Investigation on the requirements regarding the minimum embedded length of monopiles

Monopiles are open-ended steel pipe piles with diameters of usually 3 to 5m, which are used as foundation structures for offshore wind energy towers. Such foundations are subject to intensive cyclic horizontal loading due to wind and wave loads. In the practical design different requirements regarding the minimum embedded pile length are used in order to limit the lateral deformation under static and in particular cyclic loads. Such are the "critical length", "vertical-tangent" and "zero-toe-kick" criteria. For piles of very large diameter and thus high stiffness like monopiles these criteria lead to very large and hardly realistic required lengths. In this study, the different design criteria are compared with respect to piles of very large diameter. By means of the stiffness degradation method, a method which makes the determination of displacement accumulation with cyclic loading possible, the different criteria are assessed. With that, the suitability of adopting the mentioned design criteria for monopile foundations is discussed.