Float-over installation method is widely utilized for the topside integration installation. Mating operation is the most important phase of the float-over, as unacceptable consequences such as damage in jacket or topside are more likely to occur in this phase comparing to the Standby, Entry and Exit phases. This paper presents the considerations in the engineering design of the float-over installation of the fullyintegrated Liwan 3-1 topside. The topside weighed about 26,300 MT and was installed onto the jacket by the bottle shaped barge HYSY229. The emphasis of this paper is on describing the engineering considerations which is required for the safety of the float-over mating operation.