In general, it may be said that semi-submersible offshore drilling units are designed under some conditions, such as design conditions and design criteria. The former include conditions of operation site, and the latter consist of requirements for operation and restriction of regulatory bodies, and both can be largely considered as a necessary conditions for initial designing of semi-submersibles. However, it is necessary to work out some sufficient conditions for creating economical and reliable semi-submersibles. In this paper the author explain the above necessary and sufficient conditions obtained through fabrication and operation of Hakuryu series of semi-submersibes, and pick up some problems which we have to consider in the initial design stage for creating a new type of semi-submersibles.