Nonlinear Detection in Wave Loaded Structures

Abstract

When steel structures experience short duration overloads, for instance wind burst or a breaking wave, the structure might yield for a short moment and after this yielding event the reduction of stiffness could be minimal. The result is mainly a permanent deformation, which is normally detected using tilt meters. But since tilt meters are measuring angles at the location of instrumentation, and in some cases, yielding does not result in permanent changes of tilt angles, the yielding must in these cases be detected by other means. In this paper we are shortly discussing the use of the short duration changes of natural frequency and damping following ideas from the earthquake cases of period elongation. However, due to the strong influence of the external forces on the estimation of these changes, other effects are considered such as quasistatic displacement movement, slamming effect of the overload, permanent displacement using low frequency signals, and mode shape changes using principles from stochastic subspace identification. This defines a set of five non-linear detection (NLD) indicators that are studied on a case of possible yielding in a wave loaded offshore structure using simulation.