Offshore wind turbine single pile rigidity numerical simulation method considering long-term circulation and scouring

Abstract

According to the offshore wind turbine single pile rigidity numerical simulation method considering long-term circulation and scouring, the accuracy and the calculation convenience are both considered in model building through the linear elastic steel pipe pile model and the Mohr-Coulomb soil body model. By calculating the horizontal ultimate bearing capacity of the single pile, combining the ultimate analysis method and the p-y curve calculation method, the influence of different scouring depths on the fan is considered, the horizontal load bearing capacity and inherent frequency evolution of the single pile are comprehensively measured, and a basis is provided for design and safety evaluation. A sandy soil rigidity attenuation model is established for secondary development of Abaqus and verified, and the rigidity change of sandy soil under long-term cyclic loading is considered, so that the simulation precision is improved. And the numerical model is tested and verified to ensure the reliability. And the accumulated lateral displacement of the steel pipe pile can be accurately predicted, the influence rule of cyclic load times, soil elasticity modulus and embedding depth factors on displacement development is disclosed, wind power engineering design optimization is assisted, and economical efficiency and safety are improved.

Description

Technical Field

[0001] The present invention relates to the field of wind power engineering, and in particular, to a numerical simulation method for the stiffness of monopiles of offshore wind turbines considering long-term cyclic loading and scouring. Background Art

[0002] In the construction of wind power engineering, the bearing capacity and dynamic characteristics of pile foundations are the core issues to ensure the stable operation of wind turbines, which are subjected to various complex loads such as wind loads and wave loads in a complex natural environment for a long time. However, during the long-term service of wind power projects, the laws of the bearing characteristics and dynamic responses of wind turbines under the coupled action of soil stiffness evolution and scouring are not yet clear. Currently, when establishing numerical models of pile-soil structures, most methods do not fully consider the symmetry of the structure.

[0003] When calculating the horizontal ultimate load of monopile foundations in the prior art, the methods are relatively single, and the comprehensive application of multiple methods is lacking. Only using a single calculation method often cannot comprehensively and accurately describe the mechanical behavior of monopiles under horizontal loads, resulting in inaccurate assessment of the horizontal load-bearing capacity of monopiles. This may pose potential safety hazards in the design of pile foundations. Either the pile size is too large, causing waste of resources, or the bearing capacity of the pile is insufficient, and in actual projects, the pile foundation may fail due to horizontal loads, affecting the safety and stability of wind power projects. At the same time, few existing studies consider the coupling effect of foundation scouring and soil stiffness evolution on the dynamic and static forces of wind turbines. Moreover, the existing calculation methods for horizontal ultimate loads may not fully consider the actual deformation characteristics of piles and the pile tip failure modes, resulting in a large deviation between the calculation results and the actual situation. When simulating the mechanical properties of sandy soils, most existing numerical models do not consider the change in stiffness of sandy soils under long-term cyclic loading. In actual projects, the stiffness of sandy soils will decay under long-term wind loads and wave loads, and existing models cannot accurately simulate this characteristic, resulting in a large error between the simulation results and the actual situation.

[0004] Regarding the deformation and stiffness evolution of wind power foundations under long-term cyclic loading, the existing technologies lack in-depth research and accurate prediction methods. Pile foundations will generate cumulative lateral displacements under long-term and low-frequency lateral cyclic forces, but it is currently difficult to accurately predict the cumulative lateral displacement of steel pipe piles during their normal service life. This makes it impossible for engineers to fully consider the influence of long-term deformation when designing wind power foundations, which may lead to structural damage due to excessive cumulative lateral displacement during long-term use. At the same time, there is insufficient understanding of the influence laws of the number of cyclic load applications, the elastic modulus of the soil, and the embedded depth of the steel pipe pile on the development of the horizontal displacement at the pile top and the lateral displacement of the pile body, making it difficult to reasonably select parameters and optimize the design of the pile foundation according to the actual engineering situation, thus affecting the economy and safety of wind power projects. Therefore, we have made improvements in this regard and proposed a numerical simulation method for the stiffness of monopiles of offshore wind turbines considering long-term cycling and scour. Summary of the Invention

[0005] The purpose of the present invention is to address the problems raised in the existing background technology. To achieve the above-mentioned invention purpose, the present invention provides the following technical solutions: A numerical simulation method for the stiffness of monopiles of offshore wind turbines considering long-term cycling and scour, including the following steps:

[0006] Step 1, Establishment of a numerical model. A three-dimensional finite element model of the monopile foundation is established using finite element analysis software. Due to the symmetry of the stress conditions and the pile-soil structure, half of the pile-soil structure is taken for analysis;

[0007] Step 2, Calculate the horizontal ultimate load of the monopile foundation and calculate the horizontal ultimate bearing capacity H u , and measure the ability of the monopile to withstand horizontal loads through the horizontal ultimate bearing capacity;

[0008] Step 3, Specific implementation and experimental verification of the numerical model. Through secondary development of Abaqus, a stiffness degradation model of sand is established in the numerical analysis program and verified

[0009] Step 4, Analyze the deformation and stiffness evolution of the wind power foundation under long-term cycling. The pile foundation will continuously bear long-term and low-frequency lateral cyclic forces brought by wind loads and wave loads, and accurately predict the cumulative lateral displacement generated by the steel pipe pile during its normal service life through analysis.

[0010] As a preferred technical solution of the present invention, in Step 1, Establishment of a numerical model, the steel pipe pile adopts a linear elastic model, with a pile diameter D = 6.3m, a wall thickness s = 0.64mm, and an elastic modulus E p= 210 GPa; The soil body adopts the Mohr-Coulomb ideal elastoplastic model, which is simple and clear and can better reflect the loading characteristics of sandy soil. A contact surface is set between the pile and the soil. Among them, the normal contact adopts hard contact, and the tangential contact adopts frictional contact. The friction coefficient is two-thirds of the internal friction angle, the soil cohesion c = 5 kPa; the internal friction angle φ' = 32°, the dilation angle ψ = 0°, and the effective unit weight γ' = 19.4 kg / m 3 , Poisson's ratio v = 0.4, and the elastic modulus is taken according to the following formula:

[0011]

[0012] In the formula: σ at is the atmospheric pressure, and the value is 101 kPa; σ m is the average soil stress of the soil body; k and η are dimensionless constants. In this paper, k = 560 and η = 0.6 are taken.

[0013] As a preferred technical solution of the present invention, in step 2, the limit analysis method is adopted to calculate the horizontal ultimate load of the single-pile foundation

[0014] The deformation of the single pile shows the deformation characteristics of a rigid pile. The pile tip part of the single pile shows the form of toe kick failure, and the whole pile body rotates around a certain point on a certain pile tip; the horizontal ultimate bearing capacity is expressed as H u is:

[0015]

[0016] In the formula: h is the height from the mud surface to the pile top; D is the pile diameter; L is the unit weight of the soil body; γ s is the unit weight of the soil body; K P is the Rankine passive earth pressure coefficient.

[0017] As a preferred technical solution of the present invention, in step 2, the limit analysis method is adopted to calculate the horizontal ultimate load of the single-pile foundation: For pipe piles, its yield moment can be expressed as:

[0018]

[0019] In the formula: I P is the moment of inertia of the pile body cross-section; σ y is the yield strength of the pile body material.

[0020] As a preferred technical solution of the present invention, in step 2, the p-y curve calculation of the horizontal ultimate load of the single-pile foundation is as follows:

[0021] When y < 3y 50 then

[0022] When y > 3y 50 then, x > xr When p = 0.72p u

[0023] x < x r When

[0024] where y is the deflection of the pile; y 50 is the displacement of the pile when the soil resistance reaches half of the limit value; x is the depth below the mud surface; x r is the critical depth; p is the soil resistance around the pile; p u is the ultimate soil resistance of the soil around the pile, related to the undrained shear strength C u and ε 50 is relevant

[0025] As a preferred technical solution of the present invention, step 3, specific implementation and experimental verification of the numerical model: The following formula is calculated for the soil below the deep layer:

[0026] p us =(C1X + C2b)γ · X

[0027] p ud =C3bγ · X

[0028] where p u is the ultimate soil resistance per unit pile length; γ · is the effective unit weight of the soil; X is the depth below the mud surface; C1, C2 and C3 are coefficients related to the internal friction angle φ, and b is the average pile diameter within the range from the mud surface to the required depth

[0029] As a preferred technical solution of the present invention, a three-dimensional finite element model of a large-diameter single pile under horizontal loading in sandy soil is established. Geostress balance is carried out and a vertical load V is applied, and the initial major principal stress and minor principal stress

[0030] of each soil element are calculated and extracted. A horizontal load H equal to the peak value of the cyclic load is applied at the loading point, and the major principal stress and minor principal stress

[0031] under horizontal loading of each soil element are calculated and extracted The extracted c is taken as the confining pressure σ acting on each soil element. The difference between and d is obtained to get the deviator stress level σ

[0032]

[0033] Write the cyclic stress ratio of each soil element into the usdfld subroutine, and the software will call this subroutine in the analysis and calculate the mechanical behavior of the pile foundation under N cyclic loads according to the attenuation stiffness of each soil element after N cycles. x is the horizontal displacement of the loading point; H u is the horizontal static ultimate load of the pile foundation.

[0034] As a preferred technical solution of the present invention, in step 4, analyze the deformation and stiffness evolution of the wind power foundation under long-term cyclic loading. The horizontal displacement value at the pile top after N cyclic loadings decreases; as h / L increases, the decreasing trend continuously increases. When h / L is 0.294, the cumulative horizontal displacement value at the pile top decreases to 30% of the original value.

[0035] As a preferred technical solution of the present invention, in step 4, analyze the deformation and stiffness evolution of the wind power foundation under long-term cyclic loading. For a steel pipe pile with an embedment depth of 34m, under a horizontal cyclic load of 0.25Hu, analyze the development of the lateral displacement of the pile body; under the action of the horizontal load, as the cycle progresses, the horizontal displacement at the pile top continuously develops; in the initial stage of the cyclic load action, the lateral displacement of the steel pipe pile develops rapidly. After the number of cycles exceeds 100 times, the lateral displacement development slows down and gradually stabilizes.

[0036] As a preferred technical solution of the present invention, in step 4, analyze the deformation and stiffness evolution of the wind power foundation under long-term cyclic loading. The horizontal displacement at the pile top decreases as the elastic modulus of the soil increases; when the embedment depth of the steel pipe pile is small, the decreasing amplitude is significant; as the embedment depth of the steel pipe pile increases, the decreasing amplitude of its lateral displacement gradually decreases.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] In the process of establishing the numerical model of the present invention, by utilizing the stress conditions and the symmetry of the pile-soil structure, half of the pile-soil structure is taken for analysis. This simplification method greatly reduces the amount of calculation and calculation time, reduces the demand for computing resources, enables the simulation analysis to be completed more quickly under the condition of limited computing resources, improves the efficiency of the entire simulation process, and helps engineers obtain the simulation results and make decisions in a shorter time. In the present invention, the steel pipe pile adopts a linear elastic model, and the soil body selects the Mohr-Coulomb ideal elastic-plastic model. The linear elastic model can accurately reflect the mechanical properties of the steel pipe pile to a certain extent, and the calculation is relatively simple; while the Mohr-Coulomb ideal elastic-plastic model is simple and clear and can better reflect the loading characteristics of sandy soil. The reasonable combination of the two not only ensures the accuracy of the model but also takes into account the convenience of calculation, providing a reliable basis for subsequent simulation analysis. The present invention measures the horizontal ultimate bearing capacity of a single pile through calculation and uses it to evaluate the ability of a single pile to bear horizontal loads, providing an important basis for the design and safety assessment of the pile-soil structure in wind power projects. Accurate calculation of the horizontal ultimate load helps to determine the reasonable size and layout of the pile, ensuring that the pile can bear the corresponding horizontal load in actual projects, avoiding the failure of the pile foundation caused by the action of horizontal loads, and improving the safety and stability of wind power projects. The present invention uses the limit analysis method to calculate the horizontal ultimate load of the single-pile foundation and combines multiple methods such as the p-y curve calculation method to analyze the horizontal mechanical characteristics of the single pile from different angles. This way of combining multiple methods can more comprehensively and accurately describe the mechanical behavior of the single pile under the action of horizontal loads, providing richer and more reliable data for design and analysis, enabling engineers to better understand the performance of the pile foundation and optimize the design scheme.

[0039] By analyzing the deformation and stiffness evolution of the wind power foundation under long-term cycling, the present invention can accurately predict the cumulative lateral displacement generated by the steel pipe pile during its normal service life. This is crucial for the long-term stability assessment of wind power projects. Engineers can take corresponding measures according to the prediction results, such as adjusting the design parameters of the pile and strengthening the foundation reinforcement, to ensure that the wind power foundation will not cause structural damage due to excessive cumulative lateral displacement during long-term use. The analysis results of the present invention reveal the influence laws of the number of cyclic load applications, the elastic modulus of the soil body, and the embedded depth of the steel pipe pile on the development of the horizontal displacement at the pile top and the lateral displacement of the pile body. Description of the Drawings

[0040] Figure 1 It is a schematic diagram of the force analysis provided by the present invention;

[0041] Figure 2 It is a schematic diagram of several different methods provided by the present invention to determine the horizontal ultimate bearing capacity of a single pile;

[0042] Figure 3Schematic diagram for comparing the method and test results provided by the present invention;

[0043] Figure 4 Passive protection measure diagram provided by the present invention;

[0044] Figure 5 Geological section diagram provided by the present invention;

[0045] Figure 6 Single-pile foundation diagram provided by the present invention;

[0046] Figure 7 Multi-pile foundation diagram provided by the present invention. Specific implementation manners

[0047] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some but not all of the embodiments of the present invention.

[0048] Embodiment 1: A numerical simulation method for the stiffness of a monopile of an offshore wind turbine considering long-term circulation and scour. Due to the blocking effect of the foundation on the water flow around the foundation, the water flow velocity increases, resulting in the generation of an eddy structure that can carry sediment around the foundation, leading to local scour around the foundation. Since it is very difficult to completely avoid foundation scour in engineering practice, in order to improve the stability of the foundation structure and reduce the scour depth around the foundation, adopting appropriate scour protection measures has a significant effect in reducing local scour. Foundation scour prevention is an important issue that needs to be considered to ensure the safety and stability of the foundation.

[0049] The geological data mainly refers to the "Investigation Report of the 300MW Demonstration Project of the Tangshan Leting Putidao Offshore Wind Farm". The field area is located in the nearshore waters of the Bohai Sea. The water depth in the project area is about 7 - 28m, and the center of the site is about 18km away from the shoreline. In the NNE local area of the field area, affected by sand mining, the water depth changes greatly. The seabed topography in the proposed project area has large undulations, generally showing a trend of lower in the south and higher in the north, and lower in the west and higher in the east. The seabed ground elevation is -7.10 to -29.80m. The northwest side is adjacent to the Xikengtuo sandbar, with a shallower water depth. The water depth is larger on the east and south sides, and it is an open water area. The geomorphic type is an underwater sedimentary plain in the offshore waters of the Bohai Bay, located at the edge of the sandbar area.

[0050] The centrifuge data acquisition and data processing system is a special-purpose measurement device developed and produced by ANCO Corporation in the United States. It consists of a data collector, lower computer hardware, and data processing software. The data acquisition system combines static and dynamic data acquisition methods to form a 128-channel data acquisition system. The single-channel sampling rate is not less than 30K. When used in geotechnical centrifuge simulation tests, it can measure and collect various changing data of the test model, such as earth pressure, water pressure, displacement, distance, and dynamic strain. At the same time, a high-speed photography and videography system is equipped to perform PIV analysis on the test model.

[0051] Test plan

[0052] Centrifuge model scale and similarity relationship

[0053] When a physical phenomenon can be described by the functional relationship of n physical quantities, and these physical quantities include m basic dimensions, then (n - m) dimensionless groups can be obtained by dimensional analysis. And the characteristics of this phenomenon can be expressed in the form of the relationship of these (n - m) dimensionless groups. In this special topic test, there are three aspects of similarity ratio problems to be determined: the similarity problem of soil materials, the similarity problem of structural materials, and the similarity ratio problem of the model. According to the geometric dimensions of the aforementioned structures and the model box size, the acceleration of the centrifuge model test is planned to be 100g, that is, the model geometric scale is N = 100. Dimensional conversion is carried out according to the flexural rigidity.

[0054]

[0055] Centrifuge model test scale relationship

[0056] Model design

[0057] Considering the processing difficulty and precision control factors, both single-pile and multi-pile bearing wind power models are designed and processed using aluminum pipes. Since the horizontal resistance and natural vibration of the pile foundation and structure are closely related to the flexural rigidity of the structure, a reduced-scale model design is carried out according to the flexural rigidity, and the calculation formula is used to obtain the wind power structure model.

[0058]

[0059] In the formula, E m is the elastic modulus of the model material aluminum, and E p is the elastic modulus of the prototype material steel, D m , d m are the outer diameter and inner diameter of the model, and D p , d p are the outer diameter and inner diameter of the prototype.

[0060] The tower barrel of the single-pile foundation is connected to the pile foundation through a flange. There is a concentrated mass block at the upper part of the tower barrel, and the size of the concentrated mass block is 74.4 mm × 20 mm × 20 mm. The pile foundation of the multi-pile foundation is connected to the tower barrel through a bearing platform. The bearing platform is made of aluminum blocks, and the bearing platform and the pile foundation are integrally connected by glue.

[0061] Test conditions and production of scour pits

[0062] This test aims to study the change of the natural frequency and the horizontal bearing characteristics of the single-pile and multi-pile foundation offshore wind turbines under different scour depths. Test conditions for the frequency and bearing capacity of the wind turbine foundation at different scour depths. For the single-pile foundation, the natural frequency of the foundation is tested at the scour depths of 0 m, 5 m, 10 m, and 15 m, and its ultimate bearing capacity is tested at the scour depths of 5 m, 10 m, and 15 m. For the multi-pile bearing platform foundation, the natural frequency of the foundation is tested at the scour depths of 0 m, 3 m, 6 m, 10 m, 12 m, and 15 m, and its ultimate bearing capacity is tested at the scour depths of 6 m and 12 m. Two groups of wind turbine foundation models are placed in each model box to improve the test efficiency. A grinding tool is used to excavate the scour pits along the periphery of the single-pile and multi-pile foundations. The shape of the scour pit is approximately a frustum of a cone, and the excavation depth is determined according to different test conditions.

[0063] Frequency test of wind turbine foundation at different scour depths

[0064]

[0065] Bearing capacity test of wind turbine foundation at different scour depths

[0066]

[0067] Establishment of numerical model

[0068] The linear elastic model is based on Hooke's law in general, including isotropic elastic model, orthotropic model, and anisotropy. The linear elastic model is applicable to any element. In the pile-soil system of this study, the pile can be regarded as a linear elastic model.

[0069] (1) Isotropic elastic model

[0070] The stress-strain expression of the isotropic linear elastic model is:

[0071]

[0072] There are two parameters involved here, namely the elastic modulus E and Poisson's ratio v, which can vary with temperature and other field variables.

[0073] (2) Orthotropic elastic model

[0074] The independent model parameters of orthotropic materials are the Young's moduli E1, E2, and E3 in three orthogonal directions, the Poisson's ratios v12, v13, and v23, and the shear moduli G12, G13, and G23. The stress-strain expression is as follows:

[0075]

[0076] In the orthotropic model, if the properties of a certain plane of the material are the same, it is a transversely isotropic elastic body. Assuming that the 1-2 plane is the isotropic plane, then E1 = E2 = Ep, ν31 = ν32 = νtp, ν13 = ν23 = νpt, and G13 = G23 = Gt, where p and t represent the transverse and longitudinal directions of the transversely isotropic body, respectively. Therefore, the stress-strain expression of the transversely isotropic body is as follows:

[0077]

[0078] Among them, G p = E p / 2(1 + v p ). So the independent model parameters of this model are 5. The usage of the transversely isotropic elastic model is the same as that of the orthotropic model.

[0079] (3) Anisotropic elastic model

[0080] The independent model parameters of a fully anisotropic elastic model are 21, and its stress-strain expression is as follows:

[0081]

[0082] The results of data analysis are all converted to the prototype structure according to the similarity scale of the centrifuge model test, and the horizontal thrust and the horizontal displacement at the mud surface at each moment are extracted to obtain the force-displacement curve at different scour depths. At each scour depth, the greater the mud surface displacement, the greater the required horizontal thrust, and the force-displacement curve gradually flattens with the increase of the mud surface displacement. The smaller the scour depth, the greater the horizontal thrust required to reach a certain mud surface displacement. When there is no scour around the pile, the horizontal load required for the mud surface displacement to reach 0.1D, that is, the ultimate horizontal bearing capacity, is the largest, which is 45.1 MN; when the scour depth is 15 m, the ultimate horizontal bearing capacity is the smallest, which is 14.8 MN, a decrease of more than 80% compared with the non-scoured situation. The numerical calculation results are consistent with the laws of centrifuge experiments, that is, with the increase of the scour depth, the horizontal ultimate bearing capacity of the pile does not decrease significantly.

[0083] Based on the indoor dynamic triaxial test of sandy soil, Achmus proposed a research method for this stiffness decay model. Since it considers the decay of soil stiffness with the number of cycles in the calculation model and has a simple formula form, it shows good application prospects. This method not only conforms to the essence of cumulative plastic deformation of soil under long-term cyclic loading but also avoids the disadvantages of a large amount of time consumption and cumulative calculation errors brought about by using general numerical analysis methods to study long-term cyclic problems. It is an effective analysis method for the cumulative plastic deformation of cohesionless soil. Therefore, this project uses this theoretical model to study the cyclic stiffness evolution of soil;

[0084] Test scheme design

[0085] For the unit test of the cyclic stress of the soil mass this time, the confining pressure and the magnitude of the cyclic stress considered are designed according to the actual stress conditions of the offshore wind power project. According to the basic design of the pile foundation of the Putidao offshore wind farm project in Laoting, Tangshan, the maximum depth range of the pile foundation is about 66m. Considering the buoyant unit weight of the soil of 10kN / m3 and the lateral pressure coefficient of 0.5, the maximum confining pressure of the soil around the pile is about 330kPa. Therefore, the maximum confining pressure in this test is considered as 500kPa. In addition, since the horizontal cyclic load of the offshore wind power mainly comes from the upper wind, wave, and current loads, the horizontal cyclic load on the soil is mainly generated by the bending deformation of the pile foundation caused by the above external loads. According to the deformation law of the pile foundation, the horizontal deformation decreases with the increase of the pile foundation depth. Therefore, the magnitude of the cyclic stress ratio in this test is also related to the magnitude of the confining pressure. The greater the confining pressure, the smaller the cyclic stress ratio considered. The cyclic load loading test scheme for this test is shown in the following table after comprehensive consideration.

[0086]

[0087] Preparation of soil samples: Since the condition of taking soil samples on-site is not available in this study, the model soil for this test is made by mixing quartz sand with different particle size ranges according to the on-site particle gradation. The standard specimen shape of the soil for the dynamic triaxial test is a cylinder, which is made through a standard mold. The diameter of the specimen is 39.1mm and the height is 80mm. According to the basic physical parameters of the soil in the geological exploration report, the samples are prepared according to the dry density of 1.64g / cm3;

[0088] The process of specimen production is as follows:

[0089] (1) Use the dry-packing method to form a membrane cylinder for sample preparation. Compress the soil layer by layer according to the sample height and use a vibrating hammer to compact it. In this test, it is divided into four layers, and the thickness of each layer d = 100 / 4 = 25mm. A scale mark can be made on the membrane cylinder before the test, and the thickness of the rubber membrane is 0.2mm;

[0090] (2) Calculate the mass of each layer of soil sample, load the soil layer by layer, and use a vibrating fork to gently strike to reach the corresponding soil sample height and density.

[0091] The specimen saturation process is as follows: (1) During the test, the confining pressure, pore pressure, and axial pressure are calibrated according to the triaxial test principle. At the same time, to avoid the initial state of the instrument not being zero, water is injected into the pressure chamber before loading. The height of the water reaches about half of the specimen's height. Then, the pore pressure, confining pressure, and axial pressure parameters are reset to zero. Finally, the water in the pressure chamber is drained and the specimen is loaded;

[0092] (2) Carbon dioxide and back pressure are used jointly for saturation. The specific steps are as follows: 1) Before loading, the air in the GDS confining pressure and back pressure application devices is removed; 2) The base and the pore pressure sensor are first initially filled with carbon dioxide until bubbles emerge from the air outlet; 3) After loading is completed, a negative pressure of 3 - 5 kPa is applied, the ventilation hole of the base is closed, and after the specimen is made upright, the mold is removed. Water is injected into the pressure chamber and a confining pressure of 10 kPa is maintained; 4) The ventilation hole of the base is connected to carbon dioxide, and the negative pressure is released. After the negative pressure is completely released, all ventilation holes are closed; 5) The base is filled with carbon dioxide for the second time. The valve of the ventilation hole at the top of the specimen is closed, and the base and the pore pressure sensor are filled with carbon dioxide for the second time to remove the internal air for 30 minutes to 1 hour; 6) The specimen is filled with carbon dioxide. The valves of the base and the sensor are closed, and the specimen is filled with carbon dioxide for 2 hours with bubbles emerging; 7) The base is filled with water. A certain pressure of 5 kPa is applied using GDS to fill the base with water, and then the valve of the base is closed; 8) The specimen is filled with water. A certain pressure of 3 kPa is applied using GDS for a certain time to saturate the specimen for 6 hours as scheduled;

[0093] The effects of different cyclic stress ratios (X = 0.4, 0.6, 0.8, 1), corresponding to different deviator stress amplitudes (20 kPa, 30 kPa, 40 kPa, 50 kPa, 80 kPa), on the maximum plastic strain of the specimen were compared under the same initial physical state of the specimen and different confining pressure levels (50 kPa, 200 kPa). It can be seen that the maximum plastic strain of the specimen increases with the increase of the cyclic stress ratio, that is, the maximum plastic strain of the specimen increases with the increase of the dynamic stress amplitude.

[0094] The Mohr - Coulomb elastoplastic model is mainly applicable to granular materials and is widely used in geotechnical engineering. The ABAQUS finite element analysis in this paper mainly conducts relevant research in combination with the Mohr - Coulomb elastoplastic model.

[0095] Model characteristics

[0096] This constitutive model is an extension of the classical Mohr - Coulomb yield criterion. The adopted Mohr - Coulomb yield function includes the isotropic hardening and softening of cohesion. The shape of the flow potential function on the meridian plane is hyperbolic, and there are no sharp corners on the π - plane. Therefore, the potential function is completely smooth, ensuring the uniqueness of the plastic flow direction.

[0097] Yield characteristics

[0098] When expressed in terms of strain invariants, the yield surface equation of the Mohr-Coulomb model is:

[0099] F = R m q - p tan φ - c = 0

[0100] R m is the deviatoric stress coefficient of Mohr-Coulomb, defined as:

[0101]

[0102] where φ is the inclination angle of the Mohr-Coulomb yield surface in the p-Rmcq plane, generally referring to the internal...

[0103] friction angle, θ is the azimuth angle of the generalized shear stress, and p is the effective compressive stress.

[0104]

[0105] where is the Mises effective stress; is the third invariant of the stress deviator tensor.

[0106] The friction angle φ also controls the shape of the yield surface of the material on the π plane. The value range of the friction angle is 0° ≤ φ ≤ 90°. When φ = 0°, the Mohr-Coulomb model degenerates into the Tresca model independent of the confining pressure. At this time, the yield surface on the π plane is a regular hexagon; when φ = 90°, the Mohr-Coulomb model will change into the Rankine model. At this time, the yield surface on the π plane is a regular triangle.

[0107] Flow rule

[0108] The definition of the flow rule is:

[0109]

[0110] where:

[0111]

[0112] G is the flow potential function. The yield surface of the traditional Mohr-Coulomb model has sharp corners, making the plastic flow direction not unique, resulting in cumbersome numerical calculations and slow convergence. To avoid these problems, the Mohr-Coulomb model provided by ABAQUS selects a continuous and smooth flow potential function, whose shape is hyperbolic on the meridian plane and elliptical on the π plane.

[0113] The governing equation of the hyperbolic flow potential function is as follows:

[0114]

[0115] Where:

[0116]

[0117] Where ψ is the dilation angle at high confining pressure in the meridian plane; c|0 is the initial cohesion; ε and e are shape parameters defining the flow potential function in the meridian plane and in the π plane. Generally, ε is taken as 0.1, and e can be expressed as:

[0118]

[0119] According to the above formula, the requirements for the convexity and smoothness of the elliptical yield surface are 0.5 < e ≤ 1.0;

[0120] Soil stiffness degradation model

[0121] When sand is under cyclic loading under drained conditions, the axial plastic strain ε a Increases with the increase in the number of cyclic loading applications. The increase in axial plastic strain is related to the magnitude of the cyclic stress and the initial confining pressure;

[0122] Idriss (1978) pointed out that the secant elastic modulus of sand continuously decreases with the increase in the number of cycles, and gave an empirical expression for the soil stiffness degradation coefficient δ:

[0123]

[0124] E in the formula sN 、E s1 Are the secant stiffnesses under the action of cyclic stress for N times and the initial action respectively, Are the axial plastic strains under the action of N times and the initial action respectively. S is a test parameter, which is related to the soil properties and the cyclic stress level.

[0125] Achmus believes that the axial plastic strain of sand under cyclic loading can be expressed by a semi-empirical formula containing stress parameters. The ratio of axial strain increment Can be expressed as:

[0126]

[0127] Where: X is the cyclic stress ratio, and a and b are test parameters. The definition of the cyclic stress ratio X is:

[0128]

[0129] Where: σ 1,sfσ is the major principal stress at the time of soil failure under static action. 1,cyc σ 1,cyc is the maximum principal stress within a cycle.

[0130] The cyclic stress ratio proposed by Achmus takes into account the influence of the maximum static and dynamic stresses on the dynamic characteristics of the soil, and to a certain extent reflects the stress level of the soil element under cyclic loading. However, the influence of the dynamic load amplitude and confining pressure is not considered in the expression of the cyclic stress ratio. Hardin pointed out that the main factors affecting the dynamic characteristics of sand are the effective confining pressure, void ratio and amplitude. Zhou Jingxing's research shows that the magnitudes of the dynamic deviator stress and confining pressure have a great influence on the dynamic strength characteristics of sand. Based on this, this paper considers the influence of the dynamic deviator stress and confining pressure on the dynamic characteristics of sand, and establishes a soil stiffness degradation model that is more in line with its mechanical properties. The ratio of the confining pressure to the dynamic deviator stress is used to reflect the cyclic stress level, and the definition of the cyclic stress ratio X is:

[0131]

[0132] In the formula: σ d is the amplitude of the deviator stress under cyclic loading, and σ c is the confining pressure level of the soil.

[0133] Implementation of stiffness degradation

[0134] The USDFLD subroutine can define the variables obtained at a time or material point as field variables during the calculation process, and the field variables can be introduced into the program and a functional relationship can be established with the material properties related to the solution. Therefore, some property parameters of the material, such as the basic mechanical parameters elastic modulus, cohesion and internal friction angle, can change as a function of the user-defined field variables. Therefore, the USDFLD subroutine can solve the problem of weakening of soil mechanical parameters.

[0135] The soil sample for the indoor dynamic triaxial test is the fine sand taken from the site. The dynamic triaxial test is a cyclic triaxial compression test under drained conditions. The loading waveform is a half-sine wave. The above method is used to make a comparison with a working condition of the indoor dynamic triaxial test. This working condition is: relative density D = 0.68, confining pressure and deviator pressure are both 200 kPa, cyclic stress ratio is 0.1, loading frequency is 1 Hz, and the number of loading cycles is 1000.

[0136] Among them, the Mohr-Coulomb ideal elastoplastic model is selected for the soil. This model is simple and clear and can better reflect the loading characteristics of sand. The loading process of the finite element simulation is consistent with the indoor test process, where the soil cohesion c = 5 kPa, the internal friction angle φ' = 32°, the dilation angle ψ = 0°, the effective unit weight γ' = 19.4 kg / m 3 , the Poisson's ratio v = 0.4, and the elastic modulus E = 75 MPa.

[0137] According to the site address survey report, except for the silty soil on the surface of the seabed within the wind farm, the most widely distributed soil layer within the pile depth is the fine sand layer, and its particle size distribution and physical and mechanical parameters are shown in the following table.

[0138] Table of particle size distribution of fine sand

[0139] Particle size range (mm) 5～2 2～0.5 0.5～0.25 0.25～0.075 0.075～0.005 <0.005 Content (%) 0.1 1.5 12.6 72.8 12.9 0.1

[0140] Table of physical and mechanical parameters of fine sand

[0141]

[0142] This study conducts research on the cumulative deformation and stiffness evolution under long-term cyclic loading for this soil layer. Since on-site soil sampling conditions are not available, the model soil for this test is made by mixing quartz sand with different particle size ranges according to the on-site particle size distribution.

[0143] Test process

[0144] Process of static loading test

[0145] (1) Process the model of the wind turbine pile foundation and tower according to the similarity ratio design scheme, and configure the test sand according to the actual mix ratio of the borehole data.

[0146] (2) Stick strain gauges on both sides of the pile foundation. The strain gauge wires are evenly arranged along the outer wall of the pile. Epoxy resin is applied in layers around the strain gauges and wires. After applying one layer of epoxy resin, the surface is polished with sandpaper to ensure the bonding between layers. After multiple applications, the thickness of the epoxy resin is approximately between 1 - 1.5 mm.

[0147] (3) Calibrate the strain gauges to obtain the relationship between the voltage of each strain gauge and the bending moment.

[0148] (4) Conduct a calibration test on the falling height for the sand rain method of model making to calibrate the relationship between the falling height of the sand and the corresponding sand compactness. Calibrate the dry sand according to a density of 1.64 t / m3.

[0149] (5) Assemble the overall model of the wind turbine and fix it in the model box according to the designed position.

[0150] (6) Conduct model making by the sand rain method to a unified elevation. The target thickness of the soil layer is 720 mm, and it is prepared in 10 layers. When each layer of the seabed is completed, record the mass of the sand used, calculate the density and relative compactness of the soil body. The average relative compactness of the entire prepared seabed is 65%.

[0151] (7) Place the model box into the centrifuge, install the laser displacement sensor and the axial force sensor, adjust the position of the loading motor, install the camera, LED lighting and other accessory parts. Finally, lift the model box onto the centrifuge platform;

[0152] (8) Leave it static for 2 h under an acceleration of 100 g until the settlement of the soil mass is stable, and then start the test.

[0153] (9) Conduct a monotonic static loading test to obtain the ultimate bearing capacity of a single pile; push until the thrust shows a descending section, analyze and obtain the bearing capacity Fu when the mud surface has a displacement of 0.1D. At this time, the distance pushed by the hydraulic rod is L.

[0154] (10) Unload the load to 0, stop the machine, lift out the model box, and complete the test.

[0155] Process of cyclic loading test

[0156] (1) Process the model of the wind turbine pile foundation and tower according to the similarity ratio design scheme, and configure the test sand according to the actual mix ratio of the borehole data.

[0157] (2) Paste strain gauges on both sides of the pile foundation. The strain gauge wires are evenly arranged along the outer wall of the pile. Apply epoxy resin in layers around the strain gauges and the wires, and polish the surface with sandpaper to ensure the bonding between layers. The thickness of the epoxy resin after multiple applications is approximately between 1 - 1.5 mm.

[0158] (3) Calibrate the strain gauges to obtain the relationship between the voltage of each strain gauge and the bending moment.

[0159] (4) Conduct a calibration test on the falling height for the sand rain method of mold making to calibrate the relationship between the falling height of the sand and the corresponding sand density. Calibrate the dry sand according to a density of 1.64 t / m³.

[0160] (5) Assemble the overall model of the wind turbine and fix it in the model box according to the designed position.

[0161] (6) Conduct mold making by the sand rain method to a unified elevation. The target thickness of the soil layer is 720 mm, and it is prepared in 10 layers. When each layer of the seabed is completed, record the mass of the sand used, calculate the density and relative density of the soil mass. The average relative density of the entire prepared seabed is 65%.

[0162] (7) Place the model box into the centrifuge, install laser displacement sensors and axial force sensors, adjust the position of the loading motor, install cameras, LED lights and other accessory parts. Finally, lift the model box onto the centrifuge platform;

[0163] (8) Leave it static for 2 h under an acceleration of 100 g until the settlement of the soil mass is stable, and then start the test.

[0164] (9) Conduct a cyclic loading test, and conduct cyclic loading according to 20%, 30%, 40% of Fu. The loading frequency is at the highest frequency of 2 Hz for the test, and the number of cycles for each level is 2000 times.

[0165] (10) Unload the load to 0, stop the machine and lift out the model box to complete the test.

[0166] Test data analysis

[0167] For the cyclic loading test in sandy soil foundation, the unloading stiffness in the first cycle is smaller than that in the subsequent cycles, further illustrating the strong nonlinearity of undisturbed sand. LeBlanc (2009) proposed an expression for the variation of unloading stiffness with the number of cycles for rigid piles:

[0168] k N = k0 + bln(N)

[0169] Due to the large discreteness of the variation of unloading stiffness and to reduce the influence of the nonlinearity of the soil in the initial state, in this paper, the unloading stiffness in the third cycle is used to normalize the cyclic unloading stiffness, and the above formula is used for fitting.

[0170] Numerical simulation of the deformation and stiffness evolution of wind power foundations under long-term cyclic loading

[0171] Establishment of numerical model: A three-dimensional finite element model of a single-pile foundation is established using finite element analysis software. Due to the symmetry of the loading conditions and the pile-soil structure, half of the pile-soil structure is taken for analysis;

[0172] Calculate the horizontal ultimate load of the single-pile foundation and calculate the horizontal ultimate bearing capacity H u , and measure the ability of the single pile to bear horizontal loads through the horizontal ultimate bearing capacity;

[0173] Specific implementation and experimental verification of the numerical model: Through the secondary development of Abaqus, a stiffness degradation model of sand is established in the numerical analysis program and verified.

[0174] The steel pipe pile adopts a linear elastic model, with a pile diameter D = 6.3m, a wall thickness s = 0.64mm, and an elastic modulus E p = 210Gpa; the soil is selected as the Mohr-Coulomb ideal elastic-plastic model, which is simple and clear and can better reflect the loading characteristics of sand. A contact surface is set between the pile and the soil, where the normal contact is selected as hard contact and the tangential contact is selected as frictional contact, with a friction coefficient of two-thirds of the internal friction angle, a soil cohesion c = 5kPa; an internal friction angle φ' = 32°, a dilatancy angle ψ = 0°, and an effective unit weight γ' = 19.4kg / m 3 , a Poisson's ratio v = 0.4, and the elastic modulus is taken according to the following formula:

[0175]

[0176] In the formula: σ atis the atmospheric pressure, with a value of 101 kPa; σ m is the average soil stress of the soil mass; k and η are dimensionless constants. In this paper, k = 560 and η = 0.6 are taken.

[0177] The limit analysis method is used to calculate the horizontal ultimate load of a single-pile foundation

[0178] The deformation of a single pile exhibits the deformation characteristics of a rigid pile. The pile tip part of the single pile shows a form of toe kick failure, and the entire pile body rotates around a certain point on a certain pile tip; the horizontal ultimate bearing capacity is denoted as H u is:

[0179]

[0180] In the formula: h is the height from the mud surface to the pile top; D is the pile diameter; L is the unit weight of the soil mass; γ s is the unit weight of the soil mass; K P is the Rankine passive earth pressure coefficient.

[0181] Step 2: The limit analysis method is used to calculate the horizontal ultimate load of a single-pile foundation:

[0182] For pipe piles, its yield moment can be expressed as:

[0183]

[0184] In the formula: I P is the moment of inertia of the pile cross-section; σ y is the yield strength of the pile material.

[0185] Step 2: The p-y curve calculation for calculating the horizontal ultimate load of a single-pile foundation:

[0186] When y < 3y 50 then

[0187] When y > 3y 50 then, when x > x r then p = 0.72p u

[0188] When x < x r then

[0189] In the formula, y is the deflection of the pile; y 50 is the displacement of the pile when the soil resistance reaches half of the limit value; x is the depth below the mud surface; x r is the critical depth; p is the soil resistance around the pile; p u is the ultimate soil resistance around the pile, which is related to the undrained shear strength C u and ε 50 is related.

[0190] Specific implementation and experimental verification of the numerical model: The following formula is used for deep soil calculation:

[0191] p us =(C1X + C2b)γ · X

[0192] p ud =C3bγ · X

[0193] In the formula, p u is the ultimate soil resistance per unit pile length; γ · is the effective unit weight of the soil; X is the depth below the mud surface; C1, C2, and C3 are coefficients related to the internal friction angle φ, and b is the average pile diameter within the range from the mud surface to the depth in question.

[0194] Establish a three-dimensional finite element model for the horizontal loading of large-diameter single piles in sandy soil, conduct in-situ stress equilibrium and apply a vertical load V, calculate and extract the initial major principal stress and minor principal stress

[0195] Apply a horizontal load H at the loading point that is equal to the peak value of the cyclic load, calculate and extract the major principal stress and minor principal stress

[0196] The extracted is used as the confining pressure σ c received by each soil element. Subtract from to obtain the deviator stress level σ d received by each soil element. Furthermore, the expression for the cyclic stress ratio X of each soil element is:

[0197]

[0198] Write the cyclic stress ratio of each soil element into the usdfld subroutine. The software will call this subroutine during the analysis and calculate the mechanical behavior of the pile foundation under N cyclic loads based on the attenuation stiffness of each soil element after N cycles. x is the horizontal displacement of the loading point; H u is the ultimate static horizontal load of the pile foundation.

[0199] Analyze the deformation and stiffness evolution of the wind power foundation under long-term cyclic loading. The horizontal displacement value at the pile top decreases after the cyclic load acts; as h / L increases, the decreasing trend intensifies. When h / L is 0.294, the cumulative horizontal displacement value at the pile top decreases to 30% of the original value.

[0200] Step 4: Analyze the deformation and stiffness evolution of the wind power foundation under long-term cycling. For a steel pipe pile with an embedment depth of 34 m, under the action of a horizontal cyclic load of 0.25Hu, the development of the lateral displacement of the pile body; under the action of the horizontal load, as the cycling progresses, the horizontal displacement of the pile top continuously develops; at the initial stage of the cyclic load action, the lateral displacement of the steel pipe pile develops rapidly. After the number of cycles exceeds 100 times, the development of the lateral displacement slows down and gradually stabilizes.

[0201] Analyze the deformation and stiffness evolution of the wind power foundation under long-term cycling. The horizontal displacement of the pile top decreases with the increase of the elastic modulus of the soil; the embedment depth of the steel pipe pile is small, and the decreasing amplitude is significant; as the embedment depth of the steel pipe pile increases, the decreasing amplitude of its lateral displacement gradually decreases.

[0202] The above embodiments are only used to illustrate the present invention and do not limit the technical solutions described in the present invention. Although this specification has described the present invention in detail with reference to the above respective embodiments, the present invention is not limited to the above specific embodiments. Therefore, any modification or equivalent replacement to the present invention; and all technical solutions and their improvements that do not depart from the spirit and scope of the invention are covered by the scope of the claims of the present invention.

Claims

1. Numerical simulation method for the stiffness of monopiles of offshore wind turbines considering long-term cycling and scouring, characterized in that, It includes the following steps: Step 1: Establishing a numerical model. A three-dimensional finite element model of a single-pile foundation is established using finite element analysis software. Due to the symmetry of the stress conditions and the pile-soil structure, half of the pile-soil structure is taken for analysis. An erosion pit model is established according to a certain erosion depth, and the slope ratio of the erosion pit should be determined according to the actual engineering situation; Step 2: Calculate the horizontal ultimate load of the single-pile foundation and the horizontal ultimate bearing capacity H of the single pile, and measure the horizontal load-bearing capacity of the single pile through the horizontal ultimate bearing capacity; u ​ Step 3: Specific implementation and experimental verification of the numerical model. Through secondary development of Abaqus, a stiffness degradation model of sand is established in the numerical analysis program and verified; Step 4: Analyzing the deformation and stiffness evolution of the wind power foundation under long-term cyclic loading. The pile foundation will continuously bear the long-term and low-frequency lateral cyclic forces brought by wind loads and wave loads. The cumulative lateral displacement generated by the steel pipe pile during its normal service life is accurately predicted through analysis.

2. The numerical simulation method for the stiffness of a monopile of an offshore wind turbine considering long-term cycling and scouring according to claim 1, characterized in that Step 1. Establish a numerical model of seabed - wind turbine with different scouring depths. The steel pipe pile adopts a linear elastic model, with a pile diameter D = 6.3m, a wall thickness s = 0.64mm, and an elastic modulus E p = 210GPa; the soil body selects the Mohr - Coulomb ideal elastic - plastic model, which is simple and clear and can better reflect the loading characteristics of sandy soil. A contact surface is set between the pile and the soil. Among them, the normal contact selects hard contact, and the tangential contact selects frictional contact. The friction coefficient is two - thirds of the internal friction angle, the soil cohesion c = 5kPa; the internal friction angle φ' = 32°, the dilation angle ψ = 0°, the effective unit weight γ' = 19.4kg / m 3 , the Poisson's ratio v = 0.4, and the elastic modulus is taken according to the following formula: where: σ at is the atmospheric pressure, with a value of 101 kPa; σ m is the average soil stress of the soil mass; k and η are dimensionless constants, and in this paper, k = 560 and η = 0.6 are taken.

3. The numerical simulation method for the stiffness of a monopile of an offshore wind turbine considering long-term cycling and scouring according to claim 2, characterized in that, Step 2: Using the limit analysis method to calculate the horizontal ultimate load of the single-pile foundation The deformation of a single pile exhibits the deformation characteristics of a rigid pile. The pile tip part of the single pile shows a form of toe kick failure, and the entire pile rotates around a certain point on a certain pile tip; the horizontal ultimate bearing capacity is denoted as H u is as follows: Where: h is the height from the mud surface to the top of the pile; D is the pile diameter; L is the unit weight of the soil mass; γ s Unit weight of the soil mass; K P Rankine passive earth pressure coefficient.

4. The numerical simulation method for the stiffness of a monopile of an offshore wind turbine considering long-term cycling and scouring according to claim 3, characterized in that Step 2: Using the limit analysis method to calculate the horizontal ultimate load of the single-pile foundation: For pipe piles, its yield moment can be expressed as: Where: I P Moment of inertia of the pile cross-section; σ y Yield strength of the pile material.

5. The numerical simulation method for the stiffness of a monopile of an offshore wind turbine considering long-term cycling and scouring according to claim 4, wherein Step 2: Calculating the p-y curve of the horizontal ultimate load of the single-pile foundation When y < 3y 50 at this time When y > 3y 50 then x > x r then p = 0.72p u x < x r when In the formula, y is the deflection of the pile; y 50 is the displacement of the pile when the soil resistance reaches half of the ultimate value; x is the depth below the mud surface; x r is the critical depth; p is the soil resistance around the pile; p u is the ultimate soil resistance of the soil around the pile, related to the undrained shear strength C u and ε 50 is relevant.

6. The numerical simulation method for the stiffness of a monopile of an offshore wind turbine considering long-term cyclic loading and scouring according to claim 5, wherein Step 3: Specific implementation and experimental verification of the numerical model: Calculation using the following formula for soils below the deep layer p us = (C1X + C2b)γ·X p ud = C3bγ·X where p u is the ultimate soil resistance per unit pile length; γ· is the effective unit weight of soil; X is the depth below the mud surface; C1, C2, and C3 are coefficients related to the internal friction angle φ, and b is the average pile diameter within the range from the mud surface to the depth in question.

7. The numerical simulation method for the stiffness of a monopile of an offshore wind turbine considering long-term cycling and scouring according to claim 6, characterized in that, A three-dimensional finite element model of a large-diameter single pile under lateral load in sandy soil is established. The in-situ stress is balanced and the vertical load V is applied. The initial major principal stress and minor principal stress of each soil element are calculated and extracted. and minor principal stress Apply a horizontal load H equal to the peak value of the cyclic load at the loading point, calculate and extract the major principal stress and minor principal stress of each soil element under horizontal loading and minor principal stress Extracted As the confining pressure σ c received by each soil element, Subtract from it to obtain the deviator stress level σ d received by each soil element, and then the expression of the cyclic stress ratio X for each soil element is obtained as follows: Write the cyclic stress ratio of each soil element into the usdfld subroutine, and the software will call this subroutine in the analysis and calculate the mechanical behavior of the pile foundation under N cyclic loads according to the attenuation stiffness of each soil element after N cycles. x is the horizontal displacement of the loading point; H u is the horizontal static ultimate load of the pile foundation.

8. The numerical simulation method for the stiffness of a monopile of an offshore wind turbine considering long-term cycling and scouring according to claim 7, characterized in that Step 4: Analyzing the deformation and stiffness evolution of the wind power foundation under long-term cyclic loading. The horizontal displacement value at the pile top decreases after a certain number of cyclic loadings; as h / L increases, the decreasing trend becomes more significant. When h / L is 0.294, the cumulative horizontal displacement value at the pile top decreases to 30% of the original value.

9. The numerical simulation method for the stiffness of a monopile of an offshore wind turbine considering long-term cyclic loading and scouring according to claim 8, characterized in that Step 4: Analyzing the deformation and stiffness evolution of the wind power foundation under long-term cyclic loading. The development of the lateral displacement of a steel pipe pile with an embedment depth of 34m under a horizontal cyclic load of 0.25Hu; under the action of the horizontal load, the horizontal displacement at the pile top continuously develops as the cycle progresses; at the initial stage of the cyclic load action, the lateral displacement of the steel pipe pile develops rapidly. After the number of cycles exceeds 100, the lateral displacement development slows down and gradually stabilizes.

10. The numerical simulation method for the stiffness of a monopile of an offshore wind turbine considering long-term cycling and scouring according to claim 9, characterized in that Step 4: Analyzing the deformation and stiffness evolution of the wind power foundation under long-term cyclic loading. The horizontal displacement at the pile top decreases as the elastic modulus of the soil increases; when the embedment depth of the steel pipe pile is small, the decreasing amplitude is significant; as the embedment depth of the steel pipe pile increases, the decreasing amplitude of its lateral displacement gradually decreases.

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