Two types of nonlinearity in offshore structural analysis, namely the nonlinear wave-force and nonlinear stiffness, are studied by using some simple numerical models. It is pointed out that even such simple models may, in some cases, exhibit chaotic behaviour. The effect of noise on the chaotic response of a model with bilinear stiffness is also d1scussed.
Nonlinear dynamic analysis is an essential part of the design of many offshore structures. Numerical analyses, including nonlinear behavior, are performed not only for structural systems which undergo large deformations (such as TLP, compliant towers, guyed towers, semi- submersibles, mooring cables and risers) but, also for the conventional fixed-base structures when inelastic propert1es of the structural members or foundation are considered. Typically, such nonlinear dynamic analyses are carried out by using computer programs based on step-by-step time integration techniques. Even for a moderately complex structural system, nonlinear dynamic analysis is an expensive task in terms of both manpower and computer costs. Consequently, only a few "typical" cases are usually analyzed for a given system. The obtained numerical results are then treated as the "typical" nonlinear response of the system. The belief that typical behavior of a nonlinear system can be predicted by performing time history analysis using typical system parameters has now been questioned by many researchers to view of the recent advances to the study of nonlinear dynamics. This paper discusses some examples of the newly discovered nonlinear phenomena, including chaos and subharmonic oscillations, in offshore structural analyses.
One of the most interesting recent developments in nonlinear dynamics is the discovery of chaos (Gleick, 1987). Many nonlinear systems including some simple structural systems have been found to have chaotic responses under deterministic excitations (Thompson and Stewart, 1986; Moon, 1987).