Determining the Embedded Pile Length for Large-Diameter Monopiles

摘要

The use of the monopile support structure farther offshore requires a large amount of construction steel. Designing an efficient foundation can significantly reduce the amount of steel needed and thus the total costs. This paper evaluates the applicability of current foundation design criteria for large-diameter monopiles. Emphasis will be on the vertical tangent criterion as suggested by Germanischer Lloyd and the "zero-toe-kick" criterion for determining the required embedded pile length under static loading conditions. The lateral behavior of a total of 40 different design cases of monopile support structures in water depths ranging from 15 to 35 m has been studied. The soil conditions ranged from loose to very dense sand, which is typical for the North Sea. It has been concluded that the vertical tangent and the "zero-toe-kick" criteria leads to overly conservative embedded pile lengths. A preliminary design approach is presented, which is based on the knowledge that shortening the embedded pile length will decrease the natural frequency of the support structure. The results from this preliminary design approach study have been compared with the current monopile design practice, and it was concluded that the embedded monopile length can be reduced while achieving both lateral stability and maintaining small values of deflection at mudline and the pile toe.