This study aims to define a simulation strategy through Computational Fluid Dynamics (CFD) for modeling the cool down process in a 3-phase (oil/water/gas) gravitational subsea separator. It was observed that the 3-phase model obtained results with excellent agreement with the available experimental data. Then, simplifications were made to minimize the computational effort, focusing in maintaining acceptable levels of accuracy. First, it has been found that it is possible to ensure the robustness of the model even when less refined meshes were employed. Later, a set of phase grouping was applied. The 2-phase model, which combines the two liquid phases, returned robust results in a reduced processing time. However, the 1-phase and pure solid simulations, although fast, had a lower agreement with the experiment, discouraging their use without further precautions.