| A comprehensive design methodology is developed to eliminate trial and error and extensive simulations for ocean mooring systems. It is based on derivation of time independent response characteristics such as equilibria, bifurcation sequences and their singularities, and morphogeneses crossing bifurcation boundaries. The problem formulation includes most system features and environmental loads that previous studies, model tests, or observations have identified as having significant impact on the system dynamics. Specifically, the horizontal plane slow motions of the moored vessel are modeled by the third-order maneuvering equations taking into consideration current, wind, mean and slowly varying wave drift load, the effect of fast motions on slow motions modeled as hydrodynamic memory effect, and the quasi-static nonlinear riser dynamics.; The methodology first identifies the system model which includes as many of the above features as possible and still remains autonomous. Such a system does not include slowly-varying wave drift forces but includes the memory effect which is converted to an autonomous excitation by the method of extended dynamics. Stability analysis and bifurcation theory are used to produce catastrophe sets in the design space defining regions of qualitatively different dynamics. The catastrophe set can provide qualitative conclusions regarding system dynamics, thus enabling the designer to select appropriate values of design parameters. The design methodology virtually eliminates both trial and error in selecting parameter values and extensive nonlinear simulations typically used to assess a specific configuration. Catastrophe sets are derived numerically by systematic search for equilibria bifurcations. Second, the slowly-varying wave drift load is included resulting in a nonautonomous system. This system is studied numerically in comparison with the corresponding autonomous system. Numerical results reveal the interaction between slowly varying drift forces and system dynamics identifying changes in system stability characteristics. Third, the problem of coupled spread mooring and riser analysis is studied to investigate the effect of riser forces on the stability of mooring systems. Overall, it is noted that the riser should be installed as forward as necessary to limit the horizontal slow motions of the vessel at the expense of increased wave frequency motions.; The developed methodology produces conclusions of global validity in the design space and proves that methods often used in practice to stabilize mooring systems actually may have adverse effect. |
