Considerations on the superposition method’s applicability to pile group impedance functions for offshore wind turbine foundations

• The superposition method’s application for jacket pile group impedances is studied. • A matrix procedure involving the lateral, vertical and rocking terms is proposed. • The contribution of each flexibility term on each impedance function is evaluated. • The superposition method leads to an accurate impedance function matrix estimation. • The estimation of lateral-rocking coupled impedance requires all flexibility terms. The modelling of pile-soil-pile interaction using the superposition method is studied. For this purpose, a general matrix-based formulation is implemented, explicitly incorporating all flexibility components of the problem: horizontal, rocking, vertical, and their coupled terms. This model is employed to: (1) investigate the impact of each individual flexibility terms, by selectively removing them within the global formulation, on the horizontal, rocking, and cross-coupled impedances functions; and (2) evaluate the method’s applicability to polygonal pile group configurations, representative of those used in jacket-supported offshore wind turbines. A previously developed continuum model has been used as a benchmark for validation. Results show that the horizontal, cross-coupled and rocking dynamic impedances for regular pile groups are accurately reproduced through the superposition method. It is revealed that the horizontal impedances are correctly estimated when the terms related to vertical behaviour are neglected, by only considering the horizontal and rocking flexibilities. The rocking impedances are mainly influenced by the vertical component, although the flexibility terms related to the vertical-lateral coupling can be disregarded. However, to correctly reproduce the group cross-coupled impedances, all vertical, horizontal, rocking, and their coupling flexibility terms must be considered.