Aspects of the hydrodynamic and structural loading experienced by semi-submersibles are investigated in this thesis with a view to formulating an integrated approach to semi-submersible structural design. A waveloading program was developed for floating structures, comprising rectangular and circular members at any angle of incidence to the waves. It yields, the hydrostatic particulars of the structure, the wave induced forces and moments, including the distribution of the forces along the members and the resulting body motions. There is an I option for using either constant or frequency dependent coefficients. Comparison of the motion characteristics of different designs can be carried out quite easily and novel designs, e.g. ring-pontoon semi- submersibles, can be investigated. Experiments were carried out to determine the wave induced forces on models of rectangular components of semi-submersibles for a number of angles of incidence, in order to check on the validity of certain approximations inherent in the application of strip techniques to force and moment computations. These were compared with theoretical predictions, as given by the linear and non-linear approaches, as well as Frank's Close-Fit Method. Very good agreement was obtained for the heave forces, as predicted by the linear approach. Agreement for the surge and sway forces varied and a comprehensive account of the topic is given in Chapter 3. The waveloading program was linked to a 3-dimensional frame analysis program to provide a tool for investigating different structural arrangements and give design values to be used for calculating the scantlings of structural elements. A preliminary study of a number of bracing arrangements seems to justify the trend in new designs to adopt transverse bracings only, together with torsionally stiff decks. The output from the frame analysis program is input to the structural synthesis program which provides structural weights and data on the scantlings of the rectangular members of semi-submersibles. The approach to the structural design is a fundamental one in which each member is designed to withstand the combined effects of its inplane and normal loading. Checks are applied to ensure that neither an overall failure mode nor local failure mode occurs. The weight of the pontoons seems to be dominated by static load considerations, and in particular, by pressure induced loads. The complete system of programs provides a logical system by which a designer can arrive at an efficient structural arrangement both from the overall geometry (motion considerations) and local arrangements of plating and stiffeners.