A moonpool is a vertical well, open to atmosphere and hydraulically connected to the ocean, introduced in floating bodies such as drilling ships, FPSOs and offshore drilling platforms to provide a calm working environment for lowering of drill pipes, diving bells and research equipment. With advancement in marine technology, the equipment to be lowered is becoming larger which has led to a shift in focus towards large aspect-ratio rectangular moonpools. Water-column motion within a rectangular moonpool can be correlated to that occurring in multihull ships and side-by-side barges.&nbsp;The water column entrapped in a moonpool is characterized by a vertical piston motion and a horizontal sloshing motion; these modes are dramatically amplified under resonance conditions. In case of ships, large relative motions within the moonpool may lead to stability issues, horizontal sloshing loads (imparted on the walls of the moonpool) and vertical slamming loads (imparted on equipment being lowered into water). Thus, considerable research effort has been invested towards understanding the mechanisms governing water column excitation and development of geometrical appendages designed to dampen said excitation. However, most recent investigations consider moonpools interacting with regular waves.Owing to sea-level rise, the frequency of occurrence of extreme wave events has increased. These so-called focused waves result from constructive interference of wave packets with the focusing process characterized by non-linear wave-wave interactions. The interaction between a marine structure and a non-breaking focused wave may be interpreted as one involving a single, steep regular wave. Thus, focused waves may induce strong, temporally-localized excitations of the water column within a moonpool which could lead to impulsive loading as well as stability issues. As of now, the nature of excitation as well as mechanisms governing the interaction between focused waves and a moonpool are not well-understood; the present work strives to fill this research gap.A series of experiments have been carried out in a 22m&nbsp;long, 0.6m&nbsp;wide and 0.8m&nbsp;deep glass flume in the Department of Ocean Engineering at IIT Madras. Two fixed cuboidal boxes with the same geometry (0.58m&nbsp;long, 0.16m&nbsp;wide and 0.25m&nbsp;high with&nbsp;0.15m draft) representing the twin-hull of a drill-ship were placed in the flume with a still-water depth of 0.6m. The gap between the twin bodies represents a rectangular moonpool with an aspect ratio of 3.867. Focused waves based on a constant steepness spectrum were generated by a piston-type wave-paddle whose motion was modelled based on the second-order wavemaker theory. Two focused waves with steepness (H/L)&nbsp;of 0.0296&nbsp;and 0.0467&nbsp;at the point of focusing were generated. The focusing point itself was set at the centre of the moonpool and wave-focusing experiments were performed both with and without the twin-body. Wave elevations at various locations along the flume and within the moonpool were measured using five wave-gauges. The amplification of oscillations within the gap, influence of wave steepness on gap-excitation as well as viscous damping of waves downstream of the gap was quantified.To gain further insight into the hydrodynamics of focused waves interacting with the twin-body, fully viscous CFD simulations were performed using the in-house finite volume code IITM-RANS3D. High-fidelity focused wave generation was achieved by weakly coupling the RANS solver to the fully non-linear potential theory based in-house code: IITM-FNPT2D. The twin bodies were modelled using a body-fitted immersed-boundary treatment whilst influence of air was modelled using VOF. The simulations have been validated against experiments for both focused wave types with and without the twin-bodies. The dynamics of turbulence generation, vortex roll-up and shedding from the edges of the moonpool has been investigated, lending valuable insight into mechanisms governing amplification as well as damping of the gap-excitation occurring during focused waves interacting with a rectangular moonpool. &nbsp;&nbsp;&nbsp;&nbsp;

Experimental and FNPT-RANS Investigations Into Gap-Excitation and Vortex Dynamics in a Rectangular Moonpool Interacting With Focused Waves

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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