PIV Measurements in Thruster-interaction Research

During DP operation the thrusters of the vessel counteract the wind, wave and current forces, so that the vessel can maintain its required position and heading. However, the generated forces can be significantly smaller than what would be expected based on the thrusters’ bollard pull characteristics. This is a result of thruster-interaction (or thrust degradation) effects, which occur as a result of interactions with the hull, current and the wake of neighboring thrusters. The understanding and quantification of these interaction effects is essential for an accurate evaluation of the station-keeping capabilities of DP vessels.
The wake flow behind azimuthing thrusters was investigated in 5 different situations; (1) in open water, (2) under a flat plate, (3) under a schematical barge, (4) a semi-submersible with 8 azimuthing thrusters and (5) a drill ship with 6 azimuthing thrusters. PIV measurements were carried out for all 5 cases. The wake flow behind a ducted azimuthing thruster shows a circular pattern in the measured cross sections.
The wake flows horizontally for the 'Normal' thruster (90 deg gear angle) and is directed slightly downwards for the 'Tilted' thruster (97 deg gear angle). The wake of the 'Normal' thruster under the plate is affected by the presence of the plate. The crosssection is no longer circular and the wake is flattened at the top.
The wake flow attaches to the plate surface and the maximum axial velocities are higher compared to the open water situation. The effect of the plate on the wake flow of the 'Tilted' thruster is much less pronounced, or even absent. This is a result of the wake path, which is directed away from the hull by the 7 deg down-ward tilt angle of the thruster. The wake of the 'Normal' thruster under the barge deflects up as it flows from under the barge into open water. This is known as the Coanda effect. The deflection of the wake is stronger for a larger bilge radius and when the thruster is placed further away from the side of the barge. The presence of a bilge keel was found to prevent deflection of the wake, but this was not further investigated in detail. The effect of the barge on the wake of the 'Tilted' thruster is small. For a semi-submersible with 'Normal' thrusters in beam conditions, the main effect causing thruster-hull interaction losses is the obstruction of the thruster wake by the opposite pontoon. In beam directions, the Coanda effect causes the wake to deflect up, in between the platform pontoons. Current may increase the interaction losses, as the wake deflection appears to be stronger in the presence of (beam on) current.
The interaction losses are significantly smaller for the semi-submersible with 'Tilted' thrusters. The deflection of the thruster wake is almost absent. As a result, the wake flow is no longer obstructed by the opposite pontoon. Current may increase the interaction losses, as the wake of the 'Tilted' thrusters is affected by the current and travels more horizontally compared to the wake flow in still water conditions.
Based on the above observations it may be concluded that the use of 'Tilted' thrusters on a DP vessel can be an effective measure against thruster-hull interaction losses. For this reason, the application of 'Tilted' thrusters is becoming common practice on many modern DP vessels. The results of PIV measurements, such as presented in this paper, can give valuable insight in the flow patterns found in the thruster wake. This can help to improve the understanding of the physics of thrusterinteraction. Furthermore, these measurements can be used as validation material for CFD calculations.