Permanently moored large weathervaninig floaters are subjected to extreme weather conditions throughout their design life. The main objective of this PhD thesis was to better understand the mooring system of these vessels, under extreme environmental loads in a tropical cyclone environment. A more efficient methodology, of evaluating the design extreme load, was developed. Subsequently, reliability analysis of mooring system, in tropical cyclone and winter storm environment, was performed. It is shown that the Factors of Safety, recommended by current codes, are not sufficient for the mooring system of permanently manned installations in a tropical cyclone environment.