Numerical Study And Application Of Anti-rolling Free-flooding Tank

Seakeeping of ship has been paid much attention by researchers all the time.Seakeeping is of vital importance to ship habitability,ship security and operationality.Generally,ship rolling has become the most prominent issue,because both roll damping and restoring moment are too small which provides extensive research space for controlling ship rolling,so that many anti-roll device appeared.As one of the earliest device appeared in the half of the last century,free-flooding tank has supplied an anti-roll strategy to the vessels which have large beam or cannot collocate anti-roll tank inside.The research if of the concrete box-type floating platform.The marine economy is developoed rapidly.The concrete floating platform plays various roles in the industries of offshore,port shipping and coastal tourism.Based on the traditional free-flooding anti-rolling tank,this paper tries to apply this kind of tanks combined with the concrete perforated structure which is commonly used in port engineering to absorb wave to the floating platform,this paper proposed a kind of double shell form with openings tank to reduce the rolling.However,its motion characteristics are different from the motionless caisson,it is necessary to study the coupling motion between the floating platform and the flow field.The anti-rolling effect of the structure size is calculated by numerical simulation,to guide the structural design.Studies on traditional free-flooding tanks with bottom flooding ports used potential method in general,which is mainly due to its main consideration in that the exchange of water inside and outside is easy to simplify.In this study,the water tank openings in the vicinity of the waterline,the flow is extremely complex,the impact of viscosity is significant,so that the potential method is difficult to solve.CFD numerical softwares are getting developed,the threshold of viscous numerical simulation of the complex flow is lowered.In this paper,the viscous CFD method is used to simulate the motion of concrete platform applied with traditional free-flooding tanks.It is concluded that the anti-rolling effect of the tanks is limited,and then the new tank form which is on the side of the platform is proposed.Then,the new form is applied to two box types concrete platforms A and B with aspect L/B,and a series of numerical simulations of free-water decay and the motion in the regular wave are carried out.At the same time,the model tests of platform A in the test-tank were carried out,the results are consistent with the numerical simulation.For platform B,the height,width and number of openings(opening ratio)of double shell perforated tank were changed in the free-rolling decay smulations to analyse their effect on the roll damping coefficients.It was concluded that when the opening ratio was increased,The linear damping coefficient decreased and nonlinear damping coefficient increased.In the regular wave,the simulations revealed that the greater the opening ratio,the better the anti-rolling effect.In the calculation of different operating conditions of platform B,it was found that when the moment of inertia and the center of gravity increased,the roll damping coefficient increased,the RAO decreased and the effect of the anti-rolling effect decreases.Finally,a short-term forecast calculation of the series-size B-type platform in irregular waves was carried out,the new form tank achieved more than 50% anti-rolling effect in the common period(4~8s)irregular wave.In this paper,a new type of anti-rolling design is proposed for the box-type platform.For this kind of platforms with low L/B,double shell perforated tank structure couble be applied on them,whether they are of concrete,steel,plastic texture or FRP texture.This form has the advantage of good anti-rolling effect,low cost,simple construction technology,stable structure,wide range and so on.The deck area of the platform could be increased without changing the displacement.