Science and Education Publishing Journal of Ocean Research 2 1 2014 10 09 Effect of Mooring Lines Pattern in a Semi-submersible Platform at Surge and Sway Movements 17 22 EN Hadi Sabziyan Hassan Ghassemi Department of Ocean Engineering, Amirkabir University of Technology, Tehran, Iran Farhood Azarsina Saeid Kazemi JOR2014214 10.12691/jor-2-1-4 2014 09 15 2014 10 08 2014 10 09 Exposure to environmental conditions at sea for floating structures is inevitable. Environmental conditions that waves are most important of them will enter forces on structure of semi-submersible platforms. Therefore such structures should be deployed in the operational capability of their own, that one of these methods is mooring them. In this condition, structure shows different behavior compared with unmoored structure. Wave force cause motions of structure and subsequently produce tension force on mooring lines. Hence, investigation of structure movements and selection an appropriate mooring system to minimizing the structure motions must have been discussed. semi-submersible platforms mooring systems results restoring force in horizontal plane, and thus control degree of freedom on Surge, Sway and Yaw movements. This study estimated Surge and Sway movements of a semi-submersible platform when that it has been exposed to 0, 45 and 90 degrees of sea wave direction with the environmental conditions of the Caspian Sea using Flow-3d (version10.0.1) software. Also the seven symmetric mooring systems in the form of 4 and 8 numbers of mooring lines' systems have been used to investigate the best modes.