RIASSUNTO
ABSTRACT
Moonpool added resistance is a practical problem in drillship design. In the past, studies are more focused on calm water resistance but few attempts on wave added resistance. In this study, CFD simulations are performed for a drillship with a large moonpool opening to evaluate its resistance and motion performance in calm water and waves. According to the CFD results, it is found compared to the calm water transit case, moonpool induced added resistances in waves appear much higher values associated with various features depending on the environmental wave periods.
INTRODUCTION
Water motion in a drillship moonpool during transit is a complex phenomenon. The water oscillation in moonpool could cause a drastic increase in drillship resistance during transit and consequently increase the fuel consumption. Moonpool water oscillations are initiated by vortices that start at the upstream moonpool edge underwater. In calm water transit condition, the oscillation of moonpool water is dominated by the moonpool natural frequencies and the amplitude increases with ship speed. The oscillation natural modes can be piston and sloshing types. In general, the sloshing mode tends to be triggered in longer moonpools, while piston mode oscillations occur in shorter moonpools. When transiting in waves, the excitation mechanism in moonpool is changed. In addition to the piston and sloshing natural modes, the ocean wave frequency components will also participate into moonpool water agitation impacting added resistance and ship motion. Instead of free oscillation phase-locked mechanism in calm water condition, forced oscillation due to the wave excitation will control more to the water motion in moonpool. In extreme condition when the ocean wave frequency matches with moonpool natural mode frequency, significant increases of resistance and motion could appear due to resonance effect.
In this study, a drillship with large moonpool opening area ~400 m2 is selected to evaluate its resistance and motion performance in calm water and waves using CFD. For the calm water case, CFD simulation results are validated with the model test measurement and confirmed that the predicted resistances are accurate compared with the model test data, within 3% difference for a speed range around design speed. Based on the validated CFD set-up, CFD simulations are then performed for the wave cases with wave period range including the moonpool water resonance periods. Within the considered period range, it is found when a wave period is close to moonpool resonance mode periods, either piston or sloshing mode, the water motion in moonpool is phase-locked by the natural modes and water agitation is further worsen by the external waves. Due to the external wave induced resonance effect, the ship motion and moonpool added resistance increase significantly. the ship motion and resistance are alleviated although they are still with higher values compared to the calm water case. Furthermore, through the harmonic analysis of ship hull and moonpool resistances time history, it is found ship hull resistance is directly influenced by the environmental waves and consists of all the harmonic components of the environmental waves. For moonpool resistance, depending on the wave periods, it appears various features.