Stability of tapered monopile offshore wind turbine with concrete-filled double-steel tubular structure

This study presents a tapered monopile foundation for offshore wind turbines and evaluates its structural performance under extreme loading. The primary objective of this study is to assess the structural integrity and practical applicability of the monopile while examining variations in its response to different design specifications. A series of nine tapered CFDST monopile configurations is developed, and a three-dimensional finite-element model is constructed for numerical analysis. An extreme-load analysis incorporating soil–structure interaction is conducted for the installation of a tapered CFDST monopile in the East China Sea. The analysis shows that the first natural frequency of all scenarios is within the 1P–3P range of the National Renewable Energy Laboratory(NREL) 5 MW turbine, thus satisfying the soft-stiff design criterion. The maximum von Mises stresses in the outer-steel tube under ultimate loading conditions indicate safety factors between 3 and 5, thus confirming structural robustness. The maximum tensile stress in the concrete-filled section between the double-steel tubes show that all scenarios (except S7) exhibiting the lowest concrete filling maintained a safety factor of 1.5 or higher, which translates to sufficient structural performance. These findings validate the feasibility of the tapered CFDST monopile.