As the exploration of offshore oil and gas resources are moving into larger depth, many mobile drillships have been widely applied to the offshore drilling operation, thanks to the good mobility, large variable deck loads and storage volumes. Large motion responses of the drillship and the water motions inside the moonpool were reported, which introduced safety concerns. In this paper, both numerical simulations and physical experiments have been carried out to investigate the hydrodynamic performances of a newly designed deep-water drillship, and the resonant water motions inside the moonpool with submerged recess structure. The three modes of water motions coexist in the moonpool even when there is only one external excitation source provided by a regular wave train. The submerged recess structure clearly introduces the obstacle effects leading to exacerbation of the water motion inside the moonpool.
Moonpools are designed as vertical openings through the decks and the hull structures to support the required underwater operations on marine vessels and offshore platforms. The water inside moonpool is required to provide a moderate environment for operation rather than being exposed to the violent metocean conditions. However large water motions may occur inside the moonpool under some resonant conditions, including the vertical piston motions, and the longitudinal sloshing motions. The effects on 6 degrees of freedom (DOF) RAOs of the drillship which are caused by moonpool also need to be investigated. Recently, the recess type moonpool appears in drillship's moonpool design, which has advantage on drilling equipment arrangement. The impacts introduced by the recess type structure on relative water motions inside moonpool need to be clarified.
Fukuda (1974) carried out a set of physical experiments to investigate the water behavior inside the moonpool and the effects on the vessel motions in a towing tank. He obtained an empirical formulation which could be used to identify the resonant frequency of the water motions. A mathematical model was developed later to describe the water behavior inside moonpool, and also corresponding model tests with respect to the damping mechanisms and motion behavior were conducted (Aalbers, 1984). Molin (2001) treated this problem in the framework of linear potential theory under the assumption of a infinite length and beam of a barge equipped with a moonpool, and brought out the oscillation natural frequency formulation. It was believed that the water motions response inside the moonpool is overestimated with respect to the experimental results without considering the fluid viscosity (Kristiansen and Faltinsen, 2008). Kristiansen and Faltinsen (2008) set up a fully nonlinear numerical wave tank coupled with an inviscid vortex tracking method to investigate the impacts caused by fluid viscosity and the nonlinear effects that associate with free surface. More recently, computational fluid dynamics (CFD) has been utilized to study the sloshing phenomenon. Zhang (2012) applied Moving Particle Semi-Implicit (MPS) method to study the sloshing phenomenon. Large impact pressure was observed, and it was shown a periodic impact had two pressure peaks in each period.