Minimum embedded length of cyclic horizontally loaded monopiles

Monopiles are used as foundation structures for offshore wind energy towers. To ensure stable behavior of the monopile under cyclic loading conditions, a minimum embedded length is usually required. For this, different design criteria are used, some of which result in very large embedded depths for large-diameter monopiles. The suitability of these criteria is tested by means of numerical simulations. To account for cyclic loading, a new approach called the stiffness degradation method is applied. The results of a parametric study show that the design criteria used can indeed ensure optimum pile performance under static and cyclic loads. It is recommended that the requirement of a critical pile length, which leads to the minimum pile deflection under extreme load, is used as a design criterion. For the cases considered, this requirement results in only slightly greater cyclic deformations compared to the optimum case of very long piles. A further optimization of the required monopile length with respect to cyclic loading is possible, but requires specific consideration of cyclic behavior, as is done in this paper.