Spectral analysis of slender tensioned cylinder interaction

Abstract

A Tension Leg Platform (TLP) is a floating deep-water compliant structure that utilizes a system of slender vertical tensioned structural members, i.e. tendons, to provide station keeping. In the past most emphasis has been placed on understanding the global response behavior. In this study emphasis is placed upon attempting to understand the response behavior of closely spaced tendons in wave flows. In this study, two adjacent vertical tensioned cylinders are idealized as two Single Degree of Freedom (SDOF) systems and their interaction is idealized as nonlinear spring-dashpot system. A spectral analysis technique developed by Bendat and Piersol (1993), which makes use of a reverse dynamic system, is used in this study. The significant feature of the technique is conversion of the original, difficult-to-analyze nonlinear problem into a straightforward linear problem. At the same time, an analytical linear transfer function was developed under the Gaussian wave assumption for comparison with the measured data. The developed model and the interaction behavior are estimated based upon the experimental data from Rijken (1997). Three important nonlinear cases in structural vibrations were examined. Linear and nonlinear impedance function were obtained for three different spacings. Ordinary and cumulative coherence functions were estimated and compared for goodness-of-fit of the model. From the study, one observes that as the cylinder spacing increases the difference in total force measured on the upstream and downstream cylinder decreases. The simulation results show that the interaction behavior is none of the three classical nonlinear cases. This confirms that the modeling nonlinear interactions observed in the data will require a more complicated model.