NONLINEAR-SYSTEM IDENTIFICATION IN OFFSHORE STRUCTURAL RELIABILITY

Nonlinear forces acting on offshore structures are examined from a sys tem identification perspective. The nonlinearities are induced by ocea n waves and may become significant in many situations. They are not ne cessarily in the form of Morison's equation. Various wave force models are examined. The force function is either decomposed into a set of b ase functions or it is expanded in terms of the wave and structural ki nematics. The resulting nonlinear system is decomposed into a number o f parallel no-memory nonlinear systems, each followed by a finite-memo ry linear system. A conditioning procedure is applied to decouple thes e linear sub-systems; a frequency domain technique involving autospect ra and cross-spectra is employed to identify the linear transfer funct ions. The structural properties and the force transfer parameters are determined with the aid of the coherence functions. The method is veri fied using simulated data. It provides a versatile and noniterative ap proach for dealing with nonlinear interaction problems encountered in offshore structural analysis and design.