Experimental analysis of a nonlinear moored structure

Complex nonlinear and chaotic responses have been observed and demonstrated in various compliant ocean systems characterized by a nonlinear mooring restoring force and a coupled fluid-structure interaction exciting force. Such floating structures and compliant systems have caused considerable concerns for the designer because the dynamics are inherently nonlinear in nature. An experimental mooring system with two configurations is chosen for this study. The first configuration, a single-degree-of-freedom (SDOF) (horizontal or surge motion only), consists of a sphere moored by linear elastic wires, with vertical displacement restricted by a horizontal rigid rod passing through its center. In the second configuration, corresponding to the multi-degree-of-freedom (MDOF) case (both horizontal and vertical (heave) motions), the restraining rod is removed, thus allowing motion in the vertical direction. Both configurations exhibit nonlinear behavior due to geometric (large mooring line angles) and complex hydrodynamic excitations.; Three alternative multiple-input/single-output models—nonlinear-structure linearly-damped (NSLD) model, nonlinear-structure coupled hydrodynamically-damped (NSCHD) model, and nonlinear-structure nonlinearly-damped (NSND) model—distinguished by the different inputs and outputs used are derived for the SDOF system and a Reverse Multiple-Input/Single-Output (R-MI/SO) technique is adapted to determine the most suitable analytical model. The NSND model developed and validated for the SDOF configuration is extended to the MDOF system and it is found that the identified system parameters simulate a response that matches the experimental data.; A sensitivity analysis reveals that the effects of system parameters on the responses become more significant with an increase in wave excitation amplitude. For the SDOF system, the identified nonlinear structural damping coefficient varies among the tests whereas all the other system parameters remain relatively constant. With the rod restricting the vertical motion of the sphere, the horizontal motion amplitude for the SDOF system is smaller than that of the corresponding MDOF system without vertical constraint.; Applying the R-MI/SO technique with the inertia coefficient, C_m varying within a wide range shows that the identified natural frequency remains constant, but other system parameters increase with increasing C _m. Also, the subharmonic response decreases with increasing values of the inertia coefficient. Since inertia effects dominate the total forces, the response is found to be insensitive to the hydrodynamic drag coefficient, C_d.