On the piston and sloshing modes in moonpools

Abstract

So-called 'moonpools' are vertical openings through the deck and hull of ships or barges, used for marine and offshore operations, such as pipe laying or recovery of divers. In the present study rectangular moonpools of large horizontal dimensions are considered. The natural modes of oscillation of the inner free surfaces are determined, under the assumption of infinite water depth and infinite length and beam of the barges that contain the moonpools. The problem is treated in two and three dimensions, via linearized potential flow theory. Results are given for the natural frequencies and the associated shapes of the free surface, for wide ranges of the geometric parameters. Simple quasi-analytical approximations are derived that yield the natural frequencies. The most striking result is that the natural frequencies of the longitudinal sloshing modes increase without bounds when both the draught and the width decrease to zero, the length of the moonpool being kept constant. As a corollary the problem of waves travelling in a channel through a rigid ice sheet is addressed and their dispersion equation is derived. The same behaviour is obtained: the waves travel increasingly faster as both the draught and the width of the channel are reduced.

Study on Moonpool Resonance Effect on Motion of Modern Compact Drillship

“…Molin (2001) has derived theoretical formulas for resonance frequencies of a moonpool in the sloshing mode and the piston mode using velocity potential and boundary condition. He applied those formulas to a barge with a moonpool.…”

Analysis of wave resonant effects in-between offshore vessels arranged side-by-side

“…For the resonant wave frequency 8.3 rad/s, it is possible to observe that the wave elevation in the gap presents a piston type resonant mode, in which the wave behaves like a column of water moving up and down in the region between the vessels (Molin, 2001). Regarding the resonant frequency 8.5 rad/s, one may realize that instead of a piston mode, a first longitudinal mode is visualized.…”

“…They efficiently represent most of the physics involved but can lead to a poor prediction of the hydrodynamics in the gap because tend to overestimate the wave elevation. The typical resonant behavior observed in this kind of problem may be due to the onset of longitudinal and transversal sloshing modes (Lewandowski, 2008;Molin, 2001).…”

“…In potential flow numerical computations of gap flow, resonant effects appear at some frequency values (see, for example, Molin (2001)). These values are a function of the gap width and draft values.…”

“…This could be based on analytical mechanics methods or in free-surface hydrodynamics techniques, where resonant modes will appear. In order to consider the geometry of the gap, Molin (2001) provides an approximated analytical formulation based on three-dimensional gap flow analysis investigating the natural modes of oscillation of the inner free surfaces of a moonpool. More detailed about the gap resonant effects are presented in section 8.7.…”