Calculation and analysis method of extreme loads on offshore wind power generation structures

Abstract

A method for calculating and analyzing extreme loads of offshore wind power generation structures belongs to the technical field of offshore wind power generation structures. It includes three parts: first, a high-order spectrum numerical wave flume model is established, and waves are generated by inputting the target wave cross-sectional velocity at the flume inlet to simulate the generation and propagation of nonlinear extreme waves under real severe sea conditions; second, the semi-empirical and semi-theoretical Rainey slender body theory is used to directly calculate the wave loads of extreme waves acting on the wind turbine structure; finally, the aerodynamic load of the wind turbine, that is, the wind load, is calculated according to the classical blade element momentum theory. The above parts can be operated jointly or independently. The present invention can realize accurate calculation and comprehensive analysis of the extreme loads under the combined action of wind and waves on offshore wind power generation structures; and it has the characteristics of precision and high efficiency, and has bright application prospects and important development and application value in the design and engineering practice of offshore wind power generation structures.

Description

Technical Field

[0001] The present invention belongs to the technical field of offshore wind power generation structures, relates to a calculation and analysis method for wind and wave extreme loads on offshore wind power generation structures, and involves multiple fields such as hydrodynamics and aerodynamics. Background Art

[0002] At present, offshore wind power generation is an important part of my country's renewable energy sector. Clean offshore wind power can effectively reduce human dependence on fossil fuels, reduce greenhouse gas emissions, and alleviate global climate change. Because the wind speed at sea is higher and more stable than on land, offshore wind power generation can provide more stable power output, thereby improving the resilience and sustainability of the energy supply system. In addition, offshore wind power generation sites are larger and can accommodate more wind turbines, increasing overall power generation. During the entire life cycle of offshore wind power generation structures, they continue to withstand wind, wave and other loads, such as Figure 1 As shown, designing and constructing a wind turbine foundation with sufficient rigidity and strength is a necessary condition to ensure that the wind turbine can operate stably and create economic value during its service life.

[0003] Therefore, proposing a calculation and analysis method for offshore wind and wave extreme loads is of great significance for evaluating the safety of offshore wind power generation structures (Higher-harmonic response of a slender monopile to fully nonlinear focused wave groups. Jiawang Liu and Bin Teng). Summary of the invention

[0004] In view of the problems existing in the prior art and in combination with the characteristics of wind and wave loads on offshore wind power structures, the present invention provides a method for calculating and analyzing the loads of offshore wind power generation structures under the action of wind and waves. This method can efficiently and accurately calculate the extreme wave loads and wind loads on the wind turbine structure.

[0005] In order to achieve the above object, the technical solution of the present invention is:

[0006] A method for calculating and analyzing extreme loads on offshore wind power generation structures, specifically a method for calculating and analyzing extreme loads on offshore wind power generation structures under the action of wind and waves, includes three parts: first, a high-order spectral numerical wave tank model is established, and waves are generated by inputting the target wave cross-sectional velocity at the inlet of the tank to simulate the generation and propagation of nonlinear extreme waves under real severe sea conditions; secondly, the semi-empirical and semi-theoretical Rainey slender body theory is used to directly calculate the wave loads of extreme waves on the wind turbine structure; finally, the aerodynamic load of the wind turbine, that is, the wind load, is calculated according to the classic blade element momentum theory. The above parts can be operated jointly or independently. The specific steps are as follows (such as Figure 2 shown):

[0007] The first step is to simulate nonlinear extreme wave fields by establishing a high-order spectral numerical wave tank model

[0008] Real ocean waves are random and nonlinear, and irregular waves are usually used for simulation in numerical analysis. Although in linear theory, irregular waves can be expressed as the linear superposition of different wave components; but when the wave nonlinearity is strong, the results of linear theory simulation cannot meet the accuracy requirements. The present invention needs to establish a completely nonlinear high-order spectrum numerical wave tank model to simulate nonlinear extreme waves.

[0009] The high-order spectrum numerical wave tank model first decomposes the total velocity potential of the fluid in the tank into a periodic velocity potential and an additional velocity potential, uses the periodic velocity potential to process the periodic boundary conditions, and uses the additional velocity potential to process the non-periodic wave-making boundary conditions, thereby realizing the wave-making function of the numerical tank; secondly, in order to improve the calculation efficiency, the pseudo-spectral method is used to solve the two velocity potentials after decomposition, and the two are respectively expanded into Fourier series forms that satisfy the control equations and the corresponding boundary conditions; thirdly, the additional velocity potential is calculated according to the wave-making boundary conditions of the tank at each calculation moment; finally, the obtained additional velocity potential is substituted into the nonlinear free water surface boundary conditions, and time integration is performed to obtain the tank wave surface at the next moment and the periodic velocity potential at the wave surface, thereby constructing a complete high-order spectrum numerical wave tank model, and repeating the cycle until the calculation is completed. The high-order spectrum numerical wave tank model constructed by the present invention can evolve the target waves calculated theoretically into more complex and realistic nonlinear waves through nonlinear propagation, that is, it can be used to efficiently and accurately simulate random, strongly nonlinear real sea waves. The specific steps are as follows:

[0010] 1) By constructing a high-order spectrum numerical wave tank model, its tank domain is defined as the calculation domain, such as Figure 3 As shown in Figure 2, the total velocity potential of the fluid in the computational domain is decomposed into two parts, namely the periodic velocity potential and the additional velocity potential. Specifically:

[0011] φ=φ p +φ add (1)

[0012] The periodic velocity potential is φ p , which needs to satisfy the free water surface boundary conditions, the bottom boundary conditions, and the impermeable boundary conditions at both ends:

[0013]

[0014] In the formula, x represents the horizontal coordinate, represents the partial derivative with respect to x; z represents the vertical coordinate, Indicates partial derivative with respect to z; Lx is the length of the water tank, and d is the water depth of the water tank.

[0015] The additional velocity potential is φ add , which needs to satisfy the wave-making boundary conditions at the left end of the tank, the bottom boundary conditions, and the impermeable side wall boundary conditions at the right end:

[0016]

[0017] Wherein, u is the horizontal velocity of the target wave input at the inlet of the water tank. The u value is directly calculated based on the target wave parameters using the second-order Stokes wave theory. For specific methods, see: Water Wave Theory and Its Application. Zou Zhili. Science Press. 2005, Chapter 2. The target wave parameters include wave period, wave height and water depth. The horizontal velocity of the target wave can be obtained from the above parameters.

[0018] 2) Solve for the additional velocity potential φ add and the time derivative of the additional velocity potential

[0019] 2.1) Determine the additional velocity potential φ add

[0020] φ add In the extended computational domain D add Solve in, such as Figure 4 As shown, the total width of the extended domain is L z =h add +d, where h add represents the vertical coordinate of the upper boundary of the extended computational domain, d represents the water depth; the horizontal velocity of the input boundary is about the centerline z c =(h add -d) / 2 antisymmetric distribution, so the present invention selects h add =3d, centerline height z c = d, ensure the center line z c Always above the free water surface.

[0021] In order to accurately simulate steeper nonlinear waves, the present invention proposes a wave-making method that inputs the target wave horizontal velocity from the bottom of the water tank z=-d to the instantaneous wave surface z=η. Specifically:

[0022] The wave surface η(0,t) at the inlet of the water tank can be directly calculated using the second-order Stokes wave theory according to the target wave parameters (Water Wave Theory and Its Application. Zou Zhili. Science Press. 2005, Chapter 2). add According to the first step formula (3), φ add The boundary conditions that are satisfied are expanded into the following form according to the characteristic function:

[0023]

[0024] In the formula, B n (t) is the additional velocity potential φ add The amplitude of the nth mode; κ n =(2n-1)π / (h add +d) is the extended domain D add The nth eigenmode in the vertical direction, h add represents the vertical coordinate at the upper boundary of the extended computational domain, N z Indicates the total number of modes in the vertical direction; L x represents the length of the water tank. Then according to the additional velocity potential φ add The characteristic expansion of can directly obtain its x-direction partial derivative

[0025]

[0026] The partial derivative Substitute the water tank incident wave generation boundary conditions:

[0027]

[0028] Where u(z) represents the horizontal velocity of the target wave input at the inlet of the water tank, which is directly calculated according to the target wave parameters using the second-order Stokes wave theory (Water Wave Theory and Its Application. Zou Zhili. Science Press, 2005, Chapter 2), and the vertical characteristic function cos[κ n (z+d)], the coefficient B can be determined according to the vertical distribution of the horizontal velocity u(z) at the wave-making boundary: n (t), and then determine φ add .

[0029] 2.2) Determine the time derivative of the additional velocity potential

[0030] The time derivative of the additional velocity potential is The following boundary conditions are met:

[0031]

[0032] In the formula, u t (z) represents the horizontal acceleration of the target wave input at the inlet of the water tank, which is directly calculated using the second-order Stokes wave theory based on the target wave parameters (Water Wave Theory and Its Application. Zou Zhili. Science Press, 2005, Chapter 2).

[0033] Then the time derivative of the additional velocity potential According to the characteristic function, it can be expanded into the following form:

[0034]

[0035] Then the time derivative of the additional velocity potential The x-direction partial derivative of It can be parsed into the following form:

[0036]

[0037] In the formula, C n (t) Yes The amplitude of the nth mode; the partial derivative Substituting into equation (7), using the vertical characteristic function cos[κ n (z+d)], according to the time derivative of the horizontal velocity at the wave-making boundary u t The vertical distribution of (z), the coefficient of determination C n (t), and then determine

[0038] In order to avoid initial effects, u(z) and u t (z) needs to be multiplied by the buffer function R m (t), the expression is as follows:

[0039]

[0040] Where, T m To buffer the duration, the present invention takes twice the wave characteristic period.

[0041] 3) Solve for the periodic velocity potential φ p

[0042] 3.1) The periodic velocity potential φ at the free water surface of the tank p Expressed as φ which depends only on the horizontal coordinate p s form:

[0043]

[0044] Where t is the current calculation time, and η is the wave surface height at time t.

[0045] 3.2) Then the fully nonlinear free water surface boundary condition of the water tank is expressed as follows:

[0046]

[0047]

[0048] Where g represents the acceleration due to gravity; W represents the vertical velocity of the water particle at the free water surface; represents the partial derivative with respect to time; ν represents the spatial damping parameter of the wave-breaking zone added at the end of the water tank. After the wave-breaking zone is added, wave reflection can be effectively prevented. The present invention selects a spatial damping distribution function in the form of a cubic polynomial:

[0049]

[0050] In the formula, x0 represents the starting point of the wave-breaking zone, and the length of the wave-breaking zone is L. x -x0, then the dimensionless length coefficient μ=(x-x0) / (L x -x0); α is the damping strength;

[0051] 3.3) Periodic velocity potential φ p The corresponding vertical velocity of the water particle It is calculated by the following recursive formula:

[0052]

[0053] Where M is the total order of nonlinear retention, W (m) represents the mth order component, j is an integer;

[0054] According to the periodic velocity potential φ described in the first step formula (2) p The boundary conditions are satisfied, expanded into Fourier series, and calculated recursively as follows:

[0055]

[0056] In the formula, represents the mth-order periodic velocity potential The amplitude of the nth mode, k n =nπ / L x is the nth natural eigenmode of the wave tank, N x is the total number of modes in the horizontal direction.

[0057] Finally, a high-order spectrum numerical wave tank model is constructed, that is, the high-order spectrum numerical wave tank model is composed of formulas (1), (4), (8), (12), (13) and (16).

[0058] 4) The constructed high-order spectrum numerical wave tank model is used to efficiently and accurately simulate random and strongly nonlinear real sea waves, such as Figure 5 As shown, the following steps are included:

[0059] 4.1): Through the constructed numerical wave tank model, divide the tank into horizontal and vertical grids, and give the initial conditions of the tank. Set the free surface potential at each location of the numerical wave tank model at the initial time. and the free surface elevation η is as follows:

[0060]

[0061] 4.2): At the current time t, according to the wave-generating boundary condition formulas (6) and (7), the additional velocity potential φ is obtained: add and The specific steps include the following:

[0062] 4.2.1) According to the target wave parameters, the second-order Stokes wave theory is used to calculate the wave surface η, water particle velocity u and acceleration u of the target wave at the inlet of the water tank. t ;

[0063] 4.2.2) According to the wave-generating boundary conditions (6) and (7) of the flume, the additional velocity potential φ is calculated in the extended domain: add , additional velocity potential time derivative

[0064] 4.2.3) After obtaining the additional velocity potential φ add Based on the characteristic expansion coefficient, the additional velocity potential φ at the free water surface is solved by analytical method add The spatial derivative of

[0065] 4.3): Calculate the periodic velocity potential φ p and its spatial derivative, as well as the spatial derivative of the wave surface η, and calculate the velocity and acceleration of the water particles in the flow field, which specifically includes the following sub-steps:

[0066] 4.3.1) Knowing the current time According to the recursive relationship (16), the recursive calculation is performed Finally, the cumulative sum can be obtained at that moment

[0067] 4.3.2) Calculate the periodic velocity potential at the wave surface and the horizontal derivative of the wave surface respectively and the periodic velocity potential φ p The vertical derivative W of

[0068] 4.3.3) After calculating the periodic velocity potential φ p 、Additional velocity potential φ add On this basis, the spatial and temporal derivatives of the two are further calculated and summed up using analytical methods to obtain the velocity and acceleration of the water particle at any position in the flow field.

[0069] 4.4): Substitute the physical quantities calculated in steps 4.2) and 4.3) as the forcing terms into the fully nonlinear free water surface condition equations (12) and (13) and perform time integration to obtain the free surface potential at the next moment. and the free surface elevation η. Determine whether the calculation time has ended. If not, repeat steps 4.2) to 4.4) and enter the calculation of the next time t+Δt;

[0070] 4.5): When the wave simulation ends, the calculation of the high-order spectrum numerical wave tank model ends, and the numerical simulation of the whole process of nonlinear wave generation and propagation in the numerical wave tank can be realized.

[0071] The present invention uses a high-order spectrum numerical wave tank model to generate focused waves to simulate nonlinear extreme waves at sea, such as Figure 3 As shown, the inlet velocity of the nonlinear numerical wave tank is calculated using the second-order Stokes wave theory according to the simulated target wave parameters, and the wave surface elevation of each wave component of the focused wave is set to reach the maximum at time t=t0 and spatial position x=x0. Each wave component is distributed at equal intervals within the frequency range of the focused wave, and the amplitude is determined by the wave spectrum according to the NewWave model:

[0072]

[0073] In the formula, A i is the linear focused amplitude corresponding to the ith wave component; N is the total number of wave components; A is the total focused amplitude; S(f i ) is the spectrum value corresponding to the i-th wave component; S(f n ) is the spectrum value corresponding to the nth wave component. Wherein, S(f) is the spectrum of the wave, and the present invention adopts the JONSWAP spectrum:

[0074]

[0075] Where f is the wave frequency; H 1 / 3 is the effective wave height; T p is the spectrum peak period; f p is the peak frequency; γ is the peak raising factor of the spectrum,

[0076]

[0077]

[0078] The traditional second-order Stokes wave theory of irregular waves (Water Wave Theory and Its Application. Zou Zhili. Science Press. 2005, Chapter 2) can only calculate the second-order sum and difference frequency components of the focused waves, while the high-order spectrum numerical tank model established in the present invention can simulate the higher-order components in the strongly nonlinear focused waves, and the calculation results are more accurate and can simulate the real extreme waves at sea.

[0079] Waves are generated by inputting the target wave cross-sectional velocity at the inlet of the water tank to simulate the generation and propagation of nonlinear extreme waves under real severe sea conditions.

[0080] In the second step, the wave loads acting on the wind turbine structure due to the nonlinear extreme waves simulated in the first step are calculated at each moment according to the Rainey slender body theory. The wave loads are divided into two parts: the linear wave load f Rainey (t,z) contribution, and the surface concentrated force (F x ) surface Contribution section. Details are as follows:

[0081] (1) Calculation of linear wave load f Rainey (t,z) contribution

[0082] The present invention adopts Rainey slender body theory to directly calculate the wave force of extreme waves acting at any height of the pile foundation centerline, that is, the linear wave load f on the unit length of the wind turbine structure. Rainey (t,z):

[0083]

[0084] Where ρ is the density of seawater; u, are the horizontal velocity and acceleration of the water particle at the central axis of the pile in the incident wave field; w z Represents the partial derivative of the vertical velocity of the water particle in the z direction. For the focused wave incident situation, the present invention uses the focused wave simulated by the high-order spectrum numerical wave tank model to represent the extreme wave, restores the wave field in the tank at each time step and calculates the velocity and acceleration of the water particle at the fan structure in the flow field. m is the additional inertia coefficient or mass coefficient; C D is the drag coefficient or velocity force coefficient; A=πD 2 / 4 is the cross-sectional area, and D is the pile diameter. It is worth noting that compared with the classic Morison formula, the Rainey slender body theory adopted in the present invention additionally considers the axial divergence term ρAC m w z u.

[0085] The linear wave load f Rainey The wave load contributing to (t,z) needs to be calculated by formula (22) Rainey (t,z) is vertically integrated along the wind turbine structure from the seabed z = -d to the instantaneous wave surface z = η(t), and the expression is as follows:

[0086]

[0087] Where z represents the vertical coordinate; d represents the water depth; η(t) represents the wave surface at the axis of the wind turbine at the current calculation moment; Fx represents f Rainey (t,z) horizontal force of waves acting on the wind turbine structure; M y It represents the overturning moment of the wave acting on the wind turbine structure around the mud surface.

[0088] (2) Contribution of water surface concentration force

[0089] The concentrated force of the water surface (F x ) surface The wave load contribution is calculated as follows:

[0090]

[0091] In the formula, (M y ) surface Indicates the concentrated force on the water surface (F x ) surface The overturning moment acting on the fan structure around the mud surface;

[0092] By summing up the two parts of the load calculated by equations (23) and (24), the total wave load acting on the wind turbine structure at each moment can be obtained.

[0093] The third step is to calculate the wind load on the fan structure according to the blade element momentum theory, as follows:

[0094] At each moment, according to the blade parameters of the fan superstructure, the present invention uses the classic blade element momentum theory to calculate the fan aerodynamic load. This theory is based on the equality of thrust and torque calculated by momentum theory and blade element theory, and iteratively solves the axial and tangential induction factors a and a', such as Figure 6 As shown in the figure, the horizontal thrust T of the fan blade at each moment is obtained by integrating along the radial direction of the blade:

[0095]

[0096] In the formula, R hub is the hub radius, R is the fan blade radius, V0 is the wind speed, ρ air is the air density and F is the loss factor.

[0097] In summary, the present invention proposes and constructs a high-order spectrum numerical wave flume model to simulate nonlinear extreme wave fields, providing flow field information for calculating the wave loads of offshore wind turbine structures. The two are combined to form an offshore wind turbine structure wave load calculation module; parallel to this is the offshore wind turbine wind load calculation module, which directly calculates the wind load based on the wind speed using the blade element momentum theory. The total load on the wind turbine structure can be obtained by summing the wind and wave loads, comparing and analyzing the relationship between the wave load and wind load values, and finding the maximum value and its corresponding time from the total load duration curve, thereby providing a reference for the actual engineering design of offshore wind turbines.

[0098] Beneficial effects of the present invention:

[0099] The nonlinear numerical wave tank model established by the present invention is used to simulate the incident wave field, and the wave load acting on the wind turbine structure is directly calculated according to the semi-theoretical and semi-empirical Rainey slender body theory. A numerical model for efficiently calculating and analyzing the nonlinear extreme wave loads on the wind turbine structure is established, and combined with the aerodynamic load calculation module established based on the blade element momentum theory, a complete offshore wind turbine structural load time-domain calculation and analysis model is formed, thereby providing a reference for the actual engineering construction and design of offshore wind turbines.

[0100] The present invention proposes a method for calculating and analyzing the total load of an offshore wind turbine structure under the combined action of wind and waves. The method can fully consider the inherent nonlinearity of incident extreme waves, and stably, efficiently and accurately calculate the effect of extreme waves of nonlinear "wave-wave" interaction on offshore wind turbines. BRIEF DESCRIPTION OF THE DRAWINGS

[0101] Figure 1 Schematic diagram of wind and wave loads.

[0102] Figure 2 Flow chart for offshore wind turbine load calculation.

[0103] Figure 3 Schematic diagram of the numerical wave tank and fan function.

[0104] Figure 4 is the extended computational domain D add Schematic diagram.

[0105] Figure 5 Flowchart for numerical wave tank calculations.

[0106] Figure 6 Schematic diagram of wind load calculation for offshore wind turbine structures.

[0107] Figure 7 is the wavefront duration curve at the focusing position.

[0108] Figure 8 It is the time curve of horizontal force on the fan.

[0109] Fig. 9 This is the duration curve of the overturning moment of the fan around the mud surface.

[0110] In the figure, x0 is the coordinate of the starting point of the wave-breaking area of ​​the water tank, L x is the length of the water tank; L z D is the total width of the extended computational domain of the flume. add To expand the computational domain, h add is the coordinate of the upper boundary of the extended computational domain, z cis the vertical coordinate of the central axis of the extended calculation domain. DETAILED DESCRIPTION

[0111] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0112] A method for calculating and analyzing extreme loads of an offshore wind power generation structure, specifically a method for calculating and analyzing extreme loads of an offshore wind power generation structure under wind and wave action, is implemented based on an offshore wind power generation structure load calculation and analysis system. The analysis system includes three independent modules, namely, a nonlinear extreme wave simulation module, a nonlinear wave load calculation module, and a wind load calculation module. The three modules can be operated jointly or independently. The specific steps are as follows (eg Figure 2 shown):

[0113] The first step is to build a nonlinear extreme wave simulation module, that is, to build a numerical wave tank, determine the tank and wave parameters, and use the tank to simulate nonlinear extreme waves:

[0114] (1) The constructed nonlinear extreme wave simulation module is also called a numerical wave tank. The parameters of the numerical wave tank are set according to the parameters of the target wave (wave height, period or wavelength, etc.) as follows: the length of the tank is 10 times the characteristic wavelength, that is, L x =10L p , L p Indicates characteristic wavelength; 2L before the end of the water tank p Set as wave damping zone, wave damping intensity α = 1.0; buffering time is set to 2 times the characteristic period, i.e. 2T p , T p Represents the characteristic period; the number of discrete points equally spaced in the x and z directions (i.e., the number of modes in the horizontal and vertical directions) N x and N z 512 and 256 respectively. Periodic velocity potential φ in free water boundary condition p The retention order M is 7. At the initial time t = 0, the free surface potential is given and free surface elevation η = 0;

[0115] (2) At the current moment t, according to the horizontal velocity u and horizontal acceleration u of the target wave at the wave-making boundary, t Determine the additional velocity potential φ add And the time derivative of the additional velocity potential And calculate the additional velocity potential φ add The spatial derivative of

[0116] (3) At the current moment t, calculate the free water surface W. The pseudo-spectral method is used to deal with the completely nonlinear free water surface conditions, and the fourth-order Runge-Kutta method is used for time integration to obtain the free surface potential at the next moment. and the free surface elevation η, thus achieving time stepping of the numerical model;

[0117] (4) Determine whether the calculation time has ended. If not, repeat steps (2) to (4) and proceed to the calculation of the next time t+Δt.

[0118] (5) When the wave simulation ends, the numerical simulation of nonlinear wave generation and propagation can be realized.

[0119] The following is an example of extreme wave generation and propagation using the nonlinear extreme wave simulation module constructed by the present invention:

[0120] According to the target extreme wave simulation, the numerical wave tank parameters (water depth, tank size, damping zone range, etc.) and the target wave parameters (amplitude, period or wavelength, etc.) are set, and the tank model proposed in the present invention can be used to perform numerical simulation of nonlinear extreme waves. Set the high-order spectrum numerical wave tank length L = 10L p =1487.30m, the length of the damping zone at the end of the flume is 4L p , water depth d = 20m; input incident focusing wave amplitude A = 1.80m, the spectral peak frequency of the focusing wave is f p =0.085Hz, the characteristic wavelength L is determined according to the dispersion relation p =148.73m, the focused wave group consists of 25 frequencies at 0.5f p <f<1.5f p The sine components are evenly distributed within the range; the center of the single pile is set at the actual focusing position x f =1.00L p , focusing on time t f =8T p It is worth noting that the present invention also uses linear and second-order Stokes theories to directly calculate the focused wave. The wavefront duration curves of the linear and nonlinear focused waves at the focusing position are as follows: Figure 7 As shown in the figure, after comparative analysis, it is found that the high-order spectrum numerical wave tank model simulates the high-order components of the focused wave due to the nonlinear "wave-wave" interaction between the wave components, resulting in a narrow peak and a shallow trough on the focused wave surface. The peak and trough values ​​of the focused wave calculated by the second-order Stokes wave theory are greater than the results of the linear theory calculation, which is close to the simulation results of the high-order spectrum numerical wave tank. The peak value of the extreme wave focused wave simulated by the numerical wave tank is the largest.

[0121] The second step is to determine the wind turbine structural section parameters and hydrodynamic coefficients and calculate the wave loads:

[0122] The present invention calculates the wave load on the NREL 5-MW benchmark wind turbine structure, and sets the cross-sectional dimensions and hydrodynamic coefficients as follows: pile radius R = 3.0 m, additional inertia force coefficient C m =1.0, drag coefficient C D = 1.0. At each moment, the velocity and acceleration of water particles in the extreme wave field simulated by the numerical wave tank model proposed by the present invention are used to directly calculate the total wave load on the wind turbine structure using the Rainey slender body theory.

[0123] The third step is to determine the parameters of the fan blades and calculate the wind load based on the blade element momentum theory.

[0124] The present invention calculates the wind load on the NREL 5-MW benchmark wind turbine. The horizontal axis wind turbine system is mainly composed of an upper structure: blades, a hub, a nacelle, a tower, and a lower monopile foundation. Table 1 lists the overall parameters of the wind turbine system, the hub and nacelle parameters, and the structural parameters of the wind turbine blades (including blade length, single blade mass, and the position of the blade mass center, etc.). The wind turbine blades are divided into 17 blade elements along the radial direction. The corresponding aerodynamic parameters (lift, drag, and pitch moment coefficient) can be obtained according to the geometric parameters of each blade element. The blade element theory assumes that the blade elements are independent of each other along the radial direction. The load on each blade element can be calculated separately, and then the aerodynamic load of the entire blade can be obtained by integrating along the radial direction. Assuming that the incoming wind is steady and evenly distributed vertically, considering the situation where the wind and waves are in the same direction, the wind speed is set to 11.4m / s and the wind rotor speed is set to 12.1rpm. Based on this, the blade element momentum theory can be used to calculate the wind load acting on the wind turbine at each moment.

[0125] Table 1 Basic parameters of NREL5-MW benchmark wind turbine

[0126]

[0127] Step 4: Calculate and output the total load on the fan structure

[0128] The present invention superimposes the wave load and wind load calculated at each moment to obtain the total load on the wind turbine, and outputs and saves the calculation result of the total load as follows: Figure 8 , 9As shown in the figure, it can be seen that the contribution of wind load and wave load to horizontal force is not much different, but due to the towering wind turbine structure, the wind load arm is longer, and the overturning moment generated by the wind load is dominant. Compared with the total load duration curve corresponding to the linear focused wave, the wave load corresponding to the focused wave simulated by the numerical wave tank is very close to the load duration curve corresponding to the focused wave calculated by the second-order Stokes theory and is steeper. The peak (valley) value of the load duration curve is larger, indicating that the load corresponding to the nonlinear focused wave is more nonlinear and the result is more accurate. In addition, by analyzing the output total load duration curve, the extreme load (i.e., the maximum total load) and its corresponding time can be found, thereby providing a reference for the design and construction of offshore wind turbine structures.

[0129] The above-described embodiments merely express the implementation methods of the present invention, but they cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for calculating and analyzing extreme loads of offshore wind power generation structures, characterized in that: The analysis method comprises the following steps: firstly, a high-order spectrum numerical wave tank model is established, and wave generation is performed by inputting the target wave cross-sectional velocity at the inlet of the tank to simulate the generation and propagation of nonlinear extreme waves under real severe sea conditions, so as to provide flow field information for calculating the wave load of the offshore wind turbine structure; secondly, the semi-empirical and semi-theoretical Rainey slender body theory is used to calculate the wave load of the extreme wave on the wind turbine structure; finally, the wind load of the wind turbine is calculated according to the blade element momentum theory based on the wind speed; specifically: The first step is to simulate nonlinear extreme wave fields by establishing a high-order spectral numerical wave tank model The high-order spectrum numerical wave tank model first decomposes the total velocity potential of the fluid in the tank into a periodic velocity potential and an additional velocity potential, uses the periodic velocity potential to process the periodic boundary conditions, and uses the additional velocity potential to process the non-periodic wave-making boundary conditions, so as to realize the wave-making function of the numerical tank; secondly, in order to solve the two decomposed velocity potentials by using the pseudo-spectral method, the two are respectively expanded into the Fourier series form that satisfies the control equation and the corresponding boundary conditions; thirdly, the additional velocity potential is calculated according to the wave-making boundary conditions of the tank at each calculation moment; finally, the obtained additional velocity potential is substituted into the nonlinear free water surface boundary conditions, and the time integration is performed to obtain the tank wave surface at the next moment and the periodic velocity potential at the wave surface, so as to construct a complete high-order spectrum numerical wave tank model, which is repeated until the end, and can be used to efficiently and accurately simulate random and strongly nonlinear real sea waves; In the second step, the wave loads acting on the wind turbine structure due to the extreme waves simulated in the first step are calculated at each moment according to the Rainey slender body theory. The wave loads are divided into two parts: the linear wave load f Rainey (t,z) contribution, and the surface concentrated force (F x ) surface Contribution section; The third step is to calculate the wind load on the fan structure based on the blade element momentum theory; The fourth step is to sum the wind load and wave load to obtain the total load on the wind turbine structure, and compare and analyze the relationship between the wave load and wind load values ​​to provide a reference for the actual engineering design of offshore wind turbines.

2. The method for calculating and analyzing extreme loads of offshore wind power generation structures according to claim 1, characterized in that: The first step is as follows: 1) Through the constructed high-order spectrum numerical wave tank model, its tank domain is defined as the calculation domain, and the total velocity potential of the fluid in the calculation domain is decomposed into two parts, namely the periodic velocity potential and the additional velocity potential. Specifically: f=f p +φ add (1) The periodic velocity potential is φ p , which needs to satisfy the free water surface boundary conditions, the bottom boundary conditions, and the impermeable boundary conditions at both ends: In the formula, x represents the horizontal coordinate, represents the partial derivative with respect to x; z represents the vertical coordinate, Indicates partial derivative with respect to z; L x is the length of the water tank, d is the water depth of the water tank; The additional velocity potential is φ add , which needs to satisfy the wave-making boundary conditions at the left end of the tank, the bottom boundary conditions, and the impermeable side wall boundary conditions at the right end: Where u is the horizontal velocity of the target wave input at the inlet of the water tank, and the u value is directly calculated using the second-order Stokes wave theory according to the target wave parameters; 2) Solve for the additional velocity potential φ add and the time derivative of the additional velocity potential 2.1) Determine the additional velocity potential φ add φ add In the extended computational domain D add Solve in, choose h add =3d, centerline height z c = d, ensure the center line z c Always above the free water surface; In order to accurately simulate steeper nonlinear waves, a wave-making method is proposed to input the target wave horizontal velocity from the bottom of the tank z = -d to the instantaneous wave surface z = η; Specifically: The wave surface η(0,t) at the inlet of the flume is directly calculated from the target wave parameters; in the extended domain D add According to the first step formula (3), φ add The boundary conditions that are satisfied are expanded into the following form according to the characteristic function: In the formula, B n (t) is the additional velocity potential φ add The amplitude of the nth mode; κ n =(2n-1)π / (h add +d) is the extended domain D add The nth eigenmode in the vertical direction, h add represents the vertical coordinate at the upper boundary of the extended computational domain, N z Indicates the total number of modes in the vertical direction; L x represents the length of the water tank; then according to the additional velocity potential φ add The characteristic expansion of can directly obtain its x-direction partial derivative The partial derivative Substitute the water tank incident wave generation boundary conditions: At x = 0 (6) Where u(z) represents the horizontal velocity of the target wave input at the inlet of the water tank, which is directly calculated based on the target wave parameters using the second-order Stokes wave theory and the vertical characteristic function cos[κ n (z+d)], and the coefficient B is determined according to the vertical distribution of the horizontal velocity u(z) at the wave-making boundary. n (t), and then determine φ add ; 2.2) Determine the time derivative of the additional velocity potential The time derivative of the additional velocity potential is The following boundary conditions are met: At x = 0 (7) In the formula, u t (z) represents the horizontal acceleration of the target wave input at the inlet of the water tank, which is directly calculated using the second-order Stokes wave theory based on the target wave parameters; Then the time derivative of the additional velocity potential According to the characteristic function, it can be expanded into the following form: Then the time derivative of the additional velocity potential The x-direction partial derivative of It can be parsed into the following form: In the formula, C n (t) Yes The amplitude of the nth mode; the partial derivative Substituting into equation (7), using the vertical characteristic function cos[κ n (z+d)], according to the time derivative of the horizontal velocity at the wave-making boundary u t The vertical distribution of (z), the coefficient of determination C n (t), and then determine In order to avoid initial effects, u(z) and u t (z) needs to be multiplied by the buffer function R m (t), the expression is as follows: Where, T m For the buffering time, take twice the characteristic period of the wave; 3) Solve for the periodic velocity potential φ p 3.1) The periodic velocity potential φ at the free water surface of the tank p Expressed as depending only on the horizontal coordinate form: Where t is the current calculation time, η is the wave height at time t; 3.2) Then the fully nonlinear free water surface boundary condition of the water tank is expressed as follows: On z = η (12) In the formula (13) above z = η, g represents the acceleration due to gravity; W represents the vertical velocity of the water particle at the free water surface; represents the partial derivative with respect to time; ν represents the spatial damping parameter of the wave-breaking zone added at the end of the water tank. The addition of the wave-breaking zone can effectively prevent wave reflection; 3.3) Periodic velocity potential φ p The corresponding vertical velocity of the water particle It is obtained by the following formula: Where M is the total order of nonlinear retention, W (m) represents the mth order component, j is an integer; According to the periodic velocity potential φ described in the first step formula (2) p Satisfied boundary conditions, it is expanded into Fourier series, and the following formula is obtained: In the formula, represents the mth-order periodic velocity potential The amplitude of the nth mode, k n =nπ / L x is the nth natural eigenmode of the wave tank, N x is the total number of modes in the horizontal direction; Finally, a high-order spectrum numerical wave tank model was constructed; 4) The constructed high-order spectrum numerical wave tank model is used to efficiently and accurately simulate random and strongly nonlinear real sea waves, including the following steps: 4.1): Through the constructed numerical wave tank model, divide the tank into horizontal and vertical grids, and give the initial conditions of the tank; set the free surface potential at each location of the numerical wave tank model at the initial time. and the free surface elevation η is as follows: η=0, at t=0(17) 4.2): At the current time t, according to the wave-generating boundary condition formulas (6) and (7), the additional velocity potential φ is obtained: add and The specific steps include the following: 4.2.1) According to the target wave parameters, the second-order Stokes wave theory is used to calculate the wave surface η, water particle velocity u and acceleration u of the target wave at the inlet of the water tank. t ; 4.2.2) According to the wave-generating boundary conditions (6) and (7) of the flume, the additional velocity potential φ is calculated in the extended domain: add , additional velocity potential time derivative 4.2.3) After obtaining the additional velocity potential φ add Based on the characteristic expansion coefficient, the additional velocity potential φ at the free water surface is solved by analytical method add The spatial derivative of 4.3): Calculate the periodic velocity potential φ p and its spatial derivative, as well as the spatial derivative of the wave surface η, and calculate the velocity and acceleration of the water particles in the flow field, which specifically includes the following sub-steps: 4.3.1) Knowing the current time According to the recursive relationship (16), the recursive calculation is performed Finally, the cumulative sum can be obtained at that moment 4.3.2) Calculate the periodic velocity potential at the wave surface and the horizontal derivative of the wave surface respectively and the periodic velocity potential φ p The vertical derivative W of 4.3.3) After calculating the periodic velocity potential φ p 、Additional velocity potential φ add On this basis, the spatial and temporal derivatives of the two are further calculated and summed using analytical methods to obtain the velocity and acceleration of the water particle at any position in the flow field. 4.4): Substitute the physical quantities calculated in steps 4.2) and 4.3) as the forcing terms into the fully nonlinear free water surface condition equations (12) and (13) and perform time integration to obtain the free surface potential at the next moment. and the free surface elevation η; determine whether the calculation time is over, if not, repeat steps 4.2) to 4.4) over and over again, and enter the calculation of the next time t+Δt; 4.5): When the wave simulation ends, the calculation of the high-order spectrum numerical wave tank model ends, and the numerical simulation of the whole process of nonlinear wave generation and propagation in the numerical wave tank can be realized.

3. The method for calculating and analyzing extreme loads of an offshore wind power generation structure according to claim 2, characterized in that: In the step 3.2), a spatial damping distribution function in the form of a cubic polynomial is selected: In the formula, x0 represents the starting point of the wave-breaking zone, and the length of the wave-breaking zone is L. x -x0, then the dimensionless length coefficient μ=(x-x0) / (L x -x0); α is the damping strength.

4. The method for calculating and analyzing extreme loads of offshore wind power generation structures according to claim 1, characterized in that: The second step is as follows: (1) Calculation of linear wave load f Rainey (t,z) contribution The Rainey slender body theory is used to directly calculate the wave force of extreme waves at any height of the pile foundation centerline, that is, the linear wave load f on the unit length of the wind turbine structure. Rainey (t,z): Where ρ is the density of seawater; u, are the horizontal velocity and acceleration of the water particle at the central axis of the pile in the incident wave field; w z represents the partial derivative of the vertical velocity of the water particle in the z direction; for the focused wave incident case, the focused wave simulated by the high-order spectrum numerical wave tank model represents the extreme wave, and the wave field in the tank is restored at each time step and the velocity and acceleration of the water particle at the fan structure in the flow field are calculated; C m is the additional inertia coefficient or mass coefficient; C D is the drag coefficient or velocity force coefficient; A=πD 2 / 4 is the cross-sectional area, and D is the pile diameter; The linear wave load f Rainey The wave load contributing to (t,z) needs to be calculated by formula (22) Rainey (t,z) is vertically integrated along the wind turbine structure from the seabed z = -d to the instantaneous wave surface z = η(t), and the expression is as follows: Where z represents the vertical coordinate; d represents the water depth; η(t) represents the wave surface at the axis of the fan at the current calculation time; F x represents f Rainey (t,z) horizontal force of waves acting on the wind turbine structure; M y It represents the overturning moment of the wave acting on the wind turbine structure around the mud surface; (2) Contribution of water surface concentration force The concentrated force of the water surface (F x ) surface The wave load contribution is calculated as follows: In the formula, (M y ) surface Indicates the concentrated force on the water surface (F x ) surface The overturning moment acting on the fan structure around the mud surface; The total wave load acting on the wind turbine structure can be obtained by adding up the two parts of the load calculated by equations (23) and (24).

5. The method for calculating and analyzing extreme loads of offshore wind power generation structures according to claim 1, characterized in that: The third step is as follows: At each moment, the aerodynamic load of the fan is calculated based on the blade parameters of the fan superstructure using the classical blade element momentum theory; the horizontal thrust T of the fan blade as a whole at each moment is obtained by integrating along the blade radial direction: In the formula, R hub is the hub radius, R is the fan blade radius, V0 is the wind speed, ρ air is the air density and F is the loss factor.

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