Experimental And Numerical Studies Of Sloshing And Suppressing Effectiveness In A Tank With A New Floating Device

Natural gas is primarily composed of methane,which is colorless,tasteless,high calorific value and stable combustion.Besides,the weight of natural gas is lighter than air,which is volatile and unfavorable for the accumulation of gas.Liquefied natural gas(LNG)in the same amount of volume is only 1/625 of the volume of gas.Furthermore,natural gas is clean,efficient and relatively safe.The energy growth is declining further as a result of the world economy in its transition to a new normal.At the same time,structural change in energy gives rise to increasing demand for clean energy.Oil and gas drilling is moving further offshore and deeper underwater.By blending the floating liquefied natural gas(FLNG)production storage and offloading unit with the liquefied natural gas carrier(LNGC),the scale and economic development of deep-sea and marginal gas fields can be realized,and the cost and time required for the delivery of natural gas to the terminal may be effectively reduced.At sea,sloshing motion occurs unavoidably in partly filled tank,especially the floating liquefied natural gas(FLNG)production storage and offloading unit and the liquefied natural gas carrier(LNGC).Sloshing is a phenomenon of free surface fluctuations generated by external excitation,and it has strong nonlinearity and randomness.When the excitation frequency approaches the lowest natural frequency of the free surface,sloshing becomes most violent and significantly affects structural safety.Therefore,with the popularization and large-scale development of the membrane-type tanks,it is necessary and urgent to do studies on suppressing sloshing.A floating device is installed close to the side wall on the basis of the principle of floating breakwater,which is suitable for different filling rates and has a good application prospect.Each floating plate is supported by two full extension stainless steel slides.Therefore,the two floating plates can only move vertically within the tank.A series of model tests are conducted to investigate the free surface elevation and impact pressure under three different filling rates in a rigid rectangular tank with two floating plates,where a nearly two-dimensional flow is generated by moving the tank through external excitation.Comparison of the clean tank shows that adding two floating plates not only reduces effectively wave runup along the longitudinal bulkhead but also decreases significantly impact loads acting on the bulkhead for three different filling rates under prescribed harmonic rolling excitation.Furthermore,a numerical simulation based on the ANSYS FLUENT program is introduced to conduct an additional investigation.The 6DOF(Six Degree of Freedom)solver,along with the use of dynamic meshes,can be readily applied to sloshing flow interactions and to two floating plates.Except for the gravitational force,the external forces in the model also include the translational and rotational inertia forces.The accuracy of the numerical simulation is verified against the experimental results by comparing the wave elevation and impact pressure acting on the bulkhead.The velocity field in the tank with floating plates is numerically evaluated and discussed as a supplement to understand the mechanisms of suppressing sloshing.Under guaranteeing the suppressing effectiveness on sloshing,the force acted on the full extension slides should be reduced simultaneously.And so the floating plates need to be optimized.Model tests of suppressing sloshing fitted with two perforated floating plates are carried out.The study involves identification of system performance such as the suppression and the solidity ratio.Three different solidity ratios of perforated plates have been tried out as potential positive slosh damping devices.A series of sophisticated experiments have been conducted under roll harmonic excitation in the frequency range that covers the three lowest natural frequencies of a clean tank.Compared with the clean tank,all three solidity ratios can reduce significantly the runup along the vertical bulkhead and impact pressure subjected to different forcing amplitudes and frequencies for three filling rates.Among three solidity ratios considered,the maximum and the average peak impact pressure have demonstrated similar tendency,with a little increasing of the impact load as the solidity ratio decreases.The discharge capacity of the section increases accordingly with the decreasing of the solidity ratio as well.To investigate flow characteristics in the tank with two perforated floating plates,a numerical simulation is conducted using the ANSYS FLUENT program in a non-inertial reference frame.In order to reasonably reproduce the sloshing phenomenon and save the computational time,the modeling of perforated floating plates is simplified reasonably according to practical size parameters.The simulated water profiles and pressure history agree with the aforementioned experimental results.The velocity field in the tank with perforated floating plates is numerically predicted and discussed as a supplement to understand the mechanisms of suppressing sloshing.Outcomes of this study can be used as a facility of suppressing sloshing,which can be further optimized in order to achieve practical purposes for production.Meanwhile,the experimental and numerical results can be used to validate other numerical models.The numerical simulation based on the ANSYS FLUENT program can form a stable and reliable numerical prediction method,and provide an effective means for the studies on suppressing sloshing.