This paper presents experimental and theoretical research aimed at deepening the understanding of the lateral response of monopiles in sand subjected to cyclic loading. A series of 1-g model tests were performed for varying cyclic load and magnitude ratios, as well as for different pile stiffnesses. The broadly phenomenological behaviors of the monopile including accumulated displacement, cyclic secant stiffness, bending moment and reloading responses were captured. The results reveal the effects of cyclic load ratio, amplitude ratio and pile stiffness on the development of accumulated displacement and secant stiffness, and point out the action mechanism that the cyclic bending moment of rigid piles tends to increase while that of flexible piles tends to decrease. The elastic threshold of the reloading curve gradually increases with cycling, and increases with the increment of cyclic magnitude ratio. Crucially, a generalized model capable of describing the hysteretic characteristics of loading curves of monopiles was established, and the computational formulas for predicting the peak accumulated and residual displacements were derived. The reasonableness of the proposed method was verified under different loading parameters and pile-soil systems, which could be used for the preliminary design of offshore monopiles.

Monopiles serve as the foundational support for offshore wind turbines and are constructed as large, hollow, and rigid steel pipes. Given their offshore installation, these foundations experience cyclic lateral forces from wind and waves. This paper focuses on investigating the cyclic lateral capacity of monopiles through experimental and numerical analysis. The study examines varying factors such as slenderness (L/D) ratios of 2, 4, and 6, load amplitudes (xi b) of 40%, 30%, and 20%, and different densities of sand (RD) at 35%, 55%, and 75%. One-way cyclic loading at a frequency of 0.25 Hz was applied during the experiments using a pneumatic cylinder setup to the model piles. Numerical analysis was conducted on the prototype piles using PLAXIS 3D finite element software. The analysis utilised a hypoplastic model with an intergranular strain concept as the constitutive model. The model was validated against the experimental results of MLD2/LA40/RD55 and exhibited similar behaviour. The experimental findings indicate an initial 40% increase in stiffness during the first 10 cycles, leading to a higher accumulation of displacement. However, as the number of cycles increased, the rate of stiffness increase decreased due to soil getting dispersed around the pile, resulting in an increased rate of accumulated displacement. This behaviour was observed across various L/D ratios, load amplitudes, and soil densities. Additionally, an increase in load amplitude and L/D ratio, as well as a decrease in soil density, resulted in higher accumulated displacement and reduced stiffness.

The monopile-wheel composite foundation is an innovative type of offshore wind turbine (OWT) foundation with good bearing capacity. The foundation is subjected to cyclic loads from wind, waves, and tides, so it is necessary to study its horizontal cyclic characteristics. By introducing cyclic degradation constitutive and Rayleigh damping parameters, numerical models are generated and validated by comparing with laboratory experiment results. The laws of horizontal accumulated displacement of the composite foundation are then explored through changes in the cycle amplitude, number, wheel diameter, height of loading point and pre -vertical load. The main findings are that the composite foundation is more likely to reach a stable displacement under constant amplitude cyclic loading compared to that of monopile. Under transient dynamic response (74 % of the horizontal ultimate bearing capacity), accumulated displacement will increase rapidly regardless of the foundation form. The composite foundation has smaller hysteresis loop area and slower accumulation rate of bending moment. The soil pressure weakening of the monopile mainly occurs in front of the pile, while that of the composite foundation mainly occurs under the wheel. After cyclic loading, the V - H envelopes for the composite foundation contract unevenly inwards. Above these, prediction methods for the cumulative displacement, maximum bending moment and V - H envelopes for composite foundation considering cyclic degradation are proposed.