CN113268876B - Marine fan integral coupling fatigue analysis method with additional culture net cage - Google Patents
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Abstract
An integral coupling fatigue analysis method for an offshore wind turbine with an additional culture net cage belongs to the technical field of offshore wind power and seawater net cage culture. Selecting a fatigue working condition load design combination working condition; establishing an integral coupling analysis model of an offshore wind turbine rotor structure-tower barrel structure-net cage structure-foundation structure-pile foundation structure of an additional culture net cage and a servo control method; acquiring a key node hot spot stress time course; and calculating the fatigue damage of the node. The invention is suitable for the structural design of the fixed offshore wind turbine of the multi-foundation type combined mariculture, can analyze the integral coupling dynamic characteristics of the fixed offshore wind turbine with the additional culture net cage under the action of wind, wave and current, fully considers the coupling effect among the structures, and calculates the fatigue damage of each node more accurately.
Description
Technical Field
The invention relates to an offshore wind turbine integral coupling fatigue analysis method with an additional culture net cage, and belongs to the technical field of offshore wind power and seawater net cage culture.
Background
The seabed construction of the traditional fixed fan foundation for offshore wind power generation is complex, the investment of the infrastructure is much higher than that of an onshore fan, and the investment recovery period is relatively long. However, the profits of fishery farming are far higher than offshore power generation. The offshore wind power and the marine pasture fishery cultivation are combined and developed for use, and the method has important significance for shortening the cost recovery period and generating relevant profits.
Because the offshore wind turbine foundation with the additional culture net cage is arranged on the seabed, the geological structure of the foundation is complex, and the foundation needs to bear a plurality of random loads such as offshore wind, waves and ocean currents, so that the proliferation type wind turbine foundation is complex in structure, huge in volume and high in cost. It is therefore necessary to design calculations and fatigue analysis for this complex infrastructure with high investment and risk.
The traditional offshore wind turbine fatigue analysis method is divided into spectral fatigue analysis and time domain fatigue analysis. Compared with a spectral fatigue analysis method, the time domain fatigue analysis can obtain more reasonable structural fatigue accumulation. Meanwhile, in order to consider the coupling dynamic characteristics among environmental load, servo control, pile-soil interaction, net cage structure and fan structure reaction, an integral coupling model is applied to offshore fan time domain fatigue analysis, and the influence of the environmental load coupling effect on the accumulation of structural fatigue damage is fully considered.
Disclosure of Invention
The invention aims to solve the technical problem of providing an offshore wind turbine integral coupling fatigue analysis method with an additional culture net cage. The fatigue analysis method provided by the invention can realize the integral coupling fatigue analysis of the rotor structure-tower barrel structure-net cage structure-foundation structure-pile foundation structure and the servo control method under the combined action of wind, wave and current load.
The technical scheme adopted by the invention is as follows: the integral coupling fatigue analysis method of the offshore wind turbine with the additional culture net cage comprises the following steps of:
a. Selecting fatigue working conditions according to the joint probability distribution based on wind and wave, and generating a wind speed time course file required by calculation;
b. Carrying out pile foundation linearization based on nonlinear p-y, t-z and q-z pile-soil interaction curves and an elastic foundation beam model to obtain a linearized pile foundation point rigidity and quality matrix;
c. For the gravity type offshore wind turbine foundation with the high pile cap form, carrying out polycondensation on the linearized pile foundation and the foundation structure quality and rigidity matrix file by using a Guyan method, a Dynamic method, a C-B method or a SEREP method to generate a foundation structure superunit matrix file, and making the foundation structure equivalent to a superunit matrix;
d. Building a pile type offshore wind turbine integral coupling model comprising a rotor structure, a tower cylinder structure, a net cage structure, a foundation structure, a pile foundation structure and a servo control method based on the wind speed time interval file in the step a, the wave parameters and the pile head rigidity matrix file in the step b, and carrying out corresponding calculation on integral coupling power of a single pile, a jacket and a multi-pile foundation type offshore wind turbine with an additional cultivation net cage to obtain internal force time interval files of foundation rod pieces and cultivation net cage rod pieces;
e.d rotor structure-tower structure-net cage structure-foundation structure-pile foundation structure and the offshore wind turbine integral coupling model of the servo control method comprises the following characteristics: the rotor structure comprises a front third-order mode of the blade, the tower barrel structure comprises a front fourth-order mode shape, the net cage structure and the foundation structure adopt linear beam units, and the pile foundation structure adopts linear pile foundation point rigidity and mass matrix simulation; wherein, the net structure size in the net cage structure model and the material property are determined according to a mesh grouping method;
f. B, based on the basic structure superunit matrix file in the step b, establishing an offshore wind turbine coupling calculation model of a rotor structure-tower cylinder structure-superunit matrix, reading the wind speed time course file and the wave time course file in the step a, and developing gravity type high pile cap basic type offshore wind turbine integral coupling power response calculation of an additional culture net cage to obtain internal force time course files of a basic structure and a culture net cage rod piece;
g. According to the internal force time course file of the rod in the step c or the step e, the hot spot stress time course under the action of environmental load is determined according to the following rule:
g1. Firstly, the nominal stress of the pipe node under the action of axial load, in-plane and out-of-plane bending moment can be calculated according to the following formula:
wherein: σ x,σmy,σmz is the tube node axial tensile load, in-plane bending and out-of-plane bending tube node nominal stress, respectively, F z is the axial force, M i is the in-plane bending moment, M o is the out-of-plane bending moment, D is the diameter of the brace bar, D is the diameter of the chord bar, and t is the wall thickness of the brace bar.
G2. the hotspot stress concentration factor is then calculated according to the typical tube node hotspot Stress Concentration Factor (SCF) calculation equation (4-9).
SCF＝1.45βτ0.85γ(1-0.68β)(sinθ)0.7 (4)
SCF＝gt1.1(1.11-3(b-0.52)2)(sinq)1.6 (5)
SCF＝γ0.2τ(2.65+5(β-0.65)2)+τβ(C2α-3)sinθ (6)
SCF＝3+λ1.2(0.12exp(-4β)+0.011β2-0.045)+βτ(C3α-1.2) (7)
SCF＝1+0.65βτ0.4γ(1.09-0.77β)(sinθ)(0.06γ-1.16) (8)
SCF＝τ-0.54γ-0.05(0.99-0.47β+0.08β4)·γτβ(1.7-1.05β3)(sinθ)1.6 (9)
Wherein: t is the wall thickness of the chord, g is the distance between the two struts, θ is the included angle between the struts and the chord, β is the ratio of D to D, τ is the ratio of T to T, γ is the ratio of D to 2 times T, and C 2 and C 3 are chord end fixing parameters.
G3. finally, according to a tube node hot spot stress calculation formula recommended by the offshore wind turbine specification, based on the nominal internal force of the rod piece and the hot spot stress concentration factor, the fatigue stress time course of each key node can be calculated according to the following formula:
σ1＝SCFACσx+SCFMIPσmy (10)
σ3＝SCFASσx-SCFMOPσmz (12)
σ5＝SCFACσx-SCFMIPσmy (14)
σ7＝SCFASσx+SCFMOPσmz (16)
Wherein, SCF AC and SCF AS are stress concentration factors at the pipe node crown point and saddle point of the chord member under the axial load, and SCF MIP and SCF MOP are stress concentration factors under the in-plane bending moment and out-of-plane bending moment, respectively.
H. And (c) determining a fatigue stress amplitude delta sigma corresponding to the fatigue stress time course and the cycle times n i of the fatigue stress amplitude delta sigma by utilizing a rain flow counting method based on the hot spot stress time course file in the step (g).
I. The step h specifically comprises the following calculation steps:
i1. Finding out the highest peak and the lowest valley in the stress-time history, comparing the absolute values of the peak and the lowest valley, breaking the stress-time history at the point with large absolute value, and re-butting the stress-time history with the point with large absolute value as the starting point to obtain a new stress-time history;
i2. The rain flows sequentially downwards from the inner side of the peak value (valley value) in the stress-time process, and when encountering a peak value (lower valley value) which is larger than the starting point, the rain flows stop;
i3. Stopping the flow when the rain flow meets the rain flow under the upper flow;
i4. and (3) counting according to the i3 to obtain all full cycles or half cycles, and recording and calculating the fatigue stress amplitude delta sigma amplitude and the mean value of each cycle.
J. Based on the step of obtaining the fatigue stress amplitude and the cycle action number N i, the maximum allowable cycle action number N corresponding to the fatigue stress amplitude is determined by the following formula:
Where t ref is a reference thickness, t is a thickness at which cracking is most likely to occur, and if the thickness is smaller than the reference thickness, t=t ref is taken; k is a thickness index;
k. And calculating and obtaining the fatigue accumulated damage of the offshore wind turbine structure of the additional aquaculture net cage under each working condition by using the P-M linear damage accumulated criterion, wherein the fatigue accumulated damage can be calculated by a formula (19).
Wherein D i is the fatigue cumulative damage under the action of the ith stress amplitude, and N i is the ultimate fatigue damage under the action of the stress amplitude.
The invention has the beneficial effects that:
1. the method can fully consider the coupling effect among environmental load, control method, pile-soil interaction, net cage structure and fan structure reaction, and build a more perfect offshore fan integral coupling model with additional culture net cage, and the calculated structural reaction is more reasonable and accurate.
2. In the calculation, load working conditions are selected according to the wind and wave joint probability distribution, the time domain fatigue analysis method is adopted for fatigue load calculation, the influence of the environmental load coupling effect on the long-term fatigue accumulation of the structure is fully considered, and the more practical fatigue damage accumulation of the offshore wind turbine with the additional culture net cage can be obtained.
3. The invention is suitable for fatigue analysis of various foundation type fixed offshore wind turbines with additional net cages.
Drawings
The foregoing description is only an overview of the present invention, and the following drawings and detailed description provide more detailed description of the invention in order to more particularly and clearly describe the key technical means of the present invention.
FIG. 1 is a flow chart of a design of a method for analyzing the overall coupling fatigue of an offshore wind turbine with an additional aquaculture net cage.
FIG. 2 is a basic analysis model and a load diagram of an offshore single pile foundation fan with an additional aquaculture net cage.
FIG. 3 is a tower foundation bending moment time chart of a tower barrel structure of the offshore single pile foundation fan with an additional culture net cage under the action of wind and waves.
FIG. 4 is a time chart of bending moment of the structural pipe nodes of the offshore single pile foundation fan net cage of the additional aquaculture net cage under the action of wind and wave.
FIG. 5 is a thermal point stress time chart of the structural pipe nodes of the offshore single pile foundation fan net cage of the additional aquaculture net cage under the action of wind and wave.
Detailed Description
The invention relates to a marine fan integral coupling fatigue analysis method with an additional culture net cage, which mainly comprises the following steps: selecting a fatigue working condition load design combination working condition; establishing an integral coupling analysis model of an offshore wind turbine rotor structure-tower barrel structure-net cage structure-foundation structure-pile foundation structure of an additional culture net cage and a servo control method; acquiring a key node hot spot stress time course; calculate node fatigue life, etc. The analysis method corresponding to each part comprises the following steps and characteristics:
and 1, selecting a fatigue working condition based on a wind speed, wave height and period joint probability distribution table.
And 2, generating a wind speed time course file corresponding to each fatigue working condition according to the reference wind speed and the wind spectrum parameters.
And step 3, according to fan pile foundation parameters and geological survey data, suggesting a nonlinear p-y, t-z and q-z pile-soil interaction curve and an elastic foundation beam model based on an API RP 2A design specification, and carrying out pile foundation linearization analysis by adopting a finite difference method to obtain a linear rigidity and quality matrix of the pile foundation point position.
Based on a p-y curve method and a dynamic agglomeration method, soil layer information including basic information such as soil type, non-drainage shear strength of soil, soil weight, soil layer depth and the like is input in a program, a pile head rigidity matrix is agglomerated, and the pile head rigidity matrix calculated by a selected example is shown in the following formula.
And 4, performing basic superunit condensation on the linear pile foundation points and the gravity type high pile cap basic structure quality and rigidity matrix files according to a static condensation method (Guyan method) or a basic structure Dynamic condensation method (Dynamic method, C-B method and SEREP method), and calculating to obtain a basic structure superunit matrix.
The superunit mass and stiffness matrix of the single pile foundation based on SEREP method is shown below. The fundamental structural feature frequency comparisons are shown in the following table.
And 5, establishing a pile-soil coupling power analysis module, and establishing a pile type offshore wind turbine integral coupling model comprising a rotor structure, a tower cylinder structure, a net cage structure, a foundation structure, a pile foundation structure and a servo control method.
Further, the marine fan integral coupling model modeling adopted by the invention has the following characteristics:
(1) And establishing a rotor structure aero-elastic analysis model based on a phyllin momentum theory according to vane airfoil parameters, aerodynamic parameters, model parameters of the hub and the engine room.
(2) Establishing a rotor blade and tower numerical simulation model according to the rotor blade front third-order mode and the tower front fourth-order mode and geometric and material parameters
(3) And (3) establishing a finite element model and a hydrodynamic model of the net cage and the foundation structure by using linear beam units according to the materials, the geometric parameters and the hydrodynamic coefficients of the net cage structure and the foundation structure.
(4) And simulating a pile foundation structure by adopting a linearized pile foundation point rigidity and quality matrix, and establishing a pile-soil interaction model.
(5) An offshore wind turbine servo control method is applied.
Further, the pile-soil interaction model adopted by the invention has the following characteristics:
(1) Changing boundary conditions at the mud surface of the fan foundation structure equation of motion, changing the boundary conditions from fixed boundary constraint to elastic boundary constraint, and FIG. 2 is a basic analysis model and a load diagram of the offshore single pile foundation fan with an additional culture net cage after the boundary conditions are modified.
(2) The base structure mass and stiffness matrix is modified based on boundary conditions.
Furthermore, the net cage structure adopted by the invention is simulated by the linear beam unit, and the structural size of the net is determined by a mesh clustering method, and the net cage structure comprises the following characteristics:
(1) After clustering by a mesh clustering method, the quality of the netting is kept unchanged.
(2) The area covered by the mesh remains unchanged after clustering.
(3) The projected area of the mesh in the flow direction remains unchanged after the clustering.
Step 6, according to the wave parameters, the wind speed time course file in step 2 and the pile head rigidity matrix file in step 3, carrying out corresponding calculation on the integral coupling power of the pile type offshore wind turbine with the single pile, the jacket and the multi-pile foundation type additional culture net cage to obtain internal force time course files of the foundation rod pieces and the culture net cage rod pieces;
Step 8, based on the basic structure superunit matrix file in the step 4, an offshore wind turbine coupling calculation model of a rotor structure-tower cylinder structure-superunit matrix is established, and fig. 2 is a basic analysis model and a load diagram of an offshore single pile basic wind turbine with an additional culture net cage.
And step 9, carrying out gravity type high pile cap foundation type offshore wind turbine structure coupling dynamic response calculation of the additional culture net cage according to the wave parameters and the wind speed time course file in step 2, and obtaining internal force time course files of the tower barrel structure, the foundation structure and the culture net cage rod pieces. FIG. 3 is a tower foundation bending moment time chart of a tower barrel structure of the offshore single pile foundation fan with the additional culture net cage under the action of wind and waves, and FIG. 4 is a bending moment time chart of a pipe node of the structure of the offshore single pile foundation fan with the additional culture net cage under the action of wind and waves.
And step 10, developing a hot spot stress calculation module based on a calculation formula of a typical tube node hot spot Stress Concentration Factor (SCF), reading a force time course file in the rod piece, and calculating the hot spot stress time course of each key node. FIG. 5 is a thermal point stress time chart of a net cage structural pipe node of the offshore single pile foundation fan with the additional culture net cage under the action of wind and wave;
further, the hot spot stress calculation module of the present invention has the following features:
(1) Firstly, according to a nominal stress calculation formula under the action of three loads, namely an axial tensile load, an in-plane bending load and an out-of-plane bending load, an internal force time course file of a rod piece is input to calculate the nominal stress time course of the rod piece. The nominal stress of the pipe node under the action of axial load, in-plane and out-of-plane bending moment can be calculated according to the following formula:
wherein: σ x,σmy,σmz is the tube node axial tensile load, in-plane bending and out-of-plane bending tube node nominal stress, respectively, F z is the axial force, M i is the in-plane bending moment, M o is the out-of-plane bending moment, D is the diameter of the brace bar, D is the diameter of the chord bar, and t is the wall thickness of the brace bar.
G2. the hotspot stress concentration factor is then calculated according to the typical tube node hotspot Stress Concentration Factor (SCF) calculation equation (4-9).
G3. finally, according to a tube node hot spot stress calculation formula recommended by the offshore wind turbine specification, based on the nominal internal force of the rod piece and the hot spot stress concentration factor, the fatigue stress time course of each key node can be calculated according to the following formula:
(2) The hotspot stress concentration factor is then calculated according to the typical tube node hotspot Stress Concentration Factor (SCF) calculation equation (4-9).
SCF＝1.45βτ0.85γ(1-0.68β)(sinθ)0.7 (4)
SCF＝gt1.1(1.11-3(b-0.52)2)(sinq)1.6 (5)
SCF＝γ0.2τ(2.65+5(β-0.65)2)+τβ(C2α-3)sinθ (6)
SCF＝3+λ1.2(0.12exp(-4β)+0.011β2-0.045)+βτ(C3α-1.2) (7)
SCF＝1+0.65βτ0.4γ(1.09-0.77β)(sinθ)(0.06γ-1.16) (8)
SCF＝τ-0.54γ-0.05(0.99-0.47β+0.08β4)·γτβ(1.7-1.05β3)(sinθ)1.6 (9)
Wherein: t is the wall thickness of the chord, g is the distance between the two struts, θ is the included angle between the struts and the chord, β is the ratio of D to D, τ is the ratio of T to T, γ is the ratio of D to 2 times T, and C 2 and C 3 are chord end fixing parameters.
(3) And finally, calculating fatigue stress time courses of all key nodes based on nominal internal force of the rod piece and hot spot stress concentration factors according to a pipe node hot spot stress calculation formula (10-17) recommended by offshore wind turbine specifications.
σ1＝SCFACσx+SCFMIPσmy (10)
σ3＝SCFASσx-SCFMOPσmz (12)
σ5＝SCFACσx-SCFMIPσmy (14)
σ7＝SCFASσx+SCFMOPσmz (16)
Wherein, SCF AC and SCF AS are stress concentration factors at the pipe node crown point and saddle point of the chord member under the axial load, and SCF MIP and SCF MOP are stress concentration factors under the in-plane bending moment and out-of-plane bending moment, respectively.
And 11, according to the hot spot stress time course file, using a rain flow counting method to count and obtain a fatigue stress amplitude delta sigma corresponding to the fatigue stress time course and the cycle times n i of the fatigue stress amplitude delta sigma.
Further, the specific calculation steps of the fatigue stress amplitude delta sigma and the cycle times n i of the invention are as follows:
Firstly, finding out the highest peak and the lowest valley in the stress-time history based on a hot spot stress time history file, comparing the absolute values of the peak and the lowest valley, breaking the stress-time history at the point with large absolute value, and re-butting the points with large absolute value as the starting point to obtain a new stress-time history;
Then the rain flow sequentially flows downwards from the inner side of the peak value (valley value) in the stress-time process, and when encountering the peak value (lower valley value) which is larger than the initial point, the rain flow stops flowing; stopping the flow when the rain flow meets the rain flow under the upper flow;
And finally, counting to obtain all full cycles or half cycles, and recording and calculating the fatigue stress amplitude delta sigma amplitude and the average value of each cycle.
And step 12, calculating and obtaining the fatigue damage of the offshore wind turbine structure of the additional aquaculture net cage under each working condition based on the S-N curve and the Miner fatigue accumulation criterion according to the fatigue load stress amplitude delta sigma and the cycle times N i obtained in the step 11.
Further, the specific calculation steps of the fatigue damage are as follows:
firstly, counting and obtaining a fatigue stress amplitude and a cycle action number N based on a rain flow counting method, wherein the maximum allowable cycle action number N corresponding to the fatigue stress amplitude is determined by the following formula:
Wherein t ref is the reference thickness, the value of the non-welded pipe node is 25mm, the value of the welded pipe node is 32mm, and the value of the bolt is 25mm; t is the thickness at which the crack is most likely to occur, and if the thickness is less than the reference thickness, t=t ref is taken; k is the thickness index. m is the negative reciprocal of the slope of the S-N curve; loga is the intercept of the N axis;
And then, calculating and obtaining the fatigue accumulated damage of the offshore wind turbine structure of the additional aquaculture net cage under each working condition by using a P-M linear damage accumulated criterion, wherein the fatigue accumulated damage can be calculated by a formula (19).
Wherein D i is the fatigue accumulation damage under the action of the ith stress amplitude, N i is the limit fatigue damage under the action of the stress amplitude, and N i is the actual action times of the stress amplitude.
Claims (1)
1. The method for analyzing the integral coupling fatigue of the offshore wind turbine with the additional culture net cage is characterized by comprising the following steps of:
a. selecting fatigue working conditions according to the joint probability distribution based on wind and waves, and generating a wind speed time course file and a wave time course file required by calculation;
b. Carrying out pile foundation linearization based on nonlinear p-y, t-z and q-z pile-soil interaction curves and an elastic foundation beam model to obtain a linearized pile foundation point rigidity and quality matrix;
c. For the gravity type offshore wind turbine foundation with the high pile cap form, carrying out polycondensation on the linearized pile foundation and the foundation structure quality and rigidity matrix file by using a Guyan method, a Dynamic method, a C-B method or a SEREP method to generate a foundation structure superunit matrix file, and making the foundation structure equivalent to a superunit matrix;
d. building a pile type offshore wind turbine integral coupling model comprising a rotor structure, a tower cylinder structure, a net cage structure, a foundation structure, a pile foundation structure and a servo control method based on the wind speed time interval file in the step a, the wave parameters and the pile foundation point rigidity matrix file in the step b, and carrying out corresponding calculation on integral coupling power of a single pile, a jacket and a multi-pile foundation type offshore wind turbine of an additional culture net cage to obtain an internal force time interval file of a foundation structure and a culture net cage rod piece;
e. The integral coupling model of the offshore wind turbine of the rotor structure-tower structure-net cage structure-foundation structure-pile foundation structure and the servo control method in the step d comprises the following characteristics: the rotor structure comprises a front third-order mode of the blade, the tower barrel structure comprises a front fourth-order mode, the net cage structure and the foundation structure adopt linear beam units, and the pile foundation structure adopts linear pile foundation point rigidity and mass matrix simulation; wherein, the net structure size in the net cage structure model and the material property are determined according to a mesh grouping method;
f. C, based on the superunit matrix file in the step c, establishing a marine fan coupling calculation model of a rotor structure-tower cylinder structure-superunit matrix, and carrying out gravity type high pile cap foundation type marine fan integral coupling dynamic response calculation of the additional culture net cage according to the wind speed time course file and the wave time course file in the step a to obtain internal force time course files of the foundation structure and the culture net cage rod pieces;
g. According to the internal force time course file of the rod piece in the step d, the hot spot stress time course under the action of environmental load is determined according to the following regulations:
g1. Firstly, the nominal stress of the pipe node under the action of axial load, in-plane and out-of-plane bending moment is calculated according to the following formula:
Wherein: sigma x,σmy,σmz is the tube node axial tensile load, in-plane bending and out-of-plane bending tube node nominal stress, F z is the axial force, M i is the in-plane bending moment, M o is the out-of-plane bending moment, D is the diameter of the brace rod, D is the diameter of the chord rod, and t is the wall thickness of the brace rod;
g2. and then calculating the hot spot stress concentration factor according to a calculation formula of the hot spot stress concentration factor SCF of the typical pipe node:
SCF＝1.45βτ0.85γ(1-0.68β)(sinθ)0.7 (4)
SCF＝gt 1.1(1.11-3(b-0.52)2)(sinq)1.6 (5)
SCF＝γ0.2τ(2.65+5(β-0.65)2)+τβ(C2α-3)sinθ (6)
SCF＝3+λ1.2(0.12exp(-4β)+0.011β2-0.045)+βτ(C3α-1.2) (7)
SCF＝1+0.65βτ0.4γ(1.09-0.77β)(sinθ)(0.06γ-1.16) (8)
SCF＝τ-0.54γ-0.05(0.99-0.47β+0.08β4)·γτβ(1.7-1.05β3)(sinθ)1.6 (9)
Wherein: t is the wall thickness of the chord, g is the distance between the two struts, θ is the included angle between the struts and the chord, β is the ratio of D to D, τ is the ratio of T to T, γ is the ratio of D to 2 times T, and C 2 and C 3 are chord end fixing parameters;
g3. According to a pipe node hot spot stress calculation formula recommended by offshore wind turbine specifications, based on the nominal internal force of a rod piece and a hot spot stress concentration factor, the fatigue stress time course of each key node can be calculated according to the following formula:
σ1＝SCFACσx+SCFMIPσmy (10)
σ3＝SCFASσx-SCFMOPσmz (12)
σ5＝SCFACσx-SCFMIPσmy (14)
σ7＝SCFASσx+SCFMOPσmz (16)
Wherein, SCF AC and SCF AS are stress concentration factors at the crown point and saddle point of the pipe joint of the chord member under the action of axial load, and SCF MIP and SCF MOP are stress concentration factors under the action of in-plane bending moment and out-of-plane bending moment respectively;
h. Determining fatigue stress amplitude delta sigma corresponding to the fatigue stress time course by utilizing a rain flow counting method based on the hot spot stress time course file in the step g, and the cycle times n i;
i. The step h specifically comprises the following calculation steps:
i1. Finding out the highest peak and the lowest valley in the stress-time history, comparing the absolute values of the peak and the lowest valley, breaking the stress-time history at the point with large absolute value, and re-butting the stress-time history with the point with large absolute value as the starting point to obtain a new stress-time history;
i2. The rain flow flows downwards from the inner side of the peak value or the valley value in the stress-time process in sequence, and stops flowing when encountering the peak value or the valley value which is larger than the initial point;
i3. Stopping the flow when the rain flow meets the rain flow under the upper flow;
i4. Counting according to the i3 to obtain all full cycles or half cycles, and recording and calculating fatigue stress amplitude delta sigma amplitude and average value of each cycle;
j. Based on the step of obtaining the fatigue stress amplitude and the cycle action number N i, the maximum allowable cycle action number N corresponding to the fatigue stress amplitude is determined by the following formula:
Where t ref is a reference thickness, t is a thickness at which cracking is most likely to occur, and if the thickness is smaller than the reference thickness, t=t ref is taken; k is a thickness index;
k. calculating and obtaining fatigue accumulated damage of the offshore wind turbine structure of the additional aquaculture net cage under each working condition by using a P-M linear damage accumulated criterion, wherein the fatigue accumulated damage can be calculated by a formula (19);
wherein D i is the fatigue cumulative damage under the action of the ith stress amplitude, and N i is the ultimate fatigue damage under the action of the stress amplitude.
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