ABSTRACT
Open moonpool in drillship will cause additional resistance and limit the adaptability of ship. Considering these problems, a numerical method based on STAR-CCM+ was adopted to simulate the resistance of ship with moonpools. Six cases with different step positions and step shapes were calculated to find the effects of moonpools with different steps on ship resistance and hydrodynamic characteristics inside the moonpool. Three gauges were set to monitor the motions of free surface inside the moonpool. Results showed that a significant reduction of ship resistance was found when the ship with a step moonpool comparing with the one with plain moonpool. The change of the shape and position of moonpool step mainly affected the movement amplitude of free surface inside the moonpool, and had little influence on the movement frequency of free surface.
Moonpools are often used on ships and offshore platforms to facilitate the laying of cables, installation of risers, retracting and discharging equipment and other operations. While, the moonpool is an opening in the hull which is filled with water and own a free surface. Such opening hole may cause strong vibration of the fluid inside the moonpool,which will attack the device inside and cause additional resistance when the ship sailing, and this phenomenon attracted more and more attention. The research approaches of moonpool mainly include model test, theoretical study and numerical simulation. Based on two model tests series that carried out at Delft University of Technology, Riaan van 't Veer et al. (2008) constructed a resistance prediction model for piston type oscillations. The model was verified with a resistance measurement performed at MARIN, showing fair agreement between the predicted and measured resistance. Hammargren et al. (2012) investigated numerically the flow behavior in a moonpool of a drillship and compared the oscillation period of the numerical solution and a value calculated analytically, successfully simulated the flow pattern to get a deeper understanding of the flow phenomena. Sadiq et al. (2008) used CFD commercial software CFX to study two-dimensional and three-dimensional cylindrical and square moonpools under the condition of uniform flow, the results showed that the form of the moonpools had a great influence on the whirlpool shape and the hydrodynamic characteristics of the moonpools. Alsgaard (2010) used CFD method to study the two-dimensional piston motion of the fluid in moonpool, and compared the results with the model test. Heo et al. (2014) concluded that vortex shedding and friction drag act as the main sources of the damping, which depended on the sharpness of the moonpool corners and appendages should be considered for the moonpool dynamics. So it is of great interest to get a deep understanding of how the moonpool affects the resistance of the ship and how it can be improved. Considering these problems, the resistance performance of ship with moonpool was researched through both numerical and experimental methos in this paper. The effects of moonpool geometry on ship resistance in still water were focused. Six cases with different step positions and forms were calculated to find out the relationships among the geometric parameters of the steps, the hydrodynamic response inside the moonpool and the ship resistance.
