A method for analyzing the stability of wind power pile foundations under extreme wind waves, currents, and landslide loads

Abstract

The present invention provides a method for evaluating the stability of an offshore wind power pile foundation under the coupled action of submarine landslides and wind and wave impact loads, and develops a finite element model of pile-soil interaction under complex lateral loads. Based on the established quantification formula for submarine landslide impact loads, the lateral loads acting on a single pile under the coupled action of submarine landslides, wind, waves, and currents in a complex environment are given. Finally, the stability analysis of a single pile under complex lateral loads is carried out, and the stability of the single pile foundation is discussed while considering the strength of sediments in different sea areas. Through the technical solution of the present invention, this method can judge the stability evaluation of a single pile foundation under the action of wind, wave, and current loads and potential landslide loads in the study area, provide references for the site selection of offshore wind power and the establishment of offshore wind power, etc., and reduce the economic losses caused by the instability of offshore wind power.

Description

Technical Field

[0001] The present invention relates to the technical field of the study on the stability of offshore pile foundations. Specifically, it particularly relates to a method for evaluating the stability of offshore wind power pile foundations under extreme wind, wave, current and landslide loads. Background Art

[0002] Currently, the research on the stability of offshore pile foundations mainly focuses on the external dynamic loads acting on the piles, mainly including loads such as wind, waves and currents. Zeng et al. studied the influence of breaking waves on single-pile foundation offshore wind turbines at different positions on the slope edge through flume experiments and computational fluid dynamics models. It was found that when the waves break, the pressure acting on the single pile will be distributed in a wave shape, and the acting point will gradually move down. Shi et al. established a "three-section" computational model for single-pile foundation offshore wind turbines, analyzed the force conditions in soil, water and air respectively, further obtained the total dynamic impedance of the foundation to predict its dynamic response, and analyzed the displacements of the tower under different single-pile parameters. Miles et al. conducted a series of indoor experiments, analyzed the coupling effect of waves and currents, and studied the influence of waves and currents on single piles of offshore wind turbines. Ong et al. carried out a dynamic analysis of offshore single piles under the action of ocean currents through a finite element model, and analyzed the response of single piles under the coupled action of wind and waves under shallow water conditions with different parameters. Buljac et al. conducted experiments on small wind turbines in a wind-wave-current flume (WWCT), studied the influence of the combined action of wind, waves and currents on single-pile foundation offshore wind turbines, and found that among the overall loads on offshore wind turbines, the longitudinal surge force caused by the current dominates. Achmus et al. established a computational model for the interaction of ocean waves-wind turbines-seabed foundations, simplified the wind and wave loads as a concentrated horizontal force acting on the top of the tower barrel, and studied the horizontal bearing capacity characteristics of offshore wind power single piles. Wu et al. studied the high-pile foundation of offshore wind power by finite element analysis, simulated the bearing capacity of different soil strength parameters under the horizontal load of the wind power foundation, and the results showed that improving the soil, increasing the embedment depth and increasing the pile diameter can effectively improve the bearing capacity of the pile foundation.

[0003] The above research mainly focuses on the effects of wind, waves, and currents on the stability of pile foundations. However, in recent years, due to the frequent occurrence of global extreme storms and geological events, some research has begun to focus on the impact of submarine landslides, which are accidental loads, on pile foundations. Submarine landslides are characterized by phenomena such as local or large-scale seabed instability, large deformations, and sliding. The damage caused by high-speed submarine landslides to marine structures is very significant. For example, the maximum speed of the 1929 Grand Banks landslide in Canada reached 20 m / s, destroying 12 submarine pipelines. The 2006 submarine landslide in the Luzon Strait cut off 11 submarine communication cables, resulting in a long-term communication interruption. Currently, the research on the impact of submarine landslides on piles mainly focuses on the quantification of the impact load of submarine landslides. Li et al. fabricated a submarine landslide model flume and conducted 12 groups of model tests by adjusting variables such as the slope of the model flume and the mass fraction of kaolin in the landslide body, optimizing the dimensionless resistance coefficient formula acting on the pile foundation. Li et al. used computational fluid dynamics methods to study the impact force of submarine debris flows on single piles and analyzed the drag force characteristics acting on single piles. Li et al. used computational fluid dynamics methods to simulate the impact of submarine flow slides on pile foundations and proposed a prediction formula for the impact force.

[0004] However, these studies analyzed the impact force of submarine landslides on pile foundations, but did not conduct in-depth research on whether the pile foundations would become unstable after encountering the impact load of submarine landslides and the mechanism of instability. At the same time, considering that there are still few reports on the analysis of the stability of pile foundations under the action of various complex loads such as submarine landslides, wind, waves, and currents, it is urgent to carry out relevant research. Summary of the Invention

[0005] To make up for the deficiencies of the prior art, the present invention provides a method for evaluating the stability of offshore wind power pile foundations under the coupled action of submarine landslides, wind, and wave impact loads. The present invention developed a finite element model of pile-soil interaction under complex lateral loads. Based on the established quantification formula for submarine landslide impact loads, the lateral loads acting on a single pile under the coupled action of submarine landslides, wind, waves, and currents in a complex environment were given. Finally, the stability analysis of a single pile under complex lateral loads was carried out, and the stability of the single pile foundation was discussed considering the strength of sediments in different sea areas.

[0006] The present invention is realized through the following technical solutions: A method for evaluating the stability of offshore wind power pile foundations under the coupled action of submarine landslides, wind, and wave impact loads, characterized in that it specifically includes the following steps:

[0007] Step S1, generalization of the loads on the offshore wind power structure in a complex environment, specifically including the following steps:

[0008] Step S11, aerodynamic load:

[0009] Aerodynamic loads acting on offshore wind turbines: They are divided into blade wind loads and tower loads. Blade wind loads are the wind loads on the entire wind turbine impeller, and tower loads are the wind loads acting on the tower in the air. The calculation formula for blade wind loads is as follows:

[0010] (1)

[0011] In the formula, is the wind turbine load; is the air density, ; is the impeller swept area, which is calculated according to the blade size; is the impeller thrust coefficient; is the wind speed;

[0012] The wind load acting on the tower is calculated using the following formula:

[0013] (4)

[0014] In the formula: is the tower wind load; is the drag coefficient; is the monopile diameter; is the average wind speed at height Z. The exponential formula for simulating the variation of the average wind speed with height is:

[0015] (5)

[0016] In the formula, represents the average wind speed at the standard height of ; is the wind shear exponent;

[0017] Step S12, hydrodynamic loads, specifically including the following steps:

[0018] Hydrodynamic loads are the coupled wave-current loads, including wave loads and current loads. The wave loads divide the wave force into two parts: one is the inertial force term proportional to the acceleration, and the other is the drag force term proportional to the square of the velocity, as follows:

[0019] (6)

[0020] Among them, is the wave force; is the drag force term; is the inertial force term; is the projected area of the unit column height perpendicular to the wave propagation direction; is the drainage volume of the unit column height; is the density of seawater, ; is the drag force coefficient perpendicular to the axis of the cylinder; is the inertia force coefficient; is the wave velocity;

[0021] For the ocean current load, the ocean current is idealized as a uniform motion, which will not cause the acceleration of water particles. The inertia force term is omitted, and only the drag force term is calculated, that is:

[0022] (7)

[0023] where is the ocean current load; is the resistance coefficient;

[0024] Step S13, submarine landslide impact load:

[0025] Establish the relationship expression between the resistance coefficient and the Reynolds number of non-Newtonian fluid:

[0026] (8)

[0027] where is the landslide load; is the resistance coefficient; is the density of submarine landslide debris flow; is the landslide flow velocity; is the impact height of submarine landslide; is the outer diameter of the pile;

[0028] Fit the relationship between the resistance coefficient and the Reynolds number, which is:

[0029] (9)

[0030] The Reynolds number of non-Newtonian fluid is often expressed as:

[0031] (10)

[0032] where: is the shear rate of the mud near the pile; is the debris flow viscosity; is the shear stress; According to the relationship between the fluid shear stress and the shear rate, the rheological curve expression is obtained:

[0033] (11)

[0034] In the formula, ;

[0035] Step S14, superposition of environmental loads:

[0036] Different environmental loads are equivalent to horizontal loads and bending moments acting at the same height, and submarine landslides are applied to the pile side by uniformly distributing loads;

[0037] Step S2, establishing a pile-soil interaction model under complex horizontal loads, specifically includes the following steps:

[0038] Step S21, single pile model:

[0039] To solve the pile displacement and pile stress under the horizontal load of the pile head, a microelement with a length of dz of the horizontally loaded pile is selected, and through the force balance of the bending moment, the following is obtained:

[0040] (12)

[0041] Where M is the bending moment; is the vertical load; is the shear force; according to beam theory:

[0042] (13)

[0043] In the formula is the bending stiffness of the pile; the two equations can be connected to obtain the control equation when the pile is horizontally loaded:

[0044] (14)

[0045] In the formula, is the soil reaction force;

[0046] The single pile model is described by an isotropic elastic model, and the stress-strain relationship of the pile material is simulated by the Hooke theorem. Under three-dimensional conditions, the Hooke theorem is expressed as:

[0047] (15)

[0048] in, is Young's modulus; is Poisson’s ratio;

[0049] Step S22: soil model:

[0050] The soil model is described by the elastic-plastic model Mohr-Coulomb model, which includes cohesion intercept c, friction angle, Young's modulus E, Poisson's ratio and expansion angle Parameter; when mapped to the three-dimensional stress space, the Mohr-Coulomb criterion decomposes into an irregular hexagonal pyramid, and the pyramid forms the failure / yield envelope, which determines the behavior of the soil mass; when the stress point is within the envelope, the material undergoes elastic deformation, and when the stress reaches the yield surface, the material undergoes plastic deformation; in step S22, it is assumed that the soil mass is linearly elastic before failure;

[0051] Step S23, pile-soil contact model:

[0052] The "contact pair" algorithm is used to simulate the interaction between the pile and the soil. The main contact surface of the pile is set, and the soil is the slave contact surface. The discrete method of the surface adopts "face-to-face contact", and the tracking of the relative movement of the contact surface adopts "finite sliding". The calculation accuracy is improved by continuously judging whether the master-slave surface is in contact; the surface of the pile is defined as the main surface, and the surface of the soil is defined as the slave surface; the normal contact adopts the "hard" contact option, and the tangential contact adopts Coulomb contact. The calculation method of the interface shear stress is:

[0053] (16)

[0054] Where, is the shear stress of the contact surface; is the effective normal stress; is the friction angle of the pile-soil interface under the critical state;

[0055] Step S3, stability evaluation of a single-pile foundation under complex horizontal loads:

[0056] A numerical model is established through ABAQUS software. The single pile adopts a linear elastic model, and the Young's modulus, Poisson's ratio, and density are set; the seabed soil adopts the Mohr-Coulomb constitutive model, and the friction coefficient is set; the contact relationship between the single pile and the seabed is Coulomb friction contact. The surface of the single pile is the main surface, and the surface of the seabed is the secondary surface. Separation of the single pile-seabed interface is allowed during the calculation process; the mesh division adopts the sweep mode, and the mesh type is C3D8R; at the pile-soil interface, the density of the mesh is increased, and the mesh density gradually decreases from the interface to the edge; through mesh sensitivity analysis, the mesh size at the single pile-seabed interface is determined to be 0.1D; at the pile head, a horizontal load and a moment are applied to the single pile through a reference point, and a uniform load is applied;

[0057] Submit the calculation, export the odb file for result analysis; export the pile body displacement, and the displacement change curve of the pile body along the depth direction can be obtained; differentiate and take the derivative of the curve to obtain the pile body rotation angle; the tolerance of the permanent cumulative rotation of the single pile at the mud line is 0.25°; the instability of the single pile can be judged by comparing the calculated rotation angle with the specification; the point where the pile body is most likely to be unstable can be judged by slicing and exporting the pile body moment; at the same time, the failure characteristics of the single pile and the sediment are judged through the stress nephograms of the single pile and the sediment;

[0058] By changing the sediment cohesion and internal friction angle, the pile displacements and rotations under different sediment strengths are calculated, and then through fitting, the relationship curves of the single-pile displacement and rotation with the sediment strength cohesion and internal friction angle are obtained.

[0059] As a preferred solution, in step S11, considering different ranges of wind speed, the impeller thrust coefficient is obtained by the following three different methods:

[0060] When the cut-in wind speed ( ), wind speed ( ), rated wind speed ( ), the calculation formula of the impeller thrust coefficient is as follows:

[0061] (2)

[0062] When the rated wind speed ( ), wind speed ( ), cut-out wind speed ( ), the calculation formula of the impeller thrust coefficient is as follows:

[0063] (3)

[0064] When in the low wind speed condition, assuming that the impeller thrust coefficient does not exceed 1, formula (2) will overestimate the thrust coefficient, so its value is limited to 1;

[0065] As a preferred solution, in step S11 is a dimensionless number, not affected by the specific size of the object, only related to the structural shape. The tower barrel is a cylindrical cross-section, and after calculation, it takes 0.5.

[0066] As a preferred solution, in step S11 takes 10 m, and the wind shear exponent takes 0.143.

[0067] As a preferred solution, for the cylinder under the action of wave and current loads in step S12, take and .

[0068] As a preferred solution, the drag coefficient for circular members takes 0.73.

[0069] Due to the adoption of the above technical solutions, the present invention has the following beneficial effects compared with the prior art:

[0070] (1) This method can evaluate the stability of a single-pile foundation under the action of wind, wave, and current loads and potential landslide loads in the study area, and has a warning effect on offshore wind power platforms;

[0071] (2) It provides a reference for the site selection and establishment of offshore wind power, reducing the economic losses caused by the instability of offshore wind power;

[0072] (3) Through the finite element model, this method can clearly show the instability mode of a single pile and provide a reference for single-pile reinforcement.

[0073] The additional aspects and advantages of the present invention will become apparent in the following description section or be learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0075] Figure 1 is a simplified schematic diagram of the load;

[0076] Figure 2 is a model diagram of pile-soil interaction;

[0077] Figure 3 is the influence of different landslide densities on a single pile: (a) pile body displacement; (b) pile body rotation angle; (c) pile body bending moment;

[0078] Figure 4 is the soil stress nephogram under the action of submarine landslides with different densities: (a) landslide density is 1400 kg / m 3 ; (b) landslide density is 1800 kg / m 3 ;

[0079] Figure 5 The pile body stress nephogram under the action of submarine landslides with different densities (deformation coefficient is 100): (a) landslide density is 1400 kg / m 3 ; (b) landslide density is 1800 kg / m 3 . DETAILED DESCRIPTION OF THE EMBODIMENTS

[0080] In order to more clearly understand the above objects, features, and advantages of the present invention, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that, without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other.

[0081] In the following description, many specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced in other ways than those specifically described herein, and thus, the scope of protection of the present invention is not limited by the specific embodiments disclosed below.

[0082] The following specifically describes Figures 1 to 5 a method for evaluating the stability of an offshore wind power pile foundation under the coupled action of submarine landslides and wind and wave impact loads in an embodiment of the present invention.

[0083] The present invention provides a method for evaluating the stability of an offshore wind power pile foundation under the coupled action of submarine landslides and wind and wave impact loads, specifically including the following steps:

[0084] Step S1, generalization of the loads on the offshore wind power structure under complex environments, specifically including the following steps:

[0085] Step S11, aerodynamic loads:

[0086] The aerodynamic loads acting on the offshore wind power are divided into blade wind loads and tower loads. The blade wind loads are the wind loads received by the entire wind turbine impeller, which are the most important loads on the wind turbine; the tower loads are the wind loads acting on the tower in the air. Since the tower size is relatively small, the load value of this part is also small. The calculation formula for the blade wind loads is as follows:

[0087] (1)

[0088] In the formula, is the wind turbine load; is the air density, ; is the impeller swept area, calculated according to the blade size; is the impeller thrust coefficient; is the wind speed; considering different ranges of wind speed, the impeller thrust coefficient is obtained by the following three different methods:

[0089] When the cut-in wind speed ( ) wind speed ( ) rated wind speed ( ), the calculation formula for the impeller thrust coefficient is as follows:

[0090] (2)

[0091] When the rated wind speed ( ) wind speed ( ) cut-out wind speed ( ), the calculation formula for the impeller thrust coefficient is as follows:

[0092] (3)

[0093] When in the low wind speed condition, assuming that the impeller thrust coefficient does not exceed 1, Equation (2) will overestimate the thrust coefficient, so its value is restricted to 1

[0094] The diameter of the tower barrel structure is relatively small compared to the wind path, and the influence on the wind flow can be ignored. Therefore, the wind load acting on the tower barrel adopts the following calculation formula:

[0095] (4)

[0096] Where: is the wind load on the tower barrel; is the drag coefficient; is a dimensionless number, not affected by the specific size of the object, and only related to the structural shape. In engineering, it is used to represent the drag characteristics of the object's motion. The tower barrel has a cylindrical cross-section, and after calculation, it is taken as 0.5. is the single pile diameter; is the average wind speed at height Z. The exponential formula for simulating the variation of the average wind speed with height is:

[0097] (5)

[0098] Where, represents the average wind speed at the standard height of ; is taken as 10 m, is the wind shear exponent; the wind shear exponent is taken as 0.143.

[0099] Step S12, hydrodynamic load, specifically includes the following steps:

[0100] The hydrodynamic load is the wave-current coupling load, including the wave load and the ocean current load. The wave load divides the wave force into two parts: one is the inertial force term proportional to the acceleration, and the other is the drag force term proportional to the square of the velocity, as follows:

[0101] (6)

[0102] Among them, is the wave force; is the drag force term; is the inertial force term; is the projected area of the unit column height perpendicular to the wave propagation direction; is the drainage volume of the unit column height; is the density of seawater, ; is the drag force coefficient in the direction perpendicular to the axis of the cylinder, which centrally reflects the viscous effect caused by the viscosity of the fluid; is the inertial force coefficient; is the wave velocity; According to the "Regulations on Port Hydrology" (JTJ213 - 98), for a cylinder under the action of wave and current loads, take and .

[0103] For the current load, the current is idealized as a uniform motion, which will not cause the acceleration of water particles. The inertial force term is omitted, and only the drag force term is calculated, that is:

[0104] (7)

[0105] Among them, is the current load; is the resistance coefficient; The resistance coefficient is taken as 0.73 for circular members.

[0106] Step S13, submarine landslide impact load:

[0107] In order to study the impact force of submarine landslides on structures, by adjusting the different contents of kaolin and silty sand, debris flows with different rheological properties and different densities are obtained, the impact of the debris flow on the model pile is simulated, and combined with the theory of fluid mechanics, the relationship expression between the resistance coefficient and the Reynolds number of non - Newtonian fluid is established:

[0108] (8)

[0109] Among them, is the landslide load; is the resistance coefficient; is the density of the submarine landslide debris flow; is the landslide flow velocity; is the impact height of the submarine landslide; is the outer diameter of the pile;

[0110] The resistance coefficient can be used to measure the magnitude of the impact force. According to the principle of fluid mechanics, the resistance coefficient is closely related to the Reynolds number. The relationship between the resistance coefficient and the Reynolds number is fitted as:

[0111] (9)

[0112] The Reynolds number of non - Newtonian fluid is often expressed as:

[0113] (10)

[0114] Among them: is the shear rate of the near - pile mud; is the debris flow viscosity; is the shear stress; from the relationship between the fluid shear stress and the shear rate, the expression of the rheological curve is obtained:

[0115] (11)

[0116] In the formula, ;

[0117] Step S14, superposition of environmental loads:

[0118] To simplify the analysis, this patent simplifies the horizontal loads of wind, waves, and currents acting on the offshore wind power, as Figure 1 shown. The figure respectively shows the blade wind load, tower wind load, wave load, sea current load, and landslide load and their acting ranges. Due to the special structural form of the offshore wind power, the main load form of the pile foundation is the overturning moment. Different environmental loads can be equivalent to the horizontal load and moment acting at the same height, as Figure 1 shown. The submarine landslide is applied to the pile side in the form of a uniformly distributed load.

[0119] Step S2, establishment of the pile-soil interaction model under complex horizontal loads, specifically including the following steps:

[0120] Step S21, single-pile model:

[0121] Solve the pile body displacement and pile body stress of the pile under the action of the horizontal load at the pile head. Based on the bending theory of the beam, using the microelement method, select a microelement body with a length of dz of the horizontally loaded pile. Through the force balance of the bending moment, we get:

[0122] (12)

[0123] In the formula, M is the bending moment; is the vertical load; is the shear force; according to the beam theory:

[0124] (13)

[0125] In the formula is the flexural rigidity of the pile; by combining the two equations, the control equation of the horizontally loaded pile can be obtained:

[0126] (14)

[0127] In the formula, is the soil reaction force; the essence of using the finite element method to calculate the response of the pile foundation under horizontal loads is to solve the control equation of the horizontally loaded pile.

[0128] The single-pile model is described by an isotropic elastic model, and the stress-strain relationship of the pile material is simulated through Hooke's theorem. Under three-dimensional conditions, Hooke's theorem is expressed as:

[0129] (15)

[0130] where is Young's modulus; is Poisson's ratio;

[0131] Step S22, soil model:

[0132] The soil model is described by the elastoplastic Mohr-Coulomb model, involving parameters including cohesion intercept c, friction angle, Young's modulus E, Poisson's ratio and dilation angle According to Johnson (2006), the failure envelope depends only on the principal stresses () and is independent of the intermediate principal stress (). When mapped to the three-dimensional stress space, the Mohr-Coulomb criterion decomposes into an irregular hexagonal pyramid, and the pyramid forms the failure / yield envelope, which determines the behavior of the soil. When the stress point is within the envelope, the material undergoes elastic deformation, and when the stress reaches the yield surface, the material undergoes plastic deformation. In step S22, it is assumed that the soil is linearly elastic before failure;

[0133] Step S23, pile-soil contact model:

[0134] The "contact pair" algorithm is used to simulate the interaction between the pile and the soil. The main contact surface of the pile is set, and the soil is the slave contact surface. The surface discretization method uses "face-to-face contact", which can consider the geometric shapes of both the master and slave surfaces simultaneously, and has high calculation accuracy for stress and contact surface normal pressure. The tracking of the relative movement of the contact surface uses "finite sliding", and the calculation accuracy is improved by continuously judging whether the master-slave surfaces are in contact. Considering the relative stiffness difference, the pile surface is defined as the master surface and the soil surface is defined as the slave surface. The normal contact uses the "hard" contact option, and the tangential contact uses Coulomb contact. The calculation method of the interface shear stress is:

[0135] (16)

[0136] where is the contact surface shear stress; is the effective normal stress; is the friction angle of the pile-soil interface under the critical state;

[0137] Step S3, stability evaluation of a single-pile foundation under complex horizontal loads:

[0138] A numerical model was established by using ABAQUS software. The single pile adopted a linear elastic model, and the Young's modulus, Poisson's ratio, and density were set. The seabed soil adopted the Mohr-Coulomb constitutive model, and the friction coefficient was set. The contact relationship between the single pile and the seabed was Coulomb friction contact, with the single pile surface as the main surface and the seabed surface as the secondary surface. Separation of the single pile-seabed interface was allowed during the calculation process. The mesh division adopted a sweeping mode, and the mesh type was C3D8R. At the pile-soil interface, the mesh density was increased, and gradually decreased from the interface to the edge. Through mesh sensitivity analysis, the mesh size at the single pile-seabed interface was determined to be 0.1D. At the pile head, a horizontal load and a moment were applied to the single pile through a reference point, and a uniform load was also applied. The model diagram is as shown in Figure 2 shown.

[0139] The calculation was submitted, and the odb file was exported for result analysis. The displacement of the pile body was exported, and the displacement change curve of the pile body along the depth direction could be obtained. By differentiating the curve, the pile body rotation angle could be obtained. According to the regulations of DNV (Det Norske Veritas (DNV), 2018), the tolerance of the permanent cumulative rotation of the single pile at the mudline was 0.25°. By comparing the calculated rotation angle with the specification, the instability situation of the single pile could be judged. By exporting the pile body moment through slicing, the point where the pile body was most prone to instability could be judged. At the same time, the failure characteristics of the single pile and the sediment were judged through the stress nephograms of the single pile and the sediment.

[0140] By changing the cohesion and internal friction angle of the sediment, the displacement and rotation angle of the pile body under different sediment strengths were calculated, and then through fitting, the relationship curves of the pile body displacement, rotation angle with the sediment strength cohesion and internal friction angle were obtained.

[0141] The Xiangshui Wind Farm is located in the tidal flat area of Chenjiagang Town, Xiangshui County, Jiangsu Province. A total of 134 large-scale wind turbines with a single capacity of 1.5 MW are installed, with a total installed capacity of 201 MW and an annual on-grid power generation of approximately 440 million kWh. The wind farm uses a concrete single pile foundation, with a pile diameter D of 4.3 m, a pile length L of 70 m, and a seabed burial depth Ld of 60 m. The three-dimensional finite element model consists of two parts: the seabed and the wind power single pile foundation. The single pile adopts a linear elastic model, with a Young's modulus Epile of 25×106 kPa, a Poisson's ratio νpile of 0.20, and a density ρpile of 2400 kg / m3. To eliminate the boundary effect, the radius of the seabed in the calculation domain was taken as 70 m, and the height was 140 m. The seabed soil adopted the Mohr-Coulomb constitutive model. The contact relationship between the single pile and the seabed was Coulomb friction contact, with the single pile surface as the main surface and the seabed surface as the secondary surface, and the friction coefficient was 0.39. Separation of the single pile-seabed interface was allowed during the calculation process.

[0142] Field investigations (Zhang et al., 2018) showed that the gravity load of the entire wind power system of the Xiangshui Wind Farm project was 2.06×103 kN m. When the wind farm was operating normally, the wind speed at an altitude of 10 m was 13.4 m / s, the water depth was 5 m, the wave height was 2.68 m, the wave length was 74.1 m, and the flow velocity was 0.88 m / s. By converting and superimposing the environmental loads at different acting points, the horizontal load applied at the pile head was 307102 N, and the clockwise bending moment was 10891482 N·m. The density of the seabed sediment was taken as 2130 kg / m3, the Young's modulus was 80×103 kPa, the Poisson's ratio was 0.33, the cohesion was 10 kPa, and the internal friction angle was 10°.

[0143] To compare the effects of different landslide densities on the stability of a single pile, the landslide speed was set at 15 m / s, and the height of the landslide impact on the single pile was 6 m. The results of the pile body displacement, rotation angle, and bending moment under different densities are as Figure 3 shown. When the landslide density was 1400 kg / m 3 , the landslide impact force was 3561713 N, and the horizontal displacement at the pile top was 0.09 m. When the landslide density was 1800 kg / m 3 , the landslide impact force was 4574803 N, and the horizontal displacement at the pile top reached 0.13 m. Obviously, the greater the landslide density on the seabed, the greater the horizontal displacement at the pile top. However, the gradient of the horizontal displacement at the pile top with the increase in landslide density was basically unchanged. For every 200 kg / m 3 increase in landslide density, the horizontal displacement at the pile top increased by 0.02 m. Similarly, the gradient of the pile body rotation angle with the increase in landslide density was basically unchanged. For every 200 kg / m 3 increase in landslide density, the maximum rotation angle of the pile body increased by approximately 0.04°. When the landslide density was 1400 kg / m 3 , the maximum rotation angle at the mud line of the pile body was 0.19°. When the landslide density was 1800 kg / m 3 , the maximum rotation angle at the mud line of the pile body was 0.27°, which had reached the failure state. As the landslide density increased, the maximum value of the pile body bending moment increased. When the landslide density was 1600 kg / m 3 , the maximum bending moment of the pile body was 47.5 MN·m, which was 3.6 times the calculation result without applying the landslide load. When the landslide density was 1800 kg / m 3 , the maximum bending moment of the pile body could reach 61.9 MN·m, which was 4.7 times the calculation result without applying the landslide load. To analyze the complex stress state of the sediment and the single pile after being loaded, the Mises stress nephograms of the sediment and the single pile are mainly analyzed below. Comparing the Mises stress nephograms of the sediment with densities of 1400 kg / m 3 and 1800 kg / m 3 ( Figure 4) and the Mises stress nephogram of the pile shaft ( Figure 5 ), which can further explain the above results. When a single pile is subjected to lateral load, the stress on the compression surface of the sediment increases due to the lateral load. As the density increases, the lateral load increases and the sediment stress increases. It can be seen that the change in the density of the landslide body does not change the distribution of the sediment-pile interaction (SPI) failure zone. The SPI failure zone mainly includes the failure zone caused by the extrusion of the single pile at the pile head, the shear failure zone caused by the rotation of the single pile at the pile toe, and the failure zone on the pile side. When the density is 1400 kg / m 3 , the range of the SPI pile top failure zone is 6.3D, while when the density is 1800 kg / m 3 , the range of the SPI pile top failure zone is 7.3D. The stress nephogram of the pile shaft corresponds to that of the sediment. The increase in density leads to an increase in the stress of the pile shaft. The maximum stress of the pile shaft appears at about 10 m below the seabed surface, which is the same as the position of the maximum bending moment of the pile shaft and increases with the increase of the pile top displacement.

[0144] In the description of the present invention, the term "a plurality" means two or more, unless otherwise clearly defined. The terms "upper", "lower", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention; the terms "connection", "installation", "fixation", etc. should all be understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0145] In the description of this specification, the description of the terms "an embodiment", "some embodiments", "specific embodiments", etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0146] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for evaluating the stability of offshore wind power pile foundations under the coupling of submarine landslide and wind and wave impact loads, characterized in that , specifically including the following steps: Step S1, generalizing the loads on offshore wind power structures in complex environments, specifically including the following steps: Step S11, aerodynamic load: The aerodynamic loads acting on offshore wind power are divided into blade wind load and tower load. The blade wind load is the wind load on the entire wind turbine impeller, and the tower load is the wind load acting on the tower in the air. The blade wind load calculation formula is as follows: （1） In the formula, is the fan load; is the air density, ; is the impeller swept area, calculated based on the blade size; is the impeller thrust coefficient; is the wind speed; The wind load acting on the tower is calculated using the following formula: （4） Where: is the tower wind load; is the drag coefficient; is the diameter of a single pile; is the average wind speed at height Z, and the exponential formula for simulating the average wind speed changing along the height is: （5） In the formula, Indicates the standard height is The average wind speed at is the wind shear index; Step S12, hydrodynamic load, specifically includes the following steps: The hydrodynamic load is the wave-current coupled load, including wave load and current load. The wave load divides the wave force into two parts: one is the inertial force term proportional to the acceleration, and the other is the resistance term proportional to the square of the velocity, as follows: （6） in, is the wave force; is the drag force term; is the inertia force term; is the projected area per unit cylinder height perpendicular to the wave propagation direction; is the displacement volume per unit column height; is the density of seawater, ; is the drag force coefficient perpendicular to the axis of the cylinder; is the inertia force coefficient; is the wave speed; For the ocean current load, the ocean current is idealized as uniform motion, which will not cause the acceleration of the water particles. The inertia force term is omitted and only the drag force term is calculated, that is: （7） in, is the ocean current load; is the drag coefficient; Step S13: Submarine landslide impact load: Establish the relationship between the drag coefficient and the Reynolds number of non-Newtonian fluid: （8） in, is the landslide load; is the drag coefficient; is the density of submarine landslide debris flow; is the landslide flow velocity; is the impact height of submarine landslide; is the pile outer diameter; The relationship between the drag coefficient and the Reynolds number is fitted as follows: （9） The Reynolds number for non-Newtonian fluids is often expressed as: （10） in: is the shear rate of the mud near the pile; Debris flow viscosity; is the shear stress; the relationship between fluid shear stress and shear rate, the rheological curve expression is obtained: （11） In the formula, ; Step S14: superposition of environmental loads: Different environmental loads are equivalent to horizontal loads and bending moments acting at the same height, and submarine landslides are applied to the pile side by uniformly distributing loads; Step S2, establishing a pile-soil interaction model under complex horizontal loads, specifically includes the following steps: Step S21, single pile model: To solve the pile displacement and pile stress under the horizontal load of the pile head, a microelement with a length of dz of the horizontally loaded pile is selected, and through the force balance of the bending moment, the following is obtained: （12） Where M is the bending moment; is the vertical load; is the shear force; according to beam theory: （13） In the formula is the bending stiffness of the pile; the two equations can be connected to obtain the control equation when the pile is horizontally loaded: （14） In the formula, is the soil reaction force; The single pile model is described by an isotropic elastic model, and the stress-strain relationship of the pile material is simulated by the Hooke theorem. Under three-dimensional conditions, the Hooke theorem is expressed as: (15) in, is Young's modulus; is Poisson’s ratio; Step S22: soil model: The soil model is described by the elastic-plastic model Mohr-Coulomb model, which includes cohesion intercept c, friction angle, Young's modulus E, Poisson's ratio and expansion angle Parameters; when mapped to the three-dimensional stress space, the Mohr-Coulomb criterion is decomposed into an irregular hexagonal pyramid, which forms a failure / yield envelope surface and determines the behavior of the soil; when the stress point is within the envelope surface, the material undergoes elastic deformation, and when the stress reaches the yield surface, the material undergoes plastic deformation; in step S22, it is assumed that the soil is in a linear elastic relationship before failure; Step S23, pile-soil contact model: The "contact pair" algorithm is used to simulate the interaction between pile and soil. The pile is set as the main contact surface, and the soil is set as the slave contact surface. The surface discretization method adopts "face-to-face contact", and the tracking of the relative movement of the contact surface adopts "limited sliding". The calculation accuracy is improved by constantly judging whether the master-slave surface is in contact; the pile surface is defined as the main surface, and the soil surface is defined as the slave surface; the normal contact adopts the "hard" contact option, and the tangential contact adopts the Coulomb contact. The interface shear stress calculation method is: (16) in, is the contact surface shear stress; is the normal effective stress; is the friction angle of pile-soil interface under critical state; Step S3: Stability evaluation of single pile foundation under complex horizontal loads: The numerical model was established by ABAQUS software. The linear elastic model was used for the single pile, and Young's modulus, Poisson's ratio and density were set. The Mohr-Coulomb constitutive model was used for the seabed soil, and the friction coefficient was set. The contact relationship between the single pile and the seabed was Coulomb friction contact, the surface of the single pile was the primary surface, and the surface of the seabed was the secondary surface. The interface between the single pile and the seabed was allowed to separate during the calculation process. The meshing was divided in sweep mode, and the mesh type was C3D8R. At the pile-soil interface, the mesh density increased, and the mesh density gradually decreased from the interface to the edge. Through mesh sensitivity analysis, the mesh size at the single pile-seabed interface was determined to be 0.1D. At the pile head, horizontal load and bending moment were applied to the single pile through the reference point, and a uniformly distributed load was applied. Submit the calculation and export the odb file for result analysis; export the pile displacement to obtain the pile displacement variation curve along the depth direction; differentiate the curve to obtain the pile rotation angle; the tolerance of the permanent cumulative rotation of a single pile at the mud line is 0.25°; the instability of the single pile can be determined by comparing the calculated rotation angle with the specification; derive the bending moment along the pile body through slicing to determine the most vulnerable point of the pile body; at the same time, the damage characteristics of the single pile and sediment can be determined through the stress cloud diagram of the single pile and sediment; By changing the sediment cohesion and internal friction angle, the pile displacement and rotation angle under different sediment strengths are calculated. Then, the relationship curve between the single pile displacement, rotation angle and sediment strength cohesion and internal friction angle is obtained through fitting.

2. The method for evaluating the stability of offshore wind power pile foundation under the coupling effect of submarine landslide and wind and wave impact load according to claim 1 is characterized in that In step S11, considering different ranges of wind speed, the impeller thrust coefficient is obtained by the following three different methods: When the cut-in wind speed ( ) Wind speed ( ) Rated wind speed ( ), the calculation formula of impeller thrust coefficient is as follows: (2) When rated wind speed ( ) Wind speed ( ) Cut-out wind speed ( ), the calculation formula of impeller thrust coefficient is as follows: （3） When the wind speed is low, assuming that the impeller thrust coefficient does not exceed 1, equation (2) will overestimate the thrust coefficient, so its value is limited to 1.

3. The method for evaluating the stability of offshore wind power pile foundation under the coupling effect of submarine landslide and wind and wave impact load according to claim 1 is characterized in that In step S11 It is a dimensionless number and is not affected by the specific size of the object. It is only related to the shape of the structure. The tower tube has a cylindrical cross-section and is taken as 0.5 after calculation.

4. The method for evaluating the stability of offshore wind power pile foundation under the coupling effect of submarine landslide and wind and wave impact load according to claim 1 is characterized in that In step S11 Take 10 m, wind shear index Take 0.

143.

5. The method for evaluating the stability of offshore wind power pile foundation under the coupling effect of submarine landslide and wind and wave impact load according to claim 1 is characterized in that , the cylinder under the wave and current load in step S12, take and .

6. The method for evaluating the stability of offshore wind power pile foundation under the coupling effect of submarine landslide and wind and wave impact load according to claim 1 is characterized in that , the resistance coefficient in step S12 For circular components, take 0.73.

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