Spectral modeling of two incline cylinders with validation in the time domain

Abstract

Clusters of long slender members such as risers or tendons are commonly found on deep water offshore platforms. Current practice is to space these slender structural members far enough apart to assume that they behave as a single cylinder without any interaction effects. This study uses a coupled spectral and numerical simulation method to investigate the hydrodynamic interaction phenomena between two inline cylinders at various spacings and pre-tensions. The emphasis is placed on understanding the response behavior of cylinders as the inline spacing between them varies. A reverse dynamic system, a spectral analysis technique developed by Bendat and Piersol (1993), is used to estimate the nonlinear constants of three classic nonlinear equations. The power of this technique is that the nonlinear behavior can be lumped together. A nonlinear equation of motion representing the interactive response behavior is linearized using the estimated nonlinear constants from the spectral domain, then solved using the Newmark beta method. The developed model uses experimental data from Rijken (1997). It is the intent of this study to better quantify this interaction behavior and eventually provide some guidance for design of realistic configurations.