Analysis Of Positioning Characteristics For The Semi-Submersible Platform In Internal Solitary Waves

The submarine geomorphic characteristics of the South China Sea are complex,with the pheononmena of uneven density distribution and variable wave shapes,resulting in frequent occurrence of internal solitary waves(ISWs).A large number of observation results show that ISWs may pose serious hazards to oceanic platform structures.In order to ensure the safety operation of the platform in this region of the sea,an analysis for the dynamic positioning capability and motion response in ISWs conditions then appears to be essential.However,it seems that very few experiences have been reported in the literature with respect to positioning capability in the ISWs environment,which is actually a field that needs intense research.Therefore,on the basis of reviewing the relevant research progress,combined with the laboratory experiments,numerical simulation and theoretical analysis,a method to predict the ISW-induced forces acting on the caissons and columns of the semi-submersible platform is established.Focus is put on obtaining realistic external ISW forces,dynamic response characteristics,and evaluating the positioning capability under various external environment loads.Detailed work is organized as follows:A prediction method is proposed which simulates the ISW-induced loads acting on the columns and caissons of the semi-submersible platform respectively,based on the three different kinds of ISW theories(MCC,Kd V and e Kd V).Verification the applicability of the prediction method with comparison between experiments and calculation is then conducted,demonstrating that the utilized method can yield reliable results with sufficient accuracy in the analyzing the ISWs-forces in different incident wave direction.It is found that the Morison equation can be applied to analyze the ISW-horizontal forces exerted on the columns,whilst Morison equation is allowed to calculate the horizontal forces on the caissons when the direction of incoming waves is not parallel to the longitudinal profile of the platform;the horizontal forces on the caissons,if the incoming wave is in the same direction with the longitudinal profile,can be analyzed by means of the Froude-Krylov approach.No matter whether the propagation direction of the ISWs is parallel to the longitudinal profile of the platform,however,it is feasible to estimate the vertical ISW-load by adopting the Froude-Krylov equation.Based on the measured parameters of ISWs near Dongsha Island in the South China Sea,a theoretical model for the ISW-forces has been established with four kinds of ocean stratified conditions.The real-scale ISW forces was added in the traditional method.Combined with quadratic programming methods,an analysis on the capability of the dynamic positioning for the semi-submersible platform was carried out and,extreme safe amplitudes under different layer thickness ratios were proposed.It is showed from the results that the load curve of the surge is a sine curve and the load curve of the sway motion is a cosine curve,with a phase difference of 45°.Reference to the obtained results from the DP capability simulation reveals that the direction of the ISWs is about 60°,serious positioning failure will occur under different layer thickness ratios.Therefore,the heading angles should be adjusted in time to avoid encountering the ISWs in this directions,which can ensure the safe operation of the platform.Combined with the three-degree-of-freedom nonlinear motion equation,a theoretical model has been established to numerically simulate the dynamic response of the semi-submersible platform mooring in ISWs,which can be computed by the lumped mass method.The time history of the platform and the mooring lines have been analyzed for different layer thickness ratios and,extreme safe ISW-amplitudes were proposed as well.Results show that the semi-submersible platform will produce a significant horizontal displacement.However,the pitch and heave response is quite small.Therefore,the influence of ISWs on the platform will mainly be reflected in the low-frequency motion response.