Ultra-large container ship whole-ship strength analysis method based on equivalent design wave method

Abstract

The invention relates to the technical field of ultra-large container ship whole-ship strength analysis, in particular to an ultra-large container ship whole-ship strength analysis method based on an equivalent design wave method.The method comprises the steps that on the basis of long-term sea condition forecast data, equivalent regular waves are determined through a statistical analysis method; the wave height and the period of the equivalent regular wave should be matched with the average wave height and the main period in the long-term sea condition forecast; establishing a three-dimensional finite element model of a whole ship structure according to a design structure drawing of the ultra-large container ship; the three-dimensional finite element model of the whole ship structure is subjected to mesh generation, the wave load is calculated and applied to the three-dimensional finite element model, yield strength analysis is conducted on the ship, and the analysis result shows that the ship structure design is good. The method is not only suitable for the whole ship strength analysis of the ultra-large container ship, but also can be applied to the strength evaluation of other types of large ships.

Description

Technical Field

[0001] This invention relates to the field of whole-ship strength analysis technology for ultra-large container ships, and specifically to a whole-ship strength analysis method for ultra-large container ships based on the equivalent design wave method. Background Technology

[0002] Ultra-large container ships are a type of transport vessel used to carry international standard containers. In recent years, the demand for container shipping trade both domestically and internationally has been increasing; however, the actual shortage of shipping capacity has become increasingly serious. International shipping companies are paying more and more attention to the research and development and application of medium and large container ships. Today, the trend towards larger container ships is becoming increasingly apparent, with shipyards around the world building new 24,000 TEU ultra-large container ships, significantly increasing their carrying capacity. With the increase in the weight of the ships themselves, the requirements for the hull structure must also reach a higher level.

[0003] With the rapid development of the shipbuilding industry at home and abroad, and the trend of ships becoming larger and faster, the structural design of large container ships is receiving increasing attention. In hull structure design, research on hull structure safety design technology is the foundation of ship energy efficiency improvement technology, which can effectively improve ship performance and reduce ship energy consumption.

[0004] There are still certain shortcomings in the overall strength analysis of large container ships. The simplification of the model and the discrepancy between the model and the actual situation can lead to discrepancies. When conducting full-ship finite element analysis, the actual container ship structure needs to be simplified to a certain extent, which may result in deviations from the actual situation. Some small structural features may be overlooked, but these details can have a significant impact on strength and stiffness in reality. Because large container ships typically have complex structural designs, such as torsion-resistant boxes and continuous effective hatch longitudinal coamings or double-hull side structures, this increases the difficulty of the analysis, requiring precise analysis and optimization to ensure structural integrity and load-bearing capacity. Traditional analysis methods often struggle to accurately assess the overall strength of ultra-large container ships under various sea conditions. Therefore, this invention proposes a full-ship strength analysis method for ultra-large container ships based on the equivalent design wave method, which can significantly improve the accuracy and efficiency of the analysis.

[0005] Based on this, the present invention relates to a method for whole-ship strength analysis of ultra-large container ships based on the equivalent design wave method to solve the above problems. Summary of the Invention

[0006] To address the aforementioned shortcomings of current technologies, this invention provides a method for overall strength analysis of ultra-large container ships based on the equivalent design wave method.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for full-ship strength analysis of ultra-large container ships based on the equivalent design wave method, characterized in that the method includes the following steps:

[0008] (1) Using long-term sea state forecast data of the ship navigation area, obtain the wave distribution characteristics of the target sea area under different seasons and different navigation conditions. The main parameters include wave height, wave period and wave direction. Based on the long-term sea state forecast data, determine the equivalent regular wave through statistical analysis. The wave height and period of the equivalent regular wave should match the average wave height and main period in the long-term sea state forecast.

[0009] (2) Based on the design structural drawings of the ultra-large container ship, a three-dimensional finite element model of the entire ship structure is established; (3) The three-dimensional finite element model of the entire ship structure is meshed, with sufficient mesh density in stress concentration areas and weak parts. Fine mesh is used in stress concentration areas, and coarse mesh is used in other parts. The transition areas of the bow, stern, bottom and side plates are divided using tetrahedral mesh; the main deck and keel are divided using hexahedral mesh; the models of each area after meshing are merged to obtain the three-dimensional finite element model of the entire ship structure;

[0010] (4) Select equivalent regular waves and typical working conditions, calculate wave loads and apply them to the three-dimensional finite element model, calculate loading conditions according to the design specifications of the ship, combine the loading conditions with the aforementioned wave load conditions as the calculation conditions for the whole ship finite element analysis, load and solve the calculation conditions, obtain the stress and strain response results of the whole ship structure, find the stress concentration areas in the hull structure through the analysis results, and evaluate the overall strength of the ultra-large container ship.

[0011] As a further embodiment of the present invention, in step 1 above, the wave period and wavelength of the equivalent regular wave are determined by the observed values ​​of the actual irregular wave, the long-term irregular wave spectrum is determined, the wave characteristics are analyzed, statistical data is extracted, and based on the long-term irregular wave spectrum, the irregular wave is decomposed into a series of regular waves by the spectral decomposition method, and representative regular waves are selected.

[0012] As a further embodiment of the present invention, in step 2 above, the coordinate system of the three-dimensional finite element model adopts a right-handed rectangular coordinate system, and the finite element modeling of the entire ship still uses large-scale general-purpose finite element software. The three-dimensional finite element model of the ship includes the main hull structure, such as decks, outer plating, engine room, funnel, superbottom, longitudinal bulkheads, transverse bulkheads, beams and longitudinal girder. All plates are represented by shell elements, and the longitudinal ribs and stiffeners on all load-bearing plates are simulated by eccentric beams. All skeletons are represented by beam elements with axial stiffness, torsional stiffness, bidirectional shear stiffness and bending stiffness. The model should consider the eccentricity of the neutral axis. Larger profiles are simulated with plate elements to simulate their webs, and their panels are simulated with beam elements that consider eccentricity. Opening structures such as manholes are simulated and constructed according to the actual structure.

[0013] As a further aspect of the present invention, before step 4, the hull's support and boundary conditions are reasonably set to simulate the hull's stress and deformation under actual navigation conditions. The boundary conditions include the fixed constraint at the stern and the free support at the fore and aft of the hull. In the finite element strength analysis, in order to eliminate the influence of rigid displacement, it is necessary to restrict the rigid displacement of the ship and apply constraints to calculate the relative deformation of the structure. During this process, static imbalance may occur, generating large support reaction forces at the constraint points and attachments, which may affect the calculation results. Therefore, three nodes far from the center are selected to apply degree of freedom constraints, while not affecting the subsequent deformation and stress of the hull structure.

[0014] As a further aspect of the present invention, before analysis,

[0015] The empty ship weight mainly includes the weight of the steel hull of the container ship and the weight of its attached equipment. In the model, the weight of the equipment is achieved by adding mass elements, while the weight of the steel is simulated using the aforementioned finite element model.

[0016] Hydrostatic pressure and wave pressure on the outer plating: In the finite element software, a domain function 1.005e-5*(14800-z) is defined and applied to the model in the form of pressure, P={ρg(TZ)}, with units of MPa, where P is the hydrostatic pressure, ρ is the density, T is the minimum water level under the considered loading conditions, and Z is the depth of the point below the liquid surface; the hydrostatic pressure value is proportional to the height from the waterline of the hull to the bottom of the ship, and consists of two parts: one part is the hydrostatic pressure outside the hull, and the other part is the static pressure of ballast water and fuel on the bulkhead;

[0017] When performing quasi-static analysis on a finite element model, if the force system of the entire hull cannot remain in equilibrium, stress concentration will occur at the restricted free points, leading to deviations in the calculation results; therefore, an inertial balancing force must be applied.

[0018] As a further aspect of the present invention, the overall strength of the ultra-large container ship is evaluated, the yield strength of the main hull components is assessed, high-stress areas are identified and analyzed, and the yield strength of the coarse mesh model of the three-dimensional finite element model should meet the following requirements:

[0019] σ VM ≤σ MASTER

[0020]

[0021] Where, σ VM Von-Mises stress, MPa; σ MASTER The principal allowable stress is expressed in MPa; K is the material coefficient.

[0022] R is the material safety factor. For the s*s mesh model, γR is 1.05, and for the 3s*3s mesh model, γR is 1.2. γm is the deformation resistance safety factor, which is 1.02.

[0023] For fine-mesh models, the yield strength criterion can be appropriately scaled up, but the following requirements must be met:

[0024] σ VM ≤K S σ MASTER

[0025] Where Ks is the magnification factor for fine mesh evaluation, with 1.34 for elements near the weld and 1.53 for elements not near the weld.

[0026] The beneficial effects of this invention are:

[0027] This invention presents a wave load theory for whole-ship finite element analysis, including ship motion in regular waves, long-term wave load prediction, and the equivalent design wave method. Wave load calculations were performed on a container ship, considering wave conditions corresponding to different load control parameters. Subsequently, the whole-ship finite element modeling technique was explored in depth, and a whole-ship finite element analysis was conducted to obtain the stress distribution characteristics of the entire ship structure. Finally, yield strength analysis was performed on the ship, and the results show that the ship's structural design is good. This method simplifies the analysis process and effectively improves analysis efficiency, making the whole-ship strength assessment of ultra-large container ships more convenient and efficient. This method is not only applicable to the whole-ship strength analysis of ultra-large container ships but can also be applied to the strength assessment of other types of large ships. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0029] The present invention will be further described below with reference to embodiments.

[0030] Example 1: A method for overall strength analysis of ultra-large container ships based on the equivalent design wave method. The method includes the following steps:

[0031] (1) Using long-term sea state forecast data of the ship's navigation area, obtain the wave distribution characteristics of the target sea area under different seasons and different navigation conditions. The main parameters include wave height, wave period and wave direction; determine the maximum wave height, wave period and wave direction that the ultra-large container ship may encounter within its design life; based on long-term sea state forecast data, determine the equivalent regular wave through statistical analysis methods. The wave height and period of the equivalent regular wave should match the average wave height and main period in the long-term sea state forecast.

[0032] The equivalent regular wave energy represents the main effect of the actual irregular wave on the ship's structure. The wave period and wavelength of the equivalent regular wave are determined by the observed values ​​of the actual irregular wave. The long-term irregular wave spectrum is determined, wave characteristics are analyzed, and statistical data is extracted. Based on the long-term irregular wave spectrum, the irregular wave is decomposed into a series of regular waves using spectral decomposition methods, and representative regular waves are selected. Their wave height and period should cover the extreme cases in the irregular wave spectrum. The energy density of each regular wave is calculated to ensure that the total energy is consistent with the irregular wave spectrum.

[0033] The criteria for selecting equivalent regular waves include: wave height: selected based on extreme wave heights; wave period: covering waves from short to long periods.

[0034] (2) Based on the design structural drawings of the ultra-large container ship, establish a three-dimensional finite element model of the entire ship structure; obtain the hull structure drawing of the target ship, and fully consider the structural characteristics of the container ship, such as the large-open cargo hold structure and weak torsional stiffness. Through detailed modeling and analysis of the hull structure, more accurately evaluate the strength performance of the hull under various sea conditions.

[0035] The engine room, cargo hold, and bow area were directly modeled using the finite element method for ship strength, while other areas were modeled according to the dimensions of the overall ship structural drawing. The model includes major structural components such as the hull, decks, keel, transverse bulkheads, and longitudinal bulkheads.

[0036] The coordinate system of the three-dimensional finite element model adopts a right-handed rectangular coordinate system. The finite element modeling of the entire ship still uses large-scale general-purpose finite element software, MSC.PATRAN and FEMAP, while the analysis work is performed using NASTRAN software. The three-dimensional finite element model of the ship includes the main hull structure, such as decks, outer plating, engine room, funnel, superbottom, longitudinal bulkheads, transverse bulkheads, beams, and longitudinal girder, all simulated using CQUAD4 and CTRIA3 elements. All plates are represented using shell elements. For all load-bearing plates, longitudinal ribs and stiffeners are simulated using eccentric beams. All stiffeners are represented using beam elements with axial stiffness, torsional stiffness, bidirectional shear stiffness, and bending stiffness. The model should consider the eccentricity of the neutral axis. Larger profiles are simulated with plate elements for their webs and with beam elements considering eccentricity for their face plates. Opening structures such as manholes are simulated based on the actual structure; openings with a diameter less than 30cm can be ignored.

[0037] (3) The three-dimensional finite element model of the entire ship structure is meshed, with sufficient mesh density in stress concentration areas and weak points. Fine meshes are used in stress concentration areas such as longitudinal girder, transverse beam, and ribs to improve the accuracy of stress analysis. Coarse meshes are used in other parts. The transition areas between the bow, stern, bottom, and side plates are meshed using tetrahedral meshes; the main deck and keel are meshed using hexahedral meshes. The meshed models of each region are merged to obtain the three-dimensional finite element model of the entire ship structure.

[0038] (4) Determine a series of typical operating conditions based on the actual operation of the vessel. Typical operating conditions cover the vessel's working status under different loading conditions, speeds, headings, and sea states. The determination of typical operating conditions refers to the vessel design specifications and operating manuals.

[0039] Among all typical operating conditions, the conditions that have the greatest impact on the structural strength of the hull are selected as the focus of the finite element analysis. These conditions include the maximum vertical wave moment of the midship arch under full load sailing conditions, the maximum horizontal wave moment of the midship arch, the maximum wave torque near 1 / 4L, and the maximum wave torque near 3 / 4L.

[0040] Reasonable setting of hull supports and boundary conditions is used to simulate the stress and deformation of the hull under actual navigation conditions. Boundary conditions include fixed constraints at the stern and free supports at the fore and aft of the hull. In finite element strength analysis, to eliminate the influence of rigid displacement, it is necessary to restrict the rigid body displacement of the ship and apply constraints to calculate the relative deformation of the structure. During this process, static imbalance may occur, generating significant support reactions at constraint points and attachments, affecting the calculation results. Therefore, three nodes far from the center are selected to apply degree-of-freedom constraints without affecting the subsequent deformation and stress of the hull structure. Regarding boundary conditions: Node 1, where the bow perpendicular intersects the upper longitudinal midsection of the hull bottom, constrains linear displacement in three directions. Node 2, where the bottom of the starboard stern plate intersects the main deck, constrains linear displacement in the x and z directions. Node 3, where the bottom of the port stern plate intersects the main deck, constrains linear displacement in the y and z directions. See Table 1 for details.

[0041] Table 1 Boundary conditions for the finite element model of the entire ship

[0042]

[0043] The empty ship weight mainly includes the weight of the steel hull of the container ship and the weight of its attached equipment. In the model, the weight of the equipment is achieved by adding mass elements, while the weight of the steel is simulated using the aforementioned finite element model.

[0044] Hydrostatic pressure and wave pressure on the outer plating: In the PATRAN finite element software, a domain function 1.005e-5*(14800-z) is defined and applied to the model in the form of pressure, P={ρg(TZ)}, with units of MPa, where P is the hydrostatic pressure, ρ is the density, T is the minimum water level under the considered loading condition, and Z is the depth of the point below the liquid surface. The hydrostatic pressure value is proportional to the height from the waterline to the bottom of the hull and consists of two parts: one part is the hydrostatic pressure outside the hull, and the other part is the static pressure of ballast water and fuel on the bulkheads.

[0045] When performing quasi-static analysis on a finite element model, if the force system of the entire hull cannot remain in equilibrium, stress concentration will occur at the restricted free points, leading to deviations in the calculation results; therefore, an inertial balancing force must be applied.

[0046] Based on the selection of equivalent regular waves and typical operating conditions, wave loads were calculated and applied to the three-dimensional finite element model. Loading conditions were calculated according to the ship's design specifications. DNV specifications require the calculated loading conditions to meet the following requirements: uniformly distributed, partially loaded containers; fully loaded containers in the cargo hold; no ballast water; and a capacity of 28 t / FEU. During the voyage of a container ship, the loading conditions vary greatly due to the need for loading and unloading cargo at various terminals. By consulting container ship loading manuals and referring to past actual loading conditions of container ships, a uniformly loaded, fully loaded departure condition of 14 tons (14 TSD) was selected as the loading condition for the entire ship's finite element analysis. This loading condition, combined with the wave load condition described above, serves as the calculation condition for the entire ship's finite element analysis.

[0047] 1) Condition 1: Full load structural draft + speed 21.52 knots + maximum midship arch vertical wave moment;

[0048] 2) Operating Condition 2: Fully loaded structural draft + speed 21.52 knots + maximum horizontal wave moment amidships;

[0049] 3) Operating Condition 3: Fully loaded structural draft + speed 21.52 knots + maximum wave torque around 1 / 4L;

[0050] 4) Operating Condition 4: Fully loaded structural draft + speed 21.52 knots + maximum wave torque around 3 / 4L;

[0051] After applying equivalent wave loads and boundary conditions, a static finite element analysis is performed on the hull structure to calculate the stress and deformation of various parts of the hull. The dynamic stress and deformation distribution of the hull are also calculated.

[0052] Finite element analysis (FEM) of the entire ship can accurately reflect the coordination and deformation of various structural components, offering high precision. The yield strength of structural components must be based on the material's yield limit. FEM of the entire ship also provides a more intuitive view of the stress levels of the hull and structure under combined bending and torsion. In this case, a safety margin must be maintained, and the yield strength of the main hull components must be assessed. Finally, high-stress areas are identified and analyzed to facilitate further detailed analysis and node optimization.

[0053] The yield strength of the coarse mesh model in the three-dimensional finite element model should meet the following requirements:

[0054] σ VM ≤σ MASTER

[0055]

[0056] Where, σ VM Von-Mises stress, MPa; σ MASTERThe principal allowable stress is expressed in MPa; K is the material coefficient.

[0057] R is the material safety factor. For the s*s mesh model, γR is 1.05, and for the 3s*3s mesh model, γR is 1.2. γm is the deformation resistance safety factor, which is 1.02.

[0058] For fine-mesh models, the yield strength criterion can be appropriately scaled up, but the following requirements must be met:

[0059] σ VM ≤K S σ MASTER

[0060] Where Ks is the magnification factor for fine mesh evaluation, with 1.34 for elements near the weld and 1.53 for elements not near the weld.

[0061] The loading condition, combined with the aforementioned wave load condition, serves as the calculation condition for the whole-ship finite element analysis. After determining various loads, the finite element model is used to load and solve the above four calculation conditions, obtaining the stress and strain response results of the entire ship structure. The analysis results identify stress concentration areas in the hull structure. These areas are weak points in the structural design, including the end elbows of the hatch coamings and the front hatch corners of the engine room.

[0062] Based on the finite element analysis results, the overall strength of the ultra-large container ship was assessed. The assessment included whether the structural strength met design requirements and whether the stress levels in stress concentration areas were within permissible limits.

[0063] Example 2: During the design phase of an international shipping route, engineers collected historical oceanographic data for the route and selected three representative sets of equivalent regular waves (wave height 4 m, period 8 seconds; wave height 6 m, period 10 seconds; wave height 8 m, period 12 seconds) using spectral analysis. A full-ship model of the ultra-large container ship was established in finite element software and meshed. Static and dynamic finite element analyses were performed on the hull under different load conditions. The results showed that, under extreme sea conditions, the hull stress did not exceed the material's yield strength, meeting the design requirements.

[0064] Example 3: During ship operation, the original analysis model was adjusted using the latest marine forecast data. Based on the forecast data, a new equivalent regular wave group (wave height 5 meters, period 9 seconds) was added, and a new finite element analysis was performed. The results showed that although the new wave group was added, the hull structure remained within a safe range, but high stress concentrations appeared in some areas, and reinforcement treatment was recommended during the next overhaul.

[0065] The description of the embodiments disclosed above enables those skilled in the art to practice or use the invention. Various modifications to these embodiments will be readily understood by those skilled in the art, and the general principles defined herein can be applied to other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not limited to the embodiments shown herein, but should be granted the broadest scope based on the principles and innovative features disclosed herein.

Claims

1. A method for analyzing the overall strength of an ultra large container ship based on the equivalent design wave method, characterized in that: The method comprises the following steps: (1) using long-term sea state prediction data of the ship navigation area, obtaining wave distribution characteristics of the target sea area under different seasons and different navigation conditions, main parameters including wave height, wave period and wave direction; based on long-term sea state prediction data, determining equivalent regular waves by statistical analysis method, the wave height and period of the equivalent regular waves should match the average wave height and main period in the long-term sea state prediction; (2) establishing a three-dimensional finite element model of the whole ship structure according to the design structural drawings of the ultra-large container ship; (3) meshing the three-dimensional finite element model of the whole ship structure, having sufficient grid density in the stress concentration area and weak part, using fine grid in the stress concentration area and using coarse grid in the remaining part, the transition area of the bow, the stern, the bottom and the side plate adopts tetrahedral mesh for division; the main deck and the keel adopt hexahedral mesh for division; combining the models of each region after meshing to obtain the three-dimensional finite element model of the whole ship structure; (4) selecting equivalent regular waves and typical working conditions, calculating wave load and applying it to the three-dimensional finite element model, calculating loading conditions according to the design specification requirements of the ship, combining the loading conditions with the aforementioned wave load conditions as the calculation conditions of the whole ship finite element analysis, loading and solving the calculation conditions to obtain the stress and strain response results of the whole ship structure, finding out the stress concentration area in the ship structure through the analysis results, and evaluating the overall strength of the ultra-large container ship.

2. The method of claim 1, wherein, In step 1, the wave period and wavelength of the equivalent regular wave are determined by the observation value of the actual irregular wave, the long-term irregular wave spectrum is determined, the wave characteristics are analyzed, the statistical data are extracted, and the irregular wave is decomposed into a series of regular waves according to the long-term irregular wave spectrum through the spectral decomposition method, and the representative regular wave is selected.

3. The method of claim 1, wherein the method is characterized by: In step 2, the coordinate system of the three-dimensional finite element model adopts the right-hand rectangular coordinate system, and the whole ship finite element modeling still adopts the large general finite element software. The three-dimensional finite element model of the ship includes the main structures of the ship body, such as the deck, the outer plate, the engine room, the funnel, the upper bottom, the longitudinal bulkhead, the transverse bulkhead, the beam and the longitudinal girder. All the plates are represented by shell elements. The longitudinal bones and reinforcing bars on all the plates that can bear load are simulated by eccentric beams. All the bones are represented by beam elements with axial stiffness, torsional stiffness, two-way shear and bending stiffness. The model should consider the eccentricity of the neutral axis. Large-sized sections are simulated by plate elements for their web plates and by beam elements with eccentricity for their face plates. According to the actual structure, opening structures such as manholes are simulated and constructed.

4. The method of claim 1, wherein, Before step 4, the support and boundary conditions of the ship body are reasonably set to simulate the stress and deformation of the ship body under the actual navigation state. The boundary conditions include the fixed constraint of the stern and the free support of the front and back of the ship body. In the application of finite element strength analysis, in order to eliminate the influence of rigid displacement, the rigid displacement of the ship should be limited and constrained to calculate the relative deformation of the structure. In this process, it is possible to appear static imbalance, in the constraint point and the annex to produce a larger support reaction force and affect the results; so select three far from the center position of the node to apply the degree of freedom constraints, while not on the subsequent ship structure deformation and stress caused by the impact.

5. The method of claim 1, wherein, Before the operation of step 4, The weight of the empty ship mainly includes the weight of the hull steel and the weight of the equipment attached to it. In the model, the weight of the equipment is completed by adding mass units, and the weight of the steel is simulated by using the above finite element model. The outer plate static water pressure and wave pressure: define a domain function 1.005e-5*(14800-z) in the finite element software, and apply the domain function to the model in the form of pressure, P={ρg(T-Z), its unit is MPa, where P is the static water pressure, ρ is the density, T is the minimum draft of the considered loading condition, Z is the depth of the point below the water surface; The static water pressure value is proportional to the height of the ship's waterline to the bottom, composed of two parts of pressure, one is the static water pressure outside the hull, the other is the static pressure of the ballast water and fuel on the bulkhead; When performing quasi-static analysis on the finite element model, if the force system of the entire ship cannot be balanced, stress concentration will occur at the restricted free place, resulting in deviation of the calculation results; Therefore, the inertial balance force should be applied.

6. The method of claim 1, wherein, To evaluate the overall strength of the super large container ship, the yield strength of the main components of the hull, find out the high stress area and analyze it, the coarse grid model of the three-dimensional finite element model should meet the following requirements: σ VM ≤σ MASTER where σ VM is the Von-Mises stress, MPa; σ MASTER is the main allowable stress, MPa; K is the material coefficient; R is the material safety factor, for s*s grid model, γR takes 1.05, for 3s*3s, γR takes 1.2; γm is the anti-deformation safety factor, which takes 1.02; For fine grid model, its yield strength criterion can be appropriately enlarged, which needs to meet the following requirements: σ VM ≤K S σ MASTER Where Ks is the enlargement factor when evaluating the fine grid, the unit near the weld takes 1.34, and the unit not near the weld takes 1.53.

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