Offshore wind power single pile foundation scouring depth detection method based on semi-analytical model

Abstract

The invention relates to the field of scouring detection simulation of offshore wind power single pile foundation piles, in particular to an offshore wind power single pile foundation scouring depth detection method based on a semi-analytical model. According to the scheme, the method comprises the steps that a pile-water-soil interaction model is established, and a pile, water and soil dynamic control equation is obtained; analyzing model boundary conditions; the control equation is transformed into a frequency domain through Laplace for solving the control equation, and solutions of pile body displacement, seawater and seabed potential functions containing unknown numbers are obtained; solving unknown numbers by using boundary conditions; the inherent frequency and vibration mode of each order of the pile foundation under different scouring depths are obtained; external excitation is applied to the top of a pile on site, then acceleration time travel curves under different scouring depths are measured through an acceleration sensor and a vibration signal acquisition instrument of a pile body, then the inherent frequency of a pile-water-soil system can be obtained through FFT conversion, then the inherent frequency is compared with the inherent frequency of a detection model, and the inherent frequency of the pile-water-soil system is determined. And the field scouring depth can be obtained.

Description

Technical Field

[0001] The present invention relates to the field of scour detection simulation of monopile foundations for offshore wind turbines, and particularly to a method for detecting the scour depth of monopile foundations for offshore wind turbines based on a semi-analytical model. Background Art

[0002] In the face of the general trend of global warming and climate change, the development and utilization of clean energy have received attention. The development and utilization of offshore wind energy and space resources have been increased, and the construction of offshore wind power projects has been vigorously promoted. However, the complexity of the ocean current environment and seabed foundation brings huge challenges to the construction and service of offshore wind power projects, especially the monopile foundation used to accommodate the offshore wind turbine tower. In the marine environment, the combined action of waves and currents will drive the movement of seabed sediment, forming a scour pit around the pile foundation, resulting in a decrease in the effective stress of the soil around the pile foundation, and the instability and overturning of the pile foundation. Therefore, timely and effective monitoring of the depth of the scour pit around the pile foundation is an effective measure to avoid risks.

[0003] Since the scour phenomenon occurs at the seabed, it is very dangerous and costly to directly measure the scour depth. In recent years, a series of methods for predicting the scour depth of pile foundations and real-time monitoring systems have been proposed, such as Chinese Patent Publications CN117113718A, CN117113718A, CN111291470B, CN207598433U, etc. However, most of the scour depth prediction models adopted by the above technologies use empirical formulas or finite element models. Empirical formulas often have poor calculation accuracy, while finite element models have slow calculation speed, complex model establishment, and poor practicability. Summary of the Invention

[0004] A method for detecting the scour depth of a monopile foundation for offshore wind turbines based on a semi-analytical model is provided by the present invention. An external load is applied to the pile top by a vibrator for excitation, and sensors are installed on part of the pile body to obtain the modal and frequency information of the pile body for analysis. The scour depth in the actual project is detected by calculating the scour depth corresponding to the frequency through the model.

[0005] The present invention adopts the following technical solutions:

[0006] A method for detecting the scour depth of a monopile for offshore wind turbines based on a semi-analytical model comprises the following steps:

[0007] In the first step, a pile-water-soil interaction model is established by using a two-dimensional axisymmetric cylindrical coordinate system, and the control equation of the pile body in the time domain is expressed as:

[0008]

[0009] In the formula,

[0010] w r(r, z, t) and w z (r, z, t) are the horizontal and vertical displacements of the pile shaft respectively, and the dot "·" represents the derivative with respect to time;

[0011] and are the Lames constants of the pile, α p is the hysteretic damping of the pile shaft; The Lames constants are obtained through G p = E p / 2(1 + ν p ) and λ p = 2G p ν p / (1 - 2ν p );

[0012] where E p , ρ p and ν p are the elastic modulus, density, damping coefficient and Poisson's ratio of the pile respectively;

[0013] The governing equations of water and soil in the acoustic medium are expressed as:

[0014]

[0015] Second, establish the boundary conditions of the pile - water - soil model, including the boundary conditions at the pile top, the boundary conditions at the pile bottom, the natural boundary conditions of the acoustic medium inside and outside the pile, and the boundary conditions at the interface between the pile and the acoustic medium;

[0016] The boundary condition expression at the pile top is as follows:

[0017] σ pz | z＝0 = -σ F r0 ≤ r ≤ r1 (3)

[0018] In the formula, σ pz is the vertical stress of the pile, and σ F is the external excitation applied to the pile top; r0 and r1 are the inner and outer radii of the pile;

[0019] The boundary condition expression at the pile bottom is as follows:

[0020] σ pz A + k b w z | z＝L = 0 (4)

[0021] In the formula, α v = 0.68; G b and ν bThey are the shear modulus and Poisson's ratio of the elastic soil at the pile bottom respectively; the bottom area A of the pile = π(r1 2 -r0 2 );

[0022] The expressions of the natural boundary conditions of the internal acoustic medium and the external acoustic medium of the pile are as follows:

[0023]

[0024] In the formula, represents the internal acoustic medium, represents the external acoustic medium;

[0025] The boundary conditions at the interface between the pile and the acoustic medium:

[0026]

[0027] In the formula, is the velocity potential of the acoustic medium in the r direction, is the pressure in the r direction, and the subscripts i and o represent the inside and outside of the pile respectively; ρ f is the density of the acoustic medium; σ pr and τ prz are the normal stress and shear stress of the pile respectively;

[0028] The initial boundary conditions at t = 0:

[0029]

[0030] Thirdly, the control equation is solved in the frequency domain by using the Laplace transform. The Laplace transform used is expressed as:

[0031]

[0032] The control equation of the pile body established in the time domain in the first step is transformed to the frequency domain by using the Laplace transform, and the control equation formula (1) of the pile body and the control equation formula (2) of the water and soil of the acoustic medium are solved in the frequency domain;

[0033] The displacement of the pile body containing unknowns in the frequency domain is obtained. The expression of the horizontal displacement of the pile body is as follows:

[0034]

[0035] Considering the non-homogeneous boundary conditions at the pile top, the non-homogeneous boundary conditions at the pile top can be converted into the form of a particular solution plus a homogeneous boundary condition σ pz | z＝0 = 0 r0 ≤ r ≤ r1. Therefore, the expression of the vertical displacement of the pile body is as follows:

[0036]

[0037] In the formula, represents the solution in the frequency domain, represents the particular solution of the vertical displacement of the pile shaft (in the frequency domain);

[0038] B i , i = 1, 2, 3, 4 are unknowns, s = iω is the frequency domain parameter after Laplace transform, ω is the circular frequency,

[0039] β pn According to the pile bottom boundary condition, the transcendental equation needs to be satisfied, and the expression is as follows:

[0040]

[0041] Substitute into the control equation (1) of the pile shaft in the time domain established in the first step to obtain the following formula:

[0042]

[0043] In the formula,

[0044] Solve formula (12) and combine the pile top boundary condition formula (3) and the pile bottom boundary condition formula (4) to obtain the particular solution of the vertical displacement of the pile shaft The expression is as follows:

[0045]

[0046] In the formula:

[0047]

[0048] Among them, L is the pile shaft length;

[0049] Solve the control equation (2) of the water and soil of the acoustic medium to obtain the general solution of the potential function:

[0050]

[0051] Among them,

[0052] For the upper layer water acoustic medium, combined with the natural boundary condition formula (5) of the internal acoustic medium and the external acoustic medium of the pile, the boundary condition formula (6) of the interface between the pile and the acoustic medium, and the initial boundary condition formula (7), the expressions are as follows:

[0053]

[0054] For the simplified acoustic medium of the lower soil layer, the expressions combining the natural boundary conditions of the acoustic medium inside the pile and the external acoustic medium (Equation (5)), the boundary conditions at the interface between the pile and the acoustic medium (Equation (6)), and the initial boundary conditions (Equation (7)) are as follows:

[0055]

[0056] Among them, A i , i = 1, 2, 3, 4 are unknowns;

[0057] In the fourth step, solve the water and soil boundary conditions inside the pile according to the boundary conditions between the pile and the acoustic medium, and the expressions are as follows:

[0058]

[0059] Solve the water and soil boundary conditions outside the pile according to the boundary conditions between the pile and the acoustic medium, and the expressions are as follows:

[0060]

[0061] In the fifth step, solve the parametric equations (17)-(24) to obtain the pile displacement parameters. Take the second derivative of the pile displacement to calculate the frequency spectrum diagram of the pile displacement acceleration, and the vibration modes of the pile foundation under different modes can be calculated by changing the mode number n. By changing the water and soil interface positions h2 (h3), the natural frequencies and vibration modes of the pile foundation at different scouring depths can be obtained;

[0062] In the sixth step, apply an external excitation at the pile top on site, and then measure the acceleration time history curves at different scouring depths through the acceleration sensors and vibration signal collectors on the pile body. Then, the natural frequencies of the pile-water-soil system can be obtained through FFT transformation. If the scouring depth on site is consistent with the water and soil interface position of the detection model, the same natural frequencies and vibration modes of the pile foundation can be obtained. Therefore, by comparing the natural frequencies obtained from the on-site test with those of the detection model, the scouring depth on site can also be obtained.

[0063] In the method for detecting the scouring depth of a single pile of an offshore wind turbine based on a semi-analytical model described in the present invention, in the first step, the established pile-water-soil interaction model is an isotropic homogeneous model, and the pile foundation is a thin-walled cylindrical steel pipe pile. Seawater and the seabed are evenly distributed around the pile foundation.

[0064] In the method for detecting the scouring depth of a single pile of an offshore wind turbine based on a semi-analytical model described in the present invention, in the second step, the free surface of the seawater ignores the free surface waves, the contact surfaces between the pile and the water, and the pile and the seabed are in full contact without detachment. The initial state of the pile-water-soil system is static, and all initial displacements are 0.

[0065] In the third step of the method for detecting the scour depth of a monopile for offshore wind power based on a semi-analytical model according to the present invention, the Laplace transform is used to transform the governing equation in the time domain into the frequency domain for solution, and the characteristics of the harmonic load can be utilized to simplify the governing equation and the solution process. When solving the general solution of the governing equation, the method of separation of variables needs to be applied, and the general solutions and particular solutions of the governing equations of the pile, water, and soil can be obtained respectively by combining the boundary conditions, and the pile body displacement, water, and soil medium potential functions containing some unknowns can be obtained.

[0066] In the fourth step of the method for detecting the scour depth of a monopile for offshore wind power based on a semi-analytical model according to the present invention, the unknowns are solved by using the orthogonality of trigonometric functions.

[0067] In the fifth step of the method for detecting the scour depth of a monopile for offshore wind power based on a semi-analytical model according to the present invention, MATLAB software is used for programming calculation to solve the non-homogeneous equations containing 8×n unknowns.

[0068] In the sixth step of the method for detecting the scour depth of a monopile for offshore wind power based on a semi-analytical model according to the present invention, the external excitation at the pile top is applied by a vibrator, and the applied load is a distributed ideal semi-sine excitation. The sensor group includes at least three wireless acceleration signal sensors, and the wireless acceleration signal sensors are evenly distributed along the vertical direction on the side wall of the pile foundation and are all fixed above the water surface.

[0069] Beneficial effects

[0070] (1) The method for detecting the scour depth of a monopile for offshore wind power based on a semi-analytical model provided by the present invention uses the detection method of the semi-analytical model to calculate the scour depth, and the on-site operation is simple, overcoming the problem of underwater scour depth measurement. By applying an excitation load at the pile top and arranging acceleration sensors and vibration signal collectors on the pile body above the water surface, the underwater scour depth can be measured, the on-site operation is simple, and the underwater scour depth of the pile foundation can be monitored in real time, having good engineering application value.

[0071] (2) The method for detecting the scour depth of a monopile for offshore wind power based on a semi-analytical model provided by the present invention has the basic governing equations of the proposed semi-analytical model consistent with those of the finite element model, having good calculation accuracy. Compared with the finite element model, this model has a fast calculation speed and a wide calculation range. Moreover, this model uses the density and wave propagation speed of water and soil media for calculation, and the parameters can be directly obtained through on-site tests, avoiding the problem that it is difficult to determine the spring parameters of the model that assumes water and soil media as springs.

[0072] (3) The method for detecting the scour depth of a monopile foundation for offshore wind power based on a semi-analytical model provided by the present invention has good adaptability to different on-site environments. It can quickly modify the pile body radius, pile length, pile body elastic modulus, etc. to determine the scour depths of different piles on the same site separately, and can also detect the scour depths of piles on different sites by modifying the water and soil parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 It is a model diagram of the method for detecting the scour depth of a monopile foundation for offshore wind power of the semi-analytical model of the present invention.

[0074] Among them, 1 - sensor group, 2 - data acquisition instrument, 3 - data processing device, 4 - sea surface z = h1, 5 - original seabed surface z = h2, 6 - scour pit surface z = h3, 7 - pile bottom surface z = L, 8 - pile top excitation load σ F (t), 9 - pile, 10 - water, 11 - soil, 12 - elastic boundary condition at the pile bottom, 13 - pressure of internal water on the pile, 14 - pressure of external water on the pile, 15 - pressure of internal soil on the pile, 16 - pressure of external soil on the pile. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0075] To make the objectives and technical solutions of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0076] As Figure 1 shown: A method for detecting the scour depth of a monopile foundation for offshore wind power based on a semi-analytical model, the specific steps are as follows:

[0077] First step, adopt a two-dimensional axisymmetric cylindrical coordinate system to establish a pile-water-soil interaction model. Assume the pile as an elastic medium, the water as a one-dimensional thin-layer acoustic medium, and the seabed soil body is also simplified as a one-dimensional thin-layer acoustic medium.

[0078] At the same time, apply an impact load to the pile top and simplify it into a suitable expression. According to the dynamics theory, establish the control equations of each part. The control equation of the pile body in the time domain is expressed as:

[0079]

[0080] In the formula,

[0081] w r (r,z,t) and w z(r, z, t) are the horizontal and vertical displacements of the pile shaft respectively, and the dot "·" represents the derivative with respect to time;

[0082] and are the Lames constants of the pile, α p is the hysteretic damping of the pile shaft; The Lames constants are obtained through G p = E p / 2(1 + ν p ) and λ p = 2G p ν p / (1 - 2ν p );

[0083] where E p , ρ p and ν p are the elastic modulus, density, damping coefficient and Poisson's ratio of the pile respectively.

[0084] The pile - water - soil model is an isotropic homogeneous model. The pile foundation is a thin - walled cylindrical steel pipe pile with uniform mass and no splicing. Seawater and seabed are evenly distributed around the pile foundation.

[0085] According to the on - site test, the seawater density ρ1, the wave propagation speed c1 in seawater, the seabed density ρ2, and the longitudinal wave propagation speed c2 in the seabed can be measured. In addition, during the pile driving process, the sea - level depth h1 (taking the pile top after pile driving as the coordinate zero point), the original seabed depth h2, and the pile bottom surface L can also be recorded. All parameters are respectively substituted into the control equations of the pile, water, and soil.

[0086] In the second step, establish the boundary conditions of the pile - water - soil system, including the pile - top boundary condition, the pile - bottom boundary condition, the natural boundary conditions of the internal and external acoustic media of the pile, and the boundary conditions at the interface between the pile and the acoustic media; The free surface of the seawater ignores the free - surface wave, and the contacts between the pile and water, and the pile and seabed are in full contact without detachment. The initial state of the pile - water - soil system is static, and all initial displacements are 0.

[0087] The expression of the pile - top boundary condition is as follows:

[0088] σ pz | z＝0 = -σ F r0 ≤ r ≤ r1

[0089] In the formula, σ pz is the vertical stress of the pile, and σ F is the external excitation applied to the pile top; r0 and r1 are the inner and outer radii of the pile;

[0090] The expression of the pile - bottom boundary condition is as follows:

[0091] σ pz A + k b w z | z＝L =0

[0092] In the formula, α v =0.68; G b and ν b are respectively the shear modulus and Poisson's ratio of the elastic soil at the pile bottom; the bottom area A of the pile = π(r1 2 - r0 2 );

[0093] The expressions of the natural boundary conditions of the internal acoustic medium and the external acoustic medium of the pile are as follows:

[0094]

[0095] In the formula, represents the internal acoustic medium, represents the external acoustic medium;

[0096] The boundary conditions at the interface between the pile and the acoustic medium:

[0097]

[0098] In the formula, is the velocity potential of the acoustic medium in the r direction, is the pressure in the r direction, and the subscripts i and o respectively represent the inside and outside of the pile; ρ f is the density of the acoustic medium; σ pr and τ prz are respectively the normal stress and shear stress of the pile;

[0099] The initial boundary conditions at t = 0:

[0100]

[0101] The stress at the pile top is equal to the applied impact load σ pz | z＝0 = - F(t) r0 ≤ r ≤ r1;

[0102] According to the model established by the depth detection method of the present invention, the bottom soil body is set as a spring structure. Therefore, the pile bottom is supported by a spring, and the spring parameters G b , υ b can be obtained through on-site tests, σ pz A + k b w z | z＝L =0, α v =0.68, A = π(r1 2 - r02 )。

[0103] Step 3: Use the Laplace transform to transform the control equation in the time domain into the frequency domain for solution. The characteristics of the harmonic load can be utilized to simplify the control equation and the solution process. When solving the general solution of the control equation, the method of separation of variables needs to be applied, and combined with the boundary conditions, the general solutions and particular solutions of the control equations of the pile, water, and soil can be obtained respectively, and the pile body displacement, water, and soil medium potential functions containing some unknowns can be obtained.

[0104] The expression of the horizontal displacement of the pile body is as follows:

[0105]

[0106] Considering the non-homogeneous boundary condition at the pile top, the non-homogeneous boundary condition at the pile top can be transformed into the form of a particular solution plus a homogeneous boundary condition σ pz | z＝0 = 0 r0 ≤ r ≤ r1. Therefore, the expression of the vertical displacement of the pile body is as follows:

[0107]

[0108] In the formula, represents the solution in the frequency domain, represents the particular solution of the vertical displacement of the pile body;

[0109] B i , i = 1, 2, 3, 4 are unknowns, s = iω is the frequency domain parameter after Laplace transform, ω is the circular frequency,

[0110] β pn According to the pile bottom boundary condition, it is necessary to satisfy the transcendental equation, and the expression is as follows:

[0111]

[0112] Substitute into the control equation (1) of the pile body in the time domain established in the first step to obtain the following formula:

[0113]

[0114] In the formula,

[0115] Solve formula (12) and combine the pile top boundary condition formula (3) and the pile bottom boundary condition formula (4) to obtain the particular solution of the vertical displacement of the pile body The expression is as follows:

[0116]

[0117] In the formula:

[0118]

[0119] Among them, L is the length of the pile shaft;

[0120] Solve the water and soil control equations (2) of the acoustic medium to obtain the general solution of the potential function:

[0121]

[0122] Among them,

[0123] For the acoustic medium of the upper water body, combined with the natural boundary conditions of the acoustic medium inside and outside the pile (5), the boundary conditions at the interface between the pile and the acoustic medium (6), and the initial boundary conditions (7), the expressions are as follows:

[0124]

[0125] For the simplified acoustic medium of the lower soil body, combined with the natural boundary conditions of the acoustic medium inside and outside the pile (5), the boundary conditions at the interface between the pile and the acoustic medium (6), and the initial boundary conditions (7), the expressions are as follows:

[0126]

[0127] Among them, A i , i = 1, 2, 3, 4 are unknowns;

[0128] In the fourth step, according to the boundary conditions between the pile and the acoustic medium, use the orthogonality of trigonometric functions to solve for the unknowns, as shown in the following formula:

[0129]

[0130] In the fifth step, use MATLAB software for programming calculation to solve the above non-homogeneous equations, and the pile shaft displacement parameters can be obtained. Take the second derivative of the pile shaft displacement to further calculate the frequency spectrum diagram of the pile shaft displacement acceleration. By changing the mode number n, the vibration modes of the pile foundation under different modes can be calculated. Obtain the natural frequencies and vibration modes of each order of the pile foundation under different scour depths.

[0131] Step 6: At the site, apply an external excitation to the pile top, and then measure the acceleration time history curves at different scouring depths through the acceleration sensors and vibration signal collectors on the pile body. The external excitation at the pile top is applied by a vibrator, and the applied load is a distributed ideal half-sine excitation. The sensor group includes at least three wireless acceleration signal sensors, which are uniformly distributed along the vertical direction on the side wall of the pile foundation and are all fixed above the water surface. The natural frequency of the pile-water-soil system can be obtained through FFT transformation, and then compared with the natural frequency of the detection model to obtain the scouring depth at the site.

[0132] As mentioned above, the above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A method for detecting the scour depth of a monopile for offshore wind power based on a semi-analytical model, characterized in that: The steps are as follows: In the first step, a two-dimensional axisymmetric cylindrical coordinate system is adopted to establish a pile-water-soil interaction model. The control equation of the pile body in the time domain is expressed as: In the formula, w r (r, z, t) and w z (r, z, t) are the horizontal and vertical displacements of the pile shaft respectively, and the dot "·" represents the derivative with respect to time; and is the Lames constant of the pile, α p is the hysteretic damping of the pile shaft; the Lames constant is obtained through G p = E p / 2(1 + ν p ) and λ p = 2G p ν p / (1 - 2ν p ) is obtained; Among them, E p , ρ p and ν p are respectively the elastic modulus, density, damping coefficient and Poisson's ratio of the pile; The control equations of water and soil in the acoustic medium are expressed as: In the second step, establish the boundary conditions of the pile-water-soil model, including the pile top boundary condition, the pile bottom boundary condition, the natural boundary conditions of the acoustic medium inside and outside the pile, and the boundary conditions at the interface between the pile and the acoustic medium; The expression of the pile top boundary condition is as follows: σ pz | z＝0 = -σ F r0 ≤ r ≤ r1 (3) where, σ pz is the vertical stress of the pile, and σ F is the external excitation applied to the pile top; r0 and r1 are the inner and outer radii of the pile; The expression of the pile bottom boundary condition is as follows: σ pz A + k b w z | z＝L = 0 (4) In the formula, α v = 0.68; G b and ν b are respectively the shear modulus and Poisson's ratio of the elastic soil at the pile bottom; the bottom area of the pile The expression of the natural boundary conditions of the acoustic medium inside and outside the pile is as follows: In the formula, represents the internal acoustic medium, represents the external acoustic medium; The boundary conditions at the interface between the pile and the acoustic medium: In the formula, is the velocity potential of the acoustic medium in the r direction, is the pressure in the r direction, and the subscripts i and o represent the inside and outside of the pile respectively; ρ f is the density of the acoustic medium; σ pr and τ prz are the normal stress and shear stress of the pile respectively; The initial boundary condition at t = 0: In the third step, the control equation is solved in the frequency domain by using the Laplace transform. The Laplace transform used is expressed as: The control equation of the pile body established in the first step in the time domain is transformed to the frequency domain by using the Laplace transform. The control equation formula (1) of the pile body and the control equation formulas (2) of water and soil in the acoustic medium are solved in the frequency domain; The pile body displacement containing unknowns in the frequency domain is obtained. The expression of the horizontal displacement of the pile body is as follows: The expression of the vertical displacement of the pile body is as follows: In the formula, represents the solution in the frequency domain, represents the particular solution of the vertical displacement of the pile shaft; B i , where i = 1, 2, 3, 4 are unknowns s = iω is the frequency domain parameter after Laplace transform, and ω is the circular frequency β pn According to the pile bottom boundary conditions, the transcendental equation needs to be satisfied, and the expression is as follows: Bring w z * into the control equation (1) of the pile body in the time domain established in the first step to obtain the following formula: Wherein, Solve formula (12) and combine with the pile top boundary condition formula (3) and the pile bottom boundary condition formula (4) to obtain the particular solution of the vertical displacement of the pile shaft The expression is as follows: In the formula: Among them, L is the length of the pile body; Solve the control equations (2) of water and soil in the acoustic medium to obtain the general solution of the potential function: Among them, For the upper water body acoustic medium, combined with the natural boundary condition formula (5) of the acoustic medium inside and outside the pile, the boundary condition formula (6) at the interface between the pile and the acoustic medium, and the initial boundary condition formula (7), the expression is as follows: For the simplified acoustic medium of the lower soil body, combined with the natural boundary condition formula (5) of the acoustic medium inside and outside the pile, the boundary condition formula (6) at the interface between the pile and the acoustic medium, and the initial boundary condition formula (7), the expression is as follows: Among them, A i , where i = 1, 2, 3, 4 are unknowns; In the fourth step, solve the water and soil boundary conditions inside the pile according to the boundary conditions between the pile and the acoustic medium. The expression is as follows: Solve the water and soil boundary conditions outside the pile according to the boundary conditions between the pile and the acoustic medium. The expression is as follows: In the fifth step, solve the parametric equations (17)-(24) in the fourth step to obtain the pile body displacement parameters; take the second derivative of the pile body displacement to calculate the frequency spectrum diagram of the pile body displacement acceleration. By changing the mode number n, the vibration modes of the pile foundation under different modes can be calculated; by changing the position h2 (h3) of the water-soil interface, the natural frequencies and vibration modes of each order of the pile foundation under different scour depths can be calculated; In the sixth step, on-site, apply an external excitation to the pile top, and then measure the acceleration time history curve at different scour depths according to the acceleration sensor and vibration signal collector on the pile body. The natural frequency of the pile-water-soil system can be obtained through FFT transformation; if the scour depth on-site is consistent with the position of the water-soil interface of the detection model, the same natural frequency and vibration mode of the pile foundation can be obtained; compare the natural frequency obtained from the on-site test with the natural frequency of the detection model to obtain the scour depth on-site.

2. The method for detecting the scour depth of a monopile for offshore wind power based on a semi-analytical model according to claim 1, characterized in that in the first step, the established pile-water-soil interaction model is an isotropic homogeneous model, and the pile foundation is a thin-walled cylindrical steel pipe pile. Seawater and seabed are evenly distributed around the pile foundation.

3. The method for detecting the scour depth of an offshore wind turbine monopile based on a semi-analytical model according to claim 1, characterized in that, In the second step, the free surface wave of the seawater free surface is ignored, and the contact surfaces between the pile and the water, and between the pile and the seabed are in full contact without detachment. The initial state of the pile-water-soil system is delicate, and all initial displacements are 0.

4. The method for detecting the scour depth of a monopile of an offshore wind turbine based on a semi-analytical model according to claim 1, characterized in that, In the fourth step, the unknowns are solved by using the orthogonality of trigonometric functions.

5. The method for detecting the scour depth of a monopile for offshore wind power based on a semi-analytical model according to claim 1, characterized in that, In the fifth step, MATLAB software is used for programming calculation to solve the non-homogeneous equations containing 8×n unknowns.

6. The method for detecting the scour depth of a monopile of an offshore wind turbine based on a semi-analytical model according to claim 1, characterized in that, In the sixth step, the external excitation at the pile top is applied by an exciter, and the applied load is a distributed ideal semi-sine excitation. The sensor group includes at least three wireless acceleration signal sensors, which are evenly distributed along the vertical direction on the side wall of the pile foundation and are all fixed above the water surface.

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