Study On Hydrodynamic Performances Of Drillship With Large Moonpool

With the proposal of "Made in China 2025",China has accelerated the progress of development on marine engineering equipments.Drillship has become one of the research hotspots because of its wide application on water depth and good maneuverability.Drillship has a structure called moonpool that digs hole in the hull and is used for lowering operation equipment.With the continuous development of drillships,the moonpool has gradually developed into a larger one by the seventh generation of ultra-deep water drillship.The complex water motions inside the moonpool have adverse effects on the resistance and movement performance of the drillship.To study the hydrodynamic performance of drilling vessel with a large moonpool is not only for the sake of energy conservation,but also for the sake of ensuring the smooth operation.In this paper,combining with model tests and numerical calculation,the hydrodynamic performance of drillship with a large moonpool is studied comprehensively.What's more,the difference of hydrodynamic performance caused by eight different moonpools is analyzed.The phenomenon and mechanism of added resistance of moonpool are studied through towing tank test and CFD simulation.It is found that the increase of resistance of the moonpool can exceed 50% at a high speed.The main cause for added resistance is the water oscillation caused by the vortex shedding inside the moonpool.Comparing the resistance of drillships with different moonpools,respect to this drillship,it is observed that postposition of moonpool recess and reasonable inclined front and back walls of the moonpool are helpful to reduce the added resistance at a maximum extent 10% and 9% respectively.Comparing the spectrum of wave amplitude at the rear wall of these 8 moonpools,it is discovered that the water oscillation in the moonpools which have better resistance performance is more gentle,so that their drag is smaller.The RAO curves obtained from seakeeping test and potential calculation,as well as the extreme values and significant values of motion in irregular waves,are used to analyze the movement performance of the drillship with a large opening of moonpool.The influence of different moonpool shapes on the movement performance of the drillship is compared and it leads to the conclusion that the change of moonpool shapes has little effect on the movement performance of the drilling vessel when the volume of the moonpool changes little.The wave elevation in the stepped large open moonpool is studied through regular wave tests,white noise tests and potential calculation.Comparing the experimental results with the potential calculation results,it is found that the results of potential calculation is credible for estimaiting the natural periods of the moonpool and wave amplitude in the non-resonant zone.However,in the resonant zone,the wave amplitude calculated by the potential model is larger.Because of the shallow water effect,the peak wave amplitude on the recess near the wall is the highest,and the ratio of wave height to incident wave height is about 3.Comparing the wave amplitude on the recess near the wall of different moonpools,it can be seen that the inclined side walls of moonpool reduce the natural period of moonpool while postposition of the recess doesn't change the natural period.Comparing the wave amplitude in the same moonpool when the drilling vessel is moving and fixed,it can be concluded that the coupled water motion inside the moonpool and the movement of drillship change the amplitude of the water oscillation in the moonpool at the resonant zone and increase the frequency of the resonance.In conclusion,this paper analyzes the hydrodynamic performance of the drilling vessel with a large-opening moonpool systematically,and obtains a series of meaningful conclusions,which provides useful information for the study of the hydrodynamic performance of drillship and the design of the moonpool.