The authors have made extensive investigations on the stability of the moored semisubmersible platforms for past several years and have pointed out that the existing intact stability criteria considering only unmoored platforms have not been sufficient to prevent the dangerous situations of the platforms in violent sea states. In the case that the platform is moored, the various kinds of environmental loadings such as waves and currents should be generally taken into consideration in addition to the wind loadings. Besides those, the steady tilt of the platforms caused by the wave induced vertical steady force on the lower hulls has been found to be important to introduce the capasizing phenomena. These were already published in the first report. The second report shows the study on the combined environmental loadings for the semi-submersible platforms by using the time-domain simulation program. As the results of the study, it was concluded that the currents and waves induce the same order effects on the moored semi-submersible platforms as the wind's effects. In the present report, the authors summarized some problems on the existing intact stability criteria by using the results of platform behaviors obtained from the time-domain simulation. Finally the new computational approach for determining the required minimum GM ( GM_r ) is proposed to avoid the significant larger inclination. This GM_r consists of the standard GM ( GM ₀) and the several correction terms (Δ GM ) which come from the effects of various kinds of environmental lodings and mooring tensions. The present approach is believed to be useful for designers as it is easily applied to many semi-submersible platforms for determining the adequate GM . This approach also has a great potential of application for finding the alternative solution of platform's configuration.