Analysis on Effects of Varying Parameters and Hull Shapes on Parametric Instability of Spar

Parametric instability is a phenomenon which may cause excessive motions of ships and platforms. It occurs when a system parameter varies with time and the change meets a certain condition. For spar, the parameter which causes parametric resonance is usually the metacentric height, GM. In studies prior to this work, the parametric instability is mainly analyzed in regular waves. However, the real sea condition is irregular and it is found that parametric resonance also occurs in irregular waves. This study analyzes the parametric instability property of a spar platform being affected by multi-frequency waves, which corresponds to realistic sea conditions. The determinations of stability are carried out based on both analytic methods and numerical simulations. A great difference when analyzing the parametric instability due to an irregular wave is the effect of different hull shapes. The effect is determined by comparing different stability charts of each shape. The effects of the design parameters of a spar platform and wave parameters on are also analyzed by giving contours of the pitch angle. Investigations are also conducted to see how time-varying displacement affects the instability property. This study offers a method of predicting parametric resonance via numerical simulation, and also proposes some suggestions to avoid it in design by choosing appropriate design parameters and hull shape through stability charts.