the actual deep-sea slow-wave flexible riser is in a dangerous working condition, triggering the early warning program and taking emergency response measures.

[0012] Furthermore, when the actual maximum linear acceleration values ​​of the deep-sea gently corrugated flexible riser fall within the corresponding acceleration envelope diagram in step S4 or the gently corrugated flexible riser model is corrected in step S5, the reliability calculation of the deep-sea gently corrugated flexible riser is performed, which specifically includes the following steps:

[0013] D1: Analyze the different fatigue damage rates caused by parameter changes, establish the relevant response surface equations of various fatigue factors after fatigue analysis based on the slow-wave flexible riser model, and then calculate the wave-induced fatigue damage D of the slow-wave flexible riser based on the relevant response surface equations of various fatigue factors. w and vortex-induced fatigue damage D v ;

[0014] D2: Using the reliability limit state equation (1) of the slow-wave flexible riser, the Monte Carlo method is used to simulate the failure probability of the riser and obtain the Z under this set of random parameters. w 、Z vValue, if Z w 、Z v If one of the values ​​is less than 0 or both values ​​are less than 0, it means that the structure fails under this set of random parameters. Then, the failure probability under the correlation of the two failure modes is calculated according to formula (2);

[0015]

[0016] P f =P{Z w <0Z v <0} (2)

[0017] Where: Δ is the value of fatigue failure in Miner criterion, X mod is the model uncertainty coefficient, T s is the design life, D is the annual fatigue damage, Z w is the wave-induced fatigue reliability limit state equation, Z v is the wave-induced fatigue reliability limit state equation, D w is the wave-induced fatigue damage, D v is vortex-induced fatigue damage, A is the random parameter in the SN curve lgN+mlgS=lgA, S is the stress range, N is the number of cycles, X i Yes and D w 、D v A series of random variables, P f is the failure probability of the structure under the correlation of two failure modes;

[0018] D3: Repeat step D2 multiple times within the set time step to calculate the failure probability under the dual failure mode that varies with the time step, and draw the failure probability curve of the gently corrugated flexible riser under the two failure modes of wave-induced fatigue and vortex-induced fatigue;

[0019] D4: According to the failure probability curve of D3, when the failure probability in a certain year is greater than the set failure probability, the monitoring of the slow-wave flexible riser will be strengthened or the slow-wave flexible riser will be replaced when the certain year comes.

[0020] Optimally, the number of monitoring nodes selected in step S3 is 2-5.

[0021] Optimally, the monitoring period in steps S3 and S4 is 15 minutes, and the interval between adjacent monitoring periods is 1 hour.

[0022] Advantageous Effects of the Invention

[0023] The present invention provides a safety assessment method for a deep-sea gently corrugated flexible riser, which has the following advantages:

[0024] (1) The present invention proposes a method for establishing a more reasonable riser model that conforms to engineering practice. By comparing the output results with the monitoring data, the rationality of the model and the input parameters is verified, solving the problem that the environmental parameters cannot meet all the requirements of the model for environmental parameter input. (2) The present invention proposes a new system for using monitoring data. On the basis of comparing and verifying whether a reasonable riser model is established, the threshold value method is used to react to the monitoring data, making the riser working condition judgment more reasonable. The model is applied to the reliability assessment, making the riser safety assessment system more comprehensive and three-dimensional.

[0025] (3) The present invention uses reliability assessment to guide real-time monitoring. Through a logic OR gate, the failure probability is calculated by considering the correlation between the two most important failure modes, wave-induced and vortex-induced, to provide targeted guidance for monitoring and improve monitoring efficiency.

[0026] (4) The present invention organically combines the current monitoring data with the model, which has extremely high versatility in engineering practice, provides guidance for decision-making on reliability assessment and diagnosis of risers, and has great engineering application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is the overall flow chart of the present invention;

[0028] Figure 2 This is a model diagram of a gently corrugated flexible riser;

[0029] Figure 3-Figure 5 These are the riser nodes and acceleration curves in the x, y, and z directions calculated under the possible case 1 of the incoming flow reduction coefficient;

[0030] Figure 6-Figure 8 These are the riser nodes and acceleration curves in the x, y, and z directions calculated under the possible case 2 of the incoming flow reduction coefficient;

[0031] Figure 9 is the wave-induced fatigue damage rate curve of the gently corrugated flexible riser distributed along the length of the pipe as the inner diameter changes;

[0032] Figure 10 This is the failure probability curve of the gently corrugated flexible riser under the two failure modes of wave-induced fatigue and vortex-induced fatigue;

[0033] Figure 11 This is a flow chart for reliability calculation of slow-corrugated flexible risers. DETAILED DESCRIPTION

[0034] A safety assessment method for deep-sea gentle corrugated flexible riser, the overall flow chart is as shown in the attached Figure 1 As shown: It includes the following steps:

[0035] S1: Input the relevant parameters of the deep-sea gentle corrugated flexible riser and establish a gentle corrugated flexible riser model in the orcaflex software; in a specific embodiment, the relevant parameters may be as shown in Table 1 and Table 2:

[0036] Table 1 Length data of each section of riser

[0037]

[0038] Table 2 Riser parameters

[0039]

[0040]

[0041] By inputting the actual corrugated flexible riser data into the orcaflex software, a corrugated flexible riser model can be established in the software. The specific riser model is shown in the attached manual. Figure 2 As shown; S2: The environmental parameters of the two working conditions of once a year and once a hundred years are respectively input into the orcaflex software, so that the corresponding environmental parameters are applied to the gentle corrugated flexible riser model; the environmental parameters input in the specific embodiment are as follows:

[0042] Environmental parameters for a one-year return period: wave height of 3m, period of 10s, angle of 45 degrees with the positive x-axis; wind speed of 12m / s, angle of 45 degrees with the positive x-axis; surface velocity of incoming flow of 0.25m / s.

[0043] Environmental parameters for a 100-year event: wave height of 8.7m, period of 11.6s, angle with the positive x-axis of 0 degrees; wind speed of 24.8m / s, angle with the positive x-axis of 0 degrees; surface velocity of incoming flow of 1.850m / s.

[0044] The incoming flow state under these two working conditions is assumed to be step flow. There are many relationships between the reduction coefficient and water depth. Only the examples shown in Tables 3 and 4 are used for illustration.

[0045] Table 3 Relationship between the inflow reduction coefficient and water depth Possible case 1

[0046]

[0047]

[0048] Table 4. Possible case 2 of the relationship between the inflow reduction coefficient and water depth

[0049]

[0050] S3: Obtain the linear acceleration of multiple monitoring nodes of the slow-wave flexible riser model under two working environment parameters, and draw an acceleration envelope diagram of the slow-wave flexible riser model during the monitoring period of each monitoring node;

[0051] The flow velocity in different areas is shown in formula (3):

[0052]

[0053] According to the motion equation (4) for time domain dynamic analysis using orcaflex software, the initial state and boundary conditions of the static analysis are used for time domain dynamic analysis. The monitoring point locations set in actual working conditions can be 40m and 100m from the water surface. The input environmental parameters are the 40m and 100m monitoring nodes of the numerical model with a return period of one year. Finally, the analyzed acceleration results are derived to obtain the acceleration curve of the slow-wave flexible riser model within 400s-500s at the monitoring node, thereby drawing the acceleration envelope curves of the monitoring node in the x, y, and z directions. The envelope curve diagram and the location of the monitoring node in the envelope curve diagram are shown in the attached figure. Figure 3 、 4 , 5, 6, 7, and 8.

[0054] M(p,a)+C(p,v)+K(p)=F(p,v,t) (4)

[0055] Among them, M(p,a) is the system inertia load; C(p,v) is the system damping load; K(p) is the system stiffness load, and F(p,v,t) is the system external load, where p, v, and a represent the position, velocity, and acceleration of the monitoring node, respectively.

[0056] S4: Obtain the maximum linear acceleration of each monitoring node during the monitoring period, and compare the maximum value with the acceleration envelope of the corresponding monitoring node of the slow-wave flexible riser model under the one-year operating condition in step S3. Two situations may occur at this time. The first situation is to use the reduction coefficient table provided in Table 3 as an example for calculation. The numerical results of the calculation are compared with the actual monitoring data as shown in Table 5. It is found that the maximum values ​​of the monitoring data all fall within the corresponding acceleration envelope, which is within the scope of the Appendix to the Instructions. Figure 3 、 4, 5, the actual deep-sea gentle corrugated flexible riser is considered safe, and there is no need to modify the gentle corrugated flexible riser model, and the process directly jumps to step S6. The second case is calculated using the reduction coefficient table provided in Table 4 as an example. The specific riser model results calculated at this time are compared with the actual monitoring data as shown in Table 6. It is found that the maximum acceleration in the X direction at 40m and the maximum acceleration in the X and Y directions at 100m exceed the maximum acceleration of the corresponding monitoring nodes of the gentle corrugated flexible riser model under the one-year operating condition, which falls within the scope of the appendix to the manual. Figure 6 、 7 If the correction area is 8, the process jumps to step S5 to correct the gentle corrugated flexible riser model.

[0057] Table 5 Comparison between riser numerical model calculation results and monitoring data

[0058]

[0059]

[0060] Table 6 Comparison between riser numerical model calculation results and monitoring data

[0061]

[0062] By comparing the model output results with the actual monitoring data, the rationality of the model and input parameters can be verified, solving the problem that the environmental parameters cannot meet all the requirements of the model for environmental parameter input;

[0063] S5: Check whether the environmental parameters input in step S2 are the environmental parameters of the actual sea area where the deep-sea gently corrugated flexible riser is located, and determine whether the input environmental parameters are reasonable. If it is determined that the input environmental parameters are incorrect or unreasonable, re-enter the corresponding environmental parameters and repeat steps S2-S5 until the maximum values ​​of the linear accelerations of all monitoring nodes fall within the corresponding acceleration envelope graph. This completes the correction of the gently corrugated flexible riser model and jumps to step S6.

[0064] When making specific corrections, you can first check the weather conditions on the monitoring day to see whether the monitoring data is greater than the model data due to extreme conditions such as bad weather. If not, correct the environmental parameters. You can make further corrections by adjusting the flow reduction coefficient and other methods.

[0065] S6: Export the linear acceleration results of all monitoring nodes of the slow-wave flexible riser model during the monitoring period under the once-a-year operating condition, and take the maximum value of the linear acceleration as the threshold of the linear acceleration, as shown in Table 7. Monitor the linear acceleration data of each monitoring node of the actual deep-sea slow-wave flexible riser in real time and compare them with the threshold. If the linear acceleration data of the actual deep-sea slow-wave flexible riser are all less than or equal to the threshold, it means that the actual deep-sea slow-wave flexible riser is in a safe working condition. If the linear acceleration data of the actual deep-sea slow-wave flexible riser is greater than the threshold, it means that the actual deep-sea slow-wave flexible riser is in a dangerous working condition, triggering the early warning program and taking emergency response measures.

[0066] Table 7 Riser acceleration threshold settings

[0067]

[0068] By comparing and verifying whether a reasonable riser model is established, the threshold setting method is used to react to the monitoring data, making the judgment of the riser working condition more reasonable. By organically combining the data and the model, it has extremely high versatility in engineering practice, has guiding significance for the decision-making of the reliability assessment and diagnosis of the riser, and has great engineering application value.

[0069] Furthermore, when the maximum linear acceleration values ​​of the actual deep-sea gentle wave-shaped flexible riser fall within the corresponding acceleration envelope diagram in step S4 or the correction of the gentle wave-shaped flexible riser model is completed in step S5, the reliability calculation of the deep-sea gentle wave-shaped flexible riser is performed. The specific flow chart is shown in the attached figure. Figure 11 As shown, it includes the following steps:

[0070] D1: Analyze the different fatigue damage rates caused by parameter changes, establish the relevant response surface equations of various fatigue factors after fatigue analysis based on the slow-wave flexible riser model, and then calculate the wave-induced fatigue damage D of the slow-wave flexible riser based on the relevant response surface equations of various fatigue factors. w and vortex-induced fatigue damage D v ;

[0071] The parameters here include: inner diameter, outer diameter, axial stiffness, bending stiffness, weight per meter, wind speed, wave height, current velocity, SCF, added mass, drag coefficient, seabed stiffness, etc. The parameter distribution of the specific embodiment is shown in Table 8.

[0072] Table 8 Random variables and statistical parameters of reliability

[0073]

[0074] The response surface methodology is illustrated using the inner diameter as an example when considering wave-induced fatigue. The riser inner diameter varies from 0.188 to 0.282, with a mean of 0.235 and a coefficient of variation of 0.1. The riser inner diameter values ​​are shown in the first row of Table 8. Based on the new model formed for different riser inner diameters, OrcaFlex was used to analyze the riser fatigue life. The wave-induced fatigue damage rate curve along the riser length was obtained as the inner diameter varied. Figure 9 As shown, from Figure 9 The specific damage rate read out is shown in Table 9. Then the five points are fitted on the inner diameter-damage rate plane to obtain a curve, which is the response surface D w (A, d), the fitting curve is expressed as y = 3.436 × 10 -7 x-6.981×10 -8 .

[0075] Table 9 Fatigue damage rate with changes in inner diameter

[0076]

[0077] Based on the above steps, we can obtain the response surface equation D of other parameters in the same way. w 、D v , as shown in Table 10;

[0078] Table 10 Probability distribution of various fatigue factors and wave-induced fatigue response surface equation

[0079]

[0080]

[0081] D2: Using the reliability limit state equation (1) of the slow-wave flexible riser, the Monte Carlo method is used to simulate the failure probability of the riser and obtain the Z under this set of random parameters. w 、Z v Value, if Z w 、Z v If one of the values ​​is less than 0 or both values ​​are less than 0, it means that the structure fails under this set of random parameters. Then, the failure probability under the correlation of the two failure modes is calculated according to formula (2);

[0082]

[0083] P f =P{Z w <0|Z v <0} (2)

[0084] Where: Δ is the value of fatigue failure in Miner criterion, X modis the model uncertainty coefficient, T s is the design life, D is the annual fatigue damage, Z w is the wave-induced fatigue reliability limit state equation, Z v is the wave-induced fatigue reliability limit state equation, D w is the wave-induced fatigue damage, D v is vortex-induced fatigue damage, A is the random parameter in the SN curve lgN+mlgS=lgA, S is the stress range, N is the number of cycles, X i Yes and D w 、D v A series of random variables, P f is the failure probability of the structure under the two related failure modes.

[0085] The specific Monte Carlo method for simulating the probability of riser failure is as follows: Using the Monte Carlo method to simulate, according to the law of large numbers, as long as the number of simulations is large enough, the frequency of failure events in the number of simulations will be close to the probability of system failure. Specifically, the analysis period can be selected as 100 years and the number of simulations can be selected as 10,000. The following operations can be performed: Under the current period, according to X i The distribution form (i=1,2,...,M) generates a set of X i After the random array is formed, it is substituted into the riser reliability limit state function Z w 、Z v , calculate Z under this set of random parameters w 、Z v If one of the two values ​​is less than 0, it means that the structure fails under this set of random parameters. Then continue to complete the next set of random numbers X at the current time step. i The simulation is continued until the number of random groups X for the current year is completed 10,000 times. i Simulate the number of all failure events in the current time step, calculate the frequency of each failure event, and regard it as the Z under the current time step w 、Z v Failure probability. In the current time step, the two failure probabilities are related by a logic “OR” gate, that is, if wave-induced failure or fatigue failure occurs, the structure will be considered to have failed. According to (2), the failure probability under the two failure modes can be calculated.

[0086] D3: Repeat step D2 multiple times within the set time step to calculate the failure probability under the dual failure mode that varies with the time step, and draw the failure probability curve of the gentle corrugated flexible riser under the two failure modes of wave-induced fatigue and vortex-induced fatigue, as shown in the attached figure. Figure 10D4: According to the failure probability curve of step D3, when the failure probability in a certain year is greater than the set failure probability, the monitoring of the slowly corrugated flexible riser will be strengthened or the slowly corrugated flexible riser will be replaced starting in a certain year.

[0087] According to the failure probability curve, it can be determined how many years it will take for the failure probability of the slow-wave flexible riser to be greater than the set failure probability value. It is necessary to conduct more frequent measurements and assessments within a period of time before reaching this age limit, or to replace the slow-wave flexible riser in a timely manner to ensure the safety and reliability of the slow-wave flexible riser. This method has strong engineering guidance significance.

[0088] Optimally, the number of monitoring nodes selected in step S3 is 2-5.

[0089] Optimally, the monitoring period in steps S3 and S4 is 15 minutes, and the interval between adjacent monitoring periods is 1 hour.

[0090] In summary, the present invention provides a safety assessment method for a deep-sea gentle-waveform flexible riser. After using monitoring data to identify or correct a model, the corrected model is used to analyze the monitoring data and perform safety and reliability assessments, making the model more accurate and complete. The model is then reacted to the monitoring data to provide early warnings, assessments, and guidance for diagnostic decisions regarding the riser's operating status, providing a basis for triggering assessments, detection, or emergency responses, and ensuring the long-term safe and efficient operation of the riser.

[0091] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for safety assessment of a deep-sea gently corrugated flexible riser, characterized in that: The steps include: S1: Input the relevant parameters of deep-sea gentle corrugated flexible riser and establish a gentle corrugated flexible riser model in orcaflex software; S2: Input the environmental parameters of the two working conditions into the orcaflex software respectively, so that the corresponding environmental parameters are applied to the gentle corrugated flexible riser model; S3: Obtain the linear acceleration of multiple monitoring nodes of the slow-wave flexible riser model under two working environment parameters, and draw an acceleration envelope diagram of the slow-wave flexible riser model during the monitoring period of each monitoring node; S4: Obtain the linear acceleration of the monitoring nodes on the actual deep-sea gentle wave-shaped flexible riser corresponding to the gentle wave-shaped flexible riser model, obtain the maximum linear acceleration of each monitoring node during the monitoring period, and compare the maximum value with the acceleration envelope diagram of the corresponding monitoring node of the gentle wave-shaped flexible riser model under the once-a-year operating condition in step S3. If both maximum values ​​fall within the corresponding acceleration envelope diagram, the actual deep-sea gentle wave-shaped flexible riser is considered safe, and the process directly jumps to step S6. If the maximum value of any monitoring node falls outside the corresponding acceleration envelope diagram, the process jumps to step S5 to modify the gentle wave-shaped flexible riser model. S5: Check whether the environmental parameters input in step S2 are the environmental parameters of the actual sea area where the deep-sea gently corrugated flexible riser is located, and determine whether the input environmental parameters are reasonable. If it is determined that the input environmental parameters are incorrect or unreasonable, re-enter the corresponding environmental parameters and repeat steps S2-S5 until the maximum values ​​of the linear accelerations of all monitoring nodes fall within the corresponding acceleration envelope graph. This completes the correction of the gently corrugated flexible riser model and jumps to step S6. S6: Export the linear acceleration results of all monitoring nodes of the slow-wave flexible riser model during the monitoring period under the once-a-year operating condition, take the maximum value of the linear acceleration as the threshold of the linear acceleration, monitor the linear acceleration data of each monitoring node of the actual deep-sea slow-wave flexible riser in real time and compare them with the threshold. If the linear acceleration data of the actual deep-sea slow-wave flexible riser are all less than or equal to the threshold, it means that the actual deep-sea slow-wave flexible riser is in a safe working condition. If the linear acceleration data of the actual deep-sea slow-wave flexible riser is greater than the threshold, it means that the actual deep-sea slow-wave flexible riser is in a dangerous working condition, triggering the early warning program and taking emergency response measures.

2. A deep-sea gently corrugated flexible riser safety assessment method according to claim 1, characterized in that: When the actual maximum linear acceleration values ​​of the deep-sea gently corrugated flexible riser fall within the corresponding acceleration envelope diagram in step S4 or the gently corrugated flexible riser model is corrected in step S5, the reliability calculation of the deep-sea gently corrugated flexible riser is performed, which specifically includes the following steps: D1: Analyze the different fatigue damage rates caused by parameter changes, establish the relevant response surface equations of various fatigue factors after fatigue analysis based on the slow-wave flexible riser model, and then calculate the wave-induced fatigue damage D of the slow-wave flexible riser based on the relevant response surface equations of various fatigue factors. w and vortex-induced fatigue damage D v ; D2: Using the reliability limit state equation (1) of the slow-wave flexible riser, the Monte Carlo method is used to simulate the failure probability of the riser and obtain the Z under this set of random parameters. w 、Z v Value, if Z w 、Z v If one of the values ​​is less than 0 or both values ​​are less than 0, it means that the structure fails under this set of random parameters. Then, the failure probability under the correlation of the two failure modes is calculated according to formula (2); P f =P{Z w <0|Z v <0} (2) Where: Δ is the value of fatigue failure in Miner criterion, X mod is the model uncertainty coefficient, T s is the design life, D is the annual fatigue damage, Z w is the wave-induced fatigue reliability limit state equation, Z v is the wave-induced fatigue reliability limit state equation, D w is the wave-induced fatigue damage, D v is vortex-induced fatigue damage, A is the random parameter in the SN curve lgN+mlgS=lgA, S is the stress range, N is the number of cycles, X i Yes and D w 、D v A series of random variables, P f is the failure probability of the structure under the correlation of two failure modes; D3: Repeat step D2 multiple times within the set time step to calculate the failure probability under the dual failure mode that varies with the time step, and draw the failure probability curve of the gently corrugated flexible riser under the two failure modes of wave-induced fatigue and vortex-induced fatigue; D4: According to the failure probability curve of D3, when the failure probability in a certain year is greater than the set failure probability, the monitoring of the slow-wave flexible riser should be strengthened or the slow-wave flexible riser should be replaced starting from a certain year.

3. A deep-sea gently corrugated flexible riser safety assessment method according to claim 1, characterized in that: The number of monitoring nodes selected in step S3 is 2-5.

4. A deep-sea gently corrugated flexible riser safety assessment method according to claim 3, characterized in that: The monitoring period in steps S3 and S4 is 15 minutes, and the interval between adjacent monitoring periods is 1 hour.

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