The highly nonlinear responses of compliant ocean structures characterized by a large-geometry restoring force and a coupled fluidstructure interaction excitation are of great interest to ocean engineers. Practical modeling, parameter identification, and incorporation of the inherent nonlinear dynamics in the design of these systems are essential and challenging. An experimental mooring system exhibiting nonlinear behavior due to geometric nonlinearity of mooring line angles and the complexity of hydrodynamic excitations is chosen for the study. An Independent-Flow-Field (IFF) model based on Morison equation and its associated nonlinear system identification algorithm is used to evaluate the system parameters of an experimental submerged mooring system. With the input wave and output system response data known, based on multiple input/single-output linear analysis of reverse dynamic system, the methodology identifies the linear and nonlinear system properties. A sensitive analysis is conducted to investigate the coupled hydrodynamic forces modeled by the Morison equation, nonlinear stiffness from mooring lines and nonlinear response.
Complex nonlinear responses including harmonics, sub- and superharmonics and chaos, have been observed and demonstrated in various compliant ocean systems characterized by large-geometry nonlinear mooring restoring force and coupled fluid-structure interaction exciting force. When examining the responses of these highly nonlinear systems, it is important to develop sophisticated analytical models that the details of the nonlinear responses can be captured accurately. Deterministic analysis theories and numerical prediction techniques of relatively simple models have been developed to analyze the complex nonlinear phenomena for singlepoint mooring systems (Gottlieb et al 1992), ships (Bishop and Virgin 1988), and multi-point mooring (Bernitsas and Chung 1990, Gottlieb and Yim 1993). Lin and Yim (1996 and 1997) developed stochastic extensions of these techniques and corresponding analyses. They provided guidelines for interpreting field and experimental observations where randomness cannot be neglected.