Moonpools are typical structures employed on a drilling vessel and offshore platforms to facilitate drilling and other marine operations. In the present study, the evolutions of vortex structures inside a moonpool with a recess are investigated within the frameworks of numerical methods. Besides, the induced adverse effects including resonant fluid motions caused by forward speed, and added resistance for a drilling vessel due to the existence of the moonpool are also discussed in detail based on the unsteady RANS-based numerical simulations. The result shows that there is serious vortex shedding from the moonpool's inlet and the sharp corners of the recess when a drilling vessel is exposed to forward speed, which activates the resonant water body behaviors in the natural frequency of the flow system, and great added resistance compared with the full close-type vessel.
Moonpools are openings through vertically the hull of drilling vessels and offshore platforms, which are often installed at the mid-position of drilling structures to facilitate daily drilling and other marine operations (Fig. 1). When a moonpool is under resonant situations, there are two dominant types of fluid motions in the flow system known as the socalled piston and sloshing modes (Molin, 2001). The fluid motions regarding piston mode appear along the vertical direction of a moonpool while the sloshing modes are more similar to standing waves in a closed tank and are characterized by longitudinal behaviors.
Vortex shedding is one of the most common non-linear phenomena in naval architecture and ocean engineering, which also occurs inside a moonpool when the flow system is under forward speed of the ship. Vortex shedding changes the pressure field inside the moonpool, and thus enlarges the longitudinal pressure gradient of the drilling vessel, causing the so-called added resistance (Veer & Tholen, 2008).
There are many investigations implemented for the resonant behaviors in a traditional cylindrical- or rectangular- shape moonpool using both the methods of semi-analytical solutions, viscous (CFD) or inviscid (BEM) numerical simulations and physical experiment. For the semianalytical solutions, the domain decomposition based on the linear potential flow theory is always carried out to study the natural frequency and mode shape inside a moonpool. (Faltinsen et al., 2007; Faltinsen & Timokha, 2015; Kristiansen & Faltinsen, 2008; Molin, 2001, 2017; Molin et al., 2018) For the numerical investigations, many CFD packages, especially in practical engineering, are always carried out at the pre-design stage for a drilling structure as an alternative tool for the expensive model tests. (Do Vale Machado et al., 2016; Krijger & Chalkias, 2016; Lohrmann et al., 2017; Pistidda & Ottens, 2014). Some BEM packages are also carried out to study the resonant problem, but it always over-predict the response amplitude due to the neglect of nonlinear effects inside a moonpool such as the non-linear free-surface conditions, and vortex shedding from the moonpool’s leading edge or the inner sharp corners. (Ravinthrakumar et al., 2019)
