Dynamic responses of a 15 MW floating offshore wind turbine with an equivalent anchor–chain–soil boundary in clay

Accurate modeling of the mooring system is vital for ensuring the structural safety and cost-effective deployment of floating offshore wind turbines (FOWTs). However, traditional approaches that use fixed-anchor boundary conditions do not capture the nonlinear behavior of the anchor–chain–soil system, which may affect the predicted dynamic characteristics and mooring loads. Therefore, this study extends OpenFAST by introducing an equivalent anchor–chain–soil boundary into MoorDyn, rather than treating the anchor as an idealized fixed point. First, a finite element model of the anchor–chain–soil system is developed using the coupled Eulerian–Lagrangian (CEL) method in ABAQUS. Then, an equivalent stiffness and damping approach is proposed to approximate the anchor–chain–soil interactions and is implemented in the mooring module of OpenFAST. Finally, a case study of the IEA 15-MW reference turbine is conducted to investigate the influence of the equivalent anchor–chain–soil boundary on the dynamic characteristics of the FOWT. The results indicate that, compared with the fixed anchor boundary condition, the proposed equivalent-boundary model increases the maximum surge response by 18.41% on average under the rated condition and reduces the maximum ML1 fairlead tension by 10.35% on average under the extreme condition. In addition, the results show that the spring component primarily governs the increase in surge response, whereas damping provides an additional reduction in peak tension. This study provides a practical numerical framework for the preliminary assessment of FOWTs when anchor–chain–soil interactions cannot be ignored.