Abstract
The performance of offshore work from floating platforms dictates the desirability of "Minimum platforms dictates the desirability of "Minimum motions". One of the most troublesome motions, especially in the case of ship-shape or barge-shape platforms, is the roll motion because of the large platforms, is the roll motion because of the large amplitude and relatively large acceleration forces that could arise from such motion.
Different devices have been employed to minimize The roll motion of ships bilge keels, gyroscopic stabilizers, solid moss transfer, stabilizing fins, U-tanks, flume tanks, active ballast transfer tanks, etc. The systems utilizing fins are effective only when the ship is under way and are not suitable for zero-speed offshore operations. The barge keels are generally very effective in damping the roll motions, and they prevent the angle of roll from becoming too large, but their reduction of the roll is limited to angles that are too great for the satisfactory performance of offshore operations, i.e., oil drilling. performance of offshore operations, i.e., oil drilling. The gyroscopic stabilizers generally are not used because of their high cost and complexity. The U-tanks, flume tanks, and active stabilizing tanks work by transferring ballast horizontally and by creating a stabilizing moment out of phase with the exciting force.
This paper details use of the detuning tank. This system is different from others in the sense that it does not try to compensate the action of the forces imparted by the sea to the floating body with properly phased compensating forces, but tries to properly phased compensating forces, but tries to prevent the sea from imparting the forces to the prevent the sea from imparting the forces to the body. The effectiveness of the detuning tanks bas been experimentally verified in model basin motion tests and studies.
Introduction
Safety at sea is the first consideration of marine designers. Ships of conventional form or floating platforms for offshore operations must, under all platforms for offshore operations must, under all expected circumstances, float and be stable-hence, the application of suitable criteria for stability is one of the cornerstones of naval architectural design. The ability to maintain stability under design weather conditions, even after sustaining a certain amount of damage, has dictated maximum allowable heights of the center of gravity (KG) or minimum metacentric heights (GM).
This limitation is certainly necessary in order to meet the demands of safety, but it is necessary only during certain extreme weather conditions or after flooding caused by damage. But this occurs, if it ever does, only during a very small percentage of the life of the ship, and to be prepared for this eventuality we may be forced to select some "safe parameter", like minimum GM, that may affect the parameter", like minimum GM, that may affect the motion performance of the ship during the major part of her useful working life. part of her useful working life. The motion performance may be of certain importance for ships engaged in ocean trade from the standpoint of crew comfort and the ability to maintain sea speed both of which have a certain economic value.
But ships and platforms that are engaged An offshore work are more vitally affected by the motion performance, and their very effectiveness as offshore tools depends on how many days of the year they can perform their functions and the weather conditions that would force operations to be suspended.
Roll motions have been one of the reasons for discontinuing offshore operations for ship-shape and barge-shape platforms not only because the amplitude of the motions but because of the high acceleration forces in the work area that can be originated by relative small amplitudes at short motion periods. The inclination due to wave action appears to be composed of two periodic functions, the period of one being the wave period T and the other period of one being the wave period T and the other the natural rolling period of the ship T . Rolling in still water is a free oscillation and the ship will roll in its own natural period. Among waves, the impulses producing the roll are periodic and tend to set up a forced oscillation of the ship in the period of the wave. If waves of constant period act for a sufficient time upon the ship, it will roll in the period of the waves, but if the period of the waves period of the waves, but if the period of the waves is not constant, the ship roll will not follow exactly that of the waves because of the tendency of the ship to revert to roll in its own natural period.
SPEJ
P. 48
A New Semisubmersible Design for Improved Heave Motion, Vortex-Induced Motion and Quayside Stability