Method for determining the tensile strength of discontinuous areas in a type C cargo containment system structure

Abstract

A method for determining the tensile strength of discontinuous regions in a C-type cargo containment system is disclosed. The method involves establishing the tensile limit state equation for the discontinuous regions of the C-type cargo containment system; determining the model uncertainty coefficient, material, environmental load, and functional load probability characteristics; establishing a finite element model and calculating the total stress probability characteristics of the C-type containment system in the discontinuous regions using the Rosenblueth method; calculating the reliability of the tensile strength of the discontinuous regions using the first second-order moment method of reliability; establishing the expression for the tensile strength evaluation criterion; and using the least squares method to determine the partial factors for permanent loads, environmental loads, and the optimal resistance coefficient for tensile failure in the discontinuous regions of the C-type containment system design formula. This invention establishes the design criteria for determining the tensile strength of the discontinuous regions (Y-type joints) in a C-type cargo containment system.

Description

Technical Field

[0001] This invention belongs to the field of marine pressure vessel design standards and relates to a method for determining the tensile strength of discontinuous areas in a C-type cargo containment system structure. Background Technology

[0002] The International Maritime Organization (IMO) clearly defines the function and purpose of Type C cargo containment systems in its International Code for the Construction and Equipment of Liquefied Gas Ships (IGC Code). A Type C cargo containment system is a special type of pressure vessel with a complex structure, operating in an ultra-low temperature environment. The materials and loads of a Type C cargo containment system are subject to randomness, which can pose certain risks to its safety, especially in areas of structural discontinuity (Y-joint areas). These discontinuous areas experience complex stresses, resulting in high total stress values ​​that may exceed the material's yield strength, leading to plastic deformation. Therefore, establishing a design criterion for the tensile ultimate strength of Type C containment systems, particularly in areas of structural discontinuity, is crucial to ensuring their safety.

[0003] Summary of the invention.

[0004] To overcome the shortcomings of existing technologies, this invention provides a method for determining the tensile strength of discontinuous regions in a C-type cargo containment system. It determines the loads, resistances, and stochasticity of the C-type liquid cargo containment system model. The reliability of the C-type cargo containment system (twin-body tank and triple-body tank) is calculated using the first-order second-moment method, and the target reliability for tensile failure of different units in the discontinuous regions of the C-type cargo containment system structure is determined. The optimal partial factors for the tensile limit design criterion of the C-type cargo containment system are determined using the least squares method.

[0005] The technical solution adopted by this invention to solve its technical problem is: A method for determining the tensile strength of discontinuous areas in a C-type cargo containment system structure includes the following steps: Step S1: Establish the tensile limit state equation for the discontinuous region of the C-type cargo containment system structure, i.e., the Y-type joint. Step S2: Determine the model uncertainty coefficient, material, environmental load, and functional (including self-weight) load probability characteristics of the discontinuous region of the C-type enclosure system structure; Step S3: Establish a finite element model of the C-type enclosure system and use the Rosenblueth method to calculate the total stress probability characteristics of the C-type enclosure system in the structural discontinuity region. Step S4: Use the first second moment method of reliability to calculate the reliability of the tensile strength of the discontinuous area of ​​the C-type enclosure system structure, i.e. the Y-type joint. Step S5: Establish a reliability-based evaluation criterion expression for the tensile strength assessment of discontinuous areas in the structure of an independent C-type liquid cargo tank. Step S6: Using the least squares method, determine the functional (including self-weight) load partial factor, environmental load partial factor, and optimal resistance coefficient for tensile failure in the discontinuous structural areas of the C-type enclosure system design formula.

[0006] Furthermore, in step S1, the tensile limit state equation for the discontinuous region of the C-type cargo containment system structure is established: (1); in, These are the uncertainties of the tensile strength, functional (including self-weight), and environmental models of the discontinuous structural regions, respectively. These are the tensile strength of the structural discontinuity region, the functional (including self-weight) load, and the total combined stress of the tank unit caused by environmental load, respectively.

[0007] Furthermore, in step S2, a finite element model of the C-type enclosure system is established, and the Rosenblueth method is used to calculate the total stress in the discontinuous areas of the tank structure. The probabilistic properties of ), where, For unit thin film stress; For unit bending stress; This represents the peak stress of the element.

[0008] Furthermore, in step S3, the first second-order distance method is used to calculate the discontinuous area of ​​the C-type enclosure system structure and solve the reliability of equation (1).

[0009] In step S4, considering the reliability calculation results of the actual tank and the fact that the system suffers from ductile failure without strength reserve capacity, the target reliability of the discontinuous area of ​​the C-type enclosure system is 10. -4 .

[0010] In step S5, an independent strength assessment criterion for Type C liquid cargo tanks is established: (2); in, These are the functional (including self-weight), environmental, and resistance safety sub-factors for the discontinuous areas of the C-type cargo containment system structure. These represent the total stress values ​​synthesized by the element, calculated from its own weight, functional load, and environmental load. The importance coefficient is set to 1.0. To allow for first-order global membrane stress, The calibrated lower limit of tensile strength at room temperature ).

[0011] In step S6, the least squares method is used to finally determine the functional (including self-weight) load partial factor of the tensile strength design criterion for the discontinuous area of ​​the C-type enclosure system structure as 1.0, the environmental load partial factor as 1.3, and the optimal resistance coefficient of the discontinuous area of ​​the structure as 1.1.

[0012] The beneficial effects of this invention are mainly reflected in the following aspects: The solution proposed by this invention proposes a design criterion for the tensile strength of the discontinuous area (Y-type joint) of the C-type cargo containment system structure, which solves the problem of failure or accidents during the use of the C-type cargo containment system, eliminates the potential hidden dangers of the discontinuous area of ​​the C-type cargo containment system structure, ensures that the Y-type joint of the container can withstand the expected pressure, guarantees the safety of the discontinuous area of ​​the C-type cargo containment system structure, and lays the foundation for proposing C-type cargo containment system specifications that conform to Chinese standards. Attached Figure Description

[0013] Figure 1 It is a finite element model of a three-body tank; Figure 2 It is the mean resistance under the target reliability. and design point coordinates Calculation process; Figure 3 It is the result of the combination of partial factors for the structural discontinuity segment region; Figure 4 This is a flowchart illustrating the method for determining the tensile strength of discontinuous areas in a C-type cargo containment system structure. Detailed Implementation

[0014] The present invention will now be further described with reference to the accompanying drawings.

[0015] Reference Figures 1-4 A method for determining the tensile strength of discontinuous areas in a C-type cargo containment system structure includes the following steps: Step S1: Establish the tensile limit state equation for the discontinuous region of the C-type cargo containment system structure, i.e., the Y-type joint. Step S2: Determine the model uncertainty coefficient, material, environmental load, and functional (including self-weight) load probability characteristics of the discontinuous region of the C-type enclosure system structure; Step S3: Establish a finite element model of the C-type enclosure system and use the Rosenblueth method to calculate the total stress probability characteristics of the C-type enclosure system in the structural discontinuity region. Step S4: Use the first second moment method of reliability to calculate the reliability of the tensile strength of the discontinuous area of ​​the C-type enclosure system structure, i.e. the Y-type joint. Step S5: Establish a reliability-based evaluation criterion expression for the tensile strength assessment of discontinuous areas in the structure of an independent C-type liquid cargo tank. Step S6: Using the least squares method, determine the functional (including self-weight) load partial factor, environmental load partial factor, and optimal resistance coefficient for tensile failure in the discontinuous structural areas of the C-type enclosure system design formula.

[0016] This patent uses Abaqus to perform finite element calculations on the ultimate strength of a three-body tank model.

[0017] The main parameters of the three-body tank are as follows: total length 29.8m, total width 25.2m, total height 19.75m, and volume approximately 9497m³. 3 The thickness of the cylindrical wall region of the liquid cargo tank is 15.5 mm, the thickness of the hemispherical wall region is 12.8 mm, and the thickness of the longitudinal wall is 22.5 mm; for the thickness of other parts of the model, please refer to [link / reference]. Figure 2 .

[0018] The implementation process of this embodiment is as follows: Step S1: Based on the finite element method proposed by Rec 174, the first-order second-moment method is used. Based on the uncertainty coefficients of the model, the tensile limit state equation for the discontinuous region of the C-type cargo containment system structure is established: (1); in, These are the uncertainties of the tensile strength, functional (including self-weight), and environmental models of the discontinuous structural regions, respectively. These represent the tensile strength of the structural discontinuity region, the functional (including self-weight) load, and the total stress of the tank unit caused by environmental load, respectively.

[0019] Step S2: Determine the model uncertainty coefficient, material probability characteristics, environmental load, and functional (including self-weight) load probability characteristics of the C-type enclosure system.

[0020] Reliability calculations and analyses were mainly performed on the two working conditions with the highest stress values, LC03 (vertical acceleration) and LC05 (experiment), with a total of 27 three-body tank models calculated. The structural discontinuity region (Y-type joint) in the three-body tank model has three element types: 8-node shell, 1 layer of 20-node solid elements, and 4 layers of 8-node solid elements.

[0021] The size and length of the solid element are based on the thickness T. The 20-node solid element divided by T×T×T is simply referred to as T×T×T (20), the 8-node solid element divided by T×T×T / 4 is simply referred to as T×T×T / 4 (8), and the 8-node shell element is simply referred to as shell element.

[0022] Step S3: Using the Rosenblueth method, calculate the probabilistic characteristics of the total stress at the Y-type joint of the tank.

[0023] The core formula of the Rosenblueth method is as follows: (2); (3); in, The stress values ​​of the model obtained when all variables are at their mean values. For the first The average stress is obtained by adding or subtracting one standard deviation from the mean of each variable. For the first The average of the stress differences is obtained by adding or subtracting one standard deviation from the mean of each variable.

[0024] Taking the LC03 condition of the two-body tank shell unit under no temperature effect as an example, the probability characteristics obtained by the Rosenblueth method after model calculation are shown. Table 1 is the probability variable table (MPa) of the shell unit under the LC03 condition of the two-body tank.

[0025]

[0026] Table 1 Step S4 involves organizing the material's tensile strength limit state design equations, material probabilistic characteristics, model uncertainty coefficients from S1, and the probabilistic characteristics obtained from the Rosenblueth method in S2. The reliability of the type C containment system is calculated using the first second-moment method of reliability. Table 2 shows the reliability results for the liquid cargo tank at no temperature.

[0027]

[0028] Table 2 Based on this, the target reliability of the tensile strength of the C-type enclosure system is determined using ductile failure without strength reserve in the case of non-critical failure, with a target reliability of 10. -4 .

[0029] Step S5: Establish a reliability-based tensile strength evaluation criterion for the C-type enclosure system.

[0030] The expression for the design principle: (4); in, These are the functional (including self-weight), environmental, and resistance safety sub-factors for the discontinuous areas of the C-type cargo containment system structure. These represent the total stress values ​​of the element, calculated by self-weight, functional load, and environmental load, respectively. The calibrated lower limit of tensile strength at room temperature ). The importance coefficient is set to 1.0. See Table 3 for details. Table 3 is the importance coefficient table.

[0031]

[0032] Note: 1) This means a large amount of cargo was leaked and there is a high possibility of causing a large number of casualties or a large amount of cargo being leaked.

[0033] 2) This implies the possibility of cargo leakage and personal injury. 3) This has been proven through risk assessment and approved by the competent authority.

[0034] Table 3 Based on the principle of selecting the best safety factors for each component: the reliability index of the independent liquid cargo tank designed according to the design formula, and the target reliability index. The overall error is the smallest between them.

[0035] According to the second-level probabilistic design method—the approximate probabilistic limit design method—based on the given reliability indicators... and statistical parameters of each basic variable , The target reliability index can be calculated. Average resistance of liquid cargo tanks Then calculate the corresponding design point value using the following formula. .

[0036] (5); This represents the load effect coefficient.

[0037] The tensile strength of the liquid cargo tank material was obtained. and obtained Equal, that is If the structural reliability index of the liquid cargo tank designed according to the formula is equal to the specified reliability index, then... If the structural reliability index measured according to the formula is greater than the specified reliability index, then the optimal sub-item safety factor will be... The sum of squared errors should be satisfied. Minimum: (6); In the formula, j represents the sample size, i.e., the number of liquid cargo tanks; The magnitude of the value relatively reflects the selection of this group. , and The degree of difference between the designed reliability index of the liquid cargo tank and the target reliability index is determined by the design expression.

[0038] make We can obtain: (7); when and When it is pre-set, it can be obtained Therefore, in actual calculations, the value can be set in advance. and The possible values ​​for each group and For each value, the corresponding result can be calculated. value, The value, obviously makes The partial factor with the smallest value is the optimal partial factor.

[0039] When the environmental load effect follows an extreme value type I distribution, the resistance follows a log-normal distribution, and the functional (including self-weight) load effect follows a normal distribution: the mean resistance and design point coordinates The solution process, such as Figure 3 .

[0040] Step S6: After considering the uncertainty coefficient of the intensity model, the mean of each comprehensive random variable. and coefficient of variation for: (8); (9); The deviation coefficients for materials and loads are: (10); In the formula, This represents the average values ​​of the material and the load. The values ​​represent the nominal values ​​for materials and loads. For details on the material deviation coefficient and coefficient of variation of liquid cargo tanks, please refer to Table 4.

[0041]

[0042] Table 4 The functional (including self-weight) load effect of liquid cargo tanks is divided into the effect of the tank's own weight and the effect of the functional load inside the tank. By combining these two normally random distribution probabilities, the coefficient of variation and the deviation coefficient of the functional (including self-weight) load are obtained. The coefficient of variation and the deviation coefficient of the functional (including self-weight) load of the structural discontinuity region (Y-type joint) of different units are obtained. Table 5 is a table of deviation coefficients and coefficients of variation of the load effect of liquid cargo tanks.

[0043]

[0044] Table 5 Load effect ratio The load effect ratio is defined as the ratio of variable load to permanent load. The reliability calculation results will change with the change of the load effect ratio. Therefore, it is necessary to define the range of the load effect ratio. Based on the actual situation of liquid cargo tanks, this report adopts load effect ratios of 0.1, 0.25, 0.5, 1.0, 1.5 and 2.0.

[0045] The partial factor for the functional (including self-weight) load effect is taken as follows: =0.90, 1.00, 1.10, the partial factors for environmental load effects are taken as follows: =0.50, 0.60, 0.70, 0.80, 0.90, 1.00, 1.10, 1.20, 1.30, 1.40, 1.50, 1.60, 1.70, therefore different There are a total of 39 possible value combinations. The target reliability for the structural discontinuity region is 10. -4 For details on the calculation of shell elements in discontinuous regions, please refer to [link / reference]. Figure 4 When the functional (including self-weight) load partial factor When the values ​​are 0.9, 1.0, and 1.1, the location with the minimum cumulative error H is at the environmental load. The positions are 1.3, 1.4, and 1.5.

[0046] This patent uses a functional (including self-weight) load partial factor. The environmental load factor is 1.0. The value is 1.3, and the optimal resistance coefficient is obtained based on this. .

[0047] The optimal resistance coefficients of the Type C enclosure system were calculated for different unit types in the structural discontinuities. The calculation results are shown in Table 6, which is a table of optimal resistance coefficients for liquid cargo tanks.

[0048]

[0049] Table 6 The results show that the optimal resistance coefficient is found in the discontinuous structural region. It is 1.1.

[0050] The final design criterion expression calculated in this embodiment is: (11); These represent the total stress values ​​of the element, calculated by self-weight, functional load, and environmental load, respectively. The calibrated lower limit of tensile strength at room temperature ), For importance coefficients, see Table 3.

[0051] The embodiments described in this specification are merely examples of implementations of the inventive concept and are for illustrative purposes only. The scope of protection of this invention should not be considered limited to the specific forms described in these embodiments; rather, it extends to equivalent technical means conceived by those skilled in the art based on the inventive concept.

Claims

1. A method for determining the tensile strength of discontinuous areas in a C-type cargo containment system structure, characterized in that, The method includes the following steps: Step S1: Establish the tensile limit state equation for the discontinuous region of the C-type cargo containment system structure, i.e., the Y-type joint. Step S2: Determine the model uncertainty coefficient, material, environmental load, and functional load probability characteristics of the discontinuous region of the C-type enclosure system structure; Step S3: Establish a finite element model of the C-type enclosure system and use the Rosenblueth method to calculate the total stress probability characteristics of the C-type enclosure system in the structural discontinuity region. Step S4: Use the first second moment method of reliability to calculate the reliability of the tensile strength of the discontinuous area of ​​the C-type enclosure system structure, i.e. the Y-type joint. Step S5: Establish a reliability-based evaluation criterion expression for the tensile strength assessment of discontinuous areas in the structure of an independent C-type liquid cargo tank. Step S6: Using the least squares method, determine the partial factors for permanent loads, environmental loads, and the optimal resistance coefficient for tensile failure in the discontinuous structural regions of the C-type enclosure system design formula. In step S1, the tensile limit state equation for the discontinuous region of the C-type cargo containment system structure is established: （1）； in, These are the tensile, functional, and environmental model uncertainties for the structural discontinuities, respectively. These represent the tensile strength of the structural discontinuity region, the functional load, and the total combined stress of the tank unit caused by environmental load, respectively. In step S4, the first second-order distance method is used to calculate the discontinuous area of ​​the C-type enclosure system structure and solve the reliability of equation (1); In step S5, an independent strength assessment criterion for Type C liquid cargo tanks is established: （2）； in, These are the functional, environmental, and resistance safety sub-factors for the discontinuous areas of the C-type cargo containment system structure. These represent the total stress values ​​synthesized by the element, calculated from its own weight, functional load, and environmental load. The importance coefficient is set to 1.

0. This is the calibrated lower limit of tensile strength at room temperature, in N / mm. 2 .

2. The method for determining the tensile strength of discontinuous areas in the C-type cargo containment system structure as described in claim 1, characterized in that, In step S2, a finite element model of the C-type enclosure system is established, and the Rosenblueth method is used to calculate the total stress in the discontinuous areas of the tank structure. The probabilistic properties of ), where, For unit thin film stress; For unit bending stress; This represents the peak stress of the element.

3. The method for determining the tensile strength of discontinuous areas in the C-type cargo containment system structure as described in claim 1, characterized in that, In step S4, considering the reliability calculation results of the actual tank and the fact that the system suffers from ductile failure without strength reserve capacity, the target reliability of the discontinuous area of ​​the C-type enclosure system is 10. -4 .

4. The method for determining the tensile strength of discontinuous areas in a C-type cargo containment system structure as described in claim 1, characterized in that, In step S6, the least squares method is used to finally determine the permanent load partial factor of the tensile strength design criterion for the discontinuous area of ​​the C-type enclosure system as 1.0, the environmental load partial factor as 1.3, and the optimal resistance coefficient of the discontinuous area as 1.1.

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