A design method for the cross-section of a composite flexible pipe

Abstract

The present invention discloses a design method for the cross-section of a composite flexible pipe, and the steps are as follows: according to the design objectives of the flexible pipe, obtain the geometric dimensions and material parameters of the flexible pipe; according to the operating environment of the flexible pipe and the mechanical properties of the material, combined with the provisions of the design specifications, determine the design load and failure criteria of the flexible pipe; establish a finite element model and an overall analysis model of the flexible pipe, and verify their effectiveness; analyze the alternating winding angle of the fibers in the reinforcement layer of the flexible pipe, the material of the outer protective layer, the thickness of the inner lining layer and the thickness of the outer protective layer, and determine the basic design parameters of the flexible pipe in combination with the actual production and design specifications; conduct a preliminary cross-section design, a single design tensile load, a strength check of the flexible pipe, a curvature check of the flexible pipe, and a fatigue life check of the flexible pipe; the cross-section design of the flexible pipe is completed.

Description

Technical Field

[0001] The present invention belongs to the field of pipelines for offshore oil and gas equipment, and particularly relates to a design method for the cross-section of a composite flexible pipe. Background Art

[0002] Flexible pipes have many advantages over traditional steel pipes: they have a smaller bending stiffness, can undergo large deformations, can withstand large platform movements, are easy to install, have strong corrosion resistance, and can be recycled, and thus have important engineering application value in future offshore oil and gas development. By using the same design method and general production process, and selecting different material combinations and cross-section structures, customized production of flexible pipes can be achieved to meet different application requirements in the oil and gas transportation field. Summary of the Invention

[0003] The present invention aims to overcome the defects of the prior art and provides a design method for the cross-section of a composite flexible pipe, aiming to master the key technologies in the design, analysis, and production of ultra-deepwater multi-purpose flexible pipes for the needs of oil and gas exploration and development in deep water, ultra-deep water, and other complex environments, and to obtain original technical research results.

[0004] To solve the above technical problems, the present invention provides the following technical solutions:

[0005] A design method for the cross-section of a composite flexible pipe, the steps of which are as follows: 1) According to the design objectives of the flexible pipe, obtain the geometric dimensions and material parameters of the flexible pipe;

[0006] 2) According to the operating environment of the flexible pipe and the mechanical properties of the materials, and in combination with the provisions of the design specifications, determine the design loads and failure criteria of the flexible pipe;

[0007] 3) Establish a finite element model and an overall analysis model of the flexible pipe, and verify their effectiveness;

[0008] 4) Analyze the alternating winding angles of the fibers in the reinforcement layer of the flexible pipe, the material of the outer protective layer, the thickness of the inner lining layer, and the thickness of the outer protective layer, and determine the basic design parameters of the flexible pipe in combination with the actual production and design specifications;

[0009] 5) Preliminary cross-section design, and conduct preliminary cross-section design of the flexible pipe under the action of single internal pressure and external pressure loads;

[0010] 6) Single design tensile load, use the initial number of fiber layers in the cross-section to establish an overall analysis model of the flexible pipe, conduct static analysis and dynamic analysis respectively, and obtain data on effective tension, curvature, and bending moment; calculate the single design tensile load of the flexible pipe in combination with the design specifications;

[0011] 7) Strength check of the flexible pipe, curvature check of the flexible pipe, fatigue life check of the flexible pipe; the cross-section design of the flexible pipe is completed.

[0012] Preferably, in the fifth step, determine the number of reinforcing layer fibers that enables the cross-section of the flexible pipe to simultaneously meet the two combined design loads. To enable the flexible pipe to pass the strength check, bending check, and fatigue life check, estimate the number of initial cross-section fibers.

[0013] Preferably, in the seventh step, for the strength check of the flexible pipe, apply the combined load to the flexible pipe and analyze the stress-strain results; if the stress-strain results exceed the failure criteria, the flexible pipe fails, and reselect the number of initial cross-section fibers for analysis until the number of cross-section fibers enables the flexible pipe to pass the strength check.

[0014] Preferably, in the seventh step, for the curvature check of the flexible pipe, apply a bending moment to calculate its curvature and compare it with the allowable curvature obtained in the overall analysis; if the curvature is greater than the allowable curvature, replace the flexible pipe material and redesign.

[0015] Preferably, in the seventh step, for the fatigue life check of the flexible pipe, apply a fatigue load to calculate the fatigue life; if the fatigue life of the flexible pipe does not meet the design requirements, reselect the initial cross-section layer number until the number of cross-section fibers enables the flexible pipe to pass the strength check, bending check, and fatigue life check; if the number of cross-section fibers enables the flexible pipe to pass all checks, use it as the final number of cross-section fibers.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] By studying the design method of the cross-section of the thermoplastic fiber-reinforced composite flexible pipe, the present invention can provide guidance for the design and production of the thermoplastic fiber-reinforced composite flexible pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a schematic structural diagram of the composite flexible pipe of the present invention;

[0019] Figure 2 is a schematic diagram of the cylindrical coordinate system of the composite flexible pipe of the present invention;

[0020] Figure 3 is a schematic diagram of the minimum wall thickness regulation of the inner lining layer for different pipe diameters in the national standard of the present invention;

[0021] Figure 4 is a flow chart of the overall iterative analysis of the composite flexible pipe of the present invention;

[0022] Figure 5 is a flow chart of the design of the composite flexible pipe of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not intended to limit the present invention.

[0024] As Figure 1 shown, the thermoplastic composite flexible pipe is composed of a thermoplastic polymer extruded pipe and multiple layers of fiber-reinforced composite material tapes wound and bonded. Its cross-sectional structure is divided into four layers from the inside to the outside: an outer protective layer 1, a functional layer 2, a fiber-reinforced layer 3, and a lining layer 4. The functional layer is a non-main load-bearing structure. The reinforcing fibers are completely embedded in the polymer matrix, and each layer is bonded together by heating and fusion to form a fully bonded composite pipe structure. The thermoplastic polymer material can be selected from polyethylene (PE), polypropylene (PP), nylon (PA), polyvinylidene fluoride (PVDF), polyphenylene sulfide (PPS), polyether ether ketone (PEEK), etc.; the fiber material can be selected from glass fiber, carbon fiber, aramid fiber, etc. The cross-sectional design process of the flexible pipe is a process of repeated iteration. During the design process, if the cross-sectional performance designed far exceeds the design target, due to economic and actual production needs, the cross-sectional performance should be made to just meet the design target and the safety margin should be appropriately reduced.

[0025] A design method for the cross-section of a composite flexible pipe, the steps of which are as follows:

[0026] 1) According to the design target of the flexible pipe, obtain the geometric dimensions and material parameters of the flexible pipe.

[0027] 2) According to the operating environment of the flexible pipe and the mechanical properties of the material, combined with the regulations on the safety factor in the design code, determine the design load and failure criterion of the flexible pipe. The design load includes the design internal pressure load, the design external pressure load, and the design tensile load. The stress-strain failure criteria of each layer of the flexible pipe should be determined according to the material.

[0028] 3) Establish a finite element model of the flexible pipe in the finite element software Abaqus and compare it with the experimental data to verify the effectiveness of the finite element model; establish an overall analysis model of the flexible pipe in the marine engineering dynamic analysis software OrcaFlex to verify the cross-section equivalent theory.

[0029] 4) Analyze the alternating winding angle of the reinforcing layer fibers of the flexible pipe, the material of the outer protective layer, the thickness of the lining layer, and the thickness of the outer protective layer as the only variables of the finite element model of the flexible pipe one by one. Combine the analysis results with the actual production and design code to determine the best cross-sectional characteristics of the flexible pipe, including the fiber winding angle of the reinforcing layer, the best material of the outer protective layer, the best thickness of the lining layer, and the best thickness of the outer protective layer.

[0030] 5) Preliminary section design: perform preliminary section design under single internal and external pressure design loads. Apply the flexible pipe section characteristics determined above to apply the design loads to the flexible pipe finite element model. According to the flexible pipe failure criterion, determine the number of fiber layers in the flexible pipe reinforcement layer that can make the flexible pipe section meet the two design loads at the same time. This is called the initial section fiber layer number. In order for the flexible pipe to meet the through load, considering that the number of fiber layers in the section will make the section pass the subsequent combined load check, bending check and fatigue life check, combined with design experience, it is preliminarily estimated that the number of fiber layers in the section should be as large as possible.

[0031] 6) Single design tensile load: Use the initial cross-sectional fiber layer number to establish the overall analysis model of the flexible pipe, perform static analysis and dynamic analysis on it, and record its effective tension, curvature, and bending moment data; combine the effective tension with the design specifications to calculate the single design tensile load acting on the flexible pipe finite element model.

[0032] 7) Check the strength of the flexible pipe, check the curvature of the flexible pipe, and check the fatigue life of the flexible pipe; at this point, the design of the flexible pipe section is completed.

[0033] Combined load check of flexible pipe: single design tensile load can be combined with single design internal pressure load and single design external pressure load to form two combined loads. The two combined loads are applied to the finite element model of the flexible pipe for strength check and the stress-strain results are analyzed. If the stress-strain results exceed the failure standard and the flexible pipe fails, the initial cross-sectional fiber layer number is re-selected for analysis until the cross-sectional fiber layer number allows the flexible pipe to pass the strength check.

[0034] Flexible pipe curvature check: apply bending moment load to the flexible pipe finite element model for curvature check and analyze the bending deformation results. If the bending deformation exceeds the failure standard, reselect the flexible pipe material until the number of fiber layers in the cross section allows the flexible pipe to pass the strength check and curvature check.

[0035] Flexible pipe fatigue life check, use the flexible pipe finite element model parameters to establish a flexible pipe fatigue analysis model, and check the flexible pipe fatigue life. If the flexible pipe fatigue life does not meet the design requirements, re-select the initial cross-section layer number for analysis until the cross-section fiber layer number allows the flexible pipe to pass the strength check, curvature check and fatigue life check. If the cross-section fiber layer number allows the flexible pipe to pass all checks, it is called the final cross-section fiber layer number.

[0036] Cross-section equivalence, the equivalent elastic constants of the entire reinforcement layer are obtained from the elastic constants of the fiber tape through the homogenization theory, and then the three-layer structure of the flexible pipe is equivalent to a single-layer model to obtain the equivalent elastic constants of the entire flexible pipe, providing basic parameters for the overall analysis of the flexible pipe, vortex-induced fatigue analysis and stability analysis.

[0037] The equivalent theory is as follows: enhance the elastic properties of the reinforcement layer, and analyze and design the composite layer with reference to the macroscopic modulus analysis method of P.C.Chuo multi-layer boards. As Figure 2 shown in the schematic diagram of a cylinder in the cylindrical coordinate system (ρ, φ, z), where the 1 direction is the longitudinal direction of the fiber tape, and the included angle with the circumferential direction is θ; the 2 direction is the transverse direction of the fiber tape; the 3 direction is perpendicular to the fiber tape direction and is consistent with the radial direction of the flexible pipe. The radial direction in the cylindrical coordinate system is perpendicular to the plane of the fiber tape, and the axial direction of the cylindrical coordinate system coincides with the axial direction of the flexible pipe.

[0038] Since the reinforcement layer of the flexible pipe is formed by alternately winding fiber tapes at positive and negative angles, the compliance matrix of each single layer formed by the fiber tape winding can be obtained from the elastic constants of the fiber tape:

[0039]

[0040] The stiffness matrix of each single layer can be obtained by inverting the compliance matrix:

[0041] [C 0 =[S 0 -1 (2)

[0042] The stiffness matrix of each single layer in the cylindrical coordinate system is:

[0043] [S c' k =[T][S 0 [T] T (3)

[0044] where k is used to distinguish the single layer wound with a positive angle (k = 1) and the single layer wound with a negative angle (k = 2) of the fiber tape, and [T] is the coordinate transformation matrix from the material coordinate system to the cylindrical coordinate system.

[0045]

[0046] where α is the transformation angle between the material coordinate system and the cylindrical coordinate system.

[0047] The stiffness matrix of each single layer in the cylindrical coordinate system:

[0048] [C c' k =[S c' k-1 (5)

[0049] The overall equivalent elastic constants of the reinforcement layer are related to the volume ratio of each layer, and the elements in the equivalent stiffness matrix of the reinforcement layer can be calculated in the following way:

[0050] C c-ij =C​​​​c-ji = 0 (i = 1, 2, 3, 6; j = 4, 5) (6)

[0052]

[0053] where and are the volume fractions occupied by a single monolayer.

[0054]

[0055] where

[0056]

[0057] The flexibility matrix of the reinforcement layer can be obtained from the following equation:

[0058] [S c = [C c -1 (10)

[0059] Overall elastic properties of the flexible pipe

[0060] The inner lining layer and the outer protective layer of the flexible pipe are composed of polymers and are isotropic materials. Their flexibility matrix can be expressed as:

[0061]

[0062] where E p and v p are the elastic modulus and Poisson's ratio of the polymer.

[0063] The stiffness matrix of the inner lining layer and the outer protective layer can be expressed as:

[0064] [C p = [S p -1 (12)

[0065] When the flexible pipe composed of three layers is equivalent to an equivalent cross-section composed of one layer, the equivalent elastic properties of the equivalent cross-section are related to the elastic properties of the inner lining layer, the reinforcement layer, and the outer protective layer. Here, the stiffness matrices of the three layers are used: the inner lining layer [C] 1 (that is, [C p ), the reinforcement layer [C] 2 (that is, [C c ), and the outer protective layer [C] 3 (that is, [C p ) and their volume fractions (V 1 , V 2 , V 3 ​​) to calculate the elastic constants of the equivalent cross-section of the flexible pipe. The elements C in the equivalent cross-section stiffness matrix [C] ij can be obtained by the following formula:

[0066]

[0067] C ij = C ji = 0 (i = 1, 2, 3, 6, j = 4, 5) (14)

[0068]

[0069] where

[0070]

[0071] The flexibility matrix of the equivalent cross-section of the flexible pipe is:

[0072]

[0073] Then the elastic constants of the equivalent cross-section of the flexible pipe can be obtained by Equation (17).

[0074] Design Load and Failure Criterion

[0075] The design loads for the thermoplastic fiber-reinforced composite flexible pipe are the internal pressure load, the tensile load, and the external pressure load. Preliminary cross-section design is carried out under the action of a single design load. For the tensile load, the OrcaFlex software is used for calculation and iterated repeatedly with the Abaqus software to determine it, and the safety factor of 1.4 is taken with reference to the specification (DNVGL RP F119-2015) to calculate the design tensile load.

[0076] On the basis of determining the design loads, the strength failure criteria for each layer of the flexible pipe are summarized as follows:

[0077] (1) The inner liner and the outer protective layer are composed of thermoplastic materials, and the fourth strength theory is used to check whether the structure fails:

[0078]

[0079] (2) The fiber tape is a composite material. According to the specification, the design standards for the stress responses in the transverse and longitudinal directions of the fiber tape are as shown in Equation (19):

[0080]

[0081] n: the direction of the controlled stress; σ nk : the characteristic value of the local load response of the structure in the direction n; the characteristic value of the partial stress in the direction n when the matrix cracks; γ F: Local load factor; γ Sd : Local load model factor; γ M : Local resistance factor; γ Rd : Local resistance model factor, γ Rd = 1.0;

[0082] According to the specification, γ F = 1.0, γ Sd = 1.0, γ M = 1.2.

[0083] The design of the flexible pipe section is divided into basic design parameters, preliminary section design, overall analysis and checking.

[0084] For basic design parameters, analyze the winding angle of the flexible pipe reinforcement layer, the material of the outer protective layer, and the thickness of the inner lining layer and the outer protective layer to determine the optimal basic design parameters such as the winding angle of the flexible pipe reinforcement layer, the material of the outer protective layer, and the thickness of the inner lining layer and the outer protective layer.

[0085] For the winding angle of the reinforcement layer, establish a flexible pipe model using structural calculation and analysis software such as Abaqus. Take the winding angle of the flexible pipe reinforcement layer as the only variable and apply internal pressure load, tensile load, and external pressure load respectively. By comparing the ultimate load values, determine the optimal winding angle under a single design load.

[0086] For the material of the outer protective layer, under different materials of the outer protective layer, apply internal pressure load, tensile load, and external pressure load to the flexible pipe respectively, and record the ultimate internal pressure load, ultimate tensile load, and ultimate external pressure load that the flexible pipe can bear. Considering the mechanical properties and economy of the outer protective layer material, select the most suitable outer protective layer material.

[0087] For the thickness of the inner lining layer and the outer protective layer, create flexible pipe models with different thicknesses of the inner lining layer and the protective layer using structural analysis software such as Abaqus. Apply internal pressure load, tensile load, and external pressure load to the flexible pipe respectively, and record the ultimate internal pressure load, ultimate tensile load, and ultimate external pressure load that the flexible pipe can bear. On this basis, determine the thickness of the inner lining layer of the flexible pipe in combination with the national standard (Coiled reinforced plastic composite pipes - Part 2: Fibre reinforced thermoplastic composite continuous pipes). Generally speaking, the thickness of the outer protective layer is not greater than that of the inner lining layer. Therefore, according to the calculation results, the thickness of the outer protective layer is equal to that of the inner lining layer.

[0088] Preliminary cross-section design: For the thermoplastic fiber-reinforced composite flexible pipe, the internal pressure load, tensile load, and external pressure load are taken as the target loads for design analysis, and the preliminary cross-section design is carried out under the action of a single design load. The design internal pressure load and design external pressure load are designed with reference to the ABS specification (Guide for building and classing subsea pipeline system) and national standards (Coiled reinforced plastic composite pipe - Part 2: Fiberglass-reinforced thermoplastic composite continuous pipe); for the tensile load, the OrcaFlex software is used for calculation and iterated repeatedly with the Abaqus software to determine it, and a safety factor of 1.4 is taken with reference to the specification (DNVGL RP F119-2015) to calculate the design tensile load.

[0089] The Abaqus software is used to calculate the equivalent stress inside and outside the flexible pipe, the stress in the fiber direction of the reinforcement layer, and the stress perpendicular to the fiber direction under the action of the design loads (design internal pressure load, design external pressure load, design tensile load); by checking the equivalent stress of the inner lining layer and the outer protective layer, and the maximum tensile stress and maximum compressive stress in the fiber direction and the direction perpendicular to the fiber direction of the reinforcement layer, the cross-section geometric dimensions of the flexible pipe that meet the design load requirements are determined.

[0090] Overall analysis and checking: The overall analysis of the flexible pipe is carried out by the flexible pipe calculation analysis software, and the overall analysis includes static analysis and dynamic analysis. Based on the platform hydrodynamic calculation, the overall analysis of the flexible pipe is carried out to obtain parameters such as tension, Von Mises stress, and curvature, providing a basis for the cross-section design.

[0091] Static analysis: The ANSYS and other structural calculation and analysis software are used to construct a simplified platform model. The model parameters include platform size, floating body, columns, draft, column chamfer radius, displacement, and the size of the upper module, etc. The AQWA and other hydrodynamic analysis software are used to carry out hydrodynamic analysis on the platform model.

[0092] During the static analysis process, environmental parameters such as platform parameters and extreme hurricane sea condition data (waves, wind, tides, and storm surges, etc.), the distribution of extreme hurricane sea condition ocean currents along the water depth, and seabed soil parameters (seabed stiffness and friction coefficient, etc.) are considered, and the once-in-a-century working sea condition is used for calculation and analysis.

[0093] According to the density requirement of the fluid transported inside the flexible pipe, and considering the environmental load direction of 0° - 360° (such as the platform and the mooring system plus the workover pipe are axisymmetric, and the environmental load direction can be simplified to 0° - 180°), the static analysis of the flexible pipe is carried out. The tension, Von Mises stress, and curvature of the flexible pipe static analysis can be calculated, obtaining the equilibrium state configuration of the flexible pipe system under the action of gravity, buoyancy, and fluid resistance, etc. for the next dynamic analysis.

[0094] Dynamic analysis. The dynamic analysis of the flexible pipe is the key to the overall analysis. Based on the initial configuration obtained from the static analysis, referring to the platform parameters, environmental parameters (extreme hurricane sea state data, distribution of extreme hurricane sea state ocean currents along the water depth, wave parameters, submarine soil parameters, etc.), the density of the fluid inside the pipe, the direction of environmental loads, and other factors, the overall dynamic response of the flexible pipe within the time domain is analyzed. Through the dynamic analysis of the flexible pipe, the tension, curvature, and Von Mises stress distributions of the flexible pipe dynamic analysis are obtained, and compared with the static analysis to determine the maximum effective tension, curvature, and Von Mises stress, providing design parameters for the cross-section design.

[0095] Checking. After the preliminary cross-section design based on a single load, the strength of the flexible pipe is checked. The content of the strength check includes: strength check for the combined load of tension and external pressure, strength check for the combined load of tension and internal pressure, and curvature check.

[0096] The tensile load and curvature check include the calculation results of the iteration with the overall analysis. As Figure 4 shown, for the overall iterative analysis process, according to the cross-section design results and the model equivalent parameters, the cross-section parameters of the flexible pipe are recalculated to implement the overall-local-overall iterative procedure, and the cross-section parameters of the flexible pipe are determined through repeated calculations. If the designed weight of the flexible pipe is too light, it is difficult to install the pipeline in the empty pipe state, and under the action of wave and current loads, the lighter pipe weight will cause a larger response amplitude of the flexible pipe. Consider adding corresponding weights for the installation condition. At the same time, since the maximum effective tension of the flexible pipe becomes larger after adding weights, the cross-section of the flexible pipe needs to be redesigned and checked.

[0097] Fatigue analysis. The safety and reliability of the flexible pipe in deep sea or ultra-deep sea environments depend on its fatigue life. The stress fatigue analysis of the flexible pipe adopts a safety factor of 10.

[0098] Use Abaqus software to establish a finite element model of the flexible pipe, conduct time domain analysis on the flexible pipe according to the wave-period probability distribution in the working sea area, and use the Fatigue Analysis module to calculate the fatigue damage of the overall analysis model of the flexible pipe; use OrcaFlex software to conduct static analysis and modal analysis on the flexible pipe, and import its calculation results into the vortex-induced vibration (VIV) calculation software SHEAR7 to calculate the vortex-induced fatigue damage and predict the fatigue life of the flexible pipe.

[0099] Wave-induced fatigue analysis: The flexible pipe is subjected to an overall dynamic time-domain analysis based on the wave height - period probability distribution data of the working sea area. Generally, the dynamic time-domain analysis duration of the overall model is at least 1200 s. To ensure the stability of the calculation results, the analysis duration is 11300 s. The wave analysis type is selected as irregular waves, and the wave spectrum type is the Jonswap spectrum. When conducting wave-induced fatigue analysis on the flexible pipe, first design the load conditions for fatigue analysis, and focus on analyzing the fatigue conditions with the environmental load directions of 180° and 0°.

[0100] An equivalent model of the flexible pipe is established using OrcaFlex software, considering the effects of waves, ocean currents, floating platform motion, internal fluid in the pipe, and the counterweight layer. Based on the wave height - period probability distribution, the overall time-domain dynamic response of the flexible pipe equivalent model is obtained, and the comprehensive damage distribution, the most severe wave height and probability damage distribution, and the fatigue life distribution of the flexible pipe are calculated. According to the specification (API SPEC 17J - 2014), considering a fatigue safety factor of 10 times, the wave-induced fatigue life at the most dangerous location of the flexible pipe is obtained.

[0101] Vortex-induced vibration (VIV) fatigue analysis: The VIV fatigue analysis of the flexible pipe is the key and difficult point of the fatigue life analysis. SHEAR7 software is used to calculate VIV fatigue. SHEAR7 is usually used to calculate the VIV response of top-tensioned flexible pipes. For the VIV response of catenary flexible pipes, the following equivalent method is generally used for indirect calculation: The natural frequencies, modal vibration modes, and modal vibration mode curvatures of the catenary flexible pipe are calculated by finite element software and input into the file COMMON.MDS. According to the user-input natural frequencies, modal vibration modes, and modal curvatures, the root mean square displacement, root mean square stress, and fatigue damage rate of the catenary flexible pipe are calculated through the modal superposition method and the iterative method.

[0102] When calculating the VIV fatigue life, the sea current velocity with a return period of one year is adopted. An finite element model of the flexible pipe is established using OrcaFlex software and its static analysis is carried out, and the DAT input file recognizable by SHEAR7 is directly exported; the modal analysis of the flexible pipe is carried out to export the recognizable modal input file (MDS format); the two files are directly imported into SHEAR7 to calculate the VIV response of the flexible pipe.

[0103] The VIV fatigue analysis of the flexible pipe is carried out through the VIV analysis software Shear7 to obtain the annual damage rate, root mean square stress, and root mean square displacement of the flexible pipe. According to the specification (API SPEC 17J - 2014), considering a fatigue safety factor of 10 times, the VIV fatigue life at the most dangerous location of the flexible pipe is finally obtained.

[0104] The above is only to illustrate the embodiments of the present invention and is not intended to limit the present invention. For those skilled in the art, any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A design method for the cross-section of a composite flexible pipe, characterized in that, it includes the following steps: 1), According to the design objectives of the flexible pipe, obtain the geometric dimensions and material parameters of the flexible pipe; 2), Determine the design load and failure criterion of the flexible pipe according to the operating environment of the flexible pipe, the mechanical properties of the material and the provisions of the design specification; 3), Establish a finite element model and an overall analysis model of the flexible pipe, and verify the effectiveness; 4), Analyze the winding angle of the reinforcement layer of the flexible pipe, the material of the outer protective layer, and the thickness of the inner lining layer and the outer protective layer to determine the optimal design parameters for the winding angle of the reinforcement layer, the material of the outer protective layer, and the thickness of the inner lining layer and the outer protective layer of the flexible pipe; 5), Preliminary cross-section design, conduct preliminary cross-section design of the flexible pipe under the action of single internal pressure and external pressure loads; determine the number of fiber layers of the reinforcement layer for which the cross-section of the flexible pipe meets the two design loads, so that the flexible pipe passes the strength check, bending check and fatigue life check, and calculate the initial cross-section fiber layers; 6), Single design tensile load, use the initial cross-section fiber layers to establish an overall analysis model of the flexible pipe, conduct static analysis and dynamic analysis respectively, and obtain effective tension, curvature, and bending moment data; calculate the single design tensile load of the flexible pipe in combination with the design specification; 7), Conduct strength check of the flexible pipe, curvature check of the flexible pipe, and fatigue life check of the flexible pipe, and the cross-section design of the flexible pipe is completed; Strength check of the flexible pipe, apply combined loads to the flexible pipe, and analyze its stress and strain results; If the stress and strain results exceed the failure standard, the flexible pipe fails, and reselect the initial cross-section fiber layers for analysis until the cross-section fiber layers enable the flexible pipe to pass the strength check; Curvature check of the flexible pipe, apply bending moment to the flexible pipe to calculate the curvature, and compare it with the allowable curvature obtained in the overall analysis; If the curvature is greater than the allowable curvature, replace the material of the flexible pipe; Fatigue life check of the flexible pipe, apply fatigue load to the flexible pipe to calculate the fatigue life; If the fatigue life of the flexible pipe does not meet the design requirements, reselect the initial cross-section layers until the cross-section fiber layers enable the flexible pipe to pass the strength check, bending check and fatigue life check; If the cross-section fiber layers enable the flexible pipe to pass all checks, use it as the final cross-section fiber layers.

2. The design method for the cross-section of a composite flexible pipe according to claim 1, characterized in that, For the winding angle of the reinforcement layer, establish a flexible pipe model, take the winding angle of the reinforcement layer of the flexible pipe as the only variable and apply internal pressure load, tensile load, and external pressure load respectively, and determine the optimal winding angle under the action of a single design load by comparing the ultimate load values.

3. The design method for the cross-section of a composite flexible pipe according to claim 1, characterized in that, Under the condition of different outer protective layer materials, apply internal pressure load, tensile load, and external pressure load to the flexible pipe respectively, record the ultimate internal pressure load, ultimate tensile load, and ultimate external pressure load that the flexible pipe can withstand, and select the most suitable outer protective layer material.

4. The design method for the cross-section of a composite flexible pipe according to claim 1, characterized in that, Create flexible pipe models with different inner liner layer and protective layer thicknesses. Apply internal pressure loads, tensile loads, and external pressure loads to the flexible pipes respectively, and record the ultimate internal pressure load, ultimate tensile load, and ultimate external pressure load that the flexible pipes can withstand. Determine the thickness of the inner liner layer of the flexible pipe, and the thickness of the outer protective layer is equal to the thickness of the inner liner layer.

5. According to the design method of the composite flexible pipe cross-section described in claim 1, characterized in that, For fiber composite materials, the stress responses in the transverse and longitudinal directions of the fibers are: σ nk is the eigenvalue of the local load response of the structure in direction n; is the eigenvalue of the partial stress in direction n when the matrix cracks; γ F is the local load factor; γ Sd is the local load model factor; γ M is the local resistance factor; γ Rd is the local resistance model factor.

Images