A method for predicting local scour depth and fatigue damage of a goose neck at the bottom of an offshore riser

Abstract

The application relates to a method for predicting local scouring depth and fatigue damage of a marine riser bottom goose neck, and belongs to the technical field of ocean engineering. The method comprises the following steps: performing fluid dynamics analysis on the local goose neck of the marine riser bottom to obtain pipe surface pressure time history, shear stress time history and seabed bottom shear force; combining the suspended load and the sediment transport model to calculate sediment movement, and obtaining real-time changes of the bed elevation and the local dynamic scouring depth; fusing the pressure time history and the shear stress time history of the pipe surface to obtain a fused load vector; performing structure dynamics coupling analysis based on the vector, synchronously updating a time-varying stiffness matrix, solving a structure motion equation through a Newmark-beta method, and obtaining a fatigue hot spot node stress time history; processing the stress range spectrum through a rain flow counting method; combining an ocean environment S-N curve and a Miner linear cumulative damage rule to calculate total fatigue damage and predict fatigue life. The application can improve the accuracy of fatigue life prediction.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ocean engineering, and particularly relates to a method for predicting local scour depth and fatigue damage of a goose neck at the bottom of an ocean riser. BACKGROUND

[0002] The goose neck part at the bottom of the ocean platform riser is a core link for ensuring the overall stability of the platform. In the complex marine environment, the region is continuously subjected to the synergistic effect of multiple factors such as water flow, waves, and silt. Water flow and waves generate continuous hydrodynamic load acting on the surface of the goose neck structure and disturbing the seabed silt, and the movement of silt will cause local seabed scour, change the support conditions of the goose neck part, and further cause the vibration response of the structure to intensify. Long-term accumulation will cause serious structural fatigue damage, directly threatening the safe operation and long-term service of the platform. However, the existing methods for predicting scour depth and fatigue damage have significant defects: most of the methods only analyze a single influencing factor, for example, some methods only focus on the single effect of water flow on seabed scour, and some methods only evaluate the fatigue damage of the structure under fixed seabed conditions, completely ignoring the close coupling effect between fluid dynamics, silt movement, and structural dynamics. The existing methods ignore this dynamic interaction, which makes the prediction results unable to comprehensively and accurately reflect the real stress state and damage evolution process of the goose neck part in the actual marine environment, and further makes the evaluation of the long-term safety and stability of the ocean platform have obvious deviation, which is difficult to meet the high requirements of the platform design precision, risk prediction and operation science in the field of ocean engineering.

[0003] Therefore, it is a key requirement to develop a prediction method that can comprehensively integrate the coupling relationship between water flow, waves, silt and structural dynamics, and accurately calculate the scour depth and fatigue damage of the goose neck part, to ensure the safety of the ocean platform and prolong the service life. SUMMARY

[0004] The application provides a method for predicting local scour depth and fatigue damage of a goose neck at the bottom of an ocean riser.

[0005] To achieve the above purpose, the application realizes the following technical solutions:

[0006] The application provides a method for predicting local scour depth and fatigue damage of a goose neck at the bottom of an ocean riser, comprising the following steps:

[0007] S1, fluid dynamics analysis is performed on the local goose neck at the bottom of the ocean riser to obtain the pressure time history, shear stress time history of the goose neck pipe surface, and seabed shear force of the seabed surface;

[0008] Further, the wave-wind coupling model is introduced to correct the bed bottom shear force, the influence of the wind field on the wave energy is quantified by the wind speed parameter, the correction value of the bed bottom shear force is more in line with the actual complex environment of the sea, and a high-precision load basis is provided for the calculation of the sediment movement; in the application, the influence of the random wind field on the wave energy is considered, the bed bottom shear force of the seabed surface is corrected, and the correction value of the bed bottom shear force of the seabed surface is obtained, and the formula is as follows:

[0009] ,

[0010] Among them, represents the bed bottom shear force of the seabed surface; represents the correction value of the bed bottom shear force of the seabed surface; represents the wind speed measurement value; represents an empirical coefficient.

[0011] S2, based on the bed bottom shear force of the seabed surface, the suspended load and the bed load sediment transport model are combined to dynamically calculate the sediment movement, and the real-time change amount of the bed elevation and the local dynamic scour depth of the goose neck pipe are obtained;

[0012] Further, the suspended load transport rate and the bed load transport rate are calculated based on the correction value of the bed bottom shear force of the seabed surface, the sediment incipient shear stress, the fluid density, the sediment particle density and the sediment transport intensity parameter; the formula is as follows:

[0013] ,

[0014] ,

[0015] Among them, represents the sediment incipient shear stress; , represents two different empirical coefficients; represents the acceleration of gravity; represents the median particle size of the sediment; represents the fluid density; represents the sediment particle density; represents the sediment transport intensity parameter; the sediment transport intensity parameter is calculated by the Hillz parameter and the corrected critical Hillz parameter; by adding the microbial membrane influence model, the inhibition effect of the biological fouling on the sediment incipient is quantified, and the critical Hillz parameter is corrected.

[0016] Further, the bed elevation change amount control equation is constructed by the suspended load transport rate and the bed load transport rate, the real-time change amount of the bed elevation and the local dynamic scour depth of the goose neck pipe are obtained, and the formula is as follows:

[0017] ,

[0018] wherein, represents the seabed elevation; represents the gradient operator.

[0019] S3, the pressure time history of the gooseneck pipe surface, the shear stress time history is fused, the fusion load vector is obtained, the structure dynamics coupling analysis is carried out based on the fusion load vector, the time-varying stiffness matrix is updated combined with the real-time change amount of the seabed elevation, the Newmark-beta method is used to solve the structure motion equation, and the stress time history of the fatigue hot spot node of the gooseneck pipe is obtained; the real-time change amount of the seabed elevation is used as the basis, the time-varying stiffness matrix is dynamically updated through the stiffness matrix mapping function, the dynamic influence of seabed scour on the structure support is reflected in real time, and the limitation of traditional fixed stiffness analysis is broken through; the fusion load vector is formed by fusing the pressure time history and the shear stress time history of the gooseneck pipe surface, the structure dynamics coupling analysis is carried out, the structure motion equation is efficiently solved by the Newmark-beta method, the stress time history of the fatigue hot spot node is accurately captured, core data support is provided for fatigue damage calculation, and the timeliness and accuracy of stress analysis can be improved.

[0020] Further, the formula of the structure motion equation is as follows:

[0021] ,

[0022] ,

[0023] wherein, represents the mass matrix; represents the damping matrix; , , respectively represent the acceleration, velocity and displacement of the gooseneck pipe; represents the fusion load vector; represents the pressure time history of the gooseneck pipe surface; represents the shear stress time history of the gooseneck pipe surface; represents the time-varying stiffness matrix.

[0024] Further, the existing structure dynamics analysis adopts a fixed stiffness matrix, ignores the dynamic change of the structure support condition caused by seabed scour, and cannot reflect the influence of seabed elevation evolution on the structure stiffness; the real-time change amount of the seabed elevation and the initial stiffness matrix are used as the basis, and the time-varying stiffness matrix is updated through the stiffness matrix mapping function.

[0025] S4, the traditional stress time history processing method cannot accurately extract the effective stress range, and the cumulative damage rule is not combined with the load characteristics of marine engineering, resulting in large deviation of total fatigue damage calculation and low reliability of life prediction. The rain flow counting method is used to process the stress time history of the fatigue hot spot node of the goose neck pipe, and the stress range spectrum is obtained; combined with the S-N curve of marine environment and the Miner linear cumulative damage rule, the total fatigue damage and the fatigue life are calculated. The rain flow counting method is used to accurately process the stress time history, efficiently extract the stress range spectrum, exclude invalid stress data interference, and improve the accuracy of stress feature extraction; combined with the S-N curve of marine environment and the Miner linear cumulative damage rule, the quantitative calculation of total fatigue damage is realized, and the reliability and adaptability of fatigue life prediction are greatly improved, which provides a scientific basis for risk prediction, operation plan formulation and service life extension of the offshore platform.

[0026] The advantages of the present application are:

[0027] The present application can provide more accurate scour depth and fatigue damage prediction by coupling fluid dynamics, sediment movement and structural dynamics, and is more comprehensive and accurate than single analysis method; by coupling multiple physical fields, accurate optimization of platform design can be realized, field test cost can be reduced, and design reliability can be improved; long-term stability of the platform under harsh environmental conditions can be effectively predicted, scientific basis for maintenance and upgrading of the offshore platform can be provided, platform service life can be extended, and maintenance cost can be reduced. BRIEF DESCRIPTION OF DRAWINGS

[0028] The accompanying drawings are included to provide a further understanding of the present application, and constitute a part of the specification, which together with the embodiments of the present application, serve to explain the present application, and do not constitute a limitation of the present application.

[0029] Figure 1 The step flow chart of the method of the present application is shown in the figure;

[0030] Figure 2 The real-time scour depth thermal topographic map of the goose neck pipe is shown in the figure;

[0031] Figure 3 The pipe bottom scour cloud picture is shown in the figure;

[0032] Figure 4 The real-time deformation map of the structure caused by the local scour pit of the goose neck is shown in the figure;

[0033] Figure 5 The comparison of the local scour depth prediction method of the goose neck pipe of the offshore platform is shown in the figure. DETAILED DESCRIPTION

[0034] With reference to the accompanying drawings, the technical solutions in the embodiments of the present application will be described clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of the present application.

[0035] Embodiment 1

[0036] In this embodiment, as shown in the accompanying drawings, the present application provides a method for predicting local scour depth and fatigue damage of a goose neck at the bottom of a marine riser, and the specific steps include: Figure 1

[0037] Step 1: Perform fluid dynamics analysis on the local goose neck at the bottom of the marine riser to obtain the pressure time history, shear stress time history of the goose neck pipe surface and seabed surface bed shear force.

[0038] Specifically, a three-dimensional calculation domain of the riser-goose neck pipe-seabed is established, and the Reynolds-averaged Navier-Stokes equation is solved based on the Arbitrary Lagrangian-Eulerian method and the turbulence model to obtain the pressure time history, shear stress time history of the goose neck pipe surface and seabed surface bed shear force.

[0039] The formula of the Reynolds-averaged Navier-Stokes equation is as follows:

[0040] ,

[0041] wherein, denotes fluid density, unit: kg / m3; denotes time-averaged velocity component, denotes time-averaged velocity component different from, unit: m / s; denotes time, unit: s; denotes time-averaged pressure, unit: Pa; denotes dynamic viscosity, unit: Pa·s; denotes Reynolds stress term, unit: Pa; denotes momentum source term, unit: N / m3. Through the Arbitrary Lagrangian-Eulerian ALE method, the free surface and seabed boundary changes are dynamically captured, the model accurately simulates the turbulent effect in the near-wall region and provides high-precision load input for sediment movement.

[0042] ​​The prior art only considers the effect of water flow or wave on seabed, ignores the transmission effect of random wind field on wave energy, so that the calculation of seabed surface bed bottom shear force does not reflect the real marine multi-field coupling environment, and the precision is insufficient; in the process of calculating the seabed surface bed bottom shear force, the wave-wind coupling model is introduced, the influence of random wind field on wave energy is considered, the bed bottom shear force is corrected, the corrected value of seabed surface bed bottom shear force is obtained, and the formula is as follows:

[0043] ,

[0044] The seabed surface bed bottom shear force is represented by Pa. The corrected value of seabed surface bed bottom shear force is represented by Pa. The wind speed measurement value is represented by m / s. The empirical coefficient is calibrated by a wind tunnel experiment.

[0045] Step two, the traditional sand transport model does not combine the corrected bed bottom shear force, and does not cooperate with the characteristics of suspended load and bed load sand transport, so that the calculation of bed surface elevation change and local scour depth exists static deviation, and the dynamic scour process cannot be reflected. Based on the seabed surface bed bottom shear force, the suspended load and bed load sand transport model are combined to dynamically calculate the sediment movement, and the real-time change of bed surface elevation and the local dynamic scour depth of goose neck pipe are obtained.

[0046] The formula of suspended load sand transport rate is as follows:

[0047] ,

[0048] The formula of bed load sand transport rate is as follows:

[0049] ,

[0050] , The suspended load sand transport rate and the bed load sand transport rate are respectively represented by kg / (m·s). The sediment incipient shear stress is represented by Pa. , Two different empirical coefficients are represented. The gravity acceleration is represented. The median diameter of sediment is represented by m. The density of sediment particles is represented by kg / m3. The sand transport intensity parameter is calculated by the Hillz parameter and the critical Hillz parameter, and the formula is as follows:

[0051] ,​​

[0052] wherein, represents the Hjulstrm parameter, used to represent the ratio of the drag force of water flow on the sediment particles to the underwater gravity of the sediment; represents the maximum value operation; represents the modified critical Hjulstrm parameter; the inhibitory effect of biological fouling on the incipient motion of sediment is quantified by adding the microbial membrane influence model, and the formula of the modified critical Hjulstrm parameter is as follows:

[0053] ,

[0054] wherein, represents the critical Hjulstrm parameter; represents the biofilm concentration, obtained by an in-situ sensor, unit: kg / m2; represents the biological influence coefficient;

[0055] Based on the suspended load transport rate and the bed load transport rate, a bed elevation change control equation is constructed, and the formula is as follows:

[0056] ,

[0057] wherein, represents the bed elevation, unit: m; represents the gradient operator, used to perform gradient operation on the sum of the suspended load transport rate and the bed load transport rate. The local dynamic scour depth of the goose neck pipe is the local spatial integral result of the bed elevation change near the goose neck pipe. The real-time scour depth thermal topography of the goose neck pipe obtained by the method of the present application is as shown in Figure 2 , and the pipe bottom scour cloud picture is as shown in Figure 3 .

[0058] Step three, fusing the pressure time history and the shear stress time history on the surface of the goose neck pipe to obtain a fused load vector; based on the fused load vector, performing structural dynamics coupling analysis, updating the time-varying stiffness matrix combined with the real-time change of the bed elevation, and solving the structural motion equation by using the Newmark-β method to obtain the stress time history of the fatigue hot spot node of the goose neck pipe .

[0059] The formula of the structural motion equation is as follows:

[0060] ,

[0061] ,

[0062] wherein, represents the mass matrix; represents the damping matrix; , , respectively represent the acceleration (unit: m / s2), velocity (unit: m / s) and displacement (unit: m) of the gooseneck pipe; represents the fusion load vector, which is a time-dependent vector obtained by adding the pressure load component to the shear load component; the pressure load component is obtained by integrating the pressure time history over the surface area of the gooseneck pipe, and the shear load component is obtained by integrating the shear stress time history over the surface area of the gooseneck pipe; represents the pressure time history of the surface of the gooseneck pipe, with the unit being Pascal; represents the shear stress time history of the surface of the gooseneck pipe, with the unit being Pascal; represents the time-varying stiffness matrix; the updating mechanism of the time-varying stiffness matrix is:

[0063] ,

[0064] wherein, represents the real-time change amount of the bed elevation, with the unit being meters; represents the initial stiffness matrix; represents the stiffness matrix mapping function, which is responsible for mapping to the scaling factor of the stiffness matrix, which is calibrated through experiments or numerical calibration; this step can output the real-time deformation map of the structure caused by the local scour pit of the gooseneck, as shown in Figure 4 .

[0065] Step four, the rainflow counting method is used to process the stress time history of the fatigue hot spot node of the gooseneck pipe, and the stress range spectrum is obtained; combined with the marine environment S-N curve and the Miner linear cumulative damage rule, the total fatigue damage and the fatigue life are calculated;

[0066] The formula of the marine environment S-N curve is as follows:

[0067] ,

[0068] wherein, represents the stress range required for the lower structure to reach failure; represents the material correction coefficient, which is calibrated according to the type of the standpipe steel and the welding process; represents the stress range of the i-th interval in the stress range spectrum, which is the difference between the stress peak and the stress valley; represents the environmental correction coefficient, which reflects the influence of seawater corrosion, marine atmosphere and other environments on the fatigue performance;

[0069] The formula of the Miner linear cumulative damage rule is as follows:

[0070] ,

[0071] wherein, represents the stress range The corresponding actual cycle number is obtained by rain flow counting method;

[0072] represents the total fatigue damage, which is a linear superposition value of damage in each stress interval, the greater the total fatigue damage, the shorter the predicted fatigue life; the smaller the total fatigue damage, the longer the predicted fatigue life; represents that the structure has no fatigue damage; The structure has accumulated partial fatigue damage, but has not reached the damage limit; , indicating that the fatigue damage accumulates to the limit, reaches the fatigue damage state, and the life ends.

[0073] The predicted fatigue life prediction value is:

[0074] ,

[0075] wherein, represents the predicted fatigue life, unit: year; represents the average annual wave frequency; represents the average annual fatigue damage rate; the present application adopts rain flow counting method to accurately process stress time history, efficiently extracts stress range spectrum, excludes invalid stress data interference, and can improve the accuracy of stress feature extraction; combined with the S-N curve corrected by the marine environment and the Miner linear cumulative damage rule, the quantitative calculation of the total fatigue damage is realized, which greatly improves the reliability and adaptability of the fatigue life prediction, and provides a scientific basis for the risk prediction, operation plan formulation and service life extension of the offshore platform.

[0076] Example 2

[0077] In this embodiment, as Figure 5As shown, the results of the method of the present application and the traditional scour prediction method are compared, the abscissa is the service time of the offshore platform (unit: month), the ordinate is the polyline graph of the local scour depth at the bottom of the goose neck pipe (unit: meter), including three core curves: the calculation results of the method of the present application, the results of the traditional scour prediction method considering only the effect of water flow, and the results of the simplified method ignoring the influence of wave-wind coupling and microbial membrane, and the actual seabed monitoring scour depth data are labeled as verification benchmarks. The present application corrects the bed shear force through the wave-wind coupling model, constructs the bed elevation dynamic control equation by combining the suspended load and bed load sediment transport model, and corrects the critical Shields parameter through the microbial membrane influence model to quantify the inhibitory effect of biological fouling on sediment incipient motion, so the curve can reflect the nonlinear change of scour depth at different time stages (such as sudden increase of wind speed during storm period, accumulation of microbial membrane), the present application has higher agreement with actual monitoring data than other methods. The traditional method considering only water flow always underestimates the scour depth due to the neglect of the synergistic effect of wave and wind, and the simplified method overestimates the scour depth at the early stage due to the neglect of the inhibitory effect of microbial membrane on sediment incipient motion, and the time point of the later stable stage deviates significantly from the actual value. It can be seen that the scour depth prediction accuracy of the present application is much higher than that of the existing methods in complex marine environment, and can provide more reliable basis for seabed protection design.

[0078] Finally, it should be noted that: the above only describes the preferred embodiments of the present application and is not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or make equivalent replacements to some technical features. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for predicting the depth of localized scour and fatigue damage at the bottom of a marine riser, characterized in that, The method comprises the following steps: S1, fluid dynamics analysis is performed on the local goose neck of the bottom of the marine riser, to obtain the pressure time history of the surface of the goose neck pipe, the shear stress time history, and the bed shear stress of the seabed surface; S2, based on the bed shear stress of the seabed surface, the dynamic calculation of sediment movement is performed in combination with the suspended load and bed load sediment transport model, to obtain the real-time change amount of the bed elevation and the local dynamic scour depth of the goose neck pipe; the specific process is as follows: In the process of calculating the bed shear stress of the seabed surface, a wave-wind coupling model is introduced to consider the influence of random wind field on wave energy, to correct the bed shear stress, to obtain the corrected value of the bed shear stress of the seabed surface, and the formula is as follows: ， wherein, represents the bed bottom shear force of the seabed surface; represents the bed bottom shear force correction value of the seabed surface; represents the wind speed measurement value; represents the empirical coefficient; The suspended load transport rate is calculated based on the seabed surface bottom shear force correction value, sediment incipient shear stress, fluid density, sediment particle density, and sediment transport intensity parameters and the bed load transport rate ; the formula is as follows: ， ， wherein, denotes the incipient shear stress of the sediment; , denote two different empirical coefficients; denotes the gravitational acceleration; denotes the median grain size of the sediment; denotes the fluid density; denotes the sediment grain density; denotes the sediment transport intensity parameter; S3, the pressure time history and the shear stress time history of the surface of the goose neck pipe are fused to obtain a fused load vector; based on the fused load vector, structural dynamics coupling analysis is performed, the real-time change amount of the bed elevation is updated to update a time-varying stiffness matrix, a Newmark-β method is used to solve a structural motion equation, and a stress time history of a fatigue hot spot node of the goose neck pipe is obtained; S4, the rainflow counting method is used to process the stress time history of the fatigue hot spot node of the goose neck pipe, to obtain a stress range spectrum; in combination with a marine environment S-N curve and a Miner linear cumulative damage rule, total fatigue damage is calculated and fatigue life is predicted.

2. A method of predicting local scour depth and fatigue damage of a gooseneck of a marine riser bottom according to claim 1, characterized in that, The bed elevation change amount control equation is constructed by the suspended load sediment transport rate and the bed load sediment transport rate, to obtain the real-time change amount of the bed elevation and the local dynamic scour depth of the goose neck pipe, and the formula is as follows: ， wherein denotes the bed level; denotes the gradient operator.

3. The method of claim 1, wherein, The sediment transport intensity parameter is calculated from the Hilles parameter and the corrected critical Hilles parameter; by adding a microbial membrane influence model, the inhibitory effect of biological fouling on sediment starting is quantified, and the critical Hilles parameter is corrected.

4. The method of claim 1, wherein, The formula of the structural motion equation is as follows: ， ， wherein, represents a mass matrix; represents a damping matrix; , , respectively represent the acceleration, velocity, displacement of the gooseneck pipe; represents a fused load vector; represents a pressure history on the surface of the gooseneck pipe; represents a shear stress history on the surface of the gooseneck pipe; represents a time-varying stiffness matrix.

5. The method of claim 1, wherein, Based on the real-time change amount of the bed elevation and the initial stiffness matrix, the time-varying stiffness matrix is updated through a stiffness matrix mapping function.

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