Numerical Simulation Research On Parametric Resonance Of Offshore Structure

Parametric resonance of offshore structure is a hot research issue nowadays. When parametric resonance occurs, offshore structure may lose stability or even capsize due to violent movement. Among all the offshore structures, container ships and Spar ocean platforms are two typical marine structures which often suffer parametric excitation movement. When the wave length is approximate to the ship length, the rolling restoring moment of the ships is changing over time. This so-called parametric rolling phenomenon is typically a kind of parametric excitation movement. As for the Spar platform, owing to the coupled effect of between heave and pitch, large heave displacement also make the restoring moment of pitch change over time which still caused by parametric excitation movement. In this paper, taking C11container ship and a truss Spar platform as examples, factors affecting affecting parametric resonance and probability density of motion response have been discussed and analyzed.In this paper, parametric rolling problem has been analyzed by Aqwa-Naut module. This is the first time to apply mature software on this issue both home and abroad. First, it has been proved that small perturbation can make parametric rolling happen in condition of head seas. Then in the case of oblique seas, the amplitude of roll motion under different wave amplitude and speed has been calculated by using of the equivalent linear damping and nonlinear damping forms, results of which have also been contrasted by experiment results. It shows different forms of damping can decide the occurrence of parametric resonance. Meanwhile, results of equivalent linear damping are more conservative than that of experiment. Moreover, the influences of significant wave height, wave characteristic period speed and heading angle on parametric resonance problems are studied as well. At last, based on time history curve of heave, roll and pitch, the probability density analysis has been conducted. Different probability density distribution exists whether the parametric resonance occurs or not. Another study in this paper focus on the parametric excitation movement of the truss spar platform. Above all, coupled motion equation of the spar platform and mooring system has been built. Through static method (Catenary method) and dynamic method (lumped mass method), the strength of mooring has been analyzed when the platform experiences large movement because of parametric excitation. Results of dynamic method are2.76times that of static method, although both of them are relatively small comparing of restoring force. Thus, dynamic method is more reasonable to assess the cable strength for parametric resonance problem than static method. At last, probability density analysis has also been conducted on spar platform by using of time history curves of motion responses. The results show:C11container roll natural period and wave peak period ratio of2:00and pitching Truss Spar platform heave natural period and natural period ratio of2:00, are easy to parametric resonance problems. Thus, navigation and design process, due to be circumvented. Parametric resonance does not occur when the instantaneous value of the motion response corresponding probability density distribution follows a Gaussian distribution, the peak corresponding to obey Ruili distribution; when parametric resonance occurs, this phenomenon disappears.According to the research of this paper, it is easy to occur parametric resonance for C11container ship or spar platform when the ratio of roll natural period and wave encounter period or the ratio of the heave natural period and pitch natural period is2.Thus, in the process of navigation or design stage, period ratio of2should be avoided. The probability density distribution from instantaneous value of the motion response follows Gaussian distribution, and the probability density distribution from peak value of the motion response obeys Ruili distribution when parametric resonance does not occur. But this phenomenon disappears when parametric resonance occurs.