This paper deals with a numerical study of the transient effect of tendon disconnection on global performance of ETLP for harsh environmental condition in GoM (Gulf of Mexico). Twelve tendons support the platform with twelve production TTRs and one drilling riser attached by hydro-pneumatic tensioner. Charm3D, a program for global motion analysis of multiple floating hulls coupled with risers/mooring lines, is made to be capable of modeling the tendon disconnection at both top and bottom connection. The study includes the break due to high tension at the top and the unlatch due to negative tension at the bottom. A sudden disconnection of one or more tendons causes the unbalance of force and moment of the total system, only to cause the transient motion and tension as well as the mean offset. The breakage and the unlatch also make the different effects. The transient responses and the mean offsets are compared and discussed in the viewpoint of the safety of the system.