Modeling two-phase flow with offshore applications

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 anti-roll tanks, including the coupling with ship motions.

Recently, the model has been extended to incorporate two-phase 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.