Based on model test of deep-sea semi submersible platform, this paper elaborates on the hydrodynamic interaction characteristics of Dynamic Positioning (DP) thruster system, including the thruster-thruster interaction caused by the propeller race from one thruster on neighboring thrusters and the thruster-hull interaction caused by the Coanda effort. The hydrodynamic interaction of thruster system may lead to significant thrust reduction if not being taken into consideration. The research results are illustrated with a set of hydrodynamic curves, comparing the differences between in the current flow and in the calm water. Finally, some feasible ways to reduce interaction so as to improve capability of Dynamic Positioning System (DPS) are presented. Model scale equals 1:50.
Hydrodynamic interaction between thrusters or thruster-hull is important, as many offshore vessels are equipped with rotatable (azimuthing) thrusters for ease of maneuvering and dynamic positioning. Accurate prediction of available thrust to counteract horizontal environmental forces is essential in allowing the realistic station-keeping capability of these vessels to be established. Additionally such thrusters are in many cases used, possibly in conjunction with tunnel thrusters and main propeller to provide propulsive power in transit. Many tests in the past have proved that the efficiency and net thrust from thruster systems and thrusters can be significantly lower than the given and specified value from the manufacturers depending on the location of the units. Based on some experiences from testing both ships and platforms it is showing loss of thrust in the order of 30-70%. Such loss of thrust could be caused by Coanda effect or interaction efforts, such as thruster-hull, thruster-current and thruster-thruster interactions, which are the results of complex physical phenomena (Nienhuis U, 1992). Model tests are often necessary to study and document such transient and complex flow phenomena.
