A method for determining the value of seismic load suitable for a single-pile offshore wind turbine structure on a saturated seabed

Abstract

This invention discloses a method for determining seismic loads on monopile offshore wind turbine structures suitable for saturated seabeds, belonging to the field of structural seismic analysis technology. The method includes the following steps: establishing the dynamic stiffness matrix and global equilibrium equations for a saturated layered seabed; iteratively solving the dynamic response of the saturated layered seabed using an equivalent linearization method; establishing a finite element model of a monopile offshore wind turbine considering pile-soil interaction; inputting the site wavefield response into the model to calculate the structural response of the wind turbine under seismic loading, including tower top acceleration, tower base shear force, and tower base bending moment, thereby evaluating its seismic performance. This invention solves the problem of inaccurate seismic analysis results in existing methods. It also considers the site effects of saturated layered seabeds and the characteristics of obliquely incident seismic waves to conduct dynamic response analysis of offshore wind turbine structures, thereby obtaining more realistic seismic load inputs and improving the reliability of seismic assessment.

Description

Technical Field

[0001] This invention relates to a method for determining seismic load values, specifically a method for determining seismic load values ​​applicable to monopile offshore wind turbine structures on saturated seabeds, belonging to the field of structural seismic analysis technology. Background Technology

[0002] In recent years, the accelerated global energy structure transformation has driven the rapid development of the wind power industry. Offshore wind power, due to its abundant resources and great development potential, has become an important direction for my country's renewable energy construction. However, the complex geological conditions and variable marine environment in my country's coastal areas mean that offshore wind power facilities, in addition to being subjected to typical marine environmental loads such as wind, waves, and currents, may also be significantly affected by seismic activity and complex seabed conditions. Therefore, systematically studying the dynamic response of offshore wind power structures under the coupled influence of seismic action and complex seabed environment is of great significance for ensuring their seismic safety and long-term service performance.

[0003] Currently, research on the seismic response of offshore wind turbine structures mainly relies on numerical simulation methods, with related work focusing primarily on structural dynamic characteristics, soil-soil interaction (SSI), and seismic vulnerability. However, existing methods often directly input seismic motion at the foundation or mud surface location, typically using station records from strong-motion databases for direct analysis of the target site. While this approach facilitates the calculation of structural dynamic response, it often neglects the stratum effects during the propagation of seismic waves from the source to the target site. Furthermore, existing studies often simplify the saturated seabed medium as a single-phase isotropic soil, insufficiently considering factors such as the coupling between the soil skeleton and pore fluids, and wave propagation effects within the saturated seabed.

[0004] Furthermore, actual seismic waves typically propagate at an oblique angle to the site, rather than simply perpendicularly. Existing research has shown that the incident angle of seismic waves has a significant impact on the surface motion characteristics of the site, and using only the assumption of perpendicular incidence may lead to an insufficient assessment of the structural seismic requirements.

[0005] Therefore, it is necessary to study a seismic load determination method suitable for monopile offshore wind turbine structures on saturated seabeds, based on wave field propagation theory and considering the site effect of saturated layered seabeds and the characteristics of obliquely incident seismic wave input, and to conduct dynamic response analysis of offshore wind power structures in order to obtain more realistic seismic load input and improve the reliability of seismic assessment. Summary of the Invention

[0006] The purpose of this invention is to address the problem of inaccurate analysis results caused by directly inputting ground motion into the foundation and ignoring the site propagation effect and oblique incidence characteristics of seismic waves in existing seismic analysis of offshore wind turbines. This invention provides a method for determining seismic loads suitable for monopile offshore wind turbine structures on saturated seabeds, while considering the two-phase coupled wave effect of soil skeleton and pore fluid in saturated seabeds, the site propagation effect of seabeds, and the oblique incidence characteristics of seismic waves. This provides more accurate seismic load inputs for site-related seismic analysis of monopile offshore wind turbines under complex seabed conditions.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a method for determining seismic loads applicable to monopile offshore wind turbine structures on saturated seabeds, comprising the following steps: S1. Establish the dynamic stiffness matrix and global motion equilibrium equations for a saturated layered seabed site: Based on Biot's theory of saturated porous media, the saturated seabed site is simplified to a seawater layer. N The constitutive relation of the saturated soil body was constructed by the layer of saturated soil and the underlying bedrock half-space, and the dynamic equilibrium equation of the Biot saturated porous medium was established based on the principle of momentum conservation. The constitutive relation of saturated soil is: , , In the formula, s ij The components of the total stress tensor of the soil ( i , j = x , z ); G Shear modulus of the soil skeleton; e ij For the soil skeleton strain tensor components. , u i , u j For the soil skeleton displacement components, x i , x j These are spatial coordinate components; l The first Lamé constant of the soil skeleton. l =2 Gv / (1−2 n ), n Poisson's ratio for the soil skeleton e For the volumetric strain of the soil skeleton, For Biot's effective stress coefficient, p f Pore ​​fluid pressure, d ij Let Kronecker function be the function of Kronecker function, and wheni = j hour d ij =1, otherwise d ij =0; M For Biot modulus, g For fluid volume increment, g =−∇· w , w This represents the displacement component of the fluid relative to the soil skeleton. The expression for the dynamic equilibrium equation is: , , In the formula, ∇² is the Laplace operator. u i For the soil skeleton displacement components, r The total density of saturated soil r =(1− n ) r s + n r f , r s For soil particle density, r f For pore fluid density, n Porosity; m The equivalent inertial density parameter of the pore fluid. m = r a + r f / n , r a To add an inertial mass coefficient; b The viscosity coupling coefficient represents the seepage resistance between pore fluid and soil skeleton; a dot above the variable in the expression indicates the first time derivative, and two dots indicate the second time derivative. Based on Biot's theory of saturated porous media, the physical and mechanical parameters of each saturated soil layer were obtained, and the dynamic stiffness matrix of each saturated soil layer was established. K L Dynamic stiffness matrix of bedrock half-space K R The overall site stiffness matrix is ​​constructed based on the interlayer coupling relationship between each saturated soil layer and bedrock. K And based on the overall site stiffness matrix K Establish the global motion equilibrium equations for the field: In the formula, K The overall stiffness matrix of the site.U T For displacement magnitude vectors, Q T This represents the vector of external loads on the site. S2. The dynamic response of the saturated layered seabed is solved iteratively using the equivalent linearization method: Inputting obliquely incident P-waves or SV-waves from the bedrock surface, their propagation direction is... Z The angle between the axes is defined as the angle of incidence. i Determine the amplitude of incident wave displacement at the bedrock outcrop. A P0 and A SV0 is used to generate the external load vector. Q T ,Will Q T Substitute the values ​​into step S1 to solve the global motion equilibrium equations and obtain the displacements at the interfaces of each saturated soil layer. Then, the displacement of the soil skeleton at any point (x, z) within the saturated soil layer can be obtained. u , w Strain components of soil skeleton e x , e z and shear strain components e xz Maximum shear strain in saturated soil layer c max Determined by the strain components of the soil skeleton: In the formula, e x For horizontal normal strain, e z For vertical strain, e xz The shear strain component is defined as follows: 0.65 times the peak value of the maximum shear strain time history is taken as the equivalent shear strain. c eff : ; The soil skeleton shear modulus is updated using the dynamic shear modulus curve and the damping ratio curve. G With damping ratio x The parameters are used as parameters for the next iteration and substituted into step S1 to recalculate the global motion equilibrium equation, solve for the maximum shear strain and equivalent shear strain, repeat the iteration until convergence, obtain the frequency domain dynamic response of the saturated seabed site, and convert it into the time domain acceleration time history through inverse Fourier transform. The convergence criterion is: if the following conditions are met... If the result is converged, then the result is considered convergent; otherwise, let... i = i +1 Repeat step S2 above until the convergence condition is met; where, For the first iMaximum shear strain in the next iteration For the first i The maximum shear strain after −1 iterations, where Δ is the preset convergence threshold; S3. Establish a finite element model of a monopile offshore wind turbine considering pile-soil interaction: In OpenSees, the wind turbine tower is simplified into a variable cross-section beam element, and the nacelle and hub are equivalent to the concentrated mass at the top of the tower. The single pile foundation is simulated using beam elements. Distributed spring elements py, tz, and qz are arranged along the pile body below the mud surface to characterize the interaction between the pile and soil in the lateral direction, the pile side axial friction, and the pile end bearing, respectively, to establish a finite element model of a single pile offshore wind turbine. The pile-soil lateral interaction is modeled using the Py curve, and the lateral reaction force... p With lateral displacement y The relation uses a hyperbolic tangent model: In the formula, A This is a load correction factor for cyclic loading. A =0.9; p u For ultimate lateral soil resistance, k This represents the initial value of the foundation reaction modulus. z The depth below the mud surface. y This is lateral displacement; The axial frictional resistance of the pile side is modeled using the tz curve, and the expression for the ultimate frictional resistance is: In the formula, This refers to the effective vertical stress at this depth below the mud surface. d The angle of friction at the pile-soil interface; The pile end bearing capacity is modeled using the qz curve, and its expression is: , , In the formula, Q For the bearing capacity of the pile tip, Q p This refers to the ultimate bearing capacity at the pile tip. A p The cross-sectional area of ​​the pile tip. A p = πD 2 / 4, q p The limiting end resistance per unit area; z b For pile end displacement, z c For reference displacement, z c =0.07 D , D The diameter of the pile; S4. Seismic performance assessment: The horizontal and vertical acceleration time-history components of the dynamic response of the saturated seabed at the foundation embedment depth obtained in step S2 are used as seismic loads and simultaneously input into the foundation nodes of the monopile offshore wind turbine finite element model constructed in step S3, using Newmark- β The time history integral method is used to solve the structural motion equations, and the peak acceleration at the top of the tower, the peak shear force at the base of the tower, and the peak bending moment at the base of the tower are obtained respectively for the evaluation of seismic performance.

[0008] In step S1, the physical and mechanical parameters of each saturated soil layer include: saturated soil layer thickness. d Soil particle density r s Porosity n Pore ​​fluid density r f Shear wave velocity c S Compression wave speed c P Poisson's ratio of soil skeleton n and damping ratio x .

[0009] In step S1, the dynamic stiffness matrix of the saturated soil layer is: K L It is a 6×6 complex numerical symmetric matrix: , Dynamic stiffness matrix of bedrock half-space K R It is a 3×3 complex numerical symmetric matrix: , In the formula, P , R These represent the amplitudes of the horizontal and vertical external loads acting on the soil skeleton, respectively, where i is the imaginary unit. p f This refers to the orifice pressure amplitude value; u , w These represent the horizontal and vertical displacement amplitudes of the soil skeleton, respectively. W This represents the displacement amplitude of the fluid relative to the soil skeleton; subscripts 1 and 2 represent the upper and lower surfaces of the soil layer, respectively, and subscript 0 represents the bedrock outcrop. k ij ( i , j = 1~6) and r ij ( i , j =1~3) are all stiffness matrix elements.

[0010] In step S1, the overall site stiffness matrix K The expression is: , In the formula, N This represents the total number of saturated soil layers. ( n =1~ N ; i , j =1~2) is the first n Layer dynamic stiffness matrix K Ln The ij A 3×3 submatrix.

[0011] In step S3, the ultimate lateral soil resistance p u Take the smaller value calculated for shallow and deep conditions: , , In the formula, p us This represents the ultimate lateral soil resistance in the shallow layer. p ud This represents the ultimate lateral soil resistance in the deep layers. c′ The effective unit weight of the soil; z The depth below the mud surface; D The diameter of the pile; C 1. C 2. C 3 represents the angle of friction with the soil. f The relevant dimensionless coefficients.

[0012] The beneficial effects of this invention are: 1) The method of this invention solves the problem of inaccurate analysis results caused by directly inputting ground motion into the foundation and ignoring the site propagation effect of saturated seabed and the oblique incidence characteristics of seismic waves in the existing seismic analysis of offshore wind turbines; at the same time, it considers the two-phase coupled wave effect of soil skeleton and pore fluid in saturated seabed, the site propagation effect of seabed and the oblique incidence characteristics of seismic waves, and carries out dynamic response analysis of offshore wind power structure, providing more accurate seismic load input for site-related seismic analysis of monopile offshore wind turbines under complex seabed conditions, and improving the reliability of seismic assessment.

[0013] 2) The method of this invention establishes an accurate dynamic stiffness matrix of saturated soil layer based on Biot's theory of saturated porous media, which can truly reflect the two-phase coupling wave effect of soil skeleton and pore fluid in saturated seabed, and overcomes the problem of site response distortion caused by simplifying saturated seabed as single-phase soil.

[0014] 3) The method of this invention takes into account the site effect of oblique incidence propagation of seismic waves in a layered saturated seabed, avoiding the problem of inaccurate seismic load caused by directly inputting station records into the foundation in the traditional method.

[0015] 4) The method of this invention uses py, tz, and qz curve models to characterize pile-soil interaction, which can more realistically simulate the dynamic response characteristics of a single pile foundation under complex seabed conditions. Attached Figure Description

[0016] Figure 1 This is a flowchart of the steps of the method of the present invention; Figure 2 This is a finite element model of a saturated layered seabed-monopile offshore wind turbine constructed in the method of the present invention and a schematic diagram of oblique seismic wave incidence. Figure 3 This is a time history diagram of the seismic response of the wind turbine structure when the incident angle θ = 30° is tested in an embodiment of the present invention. Detailed Implementation

[0017] The present invention will be further explained and described below with reference to the accompanying drawings and specific embodiments.

[0018] Example: Figure 1 The flowchart shown illustrates a method for determining seismic loads on monopile offshore wind turbine structures suitable for saturated seabeds, comprising the following steps: S1. Establish the dynamic stiffness matrix and global motion equilibrium equations for a saturated layered seabed site: Based on Biot's theory of saturated porous media, such as Figure 2 As shown, the saturated seabed site is simplified into a seawater layer, N A layered system consisting of a layer of saturated soil and an underlying bedrock half-space; the constitutive relation of the saturated soil is constructed, and the dynamic equilibrium equation of the Biot saturated porous medium is established based on the principle of momentum conservation.

[0019] Seismic waves are input from the bedrock surface in the form of obliquely incident P-waves (compression waves) or SV waves (vertically polarized shear waves), and their propagation direction is parallel to... Z The angle between the axis (vertically upward is positive) and the incident angle is defined as the angle between the axes. i The physical and mechanical parameters of each saturated soil layer include: saturated soil layer thickness. d Soil particle density r s Porosity n Pore ​​fluid density r f Shear wave velocity c S Compression wave speed c P Poisson's ratio of soil skeleton nand damping ratio x .

[0020] Based on Biot's theory of saturated porous media, the constitutive relation of saturated soil is: , , In the formula, s ij The components of the total stress tensor of the soil ( i , j = x , z ); G The soil skeleton shear modulus (Pa); e ij For the soil skeleton strain tensor components. , u i , u j For the soil skeleton displacement components, x i , x j These are spatial coordinate components; l The first Lamé constant (Pa) of the soil skeleton. l =2 Gv / (1−2 n ), n Poisson's ratio (dimensionless) is the soil skeleton. e For the volumetric strain of the soil skeleton, e =∂ u k / ∂ x k , u k For the first k One direction ( k = x , z The soil skeleton displacement components, x k For the first k Spatial coordinates in one direction; denoted as Biot's effective stress coefficient (dimensionless). p f Pore ​​pressure (Pa) is the pressure of the pore fluid. d ij Let Kronecker function be the function of the Kronecker function, and when i = j hour d ij =1, otherwise d ij =0; M The modulus of Biot is expressed in Pa. gFor fluid volume increment, g =−∇· w , w This represents the displacement component of the fluid relative to the soil skeleton.

[0021] The expression for the dynamic equilibrium equation is: , , In the formula, ∇² is the Laplace operator. u i The displacement component of the soil skeleton (m). w i The displacement component (m) of the fluid relative to the soil skeleton; r The total density of saturated soil (kg / m³). r =(1− n ) r s + np f , r s For soil particle density, r f For pore fluid density, n Porosity (dimensionless); m The equivalent inertial density parameter of the pore fluid. m = r a + r f / n , r a For the additional inertial mass coefficient (kg / m³); b The viscosity coupling coefficient (Pa·s / m²) characterizes the seepage resistance between the pore fluid and the soil skeleton; a dot above the variable in the expression indicates the first time derivative, and two dots indicate the second time derivative.

[0022] Based on Biot's theory of saturated porous media, dynamic stiffness matrices for saturated soil layers are established respectively. K L Dynamic stiffness matrix of bedrock half-space K R ; The dynamic stiffness matrix of saturated soil layer is K L It is a 6×6 complex numerical symmetric matrix: , Dynamic stiffness matrix of bedrock half-space K R It is a 3×3 complex numerical symmetric matrix: , In the formula, P , R These represent the amplitudes (N / m) of the horizontal and vertical external loads acting on the soil skeleton, respectively, where i is the imaginary unit. p f The orifice pressure amplitude (Pa); u , w These represent the horizontal and vertical displacement amplitudes (m) of the soil skeleton. W The displacement amplitude (m) of the fluid relative to the soil skeleton; subscripts 1 and 2 represent the upper and lower surfaces of the soil layer, respectively, and subscript 0 represents the bedrock outcrop; k ij ( i , j = 1~6) and r ij ( i , j =1~3) are all stiffness matrix elements, and their derivation process is as follows: Apply a Fourier transformation to the Biot saturated porous medium wave equation with respect to the horizontal coordinate x, transforming the partial differential equation system into an ordinary differential equation system with depth z as the independent variable; solve the general solution of this ordinary differential equation system to obtain the analytical expressions for displacement, stress, and pore pressure of each layer of saturated soil in the frequency-wavenumber domain with respect to depth; establish a linear mapping relationship between the displacement vector and the force vector (including stress and pore pressure) at the upper and lower interfaces of a single layer of saturated soil, which constitutes the dynamic stiffness matrix of that layer. K L Radiation conditions are applied to the bedrock half-space, and the dynamic stiffness matrix of the bedrock is established in the same manner. K R .

[0023] The overall site stiffness matrix is ​​constructed based on the interlayer coupling relationship between each saturated soil layer and bedrock. K And based on the overall site stiffness matrix K Establish the global motion equilibrium equations for the field: In the formula, K The overall stiffness matrix of the site. U T For displacement magnitude vectors, Q T This represents the vector of external loads on the site.

[0024] Overall Stiffness Matrix of the Site K The expression is: , In the formula, N This represents the total number of saturated soil layers. ( n =1~ N ; i , j=1~2) is the first n Layer dynamic stiffness matrix K Ln The ij A 3×3 submatrix. Since the interface between the overlying seawater layer and the saturated seabed soil satisfies the permeable boundary condition, the pore water pressure at the surface of the saturated soil is zero.

[0025] S2. The dynamic response of the saturated layered seabed is solved iteratively using the equivalent linearization method: Inputting obliquely incident P-waves or SV-waves from the bedrock surface, their propagation direction is... Z The angle between the axes is defined as the angle of incidence. i Determine the amplitude of incident wave displacement at the bedrock outcrop. A P0 and A SV0 is used to generate the external load vector. Q T ,Will Q T Substitute the values ​​into step S1 to solve the global motion equilibrium equations and obtain the displacements at the interfaces of each saturated soil layer. Then, the displacement of the soil skeleton at any point (x, z) within the saturated soil layer can be obtained. u , w Strain components of soil skeleton e x , e z and shear strain components e xz Maximum shear strain in saturated soil layer c max Determined by the strain components of the soil skeleton: In the formula, e x For horizontal normal strain, e z For vertical strain, e xz The shear strain component is defined as follows: 0.65 times the peak value of the maximum shear strain time history is taken as the equivalent shear strain. c eff : ; The soil skeleton shear modulus is updated using the dynamic shear modulus curve and the damping ratio curve. G With damping ratio x The parameters are used as parameters for the next iteration and substituted into step S1 to recalculate the global motion equilibrium equation, solve for the maximum shear strain and equivalent shear strain, repeat the iteration until convergence, obtain the frequency domain dynamic response of the saturated seabed site, and convert it into the time domain acceleration time history through inverse Fourier transform. The convergence criterion is: if the following conditions are met... If the result is converged, then the result is considered convergent; otherwise, let... i =i +1 Repeat step S2 above until the convergence condition is met; where, For the first i Maximum shear strain in the next iteration For the first i The maximum shear strain after −1 iterations, where Δ is the preset convergence threshold; S3. Establish a finite element model of a monopile offshore wind turbine considering pile-soil interaction: like Figure 2 As shown, in the finite element platform OpenSees, the wind turbine tower is simplified as a variable cross-section cylindrical hollow structure and simulated using beam elements; the nacelle, hub, and impeller are equivalent to concentrated mass applied to the top of the tower; the single pile foundation is simulated using beam elements, and the structural damping is Rayleigh damping.

[0026] The pile-soil interaction was simulated by distributed springs arranged along the pile body below the seabed mud surface. In OpenSees, spring elements were built using PySimple1, TzSimple1 and QzSimple1 materials respectively to characterize the pile-soil lateral and pile side axial friction and pile end bearing interaction, and a finite element model of a monopile offshore wind turbine was established.

[0027] The py curve model is used for pile-soil lateral interaction, and the ultimate lateral soil resistance at a depth z below the mud surface is... p u Take the smaller value calculated for shallow and deep conditions: , , In the formula, p us The ultimate lateral soil resistance in the shallow layer (kN / m); p ud The ultimate lateral soil resistance in the deep layer (kN / m); c′ The effective unit weight of the soil (kN / m³); z The depth below the mud surface (m); D Pile diameter (m); C 1. C 2. C 3 represents the angle of friction with the soil. f The relevant dimensionless coefficients.

[0028] depth z Lateral reaction force of soil p With lateral displacement y The relation uses a hyperbolic tangent model: In the formula, A The load correction factor (dimensionless) is used during cyclic loading. A=0.9; p u The ultimate lateral soil resistance (kN / m). k This represents the initial value of the foundation reaction modulus (kN / m³). z The depth (m) below the mud surface. y The displacement is lateral (m).

[0029] For the axial friction of the pile side, the Tz curve model is used, and the expression for the ultimate frictional resistance is: In the formula, This represents the effective vertical stress (kPa) at this depth below the mud surface. d The friction angle at the pile-soil interface is (°).

[0030] For pile end bearing, the qz curve model is used, and the expression is: , , In the formula, Q This represents the pile end bearing capacity (kN). Q p The ultimate bearing capacity of the pile tip (kN); A p This represents the cross-sectional area of ​​the pile tip (m²). A p = πD 2 / 4, q p The limiting end resistance per unit area (kPa); z b The displacement at the pile end is (m). z c Reference displacement (m) z c =0.07 D , D The diameter of the pile is in meters (m).

[0031] S4. Seismic performance assessment: The horizontal and vertical acceleration time-history components of the dynamic response of the saturated seabed at the foundation embedment depth obtained in step S2 are used as seismic loads and simultaneously input into the foundation nodes of the monopile offshore wind turbine finite element model constructed in step S3, using Newmark- β The time history integral method is used to solve the structural motion equations, and the peak acceleration at the top of the tower, the peak shear force at the base of the tower, and the peak bending moment at the base of the tower are obtained respectively for the evaluation of seismic performance.

[0032] This embodiment selects the NREL 5-MW reference wind turbine as the research object, with a hub height of 90 m, a rotor diameter of 126 m, and an equivalent concentrated mass of 350,000 kg for the nacelle and rotor. The monopile foundation has a pile diameter of 6 m, a soil penetration depth of 36 m, and a seawater depth of 20 m. The tower is made of Q345 steel (elastic modulus 2.06 × 10⁻⁶). 5 MPa), and the structural damping ratio is taken as 0.01.

[0033] The saturated seabed consists of five layers of saturated soil from the mud surface downwards, followed by underlying bedrock. Shear wave velocities from shallow to deep depths are 100, 140, 180, 230, and 300 m / s, with the bedrock reaching 500 m / s. The porosity of each layer ranges from 0.35 to 0.55. The fluid density is taken as 1000 kg / m³, and the seawater density as 1025 kg / m³. Biot effective stress coefficient... Take 0.95. The pile-soil interaction parameters are determined according to API specifications.

[0034] The El Centro earthquake record (amplitude modulated to 0.1g) was selected as input and obliquely incident in SV wave form at an incident angle of... i Take 0°, 15°, 30°, and 45° respectively.

[0035] Figure 3 Given i The structural response time history at 30° is shown. The results indicate that the peak horizontal acceleration at the top of the tower and the internal forces at the base generally decrease with increasing incident angle; the peak vertical acceleration at the top of the tower... i The value reaches a maximum of 0.82 m / s² near 30°, exhibiting a non-monotonic variation, indicating that the vertical incidence assumption may underestimate the vertical seismic demand.

[0036] This invention addresses the problem of inaccurate analysis results caused by directly inputting ground motion into the foundation and neglecting the site propagation effect and oblique incidence characteristics of seismic waves in existing offshore wind turbine seismic analysis. Simultaneously, it considers the two-phase coupled wave effect of soil skeleton and pore fluid in saturated seabed, the site propagation effect of seabed, and the oblique incidence characteristics of seismic waves to conduct dynamic response analysis of offshore wind turbine structures. This provides more accurate seismic load input for site-related seismic analysis of monopile offshore wind turbines under complex seabed conditions, improving the reliability of seismic assessment.

[0037] The above description is only used to illustrate the technical solution of the present invention and is not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention, as long as they do not depart from the spirit and scope of the technical solution of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A method for determining seismic loads applicable to monopile offshore wind turbine structures on saturated seabeds, characterized in that: Includes the following steps: S1. Establish the dynamic stiffness matrix and global motion equilibrium equations for a saturated layered seabed site: Based on Biot's theory of saturated porous media, the saturated seabed site is simplified to a seawater layer. N The constitutive relation of the saturated soil body was constructed by the layer of saturated soil and the underlying bedrock half-space, and the dynamic equilibrium equation of the Biot saturated porous medium was established based on the principle of momentum conservation. The constitutive relation of saturated soil is: ， ， In the formula, σ ij The components of the total stress tensor of the soil ( i , j = x , z ); G Shear modulus of the soil skeleton; ε ij For the soil skeleton strain tensor components. , u i , u j For the soil skeleton displacement components, x i , x j These are spatial coordinate components; λ The first Lamé constant of the soil skeleton. λ =2 Gν / (1−2 ν ), ν Poisson's ratio for the soil skeleton e For the volumetric strain of the soil skeleton, For Biot's effective stress coefficient, p f Pore ​​fluid pressure, δ ij Let Kronecker function be the function of Kronecker function, and when i = j hour δ ij =1, otherwise δ ij =0; M For Biot modulus, ζ For fluid volume increment, ζ =−∇· w , w This represents the displacement component of the fluid relative to the soil skeleton. The expression for the dynamic equilibrium equation is: ， ， In the formula, ∇² is the Laplace operator. u i For the soil skeleton displacement components, ρ The total density of saturated soil ρ =(1− n ) ρ s + nρ f , ρ s For soil particle density, ρ f For pore fluid density, n Porosity; m The equivalent inertial density parameter of the pore fluid. m = ρ a + ρ f / n , ρ a To add an inertial mass coefficient; b The viscosity coupling coefficient represents the seepage resistance between pore fluid and soil skeleton; a dot above the variable in the expression indicates the first time derivative, and two dots indicate the second time derivative. Based on Biot's theory of saturated porous media, the physical and mechanical parameters of each saturated soil layer were obtained, and the dynamic stiffness matrix of each saturated soil layer was established. K L Dynamic stiffness matrix of bedrock half-space K R The overall site stiffness matrix is ​​constructed based on the interlayer coupling relationship between each saturated soil layer and bedrock. K And based on the overall site stiffness matrix K Establish the global motion equilibrium equations for the field: In the formula, K The overall stiffness matrix of the site. U T For displacement magnitude vectors, Q T This represents the vector of external loads on the site. S2. The dynamic response of the saturated layered seabed is solved iteratively using the equivalent linearization method: Inputting obliquely incident P-waves or SV-waves from the bedrock surface, their propagation direction is... Z The angle between the axes is defined as the angle of incidence. θ Determine the amplitude of incident wave displacement at the bedrock outcrop. A P0 and A SV0 is used to generate the external load vector. Q T ,Will Q T Substitute the values ​​into step S1 to solve the global motion equilibrium equations and obtain the displacements at the interfaces of each saturated soil layer. Then, the displacement of the soil skeleton at any point (x, z) within the saturated soil layer can be obtained. u , w Strain components of soil skeleton ε x , ε z and shear strain components ε xz Maximum shear strain in saturated soil layer γ max Determined by the strain components of the soil skeleton: In the formula, ε x For horizontal normal strain, ε z For vertical strain, ε xz The shear strain component is defined as follows: 0.65 times the peak value of the maximum shear strain time history is taken as the equivalent shear strain. γ eff : ; The soil skeleton shear modulus is updated using the dynamic shear modulus curve and the damping ratio curve. G With damping ratio ξ The parameters are used as parameters for the next iteration and substituted into step S1 to recalculate the global motion equilibrium equation, solve for the maximum shear strain and equivalent shear strain, repeat the iteration until convergence, obtain the frequency domain dynamic response of the saturated seabed site, and convert it into the time domain acceleration time history through inverse Fourier transform. The convergence criterion is: if the following conditions are met... If the result is converged, then the result is considered convergent; otherwise, let... i = i +1 Repeat step S2 above until the convergence condition is met; where, For the first i Maximum shear strain in the next iteration For the first i The maximum shear strain after −1 iterations, where Δ is the preset convergence threshold; S3. Establish a finite element model of a monopile offshore wind turbine considering pile-soil interaction: In OpenSees, the wind turbine tower is simplified into a variable cross-section beam element, and the nacelle and hub are equivalent to the concentrated mass at the top of the tower. The single pile foundation is simulated using beam elements. Distributed spring elements py, tz, and qz are arranged along the pile body below the mud surface to characterize the interaction between the pile and soil in the lateral direction, the pile side axial friction, and the pile end bearing, respectively, to establish a finite element model of a single pile offshore wind turbine. The pile-soil lateral interaction is modeled using the Py curve, and the lateral reaction force... p With lateral displacement y The relation uses a hyperbolic tangent model: In the formula, A This is a load correction factor for cyclic loading. A =0.9; p u For ultimate lateral soil resistance, k This represents the initial value of the foundation reaction modulus. z The depth below the mud surface. y This is lateral displacement; The axial frictional resistance of the pile side is modeled using the tz curve, and the expression for the ultimate frictional resistance is: In the formula, This represents the effective vertical stress at this depth below the mud surface. δ The angle of friction at the pile-soil interface; The pile end bearing capacity is modeled using the qz curve, and its expression is: , , In the formula, Q For the bearing capacity of the pile tip, Q p This refers to the ultimate bearing capacity at the pile tip. A p The cross-sectional area of ​​the pile tip. A p = πD ² / 4, q p The limiting end resistance per unit area; z b For pile end displacement, z c For reference displacement, z c =0.07 D , D Pile diameter; S4. Seismic performance assessment: The horizontal and vertical acceleration time-history components of the dynamic response of the saturated seabed at the foundation embedment depth obtained in step S2 are used as seismic loads and simultaneously input into the foundation nodes of the monopile offshore wind turbine finite element model constructed in step S3, using Newmark- β The time history integral method is used to solve the structural motion equations, and the peak acceleration at the top of the tower, the peak shear force at the base of the tower, and the peak bending moment at the base of the tower are obtained respectively for the evaluation of seismic performance.

2. The method for determining seismic loads applicable to monopile offshore wind turbine structures on saturated seabeds according to claim 1, characterized in that: In step S1, the physical and mechanical parameters of each saturated soil layer include: saturated soil layer thickness. d Soil particle density ρ s Porosity n Pore ​​fluid density ρ f Shear wave velocity c S Compression wave speed c P Poisson's ratio of soil skeleton ν and damping ratio ξ .

3. The method for determining seismic load values ​​for monopile offshore wind turbine structures on saturated seabeds according to claim 1, characterized in that: In step S1, the dynamic stiffness matrix of the saturated soil layer is: K L It is a 6×6 complex numerical symmetric matrix: ， Dynamic stiffness matrix of bedrock half-space K R It is a 3×3 complex numerical symmetric matrix: ， In the formula, P , R These represent the amplitudes of the horizontal and vertical external loads acting on the soil skeleton, respectively, where i is the imaginary unit. p f This refers to the orifice pressure amplitude value; u , w These represent the horizontal and vertical displacement amplitudes of the soil skeleton, respectively. W This represents the displacement amplitude of the fluid relative to the soil skeleton; subscripts 1 and 2 represent the upper and lower surfaces of the soil layer, respectively, and subscript 0 represents the bedrock outcrop. k ij ( i , j = 1~6) and r ij ( i , j =1~3) are all stiffness matrix elements.

4. The method for determining seismic load values ​​for monopile offshore wind turbine structures on saturated seabeds according to claim 1, characterized in that: In step S1, the overall site stiffness matrix K The expression is: ， In the formula, N This represents the total number of saturated soil layers. ( n =1~ N ; i , j =1~2) is the first n Layer dynamic stiffness matrix K Ln The ij A 3×3 submatrix.

5. The method for determining seismic load values ​​for monopile offshore wind turbine structures on saturated seabeds according to claim 1, characterized in that: In step S3, the ultimate lateral soil resistance p u Take the smaller value calculated for shallow and deep conditions: ， ， In the formula, p us This represents the ultimate lateral soil resistance in the shallow layer. p ud This represents the ultimate lateral soil resistance in the deep layers. γ′ The effective unit weight of the soil; z The depth below the mud surface; D Pile diameter; C 1. C 2. C 3 represents the angle of friction with the soil. φ The relevant dimensionless coefficients.

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