The Investigation On Wet Towing Coupling Motion Simulation Of Semi-submersible Platform

In the towing operation of semi-submersible platform,a coupling system is constituted of a tug,a towing line,and semi-submersible platform itself.Towing speed and towing line length are two important factors that have great influence on the coupling system.In this thesis,the computational fluid dynamics(CFD)software Star-CCM+ is used.Based on Reynolds timeaveraged Navier-Stokes(RANS)equations and volume of fluid(VOF)method,the hydrodynamic characteristics of the tug and the semi-submersible platform are studied.Then,the maneuverability of the tug is analyzed.Finally,the coupling system is established based on the 6 degree of freedom(DOF)nonlinear mathematical model,and the effects of towing speed and cable length on the coupling system are studied.The main research contents are as follows:(1)The CFD method is used to simulate the drag and planar motion mechanism(PMM)simulations of the tug,and the viscous hydrodynamic coefficients and inertial hydrodynamic coefficients of the tug are obtained according to the 6 DOF nonlinear maneuvering equations.The flow field distribution of velocity and vorticity are studied,and the flow separation changes caused by stem and stern bilge vortex and transverse motion are analyzed.The maneuverability of tug is analyzed via the simulation of constant turning,zig-zag,and spiral motion.(2)Hydrodynamic numerical models of semi-submersible platform are established which accompanied with the CFD simulations of drag,rotating arm and PMM motions.Based on the6 DOF nonlinear mathematical model of semi-submersible platform,the drag coefficients and added mass of the semi-submersible platform are calculated.The velocity distribution near the free surface and vorticity distribution around the floating body are analyzed,and the separation of vorticity in different transverse sections of the body under the same motion frequency and different phases are analyzed.(3)Based on the 6 DOF nonlinear models of tug and semi-submersible platform,wet towing coupling motion simulations with a simplified towing line are presented.The motion response of semi-submersible platform under typical conditions of stationary towing operations,turning with a heading angle,are studied.By comparing the attitude difference between 3 and6 DOF coupling system,the necessity of 6 DOF coupling model was verified.A quantitative method was proposed to study the towing velocity of semi-submersible platform,and the relationship between characteristic parameters and Froude number during towing rotation is studied.The effects of towing speed and cable length on the coupling system are studied respectively,and the sensitivity effects of cable length and thrust on the pose of the semisubmersible platform are analyzed according to those parameters.