Numerical Simulation on Anchored Load-Bearing Characteristics of Suction Caisson for Floating Offshore Wind Power

Abstract

Suction caisson anchor foundations have been widely applied in oil and gas platforms but remain in the exploratory stage for floating offshore wind power applications, where research on their anchor load-bearing characteristics is insufficient. This study focuses on the influence of length-to-diameter ratio, loading angle, and loading point depth on the anchor load-bearing characteristics of suction caisson anchor foundations. Through numerical simulation, the load–displacement curves, internal force distribution along the caisson body, movement mode transitions, and soil failure characteristics were obtained. The results indicate that loading point depth and loading angle alter the movement mode of the suction caisson anchor foundation, directly affecting its bearing capacity. Smaller loading angles result in higher bearing capacity, which initially increases with loading point depth, peaks at 0.6 L, and then decreases at 0.8 L due to a transition in the foundation’s movement mode. Similarly, as the length-to-diameter ratio decreases, the bearing capacity and overall movement amplitude of the foundation decrease, leading to a shift in the optimal loading point position. The circumferential soil pressure and horizontal soil resistance distributions vary significantly with loading angle and depth. The findings of this study provide valuable reference for the design and application of suction caisson anchor foundations.

1. Introduction

Floating offshore wind power effectively harnesses wind energy resources located in deep waters and far from the coast by installing wind turbines on floating structures. The positioning of floating structures typically relies on mooring systems to anchor them to the seabed, with suction caisson anchors being one of the mainstream anchoring methods, as shown in Figure 1. The operational principle of this foundation involves sinking it into place using negative pressure suction, offering a simple and efficient installation process. Its cylindrical design supports multiple reuse cycles, and it provides precise positioning along with excellent long-term uplift resistance. Due to these comprehensive advantages, the suction caisson anchor foundation has become the most extensively used anchoring type in the field of floating offshore wind power.

Current research has comprehensively elucidated the bearing characteristics of suction caissons as fixed support foundations. Allersma et al. [1] investigated the effects of the length-to-diameter ratio, cyclic loading, long-term loading, and loading rate on the horizontal bearing capacity of suction caissons under cyclic and sustained loads. The results indicated a linear relationship between the horizontal bearing capacity and soil density. Xu et al. [2] proposed a predictive calculation method for the uplift bearing capacity of suction caissons, accounting for soil stress release and differential pressure contributions. A series of numerical simulations were conducted to study the uplift response of suction caisson foundations in sandy soil, with the results validated against centrifuge test data. Supachawarote et al. [3] employed finite element methods to analyze the influence of the length-to-diameter ratio, load direction, mooring point location, and soil conditions on the ultimate bearing capacity of suction caisson foundations. Fan et al. [4] developed a simplified distribution model for soil pressure on caisson walls during instability and a limit equilibrium method for calculating the horizontal bearing capacity of large-diameter suction caissons (up to 10 m) on dense sand, based on finite element simulations. Li Dayong et al. [5] used finite element software to study the monotonic and cyclic loading performance of traditional and skirted suction foundations in saturated sandy soils, concluding that skirted suction foundations are better suited for structures dominated by horizontal loads. Lü et al. [6] simulated the large-deformation penetration process of suction caissons in clay, finding that increasing the final suction value significantly reduces penetration resistance while markedly increasing soil plug height. Wang et al. [7] analyzed, via finite element simulation, the impact of contact surfaces on structural horizontal displacement and the effect of skirted foundation size ratios on horizontal bearing capacity, noting that increasing the height of skirted foundations effectively enhances bearing capacity. Zhu [8] developed a plastic segment transfer matrix to address the convergence issues of pile-end boundary conditions in traditional methods. Sun et al. [9] used finite element methods to study the bearing characteristics of suction caisson foundations under inclined loads across different length-to-diameter ratios and strength distributions, proposing a combined load capacity calculation method. Sharma et al. [10] and Ukritchon et al. [11] utilized ABAQUS to compare the bearing characteristics of suction anchors in normally consolidated and overconsolidated soils, exploring the effects of loading methods and point locations on ultimate bearing capacity, with findings highlighting the significant impact of loading point position on failure modes. Kourkoulis [12] conducted three-dimensional nonlinear finite element analyses to investigate the response of suction caisson foundations under wave cyclic loads and seismic actions, focusing on the influence of soil–foundation interface bonding, soil heterogeneity, and seismic–environmental load coupling on wind turbine foundation performance. Large-deformation finite element models were employed in [13,14,15] to reveal the dynamic evolution of pore water pressure in clay, quantifying the dissipation of pore water pressure and its time-varying effect on bearing capacity. This body of work provides a model basis for predicting the time-dependent bearing capacity post-installation of suction caissons.

To date, suction caisson foundations have been applied in several large-scale marine engineering projects. However, the complexity of marine environments, variations in inclined anchor line pull loads, and the construction method involving negative pressure sinking result in highly complex mechanical behavior [16]. Their application in floating offshore wind power remains in the exploratory stage. Zhu et al. [17] studied the overturning resistance and angular rotation control of single suction caisson foundations for offshore wind turbines in silty and sandy seabeds in China and proposed a deflection-based bearing capacity calculation method through model tests and theoretical analysis. Kulczykowski et al. [18] conducted 1 g model tests on suction caisson foundations installed in sandy soil under single gravity, finding that the displacement rate significantly affects uplift bearing capacity but has little impact on the stress-displacement relationship. Studies such as [19,20,21] have analyzed the uplift bearing characteristics of uplift piles under different soil conditions using field ultimate load tests and theoretical methods, deriving load–displacement curves and validating predictive models through the comparison of experimental and theoretical results, thereby providing theoretical support for research on uplift piles. Byrne [22] systematically quantified the effects of water depth, loading rate, and sand permeability on the transient tensile performance of suction caisson foundations through pressure chamber model tests and addressed gaps in cavitation effect studies and providing critical experimental data for deep-water wind turbine foundation design. Houlsby et al. [23] proposed a response mechanism for suction caisson models under transient vertical loads using pressure chamber test devices. Mana et al. [24,25] systematically described the vertical bearing capacity characteristics of deeply embedded skirted foundations in heterogeneous clay through numerical simulations and model tests, revealing a unique failure mode involving reverse slip surfaces within the skirt soil. Mani et al. [26] and Huang et al. [27] conducted cyclic loading model tests and numerical simulations on suction caisson foundations, assessing the impact of soil–caisson interface friction degradation on bearing capacity under cyclic loads. Zhang et al. [28] studied penetration models and tests of suction caisson foundations, quantifying the spatiotemporal evolution of seepage fields during suction penetration and addressing the prediction of penetration resistance in heterogeneous seabeds. Zhan et al. [29] analyzed the failure mechanisms of suction caisson foundations under coupled loads using a refined friction interface model. Studies in [30,31,32] established and validated a cost-effective inferred Winkler model through the tests and numerical simulations of suction caissons under combined loads. Qin et al. [33] and Ouyang et al. [34] utilized improved SHPB (split Hopkinson pressure bar) tests to uncover the dynamic mechanical response and particle breakage evolution of calcareous sand under repeated impacts, offering a theoretical basis for assessing soil disturbance during suction caisson penetration. These studies elucidate the influence of factors such as the length-to-diameter ratio and loading point height on the bearing performance of suction caisson anchor foundations, providing valuable guidance for the numerical simulation research in this project.

Current research on suction caisson anchor foundations has rarely explored the inclined pull load-bearing mechanism, particularly the distribution characteristics of horizontal soil resistance under inclined loads. This makes it challenging for numerical models to accurately predict the inclined pullout bearing capacity of suction caisson anchor foundations. Therefore, this study employs the finite element software ABAQUS to conduct a numerical investigation into the bearing characteristics of suction caisson anchor foundations under inclined pull loads. By comparing the bearing capacity of suction caisson foundations under various conditions, this study aims to determine the influence of movement failure modes, loading depth, loading angle, and soil resistance distribution characteristics on the uplift bearing capacity of the foundation.

2. Methods and Materials

Whether through indoor model tests or field prototype tests, it is generally only possible to obtain the bearing capacity of suction caisson anchor foundations and the failure characteristics of the surface soil, making it difficult to clearly reveal the interaction characteristics between the caisson and the soil. Numerical simulation methods are a relatively suitable technical approach for conducting load-bearing simulations of suction caisson anchor foundations. Moreover, the results obtained from simulations can better observe the motion failure modes of the suction caisson and the mechanisms of pile–soil interaction.

2.1. Establishment of Finite Element Model for Suction Caisson Anchor Foundation

The suction caisson anchor foundation has a diameter of 5 m and a wall thickness of 0.1 m. Figure 2 presents the mesh model of the suction caisson anchor foundation and the soil. The main structure of the suction caisson foundation consists of a top cap and the caisson body.

This study employed the ABAQUS 2022 finite element software for modeling and analysis. The model in this study uses the element birth and death technique to achieve geostress balance (as shown in Figure 3). The loading method adopts the reference point approach, where a reference point is set above the cap and coupled with the top surface of the cap, with loads applied to the reference point during loading. The horizontal extent of the soil was set to 10 times the diameter of the suction caisson, and the vertical extent was set to 6 times the length of the caisson to eliminate boundary effects. The contact interface was modeled using a surface-to-surface contact formulation, with the tangential friction coefficient of the contact surface set as μ = tan (2/3φ), where φ is the internal friction angle of the soil. The normal behavior of the contact surface adopts hard contact. The soil mesh was refined locally, with denser meshing applied to the soil inside the caisson. For the soil outside the caisson, a single-precision local refinement method was used, where the mesh size of the soil decreases closer to the suction caisson, resulting in higher computational accuracy. The soil used was silty sand from the nearshore sea area in the northern part of Binhai County, Jiangsu Province. The Mohr-Coulomb constitutive model was selected for the soil, and the suction caisson was modeled using an elastic constitutive model. The finite element calculation parameters are shown in

Table 1. Material-related calculation parameters.

Material	Elastic Modulus/Mpa	Effective Heavy/(kN·m−3)	Cohesion/kPa	Internal Friction Angle/(°)	Poisson’s Ratio
Silty sand	33	20.0	2	33.7	0.3
Steel	210,000	78.5	-	-	0.3

.

2.2. Finite Element Simulation Conditions for Suction Caisson Anchor Foundation

This section proposes to conduct a total of 3 × 4 × 11 = 132 sets of finite element numerical simulation analyses to investigate the influence patterns of different loading point depths, loading angles, and length-to-diameter ratios on the bearing characteristics of the suction caisson anchor foundation. The simulation conditions are shown in

Table 2. Finite element model dimensions and simulation conditions.

Number	Caisson Length (m)	Caisson Body Loading Point	Loading Angle
1	10	0.2 L	Load at 11 angles: 0°, 10°, 15°, 20°, 30°, 45°, 60°, 70°, 75°, 80°, and 90°.
2		0.4 L
3		0.6 L
4		0.8 L
5	5	0.2 L
6		0.4 L
7		0.6 L
8		0.8 L
9	2.5	0.2 L
10		0.4 L
11		0.6 L
12		0.8 L

. The diameter of all simulation schemes is 5 m, with a sidewall thickness of 0.1 m.

3. Analysis of Anchor Pull Bearing Characteristics of Suction Caisson Anchor Foundation

3.1. Analysis of Anchor Pull Load–Displacement Performance Curve

Figure 4 presents the load–displacement curves of the foundation under different loading angles with the same loading depth when the length-to-diameter ratio is 2. When the displacement is less than 0.05 m, the curves for all loading angles of the suction caisson nearly coincide and exhibit a linear relationship. This is because, at this stage, the soil is in a relatively stable elastic deformation phase, indicating that the loading angle has a minimal impact on the bearing capacity of the suction anchor foundation within this range. As the external load further increases, the curves transition from the initial linear phase to a nonlinear phase, showing a decline in the foundation’s tensile bearing capacity at the same displacement as the loading angle increases. This phenomenon occurs because the horizontal force acting on the foundation decreases with the increase in loading angle, resulting in a reduction in the stress at the loading point and the stress diffusion zone of the active side soil.

The load–displacement curves under different loading depths with the same loading angle when the length-to-diameter ratio is 2 are shown in Figure 5. The load at which the suction caisson displacement reaches 0.8 m is used as a reference value. From the figure, it can be observed that the bearing capacity of the suction caisson exhibits a trend of initially increasing and then decreasing as the loading depth increases. Taking the 60° loading angle as an example, when the loading depth is 0.2 L, the bearing capacity of the suction caisson is 8 MN. As the loading depth increases, it reaches 12 MN at a depth of 0.4 L, and subsequently reaches a maximum value of 17 MN at a depth of 0.6 L. As the loading depth continues to increase, the bearing capacity drops to 16 MN at 0.8 L. A similar pattern is observed for the other three angles. This leads to the conclusion that the bearing capacity of the suction caisson is significantly correlated with the loading point depth, initially increasing and then decreasing as the loading point depth increases, with the highest bearing capacity achieved when the loading point depth is 0.6 L.

3.2. Analysis of Suction Caisson Foundation–Soil Interaction

From the stress distribution graph of soil for suction caisson under different loading depths, and under different loading angles, in Figure 6 and Figure 7, it can be seen that the soil stress diffusion zone of the suction anchor foundation is significantly related to its loading point depth and loading angle. When the loading angle is 0°, the movement mode of the suction anchor is rotational, and the soil stress near the loading point is significantly higher than at other angles, indicating that at a loading angle of 0°, the foundation has the highest bearing capacity and the largest soil stress diffusion zone. When the loading depth increases, the rotation direction of the foundation changes from clockwise to counterclockwise, and at a loading point depth of 0.6 L, the movement mode of the foundation is almost translational. Meanwhile, as the loading point depth increases, the maximum soil stress near the loading point also increases, reaching its maximum at 0.8 L. This pattern differs from the variation in bearing capacity with loading point depth, because when the loading point is at 0.8 L, the foundation undergoes counterclockwise rotation under horizontal force, with the displacement below the loading point being greater than at the loading point itself, creating a displacement difference that hinders the downward diffusion of stress, leading to stress accumulation below the loading point.

3.3. Analysis of the Influence of Length-to-Diameter Ratio on the Bearing Characteristics of Suction Caissons

To analyze the influence of the length-to-diameter ratio on the bearing capacity of the suction caisson anchor foundation, multi-directional loading tests were conducted on suction caissons with length-to-diameter ratios of 2, 1, and 0.5 (corresponding to caisson lengths of 10 m, 5 m, and 2.5 m, respectively). The load–displacement curves from high to low length-to-diameter ratios are shown in Figure 4, Figure 8 and Figure 9. After changing the length-to-diameter ratio, when the displacement is less than 0.05 m, the load at various angles increases linearly with displacement. When the displacement exceeds 0.05 m, differences in the bearing capacity of the suction caisson begin to emerge at different loading angles. Taking the loading angle of 0° as an example for the suction caisson anchor foundation, as the length-to-diameter ratio decreases, the bearing capacity of the foundation also decreases, dropping from approximately 27 MN at a length-to-diameter ratio of 2 to about 3 MN at a ratio of 0.5. Additionally, the bearing capacity of the suction caisson anchor foundation remains inversely proportional to the loading angle; it reaches its peak at a loading angle of 0° and gradually decreases as the angle increases, reaching its lowest value at 90°. Comparing Figure 4, Figure 8 and Figure 9, it is evident that the bearing capacity of the suction caisson with a length-to-diameter ratio of 2 shows a clear decline with increasing loading angle. However, as the length-to-diameter ratio decreases, the decline in bearing capacity under small-angle and horizontal loading becomes less pronounced. The bearing capacity under different loading angles varies significantly with the loading point depth, and the anchor foundation achieves its highest bearing capacity when the loading point depth is 0.6 L, a conclusion that remains unaffected by the length-to-diameter ratio.

Figure 6, Figure 10 and Figure 11 show that the displacement magnitude of the foundation failure surface is influenced by changes in the length-to-diameter ratio, and the movement modes of foundations with different length-to-diameter ratios at a loading position depth of 0.8 L exhibit significant differences. The smaller the length-to-diameter ratio, the less pronounced the movement amplitude, indicating that changes in the length-to-diameter ratio directly affect the amplitude of the movement mode after the foundation is subjected to loading. This leads to the conclusion that a change in the length-to-diameter ratio also influences the optimal loading point position of the suction caisson anchor foundation.

3.4. Analysis of Motion Failure Modes of Suction Anchors

Based on the previous analysis, it is evident that the bearing capacity of the suction caisson anchor foundation is related to its motion mode after loading. Therefore, the motion failure mode is analyzed by combining the displacement vector diagrams of the suction caisson anchor foundation after loading.

Taking the loading angle of 0° as an example, a comparative analysis was conducted on the motion modes of the suction caisson foundation under different depth loadings at the same angle. As shown in Figure 7, when the loading point depth is 0.2 L, the rotation point (The red dot in the figure represents the rotation point.) of the suction anchor after loading is located at the bottom right of the anchor body (as shown in Figure 12a). When the loading point depth is 0.6 L, the vertical displacements above and below the loading point of the suction anchor are nearly symmetric, with negligible displacement difference and almost no rotation (as shown in Figure 12c). When the loading point depth is 0.8 L, the rotation point of the suction caisson after loading is located above the right side of the top cap, and the caisson body rotates counterclockwise around this point after loading (as shown in Figure 12d). This indicates that the overall rotation point position of the foundation shifts counterclockwise around the suction caisson as the loading position increases. Furthermore, it is clearly observed that the rotation direction of the foundation displacement vector at 0.2 L is clockwise. As the loading depth increases, the overall motion direction of the foundation gradually transitions to translational motion when the loading point reaches 0.6 L and finally changes to counterclockwise rotation.

Figure 13 presents the displacement vector diagrams of the suction caisson at different loading angles when the loading depth is 0.2 L. When the suction anchor is subjected to a single horizontal load, the caisson body rotates clockwise around a rotation point (as shown in Figure 13a), with the motion mode being solely rotational at this stage. As the angle increases, under small-angle loading (0–30°), the motion mode of the suction anchor transitions to an upward diagonal translational motion combined with rotation. When the loading angle reaches 60°, the foundation’s motion mode gradually shifts from a downward clockwise rotation to a vertical upward translational motion. Subsequently, as the loading angle continues to increase, the motion mode transitions from vertical upward translation to counterclockwise rotation, with the rotation center located at a greater distance on the left side of the foundation.

By comparing Figure 6, Figure 7, Figure 12 and Figure 13, it can be concluded that the rotational amplitude of the foundation gradually decreases as the loading point depth increases, until it reaches pure translation, after which the rotational amplitude gradually increases again. The bearing capacity is related to the motion mode after loading. For suction caissons with the same length-to-diameter ratio subjected to loads in the same direction, a smaller rotational amplitude after loading corresponds to a larger bearing capacity. As the loading depth continues to increase, the rotation mode transitions from pure translation at 0.6 L to counterclockwise rotation at 0.8 L, at which point the bearing capacity of the foundation slightly decreases.

4. Bearing Capacity Analysis of Suction Caisson Anchor Foundation

4.1. Analysis of Circumferential Soil Pressure

The suction caisson bears some similarity to a rigid pile in terms of structural form; however, due to the presence of negative pressure inside the suction caisson, the interaction between the caisson and the soil is far more complex than that of a pile. This negative pressure environment alters the stress transfer mode between the soil and the structure, resulting in a stress transfer mechanism for the suction caisson that is more intricate and difficult to accurately assess compared to a rigid pile. Therefore, it is necessary to analyze the circumferential soil pressure distribution after the suction caisson is loaded, and to study the stress transfer mechanism between the suction caisson and the soil, providing critical theoretical support and data basis for the optimized design and safe, stable operation of suction caisson foundations.

As shown in Figure 14 and Figure 15, when the loading angle is 30°, the circumferential soil pressure distribution follows a distinct cosine function pattern, reaching a maximum at 0° of approximately 700 kPa and then gradually decreasing to 0 at 90°. The maximum pressure value is reduced by 30% compared to the 1000 kPa observed when the suction caisson is loaded at 0°. It is observed that the circumferential soil pressure at depths of 2–4 m is significantly higher than at other depths. Compared to the 0° loading condition, the soil pressure at the bottom of the caisson (8–10 m) increases notably. When the loading angle increases to 60°, the overall circumferential soil pressure value further decreases, with the maximum soil pressure at this point being only 50% of that at 30° loading. A comparison of Figure 15b–d reveals that the soil pressure magnitude and distribution pattern at the bottom of the suction caisson remain almost unchanged, maintaining a range of 150–200 kPa. When the loading angle reaches 90°, due to the overall counterclockwise rotation of the foundation, the passive side soil pressure at the upper part of the foundation shifts from the right to the left, with the passive side pressure value falling below 120 kPa. Additionally, a portion of soil pressure exists on the active side (right) of the upper foundation, which is attributed to deformation at the loading point of the suction caisson caused by the applied load.

In summary, the soil pressure at the loading point of the suction anchor is primarily distributed within ±30°, reaching its maximum at 0° and gradually decreasing toward both sides, dropping to 0 at ±90°. The decay trend of soil pressure distribution at non-loading points predominantly follows a cosine function form. The overall circumferential soil pressure increases with the increase in loading point depth, while it decreases as the loading angle increases.

Taking the suction caisson under a 30° loading angle as an example, a comparison of the circumferential soil pressure diagrams under the same loading angle but different loading depths reveals that as the loading depth increases, the circumferential soil pressure values around the suction caisson exhibit a rising trend, increasing from 700 kPa at 0.2 L to 1600 kPa at 0.8 L. This clearly demonstrates that as the loading depth increases, the position of the maximum circumferential soil pressure on the suction caisson also shifts downward, moving from 4.25 m at 0.2 L to a gradual development downward, reaching 6.25 m at 0.6 L, where the maximum circumferential soil pressure occurs, and finally peaking at 9.25 m at 0.8 L. This phenomenon is consistent with the previously described variation in the maximum soil stress with loading point depth and aligns with the earlier observation that the foundation failure surface appears at or below the loading point as the loading point depth increases.

4.2. Analysis of Horizontal Soil Resistance

To further investigate the interaction characteristics between suction caisson piles and soil, the distribution curves of horizontal soil resistance at different depths under varying horizontal load conditions are shown in Figure 16. It can be observed from the figure that when the loading point depth is 0.2 L, the horizontal soil resistance at the top of the foundation is zero. As the burial depth increases (beyond 1 m), the horizontal soil resistance gradually increases, reaching its maximum value at or below the loading point, and then decreases with further increases in burial depth, dropping to zero at the bottom of the caisson. When the load is large, the horizontal soil resistance at the bottom becomes negative. As the horizontal load increases (from 1.83 MN to 27.06 MN), the overall soil resistance significantly increases, and the peak soil resistance shifts downward from the loading point (2 m). For a load of 1.83 MN, the peak soil resistance occurs at the loading point, while for a load of 27.06 MN, the peak shifts to a depth of 5 m. Based on the previous analysis, this is because, at a loading depth of 0.2 L, the load primarily induces rotational displacement. Due to the differential displacement between the upper and lower parts, the soil undergoes plastic flow toward the region below the loading point, resulting in higher stresses below the loading point.

When the loading point depth is 0.4 L, the horizontal soil resistance at the top of the foundation is zero. As the burial depth increases, the horizontal soil resistance gradually increases, peaking at the loading point, and then decreases with further increases in burial depth. As the horizontal load increases (from 1.88 MN to 34.60 MN), the overall soil resistance significantly increases, and the peak soil resistance shifts downward from the loading point (4 m). For smaller loads (1.88 MN to 13.04 MN), the peak soil resistance occurs at the loading point. As the load increases further, the peak shifts downward, reaching a depth of 6 m for a load of 34.6 MN. For loading point depths of 0.6 L and 0.8 L, the distribution curves of soil resistance with respect to burial depth follow the same pattern as those for 0.2 L and 0.4 L. The soil resistance at the top of the foundation is zero, increases with burial depth, peaks at the loading point, and then decreases. The peak position continues to shift downward as the external load increases, with the soil resistance exhibiting a declining trend after the peak.

Comparing the distribution curves for different loading depths, it is evident that for smaller external loads (1.8 MN to 8 MN), the differences in peak soil resistance corresponding to different loading point depths are minimal. This indicates that, under low-load conditions, the magnitude of the external load is the dominant factor influencing the peak soil resistance.

As observed from Figure 17, during the process of increasing the loading angle from 0° to 90°, the horizontal soil resistance curve of the suction caisson foundation transitions from a positive distribution to a negative distribution. For small-angle loading (0° to 45°), the horizontal soil resistance is primarily concentrated near the loading point (1 m to 6 m). As the loading point shifts downward and the angle increases to large angles (75° to 80°), the maximum soil resistance is mainly distributed at the bottom of the caisson. When the loading angle reaches 90°, horizontal foundation resistance reverses to negative values at depths less than 8 m, unlike at other angles. It initially increases with burial depth, reaches an extreme value below the loading point (at 3 m), and subsequently, the negative soil resistance decreases and transitions back to a positive increase.

5. Discussion

Based on comparative analysis, when anchored loads are applied at the same depth on the suction caisson, the maximum bearing capacity consistently occurs at a loading angle of 0°. This capacity decreases progressively as the loading angle increases, reaching its minimum at a 90° loading angle. The anchored load decomposes into vertical and horizontal components at the loading point, corresponding to the uplift resistance and lateral bearing capacity of the suction caisson, respectively. Since the uplift resistance is weaker than the lateral bearing capacity, any non-zero loading angle decomposes the load and reduces the overall capacity (i.e., the combined uplift and lateral resistance). This occurs because the uplift component may reach its limit while the lateral capacity remains underutilized. When the loading angle remains constant but the loading depth increases from shallow to deep, the kinematic mode of the suction caisson shifts significantly: from clockwise rotation at shallow depth (lower capacity), to translational motion at mid-depth (maximum capacity), and finally to counterclockwise rotation at deep depth (higher capacity). Consequently, the anchor point should be designed at depths (0.6 L–0.8 L) that induce translational or slight counterclockwise rotation motion.

This study further reveals circumferential earth pressure distribution patterns at varying depths, providing direct reference for defining the confinement pressure distribution function in theoretical models. When constructing theoretical models for suction caisson bearing capacity, due to size effects, both frontal soil resistance and peripheral shear resistance must be considered, where the confinement pressure function is critical.

Current research predominantly focuses on ultimate bearing capacity, inherited from early oil and gas floating platforms. These platforms operate in deep waters with long mooring lines, where anchor foundation stiffness minimally affects mooring system stiffness; thus, preventing pullout failure is the primary concern. However, floating wind platforms are significantly lighter than oil and gas platforms, making them more sensitive to anchor foundation stiffness variations. Moreover, most Chinese demonstration projects for floating wind are in shallow water areas, where limited mooring line length amplifies stiffness sensitivity. This necessitates developing a theoretical solution for suction caisson foundation stiffness, extending beyond mere ultimate bearing capacity.

6. Conclusions

To address the limitations in research on the inclined load-bearing characteristics of suction caisson foundations, this study employed numerical simulation methods to investigate the bearing capacity of suction caisson anchor foundations with different length-to-diameter ratios under varying loading angles and depths. The following conclusions were drawn:

• The bearing capacity of the suction caisson is significantly correlated with the loading angle and the depth of the loading point. The bearing capacity is maximized when the loading angle is 0°, and it decreases as the loading angle increases, a pattern that is independent of the loading point depth. The bearing performance of the foundation initially increases and then decreases with increasing loading point depth, with the optimal loading point depth ranging between 0.6 L and 0.8 L. At this depth, when a horizontal load is applied, the foundation undergoes pure translation without rotation.
• The bearing capacity of the suction caisson anchor foundations is closely linked to its motion mode. During shallow loading (0.2 L–0.4 L), the foundation undergoes clockwise rotation, and the bearing capacity gradually increases with increasing loading depth. When the loading depth reaches 0.6 L, the overall motion mode of the foundation almost fully transitions to pure translation, with the bearing capacity reaching its peak. As the loading depth enters the deep range, the motion mode gradually shifts from translation to counterclockwise rotation, and the bearing capacity exhibits a decreasing trend compared to that at 0.6 L. Additionally, the loading angle of the suction caisson also influences the overall motion mode.
• Under varying length-to-diameter ratios, the bearing capacity of suction caisson anchor foundations follows similar trends with respect to loading angle and depth. However, as the length-to-diameter ratio decreases, the movement amplitude becomes less pronounced, affecting the optimal loading point position.
• The circumferential soil pressure distribution around suction caisson anchor foundations primarily exhibits two patterns. At the loading point, soil pressure is mainly concentrated within ±30°, peaking at 0° and decreasing toward both sides in a cosine function pattern. The overall circumferential soil pressure increases with loading point depth.
• The horizontal soil resistance distribution of suction caisson anchor foundations is significantly related to loading angle, loading point depth, and external load magnitude. Under horizontal loading, soil resistance exhibits a single-peak distribution along the depth, with the peak near the loading point. As the external load increases, the peak shifts downward below the loading point. Increasing loading angles induce a counterclockwise rotation mode, causing the soil resistance in the upper part of the foundation to transition to negative values.