Identification of nonlinear and parametric systems with application to offshore structures
Spanos, Pol D.
Doctor of Philosophy
Nonlinearity is characteristic of many structural and mechanical systems. For the purpose of system identification, a frequency domain method is proposed. The unknown model of the restoring force or interaction force is represented in a set of base functions. The nonlinear system is decomposed into a number of no-memory nonlinear subsystems, each followed by a finite-memory linear system. These subsystems are then decoupled. A cross-spectrum technique is employed to identify the linear transfer functions individually, from which the structural properties and the force transfer parameters are determined with the aid of coherence functions. Nonlinear systems may in addition be featured by time-variant properties. The above decomposition procedure can be employed to estimate the constant parameters that may be present and to rearrange the parametric system. Spectral relationships between the unknown time-variant parameters are established. Estimates of the power spectra of the parameters are obtained by deconvolution. The methods are expected to be applicable to general nonlinear and parametric systems and are suitable for model identification. They provide a versatile and non-iterative approach to dealing with offshore structural problems, particularly the identification of wave force model and time-variant force transfer parameters. Simulation is the primary tool in this study for the purposes of verification and application.
Civil engineering; Mechanical engineering