Numerical Simulation On Coupled Effect Between LNG-FSRU Motion And Liquid Sloshing Under Wave Action

With the increased demand of FPSO, FSRU and other large liquid carrier vessel, academia and engineering now pay great attention to tank sloshing problems again. Researching methods which developed rapidly recently include experimental research, theoretical studies and computational fluid methods. Among these methods, the experimental research is considered as taking relatively too much time and resource despite of its accuracy. Theoretical research is not satisfied on terms of nonlinear terms. On numerical simulation, compared with finite element method and computational fluid method which occupying large resources and takes long time to calculate. The boundary element method is more favored by researchers when it comes to sloshing problems. The boundary element method can deal the problem by cutting one dimension, which not only ensures accuracy, but also reduces computational requirements and time.However, the boundary element method has its disadvantage due to the singularity problem. To solve this problem, this thesis proposed another method called DBIEM(desingularized boundary integral equation method). The traditional boundary method originally arranged all odd points on the surface of the fluid computational domain, while this new method move all odd points to the outside of the computational domain. In another word the computational domain surface is separated from integrating surface in order to solve the singularity problem. In this thesis, in addition to verify the accuracy and applicability of this method, the new method is also applied to in practice. A 329 m FSRU ship is used for the research(Using a 329 m FSRU ship as reserching object), with the new method we can simulate ship motion response considering coupled effects of ship motion and tank sloshing, further, we can simulate fluid movement patterns inside the tank under the coupled movement. The main contents are as follows:(1)Establishing FSRU ship motion equations. According to free surface Green’s function method of three-dimensional potential theory in the frequency domain, wave force, radiation force, diffraction force of ship sailing in regular wave can be derivated. Then calculating the equations of ships motion in waves, to get the ship motion response in six degrees of freedom when sailing in waves.(2)Based on traditional boundary element method, using the BV hydrodynamic analysis software HydroSTAR, and considering the coupled effect of FSRU and tank, we will analyze hydrodynamic coefficients and motion response of ship with fluid tank and acquire ship motion which considers the coupling between ship motion and tank sloshing. Furthermore, we discussed other parameters that may influence the motion of FSRU ship like tank shapes, layout, etc.(3)Based on DBIE-method, the hydrodynamic numerical model of fluid sloshing problem is established. In addition, a program is developed using Fortran based on DBIEM to simulate tank sloshing. Then the reliability of the method is confirmed. Determining the scope of grid size, time step, desingularized distance. At last we simulate tank sloshing under unidirectional or bidirectional irregular exciting.(4)Simulating internal fluid movement patterns of actual FSRU ship while considering the coupling between ship movement and tank sloshing. Choosing ship movement response of typical loading condition, then using the response as boundary conditions for the liquid tank. We can acquire fluid movement patterns under tank sloshing and ship movement interaction when six degrees of freedom motion are all considered. All the work of this article will contribute a lot to the further research, and provide a fast and effective analysis method to design ships with liquid tank or anti-rolling tanks.