Investigation On Prediction Of Parametric Pitch Of Spar Platforms In Irregular Waves

The parametric instability of Spar platforms is analyzed under the condition of irregular waves. With the exploitation of oil and gas extends to deeper ocean area, the floating platforms are applied wider and wider. Parametric instability is a phenomenon that may cause excessive motion of offshore structures. Parametric instability occurs when a system parameter varies with time, and the variety meets certain conditions. For a Spar platform, the parameter that causes parametric resonance is usually the time varying metacentric height due to heave motion. And it will result in large motion in pitching or rolling, which may cause damage to the platform, the facilities and the crew. Thus prediction of this phenomenon could help avoiding loss of economy and lives. Many researchers have investigated the parametric instability of Spar platforms in condition of regular waves. However, waves in real sea conditions are irregular. In addition, some accidents and experiments have shown that parametric resonance also occurs in irregular waves. It may be dangerous to ignore the condition of irregular waves. Thus, though there are lots of challenges in predicting the parametric pitch of Spar platforms in irregular waves, it is important to analyze this phenomenon in condition of irregular waves.This paper concludes the present analysis on parametric instability of Spar platforms. Based on the heave-pitch motion equation and the linear wave theory, heave motion of the Spar platform in irregular waves is simulated. The following investigations are then conducted.1) Hill’s equation which can be applied to the condition of irregular waves is deduced. Analytic prediction of parametric pitch of Spar platforms in irregular waves is realized. Stability diagrams containing the boundaries of the stability regions in conditions of multiple frequencies are given and compared with the diagrams in conditions of single frequency. The distribution of unstable regions and the influence of damping coefficient are investigated. Stability diagrams of different hull shapes are also given based on the solution of Hill’s equation. Effects of these factors are analyzed and the hull shapes which can help avoiding parametric instability are given.2) Simulations of parametric pitch of Spar platforms in irregular waves are carried out based on the numerical simulation method. The simulations can not only determine whether the Spar platform is stable, but can also give the maximum pitch angle. The effects of the design parameters of a Spar platform and the influence of wave parameters are then analyzed by giving the contours of the maximum pitch angle. The effects of time-varying displacement are also considered in this work.This paper gives the predictions and analysis of parametric pitch of Spar platforms in irregular waves. Some suggestions are also given to avoid the parametric pitch of a Spar in design by adjusting design parameters appropriately and choose the best hull shape. In this way, the parametric instability of Spar platforms in irregular waves can be avoided, which ensures the safety of structures and crew in service.