Hydrodynamic Study on Water Column Oscillation of Varying Cross-Section Moonpool and Its Effect on Resistance of a Drill Ship

Study on the influence of the moonpool on the smooth water resistance performance of the ship

Abstract: To investigate the influence of the flow pattern of the flow field on the resistance of a ship with a moonpool and the characteristics of the flow in the moonpool during smooth water navigation, the Star-CCM + software was used. The calculation results were verified by the experimental data of the pool, showing that the flow pattern of the moonpool has a great influence on the resistance performance of the ship with the moonpool. The force and the flow field of the moonpool have a great nonlinear change in the process from transition state to turbulence state. The flow in the moonpool also affects the forces on the hull. The main factors of the resistance of the moonpool and the fluctuation of the flow field are the constant shedding of the vortex street at the front of the moonpool, which causes the periodic change of the resistance of the moonpool and the phenomena of ‘piston’ and ‘sloshing’. The Reynolds number is the key similarity number that determines the characteristics of the moonpool flow field and determines the flow pattern and vortex shedding law.

Multidimensional modal analysis of nonlinear sloshing in a rectangular tank with finite water depth

Abstract: Multidimensional modal analysis of nonlinear sloshing in a rectangular tank with finite water depth

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.

Two-dimensional resonant piston-like sloshing in a moonpool

Abstract: This paper presents combined theoretical and experimental studies of the two-dimensional piston-like steady-state motions of a fluid in a moonpool formed by two rectangular hulls (e.g. a dual pontoon or catamaran). Vertical harmonic excitation of the partly submerged structure in calm water is assumed. A high-precision analytically oriented linear-potential-flow method, which captures the singular behaviour of the velocity potential at the corner points of the rectangular structure, is developed. The linear steady-state results are compared with new experimental data and show generally satisfactory agreement. The influence of vortex shedding has been evaluated by using the local discrete-vortex method of Graham (1980). It was shown to be small. Thus, the discrepancy between the theory and experiment may be related to the free-surface nonlinearity.

Behavior of Water in Vertical Well with Bottom Opening of Ship, and its Effects on Ship-Motion

Abstract: Recently some ships have been constructed with a vertical well which have the bottom opening for the purpose of raising and lowering the equipment. Some examples are vessel-type drilling rigs, vessels carrying small submersibles and so on.This paper shows the behavior of the water in the well, and its effects on the motion of the ship, when she is running.Experimental studies were performed under two different sets of conditions.(1) The motion of the free water surface in the well was studied, when the ship-model was fixed stationary in the stream.(2) The effect of the motion of the water in the well on the motion of the ship-model was studied, when the model was towed.The following conclusions were obtained.1) When the ship-model with a well is fixed stationary in the stream, it can be observed that the water surface moves regularly in it. This phenomenon is similar in many respects to the motion of a cylinder which is elastically supported by springs in the stream.2) The type of water motion, whether up and down, right and left or forwards and backwards, depends on the velocity of the stream and the shape of the well.3) The water motion can be seen at the dimensionless stream velocity of 0.2<2U/lω0<1.6, where U is the stream velocity, lis the length of the bottom opening and ω0 is the natural circular frequency of the water moving in the well.4) The experimental values agree well with the results of the theoretical calculation on the heaving amplitude of the free water surface in the well, and on the increased drag of the ship-model by the motion of water in it.5) In order to reduce the motion of water in the well, it is very effective to fix short flanges on its vertical walls a little below the water surface.

The water motions in a moonpool

Abstract: Moonpools are vertical wells in a floating body, frequently found in drilling ships—to give passage to the drill pipe—and in diving support vessels—to launch diving bells and equipment through it. Observations showed that inside a moonpool considerable relative motions may occur, depending on the shape and depth of the moonpool and on the frequency range of the waves to which the ship is exposed. Recently, a research program has been carried out at the Netherlands Ship Model Basin in which the hydrodynamics with respect to a moonpool were investigated. A mathematical model describing the relative water motions inside a moonpool was developed, while also model tests were carried out to obtain empiric results with respect to the damping mechanisms and motion behaviour.