CN112052522B - A simplified calculation method of ship hull structure optimization based on fatigue strength - Google Patents

Abstract

The invention belongs to the technical field of ship hull structure optimization design, in particular to a simplified calculation method for ship hull structure optimization based on fatigue strength. In the present invention, the relationship between the hot spot stress of the structure and the fatigue cumulative damage degree of the wave direction, and the fatigue cumulative damage degree and the fatigue cumulative total damage of each calculated wave direction under the specific calculation frequency of a certain calculation wave direction of the typical node structure of the hull A simplified calculation method of hull structure optimization based on fatigue strength is given, which can effectively solve the problem that the position where the fatigue damage of the hull structure is large is often the part with more complex structure. Compared with the problem that the calculation process of the spectrum analysis method is complicated, which leads to the heavy calculation work, the present invention can greatly reduce the calculation workload, improve the calculation efficiency, and bring great advantages for the optimization design of structural fatigue strength based on the spectrum analysis method. Great convenience.

Description

A simplified calculation method of ship hull structure optimization based on fatigue strength

technical field

The invention belongs to the technical field of ship hull structure optimization design, in particular to a simplified calculation method for ship hull structure optimization based on fatigue strength.

Background technique

Fatigue damage is one of the main failure modes of the hull structure. Fatigue cracks in the structure need to be repaired in time, otherwise, the crack expansion to a certain extent will lead to catastrophic damage to the hull structure. Several major classification societies in the world have made relevant regulations on fatigue strength checking. At present, the fatigue strength assessment methods proposed by various classification societies can be divided into simplified calculation and direct calculation; direct calculation is divided into design wave method and spectrum. Analysis. Among them, the spectral analysis method is to first calculate the wave load and hull structure stress response of the target ship under different loading conditions, wave directions and calculation frequencies, and then obtain the stress transfer function; then, combine the wave spectrum and sea state data of a specific sea area to obtain the stress The response spectrum and the short-term distribution of stress; finally, based on the linear cumulative damage theory and selecting the corresponding curve, the total cumulative damage degree of the fatigue assessment part is calculated; the flow of the spectrum analysis method is shown in Figure 1.

In fact, not all the initial design schemes of hull structures can meet the requirements of structural fatigue strength design service life; therefore, in the actual fatigue strength assessment of typical hull joint structures, it is necessary to conduct optimization design research on hull structures that cannot meet the design requirements. The commonly used method is to continuously modify the finite element model of the hull structure according to the formed structural optimization design scheme, and then repeatedly perform the spectral analysis and calculation, the process of which is shown in Figure 2; however, the position where the fatigue damage of the hull structure is large is often the part with very complex structure, and The calculation process of spectral analysis is complicated and the workload of calculation is very heavy, which brings great difficulty to the optimal design of the typical node structure of the hull.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a simplified calculation method of ship hull structure optimization based on fatigue strength.

The object of the present invention is achieved through the following technical solutions: comprise the following steps:

Step 1: Input the hull structure model, the wave direction u to be calculated and the frequency ω of the wave direction, and convert the hull structure model into a finite element analysis model;

Step 2: Evaluate the fatigue strength of the finite element analysis model of the hull structure, and obtain the fatigue hot spots of the hull structure;

Step 3: Apply n wave directions of different angles to the fatigue hot spot of the hull structure, and obtain the fatigue cumulative damage degree D _i at the fatigue hot spot under each wave, i=1,2,...,n; select the corresponding fatigue accumulation The wave direction with the maximum damage degree is regarded as the most dangerous wave direction; the fatigue cumulative damage degree of the most dangerous wave direction to the fatigue hot spot is D _max ;

Step 4: Calculate the deterministic relationship coefficient p of the fatigue cumulative damage degree _Dmax of the fatigue hot spot and the total damage degree D under the most dangerous wave;

Step 5: Apply the most dangerous wave directions of different frequencies to the fatigue hot spot of the hull structure, obtain the RAO curve of the fatigue hot spot in the downward direction of the most dangerous wave, select the frequency corresponding to the maximum stress value of the structural hot spot as the most dangerous frequency, and obtain The maximum hot spot stress of the structure is σ _max ;

Step 6: Calculate the relationship coefficient R between the structural hot spot stress of the fatigue hot spot and the cumulative damage degree of wave fatigue;

Among them, m is the set constant;

Step 7: Calculate the structural hot spot stress σ _hot at the fatigue hot spot when the wave direction u with frequency ω is applied to the fatigue hot spot;

Step 8: Calculate the fatigue cumulative total damage degree D _u at the fatigue hot spot when the wave direction u with frequency ω is applied to the fatigue hot spot;

The present invention can also include:

In the step 7, when the wave direction u with frequency ω is applied to the fatigue hot spot, the method for calculating the structural hot spot stress σ _hot at the fatigue hot spot is as follows:

Four refined finite element mesh elements near the fatigue hot spot are selected to obtain the element stress σ _kx , σ _ky , τ _kxy , k=1, 2, 3, 4; the structural hot spot stress σ _hot at the fatigue hot spot is:

The relationship coefficient R between the structural hot spot stress of the fatigue hot spot and the cumulative damage degree of wave fatigue can also be calculated by the following method:

为设定的参数，根据船体结构疲劳规范进行选取；Γ()为伽玛函数；p_nj为第j个浪向对应的船舶装载状态n出现的概率；p_ij为第j个浪向对应的海况i出现的概率；p_j为第j个浪向出现的概率；ω_ek为船舶的遭遇频率；θ为浪向角；Q_k为在浪向j的某个计算频率k下的结构热点应力与航向j下各计算频率对应的结构热点应力的加和结果的比值，可根据有限元模型直接计算结果获得，在船体典型节点结构的结构型式不变的情况下，其值为定值，且

k＝1,2,…l，l代表计算频率的总数，根据具体船型来设定；σ_hot-jk为在航向j的频率k下的结构热点应力；

是与选定应用于该简化计算方法的计算频率相对应的结构热点应力的Q_k值；G_ηη(ω_ek,θ)为以遭遇频率ω_ek表达的与航向角θ有关的波能谱，且

ω_k为波浪频率；U为设定的船舶的航速；G_ηη(ω_k)为用有义波高H_s和平均跨零周期T_z来表达的波浪谱，且

有义波高H_s和平均跨零周期T_z通过选定的海浪谱和海况资料来确定。Among them, n is the total number of calculated wave directions; δ is the voyage coefficient; T _L is the recovery period of ship fatigue calculation;

is the set parameter, selected according to the fatigue specification of the hull structure; Γ() is the gamma function; p _nj is the probability of the occurrence of the ship loading state n corresponding to the jth wave direction; p _ij is the corresponding to the jth wave direction. The probability of occurrence of sea state i; p _j is the probability of occurrence of the jth wave direction; ω _ek is the encounter frequency of the ship; θ is the wave direction angle; Q _k is the structural hot spot stress at a certain calculation frequency k of the wave direction j The ratio of the summation results of the structural hot spot stress corresponding to each calculation frequency in the course j can be obtained directly from the finite element model. When the structural type of the typical node structure of the hull remains unchanged, its value is a fixed value, and

k=1,2,...l, l represents the total number of calculation frequencies, which are set according to the specific ship type; σ _hot-jk is the structural hot spot stress at the frequency k of the course j;

is the Q _k value of the structural hot spot stress corresponding to the calculation frequency selected to be applied to this simplified calculation method; G _ηη (ω _ek , θ) is the wave energy spectrum related to the heading angle θ expressed in the encounter frequency ω _ek , and

ω _k is the wave frequency; U is the set speed of the ship; G _ηη (ω _k ) is the wave spectrum expressed by the significant wave height H _s and the average zero-crossing period T _z , and

The significant wave height H _s and the mean zero crossing period T _z are determined from selected wave spectrum and sea state data.

The beneficial effects of the present invention are:

In the present invention, the relationship between the hot spot stress of the structure and the fatigue cumulative damage degree of the wave direction, and the fatigue cumulative damage degree and the fatigue cumulative total damage of each calculated wave direction under the specific calculation frequency of a certain calculation wave direction of the typical node structure of the hull A simplified calculation method of hull structure optimization based on fatigue strength is given, which can effectively solve the problem that the position where the fatigue damage of the hull structure is large is often the part with more complex structure. Compared with the problem that the calculation process of the spectrum analysis method is complicated, which leads to the heavy calculation work, the present invention can greatly reduce the calculation workload, improve the calculation efficiency, and bring great advantages for the optimization design of structural fatigue strength based on the spectrum analysis method. Great convenience.

Description of drawings

Figure 1 is a flow chart of spectral analysis.

Figure 2 shows the optimization flow chart of fatigue spectrum analysis.

Figure 3 is a flowchart of the simplified method for fatigue optimization.

Figure 4 is a graph of the stress gradient near the fatigue hot spot.

Figure 5 is a schematic diagram of a hot spot stress interpolation method.

Figure 6 is a schematic diagram of the location of fatigue hot spots.

Figure 7 is a finite element refinement of the fatigue hot spot.

Figure 8 shows the RAO curve of the fatigue hot spot in 180-degree wave direction.

FIG. 9 is a table showing the relationship between the cumulative damage degree D _i of wave direction fatigue and the total cumulative damage degree D of fatigue in the embodiment of the present invention.

FIG. 10 is a table of optimization results obtained by adopting a simplified calculation method for fatigue hot spots in an embodiment of the present invention.

FIG. 11 is a table of optimization results obtained by adopting the spectral analysis calculation method for fatigue hot spots in the embodiment of the present invention.

FIG. 12 is an error analysis table of an optimization result of a simplified calculation method in an embodiment of the present invention.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings.

The invention relates to the field of ship hull structure optimization design, in particular to a simplified calculation method for ship hull structure optimization design based on fatigue strength evaluation results. The present invention mainly analyzes the relationship between the structural hot spot stress and the cumulative fatigue damage degree of the wave direction under the specific calculation frequency of a certain calculation wave direction of the typical node structure of the hull, and the relationship between the fatigue cumulative damage degree and the fatigue cumulative damage degree of each calculated wave direction. The relationship of damage degree, a simplified calculation method of ship structure optimization based on fatigue strength is given. This method can effectively solve the problem that the position where the fatigue damage of the hull structure is large is often the part with more complex structure, and the calculation process of the spectrum analysis method is complicated, which leads to heavy calculation work. The optimal design of the node structure brings a very difficult problem. The simplified calculation method can greatly reduce the calculation workload, improve the calculation efficiency, and bring great convenience to the optimization design of structural fatigue strength based on the spectral analysis method.

The present invention aims to provide a simplified calculation method for optimization of ship hull structure based on fatigue strength, so as to solve the problems of long time-consuming and huge workload in the optimization process in the optimization design of typical ship hull structure using the spectral analysis method.

In order to realize the simplification of the optimization design process of the typical hull structure based on the fatigue strength using the spectral analysis method, this method uses the typical hull structure in a certain calculation frequency of a certain wave direction, the hot spot stress of the structure and the fatigue cumulative damage of the wave direction. and the relationship between the fatigue cumulative damage degree of each calculated wave direction and the fatigue cumulative damage degree, the simplified calculation method of the optimal design of the hull structure is determined. This method can greatly reduce the computational workload, improve the computational efficiency, and bring great convenience to the optimization design of structural fatigue strength based on the spectral analysis method.

In the spectral analysis method, considering N _load loading states, the fatigue cumulative damage degree of the hull structure is expressed as:

m-所用S-N曲线的两个参数，根据船体结构疲劳规范进行选取；

-伽玛函数；m_0ijn-第n个装载及海况i和航向j下，应力响应谱的零阶矩；p_n-第n个装载的时间分配系数；p_i-第i个海况出现的概率；p_j-第j个航向出现的概率；υ_ijn-第n个装载，海况i和航向j的应力响应过零率，

m_2ijn-第n个装载及海况i和航向j下，应力响应谱的二阶矩。In the formula: δ - coefficient of voyage; T _L - recovery period of ship fatigue calculation, it is stipulated that T _L = 20 years = 6.3x10 ⁸ seconds;

m - The two parameters of the SN curve used are selected according to the fatigue specification of the hull structure;

- Gamma function; m _0ijn - the zeroth moment of the stress response spectrum for the nth loading and sea state i and heading j; p _n - the time distribution coefficient of the nth loading; p _i - the probability of occurrence of the ith sea state ; p _j - the probability of occurrence of the jth heading; υ _ijn - the nth load, the zero-crossing rate of the stress response in sea state i and heading j,

m _2ijn - the second moment of the stress response spectrum for the nth load and at sea state i and heading j.

For the present invention, its principle is as follows:

1) Calculation method of hot spot stress

Because the stress at the hot spot cannot be directly extracted, it is calculated by linear extrapolation according to the stress gradient near the hot spot. As shown in Figure 4, the surface stress at 3t/2 and t/2 distances from the hot spot is extracted from the refined mesh model, and the stress at the hot spot is obtained by extrapolation according to the linear relationship.

At the same time, since the spectral analysis method is to treat the loads generated by the regular waves of different frequencies with unit amplitude as the real part and the imaginary part, and then obtain the corresponding responses σ _c , σ _s ; therefore, it is necessary to synthesize the above stresses, The stress calculation result of the hull structure at the corresponding calculation frequency is obtained, and the resultant stress can be expressed as σ=σ _c +iσ _s , thus:

Select four refined finite element mesh elements near the fatigue hot spot, and the stress interpolation method as shown in Fig. 5 can be used, and the stress σ _kx , σ _ky , τ _kxy ( k=1, 2, 3, 4) According to formula (3), the stress σ _hot,x , σ _hot,y , and τ _hot,xy at the hot spot are obtained by interpolation calculation.

Then, the hot spot stress σ _hot can be calculated and determined by formula (4):

And σ _hot is taken as the calculated value in formula (4) within 45° of the plane of the interpolation unit.

2) Simplified calculation method of cumulative damage degree of hull structure in wave direction

According to formula (1), the cumulative damage degree of wave direction fatigue of a typical node structure of a hull in a certain loading state and a certain wave down in a certain sea state can be calculated by formula (5):

In the formula: n- represents the total number of calculated wave directions; p _nj - the probability of the occurrence of the ship loading state n corresponding to the j-th wave direction; p _ij - is the probability of the occurrence of the sea state i corresponding to the j-th wave direction;

Further, formula (5) is simplified. At this time, it is only necessary to obtain the structural hot spot stress σ _hot-jk of the typical node structure of the hull at a certain calculation frequency in a certain wave direction, which can be calculated according to formula (6). The cumulative damage degree of directional fatigue of typical node structure of hull:

Among them, R _jk is defined as the relationship coefficient between the structural hot spot stress σ _hot-jk and the cumulative damage degree D _j of wave fatigue, as follows:

是与选定应用于该简化计算方法的计算频率相对应的结构热点应力的Q_k值；G_ηη(ω_ek,θ)-以遭遇频率ω_ek表达的与航向角θ有关的波能谱，且

ω_k-波浪频率；U-航速，knots；G_ηη(ω_k)-用有义波高H_s和平均跨零周期T_z来表达的波浪谱，且

其中，H_s-有义波高，T_z-平均跨零周期，二者可通过选定的海浪谱和海况资料来确定。In the formula: ω _ek - the encounter frequency of the ship; θ - the heading angle, rad; Q _k is defined as the addition of the structural hot spot stress at a certain calculation frequency k of the heading j and the structural hot spot stress corresponding to each calculation frequency on the heading j. The ratio of the result to the result can be obtained directly from the calculation result of the finite element model. Under the condition that the structural type of the typical node structure of the hull remains unchanged, its value is a fixed value, and

is the Q _k value of the structural hot spot stress corresponding to the calculation frequency chosen to apply this simplified calculation method; G _ηη (ω _ek , θ) - the wave energy spectrum expressed in the encounter frequency ω _ek related to the heading angle θ, and

ω _k - wave frequency; U - speed, knots; G _ηη (ω _k ) - wave spectrum expressed in terms of significant wave height H _s and mean zero crossing period T _z , and

Among them, H _s - significant wave height, T _z - mean zero crossing period, both of which can be determined by selected wave spectrum and sea state data.

R _jk can also be obtained directly by the finite element model, and its value is a fixed value when the structural type of the typical node structure of the hull remains unchanged; k represents the frequency, k=1, 2,...l; l represents the calculation The total number of frequencies is set according to the specific ship type; σ _hot-jk is the structural hot spot stress at the frequency k of the course j, which can be calculated according to formula (4); m is generally taken as 3.

3) Simplified calculation method of fatigue cumulative total damage degree of hull structure

In the spectral analysis method, the fatigue cumulative damage degree of the typical node structure of the hull is related to the wave direction fatigue cumulative damage degree as follows:

Among them: j represents the wave direction, j=1,2,...,n; n represents the total wave direction number, which is determined by the size of the wave direction angle. When the wave direction angle is set to 30° interval, take n=12; D _j represents the fatigue cumulative damage in the j wave direction.

Therefore, the fatigue cumulative damage degree of the typical node structure of the hull can also be calculated by the following formula:

where: σ _hot-jk is the structural hot spot stress corresponding to the calculated frequency k in the wave direction j.

Further, in this patent, for a typical node structure of a hull that satisfies the proportion of the cumulative fatigue damage degree in the wave direction to a fixed value, it is determined that the cumulative fatigue damage degree in a certain wave direction and the total fatigue cumulative damage degree are related. The following relationship:

Among them, p _nj is defined as the relationship coefficient between the wave-direction fatigue cumulative damage degree of the hull structure and its total fatigue cumulative damage degree, which can be obtained directly by the finite element model. Its value is a fixed value; j represents the wave direction, and the definition is the same as above.

At this time, the fatigue cumulative damage degree of the typical node structure of the hull can be calculated by the following formula:

此时有：Further, K _jk can be defined as the relationship coefficient between the hot spot stress of the structure and the cumulative total damage degree of fatigue, let

At this point there are:

The concrete method of the present invention is:

For a typical node structure of the hull, in the process of structural optimization design, it is only necessary to calculate the structural hot spot stress at any calculation frequency in a certain wave direction, and then the fatigue accumulation of the typical node structure under the optimization scheme can be calculated. The total damage degree does not need to be calculated by the structural fatigue spectrum analysis of all wave directions and all frequencies.

After obtaining the structural hot spot stress of the typical node structure of the hull under a certain wave direction and a certain calculation frequency, the cumulative damage degree of wave fatigue in the wave direction can be calculated according to the formula (6), which can be calculated according to the formula (8) Or formula (10) to calculate the fatigue cumulative total damage of the structure. After obtaining the fatigue cumulative damage degree of a typical node structure of the hull in a certain wave direction, and it can be determined that there is a definite relationship between the fatigue cumulative damage degree in the wave direction and the fatigue cumulative damage degree, the structure can be calculated according to formula (9). Fatigue cumulative total damage;

A simplified calculation method of hull structure optimization based on fatigue strength can be summarized as:

1) The simplified calculation of the hull structure optimization is carried out through the relationship between the structural hot spot stress and the fatigue cumulative total damage D under a certain calculation frequency of a certain wave direction of the typical node structure of the hull;

2) The simplified calculation of the optimization of the hull structure is carried out by the relationship between the cumulative damage degree D _j of fatigue in the wave direction and the total cumulative damage degree D of fatigue of the typical node structure of the hull;

3) The coefficients Q _k , R _jk are obtained according to the calculation formulas of Q _k and R _jk , or the calculation results of the spectrum analysis of the typical node structure of the hull in the full wave direction and full frequency; then, the typical hull to be optimized can be calculated according to formula (6). The cumulative damage degree D _j of the nodal structure under the direction of the studied wave.

Modify the structural model according to the optimal design scheme of the structure to be optimized, and apply the calculated load to it, and use the element stress σ _kx , σ _ky , τ _kxy (k=1, 2, 3, 4) obtained by formula (2); then , and the stress σ _hot,x , σ _hot,y , τ _hot,xy at the hot spot is obtained by interpolation calculation according to formula (3); and the hot spot stress σ _hot is obtained by calculation according to formula (4).

Combine formula (5) and formula (6), or obtain the coefficient R _jk through the spectral analysis and calculation results of the typical hull node structure in all wave directions and all frequencies; The wave-direction fatigue cumulative damage degree D _j for the downward wave.

R _jk can also be calculated by the following formula:

是与选定应用于该简化计算方法的计算频率相对应的结构热点应力的Q_k值；G_ηη(ω_ek,θ)-以遭遇频率ω_ek表达的与航向角θ有关的波能谱，且

ω_k-波浪频率；U-航速，knots；G_ηη(ω_k)-用有义波高H_s和平均跨零周期T_z来表达的波浪谱，且

其中，H_s-有义波高，T_z-平均跨零周期，二者可通过选定的海浪谱和海况资料来确定。In the formula: ω _ek - the encounter frequency of the ship; θ - the heading angle, rad; Q _k is defined as the addition of the structural hot spot stress at a certain calculation frequency k of the heading j and the structural hot spot stress corresponding to each calculation frequency on the heading j. The ratio of the result to the result can be obtained directly from the calculation result of the finite element model. Under the condition that the structural type of the typical node structure of the hull remains unchanged, its value is a fixed value, and

is the Q _k value of the structural hot spot stress corresponding to the calculation frequency chosen to apply this simplified calculation method; G _ηη (ω _ek , θ) - the wave energy spectrum expressed in the encounter frequency ω _ek related to the heading angle θ, and

ω _k - wave frequency; U - speed, knots; G _ηη (ω _k ) - wave spectrum expressed in terms of significant wave height H _s and mean zero crossing period T _z , and

Among them, H _s - significant wave height, T _z - mean zero crossing period, both of which can be determined by selected wave spectrum and sea state data.

The fatigue cumulative damage degree of the typical node structure of the hull can be calculated according to formula (8). At this time, all the calculated wave directions of the structure need to be studied; when p _nj is determined, the fatigue cumulative total damage degree of the typical node structure of the hull It can also be calculated according to formula (10).

The characteristics of the present invention are as follows:

或

即可计算得到该优化方案下的典型节点结构的疲劳累积损伤度，而不需要进行全浪向全频率的结构疲劳谱分析计算。In the process of carrying out the structural optimization design of fatigue hot spots, it is only necessary to calculate the structural hot spot stress σ _hot of the fatigue hot spot at a certain calculation frequency in a certain calculation wave direction, and then combine the formula

or

The fatigue cumulative damage degree of the typical joint structure under the optimization scheme can be calculated without the need to perform the structural fatigue spectrum analysis and calculation in the full wave direction and the full frequency.

时间完成疲劳热点的优化设计工作。1) The simplified calculation method can greatly reduce the calculation workload and improve the calculation efficiency. If there are n calculation wave directions and l calculation frequencies, then this method can at least use the spectral analysis calculation time of the original full wave direction and full frequency.

Time to complete the optimal design of fatigue hot spots.

2) The simplified calculation method has a simple calculation process and high reliability. After a large amount of data analysis and research, it is found that the error between the fatigue cumulative damage degree of fatigue hot spots calculated by this simplified calculation method and the spectrum analysis calculation method using the full wave direction and full frequency is basically within the range of 1%, and the maximum error can be controlled within 5%. about.

3) The calculation result of this simplified calculation method is on the safe side. By taking the values of the fixed coefficient R _jk and the coefficient p _nj reasonably, it can be ensured that the error between the calculation result of the simplified calculation method and the calculation result of the spectrum analysis method using the full wave direction and full frequency is within an acceptable range, and then the hull can be fully guaranteed. The fatigue strength of the structure is safe.

This simplified calculation method is valid for any wave direction and at any frequency of a wave direction; however, the accuracy of the method can be guaranteed in the most dangerous wave direction of a typical structure and the most dangerous frequency in the downward direction of the wave Higher and better.

There are certain requirements for the selection of R _jk and p _nj values. When the requirements are met, the simplified calculation method has the best effect.

R _jk , p _nj are related to the structural type of the hull structure; in the case that the structural type needs to be changed for structural optimization, the values of R _jk and p _nj need to be re-determined; while the structural type of the hull structure remains unchanged, only changing When the thickness of each structural plate of the structure, the values of R _jk and p _nj remain unchanged.

Example 1:

Research object: The target ship has a tall and long superstructure, and it terminates near the midship, resulting in obvious stress concentration at the connection between the end of the superstructure and the main deck (1 deck). When evaluating the fatigue strength of the typical node structure of the target ship, it is found that the fatigue cumulative damage of the fatigue hot spots marked in Fig. 6 is too large, far greater than 1. Therefore, this fatigue hotspot is selected as the research object; at the same time, it is found that there is a definite relationship between the cumulative damage degree of wave-direction fatigue and the total cumulative damage degree of fatigue of the structure. The schematic diagram of the fatigue hotspot research location is shown in Figure 6, and the finite element refinement model of the research location is shown in Figure 7.

The relationship between the cumulative damage degree of each wave direction and the total cumulative damage degree of fatigue in the initial design scheme of the fatigue hot spot is shown in Figure 9. Among them, the number of wave directions calculated by the spectral analysis of fatigue hot spots is n=12, and the number of calculation frequencies of each wave direction is k=18. It can be seen from Figure 9 that the most dangerous wave direction of the fatigue hot spot is 180°. Therefore, considering the security of the calculation result, according to the value principle of p _nj , first determine p _nj =p _n-7 =0.15.

The RAO curve of the fatigue hot spot in the 180° wave direction is shown in Figure 8. It can be seen from Fig. 8 that the most dangerous frequency of the fatigue hot spot in this wave direction is 0.8; correspondingly, the hot spot stress of the structure is σ _hot =57.95Mpa.

At this time, considering the safety of the simplified calculation result, R _jk =R ₇₈ =0.000006 is determined according to the calculation method and value principle of R _jk .

First, the method of increasing the thickness of the structural plate at the fatigue hot spot position is considered to carry out the structural optimization design work. Using the simplified calculation method, the final optimization result of the optimization scheme is finally obtained, as shown in Figure 10.

For this optimization scheme, a spectrum analysis and calculation in all wave directions and all frequencies is performed to obtain the fatigue cumulative total damage degree D of the fatigue hot spot and the wave direction fatigue cumulative damage degree D ₇ of the 180° wave direction, as shown in Figure 11.

For this optimization scheme, the optimization results of fatigue hot spots using the simplified calculation method and the optimization results of the spectral analysis method have been obtained. Now, the error analysis of the optimization results obtained by the simplified calculation method is carried out, as shown in Figure 12.

It can be seen from Fig. 12 that, compared with the calculation result of the spectrum analysis method in the full wave direction and the full frequency, the error of the fatigue cumulative total damage obtained by the simplified calculation method for the fatigue hot spot is 1.08%, and the error is small. It can be considered that this simplified calculation method is used. The calculation method is feasible. At the same time, it can be seen from Fig. 11 that the optimization scheme fails to make the fatigue hot spot meet the design requirements of fatigue life. Therefore, it is necessary to consider the optimization scheme of changing the structural type; for this scheme, due to the change of the structural type of the fatigue hot spot, the In this simplified calculation method, the values of R _jk and p _nj need to be re-determined, and the specific calculation process and the final optimization result will not be repeated.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims (3)

1. A ship hull structure optimization and simplification calculation method based on fatigue strength is characterized by comprising the following steps:

step 1: inputting a hull structure model, a wave direction u to be calculated and the frequency omega of the wave direction, and converting the hull structure model into a finite element analysis model;

and 2, step: carrying out fatigue strength evaluation on the finite element analysis model of the hull structure to obtain a fatigue hot spot of the hull structure;

and 3, step 3: applying n wave directions with different angles to fatigue hot spots of the hull structure, and acquiring fatigue accumulated damage degrees D at each downward fatigue hot spot _i I ═ 1,2, …, n; selecting the wave direction corresponding to the maximum value of the fatigue accumulated damage degree as the most dangerous wave direction; the fatigue cumulative damage degree of the most dangerous wave direction to the fatigue hot spot is D _max ；

And 4, step 4: calculating the fatigue accumulated damage degree D of the fatigue hot spots under the most dangerous waves _max A deterministic relationship coefficient p with the total damage degree D;

and 5: applying the most dangerous wave directions with different frequencies to the fatigue hot spots of the hull structure, acquiring an RAO curve graph of the fatigue hot spots under the most dangerous wave directions, selecting the frequency corresponding to the maximum stress value of the structure hot spot as the most dangerous frequency, and acquiring the maximum stress value sigma of the structure hot spot _max ；

Step 6: calculating a relation coefficient R of structural hot spot stress of the fatigue hot spots and wave-direction fatigue accumulated damage degree;

wherein m is a set constant;

and 7: calculating the structural hot spot stress sigma at the fatigue hot spot when applying a wave direction u with a frequency omega to the fatigue hot spot _hot ；

And 8: calculating the fatigue cumulative total damage degree D at the fatigue hot spot when the wave direction u with the frequency omega is applied to the fatigue hot spot _u ；

2. The hull structure optimization simplified calculation method based on fatigue strength as claimed in claim 1, characterized in that: in the step 7, the structural hot spot stress sigma at the fatigue hot spot when the wave direction u with the frequency omega is applied to the fatigue hot spot is calculated _hot The method comprises the following steps:

selecting four refined finite element grid units near the fatigue hot point to obtain the stress sigma of each unit _kx 、σ _ky 、τ _kxy K is 1,2,3, 4; structural hot spot stress σ at fatigue hot spot _hot Comprises the following steps:

3. the hull structure optimization and simplification calculation method based on the fatigue strength according to claim 1 or 2, characterized in that: the relation coefficient R of the structural hot spot stress of the fatigue hot spot and the wave-direction fatigue accumulated damage degree is calculated by the following method:

wherein n is the total number of wave directions; delta is the coefficient of the air velocity; t is _L A recovery period calculated for the vessel fatigue;

selecting the parameters according to the fatigue standard of the hull structure; Γ () is a gamma function; p is a radical of formula _nj The probability of occurrence of the loading state n of the ship corresponding to the jth wave direction; p is a radical of formula _ij The probability of occurrence of the sea state i corresponding to the jth wave direction; p is a radical of _j Is the probability of occurrence of the jth wave direction; omega _ek Is the encounter frequency of the ship; theta is a wave direction angle; q _k The ratio of the structural hot spot stress at a certain calculation frequency k of the wave direction j to the sum of the structural hot spot stresses corresponding to each calculation frequency in the course j can be obtained by directly calculating the result according to a finite element model, and the value is a fixed value under the condition that the structural type of the typical node structure of the ship body is not changed, in addition, the structural hot spot stress at the certain calculation frequency k of the wave direction j and the sum of the structural hot spot stresses corresponding to each calculation frequency in the course j are obtained by directly calculating the result according to the finite element model, and in addition, the value is a fixed value under the condition that the structural type of the typical node structure of the ship body is not changed

k is 1,2, … l, l represents the total number of calculation frequencies, set according to the specific ship type; sigma _hot-jk Is the structural hot spot stress at frequency k in the heading j;

q of structural hot spot stresses corresponding to the calculation frequency selected for application in the simplified calculation method _k A value; g _ηη (ω _ek θ) is at an encounter frequency ω _ek Expressed angle theta with headingRelated wave energy spectrum, and

ω _k is the wave frequency; u is the set navigational speed of the ship; g _ηη (ω _k ) Is high H with sense wave _s And average over zero period T _z To express a wave spectrum, and

height of sense wave H _s And average over zero period T _z Is determined by the selected wave spectrum and sea state data.

Images