Investigation Of Hydrodynamic Characteristics Of Floating Production Storage And Offloading Unit In Internal Solitary Waves

The South China Sea has rich oil and gas reserves.However the internal solitary waves which are characterized by large amplitude and long duration are frequent due to geological conditions and significant stratification of seawater density.Deep-sea Floating Production,Storage and Offloading unit(FPSO)is one of the major offshore engineering in the exploration of oil and gas.Some accidents,such as the large movement of the FPSO,the fracture of the mooring cables,and so on,can be caused by the internal solitary waves.These seriously affect the normal work and the safety of oil and gas resources development.At present,there are still many unknown fields about the mechanism of possible harm to FPSO caused by the internal solitary waves.Available evaluation means are lacked.In this thesis,the hydrodynamic characteristics of FPSO under the internal solitary waves are studied using the theoretical analysis,model experiments and numerical simulation.Model experiments with the attack angle of 0°～360°between the FPSO and the propagation direction of internal solitary waves were carried in the large scale density stratified tank.Based on the Kd V,e Kd V and MCC internal solitary wave theories,the characteristics of the internal solitary wave load on the FPSO was studied.The rules of the loads with the incident angle,the internal solitary wave amplitude,the upper and lower layer fluid depth were revealed.Results showed that the maximum horizontal forces linearly increased with the internal solitary wave amplitude increasing.The horizontal force amplitude increased with the ratio of upper and lower layer depth decreasing.The amplitude of horizontal forces was the largest with the attack angle of90° between the internal solitary wave and the FPSO,and other cases were less than this case.The minimum horizontal forces changed a little with the incident angle,the internal solitary wave amplitude,and the ratio of the upper and lower layer depth.The amplitude of the transversal forces with the incident angles of 45°and 135°was equivalent to that of the horizontal forces.In contrast,the values of transversal forces under other incident angle cases were very small.The vertical forces on FPSO by internal solitary waves were always positive.Its maximum values were basically constant with the incident angle of the internal solitary wave.On the basis of the internal solitary wave loads characteristics,the theoretical prediction model of the internal solitary wave loads on FPSO was established combined with Froude-Krylov formula and viscous force formula.In the prediction model,the friction coefficient and correction factor for the horizontal and transversal forces were formulated based on the serial model experiments,as well as the formula of viscous coefficient was regressed for the attack angle of 90°.The friction coefficient,correction factor and viscous coefficient were related to Reynolds number Re,Keulegan–Carpenter number KC,the ratio of the upper and lower layer depth h1/h2 and incident angle α between the internal solitary wave and FPSO.The time-history characteristics and amplitudes of internal solitary wave loads on FPSO calculated by the theoretical prediction model for different amplitudes of internal solitary wave,fluid depth ratio of upper and lower layers and wave incident angles were in good agreement with the experimental results.The average velocity of layer depth was used as the velocity inlet boundary condition.VOF method was used to track the surface between two layers of fluid.The numerical model of internal solitary wave loads on FPSO was established through considering the applicability of ISW theories.The hydrodynamic characteristics between FPSO and internal solitary wave for the models of scale ratio λ=1:1,20:1and300:1 were simulated.The differences and reasons of the internal solitary wave load components on FPSO were analyzed.Results showed that the calculation formulas of the friction coefficient and the correction factor in the theoretical prediction model were applicable for the real FPSO.The theoretical model of nonlinear coupled dynamic response of FPSO under internal solitary wave was established through combining with the theoretical prediction model in Chapter 3 and the three degrees of freedom motion equation of FPSO’s floating body.The lumped mass method was used to solve the dynamic characteristics of three-stage mooring cables.The time history characteristics of internal solitary wave load,motion and cable tension under the measured internal solitary wave in the South China Sea were calculated.The results showed that FPSO’s floating body was subjected to huge dynamic loads,large surging,small heaving and tiny pitching.The top tension of the mooring line changed sharply.The horizontal force and moment on FPSO decreased with the increase of upper fluid depth,while the vertical force increased first and then decreased.The horizontal force,vertical force and moment increased with ISW amplitude increasing.Horizontal force and moment decreased slightly with the increase of initial horizontal tension at the top of the mooring cable,while vertical force remained the same.Both the surging and heaving of FPSO decreased with the increase of the upper fluid depth and increased with ISW amplitude increasing.With the increase of initial horizontal tension at the top of the mooring cable,the surging increased,while the heaving and pitching decreased.The top dynamic tension of the FPSO’s mooring line decreased with the increase of the upper fluid depth and increased with ISW amplitude increasing.The dynamic tension at the top of the mooring line in the headstream direction decreased with the increase of the initial horizontal tension,while the one in the backstream direction increased accordingly.