Impact of soil–structure interaction models on the dynamic response of hybrid offshore wind–hydrokinetic systems

Hybrid offshore wind–hydrokinetic systems supported by a common monopile are sensitive to soil–structure interaction (SSI) modelling because small changes in foundation flexibility may alter the predicted natural frequencies and the available separation from rotor-induced excitation bands. This study systematically investigates how the selection of SSI framework propagates into the dynamic response of a dual-turbine hybrid offshore system. Four SSI representations—coupled springs, conventional p–y curves, a one-dimensional PISA-based formulation, and three-dimensional finite element modelling—are implemented within a unified two-degree-of-freedom dynamic framework. Latin Hypercube Sampling is used to generate 1000 stochastic operational scenarios covering combined wind and current loading conditions. The comparison focuses on the resulting natural frequencies, modal effective mass ratios, and frequency-avoidance force-ratio windows. The results show that SSI model selection produces appreciable differences in predicted dynamic properties, particularly for the second mode and for operating conditions close to the soft–stiff design window. The coupled-spring and conventional p–y approaches generally predict higher frequency levels relative to the PISA-based and 3D finite element solutions, whereas the 1D PISA formulation provides operational envelopes that are closely aligned with the 3D numerical reference. These findings indicate that simplified SSI assumptions may lead to different estimates of resonance-avoidance margins in hybrid offshore systems. The proposed comparison provides a systematic methodological reference for preliminary dynamic assessment, while the quantitative operating windows should be interpreted as frequency-avoidance criteria rather than complete resonance safety limits.