The behavior of monopile embedded in sand under scour and cyclic loading conditions

The large-diameter monopile, as one of the most used foundations of offshore wind turbines, is subjected to severe sea conditions, such as the cyclic lateral loads induced by wind and waves, as well as the scour induced by the flushing currents. The combined cyclic lateral loads and scour introduces additional complexity and is encountered in offshore wind turbines as well as other offshore structures. In this study, the experimental tests and numerical simulation methods are adopted to investigate the behavior of monopile subjected to the combined cyclic lateral loads and scour. Firstly, the static and one-way cyclic lateral load tests are conducted to investigate the influence of scour conditions and the characteristics of cyclic lateral loads on the lateral displacement and bending moment of monopile. Subsequently, the three-dimensional numerical model is established by employing the stiffness degradation material model, reasonable comparisons between the numerical results and the experimental results suggest that the numerical model can be used as a reliable complement of the model tests. Finally, a series of parametric studies are carried out to investigate the effect of cycle numbers, scour conditions and cyclic load ratio on the accumulated deformation and lateral bearing capacity of monopile. It is found that the accumulated deformations are dependent on the cyclic load characteristics and scour depth, the one-way cyclic load and scour have opposite effects to the lateral capacity of monopile. A framework to predict the accumulated deformation of monopile subjected to the combined scour and long-term cyclic lateral load is developed, which can be served as a basis for the practical design of monopile.