Offshore semi-submersible foundation and design method therefor

Abstract

The present application relates to the technical field of marine engineering. Disclosed are an offshore semi-submersible foundation and a design method therefor. The offshore semi-submersible foundation comprises a lower floating body, the lower floating body being in the shape of a triangular ring; edges of the lower floating body are rounded; three corners of the lower floating body are respectively provided with upright columns, the three upright columns having the same size; the cross section of each upright column is in a round shape; a connecting rod is provided between any two upright columns, the connecting rods being horizontally arranged, and the joints of the connecting rods and the upright columns being chamfered. The offshore semi-submersible foundation of the present application ensures the stability and safety of structures in various marine environments and meets the requirements for economy, environmental friendliness, compliance and long-term operation.

Description

A semi-shallow foundation for offshore use and its design method

[0001] Cross-reference to related applications

[0002] This application is based on and claims priority to Chinese Patent Application No. 202411013119.2, filed on July 26, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of marine engineering technology, and in particular to a semi-shallow offshore foundation and its design method. Background Technology

[0004] Floating foundations are an inevitable choice for deep-sea operations, while semi-submersible foundations are a mature solution suitable for water depths of 50 meters and above. Semi-submersible foundations are primarily used to support offshore structures such as offshore wind farms and oil drilling platforms. This type of foundation involves part of the structure submerged on the seabed and part floating on the surface, aiming to combine the advantages of both to adapt to specific marine environments and economic needs. However, while there are many types of semi-submersible foundations available, many are designed to be different, lacking sufficient consideration for actual conditions and failing to ensure the stability and safety of the structure in various marine environments. They cannot simultaneously meet the requirements of economy, environmental protection, compliance, and long-term operation. Summary of the Invention

[0005] This application aims to at least partially address one of the technical problems in the related art.

[0006] Therefore, embodiments of this application propose a forward-designed and customized optimized semi-shallow foundation for offshore use.

[0007] The semi-shallow marine foundation of this application includes a lower floating body, which is triangular and annular in shape. The edges of the lower floating body are rounded. A column is provided at each of the three corners of the lower floating body. The three columns are of the same size and have a circular cross-section. A connecting rod is provided between any two columns. The connecting rod is horizontally arranged and the connection between the connecting rod and the column is chamfered.

[0008] The semi-shallow marine foundation of this application embodiment ensures the stability and safety of the structure in various marine environments, while meeting the requirements of economy, environmental protection, compliance and long-term operation.

[0009] The embodiments of this application also propose a design method for semi-shallow foundations at sea.

[0010] The design method of this application embodiment is used for the offshore semi-shallow foundation described in any of the above embodiments, and the design method includes:

[0011] Determine the boundary conditions of the pre-designated site for the semi-shallow offshore foundation;

[0012] The number of columns is determined based on the boundary conditions;

[0013] The type of the lower floating body is determined based on the boundary conditions and the number of columns;

[0014] Based on the boundary conditions, determine the diameter relationship between the columns, determine the cross-sectional shape of the columns, and determine the edge shape of the lower floating body;

[0015] The connection structure between the columns is determined based on the boundary conditions.

[0016] In some embodiments, the number of columns is three or four, corresponding to a three-column floating foundation and a four-column floating foundation, respectively. Under the condition that the static water stiffness is consistent and / or the displacement is consistent, the three-column floating foundation or the four-column floating foundation is selected by comprehensive judgment.

[0017] In some embodiments, the side column diameter of the four-column floating foundation is the same as the column diameter of the three-column floating foundation, keeping the hydrostatic stiffness of both consistent, and comparing their frequency domain response characteristics.

[0018] In some embodiments, the diameter of the side columns of the four-column floating foundation is larger than the column diameter of the three-column floating foundation, while keeping the displacement of both the column and column, and comparing their frequency domain response characteristics.

[0019] In some embodiments, the diameter of the side columns of the four-column floating foundation is the same as the diameter of the columns of the three-column floating foundation, and by increasing the size of the lower floating body of the four-column floating foundation, the hydrostatic stiffness and displacement of both are kept consistent, and the frequency domain response characteristics of the two are compared.

[0020] In some embodiments, the lower float is of the form of annular bypass or strut type, corresponding to annular bypass float foundation and strut type float foundation respectively. Under the condition that the static water stiffness is consistent and / or the displacement is consistent, the annular bypass float foundation or strut type float foundation is selected by comprehensive judgment.

[0021] In some embodiments, when sea conditions are poor and the wind-wave coupling effect is significant, the diameters of the columns are the same;

[0022] When the sea conditions are good, the diameter of the column where the tower is located is D1, and the diameter of the other columns is D2, and the relationship is satisfied: D1=(1.1~1.2)D2.

[0023] In some embodiments, the cross-sectional shape of the column is circular or polygonal, corresponding to a circular column floating foundation and a polygonal column floating foundation, respectively. Under the condition of consistent drainage volume, it is determined whether to select a circular column floating foundation or a polygonal column floating foundation.

[0024] In some embodiments, the edge shape of the lower float is arc-shaped or square, corresponding to a rounded corner float foundation and a right-angle float foundation, respectively. Under the condition of consistent displacement, the rounded corner float foundation or the right-angle float foundation is selected. Attached Figure Description

[0025] Figure 1 is a schematic diagram of a semi-shallow offshore foundation according to an embodiment of this application.

[0026] Figure 2 is a flowchart of the design method for a semi-shallow offshore foundation according to an embodiment of this application.

[0027] Figure 3 is a comparative schematic diagram of a three-column floating foundation and a four-column floating foundation according to an embodiment of this application.

[0028] Figure 4 is a comparative schematic diagram of the annular bypass floating body foundation and the strut-type floating body foundation according to embodiments of this application.

[0029] Figure 5 is a comparative schematic diagram of a constant-diameter column floating foundation and a variable-diameter column floating foundation according to an embodiment of this application.

[0030] Figure 6 is a comparative schematic diagram of a circular column floating foundation and a polygonal column floating foundation according to an embodiment of this application.

[0031] Figure 7 is a comparative schematic diagram of the rounded corner floating body foundation and the right-angle floating body foundation according to an embodiment of this application.

[0032] Reference numerals: 1. Lower float; 2. Column; 3. Connecting rod; 11. Three-column float foundation; 12. First four-column float foundation; 13. Second four-column float foundation; 14. Third four-column float foundation; 21. Annular bypass float foundation; 22. First strut-type float foundation; 23. Second strut-type float foundation; 31. Variable diameter column float foundation; 32. Circular column float foundation; 41. Polygonal column float foundation; 42. Rounded corner float foundation; 51. Right-angle float foundation; 52. Detailed Implementation

[0033] The embodiments of this application are described in detail below, with examples of these embodiments illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0034] The following describes an embodiment of the semi-shallow marine foundation of this application with reference to the accompanying drawings.

[0035] As shown in Figure 1, the semi-shallow marine foundation of this embodiment includes a lower floating body 1, which is triangular and annular with rounded edges. Columns 2 are provided at the three corners of the lower floating body 1, all three columns 2 being identical in size and having a circular cross-section. A connecting rod 3 is provided between any two columns 2, the connecting rod 3 being horizontally positioned, and the connection between the connecting rod 3 and the column 2 having a chamfered angle.

[0036] Understandably, the lower float 1, being an annular bypass, provides the primary connection strength between the columns 2, thus simplifying the connection structure between them. The large volume of the lower float 1 facilitates ballast adjustment. The structural design of the three columns 2 is simple in construction and relatively easy to build. The equal-diameter columns 2, the circular columns 2, and the rounded edges of the lower float 1 all contribute to improving the hydrodynamic performance of the foundation. The columns 2 are connected by a connecting rod 3, and the chamfered joint at the connection point disperses stress, improving the stability of the connection.

[0037] Therefore, the semi-shallow marine foundation of this application embodiment ensures the stability and safety of the structure in various marine environments, while meeting the requirements of economy, environmental protection, compliance and long-term operation.

[0038] The design method of a semi-shallow offshore foundation according to an embodiment of this application is described below with reference to the accompanying drawings.

[0039] As shown in Figure 2, the design method of this application embodiment is used for offshore semi-shallow foundations in any of the above embodiments. The design method includes:

[0040] S1: Determine the boundary conditions of the pre-designed site for a semi-shallow foundation at sea, including environmental parameters such as wind, waves, and soil erosion.

[0041] S2: Determine the number of columns based on the boundary conditions. There is no significant advantage or disadvantage in stability and hydrodynamic characteristics between a three-column and a four-column structural design. The choice depends on the developer's own capabilities (whether they have the capacity for independent manufacturing, hoisting, and transportation). If the requirements for hoisting equipment and schedule at the dock (the tower is installed on the middle column, and the dock needs a crane with corresponding capabilities) can be met, then a four-column structural design can be chosen; otherwise, a three-column structural design should be selected.

[0042] S3: Determine the type of the lower floating body based on the boundary conditions and the number of columns. Furthermore, the actual conditions of the wharf must be considered when making the selection. When the wharf is uncertain, a ring-shaped bypass structure for the lower floating body is preferable.

[0043] S4: Determine the diameter relationship between the columns based on the boundary conditions. When the sea conditions are poor and the wind-wave coupling effect is significant, choose a structural design with columns of equal diameter. When the sea conditions are good, a structural design with columns of unequal diameter can be chosen. Determine the cross-sectional shape of the columns. Choose either polygonal or circular columns based on local construction conditions. When the construction conditions and equipment are good, choose a circular column structural design. When the construction conditions are average, a polygonal column structural design can be chosen. Determine the edge shape of the lower floating body. When the current velocity is low, a right-angled edge can be chosen. When the current velocity is high, an arc-shaped (streamlined) edge can be chosen.

[0044] S5: Based on the boundary conditions, topology optimization technology is used to determine the connection structure between the columns. The optimization result is a chamfer at the connection between the connecting rod and the column to improve the structural stability.

[0045] Therefore, the design method of this application embodiment can design site-customized semi-shallow offshore foundations suitable for developers with their own resources.

[0046] In some embodiments, the given site boundary conditions are: a significant wave height of 13.35 m, a spectral peak period of 14.8 s, a flow velocity of 2.37 m / s, and a wind speed of 56.7 m / s.

[0047] As shown in Figure 3, step two of the design method in this embodiment of the application is as follows:

[0048] Comparing the differences between having three or four columns, for ease of description, the structural design with three columns is defined as a three-column floating foundation 11, and the structural design with four columns is defined as a four-column floating foundation. Under the condition of consistent static water stiffness and / or consistent displacement, a comprehensive judgment is made as to whether to choose a three-column floating foundation 11 or a four-column floating foundation.

[0049] Figure 3 shows the three-column floating foundation 11, the first four-column floating foundation 12, the second four-column floating foundation 13, and the third four-column floating foundation 14 from left to right and from top to bottom.

[0050] The side column diameters of the first four-column floating foundation 12 are the same as those of the columns of the three-column floating foundation 11, maintaining consistent hydrostatic stiffness. Their frequency domain response characteristics are compared. In the RAO (Rapid Orbit) of heave, roll, and yaw, the differences between the three-column floating foundation 11 and the first four-column floating foundation 12 are small. However, in the heave, roll, and pitch directions, the natural period of the first four-column floating foundation decreases, especially the heave period, which is closer to the frequency range with higher wave energy, leading to an increase in the heave response amplitude.

[0051] The diameter of the side columns of the second four-column floating foundation 13 is larger than the column diameter of the three-column floating foundation 11. In other words, the second four-column floating foundation 13 is essentially an increased column diameter on top of the first four-column floating foundation 12. This is to maintain the same displacement as the three-column floating foundation 11. The frequency domain response characteristics of the two are compared. From the frequency domain hydrodynamic analysis, the natural periods of heave, roll, and pitch are significantly reduced, especially the heave period, which is less than 16 seconds, closer to the frequency range with higher wave energy, leading to an increase in the heave response amplitude.

[0052] The diameter of the side columns of the third and fourth column floating foundation 14 is the same as the diameter of the columns of the three-column floating foundation 11. Furthermore, by increasing the size of the lower floating body of the four-column floating foundation—that is, by adding the size of the lower floating body to the first and fourth column floating foundation 12—the static stiffness and displacement of the third and fourth column floating foundation 14 are kept consistent with those of the three-column floating foundation 11. The frequency domain response characteristics of the two are compared. From the frequency domain hydrodynamic analysis, under the condition that the static stiffness and displacement are basically the same, the hydrodynamic characteristics of the two are not significantly different.

[0053] Therefore, from the perspective of hydrodynamic characteristics alone, there is no significant advantage or disadvantage between the three-column floating foundation 11 and the four-column floating foundation. However, considering that the four-column floating foundation has a poorer ability to resist the torsional force of oblique waves, it requires additional reinforcement, making the structure more complex and the construction more difficult; at the same time, since the wind turbine of the four-column floating foundation is located in the middle, the dock hoisting resources need to have a large hoisting distance while meeting the hoisting height and weight requirements, making the hoisting more difficult. Taking all the above factors into account, the three-column floating configuration is more suitable for the site of the design method of this application embodiment.

[0054] As shown in Figure 4, step three of the design method in this embodiment of the application is as follows:

[0055] Comparing the types of floats, annular bypass and strut type, for ease of description, the annular bypass structure is defined as an annular bypass float foundation 21, and the strut type structure is defined as a strut type float foundation. Under the condition of consistent static water stiffness and / or consistent displacement, a comprehensive judgment is made as to whether to choose the annular bypass float foundation 21 or the strut type float foundation.

[0056] Figure 4 shows, from left to right, the annular bypass floating body foundation 21, the first strut-type floating body foundation 22, and the second strut-type floating body foundation 23.

[0057] The first strut-type floating foundation 22 and the annular bypass floating foundation 21 have the same column diameter, maintaining the same hydrostatic stiffness. The changes in their hydrodynamic characteristics are compared. After adopting the strut, the natural heave period drops significantly to near the peak period of the wave spectrum, which will lead to an increase in the amplitude of the heave response, indicating that the increased floating mass of the annular bypass can improve the natural heave period.

[0058] The second strut-type floating body foundation 23 increases the size of the lower floating body on top of the first strut-type floating body foundation 22 to maintain the same hydrostatic stiffness and displacement as the second strut-type floating body foundation 23 and the annular bypass floating body foundation 21. After satisfying the conditions of equal stiffness and displacement, the second strut-type floating body foundation 23 can also meet the requirements of the floating body's motion cycle and provide additional heave damping.

[0059] Therefore, the ring bypass structure design provides the main connection strength between the columns, simplifies the connection structure between the upper columns, and the large volume of the lower floating body facilitates ballast adjustment.

[0060] As shown in Figure 5, the determination of the diameter relationship between the columns in step four of the design method of this application embodiment is as follows:

[0061] Comparing the advantages and disadvantages of equal or unequal column diameters, for ease of description, equal diameter columns are defined as equal-diameter floating foundations 31, and unequal diameter columns are defined as variable-diameter floating foundations 32. When sea conditions are poor and wind-wave coupling effects are significant, equal-diameter floating foundations are used. When sea conditions are good, variable-diameter floating foundations 32 are used, with the diameter of the column containing the tower (main column) being D1, and the diameters of the other columns (secondary columns) being D2, satisfying the relationship: D1 = (1.1~1.2)D2.

[0062] Figure 5 shows, from left to right, the equal-diameter column floating foundation 31 and the variable-diameter column floating foundation 32.

[0063] From the perspective of structural force transmission path and structural optimization, the diameter of the secondary column, which does not directly bear the wind turbine load, is reduced, while the diameter of the main column is increased to improve the structural strength performance of the floating body.

[0064] First, we evaluated the variable-diameter column floating foundation 32 from a hydrodynamic perspective, maintaining a consistent displacement, and designed three comparative schemes as shown in Table 1. The main column diameters were 14.5, 15.0, and 18.0 mm, respectively, and the secondary column diameters were 14.5, 14.2, and 12.4 mm. We conducted a frequency domain hydrodynamic analysis comparing the three schemes. The hydrostatic stiffness results show that as the secondary column diameter decreases, the roll hydrostatic stiffness also decreases, which has a significant adverse impact on stability in the roll direction.

[0065] Table 1

[0066] Comparing the RAO of sway and transverse motion under several typical wave angles, the results show that the RAO of sway and transverse motion is basically the same and is less affected by the difference in column diameter.

[0067] Comparing the natural period of yaw and roll motions at several typical wave angles, it can be seen that: the yaw motion RAO increases with the degree of difference in column diameter; the natural period of roll also increases with the degree of difference in column diameter, and the change in the roll period mainly comes from the decrease in the static stiffness of roll.

[0068] Comparing the RAO of heave and pitch motions under several typical wave angles, it can be seen that there is a significant coupling effect between heave and pitch motions. Especially for pitch motion, under wave angles where the heave response is significant, the pitch response caused by this coupling effect even exceeds the response amplitude corresponding to the natural pitch period, which is extremely detrimental to the seakeeping performance of the buoy.

[0069] As can be seen from the frequency domain response results of each degree of freedom above, for the design method of this application embodiment, the equal diameter column floating body foundation 31 has better hydrodynamic performance.

[0070] As shown in Figure 6, the fourth step of the design method in this embodiment of the application involves determining the cross-sectional shape of the column:

[0071] Comparing the differences between circular and polygonal cross-sectional shapes of the columns, for ease of description, a circular shape is defined as a circular column floating foundation 41, and a polygonal shape is defined as a polygonal column floating foundation 42. Under the condition of consistent displacement, determine whether to choose the circular column floating foundation 41 or the polygonal column floating foundation 42.

[0072] Figure 6 shows, from left to right, a circular column floating foundation 41 and a polygonal column floating foundation 42.

[0073] Neglecting the effects of viscous forces, the shape of the support column has almost no impact on the inertial forces of the floating body when the drainage volume is the same. Regarding the viscous forces, a comparison of the drag coefficients of the circular support column foundation 41 and the polygonal support column foundation 42 shows that the drag coefficient of the polygonal support column foundation 42 is more than twice that of the circular support column foundation 41. This will lead to a certain increase in the wave loads and current loads experienced by the floating support column, adversely affecting the seakeeping ability of the floating body and the design of the mooring system.

[0074] Therefore, for the design method of this application embodiment, the circular column floating body foundation 41 has better hydrodynamic performance.

[0075] As shown in Figure 7, the step four of the design method in this embodiment of the application involves determining the edge shape of the lower floating body:

[0076] Comparing the effects of curved or square edge shapes on the hydrodynamic performance of the float, for ease of description, curved edges are defined as rounded-corner float foundation 51, and square edges are defined as right-angle float foundation 52 (which can be understood as a float foundation without rounded corners). Under the condition of consistent displacement, determine whether to choose rounded-corner float foundation 51 or right-angle float foundation 52.

[0077] Figure 7 shows, from left to right, the rounded corner floating body foundation 51 and the right-angled floating body foundation 52.

[0078] Without considering the effect of viscous forces, except that the float of the right-angled float base 52 has a slightly larger added mass, the two are basically the same in terms of radiation damping and motion RAO response. This shows that when the displacement volume is the same, the rounded corners of the lower float have almost no effect on the inertial force of the float.

[0079] Regarding the viscous force component, a comparison of the drag coefficients of the rounded-corner floating base 51 and the right-angle floating base 52 reveals that the drag coefficient of the rounded-corner floating base 51 is more than 50% smaller than that of the right-angle floating base 52. This significantly improves the wave load and current load on the floating body, which is beneficial for the seakeeping of the floating body and the design of the mooring system.

[0080] Therefore, for the design method of this application embodiment, the rounded corner floating body foundation 51 has better hydrodynamic performance.

[0081] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0082] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0083] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between components; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0084] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0085] In this application, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0086] Although the above embodiments have been shown and described, it is understood that the above embodiments are exemplary and should not be construed as limiting the present application. Any changes, modifications, substitutions and variations made to the above embodiments by those skilled in the art are within the protection scope of the present application.

Claims

1. A semi-shallow offshore foundation, comprising a lower floating body, the lower floating body being triangular ring-shaped, edges of the lower floating body being rounded, three columns being respectively arranged at three corners of the lower floating body, the three columns being of the same size, the columns being circular in cross-sectional shape, a connecting rod being arranged between any two columns, the connecting rod being horizontally arranged, and the connecting rod and the columns being connected at an inclined angle.

2. A design method of a semi-shallow offshore foundation, the design method being used for the semi-shallow offshore foundation according to claim 1, the design method comprising: determining boundary conditions of a preset site of the semi-shallow offshore foundation; determining a number of the columns according to the boundary conditions; determining a type of the lower floating body according to the boundary conditions and the number of the columns; determining a diameter relationship between the columns, a cross-sectional shape of the columns, and an edge shape of the lower floating body according to the boundary conditions; and determining a connecting structure between the columns according to the boundary conditions. The number of the columns is three or four, corresponding to a three-column floating body foundation and a four-column floating body foundation respectively, and the three-column floating body foundation or the four-column floating body foundation is selected by comprehensively judging under the condition that hydrostatic stiffness is consistent and / or displacement is consistent. The edge column diameter of the four-column floating body foundation is the same as the column diameter of the three-column floating body foundation, the hydrostatic stiffness of the two is kept consistent, and the frequency domain response characteristics of the two are compared. The edge column diameter of the four-column floating body foundation is greater than the column diameter of the three-column floating body foundation, the displacement of the two is kept consistent, and the frequency domain response characteristics of the two are compared. The edge column diameter of the four-column floating body foundation is the same as the column diameter of the three-column floating body foundation, and the hydrostatic stiffness and the displacement of the two are kept consistent by increasing the size of the lower floating body of the four-column floating body foundation, and the frequency domain response characteristics of the two are compared. The type of the lower floating body is ring bypass or strut type, corresponding to a ring bypass floating body foundation and a strut type floating body foundation respectively, and the ring bypass floating body foundation or the strut type floating body foundation is selected by comprehensively judging under the condition that hydrostatic stiffness is consistent and / or displacement is consistent.

3. A method of designing an offshore semi-pile foundation according to claim 2, wherein, When the sea condition is poor and the wind-wave coupling effect is significant, the diameters of the columns are the same according to claim 2.

4. A method of designing an offshore semi-pile foundation according to claim 3, wherein, When the sea condition is good, the diameter of the column where the tower is located is D1, the diameters of the other columns are D2, and the relationship D1=(1.1-1.2)D2 is satisfied.

5. The method of designing an offshore semi- shallow foundation according to claim 3, wherein, The cross-sectional shape of the column is circular or polygonal, corresponding to a circular column floating body foundation and a polygonal column floating body foundation respectively, and the circular column floating body foundation or the polygonal column floating body foundation is selected by judging under the condition that displacement is consistent.

6. The method of designing an offshore semi- shallow foundation according to claim 3, wherein, The edge shape of the lower floating body is arc-shaped or square-shaped, corresponding to a rounded corner floating body foundation and a right-angle floating body foundation respectively, and the rounded corner floating body foundation or the right-angle floating body foundation is selected by judging under the condition that displacement is consistent.

7. The method of designing an offshore semi- shallow foundation according to claim 2, wherein, ​ ​ ​ 9. The method of designing an offshore semi- shallow foundation according to claim 2, wherein, ​ 10. The method of designing an offshore semi- shallow foundation according to claim 2, wherein, ​

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