Simulation method for acquiring ellipticity of LNG (liquefied natural gas) storage tank based on sedimentation and wind load coupling

Abstract

The invention relates to a deformation evaluation method of a storage tank under the action of multiple loads, in particular to a method for obtaining the ellipticity of an LNG storage tank based on simulation of sedimentation and wind load coupling. The method comprises the steps that modeling is conducted on all parts of the LNG storage tank, a model of the LNG storage tank is obtained, corresponding material attributes and sizes are set for all the parts of the model, and meshing is conducted on the model; boundary conditions and operation loads are applied to the model, and a radial displacement distribution cloud picture of the tank wall of the outer tank of the LNG storage tank is obtained through finite element solution; wherein the load comprises a self-weight load of the tank body, a gravity load of liquid in the tank, a settlement load and a wind load; the radial displacement of the position where the maximum radial displacement of the radial displacement distribution cloud picture is located in the circumferential direction is obtained, the ovality is calculated, and the ovality of the position where the maximum radial displacement of the LNG storage tank is located is obtained. According to the invention, the ellipticity of the LNG storage tank under multiple loads can be obtained in a simulation mode, so that a worker can judge whether the LNG can safely operate under the multi-load coupling effect or not.

Description

Technical Field

[0001] The invention relates to a method for evaluating the deformation of a storage tank under multiple loads, and in particular to a method for obtaining the ovality of an LNG storage tank based on a simulation of the coupling of settlement and wind load. Background Art

[0002] LNG storage tanks are liquefied natural gas storage equipment used to store low-temperature liquefied natural gas. The types of LNG storage tanks include single-capacity storage tanks, double-capacity storage tanks and full-capacity storage tanks. The object to be studied in this paper is 200,000 m 3 Full-capacity LNG storage tanks are different from traditional crude oil storage tanks in two aspects: in terms of operation, the tank is loaded with liquefied natural gas, and the operating temperature is -165°C; in terms of structure, the tank is a double-layer structure, with an insulation layer filled between the inner tank and the outer tank, and a pile group foundation connected to the lower surface of the foundation. LNG storage tanks are an important part of the LNG transportation and storage process.

[0003] At present, the technical research on LNG tanks under multi-load coupling conditions is not comprehensive enough, and the operating conditions of the tanks are worrying. The deformation analysis of LNG tanks under multi-load coupling conditions is an aspect that needs in-depth research. From a safety perspective, due to the particularity of the storage medium of LNG tanks, the safe operation of in-service tanks must be ranked first. Once a leak or other accident occurs, it will cause great harm to the safety of operators and the surrounding environment; from an economic perspective, once an LNG tank leaks, it is not only the loss of the medium in the tank, but the damage to the ecological environment in the surrounding area is difficult to measure.

[0004] Considering that the working environment of LNG storage tanks is generally an open-air environment, when encountering strong winds, the outer tank of the LNG storage tank will be affected by a large wind load. At this time, if there is an uneven settlement state, the coupling of the two loads may cause damage to the outer tank structure of the LNG storage tank. Therefore, it is of clear practical significance to conduct deformation analysis of the outer tank of the LNG storage tank under the coupling of settlement and wind load. 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 obtaining the ovality of an LNG storage tank based on simulation of sedimentation and wind load coupling. The method can obtain the ovality of the LNG storage tank under multiple loads through simulation, which is helpful for staff to judge whether the LNG can operate safely under the coupling of multiple loads.

[0006] In order to solve the above technical problems, the technical solution of the present invention is: a method for obtaining the ovality of an LNG storage tank based on simulation of the coupling of settlement and wind load, the method comprising:

[0007] Modeling various components of the LNG storage tank to obtain a model of the LNG storage tank, setting corresponding material properties and dimensions for various components of the model, and meshing the model;

[0008] Apply boundary conditions and operating loads to the model, and obtain the radial displacement distribution cloud diagram of the outer tank wall of the LNG storage tank through finite element solution; the loads include the deadweight load of the tank, the gravity load of the liquid in the tank, the settlement load and the wind load;

[0009] The possible area of ​​the maximum radial displacement in the radial displacement distribution cloud map is selected, and the radial displacement is extracted along the annular path at preset intervals in the axial direction. By comparison, the annular path where the maximum radial displacement is located is selected, and the ellipticity is calculated to obtain the ellipticity at the maximum radial displacement of the LNG storage tank.

[0010] Further, the steps for obtaining the settlement load are:

[0011] The settlement data of each pile in the pile group foundation is obtained, and the settlement distribution formula of the lower surface of the entire LNG tank cap is obtained based on the least squares surface fitting method of singular value decomposition, which is used as the settlement load.

[0012] Furthermore, the settlement distribution formula of the lower surface of the entire LNG tank cap is obtained by the least squares surface fitting method based on singular value decomposition; specifically:

[0013] For the selected single pile settlement data point, there are three data (x, y, z), where x and y are the x and y coordinates of the pile center point; z is the settlement of the pile;

[0014] Assume a ij =z ij , then we can get the settlement matrix of pile group foundation A=(a ij )=(z ij );

[0015] Set the m-dimensional vector uk=((uk)1,(uk)2,…(uk) m ) T , for the x-coordinate of each pile, obtain the x-coordinate distribution function uk(x) (k=1,2,…,r) of the LNG tank pile foundation;

[0016] Construct an n-dimensional vector vk=((vk)1,(vk)2,…,(vk) n ) T , corresponding to the y coordinate data set of each pile of the LNG storage tank, the function vk(y) (k=1,2,…,r) is obtained;

[0017] According to the formula

[0018]

[0019] The distribution function of the settlement of the LNG tank pile foundation is obtained:

[0020]

[0021] Furthermore, the steps for obtaining wind load are:

[0022] Calculate the standard value of wind load at different heights of LNG storage tanks; the calculation formula of the standard value of wind load is:

[0023]

[0024] In the formula, ω 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 factor; μ z is the wind load height variation coefficient; ω0 is the basic wind pressure;

[0025] The distribution law of the standard value of wind load is expressed by Fourier cosine function, which is used as wind load.

[0026] Furthermore, the various components of the LNG storage tank are modeled; specifically:

[0027] 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.

[0028] The present invention also provides a method for safety assessment of LNG storage tanks, the method comprising:

[0029] The ovality of the LNG tank at the maximum radial displacement is obtained by using a method of simulating and predicting the ovality of the LNG tank based on the coupling of settlement and wind load.

[0030] The ovality is compared with the allowable ovality. If it exceeds the allowable ovality, it is determined that LNG cannot be operated safely.

[0031] After adopting the above technical solution, the present invention can obtain the radial displacement distribution of the LNG storage tank when it is under load by simulation, and obtain the ellipticity at the maximum radial displacement, so as to use the ellipticity evaluation method to evaluate the deformation of the area with large deformation. According to the evaluation results, it can not only determine whether the storage tank can continue to serve, but also effectively simulate the limit deformation degree of each structure of the LNG storage tank when it is under load, which is of great help to the safe operation of the actual storage tank when it is under load. In addition, the present invention can also apply multiple loads under extreme weather to the model, so as to predict whether the LNG storage tank can operate safely under extreme weather. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A flow chart of a method for obtaining the ovality of an LNG storage tank based on simulation of the coupling of settlement and wind load according to the present invention;

[0033] Figure 2 A schematic diagram of setting finite element boundary conditions in the present invention;

[0034] Figure 3 It is a schematic diagram of mesh division of the LNG storage tank entity model of the present invention;

[0035] Figure 4 The radial displacement distribution cloud diagram of the maximum displacement of the outer tank wall of the LNG storage tank of the present invention;

[0036] Figure 5 is the ovality of the outer tank wall of the LNG storage tank of the present invention;

[0037] Figure 6 The settlement data of some piles of the pile group foundation of the present invention;

[0038] Figure 7 It is the radial displacement data of the annular path where the maximum radial displacement of the present invention is located. DETAILED DESCRIPTION

[0039] 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.

[0040] like Figure 1 As shown, a method for obtaining the ovality of an LNG storage tank based on a simulation of the coupling of settlement and wind loads includes:

[0041] Step S1, modeling each component of the LNG storage tank with a 20×10 4 m 3 The LNG storage tank is taken as the research object. The modeling function in ABAQUS software is used to model the various components of the LNG storage tank to obtain the model of the LNG storage tank.

[0042] Specifically, the inner tank, outer tank, bottom plate and arch of the LNG storage tank are rotated by using a shell structure; the pedestal and pile group foundation of the LNG storage tank are rotated by using a solid structure.

[0043] Step S2, setting corresponding material properties and dimensions for each component of the model.

[0044] Specifically, different material properties are set for components such as the outer tank, inner tank, insulation layer, pedestal, base plate and pile group foundation of the LNG storage tank, and each component is assigned corresponding relevant material parameters such as mass density, Young's modulus, Poisson's ratio and yield strength, and the corresponding thickness is set for each component.

[0045] Step S3, set the analysis step. Set two consecutive analysis steps, Step-1 and Step-2. Analysis step Step-1 is set to inherit the initial analysis state, and the step is set to general statics to perform stress analysis on the outer tank of the LNG storage tank under normal working conditions; Analysis step Step-2 inherits the analysis results of Step-1, and the step is set to general statics. Add the field output of wind load in Step-2 for subsequent coupling calculations.

[0046] Step S4, meshing the model.

[0047] Specifically, the model is divided into multiple regions: (1) The four-node curved shell element (S4R) provided by ABAQUS is used to simulate the tank wall area. (2) The ring wall foundation is simulated using the node linear hexahedral element (C3D8R) provided by ABAQUS. The entire model contains 61511 nodes and 36624 elements. Figure 3 shown.

[0048] Step S5, fitting the distribution formula of wind load and settlement load.

[0049] The steps for obtaining wind load are as follows:

[0050] In fact, the wind load acting on the tank wall has certain rules along the height and circumference of the tank. According to GB50009-2012 "Code for Loads on Building Structures", the standard value of wind load is calculated as follows:

[0051]

[0052] 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 );ρ represents air density, v0 represents basic wind speed;

[0053] The distribution law of the standard value of wind load is expressed by Fourier cosine function, which is used as wind load.

[0054] The expression of Fourier cosine function is:

[0055]

[0056] i—Fourier function order; α i —Fourier wind pressure coefficient; P(θ)—wind pressure distribution function, λ—wind pressure amplitude.

[0057] According to the Fourier wind pressure coefficient in the relevant foreign standard EN 1993-4-1, the distribution of wind pressure in the tank circumferential direction can be calculated. The formula is as follows:

[0058] P(θ)=1997.4(-0.54+0.28cosθ+0.533cos(2θ)+1.04cos(3θ)+0.36cos(4θ)-0.14cos5(θ).

[0059] The steps to obtain the settlement load are:

[0060] First, data collection should be carried out and the settlement data of each pile should be recorded. Considering that the lower part of the LNG storage tank is a structure of a cap + pile group foundation, the least squares surface fitting method based on singular value decomposition will be used to fit the surface settlement formula of the lower surface of the LNG storage tank cap.

[0061] Some of the data collected include Figure 6 As shown. The implementation steps of the least squares surface fitting method based on singular value decomposition are as follows:

[0062] The pile group foundation of the LNG storage tank is set up according to the distribution position of each pile, and a 22x22 settlement real matrix A is constructed. At this time, there are 22-order orthogonal matrices U and 22-order orthogonal matrices V that satisfy:

[0063] A=UDV T ;

[0064] Among them, the block form of the 22x 22 matrix D is:

[0065]

[0066] Among them, ∑ satisfies the following conditions:

[0067]

[0068] σ i (i=1, 2, ..., r) are the singular values ​​of the matrix A, where A=UDV T It is the singular value decomposition of matrix A.

[0069] For the selected single pile settlement data point, there are three data (x, y, z), where x and y are the x and y coordinates of the pile center point; z is the settlement of the pile.ij =z ij , then we can get the settlement matrix of pile group foundation A=(a ij )=(z ij ). At this time, set the m-dimensional vector u k =((u k )1,(u k )2,…(u k ) m ) T For the x-coordinate of each pile, the x-coordinate distribution function u of the LNG tank pile foundation can be obtained: k (x)(k=1,2,…,r). Similarly, we can construct an n-dimensional vector v k =((v k )1,(v k )2,…,(v k ) n ) T , corresponding to the y coordinate data set of each pile of the LNG storage tank, the function v can be obtained k (y)(k=1,2,…,r). At this time, according to the following formula:

[0070]

[0071] The distribution function of the settlement of the LNG tank pile foundation can be obtained:

[0072]

[0073] u k (x) = A1 + A2x + A3x 2 +…+A n x n

[0074] in,

[0075] v k (y) = B1 + B2y + B3y 2 +…+B n y n

[0076] By combining the three formulas, we can get the specific formula parameters in this example:

[0077] z(x,y)=-0.0324703491080102-0.00000319343317944787x+0.00000503091790837466x 2 +0.000000011090799582859x 3 +0.00000000105149977262222x4 -0.00000000000507643199245267x 5 -0.000000041482618601863y+0.00000738500596304082y 2 +0.0000000000337311324374748y 3 -0.000000000214446471395421y 4

[0078] Therefore, the settlement distribution formula of the lower surface of the entire LNG storage tank cap can be obtained from the settlement of each pile of the LNG storage tank through the least squares surface fitting method based on singular value decomposition.

[0079] This embodiment uses the singular value decomposition method to calculate the settlement distribution of pile groups. Because the settlement data of pile groups we obtain is in the form of discrete points, which is the settlement amount corresponding to a single coordinate position. Through the singular value decomposition method, we can calculate the settlement distribution formula acting on the entire lower surface of the pedestal. Compared with discrete points, the formula obtained by singular value decomposition can cover the entire lower surface of the pedestal, and the data distribution curve is smooth, there will be no sudden changes in data, and it is reliable.

[0080] Step S6, applying boundary conditions and operating loads to the model, wherein the loads include the deadweight load of the tank, the gravity load of the liquid in the tank, the settlement load and the wind load; the boundary conditions are as follows:

[0081] In terms of hydrostatic pressure, the hydrostatic pressure acting on the tank wall is triangularly distributed on the inner wall of the LNG tank, increasing from top to bottom. The liquid in the tank also exerts pressure on the bottom plate of the tank, which is applied to the bottom plate of the tank in the form of a uniformly distributed load. In terms of the deadweight of the tank, the gravity acceleration in the x and y directions is set to 0, and the gravity acceleration in the z direction is set to -9.8m / s 2 The circumferential and radial displacements of the lower surface of the pedestal are set to zero (that is, the x and y directions are set to full constraints), and the settlement formula obtained in step S5 is applied to the side edge surface of the pedestal as the axial displacement condition; the wind load formula obtained in step S5 is applied to the outer wall of the tank using the point-surface coupling method, first deriving the calculation formula, then applying it using the concentrated force method, and finally setting the amplitude.

[0082] Step S7, obtain the radial displacement distribution cloud diagram of the outer tank wall of the LNG storage tank through finite element solution; Figure 4 shown.

[0083] Step S8, select the possible area of ​​maximum radial displacement in the radial displacement distribution cloud map, extract radial displacement along the annular path at preset intervals in the axial direction, select the annular path where the maximum radial displacement is located by comparison, and calculate the ellipticity. The radial displacement of the annular path where the maximum radial displacement is located is as follows: Figure 7 As shown, the polar coordinate diagram is Figure 5 shown.

[0084] The radial displacement at the maximum displacement is extracted to obtain the ellipticity at the maximum radial displacement of the LNG storage tank, as shown in the figure. Figure 5 shown.

[0085] In this embodiment, the ovality of the outer tank of the LNG storage tank can be used to evaluate the deformation degree of the outer tank of the LNG storage tank. This embodiment adopts the allowable radius tolerance in API 653, and the specific allowable values ​​are shown in Table 1.

[0086] Table 1 LNG tank radius tolerance allowable range

[0087]

[0088] API 653 also stipulates that the radius tolerance value allowed at the position above 304.8mm above the weld between the tank wall and the tank bottom is three times that shown in the table above. Since the maximum radial displacement position of the specific embodiment of the present invention is higher than 304.8mm above the weld between the tank bottom and the tank wall, and the tank diameter of this embodiment is 80m, the radius tolerance value allowed by the specific embodiment of the present invention is ±95.25mm, and the maximum allowable ovality is 0.238%. The maximum allowable ovality is calculated as follows: maximum difference / average diameter, i.e. (80190.5-80000) / 80095.25

[0089] The ovality table of the outer tank of the LNG storage tank in the specific embodiment is shown in Table 2:

[0090] Table 2 Ovality table of LNG tank at maximum radial displacement

[0091] Minimum diameter (mm) 80407.77 Maximum diameter(mm) 81200.55 Maximum difference (mm) 792.778 Average diameter(mm) 80887.22 Ovality(%) 0.98

[0092] It can be seen from the ovality table that under the coupling effect of wind load and settlement, the ovality at the maximum radial displacement of the LNG storage tank reaches 0.98% (calculation formula: ovality = (maximum diameter - minimum diameter) / average diameter), and the radius tolerance is 396.389mm, which is far more than the allowable value calculated according to API 653. Therefore, this specific embodiment does not meet the relevant standards and needs necessary maintenance.

[0093] 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 obtaining the ovality of an LNG storage tank based on simulation of sedimentation and wind load coupling, characterized in that: Methods include: Modeling various components of the LNG storage tank to obtain a model of the LNG storage tank, setting corresponding material properties and dimensions for various components of the model, and meshing the model; Apply boundary conditions and operating loads to the model, and obtain the radial displacement distribution cloud diagram of the outer tank wall of the LNG storage tank through finite element solution; the loads include the deadweight load of the tank, the gravity load of the liquid in the tank, the settlement load and the wind load; The radial displacement in the circumferential direction of the position where the maximum radial displacement of the radial displacement distribution cloud diagram is located is obtained, and the ellipticity is calculated to obtain the ellipticity at the maximum radial displacement of the LNG storage tank.

2. The method for simulating and predicting the ovality of an LNG storage tank based on the coupling of settlement and wind load according to claim 1 is characterized in that: The steps to obtain the settlement load are: The settlement data of each pile in the pile group foundation is obtained, and the settlement distribution formula of the lower surface of the entire LNG tank cap is obtained based on the least squares surface fitting method of singular value decomposition, which is used as the settlement load.

3. The method for simulating and predicting the ovality of an LNG storage tank based on the coupling of settlement and wind load according to claim 2 is characterized in that: The settlement distribution formula of the entire LNG tank cap bottom surface is obtained by the least squares surface fitting method based on singular value decomposition; specifically: For the selected single pile settlement data point, there are three data (x, y, z), where x and y are the x and y coordinates of the pile center point; z is the settlement of the pile; Assume a ij =z ij , then we can get the settlement matrix of pile group foundation A=(a ij )=(z ij ); Set the m-dimensional vector uk=((uk)1,(uk)2,…(uk) m )T, for the x-coordinate of each pile, obtain the x-coordinate distribution function uk(x) (k=1,2,…,r) of the LNG tank pile foundation; Construct an n-dimensional vector vk=((vk)1,(vk)2,…,(vk) n ) T , corresponding to the y coordinate data set of each pile of the LNG storage tank, the function vk(y) (k=1,2,…,r) is obtained; According to the formula The distribution function of the settlement of the LNG tank pile foundation is obtained:

4. The method for simulating and predicting the ovality of an LNG storage tank based on the coupling of settlement and wind load according to claim 1 is characterized in that: The steps for obtaining wind load are: Calculate the standard value of wind load at different heights of LNG storage tanks; the calculation formula of the standard value of wind load is: In the formula, ω 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 factor; μ z is the wind load height variation coefficient; ω0 is the basic wind pressure; ρ represents the air density, and v0 represents the basic wind speed; The distribution law of the standard value of wind load is expressed by Fourier cosine function, which is used as wind load.

5. The method for simulating and predicting the ovality of an LNG storage tank based on the coupling of settlement and wind load according to claim 1 is characterized in that: Model the various components of the LNG tank; specifically: 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.

6. A method for safety assessment of LNG storage tanks, characterized in that: Methods include: The method according to any one of claims 1 to 5 is used to obtain the ellipticity at the maximum radial displacement of the LNG storage tank; The ovality is compared with the allowable ovality. If it exceeds the allowable ovality, it is determined that LNG cannot be operated safely.

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