In this thesis the differences between the monohull and semi-submersible are investigated. For a general heavy lift crane operation and its operational profile, the difference is performance and total cost are calculated. These are used to determine the turning point, the point where the semi-submersible is more favourable for a heavy lift crane operation. The monohull has relatively low capital and operational costs, but also a lower workability. There is thus a balance that has to be investigated and compared between the two vessel types. This is done for the North Sea and West Africa, which have significantly different sea conditions.