CN112052522B - Ship structure optimization simplified calculation method based on fatigue strength - Google Patents

Ship structure optimization simplified calculation method based on fatigue strength Download PDF

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CN112052522B
CN112052522B CN202011007513.7A CN202011007513A CN112052522B CN 112052522 B CN112052522 B CN 112052522B CN 202011007513 A CN202011007513 A CN 202011007513A CN 112052522 B CN112052522 B CN 112052522B
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刘玉超
任慧龙
冯国庆
周学谦
李陈峰
孙士丽
刘宁
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Abstract

The invention belongs to the technical field of optimization design of hull structures, and particularly relates to a hull structure optimization and simplification calculation method based on fatigue strength. According to the invention, through the relationship between the structural hot spot stress and the wave-direction fatigue accumulated damage degree of the typical node structure of the ship body under a certain wave-direction calculation specific frequency and the relationship between each calculated wave-direction fatigue accumulated damage degree and the fatigue accumulated total damage degree, the ship body structure optimization and simplification calculation method based on the fatigue strength is provided, and the problem that the position with larger fatigue damage of the ship body structure is often a part with a more complex structure can be effectively solved. Compared with the problem that the calculation work is very heavy due to the complex calculation process of the spectral analysis method, the method can greatly reduce the calculation workload, improve the calculation efficiency and bring great convenience to the development of the optimal design of the structural fatigue strength based on the spectral analysis method.

Description

Ship structure optimization simplified calculation method based on fatigue strength
Technical Field
The invention belongs to the technical field of optimization design of hull structures, and particularly relates to a hull structure optimization and simplification calculation method based on fatigue strength.
Background
Fatigue failure is one of the main failure modes of a ship structure, and fatigue cracks in the structure need to be repaired in time, otherwise, the cracks are expanded to a certain extent to cause catastrophic failure of the ship structure. Several major classification societies in the world carry out relevant regulations on fatigue strength check, and the fatigue strength evaluation method proposed by the classification societies of various countries at present can be mainly divided into simplified calculation and direct calculation; direct calculation is further classified into design wave method and spectral analysis method. Firstly, calculating wave load and hull structure stress response under different loading conditions, wave directions and calculation frequencies of a target ship, and further obtaining a stress transfer function; secondly, combining the wave spectrum and the sea condition data of a specific sea area to obtain a stress response spectrum and short-term stress distribution; finally, based on a linear accumulated damage theory and selecting a corresponding curve, calculating to obtain the total accumulated damage degree of the fatigue evaluation part; the flow of the spectral analysis method is shown in FIG. 1.
In fact, the initial design scheme of the hull structure cannot meet the requirement of the fatigue strength design service life of the structure; therefore, when the fatigue strength of the actual ship typical node structure is evaluated, the ship structure which cannot meet the design requirements needs to be optimized and designed, the common method is to continuously modify a finite element model of the ship structure according to the formed structure optimization design scheme and then repeatedly perform spectral analysis and calculation, and the flow is shown in fig. 2; however, the position with large fatigue damage of the hull structure is often a part with a complex structure, the spectral analysis and calculation process is complex, and the calculation workload is very heavy, which brings great difficulty to the optimization design of the typical node structure of the hull.
Disclosure of Invention
The invention aims to provide a hull structure optimization and simplification calculation method based on fatigue strength.
The purpose of the invention is realized by the following technical scheme: the method comprises 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;
step 2: 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 is 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 most dangerous wavesThe degree of accumulated fatigue damage 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;
Figure BDA0002696469390000021
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 ；
And 6: calculating a relation coefficient R of structural hot spot stress of the fatigue hot spots and wave-direction fatigue accumulated damage degree;
Figure BDA0002696469390000022
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 accumulated total damage degree D of the fatigue at the fatigue hot spot when the wave direction u with the frequency of omega is applied to the fatigue hot spot u ；
Figure BDA0002696469390000023
The present invention may further comprise:
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 spot 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:
Figure BDA0002696469390000024
Figure BDA0002696469390000025
Figure BDA0002696469390000026
Figure BDA0002696469390000027
the relation coefficient R of the structural hot spot stress of the fatigue hot spot and the wave-direction fatigue accumulated damage degree can be calculated by the following method:
Figure BDA0002696469390000031
wherein n is the total number of wave directions; delta is the air rate coefficient; t is L A recovery period calculated for the vessel fatigue;
Figure BDA0002696469390000032
selecting the parameters according to the fatigue standard of the hull structure; Γ () is a gamma function; p is a radical of nj The probability of occurrence of the loading state n of the ship corresponding to the jth wave direction; p is a radical of 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 the 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 in the wave direction j to the sum of the structural hot spot stresses corresponding to each calculation frequency in the course direction j can be obtained by directly calculating the result according to a finite element model, and the structural form of the typical node structure of the ship body is not changedUnder the condition of constant value, and
Figure BDA0002696469390000033
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;
Figure BDA0002696469390000036
q of structural hot spot stress 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 A wave energy spectrum expressed in relation to the heading angle theta, and
Figure BDA0002696469390000034
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
Figure BDA0002696469390000035
height of sense wave H s And average over zero period T z Determined by the selected wave spectrum and sea state data.
The invention has the beneficial effects that:
according to the invention, through the relationship between the structural hot spot stress and the wave-direction fatigue accumulated damage degree of the typical node structure of the ship body under a certain wave-direction calculation specific frequency and the relationship between each calculated wave-direction fatigue accumulated damage degree and the fatigue accumulated total damage degree, the ship body structure optimization and simplification calculation method based on the fatigue strength is provided, and the problem that the position with larger fatigue damage of the ship body structure is often a part with a more complex structure can be effectively solved. Compared with the problem that the calculation work is very heavy due to the complex calculation process of the spectral analysis method, the method can greatly reduce the calculation workload, improve the calculation efficiency and bring great convenience to the development of the optimal design of the structural fatigue strength based on the spectral analysis method.
Drawings
FIG. 1 is a flow chart of spectral analysis.
Fig. 2 is a flowchart of fatigue spectrum analysis optimization.
Fig. 3 is a flow chart of a simplified method of fatigue optimization.
FIG. 4 is a graph of stress gradient near a fatigue hot spot.
Fig. 5 is a schematic diagram of hot spot stress interpolation.
Fig. 6 is a schematic diagram of the location of a fatigue hot spot.
FIG. 7 is a finite element refinement of fatigue hot spots.
Fig. 8 is a RAO plot of fatigue hot spots in 180 degrees wave direction.
FIG. 9 shows the cumulative damage D of wave-direction fatigue in the embodiment of the present invention i And the fatigue accumulation total damage degree D is related to the table.
Fig. 10 is an optimization result table of the fatigue hot spot using the simplified calculation method in the embodiment of the present invention.
Fig. 11 is an optimization result table of the fatigue hot spot calculation method by using spectral analysis in the embodiment of the present invention.
Fig. 12 is an error analysis table of the optimization result of the simplified calculation method according to the embodiment of the present invention.
Detailed Description
The invention is further described below with reference to the accompanying drawings.
The invention relates to the field of optimization design of hull structures, in particular to a simplified calculation method of the optimization design of the hull structures based on fatigue strength evaluation results. The invention provides a simplified calculation method for a ship structure based on fatigue strength, which is mainly based on the relationship between structural hot spot stress and the fatigue accumulated damage degree of a ship structure in a wave direction and the relationship between each calculated wave direction fatigue accumulated damage degree and the fatigue accumulated total damage degree under a certain wave direction specific calculation frequency of the ship structure. The method can effectively solve the problems that the position with larger fatigue damage of the hull structure is often a part with a more complex structure, and the calculation process of the spectrum analysis method is more complex, so that the calculation work is very heavy, and great difficulty is brought to the optimization design of the typical node structure based on the fatigue strength by adopting the spectrum analysis method. The simplified calculation method can greatly reduce the calculation workload, improve the calculation efficiency and bring great convenience to the development of the optimal design of the structural fatigue strength based on the spectral analysis method.
The invention aims to provide a simplified calculation method for optimizing a hull structure based on fatigue strength, so as to solve the problems of long time consumption and huge workload in an optimization process in the optimization design of a typical hull structure by adopting a spectral analysis method.
In order to simplify the optimization design process of the typical structure of the ship body based on the fatigue strength by adopting a spectral analysis method, the simplified calculation method of the optimization design of the ship body structure is determined by the relationship between the structural hot spot stress and the fatigue cumulative damage degree of the typical structure in a wave direction and the relationship between the fatigue cumulative damage degree of each calculated wave direction and the fatigue cumulative total damage degree of the typical structure of the ship body under a certain wave direction calculation specific calculation frequency. The method can greatly reduce the calculation workload, improve the calculation efficiency and bring great convenience for developing the optimal design of the structural fatigue strength based on the spectral analysis method.
In the spectral analysis method, N is considered load The fatigue cumulative damage degree of the hull structure in each loading state is expressed as:
Figure BDA0002696469390000051
in the formula: delta-the air velocity coefficient; t is L Recovery period of vessel fatigue calculation, stipulated T L 20 years ═ 6.3x10 8 Second;
Figure BDA0002696469390000052
m-two parameters of the S-N curve are selected according to the fatigue specification of the hull structure;
Figure BDA0002696469390000053
-a gamma function; m is 0ijn -zero order moment of the stress response spectrum at nth loading and sea state i and heading j; p is a radical of formula n -time distribution coefficient of nth loading; p is a radical of i The ithProbability of occurrence of sea state; p is a radical of j -probability of occurrence of jth heading; upsilon is ijn -the nth load, the stress response zero crossing rate of sea state i and heading j,
Figure BDA0002696469390000054
m 2ijn -second moment of the stress response spectrum at nth loading and sea state i and heading j.
For the present invention, the principle is as follows:
1) hot spot stress calculation method
Because the stress at the hot spot cannot be directly extracted, the stress is calculated by a linear extrapolation method according to the stress gradient near the hot spot. As shown in FIG. 4, surface stresses at 3t/2 and t/2 from the hot spot are extracted from the refined grid model, and the stresses at the hot spot are obtained by extrapolation according to a linear relation.
Meanwhile, the spectral analysis method is to process each load generated by unit amplitude regular waves with different frequencies according to the real part and the imaginary part respectively so as to obtain corresponding response sigma c 、σ s (ii) a Therefore, it is necessary to perform a synthesis process on the stresses to obtain a stress calculation result of the hull structure at a corresponding calculation frequency, and the synthesized stress can be expressed as σ ═ σ c +iσ s Thus, there are:
Figure BDA0002696469390000055
selecting four refined finite element grid units near the fatigue hot spot, and performing stress sigma calculation on each unit obtained by processing the formula (2) in a stress interpolation mode as shown in fig. 5 kx 、σ ky 、τ kxy (k is 1,2,3,4) interpolating according to equation (3) to obtain the stress sigma at the hot spot hot,x 、σ hot,y 、τ hot,xy 。
Figure BDA0002696469390000056
Then, the hot spot stress σ hot Can be determined by calculation from equation (4):
Figure BDA0002696469390000057
and sigma hot Taken as the calculated value in equation (4) within 45 ° of the plane of the interpolation unit.
2) Simplified calculation method for wave-direction fatigue accumulated damage degree of hull structure
According to the formula (1), the wave-direction fatigue accumulated damage degree of the typical node structure of the ship body in a certain loading state and a certain wave-direction of a certain sea state can be calculated by the formula (5):
Figure BDA0002696469390000061
in the formula: n-represents the total number of wave directions counted; p is a radical of nj -the probability of occurrence of the jth wave towards the corresponding vessel loading state n; p is a radical of ij -the probability of occurrence of the corresponding sea state i for the jth wave;
further, the formula (5) is simplified, and only the structural hot spot stress sigma of the typical node structure of the ship body at a certain calculation frequency of a certain wave direction of the typical node structure of the ship body needs to be obtained hot-jk The course fatigue accumulated damage degree of the typical node structure of the ship body can be calculated according to the formula (6):
Figure BDA0002696469390000062
wherein, R is defined jk For structural hot spot stress sigma hot-jk Cumulative damage degree to wave direction fatigue D j The relation coefficient of (1) is as follows:
Figure BDA0002696469390000063
in the formula: omega ek -an encounter frequency of the vessel; theta-heading angle, rad; definition of Q k Is somewhere in the heading jThe ratio of the structural hot spot stress at the calculated frequency k to the sum of the structural hot spot stresses corresponding to each calculated frequency at the course j can be obtained by directly calculating the result according to a finite element model, and under the condition that the structural type of the typical node structure of the ship body is not changed, the value is a fixed value, and
Figure BDA0002696469390000067
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) -at encounter frequency ω ek A wave energy spectrum expressed in relation to the heading angle theta, and
Figure BDA0002696469390000065
ω k -wave frequency; u-speed, hits; g ηη (ω k ) Using the sense wave height H s And average across zero period T z To express a wave pattern, and
Figure BDA0002696469390000066
wherein H s -sense wave height, T z Average zero crossing period, both of which can be determined from selected wave spectra and sea state data.
R jk The method can also be obtained by directly calculating the result through a finite element model, and the value is a fixed value under the condition that the structural style of the typical node structure of the ship body is not changed; k represents frequency, k is 1,2, … l; l represents the total number of the calculated frequencies and is set according to a specific ship type; sigma hot-jk The structural hot spot stress under the frequency k of the heading j can be obtained by calculation according to a formula (4); m is generally taken to be 3.
3) Simplified calculation method for fatigue accumulation total damage degree of hull structure
In the spectrum analysis method, the fatigue accumulated total damage degree and the wave-direction fatigue accumulated damage degree of the typical node structure of the ship body are in the following relations:
Figure BDA0002696469390000071
wherein: j represents wave direction, j is 1,2, …, n; n represents the total wave direction number, is determined by the size of the wave direction angle, and when the wave direction angle is set to be 30 degrees, n is taken as 12; d j Representing the fatigue cumulative damage degree of j wave direction.
Therefore, the fatigue accumulated damage degree of the typical node structure of the ship body can be calculated by the following formula:
Figure BDA0002696469390000072
in the formula: sigma hot-jk Is the structural hot spot stress corresponding to the calculated frequency k at the wave direction j.
Further, in this patent, for a typical node structure of a ship hull that satisfies the requirement that the wave direction fatigue accumulated damage degree accounts for the fatigue accumulated total damage degree as a fixed value, the following relationship is determined between the fatigue accumulated damage degree and the fatigue total accumulated damage degree in a certain wave direction:
Figure BDA0002696469390000073
wherein, define p n-j The relation coefficient of the wave-direction fatigue accumulated damage degree of the ship structure and the fatigue accumulated total damage degree of the ship structure can be obtained by directly calculating a result through a finite element model, and the value is a fixed value under the condition that the structural style of the typical node structure of the ship is not changed; j represents wave direction, as defined above.
At this time, the fatigue accumulated damage degree of the typical node structure of the hull can be calculated by the following formula:
Figure BDA0002696469390000074
further, K may be defined jk Is a relation coefficient between the structural hot spot stress and the fatigue accumulated total damage degree
Figure BDA0002696469390000075
This time is:
Figure BDA0002696469390000076
the specific method of the invention is as follows:
for a certain typical node structure of a ship body, in the process of carrying out structure optimization design, the fatigue accumulation total damage degree of the typical node structure under the optimization scheme can be calculated only by calculating the structural hotspot stress of the typical node structure under any calculation frequency in a certain wave direction, and the analysis and calculation of a structural fatigue spectrum in the full wave direction and full frequency are not needed.
After the structural hot spot stress of the typical node structure of the ship body at a certain wave direction and a certain calculation frequency is obtained, the wave direction fatigue accumulated damage degree of the typical node structure under the wave direction can be calculated by combining the formula (6), and the fatigue accumulated total damage degree of the structure can be calculated according to the formula (8) or the formula (10). After the fatigue accumulated damage degree of the typical node structure of the ship body in a certain wave direction is obtained, and the wave direction fatigue accumulated damage degree and the fatigue accumulated total damage degree can be determined to have a determined relation, the fatigue accumulated total damage degree of the structure can be calculated according to a formula (9);
a ship structure optimization simplified calculation method based on fatigue strength can be summarized as follows:
1) simplified calculation of optimization of the hull structure is carried out according to the relation between structural hot spot stress and fatigue accumulation total damage degree D of the typical node structure of the hull at a certain wave direction and a certain calculation frequency;
2) wave-direction fatigue accumulated damage degree D through typical node structure of ship body j Simplified calculation of the optimization of the hull structure is carried out according to the relation with the fatigue accumulation total damage degree D;
3) according to Q k 、R jk Or obtaining the coefficient Q through the spectral analysis calculation result of the full wave direction full frequency of the typical node structure of the ship body k 、R jk (ii) a Then, calculating to obtain the heading fatigue accumulated damage degree D of the typical node structure of the ship body to be optimized under the research wave direction according to the formula (6) j 。
Optimization design method according to structure to be optimizedModifying the structural model, applying a calculation load to the structural model, and processing the obtained unit stress sigma by adopting a formula (2) kx 、σ ky 、τ kxy (k ═ 1,2,3, 4); then, the stress sigma at the hot spot is obtained by interpolation calculation according to the formula (3) hot,x 、σ hot,y 、τ hot,xy (ii) a Then calculating according to the formula (4) to obtain the hot spot stress sigma hot 。
Obtaining the coefficient R by combining the formula (5) and the formula (6) or through the spectral analysis and calculation result of the full wave-direction full frequency of the typical node structure of the ship body jk (ii) a Then, according to the formula (6), the wave fatigue accumulated damage degree D of the typical node structure of the ship body to be optimized under the research wave direction can be calculated and obtained j 。
R jk It can also be calculated by the following formula:
Figure BDA0002696469390000081
in the formula: omega ek -an encounter frequency of the vessel; theta-heading angle, rad; definition of Q k The ratio of the structural hot spot stress under a certain calculation frequency k of the heading j to the sum result of the structural hot spot stresses corresponding to each calculation frequency under the heading 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 style of the typical node structure of the ship body is not changed, in addition, the structural hot spot stress under the certain calculation frequency k of the heading j is the sum result of the structural hot spot stresses
Figure BDA0002696469390000091
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) -at encounter frequency ω ek A wave energy spectrum expressed in relation to the heading angle theta, and
Figure BDA0002696469390000092
ω k -wave frequency; u-speed, hits; g ηη (ω k ) Using the sense wave height H s And average over zero period T z To express a wave spectrum, and
Figure BDA0002696469390000093
wherein H s Height of sense wave, T z Average zero crossing period, both of which can be determined from selected wave spectra and sea state data.
The fatigue accumulation total damage degree of the typical node structure of the ship body can be obtained by calculation according to a formula (8), and all calculation wave directions of the structure need to be researched at the moment; in the determination of p n-j In this case, the fatigue cumulative total damage degree of the typical node structure of the hull can also be calculated according to the formula (10).
The invention has the following characteristics:
in the process of developing the structural optimization design of the fatigue hot spot, only the structural hot spot stress sigma of the fatigue hot spot under a certain calculation frequency of a certain calculation wave direction is needed to be calculated hot And then combining the formulas
Figure BDA0002696469390000094
Or
Figure BDA0002696469390000095
The fatigue accumulated damage degree of the typical node structure under the optimization scheme can be calculated without performing structural fatigue spectrum analysis calculation of full wave direction and full frequency.
1) The simplified calculation method can greatly reduce the calculation workload and improve the calculation efficiency. With n calculation wave directions and l calculation frequencies, the method can calculate time by using the spectrum analysis of the original full wave direction full frequency at least
Figure BDA0002696469390000096
And finishing the optimization design work of the fatigue hot spots.
2) The simplified calculation method has the advantages of simple calculation process and high reliability. Analysis and research of a large amount of data show that the error between the fatigue accumulated damage degree of the fatigue hot spot calculated by the simplified calculation method and the error of the spectrum analysis and calculation method adopting the full wave direction and the full frequency is basically within 1 percent, and the highest error can be controlled to be about 5 percent.
3) The calculation result of the simplified calculation method is biased to safety. Tong (Chinese character of 'tong')Reasonably over-determining the coefficient R jk Coefficient p n-j The value of (2) can ensure that the error between the calculation result of the simplified calculation method and the calculation result of the full wave direction full frequency spectrum analysis method is within an acceptable range, thereby fully ensuring the fatigue strength safety of the ship structure.
The simplified calculation method is true for any wave direction and at any frequency of a certain wave direction; but at the most dangerous wave direction of a typical structure and at the most dangerous frequency downwards from the wave direction, the method can be ensured to have higher accuracy and better effect.
R jk 、p n-j The value is selected with certain requirements, and the simplified calculation method has the best effect under the condition of meeting the requirements.
R jk 、p n-j In relation to the structural type of the hull structure; in the case of structural optimization by changing the structural style, R needs to be determined again jk 、p n-j A value of (d); and when the structural style of the ship structure is not changed and the thickness of each structural plate of the structure is only changed, R jk 、p n-j The value of (c) remains unchanged.
Example 1:
study subjects: the target vessel has a tall and long superstructure which ends near the midship, causing significant stress concentrations at the connection of the superstructure ends with the main deck (1 deck). When the fatigue strength of the typical node structure of the target ship is evaluated, the fatigue accumulated damage degree of the fatigue hot spot at the marked position of the figure 6 is found to be too large and is far larger than 1. Therefore, the fatigue hotspot is selected as a research object; meanwhile, research finds that: the wave-direction fatigue accumulated damage degree of the structure has a definite relation with the fatigue accumulated total damage degree. The fatigue hot spot research position is shown in a schematic diagram in figure 6, and a finite element refinement model of the research position is shown in figure 7.
The relationship between the cumulative damage degree of each wave direction and the cumulative total damage degree of fatigue of the initial design scheme of the fatigue hot spots is shown in figure 9. The number n of the calculated wave directions calculated by the spectrum analysis of the fatigue hot spots is 12, and the number k of the calculated frequencies of the wave directions is 18. As can be seen from fig. 9, the most dangerous wave direction of fatigue hot spots is 180 °. Thus, security taking into account the results of the calculationsAccording to p n-j First, determine p n-j ＝p n-7 ＝0.15。
The RAO curve of the fatigue hot spot at 180 ° wave direction is shown in fig. 8. As can be seen from fig. 8, the most dangerous frequency of the fatigue hotspot in the wave direction is 0.8; correspondingly, the hot spot stress σ of the structure hot ＝57.95Mpa。
At this time, considering the safety of the simplified calculation result, according to R jk Determining R according to the calculation method and the value-taking principle jk ＝R 78 ＝0.000006。
First, it is considered that the structure optimization design work is performed by increasing the thickness of the structural plate at the fatigue hot spot position. The final optimization result of the optimization scheme is finally obtained by adopting the simplified calculation method, which is specifically shown in fig. 10.
Performing one-time full wave-direction full-frequency spectrum analysis calculation on the optimization scheme to obtain the fatigue accumulated total damage degree D of the fatigue hot spots and the wave-direction fatigue accumulated damage degree D of 180 DEG wave-direction 7 See fig. 11 in particular.
For the optimization scheme, the optimization result of the fatigue hot spot by using the simplified calculation method and the optimization result by using the spectrum analysis method are obtained, and the error analysis is performed on the optimization result obtained by using the simplified calculation method, which is specifically shown in fig. 12.
As can be seen from fig. 12, compared with the total fatigue accumulated damage degree obtained by the simplified calculation method for the fatigue hot spot, the error of the total fatigue accumulated damage degree obtained by the simplified calculation method is 1.08%, and the error is small, so that the simplified calculation method is considered to be feasible. Meanwhile, as can be seen from fig. 11, the optimization scheme fails to make the fatigue hot spot meet the design requirement of the fatigue service life, and therefore, an optimization scheme for changing the structural style needs to be considered; for this solution, since the structural pattern of the fatigue hot spot is changed, when using the simplified calculation method, R needs to be re-determined jk And p n-j The specific calculation process and the final optimization result are not described in detail.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in 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;
Figure FDA0003708038300000011
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;
Figure FDA0003708038300000012
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 ；
Figure FDA0003708038300000013
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:
Figure FDA0003708038300000021
Figure FDA0003708038300000022
Figure FDA0003708038300000023
Figure FDA0003708038300000024
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:
Figure FDA0003708038300000025
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;
Figure FDA0003708038300000026
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
Figure FDA0003708038300000027
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;
Figure FDA0003708038300000028
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
Figure FDA0003708038300000029
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
Figure FDA0003708038300000031
height of sense wave H s And average over zero period T z Is determined by the selected wave spectrum and sea state data.
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