Semi-submersible platforms are used in drilling deep water and in Iran. A deepwater semisubmersible platform is operating in the Caspian Sea. Present work is devoted to some hydrodynamical tests of a simplified platform model. Hydrodynamical tests are performed in a towing tank in Isfahan University of Technology (Subsea Research Development Center). Underwater semi-submersible platform components are divided to underwater pontoons and columns that connect pontoons to platform deck. For this reason at first, experimental tests were designed to measure hydrodynamical forces for pontoons and columns separately, and secondly, the feasibility of using superposition method to calculate total exerted forces on platform model are investigated. The used platform model is built based on the similarity of the Froude and similarity ratio is 1:85. Furthermore, according to the towing tank produced wave characteristics, experimental results are presented for vertical cylinder in range of 0.263 Fr 0.981, for pontoon model in range of 0.2 Fr 1.181 and for the semi-submersible platform model in range of 0.181 Fr 0.703. Experimental tests on platform were arranged in three different draft ,three different angle( zero, 30 and 60 degrees) and with four different waves frequency (0.8 to 1.13 Hz). Most of the test were done in the range of linear wave theory and therefore, drag and inertia coefficients of platform and components are calculated using deep water theory, measured horizontal forces and Morrison’s equation. According to the results of vertical circular cylinder and other experimental works there are 15% Average differences betwee them. Average differences between measured forces for the platform and calculated forces using superposition method show 14.8% in different depths. Comparing the results of measured forces for the platform and calculated forces using superposition method show that average difference between them are 15.15% in different angle. Key words: Hydrodynamic coefficients, Semi-submersible platform, Morrison equation, Regular waves, Froude number
