A spar-type offshore platform includes a buoyant upper hull structure supporting a deck and having lower end in which is received a buoyant lower mooring module. The upper hull structure is connected to the mooring module by connection lines. The upper hull structure is removed from the mooring module by disconnecting the connection lines from the upper hull structure while leaving the connection lines attached to the mooring module and while the mooring module remains moored to the seabed. The mooring module is lowered relative to the upper hull structure, allowing the latter to be moved away. The upper hull structure may be re-positioned over the mooring module, and the mooring module may be hauled upward into engagement with the lower end of the upper hull structure, so that the connection lines can be recovered and re-attached to the upper hull structure.