Effect of simultaneous three-angular motion on the performance of an air-water-oil separator under offshore operation

A three-dimensional (3D) multiphase Eulerian computational fluid dynamics (CFD) model was developed to investigate the effect of a simultaneous three-angular motion (roll, yaw and pitch) on the performance of an air-water-oil separator. The dynamic mesh method with a 3D rotational matrix was used to simulate the three-phase separator under simultaneous three-angular motion. The pressures at water and oil outlets were adjusted by a user-defined function based on a proportional control algorithm. The CFD model was validated against experimental free oscillations of an immiscible two-fluid system under a pitching motion. The simultaneous three-angular motion with an amplitude 2 degrees and a period of 8 s showed an oil purity of 98% and an oil recovery of 99%, which were comparable to those of the onshore separator. However, the 4 degrees simultaneous three-angular motion deteriorated the oil recovery to 92% and the water purity to 75%. As the separation performance of the pitching motion was similar to that of the simultaneous three-angular motion, the performance of the three-phase separator can be tested simply by the pitching motion.