Method for evaluating structural stress of outer tank of LNG (liquefied natural gas) storage tank in extreme weather based on thermosetting coupling

Abstract

The invention relates to the field of petroleum and natural gas engineering, in particular to an LNG storage tank outer tank structure stress evaluation method under extreme weather based on thermosetting coupling. The method comprises the steps that S1, a 3D finite element model of the LNG storage tank is established; s2, setting material attributes and sizes of all the parts, and assembling the parts; s3, setting two continuous analysis steps; s4, establishing an interaction relationship among different components; s5, carrying out grid division, and carrying out encryption processing on the outer tank; s6, setting loads and boundary conditions, wherein the loads comprise gravity, wind load, snow load, hydrostatic pressure and temperature field stress; s7, performing thermal analysis and stress field analysis in sequence to obtain a stress distribution cloud picture of the outer tank; and S8, extracting the stress value of the maximum stress position of the stress distribution cloud picture, comparing allowable stress, and judging the reliability of the outer tank under the extreme wind and snow working condition. According to the method, the structural stress of the outer tank of the LNG storage tank in the extreme windy and snowy weather can be well evaluated, and the safety and reliability of the LNG storage tank are guaranteed.

Description

Technical Field

[0001] The present invention relates to the field of petroleum and natural gas engineering, and in particular to a stress assessment method for an outer tank structure of an LNG storage tank under extreme weather conditions based on thermo-solid coupling. Background Art

[0002] LNG storage tanks are special equipment for storing liquid natural gas. In terms of operation, they have the characteristic of low working environment temperature (-165℃). In terms of structure, LNG storage tanks are double-layer structures with pile foundations on the lower surface of the pedestal.

[0003] LNG storage tanks are usually operated outdoors, so when encountering extreme snow and wind weather, the outer tank of the LNG storage tank is easily accumulated with snow and affected by extreme wind loads. For LNG storage tanks, the stress state of the outer tank in extreme snow and wind weather is related to the safe operation of the entire storage tank. Considering the volatility, flammability and explosiveness of LNG, once an accident occurs, it will cause great safety hazards and human and economic losses.

[0004] It can be seen that it is of practical significance to evaluate the structural stress of the outer tank of the LNG storage tank under extreme wind and snow weather. Therefore, it is urgent to design a method for evaluating the structural stress of the outer tank of the LNG storage tank under extreme weather based on thermal-solid coupling. Summary of the invention

[0005] The technical problem to be solved by the present invention is to overcome the defects of the prior art and provide a method for evaluating the structural stress of the outer tank of an LNG storage tank under extreme weather conditions based on thermo-solid coupling, which can well perform structural stress evaluation on the outer tank of the LNG storage tank under extreme wind and snow weather to ensure the safety and reliability of the LNG storage tank.

[0006] In order to solve the above technical problems, the technical solution of the present invention is: a method for evaluating the stress of the outer tank structure of an LNG storage tank under extreme weather conditions based on thermo-solid coupling, the method comprising:

[0007] Step S1: Establishing a 3D finite element model of the LNG storage tank;

[0008] Step S2, setting the material properties and dimensions of each component in the 3D finite element model and assembling them;

[0009] Step S3, setting two consecutive analysis steps Step-1 and Step-2, the analysis step Step-1 is set to inherit the initial analysis state, the step is set to general statics, and the stress analysis of the outer tank of the LNG storage tank under normal working conditions is performed; the analysis step Step-2 inherits the analysis results of Step-1, the step is set to general statics, and on this basis, the field output of the temperature field is added;

[0010] Step S4, establishing the interaction relationship between different components in the 3D finite element model according to the connection mode between the various components of the LNG storage tank;

[0011] Step S5, meshing the 3D finite element model and encrypting the outer tank in the 3D finite element model;

[0012] Step S6, setting loads and boundary conditions, where the loads include gravity, wind load, snow load, hydrostatic pressure, and temperature field stress;

[0013] Step S7, first perform thermal analysis, then bring the obtained temperature field stress into the 3D finite element model to perform stress field analysis, run finite element numerical calculation of ABAQUS, and obtain the stress distribution cloud map of the outer tank of the LNG storage tank under extreme weather conditions;

[0014] Step S8, extracting the stress value at the maximum stress position of the stress distribution cloud map, comparing the extracted stress value with the allowable stress, and evaluating the reliability of the outer tank of the LNG storage tank under extreme wind and snow conditions.

[0015] Further, step S1 specifically includes:

[0016] The inner tank, outer tank, bottom plate and dome of the LNG storage tank are rotated and modeled using shell structure;

[0017] The solid structure is used to perform rotational modeling on the cap and pile group foundation of the LNG storage tank;

[0018] After the inner tank is modeled, the split surface function is used to split the inner tank part into a 10-layer component structure.

[0019] Further, step S4 specifically includes:

[0020] The lowest layer of the inner tank structure is connected to the bottom plate by iron binding; the outer tank is connected to the bottom plate by iron binding; the outer tank is connected to the arch by iron binding; the lower surface of the bottom plate is in surface contact with the upper surface of the cap, and the interaction relationship is a friction relationship; the pile group foundation is connected to the lower surface of the cap by iron binding.

[0021] Further, in step S6, the wind load is not uniformly distributed, and the method for obtaining the distribution formula is:

[0022] The standard values ​​of wind loads at multiple locations are calculated according to the definition formula of wind loads; wherein the definition formula of wind loads is:

[0023] w k =β z μ s μ z w0

[0024] In the formula, wk is the standard value of wind load; β z is the wind vibration coefficient at height z, μ s is the wind load shape coefficient; μ z is the wind pressure height variation coefficient; w0 is the basic wind pressure (kN / m2);

[0025] The standard values ​​of wind loads at multiple locations are brought into the discrete field and the ABAQUS calculation formula is used to calculate the external tank wind load distribution formula of the LNG storage tank.

[0026] Further, in step S6, the method for obtaining the distribution formula of the snow load is:

[0027] The snow load data of multiple locations are calculated according to the snow load definition formula; wherein the snow load definition formula is:

[0028] μ r =l / (8f)

[0029] In the formula, μ r is the snow load data, l is the diameter of the bottom surface of the arch; f is the height of the point relative to the bottom surface of the arch;

[0030] The snow load data of multiple locations are brought into the discrete field and the snow load distribution formula of the dome of the LNG storage tank is calculated using the ABAQUS calculation formula.

[0031] After adopting the above technical scheme, the present invention performs finite element stress analysis on the outer tank components of the LNG storage tank that will be subjected to wind loads and snow loads based on simulation, and can evaluate the reliability of the outer tank at the beginning of design and construction, thereby improving the safety and reliability of the outer tank of the LNG storage tank and even the entire storage tank. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a flow chart of the stress assessment method of the outer tank structure of the LNG storage tank under extreme weather conditions based on thermal-solid coupling of the present invention;

[0033] Figure 2 It is a 3D modeling assembly drawing of the tank body of the LNG storage tank of the present invention;

[0034] Figure 3 A 3D modeling diagram of a pile group foundation of an LNG storage tank of the present invention;

[0035] Figure 4 The load and boundary condition distribution diagram of the LNG storage tank under extreme weather conditions;

[0036] Figure 5 This is the equivalent stress cloud diagram of the outer tank wall of the LNG storage tank under extreme weather conditions;

[0037] Figure 6The hoop stress cloud diagram of the outer tank wall of the LNG storage tank under extreme weather conditions

[0038] Figure 7 This is the axial stress cloud diagram of the outer tank wall of the LNG storage tank under extreme weather conditions;

[0039] Figure 8 It is the equivalent stress curve of all unit points on one of the Z-direction paths of the outer tank wall of the LNG storage tank under extreme weather conditions;

[0040] Fig. 9 It is the axial stress curve of all unit points on one of the Z-direction paths of the outer tank wall of the LNG storage tank under extreme weather conditions;

[0041] Fig.10 It is the hoop stress curve of all unit points along one of the Z-direction paths of the outer tank wall of the LNG storage tank under extreme weather. DETAILED DESCRIPTION

[0042] In order to make the contents of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments in conjunction with the accompanying drawings.

[0043] like Figure 1 As shown, a stress assessment method for the outer tank structure of an LNG storage tank under extreme weather conditions based on thermo-solid coupling is provided, the method comprising:

[0044] Step S1: Establishing a 3D finite element model of the LNG storage tank;

[0045] 200,000 m 3 Taking the LNG storage tank as an example, the inner tank, outer tank, bottom plate and arch of the LNG storage tank are rotated by using shell structure; the pedestal and pile group foundation of the LNG storage tank are rotated by using solid structure; after the inner tank is modeled, the split surface function is used to split the inner tank part into 10 layers of component structures in the height direction, so as to omit the subsequent use of Tie binding for each layer of the inner tank wall and improve the reliability of the model. It should be noted that the inner tank of the LNG storage tank is a shell structure with a thickness that gradually decreases from bottom to top.

[0046] Step S2, set the material properties of each component in the 3D finite element model, assign the corresponding mass density, Young's modulus, expansion coefficient, Poisson's ratio, yield strength, heat capacity, thermal conductivity and other related material parameters to each component, and set the corresponding thickness for each component. The focus is on the thermodynamic properties of the outer tank, as follows:

[0047] temperature Thermal diffusivity Specific heat density Thermal conductivity -125 3.801 0.344 8.383 10.961 -100 3.577 0.374 8.377 11.207 -50 3.405 0.411 8.363 11.704 0 3.397 0.436 8.348 12.364 25 3.428 0.444 8.340 12.694 50 3.519 0.453 8.332 13.283 100 3.653 0.467 8.315 14.184

[0048] Step S3, after assembling the components with assigned properties, it is necessary to set up the analysis steps for the subsequent mechanical property analysis. Considering that this embodiment needs to analyze the state of the LNG storage tank under the coupling of wind load and snow load, and also involves thermo-solid coupling, two consecutive analysis steps are set to perform relevant analysis. Analysis step Step-1 is set to inherit the initial analysis state, and the step is set to general statics. Under this analysis step, the stress analysis of the outer tank of the LNG storage tank under normal working conditions will be performed; analysis step Step-2 inherits the analysis results of Step-1, and the step is set to general statics. On the basis of the display of general statics, the field output of the temperature field is added. This is to substitute the temperature field data here as known conditions into the calculation of the mechanical field when performing subsequent temperature field coupling calculations.

[0049] Step S4, establishing the interaction relationship between different components in the 3D finite element model according to the connection mode between the various components of the LNG storage tank;

[0050] Specifically, since the modeling of the inner tank component uses the surface of the cut component to classify the different thicknesses of the inner tank layer, it is no longer necessary to perform Tie binding between each inner tank layer in this step. This link requires the establishment of a Tie binding connection between the bottom layer of the inner tank component and the bottom plate; Tie binding connection between the outer tank and the bottom plate; Tie binding between the outer tank and the arch; surface-to-surface contact between the lower surface of the bottom plate and the upper surface of the cap, the interaction relationship is a friction relationship, and the friction coefficient is set to 0.5; Tie binding between the pile group and the lower surface of the cap.

[0051] Step S5, meshing the 3D finite element model. At this time, because the outer tank of the LNG storage tank is the main research object, the mesh of the outer tank of the LNG storage tank is encrypted. The mesh unit of the base component is selected as C3D8R, and the mesh unit of the other shell structure components is selected as S4R.

[0052] Step S6, setting loads and boundary conditions, where the loads include gravity, wind load, snow load, hydrostatic pressure, and temperature field stress.

[0053] For the tank’s own weight, the acceleration components in the x and y directions are set to 0, and the component in the z direction is set to -9.8 m / s. 2 , the entire model is selected as the action range. In the hydrostatic pressure setting, the maximum hydraulic pressure is at the bottom plate, which is set to 196500Pa. The area selects the inner tank wall and bottom plate of the LNG storage tank, and the liquid level height is set to 40m. The wind load setting position is the outer tank wall of the LNG storage tank. The snow load action position is the top of the LNG storage tank.

[0054] Gravity and hydrostatic pressure are not modified in either analysis step and the values ​​in Step-1 are maintained.

[0055] In terms of wind load, the wind load is a non-uniformly distributed load. The distribution formula is obtained as follows:

[0056] The standard values ​​of wind loads at multiple locations are calculated according to the definition formula of wind loads; the definition formula of wind loads (refer to GB 50009-2012 Code for Loads on Building Structures) is:

[0057] w k =β z μ s μ z w0

[0058] In the formula, w k is the standard value of wind load; β z is the wind vibration coefficient at height z. According to the concept in GB 50009-2012 Code for Loads on Building Structures, it can be calculated as 100-10000m 3 The wind vibration coefficient of LNG storage tanks is generally 0.8; μ s is the wind load shape coefficient. Considering that the LNG storage tank is a closed structure, μ s The value is 0.8; μ z is the wind pressure height variation coefficient, which is generally 1.0 at 10m; w0 is the basic wind pressure (kN / m 2 ).

[0059] The wind load standard values ​​at multiple locations are brought into the discrete field and the wind load distribution formula of the outer tank of the LNG storage tank is calculated using the ABAQUS calculation formula. The wind load action position is selected as the direction of the outer tank wall of the LNG storage tank, and the load coefficient is set to 1.

[0060] In terms of snow load, the distribution formula of snow load is obtained as follows:

[0061] The snow load data of multiple locations are calculated according to the snow load definition formula; the snow load definition formula (refer to GB 50009-2012 Code for Loads on Building Structures) is:

[0062] μ r =l / (8f)(0.4≤μ r ≤1.0)

[0063] In the formula, μ r is the snow load data, l is the diameter of the bottom surface of the arch, and f is the height of the point relative to the bottom surface of the arch.

[0064] The snow load data of multiple locations are brought into the discrete field and the snow load distribution formula of the LNG tank dome is calculated using the ABAQUS calculation formula. The snow load action position is set to the LNG tank dome, and the load coefficient is set to 1.

[0065] It should be noted that extreme weather generally needs to consider local meteorological conditions. Extreme wind weather is generally manifested as severe weather conditions such as hurricanes, typhoons, and strong winds caused by extreme air pressure changes; extreme snow weather is generally manifested as blizzards, heavy snowfall accumulation, low temperatures, and wind chill effects. The extreme weather considered in this embodiment is the weather condition where extreme wind weather and extreme snow weather occur at the same time, that is, extreme wind and snow weather, which is generally manifested as blizzards and sleet. Under such extreme weather working conditions, the wind load and snow load on the LNG storage tank will be very different from usual. Therefore, stress analysis of LNG storage tanks under extreme weather conditions is crucial.

[0066] Generally speaking, when building in a certain place, it is necessary to consider the worst weather conditions that the local area has experienced in the past 50-100 years. The worst weather conditions in the past 50-100 years are used to determine the two key physical quantities, basic wind pressure and basic snow pressure. Among them, the basic wind pressure is not only used in the calculation formula of the standard value of wind load, but also in the ABAQUS calculation formula brought into the discrete field by the standard value of wind load at multiple locations. The basic snow pressure is applied to the ABAQUS calculation formula brought into the discrete field by the snow load data at multiple locations.

[0067] The extreme weather conditions considered in this embodiment are to take the largest wind load and snow load in the past 100 years as the basic wind pressure and basic snow load. In this embodiment, by measuring the wind pressure and snow pressure in the distribution area of ​​domestic LNG storage tanks, the basic wind pressure under extreme weather conditions is 0.69kN / m 2 , basic snow pressure is 0.24kN / m 2 .

[0068] In terms of boundary conditions, select the displacement / rotation module in the Intial step, set full constraints of U1, U2, U3, UR1, UR2, and UR3 (that is, the displacement and angular displacement in the x, y, and z directions are 0, among which U1, U2, and U3 represent the displacement in the x, y, and z directions respectively, and UR1, UR2, and UR3 represent the angular displacement in the x, y, and z directions respectively), and select the lower surface of the pedestal as the area; the boundary conditions remain unchanged in the Step-1 and Step-2 analysis steps, so the data is not modified.

[0069] In order to improve the accuracy of the data, this patent will use the local wind and snow load distribution data within one year, and use the physical information neural network (PINN) to perform data fitting. The specific operation steps are as follows:

[0070] (1) Define the physical problem and the corresponding physical laws: Taking into account the particularity of wind loads, the Navier-Stokes equation is added to the calculation of standard Newtonian statics (for snow loads). This is the key equation for solving fluid mechanics (for wind loads) problems.

[0071] (2) Constructing a neural network

[0072] (3) Define the loss function: It mainly includes two aspects. The first is the data error term, which is used to measure the difference between the network prediction output and the actual observation data, with the aim of making the network fit the data as closely as possible. In this example, the mean square error will be used as the data error term. The second is the physical information error term, which is a unique error term of PINN. It considers whether the network prediction results meet the prescribed physical laws. The residual calculated by substituting the network prediction results into the corresponding physical laws (differential equations in this example) constitutes the loss function of this part to ensure physical consistency. Taking the loss function of snow load as an example, the heat conduction equation is:

[0073]

[0074] Where u(x,t) is the temperature distribution and α is the thermal diffusion coefficient.

[0075] The final loss function expression is:

[0076] L=λ PDE L PDE +λ data L data

[0077] Among them, λ PDE and λ data It is a hyperparameter that measures the importance of the error term. Through appropriate hyperparameters, it can be ensured that the predicted physical quantity is within the range allowed by physical laws.

[0078] (4) Network training: The wind and snow load distribution data of the local area in one year is used as the training set, and the network weights are adjusted using the gradient descent optimization algorithm to minimize the overall loss function in order to achieve data fitting and compliance with physical laws.

[0079] (5) Verify the neural network model to ensure that the model can make accurate and physically accurate predictions on data outside the training set.

[0080] Based on the above trained neural network model, on the one hand, after it is put into normal use in the later stage, the wind and snow loads in the future preset time period can be predicted, so as to evaluate whether the outer tank of the LNG storage tank can operate safely in the future preset time period; on the other hand, it can be used to determine the unknown coefficients in the wind and snow load formula, such as recalculating the standard value of the wind and snow load, wind vibration coefficient, wind load shape coefficient and other data in the wind and snow load according to the local information provided, so as to adapt to different actual working conditions, so as to accurately evaluate the safety of the outer tank of the LNG storage tank.

[0081] Step S7, considering that the temperature field has a great influence on the stress field, while the stress field has almost no influence on the temperature field, when performing stress analysis under extreme weather conditions, it is necessary to give priority to thermal analysis, and then bring the obtained temperature field stress into the finite element model of the LNG storage tank for stress field analysis. In the material properties, add the Young's modulus and expansion coefficient that change with temperature for each material; add heat capacity and thermal conductivity parameters to the material properties in step 2; then calculate according to the load setting in step 6, and inherit the calculation results to Step-2 to participate in the subsequent stress analysis of the outer tank wall of the LNG storage tank under extreme weather conditions, and obtain the stress distribution cloud map of the outer tank of the LNG storage tank under extreme weather conditions;

[0082] Step S8, extracting the stress value at the maximum stress position of the stress distribution cloud map, comparing the extracted stress value with the allowable stress, and evaluating the reliability of the outer tank of the LNG storage tank under extreme wind and snow conditions.

[0083] in, Figure 5 , Figure 6 and Figure 7 The equivalent, circumferential and axial stress cloud maps of the outer tank wall of the LNG storage tank under extreme weather conditions are respectively extracted. The equivalent stress value in the maximum stress area of ​​the equivalent stress cloud map, the circumferential stress value in the maximum stress area of ​​the circumferential stress cloud map, and the axial stress value in the maximum stress area of ​​the axial stress cloud map are respectively extracted, and then compared with the corresponding allowable stresses. If any one of them exceeds the allowable stress, the LNG storage tank is a risk tank.

[0084] Among them, for the allowable stress of each component material of the LNG storage tank, please refer to Table 1. It is known that the outer tank material of the LNG storage tank is basically C50 concrete. According to the data in Tables 1 and 2, the extracted stress value is compared with the corresponding allowable stress, and the reliability of the LNG storage tank under normal working conditions can be obtained. The maximum circumferential stress of the outer tank of the LNG storage tank implemented in the present invention is 27.13MPa, the maximum axial stress is 7.703MPa, and the maximum equivalent stress is 27.13MPa, which are all lower than the standard value of the compressive strength of concrete C50 of 32.4MPa. Therefore, it can be concluded that the outer tank of the LNG storage tank can operate safely under the working conditions of the coupling of the design-allowed extreme wind load and snow load.

[0085] Table 1 Standard values ​​of concrete axial compressive strength (N / mm 2 )

[0086]

[0087] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A method for evaluating the stress of the outer tank structure of an LNG storage tank under extreme weather conditions based on thermo-solid coupling, characterized in that: Methods include: Step S1: Establishing a 3D finite element model of the LNG storage tank; Step S2, setting the material properties and dimensions of each component in the 3D finite element model and assembling them; Step S3, setting two consecutive analysis steps Step-1 and Step-2, the analysis step Step-1 is set to inherit the initial analysis state, the step is set to general statics, and the stress analysis of the outer tank of the LNG storage tank under normal working conditions is performed; the analysis step Step-2 inherits the analysis results of Step-1, the step is set to general statics, and on the basis of the general statics setting, the field output of the temperature field is added; Step S4, establishing the interaction relationship between different components in the 3D finite element model according to the connection mode between the various components of the LNG storage tank; Step S5, meshing the 3D finite element model and encrypting the outer tank in the 3D finite element model; Step S6, setting loads and boundary conditions, where the loads include gravity, wind load, snow load, hydrostatic pressure, and temperature field stress; Step S7, first perform thermal analysis, then bring the obtained temperature field stress into the 3D finite element model to perform stress field analysis, run finite element numerical calculation of ABAQUS, and obtain the stress distribution cloud map of the outer tank of the LNG storage tank under extreme weather conditions; Step S8, extracting the stress value at the maximum stress position of the stress distribution cloud map, comparing the extracted stress value with the allowable stress, and evaluating the reliability of the outer tank of the LNG storage tank under extreme wind and snow conditions.

2. The stress assessment method for the outer tank structure of an LNG storage tank under extreme weather conditions based on thermo-solid coupling according to claim 1 is characterized in that: Step S1 specifically includes: The inner tank, outer tank, bottom plate and dome of the LNG storage tank are rotated and modeled using shell structure; The solid structure is used to perform rotational modeling on the cap and pile group foundation of the LNG storage tank; After the inner tank is modeled, the split surface function is used to split the inner tank part into a 10-layer component structure.

3. The method for evaluating the stress of the outer tank structure of an LNG storage tank under extreme weather conditions based on thermo-solid coupling according to claim 2 is characterized in that: Step S4 specifically includes: The lowest layer of the inner tank component is connected to the bottom plate by iron binding; the outer tank is connected to the bottom plate by iron binding; the outer tank is connected to the arch by iron binding; the lower surface of the bottom plate is in surface contact with the upper surface of the cap, and the interaction relationship is a friction relationship; the pile group foundation is connected to the lower surface of the cap by iron binding.

4. The method for evaluating the stress of the outer tank structure of an LNG storage tank under extreme weather conditions based on thermo-solid coupling according to claim 1 is characterized in that: In step S6, the wind load is not uniformly distributed, and the distribution formula is obtained as follows: The standard values ​​of wind loads at multiple locations are calculated according to the definition formula of wind loads; wherein the definition formula of wind loads is: w k =b z m s m z w0 In the formula, w k is the standard value of wind load; β z is the wind vibration coefficient at height z, μ s is the wind load shape coefficient; μ z is the wind pressure height variation coefficient; w0 is the basic wind pressure (kN / m2); The standard values ​​of wind loads at multiple locations are brought into the discrete field and the ABAQUS calculation formula is used to calculate the external tank wind load distribution formula of the LNG storage tank.

5. The method for evaluating the stress of the outer tank structure of an LNG storage tank under extreme weather conditions based on thermo-solid coupling according to claim 4 is characterized in that: In step S6, the method for obtaining the distribution formula of the snow load is: The snow load data of multiple locations are calculated according to the snow load definition formula; wherein the snow load definition formula is: μ r =l / (8f), where μ r is the snow load data, l is the diameter of the bottom surface of the arch; f is the height of the point relative to the bottom surface of the arch; The snow load data of multiple locations are brought into the discrete field and the snow load distribution formula of the dome of the LNG storage tank is calculated using the ABAQUS calculation formula.

Images