The Analysis Of Parametric Rolling Of ONR Tumblehome Ship In Head Seas And Study On Stabilization Measures

When navigating in waves,ships will encounter different sea conditions,and the effect of nonlinear wave loads will make the ships very prone to transverse parametric roll.The tumblehome hull has a wave piercing hull design with tumblehome sides and transom stern,which making the variation of the water plane area and the restoring moment of the tumblehome hull are very different from other conventional ship’s.The existing research shows that the tumblehome hull also will sail in parametric rolling in head waves.This paper’s research object is based on the ONR Tumblehome ship,bringing forward two numerical prediction methods of parametric roll motion.One numerical prediction method uses the commercial CFD solver STAR-CCM+ which is based on viscous flow theory,and the other one uses water dynamics software AQWA which is based on the potential theory.In this paper,the parametric roll in waves is calculated and analyzed by the numerical simulations,and then the stabilization measures of the ONR Tumblehome ship are studied.Firstly,two numerical prediction methods of parametric roll motion are studied in this paper,and the feasibility of the numerical methods is proved by an example.Then,numerical simulations of the motion of the ONR Tumblehome ship are carried out under a series of wave conditions.Observing the phenomenon and recording data,this paper analyses and discusses the needing conditions for parametric rolling(speed,initial disturbance),the occurring mechanism and the wave conditions(wave period,wavelength,wave height).The study finds that adding the initial disturbance can’t decide whether parametric rolling takes place or not.An appropriate initial disturbance can effectively shorten calculation time and improve the computing efficiency.The occurrence of parameter rolling needs to meet some wave conditions and encounters frequency range.The parameter rolling is very sensitive to the roll damping and can be suppressed by the method of increasing the roll damping.The swaying motion of the ONR Tumblehome ship shows a strong nonlinearity in irregular head waves,and the number of random seeds has a great influence on the calculation of parameter rolling in numerical simulation.Finally,this paper selects the bilge keel as the ONR Tumblehome ship’s rolling control measure.The effect of the installation angle and the width of the bilge keel is calculated.It is found that the installation angle has little effect on the roll damping,while the width of the bilge keel is a key factor of the roll damping.This paper also proposes a retractable bilge keel device which uses the guide structure,and then the effect of different bilge keel schemes is studied by numerical simulations.The results show that the bilge keel,especially the retractable bilge keel device can effectively reduce the rolling movement of the ONR Tumblehome ship.In the end of this paper,the developing process of the vortices near the bilge keel is analyzed by using CFD software simulation.