Performance of Thrusters

ABSTRACT The large powers required for dynamic station keeping of offshore drilling platforms have to be transmitted into the highest possible thrust, exerted at the location and in the direction required. This paper gives information on hydrodynamic characteristics of controllable pitch propellers and thrusters for offshore drilling platforms with dynamic positioning. Main aspects discussed are merits of the various thruster types, design point, offdesign performance and cavitation aspects. INTRODUCTION The large powers required for dynamic positioning of offshore drilling platforms have to be transmitted into the highest possible thrust, exerted at the location and in the direction required. The total power required for dynamic positioning has a large influence on deadweight. Moreover fuel is to be supplied by special vessels. It is important to analyze the efficiency of the various thruster types and the influence of size, both in survival and part-load condition. The results of this analysis can be used to select not only thruster types but also total thruster configurations. There is little published information on performance of ducted controllable pitch propellers (CPp) at very low or zero speed of advance. The aim of this paper is to assess the main parameters for CPP thruster configuration selection, and the thruster types reviewed are as follows:–ducted propeller with predominant thrust direction, i.e. nonsymmetrical nozzle with twisted blades for bollard condition ahead.–ducted propeller without predominant thrust direction, i.e. symmetrical nozzle, or tunnel with flat-bladed propeller.–Non-ducted propeller with predominant thrust direction, i.e. propeller with twisted blades for bollard condition ahead.–non-ducted propeller without predominant thrust direction, i.e. propeller with flat blades. Whether thrusters are fixed in position, retractable, or of the azimuthing type, is irrelevant for this presentation. It is clear, however, that these and other practical aspects do have a bearing on thruster performance and, thereby, influence the selection of total thruster configuration. MERIT COEFFICIENTS An important factor in the selection of thrusters is the thrust-power relation in bollard condition. This relationship is determined by two parameters, the thruster load and the merit coefficient nO as follows (Mathematical Equation Available In Full Paper) nO is different for each thruster type mentioned in the introduction. The thrust per unit horse power is high for high merit coefficient and low thruster load, see Formula (1). Moreover, increase of thruster load increases thrust per unit disc area, as can be seen Formula (3), but unfortunately less than proportionally. As a more general approach it is proper to utilize two different merit coefficients:–one which can be calculated when SHP and D are known in order to determine the optimum rotational speed n:nO as defined above.–one which can be calculated when SHP and rotational speed n are known, as is normally the case for right-angle drives, to determine the optimum diameter. This second coefficient can be defined as. (Mathematical Equation Available In Full Paper)