Moonpools are the vertical openings on drillships and offshore structures applied to lower or lift equipment for operation. Open moonpools have an adverse impact on ship's resistance performance. In this paper, added resistance of two moonpool configurations is investigated by numerical method using OpenFOAM. Numerical results suggest that the notch on recess in the moonpool can effectively reduce moonpool added resistance, especially for higher navigation speed. Meanwhile, compared with the configuration without notch, pitch and heave amplitudes of drillship decrease significantly, which is advantageous for seakeeping performance of a drillship.
Moonpools are vertical openings employed on drillship and offshore structures through the decks and the hull bottom structures. Due to the presence of free surface in the moonpool, open moonpool has an adverse impact on drillship resistance performance. The fluid periodic movement in the moonpool brings a significant increase in total resistance of drillship, which leads to extra fuel consumption during ship's navigation.
During the last two decades, researches on moonpool resonance frequencies have drawn high interest. Many studies have focused on the prediction of moonpool resonance frequencies. Molin derived piston and sloshing modes of moonpools in infinite and finite depth in the framework of potential theory (Molin, 2001, 2017; Molin et al., 2018). Newman (2018) investigated the resonant frequencies of moonpool with a recess using numerical simulations based on WAMIT and theoretical solution analyses. Zeyu Shi proposed a method to analyze the natural oscillation characteristics of fluid in the moonpool based on modified Fourier series method (Shi et al., 2019; Shi, 2020).
On account of long distances form operation sea regions to coastal areas, the moonpool is generally let open during drillship transit. Due to the presence of ship motions and navigation speeds, violent fluid motions occur in the moonpool, which increases the fuel consumption and endangered operation safety. Several attempts have been made to improve the hydrodynamic performance of drillship moonpool. Do Vale Machado et al. (2016) investigated the influence of shape and size of the moonpool on the resistance of a drillship and proposed a fKN3J configuration with the best resistance performance. Son et al. (2008), Hammargren and Törnblom (2012) discussed the effect of recess height on flow pattern in the moonpool and moonpool resistance performance based on Fluent and OpenFOAM, respectively. Through RANS simulations based on Shipflow, Sivabalan predicted the total resistance of a drillship with moonpool at low forward speed and investigated the effect of varying cross-section on resistance of a drillship (Sivabalan, Surendran and Balakrishnan, 2016; Sivabalan and Surendran, 2017). Through the comprehensive CFD parametric study, Ma et al.(2016) studied the effects of moonpool dimensions on added resistance of rectangular moonpool. CFD results suggested that moonpool induced added resistance is mainly attributed to the vortices shed from the front wall of moonpool, which enter into the moonpool and impinge on the moonpool rear wall. To study the modified moonpool forms on resistance performance of a drillship, Mao et al. (2019) performed numerical calculations on a drillship at different transit speeds. Meanwhile, the rules and causes of moonpool forms on ship resistance and flow pattern characteristics were analyzed.
