Semi-submersible platforms face risks of mooring failure and motion instability caused by the combined action of waves and currents in exposed sea conditions. To verify the hydrodynamic performance of a retractable aquaculture platform, physical model tests were conducted to evaluate its behavior under three typical operational conditions: maintenance, floating, and seabed-resting. A 1:75 scale model was used to measure the platform’s six-degree-of-freedom motion responses and the dynamic characteristics of mooring line tensions. The results show that under maintenance and floating conditions, the combined action of waves and currents leads to the maximum mooring forces, while different load combinations have little impact on the platform’s motion. The wave period significantly affects mooring forces and pitch motion but has a minor influence on linear displacements. Under the combined action of waves and currents, mooring forces and linear displacements increase with wave period, whereas the opposite trend is observed under pure wave conditions. In the seabed-resting condition, the platform’s surplus weight provides strong resistance to sliding and overturning, with negligible motion and mooring forces. A comparison of the platform’s dynamic responses under maintenance and floating conditions reveals that under pure current or combined wave-current conditions, increasing the draft heightens mooring forces and exacerbates pitch motion while limiting linear displacements. In contrast, under pure wave conditions, increasing the draft reduces motion responses and mooring forces. The study confirms that the stability of the six-point mooring system and the strength of the mooring chains meet the design requirements, providing a basis for platform construction.
