Crack initiation and propagation prediction method for LNG (Liquefied Natural Gas) conveying flexible pipeline

Abstract

The invention provides an LNG (Liquefied Natural Gas) conveying flexible pipeline crack initiation and extension prediction method, which comprises the following steps of: constructing a multilayer composite structure geometric model, and defining thermal parameters and mechanical parameters of each layer of material in the model; presetting an initial crack in the model, and establishing a thermal-mechanical coupling control equation containing an interface thermal resistance effect; applying an internal ultralow temperature boundary condition, an external environment temperature boundary condition and a mechanical load boundary condition; solving the thermal-mechanical coupling control equation to obtain temperature field distribution of the section of the pipeline, applying a temperature field as a body load to the multilayer composite structure geometric model, and calculating to obtain a stress field and a stress intensity factor of a crack tip; constructing a crack propagation criterion considering the interface thermal resistance effect; and comparing the stress intensity factor obtained by calculation with a crack propagation criterion, and judging whether the crack is propagated or not. According to the method, the physical authenticity and calculation precision of crack initiation and propagation prediction of the LNG flexible pipeline in the ultralow temperature environment are improved.

Description

Technical Field

[0001] This invention relates to a simulation analysis method for flexible LNG transportation pipelines, specifically a method for predicting crack initiation and propagation in flexible LNG transportation pipelines, belonging to the technical field of marine oil and gas engineering and pipeline structure safety assessment. Background Technology

[0002] As a clean energy source, liquefied natural gas (LNG) is experiencing a continuous increase in demand globally. In the development and transshipment of LNG in deep-sea areas, flexible LNG pipelines are crucial equipment connecting floating production storage and offloading (FPSO) units to transport vessels. Due to the extremely low temperature at which LNG is transported, and the fact that the pipeline is located in a relatively warm seawater environment, the pipeline structure must withstand a significant radial temperature gradient. Simultaneously, the pipeline is subjected to dynamic cyclic loads over extended periods due to the combined effects of waves, currents, and ship movement.

[0003] Existing methods for designing and assessing the lifespan of LNG flexible pipelines are typically based on classical fracture mechanics theory, treating the temperature field as a steady state or performing only unidirectional thermal-structural coupling analysis. This approach ignores the interfacial thermal resistance effect, lacks a dynamic evolution mechanism, and is insufficiently coupled to extreme operating conditions.

[0004] Existing models typically assume perfect thermal contact between the material layers in a pipe, meaning that temperature and heat flow are continuous at the interface. In reality, when microcracks or interlayer debonding exist inside the pipe, the crack surface itself creates significant thermal resistance, forcing heat flow to detour. This leads to drastic abrupt changes in local temperature gradients and additional thermal stress concentration at the crack tip. Ignoring this effect results in severely distorted estimates of the stress intensity factor.

[0005] Traditional analyses often treat interfacial thermal properties as constants. However, in actual operation, the radiative and convective heat transfer capabilities of the crack surface change with the crack opening displacement. Simultaneously, changes in microscopic surface roughness due to fatigue accumulation alter the interfacial thermal resistance before crack initiation. Current technologies cannot capture this dynamic thermodynamic feedback mechanism that evolves over time.

[0006] Under transient conditions such as thermal shock or water hammer, due to the presence of thermal resistance, the internal temperature response of the pipe wall exhibits a significant hysteresis effect relative to the fluid temperature change. The peak thermal stress generated by this transient hysteresis is often the culprit for brittle fracture of the pipeline, and conventional quasi-static models are unable to accurately predict this. Summary of the Invention

[0007] Based on the above background, the purpose of this invention is to provide a method for predicting crack initiation and propagation in flexible LNG transportation pipelines. By constructing interface thermal contact models under multiple states and introducing microscopic morphology evolution and dynamic thermal hysteresis algorithms, a two-way thermo-mechanical coupling crack propagation criterion is established. This improves the accuracy of pipeline damage evolution law and remaining life prediction under extreme operating conditions, and solves the problems mentioned in the background art, such as low life prediction accuracy, inability to effectively warn of crack initiation, and difficulty in dealing with transient thermal shocks caused by the neglect of interface thermal resistance effects in existing models.

[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0009] A method for predicting crack initiation and propagation in flexible LNG transportation pipelines, comprising the following steps:

[0010] S1. Construct a multi-layer composite structure geometric model of the LNG transportation flexible pipeline, and define the thermal and mechanical parameters of each layer material in the multi-layer composite structure geometric model;

[0011] S2. Pre-set initial cracks in the geometric model of the multi-layer composite structure, and establish a thermo-mechanical coupling control equation that includes the interface thermal resistance effect based on the interface thermal contact theory.

[0012] S3. Based on the actual operating conditions of the LNG transportation flexible pipeline, apply internal cryogenic boundary conditions, external ambient temperature boundary conditions, and mechanical load boundary conditions.

[0013] S4. Solve the thermo-mechanical coupling control equation to obtain the temperature field distribution of the LNG transportation flexible pipeline section, and apply the temperature field as a body load to the geometric model of the multi-layer composite structure to calculate the stress field and stress intensity factor at the crack tip.

[0014] S5. Introduce an interface thermal resistance correction coefficient to construct a crack propagation criterion that considers the interface thermal resistance effect.

[0015] S6. Compare the stress intensity factor calculated in step S4 with the crack propagation criterion in step S5 to determine whether the crack has propagated. If it has propagated, update the crack geometry and the thermal resistance boundary conditions of the crack surface, and return to step S4. Repeat steps S4 to S6 until the LNG transport flexible pipeline fails or reaches the preset lifespan.

[0016] Preferably, in step S2, establishing the thermo-mechanical coupling control equations that include the interfacial thermal resistance effect specifically includes defining the following four interfacial thermal contact models to describe the interlayer heat conduction behavior under different crack states:

[0017] For undamaged interlayer interfaces, a continuous thermal contact model is defined that assumes that temperature and heat flow remain continuous.

[0018] For a fully open crack interface, a jump-type thermal contact model is defined that reflects the heat flow through the gas medium in the crack gap by conduction or radiation and that there is a temperature step at the interface.

[0019] For crack tips or closed crack regions, a local thermal contact model is defined, which is composed of a microscopic contact thermal conduction region and a gap thermal resistance region.

[0020] For the debonding region between layers, an incomplete thermal contact model is defined and introduced, which uses contact thermal conductivity as a function of contact pressure.

[0021] Preferably, in the jump-type thermal contact model, the interfacial thermal resistance at the crack surface is defined as a dynamic function of the crack opening displacement, and its functional expression is:

[0022]

[0023] In the formula, The interfacial thermal resistance at the crack surface. For crack opening displacement, The thermal conductivity of the medium filling the crack. The emissivity of the crack surface.

[0024] As the crack propagates, the crack opening displacement... Increased, leading to local thermal resistance This increases, which in turn causes a redistribution of the temperature gradient at the crack tip.

[0025] Preferably, the calculation of the interface thermal resistance effect in steps S2 and S5 specifically includes:

[0026] Initial surface roughness and material microhardness are set for the interlayer interfaces of the multilayer composite structure geometric model;

[0027] In each load analysis step, the stress-strain response history at the interface is extracted, and the cumulative fatigue damage is calculated.

[0028] Based on the accumulated fatigue damage, the dynamic roughness at the current cycle number is calculated using the roughness evolution function. The calculation formula is as follows:

[0029]

[0030] In the formula, For dynamic roughness, The initial surface roughness, Roughness evolution coefficient This refers to the cumulative amount of fatigue damage.

[0031] Extract the interfacial contact pressure of the current analysis step, and substitute the dynamic roughness into the Cooper-Mikic-Yovanovich contact theory model to calculate the real-time interfacial contact thermal conductivity. The calculation formula is as follows:

[0032]

[0033] In the formula, For real-time interface contact thermal conductivity, To harmonize the thermal conductivity of the interface, The average slope of the surface micro-protrusions. The effective root mean square roughness is related to dynamic roughness, where P is the interfacial contact pressure and H is the material microhardness.

[0034] The calculated real-time interface contact thermal conductivity is updated in the local thermal contact model or the incomplete thermal contact model as the thermal conduction boundary condition for the next incremental step, and the initiation location of microcracks is predicted based on the calculated local thermal resistance anomaly increase region.

[0035] Preferably, step S6, updating the crack geometry and crack surface thermal resistance boundary conditions, specifically includes:

[0036] The location of the crack tip is updated using the extended finite element method or mesh reconstruction technology;

[0037] For newly generated cracked surfaces, their boundary conditions are modified from internal material heat conduction to a jump thermal contact model or a local thermal contact model, thereby blocking direct heat transfer in the next iteration.

[0038] Preferably, in step S5, constructing a crack propagation criterion that considers the interfacial thermal resistance effect specifically includes:

[0039] Define the equivalent thermodynamic stress intensity factor :

[0040] In the formula, The stress intensity factor generated by the mechanical load boundary conditions; The thermal stress intensity factor is caused by the local temperature difference due to the interfacial thermal resistance R, which is a function of the interfacial thermal resistance. These are the thermo-mechanical coupling weighting coefficients;

[0041] when At that time, it was determined that the crack had started to propagate. This refers to the fracture toughness of the material under low-temperature conditions.

[0042] As a preferred embodiment, constructing a crack propagation criterion that considers the interfacial thermal resistance effect also includes the following steps for correcting heat dissipation in the plastic zone at the crack tip:

[0043] Considering the plastic work-to-heat effect at the crack tip during cyclic propagation, a modified model of the local temperature field at the crack tip is constructed:

[0044]

[0045] In the formula, This is the corrected local temperature. The temperature calculated considering only heat conduction, The coefficient of heat transfer from plastic work, The cyclic plastic work density at the crack tip. For density, Specific heat capacity;

[0046] Based on the corrected local temperature, the fracture toughness of the material is dynamically adjusted. When the local temperature increases due to plastic heat dissipation, causing a change in the local material toughness, the critical expansion threshold is updated in real time.

[0047] Preferably, when the actual operating condition of the LNG transport flexible pipeline is a combination of thermal shock and water hammer caused by the switching on / off mechanism, the temperature field solution in step S4 adopts the following dynamic thermal-fluid-structure interaction algorithm:

[0048] A thermal shock hysteresis factor is defined to characterize the hysteresis effect of the internal temperature response of the LNG transport flexible pipeline wall relative to the fluid temperature change due to the presence of interfacial thermal resistance.

[0049] When calculating the dynamic stress intensity factor, the transient thermal stress component caused by thermal shock hysteresis is superimposed. The calculation formula is:

[0050]

[0051] In the formula, E is the elastic modulus. The coefficient of thermal expansion is Poisson's ratio, The temperature of the inner wall of the LNG transport flexible pipeline. This refers to the crack tip temperature after being hysteresis due to interfacial thermal resistance. It is the thermal shock hysteresis factor.

[0052] Preferably, in step S3, the mechanical load boundary conditions include wave and ocean current cyclic loads calculated based on the hydrodynamic model, and dynamic pressure fluctuation loads generated by the thermal shock of the gate and water hammer effect; during the calculation process, the dynamic pressure fluctuation loads are applied as a time-dependent function to the inner wall of the LNG transport flexible pipeline.

[0053] Preferably, the method also includes defining and calculating the following damage assessment indices based on the simulation of the entire crack propagation process: crack length growth rate, temperature gradient change rate index, and remaining life prediction value.

[0054] Compared with the prior art, the present invention has the following advantages:

[0055] This invention presents a method for predicting crack initiation and propagation in flexible LNG pipelines. Breaking away from the idealized assumptions of unidirectional thermo-mechanical decoupling or perfect interfacial contact in traditional methods, it innovatively proposes a prediction method based on interfacial thermal behavior. By incorporating the interfacial thermal resistance effect into the mechanical model of crack propagation, this invention achieves iterative calculations of crack geometry altering thermal resistance, thermal resistance altering the temperature field, the temperature field altering thermal stress, and thermal stress driving crack propagation. This method significantly improves the physical realism and computational accuracy of predicting crack initiation and propagation in flexible LNG pipelines under cryogenic conditions, effectively avoiding design biases caused by underestimating local thermal stress concentration, and providing reliable theoretical support for the long-term safe operation of deep-sea LNG transportation systems.

[0056] This invention defines four types of interfacial thermal contact models: continuous, skip, local, and incomplete. These models can cover the entire life cycle of damage, from intact interlaminar layers, microcrack initiation, crack opening to interlaminar debonding. Compared with the single contact assumption of existing technologies, this hierarchical modeling strategy can more accurately describe the heat flow transmission path at different damage stages, thereby capturing the temperature gradient anomaly at the crack tip.

[0057] This invention defines the thermal resistance of the crack surface as a dynamic function of the crack opening displacement. This means that as the crack opens and closes under cyclic loading, the model can automatically adjust the magnitude of the thermal resistance, realistically simulating the blocking and conduction effect of the breathing crack on the heat flow, and solving the problem that the static thermal resistance model cannot reflect the dynamic fatigue process.

[0058] This invention introduces a computational step based on the evolution of microscopic surface morphology, which links fatigue damage accumulation with interface roughness and contact thermal conductivity. It can detect the accumulation of microscopic damage by monitoring the abnormal increase of interface thermal resistance before macroscopic cracks are formed, thereby achieving early prediction and location of crack initiation, and making up for the shortcoming of traditional fracture mechanics that can only deal with existing cracks.

[0059] This invention considers the plastic work-to-heat effect at the crack tip in the crack propagation criterion and constructs a local temperature field correction model. This feature fully considers the competition mechanism between low-temperature brittle fracture and local plastic heat generation toughening, making the fracture criterion more consistent with the material physical properties at extremely low temperatures, and further improving the scientific nature of life prediction. Attached Figure Description

[0060] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0061] Figure 1 This is a schematic flowchart of a method for predicting the initiation and propagation of cracks in a flexible LNG transportation pipeline according to the present invention. Detailed Implementation

[0062] The technical solution of the present invention will be further described in detail below through specific embodiments and in conjunction with the accompanying drawings. It should be understood that the implementation of the present invention is not limited to the following embodiments, and any modifications and / or alterations made to the present invention will fall within the protection scope of the present invention.

[0063] In this invention, unless otherwise specified, all parts and percentages are by weight, and the equipment and raw materials used are commercially available or commonly used in the art. Unless otherwise specified, the methods in the following embodiments are conventional methods in the art. Unless otherwise specified, the components or equipment in the following embodiments are general standard parts or components known to those skilled in the art, and their structures and principles can be learned by those skilled in the art through technical manuals or conventional experimental methods.

[0064] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In this detailed description, numerous specific details are set forth to facilitate explanation and provide a thorough understanding of the embodiments of the present invention. However, one or more embodiments may be practiced by those skilled in the art without these specific details.

[0065] like Figure 1 As shown in the figure, an embodiment of the present invention discloses a method for predicting the initiation and propagation of cracks in flexible LNG transportation pipelines. This method is based on the heat conduction equation, the elasticity equation, and the fracture mechanics equation, establishing a thermo-mechanical coupling theoretical model considering the combined effects of external cyclic loads such as waves and ocean currents, and low-temperature environmental heat sources. The specific implementation steps of this method are as follows.

[0066] S1. Construct a multi-layer composite geometric model of the LNG transportation flexible pipeline, and define the thermal and mechanical parameters of each layer material in the multi-layer composite geometric model.

[0067] A three-dimensional solid model of the LNG transportation flexible pipeline was established using finite element analysis software. The model structure includes a polymer inner lining, an internal pressure sealing layer, a stainless steel interlocking pressure-resistant armor layer, a tensile armor layer, and an outer protective layer.

[0068] The physical and mechanical parameters such as elastic modulus, Poisson's ratio, and density, as well as the thermal properties such as thermal conductivity, specific heat capacity, and coefficient of thermal expansion, are defined for each layer of material. Considering that LNG is transported at temperatures as low as -162℃, all material parameters are set as nonlinear functions that change with temperature.

[0069] S2. Pre-set initial cracks in the geometric model of the multi-layer composite structure, and establish a thermo-mechanical coupling control equation that includes the interface thermal resistance effect based on the interface thermal contact theory.

[0070] To accurately describe the heat transfer mechanism under different crack morphologies, this embodiment derives the corresponding temperature field control equations and boundary conditions for four typical crack states based on the Fourier integral transform method, and defines the interfacial thermal contact behavior in the finite element model accordingly:

[0071] 1. For undamaged interlayer interfaces, a continuous thermal contact model is defined that assumes continuous temperature and heat flow, i.e., a single-material internal crack analysis model under uniform heat flow load:

[0072] When a crack is located in a single material of a flexible tube and subjected to a uniform heat flow load, it can be analyzed using integral equations and Fourier integral transforms. The focus is on analyzing the influence of characteristic parameters of cracks inside a single material on the temperature and stress field distribution, stress intensity factor, and deformation under different functional layer materials and thicknesses.

[0073] Let T1 and T2 represent the temperature fields of the material regions on the upper and lower surfaces of a single material, respectively, and they satisfy the following differential equations of heat conduction:

[0074]

[0075] Considering the partial thermal insulation properties of the crack, the temperature field boundary conditions in the material can be expressed as follows:

[0076]

[0077] The general expression for the temperature field after crack perturbation is solved using the Fourier integral transform method:

[0078]

[0079] And the dimensionless expression for the temperature difference between the upper and lower surfaces of the crack along the crack direction is obtained:

[0080]

[0081] In the finite element model, this derivation is used to determine the local thermal resistance setpoint for surfaces that do not penetrate the crack.

[0082] 2. For fully opened crack interfaces, a jump-type thermal contact model is defined to represent heat flow through the gas medium in the crack gap via conduction or radiation, and a temperature step exists at the interface. This is the crack analysis model located near the interlayer interface:

[0083] When a crack is located near the interface of two interconnected isotropic plates, the effect of interfacial thermal resistance on the temperature changes of the upper and lower surfaces of the crack needs to be considered. Furthermore, the distance between the crack and the material interface also affects the crack's stress intensity factor, as well as its opening and slip displacement.

[0084] Temperature field equation:

[0085]

[0086] Assuming the crack is an adiabatic crack, meaning no heat flows through the crack, the temperature field boundary conditions in the material can be expressed as follows:

[0087]

[0088] The expression for the temperature field solution after crack perturbation can be derived using the Fourier integral transform method:

[0089]

[0090] and the temperature field along the crack direction

[0091]

[0092] In the finite element model, this model corresponds to jump thermal contact, which simulates the physical process in which heat flow at a fully opened crack must jump across the air gap or be transferred through radiation.

[0093] 3. For crack tips or closed crack regions, a local thermal contact model is defined, consisting of a microscopic contact thermal conduction region and a gap thermal resistance region, i.e., an interface crack analysis model located at the interface of composite materials:

[0094] At the interface of composite materials, factors such as the thermal insulation coefficient of the crack, the thermal resistance of the plastic region at the crack tip, and the interfacial thermal resistance all affect the stress intensity factor of the crack and the temperature change of the crack surface.

[0095] The boundary condition expression for the temperature field in this model is as follows:

[0096]

[0097] And the expressions for the temperature changes on the upper and lower surfaces of the crack:

[0098]

[0099] In the finite element model, this corresponds to the incomplete thermal contact model, which is used to simulate the debonding region between layers. The contact thermal conductivity is introduced as a function of the contact pressure for dynamic calculation.

[0100] 4. For the interlayer debonding region, an incomplete thermal contact model is defined and introduced, which uses contact thermal conductivity as a function of contact pressure. This model considers the interfacial crack analysis model including the plastic region at the tip.

[0101] For interfacial cracks containing a tip-type plastic region, under uniform heat flux loading, it is necessary to comprehensively consider factors such as the crack's adiabatic coefficient, the thermal resistance of the plastic region at the crack tip, and the interfacial thermal resistance. In this case, the length of the plastic region under force-heat coupling conditions can be solved using Fourier integral transform and simple iterative methods. The effects of heat flux loading, the adiabatic coefficient of the crack portion, and the thermal resistance of the plastic region on the length of the plastic region and the temperature change at the crack surface can then be discussed.

[0102] Considering the thermal resistance of the plastic region at the crack tip and the interface, as well as the thermal conductivity of the crack, the general expression for the temperature field under the condition of no interface crack is as follows:

[0103]

[0104] And the temperature difference between the upper and lower surfaces of the crack:

[0105]

[0106] In the finite element model, a local thermal contact model is defined, and an equivalent thermal resistance property that varies with plastic work is assigned to the mesh region at the crack tip.

[0107] S3. Apply internal cryogenic boundary conditions, external ambient temperature boundary conditions, and mechanical load boundary conditions based on the actual operating conditions of the LNG transportation flexible pipeline.

[0108] The thermal boundary conditions are: the LNG operating temperature is applied to the inner wall of the pipeline, and the seawater ambient temperature is applied to the outer wall.

[0109] The mechanical load boundary conditions are calculated based on the hydrodynamic model to determine the wave and ocean current cyclic loads. Meanwhile, considering the thermal shock of the gate and the water hammer effect, the dynamic pressure fluctuation load is applied to the inner wall of the pipe as a time-dependent function.

[0110] S4. Solve the thermo-mechanical coupling control equations to obtain the temperature field distribution of the LNG transportation flexible pipeline section, and apply the temperature field as a body load to the multi-layer composite structure geometric model to calculate the stress field and stress intensity factor at the crack tip.

[0111] Secondary development was performed using ANSYS APDL or User Defined Materials to integrate the four thermal contact models derived in step S2 into the finite element solver. During the solution process, a thermal shock hysteresis factor was introduced for dynamic conditions. Calculate the transient thermal stress components caused by thermal shock hysteresis. The calculation formula is:

[0112]

[0113] In the formula, E is the elastic modulus. The coefficient of thermal expansion is Poisson's ratio, The temperature of the inner wall of the LNG transport flexible pipeline. is the crack tip temperature after being delayed by interfacial thermal resistance, and is the thermal shock hysteresis factor.

[0114] The calculated temperature field is applied as a body load to the structural model, and combined with the mechanical load, the stress field and stress intensity factor at the crack tip are calculated.

[0115] S5. Introduce an interface thermal resistance correction coefficient to construct a crack propagation criterion that considers the interface thermal resistance effect.

[0116] 5.1 Dynamic updating of interfacial thermal resistance effect:

[0117] Initial surface roughness and material microhardness are set for the interlayer interfaces of the multilayer composite structure geometric model;

[0118] In each load analysis step, the stress-strain response history at the interface is extracted, and the cumulative fatigue damage is calculated.

[0119] Based on the cumulative fatigue damage, the dynamic roughness at the current cycle number is calculated using the roughness evolution function. The calculation formula is as follows:

[0120]

[0121] In the formula, For dynamic roughness, The initial surface roughness, Roughness evolution coefficient This refers to the cumulative amount of fatigue damage.

[0122] Extract the interfacial contact pressure of the current analysis step, and substitute the dynamic roughness into the Cooper-Mikic-Yovanovich contact theory model to calculate the real-time interfacial contact thermal conductivity. The calculation formula is as follows:

[0123]

[0124] In the formula, For real-time interface contact thermal conductivity, To harmonize the thermal conductivity of the interface, The average slope of the surface micro-protrusions. The effective root mean square roughness is related to dynamic roughness, where P is the interfacial contact pressure and H is the material microhardness.

[0125] The calculated real-time interfacial thermal conductivity is updated in the local thermal contact model or the incomplete thermal contact model as the thermal conduction boundary condition for the next incremental step, and the initiation location of microcracks is predicted based on the calculated local thermal resistance anomaly increase region.

[0126] For existing cracks, a dynamic correction using a jump-type thermal contact model is employed. The interfacial thermal resistance at the crack surface is defined as a dynamic function of the crack opening displacement, with the following expression:

[0127]

[0128] In the formula, The interfacial thermal resistance at the crack surface. For crack opening displacement, The thermal conductivity of the medium filling the crack. The emissivity of the crack surface.

[0129] As the crack propagates, the crack opening displacement... Increased, leading to local thermal resistance This increases, which in turn causes a redistribution of the temperature gradient at the crack tip.

[0130] 5.2 Construction Extension Criteria:

[0131] Define the equivalent thermodynamic stress intensity factor :

[0132] In the formula, The stress intensity factor generated by the mechanical load boundary conditions; The thermal stress intensity factor is caused by the local temperature difference due to the interfacial thermal resistance R, which is a function of the interfacial thermal resistance. This is the thermo-mechanical coupling weighting coefficient.

[0133] Simultaneously, considering the plastic work-to-heat effect at the crack tip during cyclic propagation, a modified model of the local temperature field at the crack tip is constructed:

[0134]

[0135] In the formula, This is the corrected local temperature. The temperature calculated considering only heat conduction, The coefficient of heat transfer from plastic work, The cyclic plastic work density at the crack tip. For density, Specific heat capacity;

[0136] Based on the corrected local temperature, the fracture toughness of the material is dynamically adjusted. When the local temperature increases due to plastic heat dissipation, causing a change in the local material toughness, the critical expansion threshold is updated in real time.

[0137] S6. Compare the stress intensity factor calculated in step S4 with the crack propagation criterion in step S5 to determine whether the crack has propagated. If it has propagated, update the crack geometry and the thermal resistance boundary conditions of the crack surface, and return to step S4. Repeat steps S4 to S6 until the LNG transport flexible pipeline fails or reaches the preset lifespan.

[0138] when At that time, it was determined that the crack had started to propagate. This refers to the fracture toughness of the material under low-temperature conditions.

[0139] The crack tip location is updated using the extended finite element method or mesh reconstruction technique. For the newly generated crack surface, its boundary conditions are modified from internal material heat conduction to a jump thermal contact model or a local thermal contact model, thereby blocking direct heat transfer in the next iteration.

[0140] S7. Damage assessment and life prediction.

[0141] Based on simulation data of the entire crack propagation process, the following damage assessment indices are calculated:

[0142] Crack characteristic parameter influence analysis: Analyze the sensitivity of crack depth and length to interfacial thermal resistance and stress concentration factor.

[0143] Temperature gradient rate of change index: characterizes the intensity of thermal flow contraction effect caused by crack propagation.

[0144] Residual strength evaluation: Circumferential bending strain, axial bending strain, and axial membrane strain are calculated at the damaged area of ​​the pipeline. The maximum strain values ​​of the inner and outer walls of the pipeline are taken as the strain values ​​at the dent. When the maximum strain at the damaged deformation point reaches a critical value, the pipeline is deemed to have failed, thus obtaining the predicted remaining life value.

[0145] The method described in this embodiment can reveal the relationship between the interfacial thermal resistance and crack distribution characteristics of multilayer materials in flexible pipelines and external cyclic loads and internal and external temperature differences, thereby improving the accuracy of life prediction for flexible pipelines used in deep-sea LNG transportation under extreme operating conditions.

[0146] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A method for predicting crack initiation and propagation in flexible LNG transportation pipelines, characterized in that: The method includes the following steps: S1. Construct a multi-layer composite structure geometric model of the LNG transportation flexible pipeline, and define the thermal and mechanical parameters of each layer material in the multi-layer composite structure geometric model; S2. Pre-set initial cracks in the geometric model of the multi-layer composite structure, and establish a thermo-mechanical coupling control equation that includes the interface thermal resistance effect based on the interface thermal contact theory. S3. Based on the actual operating conditions of the LNG transportation flexible pipeline, apply internal cryogenic boundary conditions, external ambient temperature boundary conditions, and mechanical load boundary conditions. S4. Solve the thermo-mechanical coupling control equation to obtain the temperature field distribution of the LNG transportation flexible pipeline section, and apply the temperature field as a body load to the geometric model of the multi-layer composite structure to calculate the stress field and stress intensity factor at the crack tip. S5. Introduce an interface thermal resistance correction coefficient to construct a crack propagation criterion that considers the interface thermal resistance effect. S6. Compare the stress intensity factor calculated in step S4 with the crack propagation criterion in step S5 to determine whether the crack has propagated. If it has propagated, update the crack geometry and the thermal resistance boundary conditions of the crack surface, and return to step S4. Repeat steps S4 to S6 until the LNG transport flexible pipeline fails or reaches the preset lifespan.

2. The method for predicting crack initiation and propagation in a flexible LNG transportation pipeline according to claim 1, characterized in that: In step S2, establishing the thermo-mechanical coupling control equations that include the interfacial thermal resistance effect specifically includes defining the following four interfacial thermal contact models to describe the interlayer heat conduction behavior under different crack states: For undamaged interlayer interfaces, a continuous thermal contact model is defined that assumes that temperature and heat flow remain continuous. For a fully open crack interface, a jump-type thermal contact model is defined that reflects the heat flow through the gas medium in the crack gap by conduction or radiation and that there is a temperature step at the interface. For crack tips or closed crack regions, a local thermal contact model is defined, which is composed of a microscopic contact thermal conduction region and a gap thermal resistance region. For the debonding region between layers, an incomplete thermal contact model is defined and introduced, which uses contact thermal conductivity as a function of contact pressure.

3. The method for predicting crack initiation and propagation in a flexible LNG transportation pipeline according to claim 2, characterized in that: In the aforementioned jump-type thermal contact model, the interfacial thermal resistance at the crack surface is defined as a dynamic function of the crack opening displacement, and its functional expression is: ， In the formula, The interfacial thermal resistance at the crack surface. For crack opening displacement, The thermal conductivity of the medium filling the crack. The emissivity of the crack surface.

4. The method for predicting crack initiation and propagation in a flexible LNG transportation pipeline according to claim 2, characterized in that: In steps S2 and S5, the calculation of the interface thermal resistance effect specifically includes: Initial surface roughness and material microhardness are set for the interlayer interfaces of the multilayer composite structure geometric model; In each load analysis step, the stress-strain response history at the interface is extracted, and the cumulative fatigue damage is calculated. Based on the accumulated fatigue damage, the dynamic roughness at the current cycle number is calculated using the roughness evolution function. The calculation formula is as follows: ， In the formula, For dynamic roughness, The initial surface roughness, Roughness evolution coefficient This refers to the cumulative amount of fatigue damage. Extract the interfacial contact pressure of the current analysis step, and substitute the dynamic roughness into the Cooper-Mikic-Yovanovich contact theory model to calculate the real-time interfacial contact thermal conductivity. The calculation formula is as follows: ， In the formula, For real-time interface contact thermal conductivity, To harmonize the thermal conductivity of the interface, The average slope of the surface micro-protrusions. The effective root mean square roughness is related to dynamic roughness, where P is the interfacial contact pressure and H is the material microhardness. The calculated real-time interface contact thermal conductivity is updated in the local thermal contact model or the incomplete thermal contact model as the thermal conduction boundary condition for the next incremental step, and the initiation location of microcracks is predicted based on the calculated local thermal resistance anomaly increase region.

5. The method for predicting crack initiation and propagation in a flexible LNG transportation pipeline according to claim 2, characterized in that: In step S6, updating the crack geometry and crack surface thermal resistance boundary conditions specifically includes: The location of the crack tip is updated using the extended finite element method or mesh reconstruction technology; For newly generated cracked surfaces, their boundary conditions are modified from internal material heat conduction to a jump thermal contact model or a local thermal contact model, thereby blocking direct heat transfer in the next iteration.

6. The method for predicting crack initiation and propagation in a flexible LNG transportation pipeline according to claim 1, characterized in that: In step S5, constructing a crack propagation criterion that considers the interfacial thermal resistance effect specifically includes: Define the equivalent thermodynamic stress intensity factor : , In the formula, The stress intensity factor generated by the mechanical load boundary conditions; The thermal stress intensity factor is caused by the local temperature difference due to the interfacial thermal resistance R, which is a function of the interfacial thermal resistance. These are the thermo-mechanical coupling weighting coefficients; when At that time, it was determined that the crack had started to propagate. This refers to the fracture toughness of the material under low-temperature conditions.

7. The method for predicting crack initiation and propagation in a flexible LNG transportation pipeline according to claim 6, characterized in that: The construction of crack propagation criteria that takes into account interfacial thermal resistance effects also includes the following steps for correcting heat dissipation in the plastic zone at the crack tip: Considering the plastic work-to-heat effect at the crack tip during cyclic propagation, a modified model of the local temperature field at the crack tip is constructed: ， In the formula, This is the corrected local temperature. The temperature calculated considering only heat conduction, The coefficient of heat transfer from plastic work, The cyclic plastic work density at the crack tip. For density, Specific heat capacity; Based on the corrected local temperature, the fracture toughness of the material is dynamically adjusted. When the local temperature increases due to plastic heat dissipation, causing a change in the local material toughness, the critical expansion threshold is updated in real time.

8. The method for predicting crack initiation and propagation in a flexible LNG transportation pipeline according to claim 1, characterized in that: When the actual operating condition of the LNG transportation flexible pipeline is a combination of thermal shock and water hammer caused by the switching on / off mechanism, the temperature field solution in step S4 adopts the following dynamic thermal-fluid-structure interaction algorithm: A thermal shock hysteresis factor is defined to characterize the hysteresis effect of the internal temperature response of the LNG transport flexible pipeline wall relative to the fluid temperature change due to the presence of interfacial thermal resistance. When calculating the dynamic stress intensity factor, the transient thermal stress component caused by thermal shock hysteresis is superimposed. The calculation formula is: ， In the formula, E is the elastic modulus. The coefficient of thermal expansion is Poisson's ratio, The temperature of the inner wall of the LNG transport flexible pipeline. This refers to the crack tip temperature after being hysteresis due to interfacial thermal resistance. It is the thermal shock hysteresis factor.

9. The method for predicting crack initiation and propagation in a flexible LNG transportation pipeline according to claim 1, characterized in that: In step S3, the mechanical load boundary conditions include wave and ocean current cyclic loads calculated based on the hydrodynamic model, and dynamic pressure fluctuation loads generated by the thermal shock of the gate and water hammer effect. During the calculation process, the dynamic pressure fluctuation loads are applied as a time-dependent function to the inner wall of the LNG transport flexible pipeline.

10. The method for predicting crack initiation and propagation in a flexible LNG transportation pipeline according to claim 1, characterized in that: The method also includes defining and calculating the following damage assessment indices based on a full-process crack propagation simulation: crack length growth rate, temperature gradient change rate index, and remaining life prediction.

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