The ocean platform described here is formed by two coaxial cylinders with a variable separation distance and is similar to some currently used offshore "jack-up" oil drilling platforms. The variable separation distance allows the platform natural frequencies to be de tuned, in certain sea state ranges, from that of the exciting waves. This de tuning from resonance with the ocean waves limits the amplitudes of some components of the platform motion to certain prescribed values.
After a brief introductory review of the applicability of one degree of freedom oscillation theory to ocean wave excited systems, the paper develops simple equations for the heave and pitch natural frequencies of the platform in terms, of its physical properties and hydrodynamic mass coefficients. The equations are then non-dimensionalized using appropriate dimensionless scaling coefficients which generalize all results to a family of dynamically similar platforms. The concluding portion of the paper presents the results of model tank tests of a 12.5 inch diameter jack-up platform to determine its hydrodynamic mass coefficients. The hydrodynamic mass coefficients are then used to calculate the pitch and heave natural frequencies of a specific family of platforms as a function of separation distance.
The effect on the motion of the platform of detuning its pitch natural frequency from the frequency of the exciting waves is dramatically illustrated in a movie to be shown at the meeting.
Many offshore operations such as oil drilling, oceanographic instrumentation and surveillance require surface platforms which are relatively motionless under the effects of ocean waves. These waves excite a complicated pitching/heaving oscillatory motion in the platform which may develop very large amplitudes.
Consider the forced oscillation of a single degree of freedom system with damping present. Figure (1) shows the well known envelope of possible steady state oscillatory motions for such a system. The amplification factor which is the ratio of the amplitude of the oscillation of the system to the deflection of the system which would be caused by the static application of the exciting force is plotted on the ordinate. The abscissa shows the ratio of the frequency of the exciting 'force to the natural frequency of the system. The upper curve of the envelope corresponds to the oscillatory motion of an undamped system, while the lower curve of the envelope is for a system in which the damping is so critical that any further increase in the damping would result in a non-oscillatory motion. Oscillatory motions with various amounts of damping will occur inside the envelope.
Resonance is exhibited when the natural frequency of the system is close to that of the exciting force, resulting in very large amplitude motions. It can be seen from Figure (1) that resonance may be avoided by detuning the natural frequency of the system away from the frequency of the exciting force.
