Behavior of Monopile Foundations for Offshore Wind Farms in Sand

Abstract

Most offshore wind-turbine foundations are monopile foundations. Monopile foundations have the advantages of simple structures and are convenient to construct. In recent years, large-diameter monopile foundations, with diameters larger than 4 m, have been used in engineering applications to increase the bearing capacity of wind turbines. Increasing the foundation pile diameter changes the bearing characteristics from those of traditional small-diameter steel-pipe piles. However, research on the bearing characteristics of large-diameter steel-pipe piles is lacking; no mature method exists to estimate the bearing capacity. On the basis of numerical simulations, the differences between the stress and deformation characteristics of the pile body and the soil around it between large-diameter and small-diameter steel-pipe piles subjected to vertical loads and horizontal loads in sand are investigated in this study. A method to calculate the vertical bearing capacity, considering the soil-arching effect, is proposed. In addition, a method to calculate the lateral displacement, based on the shell theory, is proposed. The applicability of the proposed method is verified using laboratory experiments. The result shows that the vertical ultimate bearing capacity calculated by the method considering the soil-arching effect is significantly higher than that predicted by the American Petroleum Institute (API) code method, and the lateral displacement at the pile top using the method based on the shell theory is 5–45% greater than that predicted by the