A method for evaluating multi-directional coupling bearing capacity of suction bucket foundation considering associated plastic flow of marine clay

Abstract

The present application relates to a kind of suction bucket foundation multidirectional coupling bearing capacity evaluation method considering associated plastic flow of marine clay.The method is based on the associated flow characteristics of undrained clay plastic material,firstly, the numerical model of static loading of suction bucket foundation in undrained clay is established;Secondly, by displacement probe method, the bearing capacity of foundation under different proportional displacement loading is obtained;Finally, based on the rule of associated plastic flow, the same as the outer normal of bearing capacity envelope surface and displacement increment direction vector is used as the criterion, the bearing capacity envelope surface of suction bucket foundation (i.e. the maximum bearing capacity of foundation under multidirectional coupling load) is determined.Compared with prior art, the present application has the effect of considering the associated plastic flow of undrained clay plastic material, the method has clear physical meaning and is easy to implement, and can solve the problem that the existing model cannot accurately evaluate the bearing capacity envelope surface.

Description

Technical Field

[0001] This invention relates to the field of suction bucket foundation bearing characteristics, and in particular to a method for evaluating the multi-directional coupled bearing capacity of suction bucket foundations that takes into account the associated plastic flow of marine clay. Background Technology

[0002] In the construction of offshore wind farms, the foundation of the offshore wind turbine is one of the most important components. Unlike the terrestrial environment, the external loads in the marine environment are more complex, such as wave loads and wind loads, and often have cyclic loading characteristics. In addition, the foundation soil in the marine environment, especially in the coastal areas of my country, often has a deep layer of saturated soft clay, with extremely low foundation bearing capacity. Furthermore, the installed capacity of a single offshore wind turbine is constantly increasing, and the bearing capacity required by the design is also constantly increasing. This brings many design problems and also brings considerable expenses to the project.

[0003] Among various foundation types, suction bucket foundations, due to their advantages of not requiring piling, ease of transportation, rapid construction, and economical installation, have been widely used in many projects, leading to the development of multi-bucket jacket foundations. For bucket foundations, the suction buckets are generally large, with a relatively small length-to-diameter ratio. In contrast, the suction buckets in multi-bucket jacket foundations are often smaller, and their length-to-diameter ratio can be set larger. More importantly, while the loads transmitted from the superstructure to a typical suction bucket foundation are primarily horizontal, for multi-bucket foundations, under certain combinations of vertical loads and bending moments, a single suction bucket may experience a coupled effect of uplift and horizontal loads. If one suction bucket foundation fails or becomes unstable, the entire multi-bucket jacket foundation will be significantly affected, potentially leading to successive failures or instability. To fully understand and address these issues, we need a thorough understanding of suction bucket foundations and to establish methods for assessing the bearing capacity of suction bucket foundations under coupled loads.

[0004] The bearing characteristics of suction bucket foundations under vertical-horizontal coupled loads, i.e., the shape of the bearing capacity envelope, is crucial for the application and understanding of suction bucket foundations. To address this issue, since laboratory or field tests require significant time and effort, numerical simulation is now widely used to assist in research. However, existing numerical methods lack consideration for the mechanical properties of marine clay, lack clear physical support, and lack effective methods for evaluating the accuracy of the bearing capacity envelope. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art by providing a method for evaluating the multi-directional coupling bearing capacity of suction bucket foundations that considers the associated plastic flow of marine clay.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] First, a static loading numerical model of a suction bucket foundation in marine clay is established using finite element numerical simulation software (such as ABAQUS and ANSYS). Second, the displacement probe method is applied to perform numerical simulation to determine the multi-directional coupled bearing capacity of the suction bucket foundation under different displacement direction vector conditions. Finally, the bearing capacity envelope equation is fitted to the numerical simulation analysis data. Using the associated plastic flow law, the outward normal vector of the envelope is compared with the displacement increment direction vector given in the numerical simulation. The envelope equation is changed until the above vector directions are consistent. This process is used to evaluate the accuracy of the bearing capacity envelope.

[0008] Based on the above technical solutions, the present invention may also employ the following further technical solutions, or combine these further technical solutions:

[0009] The specific content of step S2 is as follows:

[0010] Given a specific proportional displacement loading condition (i.e., displacement direction vector) ,in For vertical displacement, (For horizontal displacement), based on this direction vector, a sufficiently large displacement with a given direction is applied to the suction bucket foundation in the numerical simulation to ensure that the calculation results obtained after the numerical simulation can represent the vertical (V)-horizontal (H) multi-directional coupled bearing capacity under this displacement direction condition. After this step is completed, the above operation needs to be repeated until the calculation results have a certain representativeness, and the calculation results are normalized and analyzed.

[0011] The specific content of step S3 is as follows:

[0012] Based on a given envelope equation, the VH bearing capacity data processed in step S2 is fitted. The accuracy of the bearing capacity envelope equation is evaluated according to the correlation flow law of plastic materials. That is, the outer normal vector of the envelope equation is compared with the displacement increment direction vector. When the above vector directions are consistent, the accurate bearing capacity envelope equation under multi-directional coupled load is obtained; otherwise, this step is repeated.

[0013] In step S2, the numerical simulation results are normalized. This method considers the heterogeneity of the soil layers and requires the numerical simulation results to be combined with the mean undrained shear strength of the soil layers for dimensionless processing. Then, each set of data is proportionally scaled to the interval [0,1], that is:

[0014] The formula for dimensionless processing of numerical simulation results is:

[0015]

[0016]

[0017] Where H represents the horizontal component of the maximum bearing capacity obtained from each group of tests in step S2; V represents the vertical component of the maximum bearing capacity obtained from each group of tests in step S2. This represents the dimensionless horizontal bearing capacity. This represents the dimensionless vertical bearing capacity. Indicates the outer diameter of the suction bucket base; Indicates the length of the suction bucket base; This represents the average undrained shear strength of the soil within the depth range of the suction bucket insertion point.

[0018] The formula for normalizing numerical simulation results is:

[0019]

[0020]

[0021] in This refers to the normalized horizontal bearing capacity. The normalized vertical bearing capacity; Indicated as in (That is, the horizontal bearing capacity obtained from this set of numerical simulations of pure horizontal loading) Indicated as in The vertical bearing capacity obtained from this set of numerical simulations (i.e., pure vertical loading).

[0022] In step S3, the associated flow law represents the internal friction angle of the saturated undrained marine clay. Meanwhile, saturated undrained clay can be considered an ideal elastic-plastic body. Therefore, the plastic equipotential surface of the soil is equivalent to the bearing capacity envelope of the soil, i.e.

[0023]

[0024] in These are the plastic equipotential surfaces of the soil. This is the bearing capacity envelope of the soil.

[0025] At this point, the normal vector of the soil at a point on the bearing capacity envelope is equal to the increment of plastic strain, which is also the increment of displacement.

[0026]

[0027] In the formula , These represent the vertical and horizontal displacement increments of the soil. , For the vertical and horizontal plastic strain increments of the soil; , These are the vertical and horizontal stress vectors of the soil.

[0028] In step S3, the main method for evaluating the accuracy of the bearing capacity envelope obtained from the test results is to compare the outer normal vector of the envelope equation with the displacement increment direction vector. The more consistent these two sets of vectors are, the more accurate the bearing capacity envelope obtained from the test results is, and the closer it is to the true bearing capacity envelope. Conversely, the greater the difference between these two sets of vectors, the less accurate the bearing capacity envelope obtained from the test results is, and the greater the difference between it and the true bearing capacity envelope.

[0029] The beneficial effects of this invention are:

[0030] This invention provides a method for evaluating the multi-directional coupled bearing capacity of suction bucket foundations considering the associated plastic flow of marine clay, which has the following beneficial effects:

[0031] 1. The evaluation model of this invention supplements the process of obtaining the bearing capacity envelope of the model after obtaining the bearing capacity under coupled load using the displacement probe method and then evaluating it.

[0032] 2. The evaluation model of this invention fully considers the physical and mechanical properties of marine clay and applies the associated plastic flow law to theoretically establish the relationship between the outer normal vector of the bearing capacity envelope equation and the direction of displacement increment. The model is more physically meaningful in theory and fills the gap in the existing methods that cannot evaluate the accuracy of the envelope surface. Attached Figure Description

[0033] Figure 1 This is a flowchart of the method of the present invention;

[0034] Figure 2 The diagram shows the relationship between horizontal and upward pull-out bearing capacity after processing in each group of tests.

[0035] Figure 3 This is a schematic diagram of the fitted curve;

[0036] Figure 4 The results of the model evaluation are displayed in graphical form. Detailed Implementation

[0037] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0038] This embodiment uses a numerical simulation test conducted in the finite element numerical simulation software ABAQUS2021 environment to demonstrate the applicability of the evaluation model of this invention and the specific calculation and analysis steps. This is an example of the uplift and horizontal bearing characteristics of a suction bucket foundation in saturated undrained clay.

[0039] like Figure 1 As shown, this invention relates to a method for evaluating the multi-directional coupled bearing capacity of suction bucket foundations considering associated plastic flow in marine clay. The method includes the following steps:

[0040] Step 1: Establish a numerical model of static loading on a suction bucket foundation in marine clay;

[0041] A finite element method (FEM) numerical simulation software was used to establish the relevant model. The model should employ the undrained total stress analysis method to simplify the calculations. Considering the symmetry of the suction bucket foundation and the symmetry of the horizontal-uplift coupled load, only half of the finite element calculation model needs to be established. The entire model consists of two parts: the suction bucket foundation and the soil within the calculation range. The suction bucket foundation is designed with steel, whose material strength is much higher than that of the foundation soil. Therefore, the material can be considered to use a linear elastic constitutive model, with an elastic modulus of... Poisson's ratio The soil surrounding the foundation is saturated soft clay. An ideal elastoplastic constitutive model based on the Mohr-Coulomb yield criterion is adopted, with a Poisson's ratio of... The soil is approximated as incompressible. The internal friction angle of the soil... The shear strength of the soil is set to increase linearly with depth. The undrained shear strength of the soil at the mud surface is... Its linearly increasing rate The depth below the mud surface is The undrained shear strength of the soil element is:

[0042]

[0043] Assuming the deformation modulus of soft soil is approximately proportional to its undrained shear strength, and neglecting the influence of foundation stress level on the deformation modulus, we assume... The overall deformation modulus is relatively small. While the value of the deformation modulus has a relatively small impact on the bearing capacity, it has a significant impact on the structural displacement that occurs when the bearing capacity is reached. Therefore, a larger displacement needs to be considered when applying displacement loads. The entire model uses three-dimensional eight-node linear hexahedral reduced integral elements. It is worth noting that since the self-weight of the soil and suction bucket foundation has a relatively small impact on the bearing capacity, this model ignores the self-weight of the soil and suction bucket foundation, and also ignores the influence of ground stress on the foundation bearing capacity.

[0044] Assuming that the soil disturbance at the contact boundary between the foundation and the soil is small and the interface is completely rough, the maximum frictional force at the pile-soil interface is equal to the soil strength, meaning that no cracks will be generated between the soil and the foundation during the test.

[0045] Step 2: Conduct tests using the displacement probe method to determine the bearing capacity of the foundation under different displacement conditions;

[0046] According to the displacement probe method, the process of solving the bearing capacity of a suction bucket foundation under a specified displacement condition is as follows:

[0047] First, a representative reference point in space is selected as the point of application of the external force on the suction bucket foundation. In this example, it is the center point of the top surface of the suction bucket foundation. Second, according to the given displacement direction vector, a large displacement is applied to this point of application in proportion to ensure that the force applied to the foundation under this displacement is close enough to the bearing capacity. Finally, numerical simulation is performed to obtain the bearing capacity of the suction bucket foundation under this displacement condition.

[0048] Therefore, by changing the given displacement direction vector, the bearing capacity of the suction bucket foundation under a series of different displacement conditions can be obtained, as shown in the table below.

[0049] Table 1 Numerical simulation of bearing capacity under different displacement direction vectors

[0050] in Let represent the direction vector given in this experiment, where Indicates the vertical direction, with upward pulling being the positive direction. Indicates the horizontal direction. In the experiment, it specifically refers to the ratio of the components of the actual displacement of the point of application in these two directions; This represents the horizontal component of the final bearing capacity obtained in this test; This represents the component of the final load-bearing capacity obtained in this test in the upward direction. It should be noted that... and These two sets of tests are indispensable because they represent the load-bearing capacity in two single directions: horizontal and vertical.

[0051] Step 3: Process and analyze the experimental data;

[0052] The experimental data obtained by the displacement probe method need to be normalized to ensure the accuracy and standardization of the model.

[0053] The formula for dimensionless processing of experimental data is:

[0054]

[0055]

[0056] in This represents the dimensionless horizontal bearing capacity. This represents the dimensionless vertical bearing capacity. Indicates the outer diameter of the suction bucket base; Indicates the length of the suction bucket base; This represents the average value of the undrained shear strength of the soil within the depth range of the suction bucket's insertion.

[0057] The formula for normalizing experimental data is:

[0058]

[0059]

[0060] in This refers to the normalized horizontal bearing capacity. The normalized vertical bearing capacity; Indicated as in The horizontal bearing capacity obtained from this set of tests; Indicated as in The vertical bearing capacity obtained from this set of tests.

[0061] After performing the above processing on all the data, the final data is as follows: Figure 2 As shown.

[0062] Step 4: Evaluate the form of the load-bearing capacity envelope;

[0063] To evaluate the form of the bearing capacity envelope, it is necessary to find the true bearing capacity envelope. Below, we consider the soil flow criterion, and we can derive the relationship between the plastic displacement increment and stress components of the soil on the failure envelope, i.e.

[0064]

[0065] in This represents the increment of plastic displacement of the soil on the failure envelope surface; denoted by , which represents the stress components of the soil on the failure envelope; g is the plastic equipotential surface of the soil. This is the proportionality coefficient.

[0066] As for For saturated soft clay foundations, the yield line described by the Mohr-Coulomb failure criterion appears as a straight line parallel to the coordinate axes on a plane. In this case, the soil can be considered to obey the Tresca yield criterion. However, when assuming the soil is an ideal elastoplastic body, we can obtain that the plastic equipotential surface of the soil is equivalent to the failure envelope surface of the soil, i.e.

[0067]

[0068] in This represents the failure envelope of the soil.

[0069] At this point, it is not difficult to determine that the stress vector at a point on the failure envelope of the soil is in the same direction as the plastic strain increment, i.e., the displacement increment, i.e., under the specific premises of this invention.

[0070]

[0071] In the formula , These represent the vertical and horizontal displacement increments of the soil. , For the vertical and horizontal plastic strain increments of the soil; , These are the vertical and horizontal stress vectors of the soil.

[0072] Therefore, the method of this invention for evaluating the similarity between the bearing capacity envelope obtained from the numerical simulation and the actual bearing capacity envelope lies in comparing whether the outward normal vector of the bearing capacity envelope and the displacement increment direction vector are consistent. If they are consistent, it can be considered that the bearing capacity envelope obtained from the model simulation is consistent with the actual bearing capacity envelope. The greater the difference between the two, the greater the gap between the bearing capacity envelope obtained from the model simulation and the actual bearing capacity envelope, and the less accurate the bearing capacity envelope obtained from the simulation is.

[0073] Therefore, it is first necessary to assume the bearing capacity envelope of the suction bucket foundation under the action of the horizontal-uplift coupled load. In this case, we will use the most widely used and simplest form:

[0074]

[0075] in All of these are fitting parameters.

[0076] Substitute the normalized data obtained in step 3 into the formula to perform curve fitting, and the resulting curve is as follows. Figure 3 As shown, its function expression is

[0077]

[0078] After obtaining the expression for the fitted curve, it is necessary to calculate the outward normal vector corresponding to each point, which is the reciprocal of the slope of the corresponding point on the fitted curve:

[0079]

[0080] By comparing the above vector with the displacement increment direction vector corresponding to each point, we obtain... Figure 4 . Figure 4The blue arrow represents the direction vector of the displacement increment at that point, and the yellow arrow represents the outward normal direction vector at that point on the envelope equation. Under ideal conditions, if all sets of vectors are consistent, the actual curve will be the same as the ideal curve. The greater the difference between the two, the more the envelope surface obtained from the experimental data deviates from reality. Figure 4 The evaluation results of the load-bearing capacity envelope in this example are presented in a more intuitive and approximate manner.

[0081] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and such modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for evaluating the multi-directional coupled bearing capacity of suction bucket foundations considering associated plastic flow in marine clay, characterized in that, Includes the following steps: S1: Establish a numerical model of static loading of suction bucket foundation in marine clay; S2: Numerical simulation using the displacement probe method was performed to determine the multi-directional coupling bearing capacity of the suction bucket foundation under different displacement direction vector conditions; S3: Fit the bearing capacity envelope equation to the numerical simulation analysis data, use the associated plastic flow law, compare the outward normal vector of the envelope with the displacement increment direction vector given in the numerical simulation, change the envelope equation until the above vector directions are consistent, and use this process to evaluate the accuracy of the bearing capacity envelope.

2. The method for evaluating the multi-directional coupled bearing capacity of a suction bucket foundation considering associated plastic flow of marine clay, as described in claim 1, is characterized in that... The specific content of step S2 is as follows: Given a specific proportional displacement loading condition (i.e., displacement direction vector) ,in For vertical displacement, (For horizontal displacement), based on this direction vector, a sufficiently large displacement with a given direction is applied to the suction bucket foundation in the numerical simulation to ensure that the calculation results obtained after the numerical simulation can represent the vertical (V)-horizontal (H) multi-directional coupled bearing capacity under this displacement direction condition. After this step is completed, the above operation needs to be repeated until the calculation results have a certain representativeness, and the calculation results are normalized and analyzed.

3. The method for evaluating the multi-directional coupled bearing capacity of a suction bucket foundation considering associated plastic flow of marine clay, as described in claim 1, is characterized in that... The specific content of step S3 is as follows: Based on a given envelope equation, the VH bearing capacity data processed in step S2 is fitted. The accuracy of the bearing capacity envelope equation is evaluated according to the correlation flow law of plastic materials. That is, the outer normal vector of the envelope equation is compared with the displacement increment direction vector. When the above vector directions are consistent, the accurate bearing capacity envelope equation under multi-directional coupled load is obtained; otherwise, this step is repeated.

4. The method for evaluating the multi-directional coupled bearing capacity of a suction bucket foundation considering associated plastic flow of marine clay, as described in claim 2, is characterized in that... In step S2, the numerical simulation results are normalized. This method considers the heterogeneity of the soil layers and requires the numerical simulation results to be combined with the mean undrained shear strength of the soil layers for dimensionless processing. Then, each set of data is proportionally scaled to the interval [0,1], that is: The formula for dimensionless processing of numerical simulation results is: Where H represents the horizontal component of the maximum bearing capacity obtained from each group of tests in step S2; V represents the vertical component of the maximum bearing capacity obtained from each group of tests in step S2. This represents the dimensionless horizontal bearing capacity. This represents the dimensionless vertical bearing capacity. Indicates the outer diameter of the suction bucket base; Indicates the length of the suction bucket base; This represents the average undrained shear strength of the soil within the depth range of the suction bucket insertion point. The formula for normalizing numerical simulation results is: in This refers to the normalized horizontal bearing capacity. The normalized vertical bearing capacity; Indicated as in (That is, the horizontal bearing capacity obtained from this set of numerical simulations of pure horizontal loading) Indicated as in The vertical bearing capacity obtained from this set of numerical simulations (i.e., pure vertical loading).

5. The method for evaluating the multi-directional coupled bearing capacity of a suction bucket foundation considering associated plastic flow of marine clay, as described in claim 3, is characterized in that... In step S3, the associated flow law represents the internal friction angle of the saturated undrained marine clay. Meanwhile, saturated undrained clay can be considered an ideal elastic-plastic body. Therefore, the plastic equipotential surface of the soil is equivalent to the bearing capacity envelope of the soil, i.e. in These are the plastic equipotential surfaces of the soil. This is the bearing capacity envelope of the soil. At this point, the normal vector of the soil at a point on the bearing capacity envelope is equal to the increment of plastic strain, which is also the increment of displacement. In the formula , These represent the vertical and horizontal displacement increments of the soil. , For the vertical and horizontal plastic strain increments of the soil; , These are the vertical and horizontal stress vectors of the soil.

6. The method for evaluating the multi-directional coupled bearing capacity of a suction bucket foundation considering associated plastic flow of marine clay, as described in claim 3, is characterized in that... In step S3, the main method for evaluating the accuracy of the bearing capacity envelope obtained from the test results is to compare the outer normal vector of the envelope equation with the displacement increment direction vector. The more consistent these two sets of vectors are, the more accurate the bearing capacity envelope obtained from the test results is, and the closer it is to the true bearing capacity envelope. Conversely, the greater the difference between these two sets of vectors, the less accurate the bearing capacity envelope obtained from the test results is, and the greater the difference between it and the true bearing capacity envelope.

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