Coupled Surge-Heave Motions of a Moored System. I: Model Calibration and Parametric Study

Model calibration and parametric studies of the coupled, complex surge-heave motions of a medium-scale experimental, nonlinear, submerged, moored structural system are presented here. The experimental system excited by periodic wave fields consists of a spherical buoy and attached multipoint mooring lines. Sources of nonlinearity include complex geometric restoring forces and coupled fluid-structure interaction exciting forces. The sphere moves mainly in a two-dimensional fashion [or two degrees of freedom (2DOF)] of surge and heave, with negligible pitch. Characteristic experimental results include harmonic, subharmonic, and superharmonic responses. As an extension of single-degree-of-freedom, independent-flow-field (IFF) models, a 2DOF, IFF model is derived and employed. Good agreement is shown between the analytical predictions and experimental results. Existence of complex nonlinear responses, including chaos and multiple coexisting steady states, are numerically identified when the coupling is strong and damping is light. Degree of complexity and nonlinearity of responses diminish with decreasing coupling and/or increasing damping.