STAB1997_Liquid Cargo and Its Effect on Ship Motions

J.M.J. Journée, Liquid Cargo and Its Effect on Ship Motions, STAB 1997, Varna, Bulgaria / DUT-SHL Report 1092-P


Liquid Cargo and Its Effect on Ship Motions
J.M.J. Journée, Delft University of Technology

ABSTRACT
When a double bottom tank, a cargo tank or a space in a rolling vessel contains a fluid,
gravity waves will appear at the surface of this fluid. These gravity waves will cause exciting
roll moments on the vessel. At lower water depths, resonance frequencies can be obtained
with high wave amplitudes. A hydraulic jump or bore, which is a strongly non-linear
phenomenon, travels periodically back and forth between the walls of the tank. A theory,
based on gas-dynamics for the shock wave in a gas flow, has been used to describe the
motions of the fluid. At higher water depths, the behaviour of the fluid tends to be more
linear. Then, a linear potential theory, as used in strip theory ship motions computer
programs, is used to describe the motions of the fluid in the tank.
Available experimental data on the behaviour of the fluid in free surface anti-rolling tanks
have been used to verify the theoretical approaches for low water depths.
Also, forced roll oscillation tests were carried out with a 2-D model a cargo tank of an LNG
carrier for a wide range of filling levels. These experimental data have been used to verify the
theoretical results for all water depths.
Finally, a ship model was equipped with liquid cargo tanks and it has been tested in beam
waves at zero forward speed. The measured roll data of the model have been compared with
the results of strip theory calculations.
It has been concluded that the approaches given in this paper, based on theories developed
already in the sixties, can be used to include the effect of non-viscous liquid cargo in ship motions calculations.

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