A method for determining equivalent design wave of ship wave loads under six sea conditions

Abstract

The present invention discloses a method for determining the equivalent design wave of a ship's wave load under level 6 sea conditions, relating to the field of ship structural safety. The method comprises: treating the hull as a variable-section beam, and for a ship at a constant speed, calculating the wave load transfer function at the hull beam section under different wave conditions, wherein the wave conditions include different wave direction angles and different circular frequencies of the incident wave; based on the wave load transfer function at the hull beam section, calculating the short-term predicted extreme values ​​of the wave load under different wave direction angles and different zero-crossing periods within the significant wave height range of level 6 sea conditions; selecting a maximum value from all the short-term predicted extreme values ​​of the wave load, and determining the equivalent design wave parameters of the wave load under level 6 sea conditions based on the maximum value. The equivalent design wave is a sinusoidal regular wave, and its parameters include wave amplitude, circular frequency, and wave direction. By converting the study of the load on the hull under random waves into the study of the hull load under regular waves, this method is of great significance for subsequent research on hull load levels and structural safety.

Description

Technical Field

[0001] The present invention relates to the field of ship structure safety, and in particular to a method for determining an equivalent design wave of a ship wave load under level six sea conditions. Background Art

[0002] Sea State VI has a significant wave height range of 4 to 6 meters, which is considered a medium wave level and occurs relatively frequently in the ocean. The structural safety of ships operating under these conditions is a major concern for ship designers. According to first principles, the safety of a ship's structure requires a clear understanding of the environmental external loads to which the hull is subjected. Wave loads, or the water pressure exerted by waves on the hull's outer plating, are the primary external loads on the hull. Clarifying these external wave loads in Sea State VI is a natural step in conducting a structural safety analysis.

[0003] However, the current hull structure design specifications do not provide for the wave loads that the hull will be subjected to under sea conditions of level 6. The hull structure design specifications generally stipulate the hull wave design load value in the longitudinal strength chapter, but this design load corresponds to the load that the ship will be subjected to throughout its entire service life cycle and is not applicable to a specific sea condition encountered by the ship (such as sea conditions of level 6). The characteristic wave loads that the hull will be subjected to under sea conditions of level 6 are significantly smaller than the design loads in the specifications, so it is inappropriate to use the design loads in the specifications when evaluating the safety of the hull structure under sea conditions of level 6, which will significantly overestimate the hull structure response. Therefore, in order to accurately evaluate the hull structure response under sea conditions of level 6, it is necessary to clarify the characteristic values ​​and calculation methods of the hull wave loads under sea conditions of level 6. Summary of the Invention

[0004] In response to the above problems and technical requirements, the inventors have proposed a method for determining the equivalent design wave of ship wave loads under level 6 sea conditions. The technical solution of the present invention is as follows:

[0005] A method for determining the equivalent design wave of a ship wave load under level 6 sea conditions comprises the following steps:

[0006] Considering the hull as a variable cross-section beam, for a ship with a constant speed, the wave load transfer function at the hull beam section is calculated under different wave conditions, including different wave direction angles and different circular frequencies of the incident wave.

[0007] Based on the wave load transfer function at the hull girder section, the short-term extreme values ​​of wave loads are calculated for different wave angles and different zero-crossing periods within the significant wave height range of sea state 6.

[0008] A maximum value is selected from all extreme values ​​of short-term wave load forecasts, and the equivalent design wave parameters of wave loads under sea conditions of level 6 are determined based on the maximum value. The equivalent design wave is a regular sinusoidal wave, and its parameters include amplitude, circular frequency and wave direction.

[0009] A further technical solution is to calculate the wave load transfer function at the hull girder section under different wave conditions, including:

[0010] The hull girder is divided into sections using imaginary sections. The divided hull girder sections are balanced under the action of gravity / gravity moment, wave force / wave moment, inertia force / inertia moment, and wave loads at the hull girder sections. The wave loads at the hull girder sections include forces and moments along the three directions of the xyz coordinate axes.

[0011] Under each wave condition, the wave force and wave moment caused by seawater pressure on the hull girder section are calculated, as well as the inertia force and inertia moment caused by the mass of the hull girder section. These are then substituted into the force / moment balance equation together with the gravity force and gravity moment of the hull girder section to obtain the wave load at the hull girder section under different wave conditions.

[0012] The wave load per unit amplitude is calculated as the wave load transfer function at the hull girder section.

[0013] A further technical solution is to calculate the wave forces and wave moments caused by seawater pressure on the hull beam sections, including:

[0014] The wet surface of the hull is discretized into grids to obtain the velocity potential at the center of each grid unit on the wet surface of the hull;

[0015] Substituting the velocity potential into the Bernoulli equation, we can obtain the seawater pressure at the center of each grid cell;

[0016] Then the force and moment of seawater pressure on the hull beam segment are:

[0017]

[0018] Among them, F and M are the wave force and wave moment caused by seawater pressure, S is the wet surface of the hull, n is the normal vector of the surface element of the hull wet surface, p is the seawater pressure at the center of each grid on the hull wet surface, and r is the bending moment radius.

[0019] A further technical solution is to calculate the inertia force and inertia moment caused by the mass of the hull beam segment, including:

[0020] Assuming the hull beam is a rigid body, the equation of motion is established using Newton's second law:

[0021]

[0022] Where m is the hull beam segment mass matrix, are the segment center of mass velocity and displacement, respectively, and I is the segment moment of inertia of the hull beam;

[0023] The wave force caused by the calculated seawater pressure on the hull beam section and wave torque Substitute into the equation of motion to find the linear acceleration of the segmented center of mass and angular acceleration

[0024] The hull beam segment mass and segment centroid linear acceleration Multiply the inertia force caused by the mass of the hull beam segment and the angular acceleration of the segment center of mass Multiplying them together gives the moment of inertia caused by the mass of the hull beam segment.

[0025] A further technical solution is to obtain the velocity potential at the center of each grid cell on the wet surface of the hull, including:

[0026] The velocity potential at the center of each grid cell on the wet surface of the hull is the incident velocity potential φ II , diffraction velocity potential φ D and the radiation velocity potential φ R sum;

[0027] Among them, the incident velocity potential φ I The expression is:

[0028]

[0029] Where g is the acceleration of gravity, A is the incident wave amplitude, (x, y, z) is the coordinates of the field point, H is the sea depth, ω is the incident wave circular frequency, β is the incident wave direction angle, and k is the wave number;

[0030] Radiation velocity potential φ R Decomposed into components on 6 degrees of freedom:

[0031]

[0032] Where, ξ j Represents the linear displacement along the xyz coordinate axes and the angular displacement around the three coordinate axes; φ j represents the radiation velocity potential under unit displacement;

[0033] Set the radiation velocity potential and diffraction velocity potential φ under unit displacement respectively D The boundary conditions satisfied are used to solve the diffraction velocity potential φ at the center of each grid cell on the wet surface of the hull D and the radiation velocity potential φ R .

[0034] A further technical solution is that when calculating the wave load at the hull girder section:

[0035] The cutting position is in the middle of the hull girder. Under each wave condition, the y-axis component of the wave moment caused by the sea water pressure on the hull girder section is calculated, as well as the y-axis component of the inertia moment caused by the mass of the hull girder section. These are substituted into the moment balance formula together with the y-axis component of the gravity moment of the hull girder section to obtain the vertical moment at the middle section of the hull girder under different wave conditions as the wave load.

[0036] A further technical solution is to calculate the short-term extreme value of wave load prediction at different wave angles and different zero-crossing periods within the significant wave height range of level 6 sea conditions based on the wave load transfer function at the hull girder section, including:

[0037] Based on the wave load transfer function at the hull girder section, the wave load response spectrum under different wave direction angles and different zero-crossing periods within the significant wave height range of sea state level 6 is calculated:

[0038] S response =RAO 2 ·S wave

[0039] Where S response is the wave load response spectrum density at the hull girder section, RAO is the wave load transfer function at the hull girder section, S wave The significant wave height H in the designated sea state level 6 s and zero-crossing period T z Wave spectral density under wave action;

[0040] The short-term extreme value prediction of wave loads under level 6 sea conditions is calculated based on the wave load response spectrum.

[0041] A further technical solution is to calculate the short-term extreme value of wave load prediction under level 6 sea conditions based on the wave load response spectrum, including:

[0042] The wave load in the short-term level 6 sea state follows the Rayleigh distribution, so the wave load at the hull girder section has a significant value M of one-third. sig The expression is:

[0043]

[0044] Where m0 is the 0th order moment of the wave load response spectrum, and its expression is:

[0045]

[0046] Where ω is the incident wave circular frequency; then the short-term forecast extreme value of wave load under level 6 sea state M max for:

[0047] M max =2.12M sig .

[0048] A further technical solution is to select a dual-parameter PM spectrum for the wave spectrum density, which is expressed as:

[0049]

[0050] Where ω is the incident wave circular frequency.

[0051] A further technical solution is to determine the equivalent design wave parameters of the wave load under sea state level 6 based on the maximum value, including:

[0052] The wave direction angle corresponding to the maximum value is the equivalent design wave direction;

[0053] The circular frequency at the peak of the wave load transfer function corresponding to the maximum value is the equivalent design wave circular frequency;

[0054] The ratio of the maximum value to its corresponding peak value of the wave load transfer function is the equivalent design wave amplitude.

[0055] The beneficial technical effects of the present invention are:

[0056] In this method, the wave load transfer function at the hull beam section under different wave conditions is first calculated, thereby calculating the short-term forecast extreme values ​​of wave loads at different wave direction angles and different zero-crossing periods within the significant wave height range of the six-level sea conditions, and selecting a maximum value from them to determine the equivalent design wave parameters under the six-level sea conditions. This application uses a regular wave model to describe ocean waves. Regular waves are simple sinusoidal waves that can be defined by parameters such as amplitude, circular frequency, wave direction, and phase. By constructing a regular wave to simulate the most unfavorable state suffered by the hull, the study of the load problem suffered by the hull under random waves is converted into the study of the hull load problem under regular waves. This not only simplifies the calculation, but also has important significance for the subsequent study of hull load levels and structural safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 This is a flow chart of the method for determining the equivalent design wave of ship wave load under level 6 sea conditions provided by this application.

[0058] Figure 2 It is a schematic diagram of the hull girder section force and moment provided in this application.

[0059] Figure 3 The vertical moment M of the midship section at a hull speed of 7.6 m / s and a wave direction of 180° provided in this application is y Transfer function diagram of .

[0060] Figure 4 is the significant wave height H provided by this application s =6m, zero-crossing period T z = Wave spectrum density diagram under 10s.

[0061] Figure 5 is the significant wave height H provided by this application s =6m, zero-crossing period T z = Vertical moment M of midship section under 10s y Response spectrum of . DETAILED DESCRIPTION

[0062] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0063] The waves in the actual sea are often irregular, so an irregular wave model should be used to simulate the waves. In order to simplify the calculation, this application provides an equivalent design wave method, which is of great significance to describe the wave loads suffered by the hull under irregular waves in the sixth level sea state by defining a simple sine regular wave. Please refer to Figure 1 As shown, an embodiment of the present application provides a method for determining the equivalent design wave of a ship wave load under level 6 sea conditions, which specifically includes the following contents:

[0064] Step 1: Consider the hull as a variable-section beam. For a ship with a constant speed, calculate the wave load transfer function at the hull girder section under different wave conditions. The specific calculation method includes: using imaginary sections to divide the hull girder, and then balancing the divided hull girder sections under the effects of gravity / gravity moment, wave force / wave moment, inertia force / inertia moment, and wave loads at the hull girder section (D'Alembert principle):

[0065] F fluid +F grav +F secLoad +F I =0 (1)

[0066] M fluid +M grav +M secLoad +M I =0 (2)

[0067] Among them: F fluid 、M fluid are the wave force and wave moment caused by seawater pressure on the hull beam segment; F grav 、M grav are the hull girder segment gravity and gravity moment respectively; F secLoad 、M secLoad The wave load on the hull girder section caused by another part of the hull girder segment includes the force F in the three directions along the xyz coordinate axis. x 、F y and F z and moment (bending moment) M x 、M y and Mz , 6 section forces and bending moments, called hull girder section loads or wave loads, such as Figure 2 As shown; F I 、M I are the inertia force and inertia moment caused by the mass of the hull beam segment, respectively.

[0068] Under each wave condition, calculate the wave force F caused by seawater pressure on the hull beam section fluid and wave moment M fluid , including: firstly, the hull wet surface is discretized into grids to obtain the velocity potential at the center of each grid unit on the hull wet surface, which is the incident velocity potential φ I , diffraction velocity potential φ D and the radiation velocity potential φ R When the incident wave is an Airy wave (sine regular wave), the incident velocity potential φ I The expression is:

[0069]

[0070] Where g is the acceleration of gravity, in meters per second squared (m / s^2), A is the incident wave amplitude, in meters (m); (x, y, z) are the coordinates of the field point, H is the sea depth, in meters (m), ω is the incident wave circular frequency, in radians per second (rad / s), β is the incident wave angle, k is the wave number, and when the sea area is infinitely deep, the relationship between the wave number k and the wave circular frequency ω is:

[0071] Diffraction velocity potential φ D The boundary conditions that are satisfied are:

[0072]

[0073] Where n represents the normal direction of the wet surface of the hull (the surface below the waterline of the hull), represents the partial derivative along the normal direction of the hull wet surface, S body Represents the wetted surface of the hull, S bottom represents the seabed boundary condition, R represents the infinite radiation control surface, represents the radial partial derivative along the control surface.

[0074] Radiation velocity potential φ R Decomposed into components on 6 degrees of freedom:

[0075]

[0076] Where, ξ jRepresents the linear displacement along the xyz coordinate axes (j=1-3), and the angular displacement around the three coordinate axes (j=4-6). j represents the radiation velocity potential under unit displacement, and the boundary conditions it satisfies are:

[0077]

[0078] The diffraction velocity potential φ at the center of each grid D and the radiation velocity potential φ under unit displacement j Both satisfy equations (4) and (6), with φ D and φ j The diffraction velocity potential φ at the center of each grid unit on the wet surface of the hull can be solved by establishing the equation D and the radiation velocity potential φ R The incident velocity potential φ I It is related to factors such as the location of the field point and the amplitude of the incident wave, which are known, so the total velocity potential at the center of each grid on the wet surface of the hull can be obtained.

[0079] Substitute the obtained total velocity potential Φ into the Bernoulli equation to obtain the seawater pressure at the center point of each grid cell on the wet surface of the hull:

[0080]

[0081] where ρ is the density of seawater.

[0082] Then the force and moment of seawater pressure on the hull beam segment are:

[0083]

[0084] Among them, F and M are the wave force and wave moment caused by sea water pressure, S is the wet surface of the hull, n is the normal vector of the wet surface element of the hull, and r is the bending moment radius.

[0085] Next, under each wave condition, the inertial force F caused by the mass of the hull beam segment needs to be calculated. I and the moment of inertia M I , including: assuming the hull beam is a rigid body, using Newton's second law to establish the equation of motion:

[0086]

[0087] Where m is the hull beam segment mass matrix, are the velocity and displacement of the segment center of mass respectively, and I is the moment of inertia of the hull beam segment. The wave force caused by the calculated seawater pressure on the hull beam segment is and wave torque Substitute into the motion equation (9) to calculate the segment center of mass linear acceleration and angular acceleration

[0088] The hull beam segment mass and segment centroid linear acceleration Multiply them to get the inertial force F caused by the mass of the hull beam segment I , the hull beam segment moment of inertia and segment center of mass rotation angular acceleration Multiply to get the inertia moment M caused by the mass of the hull beam segment I The wave force F calculated according to formula (8) fluid and wave moment M fluid , inertial force F I and the moment of inertia M I , and the hull girder segment gravity F grav and gravity moment M grav Substituting them into the force / moment balance equation (1) / (2), we can obtain the wave load F at the hull beam section under different wave conditions. secLoad With M secLoad Finally, the wave load under unit amplitude is calculated as the wave load transfer function at the hull girder section, with the force F at the hull girder section as secLoad For example, its transfer function RAO is expressed as:

[0089]

[0090] When a ship is actually sailing, the incident wave angle β, incident wave circular frequency ω, incident wave amplitude A, and ship sailing speed V will all affect the hull beam section load. At the same time, along the length of the ship, from the bow to the stern, the hull beam section load at different sections is also different. For a conventional hull, the vertical moment M at the middle section of the hull is y It is particularly important for the safety of the hull structure and is the most concerned by the hull structure designers. Therefore, the vertical moment M at the middle section of the hull beam is generally y To select the equivalent design wave parameters. In this embodiment, the above method is used to calculate the y-axis component of the wave moment caused by seawater pressure on the hull beam segment under each wave condition. And calculate the y-axis component of the inertia moment caused by the mass of the hull beam segment The y-axis component of the girder segment gravity moment Substituted into the moment balance equation (2), the vertical moment M at the middle section of the hull beam under different wave conditions is obtained. y As wave load. For a ship with a certain speed, multiple incident wave directions (0° to 180° multiple wave directions β), multiple incident wave frequencies ω, and the vertical moment M at the middle section of the hull beam are carried out. y The hull girder load transfer function is calculated. This example gives the vertical moment M when the hull speed is 7.6 m / s and the wave direction is 180°.y The transfer function diagram is as follows Figure 3 As shown in the figure, the horizontal axis is the incident wave circular frequency ω (unit: rad / s), and the vertical axis is the vertical moment M at the middle section of the hull beam. y Transfer function RAO.

[0091] Step 2: Based on the wave load transfer function at the hull girder section, calculate the short-term forecast extreme values ​​of wave loads at different wave direction angles (0° to 180°) and different zero-crossing periods within the range of significant wave heights in the six sea conditions. The specific calculation method includes: first, based on the wave load transfer function at the hull girder section, calculate the wave load response spectrum at different wave direction angles and different zero-crossing periods within the range of significant wave heights in the six sea conditions:

[0092] S response =RAO 2 ·S wave (11)

[0093] Where S response is the wave load at the middle section of the hull girder (vertical moment M y ) response spectrum density, RAO is the vertical moment M at the middle section of the hull girder y Transfer function. S wave The significant wave height H in the designated sea state level 6 s and zero-crossing period T z The wave spectrum density under the action of waves, in this embodiment S wave Select the two-parameter PM spectrum, expressed as:

[0094]

[0095] The significant wave height range of irregular waves in the sixth level sea condition is 4m~6m. In order to ensure the safety of the calculation results, the upper limit of the significant wave height of the sixth level sea condition is selected as 6m, and the zero crossing period range is selected as 5s~18s. The wave combination of the sixth level sea condition is shown in Table 1. s =6m, zero-crossing period T z =10s wave spectrum density diagram under the action of wave Figure 4 As shown, the vertical moment M in the ship corresponding to the wave spectrum density is y The response spectrum density plot is as follows Figure 5 shown.

[0096] Table 1 Wave combinations for level six sea conditions

[0097]

[0098] Secondly, the short-term extreme value of the wave load under the level 6 sea condition is calculated based on the wave load response spectrum. Specifically, the wave load under the level 6 sea condition is generally considered to obey the Rayleigh distribution. The wave load at the hull beam section (vertical moment M y ) one-third of the meaningful value M sig (single amplitude) expression is:

[0099]

[0100] Where m0 is the wave load (vertical moment M y ) The zero-order moment of the response spectrum is expressed as:

[0101]

[0102] The wave load (vertical moment M) under the sixth sea state is y ) Short-term forecast extreme value M max M sig 2.12 times:

[0103] M max =2.12M sig (15)

[0104] Therefore, the ship is sailing at a specified speed of 7.6 m / s in the sixth sea state, and the vertical moment M under different wave directions (0° to 180° selected at a certain interval) is calculated. y Transfer function RAO. Select the dual-parameter PM spectrum and the significant wave height H s The vertical moment M in the ship under different wave directions and different zero-crossing periods is calculated. y Short-term extreme value M max , as shown in Table 2:

[0105] Table 2 Significant wave height H at a speed of 7.6 m / s and sea state level 6 s = Vertical moment M corresponding to different wave directions and zero-crossing periods under 6m y Short-term forecast extreme values

[0106]

[0107]

[0108] Step 3: Select a maximum value from all extreme values ​​of the short-term wave load forecast, and determine the equivalent design wave parameters under sea conditions of level 6 based on the maximum value. The equivalent design wave is a regular sinusoidal wave, and its parameters include amplitude, circular frequency, and wave direction.

[0109] From Table 2, we can see that when the wave direction is 180 degrees and the zero crossing period is 7s, the short-term extreme value of the vertical moment M maxThe maximum is 1.15E+07N.m. The wave direction angle corresponding to this maximum value is the equivalent design wave direction, so the wave direction of 180° is the dangerous wave direction for vertical moment. The wave load corresponding to this maximum value (vertical moment M y ) The circular frequency ω at the peak of the transfer function is the equivalent design wave circular frequency, as shown in Figure 3 As shown, the vertical moment M y The response is maximum when the wave direction is 180° and the incident wave circular frequency is 1.1 rad / s, with the maximum RAO value being 2.66E+06 N.m. The ratio of this maximum value to the corresponding peak value of the wave load transfer function is the equivalent design wave amplitude:

[0110]

[0111] Where A is the equivalent design wave amplitude, M max H s =6m, zero-crossing period T z The vertical moment M when the wave direction is 180 degrees and the time is 7s y Short-term extremes, RAO max is the downward vertical moment M due to dangerous waves y The peak value of the transfer function response.

[0112] The vertical moment M in the ship under the sixth sea condition is y The equivalent design wave parameters are shown in Table 3:

[0113] Table 3 Vertical moment M in the ship y Equivalent design wave parameters

[0114]

[0115] The above description is only a preferred embodiment of the present application, and the present invention is not limited to the above embodiment. It is understood that other improvements and variations directly derived or imagined by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included in the scope of protection of the present invention.

Claims

1. A method for determining the equivalent design wave of ship wave loads under level 6 sea conditions, characterized in that: The method comprises: Considering the hull as a variable-section beam, for a ship with a constant speed, the wave load transfer function at the hull beam section is calculated under different wave conditions, including different wave angles and different circular frequencies of the incident wave. Based on the wave load transfer function at the hull girder section, the short-term prediction extreme values ​​of wave loads at different wave direction angles and different zero-crossing periods within the significant wave height range of level 6 sea conditions are calculated; A maximum value is selected from all extreme values ​​of the short-term wave load forecast, and based on the maximum value, equivalent design wave parameters of the wave load under sea state level 6 are determined, wherein the equivalent design wave is a regular sinusoidal wave, and the parameters include wave amplitude, circular frequency, and wave direction; The method of calculating the wave load transfer function at the hull girder section under different wave conditions includes: The hull girder is divided into sections using imaginary sections, and the divided hull girder sections are balanced under the action of gravity / gravity moment, wave force / wave moment, inertia force / inertia moment, and wave loads at the hull girder sections, where the wave loads at the hull girder sections include forces and moments along the three directions of the xyz coordinate axes; Under each wave condition, the wave force and wave moment caused by seawater pressure on the hull girder section are calculated, as well as the inertia force and inertia moment caused by the mass of the hull girder section. These are then substituted into the force / moment balance equation together with the gravity force and gravity moment of the hull girder section to obtain the wave load at the hull girder section under different wave conditions. The wave load per unit amplitude is calculated as the wave load transfer function at the hull girder section.

2. The method for determining the equivalent design wave of ship wave load under level 6 sea conditions according to claim 1 is characterized in that: The calculation of wave forces and wave moments caused by seawater pressure on the hull beam sections includes: The wet surface of the hull is discretized into grids to obtain the velocity potential at the center of each grid unit on the wet surface of the hull; Substituting the velocity potential into the Bernoulli equation, the seawater pressure at the center of each grid cell is obtained; Then the force and moment of seawater pressure on the hull beam segment are: in, 、 are the wave force and wave moment caused by the sea water pressure, S is the wetted surface of the hull, is the normal vector of the wet surface element of the hull, p is the seawater pressure at the center of each grid on the wet surface of the hull, is the bending moment radius vector.

3. The method for determining the equivalent design wave of ship wave load under level 6 sea conditions according to claim 1 is characterized in that: The calculation of the inertia force and inertia moment caused by the mass of the hull beam segment includes: Assuming the hull beam is a rigid body, the equation of motion is established using Newton's second law: in, m is the hull girder segment mass matrix, are the segment center of mass velocity and displacement, is the segmental moment of inertia of the hull girder; The wave force caused by the calculated seawater pressure on the hull beam section and wave torque Substitute into the equation of motion to find the linear acceleration of the segmented center of mass and angular acceleration ; The mass of the hull beam segment and the linear acceleration of the segment center of mass Multiply the inertia force caused by the mass of the hull beam segment and the angular acceleration of the segment center of mass to obtain the inertia force caused by the mass of the hull beam segment. Multiplying them together gives the moment of inertia caused by the mass of the hull beam segment.

4. The method for determining the equivalent design wave of ship wave load under level 6 sea conditions according to claim 2 is characterized in that: The obtaining of the velocity potential at the center of each grid unit on the wet surface of the hull comprises: The velocity potential at the center of each grid cell on the wet surface of the hull is the incident velocity potential , diffraction velocity potential and the radiation velocity potential sum; Wherein, the incident velocity potential The expression is: Where, g is the acceleration due to gravity, A is the incident wave amplitude, ( x , y , z ) are the field point coordinates, H is the sea depth, is the incident wave circular frequency, β Incident wave angle, k is the wave number; The radiation velocity potential Decomposed into components on 6 degrees of freedom: Where, It represents the linear displacement along the xyz three coordinate axes, and the angular displacement around the three coordinate axes; represents the radiation velocity potential under unit displacement; Set the radiation velocity potential and the diffraction velocity potential under the unit displacement respectively The boundary conditions satisfied are used to solve the diffraction velocity potential at the center of each grid cell on the wet surface of the hull and radiation velocity potential .

5. The method for determining the equivalent design wave of ship wave load under level 6 sea conditions according to claim 1 is characterized in that: When calculating the wave loads at the hull girder section: The cutting position is in the middle of the hull girder. Under each wave condition, the y-axis component of the wave moment caused by the sea water pressure on the hull girder section is calculated, as well as the y-axis component of the inertia moment caused by the mass of the hull girder section. These are substituted into the moment balance formula together with the y-axis component of the gravity moment of the hull girder section to obtain the vertical moment at the middle section of the hull girder under different wave conditions as the wave load.

6. The method for determining the equivalent design wave of ship wave load under level 6 sea conditions according to claim 1 is characterized in that: Based on the wave load transfer function at the hull girder section, the short-term forecast extreme values ​​of wave loads at different wave direction angles and different zero-crossing periods within the significant wave height range of level 6 sea conditions are calculated, including: Based on the wave load transfer function at the hull girder section, the wave load response spectrum under different wave direction angles and different zero-crossing periods within the significant wave height range of the sixth sea state is calculated: Where, is the wave load response spectrum density at the hull girder section, RAO is the wave load transfer function at the hull girder section, The significant wave height in the designated sea state level 6 H s and zero-crossing cycles T z Wave spectral density under wave action; The short-term forecast extreme value of the wave load under level 6 sea conditions is calculated based on the wave load response spectrum.

7. The method for determining the equivalent design wave of ship wave load under level 6 sea conditions according to claim 6 is characterized in that: Calculating the extreme short-term forecast value of wave load under level 6 sea state based on the wave load response spectrum includes: The wave load in the short-term level 6 sea state follows the Rayleigh distribution, so one-third of the wave load at the hull girder section has a significant value of The expression is: in is the 0th order moment of the wave load response spectrum, and its expression is: in is the incident wave circular frequency; the short-term forecast extreme value of wave load under level 6 sea state is for: 。 8. The method for determining the equivalent design wave of ship wave load under level 6 sea conditions according to claim 6 is characterized in that: The wave spectrum density selects the dual-parameter PM spectrum, which is expressed as: in, is the incident wave circular frequency.

9. The method for determining the equivalent design wave of ship wave load under level 6 sea conditions according to claim 1 is characterized in that: The equivalent design wave parameters of the wave load under sea state level 6 are determined based on the maximum value, including: The wave direction angle corresponding to the maximum value is the equivalent design wave direction; The circular frequency at the peak of the wave load transfer function corresponding to the maximum value is the equivalent design wave circular frequency; The ratio of the maximum value to the corresponding peak value of the wave load transfer function is the equivalent design wave amplitude.

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