Investigation On The Interaction Between Deep-Sea Floating Platforms With Internal Solitary Waves

Deep-sea floating platforms are the main high-tech marine engineering equipments for oil and gas exploration.A great amount of ocean observations show that internal solitary waves?ISWs?are ubiquitous features in the South China Sea because of the complex geological condition and the seawater stratification.ISWs are often very energetic with remarkable amplitudes so that they can exert considerable loads on these structures.It will cause severe damages to the deep-sea floating platforms,such as a large drift and the rupture of mooring lines.As the second severe disaster after typhoon,ISW has to be considered for the safety of marine structures in the oil and gas exploration and development activities.However,the damage mechanism on deep-sea floating platforms caused by ISWs are lack of effective analysis approaches yet.Wave loads and dynamic responses of deep-sea floating platforms induced by ISWs have been investigated with theoretical,experimental and numerical methods.In the second chapter,a theoretical model has been established for estimating the loads on the deep-sea floating platforms at different wave direction angles in ISWs.Taking the semi-submersible and the tension leg platform?TLP?as research objects,the Morison equation is applied to calculate the inertia and drag forces while the pressure-integral method is used to calculate the Froude-Krylov force.The interface displacement is obtained by three types of stationary internal solitary wave theory KdV?eKdV and MCC.Meanwhile,experiments were conducted in a density stratified two-layer fresh/saline fluid system in a steel-framed wave flume.Depression ISWs were generated by a piston-type wave maker.The forces on the deep-sea floating platform models were measured.Based on the experimental results,the inertia and drag coefficients for columns and pontoons are figured out as a function of KC,Reynolds number and the layer depth h₁/h.Moreover,a series of experiments with different parameters,such as wave amplitude,layer depth and wave direction angle,were designed to study the time history and amplitude characteristics of internal solitary wave forces exerted on the platforms.Finally,the influences induced by wave amplitude,layer depth and wave direction angle on force amplitude have been investigated.In the third chapter,numerical simulations of the interaction between ISWs and the TLP are presented.The governing equation is Navier-Stokes equation,and the velocity inlet is specified as the layer-mean velocity of the upper/lower layer.The force characteristics of different scale platform models have been studied and the scale effect of wave forces has been analyzed further.It is demonstrated that the theoretical model of internal solitary wave forces on the deep-sea floating platforms and the determination method of inertia and drag coefficients in Chapter 2 are also suitable for large scale cases at the real ocean circumstance.In the fourth chapter,in order to study the dynamic response of NH-8Semi-submersible Platform moored in ISWs,a nonlinear theoretical model has been established by combining the internal solitary wave force theory with nonlinear motion equations.Mooring lines are calculated by the lumped mass method.The histories of the semi-submersible motion and the tension force exerted by the mooring system have been analyzed numerically for differnent wave directions.Results show a large-amplitude surge motion.And the mooring line tension in the head sea direction increases while that of the opposite direction decreases.For the same layer depth and wave amplitude,the wave direction affects obviously on the horizontal wave force and moment,surge motion and the mooring line's tensions,but slightly on the vertical wave focre,heave and pitch motion.Furthermore,the influences of wave amplitude and layer depth on force amplitude,motion response and tension increments of the mooring lines for different wave directions 0°and 90°have been investigated.In the fifth chapter,a nonlinear theoretical model has also been established by combining the internal solitary wave force theory with nonlinear motion equations to numerically simulate the dynamic response of the TLP platform moored in ISWs.The nonlinear tendon model is applied to solve the vertical tension leg system.The histories of the TLP motion and the tension force of tension legs have been calculated for differnent wave directions based on ISW measurements in the northern South China Sea.It is found that the TLP has a large-amplitude surge motion,and the heave motion is very small because of the enormous pretension provided by the tension legs.The legs'tension acting on the floating platform increases rapidly,and the tension increment ofthe head sea direction is much larger than that of the other side.For the same layer depth and wave amplitude,the wave direction influences significantly on the horizontal wave force and moment,surge motion,and the leg's tensions,while it has little influence on the vertical wave focre,heave and pitch motion.In addition,the effects of wave amplitude and layer depth on force amplitude,motion response and tension increments of the tension legs for different wave directions 0°and 45°are discussed.