An experimental study on the extreme motion responses of a SPAR platform in the heave resonant waves

The prediction of motions in resonant waves is essential to design offshore structures. The extreme motions of a spar platform in heave resonant waves are investigated by an experimental approach. The prototype spar platform is a 'classic' spar which is a vertical steel cylinder with 14 equally spread catenary mooring lines. Model tests were conducted under various regular wave conditions in the Samsung Ship Model Basin. In the model tests, the spread mooring lines are simulated by using four spring-wire lines with the same equivalent horizontal system stiffness. 6-DOF motions of the spar model, relative wave heights and mooring tensions at the fairleader points were measured and analyzed. The objective of the tests is to assess global responses of offshore floaters and mooring lines of a typical spar. Four spar models with different characteristics are tested; sparl is prototype model, spar2 is the model of which natural period of pitch motion is about twice compared to the heave resonant period, two heave damping plates are attached at the bottom of spar3 and three strakes are removed in spar4 model. It was observed that unstable roll and pitch motions were occurred when the periods of encounter wave are close to the heave resonant period and twice of the roll/pitch natural periods. The effect of time-varying GM value in view of 'Mathieu-type instability' is investigated for the prediction of the extreme heave-induced pitch and roll motions. The results of model tests showed superior agreements with damped Mathieu stability diagram, and the positive effects of strake and heave damping plates were confirmed.