A medium-scale experimental study is conducted to investigate the stability and bifurcation behavior of nonlinear responses of a submerged, moored ocean structural system. The experimental model consists of a spherical buoy and attached multipoint mooring lines. The nonlinear structural system is excited by periodic wave fields in a closed channel. "Sources of nonlinearity include complex geometric restoring (stiffness) force and coupled fluid-structure interaction exciting forces. Experimental set-up, configuration and classification of the measured results are reported. Characteristic motions observed include harmonic, subharmonic and superharmonic responses. Response bifurcations are frequently observed and nonlinear primary and secondary resonances in the responses are identified in frequency response diagrams. The experimental results reveal underlying intricate patterns of the nonlinear global behavior, and infer possible existence of higher order nonlinear responses.
Complex nonlinear response phenomena of ocean structural systems subjected to periodic excitations, including non harmonic responses, instability and sensitivity to initial conditions have been studied and demonstrated analytically and numerically (e.g. Thompson and Stewart, 1986). Some of the analytical and numerical predictions have been verified through small-scale experiments where the system parameters can be accurately controlled, and the domains of attraction are easily defined and measured (e.g., Isaacson and Phadke, 1994). However, design of large-scale models in searching for chaotic ocean structural responses is difficult. This study, nonetheless, presents a medium-scale experimental investigation of the nonlinear response of a submerged, moored ocean structural system with an attempt to:
experimentally determine, as much as possible, the existence of nonlinear responses (e.g. subharmonic, superhannonic, quasi-periodic and chaotic similar to those predicted in Gottlieb and Yim, 1992), and response transition patterns in bifurcation sets predicted in the analytical study (Gottlieb et al, 1997); and
assess the validity of the analytical model of a symmetric mufti-point moored structural system subjected to a deterministic exciting field described by small amplitude waves and weak collinear current.