Conventionally, whether the applied analytical method is either of frequency domain or time domain, the motion, mooring or stability analysis of a semisubmersible has been performed independently without any considerations given to the effect of sea interaction among three phenomena, motion, mooring and stability. Therefore, each analysis reveals partial information lacking an overall sense of the unit's behavior in real seas and often produces conservative results. This paper presents the results of a real time motion simulation of an Aker H-3 unit as moored for investigating the effect of rig motions and mooring line behaviors on the unit's dynamic stability in a random sea with a varying wind.
The dynamic stability of a semisubmersible depends on the results of the unit's heave motion and angular motions such as rolling and pitching in random seas with a varying wind of the most critical environmental condition under which the unit is designed to operate. What we would like to see from a dynamic stability analysis are the unit's heeling and restoring behaviors and the relative positions of the down flooding locations on the deck with respect to water surface elevation under various operational conditions. It is a common belief that a varying wind and a spread mooring system would influence all six degrees of freedom of rigid body motion. Nevertheless, the motion characteristics of a semi-submersible has normally been investigated without considerations given to the effect of the varying wind or the contribution of the station keeping mooring system. The reasons for performing an uncoupled independent analysis of motion, mooring or stability may be attributed to the computational means and time requirements. However, these obstacles can now be overcome with computational techniques utilizing parallel processing and relatively inexpensive PC based machines.