Numerical Simulation On Slow Drift Motion Of Ocean Floating Structures In Two-Layer Fluid

The motion of floating marine structures under wave loads includes two parts:first-order wave frequency motion and second-order slow drift motion with long period on horizontal plane.The second-order low frequency wave force,which induces the slow drift motion,is much smaller than linear wave force.At the same time,since horizontal mooring stiffness is small in the deep water,the amplitude of slow drift is larger than the amplitude of linear response.When the resonance happens,the structures will have a large displacement and would be destroyed.Nowadays,the most researches about interaction between waves and floating structures are under the hypothesis that the distribution of sea-water density is homogeneous.In fact,the density of fluid in the vertical direction of the ocean are stratified.When a floating structure moves in the stratified fluid,it will induce waves on the free surface and internal waves at the interface of fluid density changed,the generated waves will influence the structures' responses and stress,which are much more complicated than the response and stress under the condition of the single-layer ocean.According to the previous researches about stratified fluid,the stratification has a little effect on hydrodynamic characteristic of the first-order wave frequency motion.However,the effects of stratification on the slow drift motion of floating structure have not been drawn attention.It is difficult to directly solve slow drift motion by numerical methods based on the current wave theories.There is great difference between wave frequency motion and slow drift motion in the time scale.Therefore,the two motion can be decoupled and the equation of slow drift motion with low-frequency can be obtained.The resistance,which derived from the experiment research,will be substitute into the equation of slow drift motion of floating structures in the two-layer fluid region.Then,the slow drift motion can solved by the numerical method in the time domain to get the time series of slow drift motion.By comparing the slow drift motion in the two-layer fluid with the motion under the conditions of single-layer,the effect of stratification on the slow drift motion will be discussed in detail.Firstly,the numerical research of the forced surge motion of the floating structures in the two-layer fluid has been conducted.The results show that the stratification has no significant influence on the added mass and the damping coefficient in the range of low frequency,compared with the results of single-layer fluid.Secondly,the towing experiments of the floating box were conducted in the gravity stratified flow tank of PLA University of Science and Technology.The towing resistances in the single-layer and the two-layer fluid are obtained.Finally,the resistances,obtained from the experiment,were substituted into the equation of slow drift motion,and then,the response of floating box under the dichromatic wave in the two-layer fluid will be researched by the validated numerical wave tank.The effect of the stratification on slow drift motion will be discussed below in detail.