Evaluation method for predicting fatigue life of deep sea flexible pipeline

Abstract

The invention discloses an evaluation method for predicting the fatigue life of a deep-sea flexible pipeline, and the method comprises the steps: calculating a product of a preset corrosion coefficient, a preset flow velocity sensitive index power of a seawater flow velocity and a correction index factor of a seawater temperature, so as to obtain an instantaneous corrosion damage rate and accumulated corrosion damage, calculating the product of the reference elastic modulus and the temperature sensitive correction term to obtain the elastic modulus, and calculating the ratio of the product of a preset stress coefficient and the square of the seawater flow velocity to the elastic modulus to obtain the bending stress, and calculating the product of the preset fatigue damage coefficient, the seawater flow velocity and the preset material fatigue sensitivity index power of the bending stress to obtain a fatigue damage rate and accumulated fatigue damage, obtaining total accumulated damage through a nonlinear interactive coupling model, and obtaining the total service life through a linear correlation model. Therefore, the coupling effect of the seawater flow velocity and temperature and the interaction effect of corrosion and fatigue damage can be comprehensively considered, and the precision of predicting the fatigue life of the deep-sea flexible pipeline is improved.

Description

Technical Field

[0001] This invention relates to the field of marine engineering, specifically to an assessment method for predicting the fatigue life of deep-sea flexible pipelines. Background Technology

[0002] In the deep-sea environment, flexible pipelines are exposed to high pressure, low temperature and high salinity seawater for a long time, and face complex corrosion and fatigue problems.

[0003] Corrosion is one of the main causes of failure in flexible pipelines. Increased seawater flow velocity exacerbates scouring corrosion of pipelines, while seawater temperature affects the corrosion rate. At the same time, flexible pipelines in the marine environment are subjected to bending stress due to seawater flow velocity, leading to fatigue damage. Changes in seawater temperature affect the elastic modulus of the pipeline material, which in turn affects the magnitude of bending stress and the rate of fatigue damage.

[0004] Therefore, developing a fatigue life prediction method for deep-sea flexible pipelines that can comprehensively consider the coupling effect of seawater flow velocity and seawater temperature is of great significance for improving prediction accuracy and ensuring the safe operation of marine engineering. Summary of the Invention

[0005] The present invention aims to at least partially solve the technical problems in the above-mentioned technologies.

[0006] Therefore, this invention discloses an evaluation method for predicting the fatigue life of deep-sea flexible pipelines, comprising the following steps:

[0007] S1: Calculate the product of the preset corrosion coefficient of the deep-sea flexible pipeline, the preset velocity sensitivity index power of the seawater flow velocity in the deep-sea environment where it is located, and the correction index factor of the seawater temperature in the deep-sea environment where it is located, so as to obtain the instantaneous corrosion damage rate of the deep-sea flexible pipeline, and perform time integration on the instantaneous corrosion damage rate to obtain the cumulative corrosion damage.

[0008] S2: Calculate the product of the reference elastic modulus and the temperature-sensitive correction term of the deep-sea flexible pipeline to obtain the elastic modulus of the deep-sea flexible pipeline;

[0009] S3: Calculate the ratio of the product of the preset stress coefficient of the deep-sea flexible pipeline and the square of the seawater flow velocity to the elastic modulus, so as to obtain the bending stress of the deep-sea flexible pipeline.

[0010] S4: Calculate the product of the preset fatigue damage coefficient of the deep-sea flexible pipeline, the seawater flow velocity, and the preset material fatigue sensitivity index power of the bending stress to obtain the fatigue damage rate of the deep-sea flexible pipeline, and perform time integration on the fatigue damage rate to obtain the cumulative fatigue damage.

[0011] S5: Based on the cumulative corrosion damage and the cumulative fatigue damage, the total cumulative damage of the deep-sea flexible pipeline is obtained through a nonlinear interactive coupling model;

[0012] S6: Based on the total cumulative damage and the service life, the total service life of the deep-sea flexible pipeline is obtained through a linear correlation model.

[0013] The evaluation method for predicting the fatigue life of deep-sea flexible pipelines disclosed in this invention can comprehensively consider the coupling effect of seawater flow velocity and temperature, as well as the interaction between corrosion and fatigue damage, thereby improving the accuracy of fatigue life prediction for deep-sea flexible pipelines.

[0014] In addition, the evaluation method for predicting the fatigue life of deep-sea flexible pipelines disclosed in this invention may also have the following additional technical features:

[0015] In one embodiment of the present invention, the following steps are also included:

[0016] S7: Randomly generate 4000~5000 sets of the seawater flow velocity and the seawater temperature, wherein the seawater flow velocity satisfies a random distribution between [0.1, 5] and the seawater temperature satisfies a random distribution between [0.5, 4].

[0017] S8: Calculate the corresponding total service life based on the seawater flow rate and seawater temperature of each group;

[0018] S9: Calculate the average value of the total service life based on the 95% confidence interval as the final total service life, and subtract the service life already used from the final total service life to obtain the remaining service life of the deep-sea flexible pipeline.

[0019] In one embodiment of the present invention, the instantaneous corrosion damage rate in step S1 is specifically: D1(V,T)=K1·V n ·exp[b·(T-T0)], where D1(V,T) is the instantaneous corrosion damage rate, K1∈[1.85,2.3] is the preset corrosion coefficient, V is the seawater flow velocity, n∈[1.85,2.3] is the preset flow velocity sensitivity index, exp[b·(T-T0)] is the correction index factor, b∈[1.7,2.1] is the corrosion temperature coefficient, T is the seawater temperature, and T0 is the reference seawater temperature.

[0020] In one embodiment of the present invention, in step S2, the elastic modulus is specifically: E(T) = E0·[1-c·(T-T0)], where E(T) is the elastic modulus, E0 is the reference elastic modulus, [1-c·(T-T0)] is the temperature-sensitive correction term, and c∈[0.27,0.31] is the preset elastic modulus temperature coefficient.

[0021] In one embodiment of the present invention, in step S3, the bending stress is specifically: σ(V,T)=(K2·V 2 ) / E(T), where σ(V,T) is the bending stress, and K2∈[2.2,2.31] is the preset stress coefficient.

[0022] In one embodiment of the present invention, in step S4, the fatigue damage rate is specifically: D2(V,T)=K3·V·[(K2·V 2 ) / E(T)] m Wherein, D2(V,T) is the fatigue damage rate, K3∈[0.157,0.315] is the preset fatigue damage coefficient, and m∈[7.7,8.71] is the preset material fatigue sensitivity index.

[0023] In one embodiment of the present invention, in step S5, the nonlinear interactive coupling model is specifically: Λ total (t)=Λ1(t)+Λ2(t)+α·Λ1(t)·Λ2(t), where, Λ total Λ1(t) represents the total cumulative damage, Λ2(t) represents the cumulative corrosion damage, Λ2(t) represents the cumulative fatigue damage, and α∈[1.95,2.05] represents the damage interaction coefficient.

[0024] In one embodiment of the present invention, in step S6, the linear correlation model is specifically: RL(t) = RL0·[1-Λ total (t) / Λ c ], where RL(t) is the total service life, RL0 is the service life already completed, and Λ c For reference, the total cumulative damage.

[0025] Additional features and advantages of this invention will be set forth in the description which follows, or may be learned by practicing the invention. Attached Figure Description

[0026] The technical solution and beneficial effects of the present invention will become apparent and readily understood from the following description in conjunction with the accompanying drawings, wherein:

[0027] Figure 1 This is a schematic flowchart of the evaluation method for predicting the fatigue life of deep-sea flexible pipelines according to the present invention.

[0028] Figure 2 This is another flowchart illustrating the evaluation method for predicting the fatigue life of deep-sea flexible pipelines according to the present invention. Detailed Implementation

[0029] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0030] The evaluation method for predicting the fatigue life of deep-sea flexible pipelines disclosed in this invention will now be described with reference to the accompanying drawings.

[0031] like Figure 1 and Figure 2 As shown, the evaluation method for predicting the fatigue life of deep-sea flexible pipelines includes the following steps:

[0032] S1: Calculate the product of the preset corrosion coefficient of the deep-sea flexible pipeline, the preset velocity sensitivity index power of the seawater velocity in the deep-sea environment, and the correction index factor of the seawater temperature in the deep-sea environment to obtain the instantaneous corrosion damage rate of the deep-sea flexible pipeline, and perform time integration on the instantaneous corrosion damage rate to obtain the cumulative corrosion damage.

[0033] S2: Calculate the product of the reference elastic modulus and the temperature-sensitive correction term for the deep-sea flexible pipeline to obtain the elastic modulus of the deep-sea flexible pipeline.

[0034] S3: Calculate the ratio of the product of the preset stress coefficient of the deep-sea flexible pipeline and the square of the seawater flow velocity to the elastic modulus, so as to obtain the bending stress of the deep-sea flexible pipeline.

[0035] S4: Calculate the product of the preset fatigue damage coefficient, seawater flow velocity, and the preset material fatigue sensitivity index power of the bending stress of the deep-sea flexible pipeline to obtain the fatigue damage rate of the deep-sea flexible pipeline, and perform time integration on the fatigue damage rate to obtain the cumulative fatigue damage.

[0036] S5: Based on cumulative corrosion damage and cumulative fatigue damage, the total cumulative damage of the deep-sea flexible pipeline is obtained through a nonlinear interactive coupling model.

[0037] S6: Based on the total cumulative damage and the service life, the total service life of the deep-sea flexible pipeline is obtained through a linear correlation model;

[0038] S7: Randomly generate 4000~5000 sets of seawater flow velocity and seawater temperature, with the seawater flow velocity satisfying a random distribution between [0.1,5] and the seawater temperature satisfying a random distribution between [0.5,4].

[0039] S8: The total service life is calculated based on the seawater flow velocity and seawater temperature for each group;

[0040] S9: The average of the total service life is statistically calculated based on the 95% confidence interval as the final total service life, and the service life already used is subtracted from the final total service life to obtain the remaining service life of the deep-sea flexible pipeline.

[0041] It should be noted that:

[0042] Step S1 addresses the corrosion damage to deep-sea flexible pipelines by first determining the instantaneous corrosion damage rate by presetting the corrosion coefficient, the velocity sensitivity index power of the seawater flow velocity, and the correction index factor of the seawater temperature. Then, the instantaneous damage is accumulated through time integration to obtain the cumulative corrosion damage, thereby quantifying the impact of corrosion on the pipeline at different times.

[0043] Step S2 calculates the elastic modulus adapted to the current deep-sea temperature based on the material property parameters of the deep-sea flexible pipeline, combined with the reference elastic modulus and temperature-sensitive correction term, to eliminate the interference of temperature on the elastic properties of the material.

[0044] Step S3 addresses the fluid interaction and material properties of the deep-sea flexible pipeline in the deep sea. It calculates the bending stress by dividing the product of the preset stress coefficient and the square of the seawater flow velocity by the elastic modulus obtained in S2, reflecting the effect of the seawater flow velocity on the mechanical damage of the pipeline.

[0045] Step S4 addresses fatigue damage by determining the instantaneous fatigue damage rate using preset fatigue damage coefficients, seawater flow velocity, and the material fatigue sensitivity index power of bending stress. The cumulative fatigue damage is then obtained through time integration, quantifying the cumulative effect of fatigue.

[0046] After completing the above basic damage calculation, step S5 combines the cumulative corrosion damage and cumulative fatigue damage through a nonlinear interactive coupling model, considering the mutual influence between the two, to obtain a more realistic total cumulative damage. Step S6, based on the total cumulative damage and service life of the deep-sea flexible pipeline, preliminarily calculates the total service life through a linear correlation model and establishes the correlation between damage and service life.

[0047] It should also be noted that:

[0048] To improve the reliability of the prediction results, random data verification is introduced in steps S7 to S9, specifically as follows:

[0049] 4000-5000 sets of seawater flow velocity ([0.1,5]) and seawater temperature ([0.5,4]) data that conform to the actual range of deep-sea environment are randomly generated. The total service life of each set of data is calculated according to the process from S1 to S6. Then, the average value of all total service lives is taken based on the 95% confidence interval as the final total service life. Finally, the service life is subtracted to obtain the remaining service life, ensuring that the prediction results can cover most deep-sea environmental conditions.

[0050] In step S1, the instantaneous corrosion damage rate is specifically: D1(V,T)=K1·V n·exp[b·(T-T0)], where D1(V,T) is the instantaneous corrosion damage rate, K1∈[1.85,2.3] is the preset corrosion coefficient, V is the seawater flow velocity, n∈[1.85,2.3] is the preset flow velocity sensitivity index, exp[b·(T-T0)] is the correction index factor, b∈[1.7,2.1] is the corrosion temperature coefficient, T is the seawater temperature, and T0 is the reference seawater temperature.

[0051] It should be noted that:

[0052] K1 is the preset corrosion coefficient, with a value of [1.85, 2.3], which can be adapted to most pipe materials;

[0053] V represents the actual measured deep-sea water flow velocity, which directly reflects the scouring effect of the fluid on the pipeline.

[0054] n is a preset flow rate sensitivity index, with values ​​ranging from [1.85, 2.3], used to quantify the influence of flow rate on corrosion damage. The higher the flow rate, the more significant the contribution of the power of this index to the instantaneous corrosion damage rate.

[0055] exp[b・(T-T0)] is used as the temperature correction exponential factor. It is in exponential form because the effect of temperature on the corrosion rate is nonlinear.

[0056] b is the corrosion temperature coefficient, with values ​​ranging from [1.7, 2.1], based on pipeline corrosion data at different temperatures;

[0057] T represents the actual seawater temperature, and T0 represents the reference seawater temperature (usually a common reference temperature in deep-sea environments). The deviation between the actual temperature and the reference temperature is reflected by (T-T0), thereby correcting the effect of temperature on the corrosion damage rate.

[0058] After calculating the instantaneous corrosion damage rate, it is integrated at fixed time intervals, and the instantaneous corrosion damage rate of each time interval is accumulated to finally obtain the cumulative corrosion damage.

[0059] In step S2, the elastic modulus is specifically: E(T) = E0·[1-c·(T-T0)], where E(T) is the elastic modulus, E0 is the reference elastic modulus, [1-c·(T-T0)] is the temperature-sensitive correction term, and c∈[0.27,0.31] is the preset elastic modulus temperature coefficient.

[0060] It should be noted that:

[0061] E0 is the reference elastic modulus of the pipe material at the reference temperature T0, which is an inherent property of the material.

[0062] [1-c・(T-T0)] is a temperature-sensitive correction term used to adjust the effect of the deviation between the actual temperature T and the reference temperature T0 on the elastic modulus. When T is higher than T0, the value of the correction term decreases, and the elastic modulus decreases accordingly.

[0063] c is the preset temperature coefficient of elastic modulus, with a value of [0.27, 0.31]. This range is determined based on the temperature and elastic modulus data of commonly used materials for deep-sea flexible pipelines, ensuring that the correction term can accurately reflect the degree of influence of temperature on elastic modulus.

[0064] The E(T) calculated by this formula can accurately reflect the actual elastic modulus of the pipeline at the current deep-sea temperature, providing material property parameters for S3 to calculate bending stress.

[0065] In step S3, the bending stress is specifically: σ(V,T)=(K2·V 2 ) / E(T), where σ(V,T) is the bending stress, and K2∈[2.2,2.31] is the preset stress coefficient.

[0066] It should be noted that:

[0067] K2 is a preset stress coefficient with a value of [2.2, 2.31]. It is determined based on the structural dimensions and fluid dynamics of the deep-sea flexible pipeline and is used to convert the seawater flow velocity into the corresponding stress coefficient.

[0068] V represents the seawater flow velocity. The V² term reflects the influence of flow velocity on the fluid force. When the flow velocity doubles, the force exerted by the fluid on the pipe increases by a square factor, which in turn leads to a significant change in bending stress.

[0069] E(T) is the elastic modulus at the current temperature calculated in S2. It is introduced as a denominator because the elastic modulus of the material directly affects the degree of deformation of the pipeline under stress. The smaller the elastic modulus, the greater the bending deformation of the pipeline under the same force, and the greater the bending stress. Through this formula, the seawater flow velocity, temperature and bending stress can be correlated to obtain the bending stress value that reflects the mechanical damage of the pipeline in the actual deep sea environment.

[0070] In step S4, the fatigue damage rate is specifically: D2(V,T)=K3·V·[(K2·V 2 ) / E(T)] m Where D2(V,T) is the fatigue damage rate, K3∈[0.157,0.315] is the preset fatigue damage coefficient, and m∈[7.7,8.71] is the preset material fatigue sensitivity index.

[0071] It should be noted that:

[0072] K3 is a preset fatigue damage coefficient with a value of [0.157, 0.315], used to quantify the basic sensitivity of a material to fatigue damage;

[0073] V represents the seawater flow velocity, which directly reflects the frequency of the dynamic interaction between the fluid and the pipeline. The higher the flow velocity, the higher the frequency of fluid impact on the pipeline, and the faster the fatigue damage rate.

[0074] m is a preset material fatigue sensitivity index, with a value of [7.7, 8.71], determined based on the SN curve data of the pipe material. The larger the index, the more significant the influence of bending stress on the fatigue damage rate.

[0075] After calculating the instantaneous fatigue damage rate, it is integrated at time intervals consistent with S1. The instantaneous fatigue damage rates of each time interval are accumulated to obtain the cumulative fatigue damage, thus quantifying the cumulative effect of fatigue over time.

[0076] In step S5, the nonlinear interactive coupling model is specifically: Λ total (t)=Λ1(t)+Λ2(t)+α·Λ1(t)·Λ2(t), where, Λ total Λ(t) represents the total cumulative damage, Λ1(t) represents the cumulative corrosion damage, Λ2(t) represents the cumulative fatigue damage, and α∈[1.95,2.05] represents the damage interaction coefficient.

[0077] It should be noted that:

[0078] The cumulative corrosion damage Λ1(t) and cumulative fatigue damage Λ2(t) are first directly added together to initially reflect the superposition effect of the two types of damage;

[0079] α·Λ1(t)·Λ2(t) is the damage interaction term, and α∈[1.95,2.05] is the damage interaction coefficient, which is used to quantify the synergistic effect of corrosion and fatigue. Corrosion will destroy the protective layer on the surface of the pipeline and accelerate the initiation and propagation of fatigue cracks. In turn, fatigue cracks will increase the corrosion area and aggravate corrosion damage. Through the interaction term, this synergistic effect can be included in the total cumulative damage calculation, making the result more consistent with the actual damage state of the pipeline.

[0080] In step S6, the linear correlation model is specifically: RL(t) = RL0·[1-Λ total (t) / Λ c ], where RL(t) is the total service life, RL0 is the service life already completed, and Λ c For reference, the total cumulative damage.

[0081] It should be noted that:

[0082] RL0 represents the service life, obtained through actual operation records;

[0083] Λtotal (t) represents the total cumulative damage obtained from S5, Λ c The total cumulative damage, which is the total cumulative damage that the pipeline material suffers when it reaches the failure state, is a key threshold for determining the end of the pipeline's life.

[0084] When Λ total As (t) increases, 1-Λ total (t) / Λ c As the term decreases, the total service life RL(t) decreases accordingly. total (t) reaches Λ c At that time, 1-Λ total (t) / Λ c A value of 0 indicates that the total service life equals the service life already served, signifying that the pipeline has reached a failure state.

[0085] In summary, the evaluation method for predicting the fatigue life of deep-sea flexible pipelines disclosed in this invention can comprehensively consider the coupling effect of seawater flow velocity and temperature, as well as the interaction between corrosion and fatigue damage, thereby improving the accuracy of fatigue life prediction for deep-sea flexible pipelines.

[0086] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. An evaluation method for fatigue life prediction of deep-sea flexible pipes, characterized by, The method comprises the following steps: S1: calculating the product of a preset corrosion coefficient of the deep-sea flexible pipe, a preset flow rate sensitive index power of a seawater flow rate of the deep-sea environment where the deep-sea flexible pipe is located, and a correction index factor of a seawater temperature of the deep-sea environment where the deep-sea flexible pipe is located, to obtain an instantaneous corrosion damage rate of the deep-sea flexible pipe, and time-integrating the instantaneous corrosion damage rate to obtain a cumulative corrosion damage; S2: calculating the product of a reference elastic modulus of the deep-sea flexible pipe and a temperature sensitive correction term, to obtain an elastic modulus of the deep-sea flexible pipe; S3: calculating the ratio of the product of a preset stress coefficient of the deep-sea flexible pipe and the square of the seawater flow rate to the elastic modulus, to obtain a bending stress of the deep-sea flexible pipe; S4: calculating the product of a preset fatigue damage coefficient of the deep-sea flexible pipe, the seawater flow rate, and the preset material fatigue sensitive index power of the bending stress, to obtain a fatigue damage rate of the deep-sea flexible pipe, and time-integrating the fatigue damage rate to obtain a cumulative fatigue damage; S5: based on the cumulative corrosion damage and the cumulative fatigue damage, obtaining a total cumulative damage of the deep-sea flexible pipe through a nonlinear interactive coupling model; S6: based on the total cumulative damage and the used service life, obtaining a total service life of the deep-sea flexible pipe through a linear correlation model.

2. The evaluation method for fatigue life prediction of deep-sea flexible pipe according to claim 1, wherein, The method further comprises the following steps: S7: randomly generating 4000-5000 groups of the seawater flow rate and the seawater temperature, the seawater flow rate satisfying a random distribution between 0.1 and 5, and the seawater temperature satisfying a random distribution between 0.5 and 4; S8: based on each group of the seawater flow rate and the seawater temperature, calculating a corresponding total service life; S9: based on a 95% confidence interval, statistically taking the average value of the total service life as a final total service life, and subtracting the used service life from the final total service life to obtain a remaining service life of the deep-sea flexible pipe.

3. The evaluation method for fatigue life prediction of deep-sea flexible pipe according to claim 1, wherein, In step S1, the instantaneous corrosion damage rate is specifically: D1(V, T) = K1·V n ·exp[b·(T-T0)], wherein D1(V, T) is the instantaneous corrosion damage rate, K1∈[1.85, 2.3] is the preset corrosion coefficient, V is the seawater flow rate, n∈[1.85, 2.3] is the preset flow rate sensitivity index, exp[b·(T-T0)] is the correction index factor, b∈[1.7, 2.1] is the corrosion temperature coefficient, T is the seawater temperature, and T0 is the reference seawater temperature.

4. The evaluation method for fatigue life prediction of deep-sea flexible pipe according to claim 3, wherein, In step S2, the elastic modulus is specifically: E(T)=E0·[1-c·(T-T0)], wherein E(T) is the elastic modulus, E0 is the reference elastic modulus, [1-c·(T-T0)] is the temperature sensitive correction term, and c∈[0.27, 0.31] is a preset elastic modulus temperature coefficient.

5. The evaluation method for fatigue life prediction of deep-sea flexible pipe as claimed in claim 4, characterized by, In step S3, the bending stress, in particular: σ(V, T) = (K2·V 2 ) / E(T), wherein σ(V, T) is the bending stress, K2∈[2.2, 2.31] is the preset stress coefficient.

6. The evaluation method for fatigue life prediction of deep-sea flexible pipe as claimed in claim 5, characterized by, In step S4, the fatigue damage rate, specifically: D2(V, T) = K3·V·[(K2·V 2 ) / E(T)] m , wherein D2(V, T) is the fatigue damage rate, K3∈[0.157, 0.315] is the preset fatigue damage coefficient, and m∈[7.7, 8.71] is the preset material fatigue sensitivity index.

7. The evaluation method for fatigue life prediction of deep-sea flexible pipes according to claim 6, wherein, In step S5, the nonlinear interaction coupling model is specifically: total (t) = Λ1(t) + Λ2(t) + α·Λ1(t)·Λ2(t), wherein, Λ total (t) is the total cumulative damage, Λ1(t) is the cumulative corrosion damage, Λ2(t) is the cumulative fatigue damage, and α ∈ [1.95, 2.05] is a damage interaction coefficient.

8. The evaluation method for fatigue life prediction of deep-sea flexible pipe as claimed in claim 7, characterized by, In step S6, the linear correlation model is determined, in particular: RL(t) = RL0·[1-Λ total (t) / Λ c ], wherein RL(t) is the total service life, RL0is the used service life, Λ c is the reference total cumulative damage.

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