LAUSR

Abstract This paper presents a hybrid monopile foundation for offshore wind turbines. It is an ultra-high performance concrete-filled double skin steel tubular structure (CFDST) used as a replacement of the conventional steel tube between the water level and the mudline so as to reduce the monopile diameter and thereby reducing the wave loads on the pile. To study the feasibility of this monopile, the NREL 5 MW wind turbine supported by a conventional monopile is selected as a reference and a three-dimensional (3D) finite element model is developed. The natural frequency, the various responses under the serviceability limit state (SLS) and the ultimate limit state (ULS) of the hybrid monopile are presented. Particularly, the effect of varying outer diameter of the CFDST on the structural performance is investigated. By applying a consistent accumulated rotation at the mudline under the SLS, the natural frequency is found to be within a desired range, and an optimized embedded length of the hybrid monopile is determined. The results indicate that the proposed hybrid monopile is able to meet the design requirements for both SLS and ULS and the optimization of pile embedded length leads to an efficient and economic monopile foundation for offshore wind turbines.