Analysis of Bearing Performance of Monopile and Single Suction Bucket Foundation for Offshore Wind Power under Horizontal Load
In conjunction with the geological conditions of the East China Sea, the bearing performance of monopile and single-suction bucket foundations is compared and analyzed in shallow and deep-sea conditions under static horizontal loads. Furthermore, the statistical data of wind and wave from 2010 to 2020 in the East China Sea were tabulated into amplitude curves applied to the two foundations in the form of dynamic loads, and the bearing performances of the two foundations under dynamic loads were analyzed. The results show that the typical suction bucket foundation for a wind turbine currently designed in the shallow sea is destabilized under static horizontal loads, while the pile foundation is more stable; both foundations are stable in the deep-sea area. However, the suction bucket foundation displacement is less than the pile foundation. Under dynamic loading, the maximum displacement of monopile in the shallow sea was 127 mm. The maximum displacement of the suction bucket foundation was 434 mm, and the foundation was unstable. Both foundations are stable in deep-sea conditions, and the maximum displacement of the pile foundation is 1.4 times the maximum displacement of the suction bucket. Considering the difficulty construction in the deep sea, it is recommended to use suction bucket foundations.
With continuous economic improvement, the annual growth of energy demand has accelerated, and the calls for breakthroughs in key core technologies and large-scale utilization in the field of marine energy are increasing . Currently, China’s offshore wind power is relatively trailing compared to foreign leaders in the construction of offshore wind power. Some core key technologies need to be addressed and widely applied. Ensuring the stability of foundations for offshore wind turbines is one of the critical technologies that needs to be addressed.
The main loads on a foundation for offshore wind power are wind, waves, ocean currents, and the superstructure’s self-weight. The horizontal loads have the most severe impact on the structure. The structure is tilted or twisted by the horizontal load, and the wind turbine cannot be used normally. A higher horizontal load can even cause the overall offshore wind power structure to overturn. In applying pile and suction bucket foundations, the economic advantages and disadvantages of suction bucket foundations have been compared with those of pile foundations in the literature [2–6], and suction bucket foundations are more suitable for the development of offshore wind power applications overall. In terms of wind power foundation bearing performance, some scholars have used finite element software to simulate various bearing performances of suction buckets. For example, Li-qiang et al.  studied the calculation method of suction bucket bearing capacity under different vertical and horizontal loads. Chen et al.  further studied the relationship between vertical loads, horizontal loads, and bending moment by the radial slip test. Luo et al.  studied the force state of the suction bucket under dynamic loads. Luo and Feng  studied the influence of a horizontally inclined load and a vertically inclined load on the displacement of suction buckets under sandy soil conditions. Yang  assessed the impact of offshore earthquakes on suction buckets. Zheng et al.  studied the bearing characteristics of a monopile after vertical loads were applied. According to Kong et al.  under the action of horizontal cyclic load, the maximum point of pile bending moment is located in shallow soil. Liang et al.  proposed using the - curve method to predict the lateral load-bearing capacity of piles through finite element simulations.
Several other scholars also simulated the stability of wind power foundations through indoor experiments. For example, Luo et al.  studied the horizontal ultimate bearing capacity through the change characteristics of the soil around the suction bucket foundation under cyclic loading. Zhang  studied the pullout ability of the offshore suction buckets under loading. Xie et al.  studied the relationship between the penetration depth and the negative penetration pressure during the installation of suction buckets. Zhang et al.  confirmed that the previous single pile specification is relatively conservative through indoor experiments. Kim et al.  determined that the bearing capacity of HBF under vertical load and combined load is considerably higher than that of traditional suction bucket SBF. Lian et al.  studied the difference between the internal and external earth pressures acting on the bucket wall. Kim J.H. and Kim D.S.  studied changes in the soil state inside the bucket due to the suction device. Karasev et al.  found that preapplying vertical loads could reduce the lateral displacement at the top of piles under horizontal loads. Tak Kim et al.  proposed modified parameters of the given - curve to predict the lateral bearing capacity of piles under different construction methods through field experiments.
Other scholars have further studied the stability of wind power foundations through indoor experiments combined with numerical simulations. For example, Ahn et al.  and Wang and Cheng  conduct numerical simulations and model tests to study the ultimate bearing capacity of the suction bucket foundation under vertical loads, horizontal loads and bending moments, and combinations of these loads. Zhu et al.  studied the bearing capacity, failure mechanisms, and load-displacement response of the suction bucket foundation under cyclic loads caused by wind, waves, and water currents. Wang et al.  obtained the soil layer parameters around the offshore piles through indoor static load experiments and back-calculation simulations using the finite element software. In terms of theory, Fu et al.  obtained an analytical solution for the horizontal vibration of ocean high-pile foundations by establishing equations and considering the influence of hydrodynamic pressure on the pile body. Zhu and Fu  analyzed the calculated length used to evaluate instability more accurately based on the method. Lei and Wei  proposed a method to approximately calculate the horizontal dynamic impedance of a single pile by equivalent layered soil to uniform soil. Li et al.  proposed the analytical expression of the foundation bearing capacity of the suction bucket foundation through theoretical derivations. Yao et al.  put forward the calculation method of horizontal bearing capacity of skirt suction bucket in practical engineering. Zhu et al.  proposed a method to calculate the overturning moment bearing capacity of single bucket foundation based on deformation by determining the bearing capacity of suction bucket through the limit state.
Several studies have been conducted on the effects of horizontal loads on pile and suction bucket foundations. However, it is necessary to study the differences in pile and suction bucket foundation bearing capacities under the horizontal and dynamic loads caused by wind and waves in shallow and deep-sea conditions. Two types of foundations typically designed in China, monopile and suction buckets, have been selected to compare and analyze the bearing performance in shallow and deep-sea conditions under horizontal loads using numerical modeling. Furthermore, the monthly wind and wave data for the same period in the East China Sea from 2010 to 2020 were used to create an amplitude curve and applied to the two types of foundations in the form of dynamic loads to compare and analyze the bearing performance of the two types of foundations in the shallow and deep sea under dynamic loads. This work provides a technical reference for promoting and applying offshore wind power.
1.1. Comparative Analysis of Bearing Performance of Two Types of Foundations in the Shallow Sea
1.1.1. Model and Related Parameters
Based on the common force model of offshore wind turbines in the shallow sea of China’s east coast  as the prototype, the three-dimensional finite element analysis models of single pile and single-suction bucket foundations under a horizontal load (Figure 1) are established, respectively.
The two models were created as follows:
1.1.2. Model 1: Monopile Foundation
(1)Establish a single pile foundation with a pile radius of 3.0 m(2)Consider the influence of the boundary. The radial size of the soil model was 20 times the pile diameter. The soil body of the single pile foundation was 120.0 m radially. The soil body thickness was 96 m, of which the thickness of the silty clay was 8 m, the thickness of the sand was 4 m, and under the sandy soil was granite. The physical and mechanical parameters of piles and rocky soil are shown in Table 1(3)Create the model. The pile foundation adopted the DP constitutive model. The friction angle was set to 33.1°, the flow stress ratio was 1, and the expansion angle was 0°. The hardening behavior was compression, and the mesh used a C3D8R solid model, all of which were hexahedral elements. The soil mesh model also used the C3D8R solid model. The normal direction of the contact surface between the pile foundation and the soil was set to “hard” contact, and separation after contact was allowed. The master and slave contact surfaces were not allowed to invade each other. The tangential behavior was set to penalty contact, and the friction coefficient was 0.2. A horizontal load was applied, as shown in Figure 1. The finite element calculation was performed to obtain the deformation of the single pile under the ultimate static load of normal use. The bearing capacity of the pile structure was then analyzed. In order to facilitate the application of horizontal loads, a reference point was added at the center of the pile body as the load acting position. The reference point and the pile body were rigidly restrained (Figure 2)
1.1.3. Model 2: Single Suction Bucket Foundation
(1)Establish a single bucket foundation model with a bucket wall thickness of 0.3 m and an outer diameter of the bucket foundation of 12.0 m(2)Consider the influence of the boundary. The radial size of the soil model was 20 times the bucket diameter. The radius of the soil of the single bucket foundation was 240.0 m, and the thickness of the soil was set to 80.0 m. The physical and mechanical parameters of bucket foundation and soil are shown in Table 1(3)Create the model. The suction bucket body structure also adopted the DP constitutive model. The friction angle was set to 33.1°. The flow stress ratio was 1. The expansion angle was 0°. The hardening behavior was compression. The mesh model used the C3D8R solid model, all of which were composed of hexahedral units. Compared with a single pile foundation under the same conditions, the soil thickness setting was the same as the pile foundation soil setting. The body of the suction bucket was also made up of upper and lower parts. The upper part was the top structure of the bucket, with a thickness of 1.0 m. The burial depth of the bucket was 6 m. According to Figure 1, a reference point was also added at the center of the bucket at the height of 20.7 m as the load position, the reference point, and the bucket were rigidly constrained (Figure 3)
1.2. Applied Load
This paper used two types of loads applied to both foundations. The first load (Figure 1) was a static horizontal load of 4MN applied to the two foundations in the shallow sea area. This load compares and analyzes the displacement and torsion characteristics of the two foundations.
Considering that both foundations are subjected to dynamic loads such as wind and waves throughout the year, the second load used the maximum wind and wave data at the Beishuang station in the shallow sea of the East China Sea from 2010 to 2020 . Wind loads and wave loads (Figure 4) were calculated according to the literature : where is the standard value of wind load (kN/m2), is the wind vibration coefficient at height , is the wind load carrier type coefficient, is the wind pressure height variation coefficient, and is the basic wind pressure (kN/㎡). where is the wind pressure (kN/m2), is the air density (kg/m3), and is the wind speed (m/s).
The wind amplitude curve was created according to the power of the monthly wind speed in the eastern sea and the maximum static load wind speed. The ratio of the monthly wave load to the limit static wave load was calculated according to formulas (3) and (4), so an amplitude curve could be created.
In formula (3) and formula (4), is the maximum wave resistance; is the maximum inertial force; is the weight of seawater, (kN/m3); is the density of seawater (kg/m3); is the water depth (m); ; is the wavelength (mm); is the drag coefficient (dimensionless); is the inertia coefficient (dimensionless), ; is the mass coefficient; is the pile diameter (m); is the wave height (m); is the hyperbolic sine function; and is the hyperbolic tangent function.
The wind wave amplitude curve was set as the amplitude curve in Abaqus. The curves were then applied to the two foundations to compare and analyze the bearing characteristics under the action of wind and wave loads in different months of the year.
1.3. Comparative Analysis of Bearing Performance of Both Foundations under Static Horizontal Load
1.3.1. Displacement in the Direction of Head Force and Back Force
The direction in which the foundation was positively loaded was set as the head-on direction (Figure 1). The face on which it was located was the head-on face; its backside was the back-on direction, also called the back-on face. Figure 5 shows the deformation of the monopile foundation under a horizontal load (a triple deformation scaling factor was set for more straightforward observations). Figure 6 shows the horizontal displacements of the monopile foundation at different depths in the direction of the head and back forces.
As shown in Figures 5 and 6, the horizontal displacement of the monopile foundation gradually decreases as the depth of penetration increases. Also, the pile body is not affected by torque and does not rotate. The maximum displacement of the pile foundation under a horizontal load is where the pile body enters the soil (14.5 mm). The displacements of the head force face and the back force face are equivalent. The horizontal displacement reversal point appears at 8 m into the soil. The upper displacement is consistent with the load-acting direction. The lower displacement is opposite to the load-acting direction. The length of the pile in the soil under the influence of the load is about 1.8 times the length of the pile outside the upper soil.
The horizontal displacement of the pile can be divided into three parts: the first part is 0 m-8 m into the soil; the horizontal displacement of this section is negatively linearly related to the depth of the soil. The second part is 8 m-20 m, and this section shows a negative exponential distribution. The third part is 20 m-36 m; the displacement increases linearly to zero at this stage. At a burial depth of 8 m-12 m, the rate of displacement increase in the direction of the head and back forces gradually slows, and the displacement reaches -30.13 mm at a depth of 16 m which is the maximum negative displacement. At this time, there is a slight difference in the displacement of the front and back forces. After 16 m, the negative displacement on the head and backside slowly decreases. Overall, the displacement is linearly related to the depth until 36 m when displacement is 0. It is recommended that the burial depth of the pile foundation soil should be greater than 1.8 times of the pile body outside the upper soil in an actual project to prevent excessive horizontal displacement of the pile foundation from causing instability and damage.
As shown in Figures 7 and 8, the maximum horizontal displacement of the single-suction bucket foundation under the load (Figure 1) is 550 mm. According to the specification, the maximum displacement does not exceed 0.02 ( is the diameter of the foundation), so it can be determined that the foundation is unstable and overturned. Unlike the pile foundation, the displacements of the suction bucket foundation in the head and back force direction are not the same. At a burial depth of 5 m, the suction bucket foundation has a horizontal displacement reversal point. The difference between the top and bottom displacement is 666 mm, indicating that the horizontal load pulls the bucket foundation up in the head-on direction.
From the horizontal displacement of the head-on and back-on direction under static horizontal loads, the bearing capacities of the pile foundation are better in shallow seas compared to the bucket foundation. It is recommended to adopt pile foundations in shallow seas. If the suction bucket foundation is adopted, the geometry of the suction bucket foundation needs to be increased to enhance its pullout ability and achieve foundation stability.
1.3.2. Side Horizontal Displacements with Depth
The horizontal displacements on both sides of the pile foundation are the same under the static horizontal load, indicating that there is no torsion. The variation of the displacement on both sides of the pile with depth is consistent with the variation of the displacement of the head and back force surface with depth, indicating that the overall effect of the pile foundation structure is apparent.
However, the suction bucket foundation displacement is different on both sides under a static horizontal load. At a burial depth of about 4.5 m, the horizontal displacements in direction side 1 and direction side 2 are both positive displacements. At the same time, the displacement on direction side 2 is unstable, as shown by the broken line, indicating that the bucket on direction side 2 is subjected to an extreme horizontal load. The bucket wall is irregularly deformed, twisting the whole bucket. At a burial depth of ~5 m, the displacement on both sides of the bucket is positive and negative, respectively, indicating that the bucket rotation intensified at this buried depth. Compared with the monopile foundation, the suction bucket foundation is relatively small and susceptible to torsion under a load. Therefore, it is necessary to increase the geometry of the suction bucket foundation to enhance its torsional capacity.
1.3.3. Plastic Deformation of the Soil around the Foundation
To further analyze the state of the soil surrounding the pile and bucket foundation under static horizontal loads, the plastic deformation of the soil around the pile and bucket foundation at a buried depth of 0 m was extracted (Figures 11–13).
Due to its deeper penetration depth, the pile body is subjected to a small displacement by the load, and the impact on the soil around the pile body is small, with a maximum plastic deformation of 8.75 mm (Figure 11). The maximum plastic deformation is 8.75 mm. Due to its small size, the suction bucket foundation is subjected to a significant overall displacement by the ultimate horizontal load (Figures 12 and 13). Soil both inside and outside the bucket undergoes plastic deformation. The maximum plastic deformation of the internal soil is 113.41 mm at the internal soil head-on surface. The maximum plastic deformation of the external bucket soil is 63.45 mm, located in the external soil back-on surface. The maximum internal plastic deformation is 1.79 times the external maximum plastic deformation. Ensuring the stability of the internal soil can improve the bearing capacity of the suction bucket foundation.
In summary, the monopile foundation has an overall coordinated displacement variation and high bearing capacity under a load. In contrast, the deformation between the suction bucket foundation is not coordinated, and the typical suction bucket has weaker bearing performance in shallow seas than a typical pile foundation.
1.4. Comparative Analysis of Bearing Performance under a Dynamic Load
Considering that wind power foundations are subjected to dynamic loads such as wind and waves throughout the year, the dynamic loads described in Section 1.2 were applied to the two foundations, respectively, and the bearing performances of both foundations are compared and analyzed. Figures 14 and 15 are the displacement curves at different depths of the two foundations’ head-on and back-on force directions in the shallow sea under dynamic loads.
The displacement of a pile foundation gradually decreases with an increase in burial depth (Figure 14). In April, when the wind and wave loads are small, the displacement is 46 mm at a burial depth of 0 m. While in October, when the wind and wave loads are large, the displacement is 127 mm, representing an overall increase of 176%. In July, when the wind and wave loads are the second largest, the displacement is 100 mm. The magnitude of the wind and wave dynamic load has a more significant impact on the deformation of the pile foundation.
The displacement of the suction bucket foundation in April when the wind and wave load is small is 5 mm, which is 11% of the pile foundation’s displacement with the same burial depth (Figure 15). In October, the displacement is 434 mm when the wind and wave load is relatively large. The maximum displacement is 85.6 times greater than the minimum displacement, which exceeds the standard displacement limit of 0.02. The foundation becomes unstable at this point. When the wind and wave loads were the second largest in July, the displacement was 192 mm. In October, the maximum displacement was 126% higher than in July, when the loads were the second largest. In October, the wind and wave loads were only 13.1% and 7% larger than the wind and wave loads in July. The bucket body is deformed under extreme wind and wave loads, and the foundation is susceptible to instability.
1.5. Comparative Analysis of the Bearing Performance of Both Foundations in the Deep Sea
1.5.1. Parameters and Loads
The model of a monopile foundation and a suction bucket foundation in a water depth of 50 m was then established (Figure 16).
There were two types of loads used:
The first was a static horizontal load of 6WN (Figure 16).
The second was a combination of wind and wave dynamic loads. The monthly wind speed and wave height at the Nanji station in the deep sea of the National Oceanographic Data Center of China were calculated (Figure 17). According to formulas (1)–(4) in Section 1.2, the wind-wave load amplitude curve was created and added to the Abaqus software. It was applied to both foundations as a dynamic load.
The physical and mechanical parameters of the rock and soil are shown in Table 1. Due to the different wind and wave loads between the deep and shallow sea, the geometric dimensions of the two foundations were changed, as shown in Table 2.
1.6. Analysis of Foundation Bearing Performance under Static Horizontal Load
1.6.1. Displacement in the Direction of the Head-On and Back-On Forces
The displacement of the pile foundation due to the head-on and back-on forces in the deep sea causes the same phenomenon as the displacement generated in the shallow sea (Figure 18). They both deform first along the direction of the load and gradually appear negative displacement as the depth increases. The displacement is 0 mm. The horizontal displacement reversal point starts to appear at a depth of 10.0 m and reaches a maximum value of -2.372 mm at 12.0 m. After 12.0 m, the displacement gradually reaches 0 mm with increasing depth. Due to the larger size of the pile foundation structure, there is a slight deformation difference between the front and back sides of the pile foundation when at burial depths less than 6 m. The main reason for the difference is the elastic deformation of the pile structure itself. The pile foundation is overall stable.
The displacements of the bucket foundation in the direction of head-on and back-on force are different in the deep and shallow sea conditions (Figure 19). The displacements are all positive and tend to zero as the depth of the foundation increases. At a burial depth ranging from 2 to 7 m, the head-on and back-on force surfaces of the suction bucket foundation appear oscillatory, mainly due to the inconsistent deformation of the front and back caused by the thin wall of the bucket. At the same time, the bucket foundation in the deep-water area was not unstable under a horizontal load.
Compared with pile foundation, bucket foundation has less deformation. The maximum displacement of the pile in the direction of head-on forces is ~6.5 times the maximum displacement of the bucket, and the maximum displacement of the pile in the direction of back-on forces is ~6.4 times the maximum displacement of the bucket. Under the deformation requirements of relevant specifications (i.e., China (NB/T10105-2018) requires that the base angle of wind turbines with a valley height of more than 100 meters should not be greater than 0.17°, while the DNVGL-ST-0126 specification uses 0.5° as the control angle of offshore wind turbines), the suction bucket structure compared to the single-pile foundation structure that needs to be driven into the supporting layer is more economical. It is also more stable and has less displacement with little change in size. Therefore, it is recommended that the suction bucket foundation is selected as the foundation of offshore wind power in deep-sea conditions.
1.6.2. Horizontal Displacements at Different Depths on Both Sides of the Foundation
The pile foundation has the same displacement on both sides, and no torsion occurs (Figure 20). The horizontal displacement reversal point forms at a burial depth of 8-10 m. The bucket foundation has the same displacement on both sides without twisting (Figure 21). The bucket body forms a horizontal displacement reversal point at a depth of ~8 m. The load-bearing performance of both foundations is within the allowable range of the specification, although the deformation of the suction bucket foundation is less.
1.7. Analysis of Foundation Bearing Performance under Dynamic Load
The dynamic load of Section 2.1 was applied to the two foundations with a water depth of 50 m, and the bearing performance of the foundation under dynamic load was further analyzed.
The maximum horizontal displacement of the pile under dynamic load is 20.9 mm (Figure 22), which occurs when the pile body enters the soil in February. As the depth of the pile into the soil increases, the displacement decreases and tends to stabilize with shock. The displacement of the pile body in the direction of the head-on force and the back-on forces tends to be the same. The different deformation of the pile causes a slight difference in deformation under tension and compression, and the pile foundation is overall stable.
The displacements of the suction bucket foundation in the head-on and back-on force directions are the same as the pile foundation (Figure 23). The maximum horizontal displacement of the suction bucket foundation under a dynamic load is 14.9 mm, which occurred at the entry point of the bucket foundation in February. The suction bucket foundation is overall stable. The maximum displacement of the pile foundation is 1.4 times the maximum displacement of the suction bucket. The bearing capacity of the suction bucket under a dynamic load in deep-sea conditions is better than that of a monopile foundation. Considering the complexity and cost of constructing pile foundations and bucket foundations in the deep sea, suction buckets are more suitable for development in deep-sea conditions than pile foundations.
The bearing performance was compared and analyzed for typical pile and suction bucket foundations in shallow and deep-sea conditions by considering the static horizontal load effect and the dynamic load effect made by statistical 11-year wind and wave data. The conclusions are as follows:
(1) Under a static horizontal load, the displacements in the direction of head and back side are distinct at different depths of the suction bucket. After the suction bucket bears the load, the pullout effect produced by the bending moment on the head side is more significant than that on the backside. The soil around the suction bucket was plastically deformed, and the foundation became unstable. However, the bucket foundation in the deep-sea area is stable, and the deformation is smaller than that of the pile foundation due to the increased geometry of the suction bucket foundation. The pile foundation is stable in both shallow and deep seas. However, considering the difficulty of construction and high cost in deep seas, it is recommended to use suction bucket foundations in deep seas
(2) The deformation analysis shows that the burial depth of the pile foundation soil should be greater than 1.8 times that of the pile body outside the upper soil to prevent the pile body from instability and damage. If the suction bucket foundation is used in shallow seas, it is necessary to increase the geometry of the suction bucket foundation to improve the pullout ability under extreme loads
(3) The pile displacement in the shallow sea gradually decreases with increased burial depth under a dynamic load. In April, when the wind and wave loads are small, the burial displacement is 46 mm, while in October, when the wind and wave loads are large, the displacement is 127 mm. The smallest displacement increased by 176%. The displacement of the suction bucket foundation was 5 mm in April when the wind and wave load was small, which was 11% of the pile foundation’s displacement with the same burial depth. In October, the displacement was 434 mm when the wind and wave load was large, causing instability of the foundation. Wind and wave dynamic loads have a more significant impact on both foundations. In the deep sea, both foundations are stable. The maximum displacement of the pile foundation is 1.4 times the maximum displacement of the suction bucket. Therefore, the suction bucket has better bearing performance under dynamic load than the monopile foundation
The data that support the findings of this study are openly available at http://nmdis.org.cn (reference number, CSTR:13452.11.01.01.2021.19).
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The National Natural Science Foundation of China (Grant Nos. 52071301, 51909238, and 51939002), the Zhejiang Provincial Natural Science Foundation of China (No. LHY21E090001), and the Open Fund of Zhejiang Provincial Key Laboratory of Deep-Sea Wind Power Technology Research (Grant No. ZOE2020006) supported the work described in the paper. The financial support is gratefully acknowledged.
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