Lowering a suction-caisson-type subsea structure through the splash zone is a difficult modelling problem because the governing response is controlled by rapidly changing hydrodynamic conditions at free-surface crossing. This paper presents a time-domain OrcaFlex analysis of the lifting and lowering of a Subsea Distribution Unit (SDU) Suction Caisson foundation. The hydrodynamic representation combines a main hollow-body spar-buoy model with two auxiliary buoy corrections: one to account for the additional axial added mass that develops when the caisson cap crosses the free surface, and one to account for the inertia of the water entrapped within the structure after submergence. Environmental screening was performed in the time domain using deterministic regular waves over a matrix of wave heights, periods, and headings. The operability maps show that the splash-zone stage governs the operation. A dedicated case study for wave direction 0°, H = 0.5 m, and T = 6 s further demonstrates the sensitivity of the response to the auxiliary buoy modelling. Relative to the base model without the two auxiliary buoys, activating the entrapped-water buoy increases the peak crane-cable tension from about 680 kN to 779 kN, while activating both the entrapped-water and axial-added-mass buoys raises it to about 907 kN and produces the longest post-entry oscillations. The results confirm that realistic suction-caisson lowering analysis requires explicit treatment of both entrapped-water inertia and cap-entry axial added mass, because these mechanisms control the transient splash-zone response and the resulting operability limits.