Resonant water motions within a recessing type moonpool in a drilling vessel

The wide employment of the moonpool on marine vessels and offshore platforms shelters the activities and operations from the external wave environment to a high degree. However, under certain resonant conditions, violent water motions with relatively large amplitudes can be excited within the moonpool. This paper describes the numerical simulations and physical experiments in an offshore basin carried out specifically to investigate the behaviour and characteristics of the resonant water motions within a large rectangular moonpool of a typical drilling vessel. By tracking the dynamic free surface evolution in the moonpool, it was observed that complex water motions consisting of vertical piston motions, longitudinal sloshing motions, wave run-up and impacts are coupled in the moonpool. The relative water motion amplitudes were quantified. It was identified that multiple water motions coexist even when there is only one external excitation source provided by a regular wave train. It was further demonstrated that the encountered waves travelling along the vessel have significant effects on the excitation of the resonant water motions in the moonpool. Close coherences were found revealed between the water elevations inside and outside the moonpool.