Bo Liu

In the marine environment, the suction caisson foundation (SCF) is often subjected to combined inclined and cyclic loading from wind and waves, which may significantly affect its ultimate bearing capacity. Under combined loading conditions, the evolution of ultimate bearing capacity is influenced by multiple factors, and the corresponding bearing capacity envelopes have become key issues that urgently need to be addressed. In this study, a series of model tests and numerical simulations were conducted considering the effects of load inclination angle, loading position, aspect ratio, soil undrained shear strength, and interface friction coefficient. The results show that under static loading conditions, as the loading depth increases, the load inclination angle corresponding to the maximum bearing capacity decreases from 45° to 0°. As the cyclic load ratio and static load ratio increase, cyclic loading significantly intensifies displacement accumulation and the degradation of ultimate bearing capacity. As the loading depth increases, the failure mechanism transitions from rotation-dominated to translation-dominated behavior. In addition, the ultimate bearing capacity increases monotonically with increasing aspect ratio, interface friction coefficient, and soil undrained shear strength. A normalized V–H bearing capacity envelope was established, which shows good agreement with the experimental and numerical results. By introducing a cyclic bearing capacity degradation coefficient, a modified envelope was proposed to describe the evolution of ultimate bearing capacity under cyclic loading conditions. The bearing capacity evolution patterns and envelope method proposed in this study provide a useful reference for the engineering design of SCF.

Pile-anchor foundations, serving as one of the anchoring solutions to ensure the safety and stability of floating offshore wind turbines, are primarily subjected to inclined loading induced by anchor chain forces, resulting in significantly different bearing behavior compared to conventional vertically loaded pile foundations. However, experimental research on the inclined pullout performance of anchor piles remains insufficient. To address this gap, this study employs a self-developed servo-controlled loading system to investigate the pullout bearing characteristics of anchor piles in dry and saturated sand, considering factors such as pullout angle and loading point depth. The research results show that from the load–displacement curve of the model pile, it can be found that with the increase in displacement, the load it bears first gradually increases to the peak, then decreases, and then gradually stabilizes. The loading angle has a significant impact on the bearing performance of pile-anchor foundations. As the loading angle increases, the failure mode shows pullout failure. When the loading angle increases from 30° to 60°, the bearing performance of the pile foundation decreases by approximately 63%. When the depth of the loading point increases from 0.22 times the pile length to 0.78 times the pile length, the diagonal anchor tensile bearing capacity of the model pile increases by approximately 45%. When the depth of the loading point is the same, the distribution patterns of bending moment and shear force are basically similar. However, the smaller the loading angle, the larger the value. This is because the horizontal load component plays a dominant role. The compression of the piles above and below the loading point, as well as the bending moment, shear force and axial force under saturated sand conditions, are similar to those in dry sand, but their values are reduced by about 50%. It can be seen that the soil conditions have an influence on the bearing characteristics of pile foundations.

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.

As one of the mooring foundation types for floating wind turbine platforms, research on the anchor pullout bearing characteristics of mooring pile foundations remains insufficient, and the underlying mechanism of anchor pullout bearing capacity needs further investigation and clarification. This paper conducts a numerical study on the bearing characteristics of mooring pile foundations under tensile anchoring forces with loading angles ranging from 0° to 90° and loading point depths of 0.2L, 0.4L, 0.6L, and 0.8L (where L is the pile length). The research findings indicate that the anchor pullout bearing capacity decreases as the loading angle increases from 0° to 90°, and exhibits a trend of first increasing and then decreasing with the increase in loading point depth. For rigid pile-anchors, the maximum anchor pullout bearing capacity occurs at a loading point depth of 0.6–0.8L, while for flexible piles, it appears at 0.4–0.6L. Both the bending moment and shear force of the pile shaft show abrupt changes at the loading point, where their maximum values also occur. This implies that the structural design at the loading point of the mooring pile foundation requires reinforcement. Meanwhile, the bending moment and shear force of the pile shaft gradually decrease with the increase in the loading angle, which is attributed to the gradual reduction of the horizontal load component. The axial force of the pile shaft also undergoes an abrupt change at the loading point, presenting characteristics where the upper section of the pile is under compression, the lower section is in tension, and both the pile top and pile tip are subjected to zero axial force. The depth of the loading point significantly influences the movement mode of the pile shaft. Shallow loading (0.2–0.4L) induces clockwise rotation, and the soil pressure around the pile is concentrated in the counterclockwise direction (90–270°). In the case of deep loading, counterclockwise rotation or pure translation of the pile shaft results in a more uniform stress distribution in the surrounding foundation soil, with the maximum soil pressure concentrated near the loading point.