Modeling Two-Phase Flow With Offshore Applications

Abstract

With the trend towards offshore LNG production and offloading, sloshing of LNG in partially filled tanks has become an important research subject for the offshore industry. LNG sloshing may induce impact pressures on the containment system and may affect the motions of the LNG carrier.So far, LNG sloshing has been studied mainly using model experiments with an oscillation tank. However, the development of Navier-Stokes solvers with a detailed handling of the free surface allows the numerical simulation of sloshing. It should be investigated, however, how accurate the results of this type of simulations are for this complex flow problem.The paper first presents the details of the numerical model, an improved Volume Of Fluid (iVOF) method. The program has been developed initially to study the sloshing of liquid fuel in satellites. Later, the numerical model has been used for calculations of green water loading and the analysis of antiroll tanks, including the coupling with ship motions. Recently, the model has been extended to incorporate twophase flow. This extension improves its ability to simulate the effect of gas bubbles of different sizes. Gas bubbles are present in virtually all relevant offshore situations; not only at LNG sloshing but also during green water events, bow slamming and water entry.In a two-phase flow model, both the liquid and the gas phase can have their own continuity and momentum equations. The handling of the compressibility of the gas phase is a major issue in the design of a two-phase flow model. However, as a first step in the modeling process, the gas phase is considered as incompressible.For a dambreak experiment, results of the one-phase model, the incompressible two-phase model and model experiment results have been compared. It is shown that the physics are more accurately simulated with the incompressible two-phase model. Furthermore, the paper will show results of the incompressible model for LNG sloshing.The physics of LNG sloshing and several other applications can be approached better by taking the compressibility into account. Therefore, as a second step, a compressible model is currently under construction, involving adiabatic compression of the gas phase.

Application of a VOF Method to Model Compressible Two-Phase Flow in Sloshing Tanks

“…By doing this in the right way, spurious velocities are prevented. For different test cases, it has been shown earlier [7] that the simulation results from the two-phase model have a better agreement with available measurement results than the results from the corresponding one-phase model. Now the numerical model has been tested on more elaborate sloshing experiments on 1:10 model scale.…”

“…The simulations are validated on a series of 1:10 scale sloshing model experiments. In comparison with earlier sloshing model experiments [7], the current validation experiments are on a larger model scale. Moreover, the numerical model is able to use the experimental global motion time-traces from the sloshing tank as input for the simulations.…”

CFD Simulations of a Semi-Submersible With Absorbing Boundary Conditions

“…As described below, an adiabatic relation between pressure and density closes the system of Navier-Stokes equations (1) and (2), see also e.g. [10][11][12][13].…”

“…The numerical method COMFLOW is based on the Navier-Stokes equations, in which a VOF method is applied to describe the evolution of the free surface, see e.g. [1][2][3][4][5][6][7][8][9][10][11][12][13]. A local height function near the free surface yields improved performance in terms of mass conservation and the number of disconnected droplets, compared to the original VOF method introduced in [14].…”

Numerical simulation of 3D sloshing flow in partially filled LNG tank using a coupled level-set and volume-of-fluid method

“…Loots et al (2004) presented an improved Volume of Fluid (iVOF) method to numerically produce the dynamics of sloshing in LNG tanks, with several improvements in the treatment of spikes for the pressure signals. Wemmenhove et al (2005) extended iVOF method to incorporate two-phase flow and improved the method to simulate the effect of gas bubbles of different sizes. Yu et al (2007) performed the Level-Set Reynolds-Averaged Navier-Stokes (RANS) method for the simulation of liquid sloshing in 2D and 3D LNG tanks.…”