Research On Parametric Resonance Of Deep Draft Semi-submersible Platform

Deep Draft Semi-submersible platform(DDS)is a new kind of floating structure.It is different from traditional semi-submersible platform,the draft of DDS is greater than traditional semi-submersible platform,so heave motion of DDS is more moderate than traditional semi-submersible platform.Deep Draft Semi-submersible platform not only inherits the advantages of traditional semi-submersible platform,but also improves its heave performance.So Deep Draft Semi-submersible platform will have a great value in deep water oil and gas exploitation.Parametric resonance is a phenomenon caused by parameters of periodic change over time.It is a common phenomenon existing on offshore structures,such as ships,top-tensioned risers(TTR),the tethers of tension-leg platforms(TLPs)and Spar platforms.When Deep Draft Semi-submersible platform works on the sea,heave motion will lead to its metacentric height changes over time,thus causes parametric pitch to occur.When parametric pitch happens,motions of Deep Draft Semi-submersible platform in some degree of freedom will increase largely so that structures and crews will suffer harm.So it has a really important practical significance to investigate parametric resonance of Deep Draft Semi-submersible platform.Many scholars have investigated parametric resonance of floating structures in regular waves,but the realistic sea conditions are irregular waves,which is more complex and random than regular waves.Some scholars also confirm parametric resonance in irregular waves in experiments.So it is necessary to investigate parametric resonance of Deep Draft Semi-submersible platform in regular waves and random waves.In this study,parametric resonance of Deep Draft Semi-submersible platform is investigated in regular waves and random waves.Pitch motion equation of Deep Draft Semi-submersible platform is established.Through analytic prediction,stability chart is obtained.By using numerical simulation,influences of different parameters are analyzed,and the differences of parametric resonance in regular waves and random waves are also compared.The main research results are as follows:(1)Analysis of parametric resonance of Deep Draft Semi-submersible platform in regular waves and random wavesPitch motion equation of Deep Draft Semi-submersible platform in regular waves and random waves is established.By using analytic method,stability chart is obtained.It can predict if DDS is stable or not by using stability chart.Through numerical simulation,different parameters’ effects are analyzed,and design proposals to avoid parametric resonance are recommended.(2)Analysis of influences of wave parameters on parametric pitch of Deep Draft Semi-submersible platform in random wavesInfluences of significant wave height,wave peak period and wave phase on parametric pitch of Deep Draft Semi-submersible platform in random waves are investigated.Using the control variable research methods,making one wave parameter changes within reasonable scope and other wave parameters remain the same,then solving pitch motion equation of Deep Draft Semi-submersible platform,stability chart can be obtained.By comparing stability charts under different wave parameters,influences of wave parameters on parametric pitch of DDS are analyzed,then several measures are recommended to avoid DDS parametric pitch.(3)Comparison of parametric pitch of Deep Draft Semi-submersible platform in regular waves and random wavesThe differences of parametric pitch of Deep Draft Semi-submersible platform in regular waves and random waves are analyzed.The stability charts in regular waves and random waves are drawn in one diagram,then several typical points are chosen to analyze their stabilities.Through numerical simulation,comparing in time domain,phase plane and frequency domain,the different properties of parametric pitch of Deep Draft Semi-submersible platform are analyzed.