Experimental study on the influence of fluid motion on hull resistance in stepped moonpool

Abstract

A series of model tests are carried out to explore the influence of fluid motion on the hull resistance in the non-resonant state of the moonpool. The variations in total hull resistance under various wavelengths, wave steepness, and speed conditions are investigated based on regular waves. Considering the three navigational conditions of low speed (Fr=0.097), medium speed (Fr=0.13), and high speed (Fr=0.162). The two incident wave frequencies are consistent with the moonpool piston and the sloshing mode, respectively. In order to ascertain the effect of fluid motion on hull resistance in the non-resonant state of the moonpool, a new damping device is designed to reduce the amplitude of the fluid motion response and dissipate the energy of fluid motion in the moonpool. The variations of fluid motion response and hull resistance with and without a damping device in the moonpool are compared and analyzed. It can be discovered that vortex structure in the moonpool has a significant effect on the hull's additional resistance. During the test conditions of λ/LPP=0.46, the average added resistance effect of the moonpool in the case of Fr=0.097 is 5.76%; in the case of Fr=0.13, it is 7.23%; and in the case of Fr=0.162, it is 17.33%. The average decreases in fluid motion response amplitude in the moonpool are 54.52%, 62.8%, and 72.55%, respectively. Under the conditions of λ/LPP=1.43, the average added resistance effect of the moonpool in the case of Fr=0.13 is 8.7%, and in the case of Fr=0.162, it is 12.48%. The average decreases in fluid motion response amplitude in the moonpool are 15.10% and 43.70%, respectively. With the dissipation of the vortex structure and the decrease of the fluid motion response in the moonpool, the total resistance of the hull is also reduced. This study will provide a reference for the green and energy-saving design of drilling ships.