Behaviour of monopile-gravity-shallow bucket hybrid foundation for offshore wind turbine under cyclic load in sandy soil

• Characteristics of a novel hybrid monopile-gravity-shallow bucket (MGB) foundation for offshore wind turbine under long-term cyclic load were investigated by 1 g model tests. • The influence of geometry of the MGB foundations on its behaviour of cyclic loading was studied. • Cyclic load with different characteristic parameters were performed on MGB foundation to examine its affect on the accumulative rotation. • A method for predicting the accumulative rotation for the MGB foundation was presented. • The variation of stiffness and post-cyclic bearing capacity of the hybrid system were investigated. This paper investigates the behavior of a monopile-gravity-shallow bucket (MGB) hybrid foundation for offshore wind turbines (OWTs) under cyclic loading conditions. The study aims to understand the long-term performance of the hybrid foundation, focusing on accumulative deflection, rotation prediction, stiffness degradation, and post-cyclic bearing capacity. A series of 1 g model tests were conducted to evaluate the foundation's response to both monotonic and cyclic lateral loads in dense sand, with varying aspect ratios and cyclic load parameters. The results indicate that the bucket diameter significantly influences the accumulative rotation, as larger diameters leading to reduced deflection under cyclic loading. The study also examines the effects of cyclic load magnitude ratio ( ζ b ) and symmetry ratio ( ζ c ) on the foundation's performance, revealing that higher ζ b values lead to greater accumulative rotation, while the two-way loading results in larger deflections compared to the one-way loading. The dimensionless analysis demonstrates that the MGB foundation exhibits a significantly slower cumulative deformation rate than monopile and suction bucket foundations, with regression models incorporating the two loading parameters further developed to predict rotation angles. Additionally, the foundation's stiffness tends to increase after long-term cyclic loading in dense soil condition. The post-cyclic monotonic bearing capacity of the hybrid foundation was found to be higher than its pre-cyclic capacity, suggesting improved soil densification around the foundation. The findings provide some insights for the design and optimization of hybrid foundations for OWTs under long-term cyclic loading.