Abstract: The mooring system of semi-sub mobile units can usually withstand the ocean environmental loads with return period of 5 or 10 years. How to optimize the mooring system in typhoon condition is a very important issue for platform operators. Taking an active semi-sub drilling unit as an example,combined with the typical environmental parameters of several areas in the South China Sea,the mooring system capability is analyzed and compared under different water depths,mooring arrangements and pre-tension settings. The factors affecting the ability of mooring against typhoon are summarized,which provide ideas for scientific typhoon protection for semi-sub unit.

Key words: semi-sub unit, mooring system, typhoon, survival condition