A corrosion fatigue analysis method for a deep-water variable stiffness non-bonded flexible riser

Abstract

The present invention relates to the field of deep - water riser research, and particularly to a corrosion fatigue analysis method for a deep - water variable - stiffness non - bonded flexible riser. This method adopts a scheme of interactive solution of the sectional mechanics and the overall hydrodynamics of the non - bonded flexible riser. When analyzing, it considers the changes in the tensile, torsional, bending, and compressive stiffnesses of the riser along the arc length direction caused by the riser's self - weight, water depth, internal pressure, and riser configuration. The changes in the stiffnesses of each direction of the riser are assigned and updated to the Line element of the overall hydrodynamic model. At the same time, it considers the influence of the riser annulus on the tensile armor layer under the corrosion of acidic medium or seawater, establishes a numerical model for the corrosion fatigue of the variable - stiffness non - bonded flexible riser considering random wave loads and annulus corrosion. Through the corrosion fatigue analysis and calculation of the tensile armor layer, a dynamic corrosion fatigue prediction method for the deep - water variable - stiffness non - bonded flexible riser is formed, thereby providing support for improving the structure of the non - bonded flexible riser, the structure of the anti - bending device, and the laying method, etc.

Description

Technical Field

[0001] The present invention relates to a corrosion fatigue analysis method for a deep - water variable - stiffness non - bonded flexible riser, belonging to the technical field of deep - water riser research. Background Technique

[0002] The riser connects the offshore floating device and the subsea wellhead, and is one of the key equipment for deep - sea oil and gas resource development. At present, China has the ability to design and develop static flexible pipes, but the mechanical behavior of dynamic non - bonded flexible risers is complex, and the design and analysis are difficult, and still completely rely on imports.

[0003] There are mainly two problems in the previous fatigue life research of non - bonded flexible risers: 1) In the previous global response analysis of non - bonded flexible risers, the stiffness in each direction was regarded as a constant value, ignoring the change of stiffness in each direction and mechanical properties caused by the coupling action of multiple loads, resulting in inaccurate dynamic response analysis of non - bonded flexible risers; 2) The fatigue research of non - bonded flexible risers is based on the traditional rain - flow counting method and linear cumulative damage theory, making it difficult to accurately describe the interaction between corrosion and fatigue of non - bonded flexible risers. To solve the technical bottleneck of corrosion failure analysis of deep - water dynamic non - bonded flexible risers, it is necessary to carry out research on the fatigue failure of the tensile armor layer induced by corrosion and alternating loads.

[0004] In summary, there is an urgent need for a corrosion fatigue analysis method for deep - water variable - stiffness non - bonded flexible risers in the current deep - sea non - bonded flexible risers, to establish a dynamic response analysis model of deep - water variable - stiffness non - bonded flexible risers under annulus corrosion and various coupling loads, reveal the corrosion fatigue failure mechanism of non - bonded flexible risers, establish a prediction method considering the corrosion fatigue of non - bonded flexible risers, provide theoretical support for the localization of non - bonded flexible risers, and ensure the safe exploitation of deep - sea oil and gas resources. Summary of the Invention

[0005] The purpose of the present invention is to provide a corrosion fatigue analysis method for a deep - water variable - stiffness non - bonded flexible riser, aiming at the problem that most of the current global response analyses of non - bonded flexible risers regard the stiffness in each direction as a constant value, ignoring the change of stiffness in each direction and mechanical properties caused by the coupling action of multiple loads, resulting in inaccurate dynamic response analysis of non - bonded flexible risers. This method considers the changes in tension, torsion, bending, and bending stiffness caused by the action of suspended weight, water depth, internal pressure, riser configuration, etc. during the overall hydrodynamic analysis of non - bonded flexible risers, which is closer to the actual working conditions of the riser.

[0006] To achieve the above purpose, the technical solution provided by the present invention is:

[0007] A corrosion fatigue analysis method for a deep - water variable - stiffness non - bonded flexible riser, characterized by considering the changes in stiffness caused by the riser's self - weight, water depth, internal pressure, and riser configuration, adopting a method of interactive solution of the overall hydrodynamics and cross - section mechanics of the non - bonded flexible riser to calculate the dynamic load - bearing capacity, vibration characteristics, and stress and strain of the tensile armor layer of the non - bonded flexible riser, conducting corrosion fatigue experiments on the tensile armor layer material under different annulus corrosion environments and alternating loads, establishing a finite - element model of the UMAT subroutine for damage evolution of the tensile armor layer, and forming a new method for predicting the corrosion fatigue of non - bonded flexible risers, which specifically includes the following steps:

[0008] 1) Comprehensively apply the method of interactive solution of the overall hydrodynamics and cross - section mechanics of the non - bonded flexible riser to calculate the changes in stiffness at different positions of the riser under different combined loads;

[0009] 2) Assign and update the stiffness change law along the arc length direction of the riser to the Line element of the overall hydrodynamic model, and establish an overall hydrodynamic model of the deep - water variable - stiffness non - bonded flexible riser;

[0010] 3) Use the method of indoor model simulation experiment to correct the overall hydrodynamic model of the deep - water variable - stiffness non - bonded flexible riser;

[0011] 4) Use the corrected overall hydrodynamic model of the variable - stiffness non - bonded flexible riser to calculate the dynamic load - bearing capacity at the most dangerous position of the riser, and use the cross - section mechanics model of the non - bonded flexible riser to calculate the stress and strain of the tensile armor layer;

[0012] 5) Take the dynamic load - bearing capacity at the most dangerous position as the loading value, and respectively consider different environments where the annulus of the riser is seawater, condensate water, and acidic corrosion media (CO2, H2S), conduct corrosion fatigue experiments on the tensile armor layer of the non - bonded flexible riser, and obtain key parameters;

[0013] 6) Establish a continuous damage constitutive model for the non - bonded flexible riser, develop and compile the UMAT subroutine for damage evolution of finite - element software, and establish a corrosion fatigue finite - element model of the tensile armor layer;

[0014] 7) Conduct corrosion fatigue simulation calculations on the tensile armor layer under the coupling of corrosion and alternating stress of the variable - stiffness non - bonded flexible riser, and establish a method for predicting the corrosion fatigue of non - bonded flexible risers.

[0015] Furthermore, the change in stiffness includes changes in tensile stiffness, torsional stiffness, bending stiffness, and compressive stiffness. The interactive solution method for the overall hydrodynamics and sectional mechanics of the unbonded flexible riser refers to first calculating the comprehensive load along the arc length direction of the riser using the overall hydrodynamics model of the riser, updating this comprehensive load characteristic to the sectional mechanics model of the riser, and thus calculating the change in stiffness at different positions of the riser under the comprehensive load. When the change value of the stiffness is greater than the set value of 5% - 10%, the stiffness of the riser is updated and then brought back into the overall hydrodynamic model of the riser to calculate the comprehensive load along the arc length direction. This process is repeated interactively until the change value of the riser stiffness is less than the set value of 5% - 10%. At this time, it is approximately considered that the stiffness at this time is the stiffness of the riser under the comprehensive load, and finally, an overall hydrodynamic model of the variable-stiffness unbonded flexible riser with stiffness changing along the arc length direction is established. The unbonded flexible riser continuous damage constitutive model is a model considering the coupled action of corrosion and alternating loads.

[0016] Advantages of the present invention:

[0017] 1. This method comprehensively applies the interactive solution of the overall hydrodynamics and sectional mechanics of the unbonded flexible riser, fully considering the change in stiffness along the arc length direction of the riser caused by the coupled load, and realizes the accurate solution of the dynamic response of the variable-stiffness unbonded flexible riser under complex loads such as ocean currents and waves.

[0018] 2. This method uses the UMAT subroutine method to correlate the elastoplastic constitutive equation, the pitting evolution model in the elastoplastic stage, and the elastoplastic fatigue damage evolution model of the unbonded flexible riser, and embeds them into the finite element software to realize the simulation analysis of the corrosion fatigue of the tensile armor layer.

[0019] 3. This method determines the most dangerous position of the unbonded flexible riser through overall hydrodynamics analysis, studies the sectional load-bearing response through local nonlinear static mechanics analysis, and then studies the elastoplastic continuous damage corrosion fatigue of the tensile armor layer, forming a progressive analysis path from the overall to the section and then to a specific layer, revealing the fatigue failure mechanism of the unbonded flexible riser, exploring the main control factors affecting the failure of the unbonded flexible riser, and helping to propose measures to reduce the corrosion fatigue of the unbonded flexible riser or increase the fatigue life of the unbonded flexible riser. Description of the drawings

[0020] Figure 1 is the flowchart of the method of the present invention; Specific embodiments

[0021] The following will make a detailed description of a method for analyzing the corrosion fatigue of a deep-water variable-stiffness unbonded flexible riser of the present invention in combination with specific embodiments and drawings.

[0022] The non-bonded flexible riser is formed by non-bonded joining of metal layers and polymer layers with different functions, structures and materials, and includes a skeleton layer and a compressive armor layer with a large-deflection special-shaped structure, and a helically wound tensile armor layer structure. Therefore, its cross-sectional mechanical behavior is complex, involving contact non-linearity, material non-linearity and structural non-linearity. The non-bonded flexible riser will affect the interlayer non-linear contact behavior under different combined loads, resulting in changes in mechanical response and anisotropic stiffness. In the past, the global response analysis of non-bonded flexible risers regarded the anisotropic stiffness as a constant value, ignoring the changes in anisotropic stiffness and mechanical properties caused by the multi-load coupling effect, resulting in inaccurate dynamic response analysis of non-bonded flexible risers. Moreover, the fatigue research of non-bonded flexible risers is based on the traditional rainflow counting method and linear cumulative damage theory, making it difficult to accurately describe the interaction between corrosion and fatigue of non-bonded flexible risers. The present invention provides a corrosion fatigue analysis method for deep-water variable-stiffness non-bonded flexible risers, which can carry out corrosion fatigue simulation calculations of the tensile armor layer under the coupling of annulus corrosion and alternating stress of variable-stiffness non-bonded flexible risers. In this embodiment, the laying method of the non-bonded flexible riser is a catenary riser.

[0023] Specifically, Figure 1 is the method flow chart of the present invention. A corrosion fatigue analysis method for deep-water variable-stiffness non-bonded flexible risers, characterized in that considering the changes in stiffness caused by the riser self-weight, water depth, internal pressure and riser configuration, adopting the method of interactive solution of the overall hydrodynamics and cross-sectional mechanics of non-bonded flexible risers, calculating the dynamic load-bearing, vibration characteristics and stress-strain of the tensile armor layer of non-bonded flexible risers, carrying out corrosion fatigue experiments on the materials of the tensile armor layer under different annulus corrosion environments and alternating loads, establishing a finite element model of the UMAT subroutine for damage evolution of the tensile armor layer, and forming a new method for predicting the corrosion fatigue of non-bonded flexible risers, specifically including the following steps:

[0024] 1) Comprehensively apply the method of interactive solution of the overall hydrodynamics and cross-sectional mechanics of non-bonded flexible risers to calculate the changes in stiffness at different positions of the riser under different combined loads;

[0025] 2) Assign and update the stiffness change law along the arc length direction of the riser to the Line element of the overall hydrodynamic model, and establish an overall hydrodynamic model of deep-water variable-stiffness non-bonded flexible risers;

[0026] 3) Adopt the method of indoor model simulation experiment to correct the overall hydrodynamic model of deep-water variable-stiffness non-bonded flexible risers;

[0027] 4) Use the corrected overall hydrodynamic model of variable-stiffness non-bonded flexible risers to calculate the dynamic load-bearing at the most dangerous position of the riser, and use the cross-sectional mechanics model of non-bonded flexible risers to calculate the stress-strain of the tensile armor layer;

[0028] 5) Taking the dynamic load at the most dangerous position as the loading value, consider different environments where the riser annulus is seawater, condensate water, and acidic corrosion media (CO2, H2S) respectively, and conduct corrosion fatigue experiments on the non-bonded flexible riser's tensile armor layer to obtain key parameters;

[0029] 6) Establish a constitutive model for continuous damage of non-bonded flexible risers, develop and compile a UMAT subroutine for damage evolution in finite element software, and establish a finite element model for corrosion fatigue of the tensile armor layer;

[0030] 7) Conduct simulation calculations on the corrosion fatigue of the tensile armor layer under the coupling of corrosion and alternating stress for non-bonded flexible risers with variable stiffness, and establish a prediction method for the corrosion fatigue of non-bonded flexible risers.

[0031] The changes in the stiffness include changes in tensile stiffness, torsional stiffness, bending stiffness, and compressive stiffness; the interactive solution method for the overall hydrodynamics and sectional mechanics of non-bonded flexible risers means first using the overall hydrodynamics model of the riser to calculate the comprehensive load along the arc length direction of the riser, updating this comprehensive load characteristic to the sectional mechanics model of the riser, so as to calculate the stiffness changes at different positions of the riser under the comprehensive load. When the stiffness change value is greater than the set value of 5% - 10%, update the riser stiffness and bring it back into the overall hydrodynamic model of the riser to calculate the comprehensive load along the arc length direction again. Repeat this interactive process until the riser stiffness change value is less than the set value of 5% - 10%. Then approximately consider the current stiffness as the stiffness of the riser under the comprehensive load, and finally establish an overall hydrodynamic model of non-bonded flexible risers with variable stiffness whose stiffness changes along the arc length direction; the constitutive model for continuous damage of non-bonded flexible risers is a model considering the coupling effect of corrosion and alternating loads.

[0032] The sectional mechanics model of non-bonded flexible risers determines the equilibrium equations of each layer by considering the interlayer slip behavior and hysteresis phenomenon of the tensile armor layer when the non-bonded flexible riser bends through methods such as elasticity theory, bending beam theory, and virtual work principle, considering the interlayer contact characteristics and geometric characteristics, and using variational equations to combine the equilibrium equations to establish a theoretical model for the sectional mechanics of non-bonded flexible risers. At the same time, through an equivalent method with equal bending moment stiffness per unit area, the skeleton layer and compressive armor layer with special-shaped structures are equivalent to regular cylindrical layers, and a numerical model for sectional mechanics considering various non-linearities is established. The correctness of the model is verified by comparing the theoretical model, numerical model, and existing experimental results.

[0033] To verify the correctness of the overall hydrodynamic model of the variable-stiffness non-bonded flexible riser, the model was modified through indoor simulation experiments. A catenary riser was selected as the research object, and a transparent silicone tube with an outer diameter of 8 mm and an inner diameter of 6 mm was used as the model pipeline. An FPSO model with a length of 93 mm, a width of 52 mm, and a height of 40 mm was used to simulate an offshore platform. A rectangular cross-section variable-slope straight flume was used to simulate environmental conditions such as ocean wave currents. The flow velocity in the experimental flume was measured using an ultrasonic Doppler velocimeter, and the time-averaged flow velocity at each velocity measurement point was monitored in real time. The top of the suspended section of the riser model was fixed to a hinge joint extending to the water surface, and the hinge joint used a fixed support at the top of the flume and a liftable support rod to achieve the free suspension of the riser. During the experiment, the top of the platform model was fixed to the bottom of a carbon fiber lift rod. The top of the carbon fiber lift rod was connected to a universal joint, and the middle was connected to the fixed support by four springs, enabling the platform model to perform two-degree-of-freedom motions of heaving and swaying (i.e., in the X and Y directions) only in the horizontal direction. The model pipeline was marked along the axial direction of the riser using an oil-based black marker pen, and the method of using a high-speed camera to track the marked points was selected to achieve non-invasive and non-interfering vibration testing of the flexible riser. To be able to capture the vibration displacements of the riser in both the transverse and longitudinal directions simultaneously, two high-speed cameras were installed on one side and diagonally below the flume to capture the displacements of the riser in the XOZ and XOY planes. Using the established indoor riser dynamic simulation experimental platform, dynamic response experiments of the riser were carried out under different speeds and configurations to provide experimental data for model verification.

[0034] Based on the annulus corrosion environment of the non-bonded flexible riser, environments such as seawater, condensate water, and acidic corrosion media (CO2, H2S) in the annulus of the non-bonded flexible riser were considered separately. Through the MTS experimental platform, corrosion fatigue experiments of the tensile armor layer were carried out. Standard tensile specimens and an annulus simulation environment of the non-bonded flexible riser were prepared. Referring to the national standard "GB20120.1-2020", the lift method was used to obtain the corrosion fatigue limit of the tensile armor layer under different annulus corrosion environments. Then, the group experiment method was used to obtain the fatigue life curve of the tensile armor layer in the simulation environment. During the experiment, the loading magnitude was derived from the dynamic load-bearing of the tensile armor layer at the most dangerous positions (the top suspension point and the bottom touchdown point) obtained from the dynamic response analysis of the non-bonded flexible riser. After the specimen fractured, the fracture surface was cut and cleaned, corrosion fatigue failure analysis was carried out, and the fracture surface morphology and corrosion morphology of the specimen were observed and analyzed using a scanning electron microscope. The least squares method was used for fatigue life fitting, and the damage parameters were calibrated through explicit relationships to obtain the continuous damage constitutive model of the tensile armor layer of the non-bonded flexible riser. By carrying out fatigue experiments of the tensile armor layer under multiple parameters such as different corrosion media (seawater, condensate water, permeating gas, etc.), environmental parameters (temperature, pressure), and stress amplitudes, the fatigue failure mechanism of the tensile armor layer was revealed.

[0035] Specifically, a finite element model of the tensile armor layer is established by finite element software. The obtained elastoplastic constitutive equation, the pitting evolution model in the elastoplastic stage, and the elastoplastic fatigue damage evolution model are correlated with each other by using the UMAT subroutine method and embedded in the finite element software. During each iterative calculation, after continuously updating the damage value and the size of the corrosion pit, it is judged whether the elements of the tensile armor layer are damaged (especially the elements near the corrosion pit). If the element is not damaged, the constitutive model is updated; if the element is damaged, it is removed. Then, the tensile armor layer is loaded, and the load value is derived from the dynamic load of the tensile armor layer at the most dangerous position obtained from the hydrodynamic analysis of the unbonded flexible riser and the cross-sectional mechanics interaction simulation calculation. When the hydrodynamic analysis of the unbonded flexible riser uses random wave loading, the load borne by the tensile armor layer is also a random load and cannot be cyclically loaded. To reduce the calculation cost, a value range t W of the calculation wave period is given during the hydrodynamic analysis of the unbonded flexible riser. During each loading calculation of the tensile armor layer, it is cyclically iterated with t W as the period until the tensile armor layer fractures and fails, and the fatigue life of the tensile armor layer is obtained.

[0036] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A corrosion fatigue analysis method for a deep-water variable stiffness non-bonded flexible riser, characterized in that, Considering the changes in stiffness caused by the suspended weight of the riser, water depth, internal pressure, and riser configuration, a method of interactive solution of the overall hydrodynamics and sectional mechanics of the unbonded flexible riser is adopted to calculate the dynamic load-bearing capacity, vibration characteristics, and stress and strain of the tensile armor layer of the unbonded flexible riser. Corrosion fatigue experiments of the tensile armor layer material are carried out under different annulus corrosion environments and alternating loads, and a finite element model of the UMAT subroutine for the damage evolution of the tensile armor layer is established to form a new method for predicting the corrosion fatigue of the unbonded flexible riser, which specifically includes the following steps: 1) Comprehensively apply the method of interactive solution of the overall hydrodynamics and sectional mechanics of the unbonded flexible riser to calculate the changes in stiffness at different positions of the riser under different combined loadings; 2) Assign and update the stiffness change law along the arc length direction of the riser to the Line element of the overall hydrodynamic model to establish an overall hydrodynamic model of the deep-water variable-stiffness unbonded flexible riser; 3) Use the method of indoor model simulation experiment to correct the overall hydrodynamic model of the deep-water variable-stiffness unbonded flexible riser; 4) Use the corrected overall hydrodynamic model of the variable-stiffness unbonded flexible riser to calculate the dynamic load-bearing capacity at the most dangerous position of the riser, and use the sectional mechanics model of the unbonded flexible riser to calculate the stress and strain of the tensile armor layer; 5) Take the dynamic load-bearing capacity at the most dangerous position as the loading value, and consider different environments where the annulus of the riser is seawater, condensate water, acidic corrosion medium CO2, and H2S respectively, and carry out corrosion fatigue experiments on the tensile armor layer of the unbonded flexible riser to obtain key parameters; 6) Establish a continuous damage constitutive model of the unbonded flexible riser, develop and compile the UMAT subroutine for damage evolution of the finite element software, and establish a corrosion fatigue finite element model of the tensile armor layer; 7) Carry out corrosion fatigue simulation calculations of the tensile armor layer under the coupling of annulus corrosion and alternating stress of the variable-stiffness unbonded flexible riser, and establish a method for predicting the corrosion fatigue of the unbonded flexible riser.

2. The corrosion fatigue analysis method of a deep - water variable - stiffness non - bonded flexible riser according to claim 1, wherein The changes in stiffness include changes in tensile stiffness, torsional stiffness, bending stiffness, and compressive stiffness.

3. A corrosion fatigue analysis method for a deep - water variable - stiffness non - bonded flexible riser according to claim 1, characterized in that, The method of interactive solution of the overall hydrodynamics and sectional mechanics of the unbonded flexible riser means that first, use the overall hydrodynamic model of the riser to calculate the comprehensive load along the arc length direction of the riser, update the comprehensive load characteristics to the sectional mechanics model of the riser, so as to calculate the changes in stiffness at different positions of the riser under the comprehensive load. When the stiffness change value is greater than the set value of 5% - 10%, update the riser stiffness and bring it into the overall hydrodynamic model of the riser again to calculate the comprehensive load along the arc length direction. Repeat this interactive process until the riser stiffness change value is less than the set value of 5% - 10%. Then, it is approximately considered that the stiffness at this time is the stiffness of the riser under the comprehensive load, and finally, an overall hydrodynamic model of the variable-stiffness unbonded flexible riser with stiffness changing along the arc length direction is established.

4. A corrosion fatigue analysis method for a deep-water variable stiffness non-bonded flexible riser according to claim 1, characterized in that The continuous damage constitutive model of the unbonded flexible riser is a model considering the coupling effect of corrosion and alternating load.

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