Coupled Method Of Sloshing Simulation And Structural Response Evaluation In VLCC Tanks

With the R&D and application of liquid carries, such as VLCC, LPG and LNG, sloshing in tanks is becoming main interest in tank structure design. Liquid carries sometimes unavoidably sail with partially filled tanks. Fluid in these tanks moves duo to ship motion in waves. Violent fluid motion (sloshing) is found in such partially filled tanks when ship motion period is close to fluid natural oscillation period in these tanks. Sloshing causes violent fluid impacts on tank boundary structure with high pressure. Therefore, to investigate tank structure response and any possible damage induced by sloshing impact pressure is a big problem in ship design. The major purpose of this thesis is to present a practical method for VLCC sloshing simulation and structural response and assessment in tanks. For this reason, the following problems are emphatically investigated.Firstly, the method for ship's 6-DOF time history motion is presented. By comparison of all sea-keeping theories available for ship motion prediction, 2D slice theory is thought as the suitable one and selected for its simpleness and efficiency in this calculation. Basing on ship time history motion, formulas for tank time history motion is derived. In order to have the ship be motionless at the beginning of simulation, a method is given for modifying ship time history curve of 6-DOF with different initial phases.Secondly, the approach (pressure-based method for sloshing in tanks) presented by DNV is discussed, and also its uncertain problems. Then a coupled sloshing simulation and structural strength assessment method is presented by the procedure of short term scanning, long term analysis and structural response where fluid and structure interactions are considered. In this method, time and space zooming technique and reduce of bulk modulus are introduced. By this, time expense for simulation is decreased dramatically, and we could not only obtain stress distribution in details but also avoid the difficulties of coupled simulation for the whole tank. Also, three tank model strategies are given for shot term scanning and long term analysis. Taking advantage of feature of fluid flowing in tanks and general distribution of tank in VLCC, 2D model is a good selection among the three strategies. By this, we could not only improve simulation efficiency, but also could consider sloshing effects on tank structures sufficiently during technique design. And structure optimization could be possible.On the basis of above studying work, taking a VLCC as an example, the sloshing simulation and structure assessment is carried out. Some conclusions are made for its structure design.