Low-frequency dynamics of moored vessels

Oppenheim, Bohdan W (1980) Low-frequency dynamics of moored vessels. University of Southampton, Doctoral Thesis.

Abstract

Both linear and non-linear theories of low-frequency dynamics of large vessels moored in deep water are derived and evaluated. Procedures are given for combining their results with the high frequency parameters for design applications. The non-linear coupled equations of the horizontal vessel motions include feedback, non-linear mooring forces and cubic damping. Mooring forces are derived for a broad class of hardware parameters. The mooring system is limited to the multi-leg, multi-segment catenary type. Transfer functions and spectra of wave drift forces are derived for ships and circular discs. Non-linear equations of motion are solved by digital simulation. Linear equations are solved in the frequency domain. Spectral analysis of random records is included. Periodic and transient non-linear responses are examined. Analysis of sensitivity to weather and stiffness is given. Special cases are examined. It is shown that the linear theory is valid for small motions, not exceeding 6 - 8% water depth for the vessels having the fairleads located sufficiently far from the centre of gravity. Turret configuration of fairleads may cause violent motions. Detected non-linearities include superharmonics, subharmonics, frequency shift, relaxation oscillations, jump, excitation modulation on ships by feedback and dynamic DC levels. All non-linearities are weak in the range of practical interest. Some non-linearities occur only when motions are small. Motion probability density functions are approximately Gaussian when the motions are small.

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