Use of Operational Modal Analysis to Identify Systems with Oscillatory Masses

Abstract

Structures experience frequently moderate or even excessive structural vibrations due to their exposure to dynamic actions such as wind- and/or wave-induced loads. To mitigate the amplitude of those oscillations, passive damping systems can be used and their efficient performance is dependent on their tuning to the vibration properties of the structure, in which they have been installed. The Tuned Liquid Damper (TLD), being commonly a container including some kind of liquid, has been already used to damp excessive vibrations of different structural systems. However, there are structures that are expected to carry containers of varying size filled with liquids during their lifetime and hence, this inevitable, vibration-wise interaction between the structure and the container may eventually affect, either favorably or adversely, the structural response. The offshore platforms, being usually equipped with a tank at its top for the temporary storage of the extracted oil, constitute an example of structures with oscillatory mass at the top. Hence, it is rather appealing to investigate the dynamic behavior of this coupled system (offshore platform and the oil tank at its deck) and especially, to identify its modal properties. To do so, a thorough study, including both experimental and numerical investigation respectively, is undertaken and the robust Operational Modal Analysis is applied herein to estimate the effect of the oscillatory mass of the TLD on the dynamic properties of the offshore platform.