I. A. Sibetheros

Volterra series-based system analysis of random wave interaction with a horizontal cylinder

Abstract The response of a long flexible cylinder excited by random waves in a large model basin was investigated. The linear and non-linear physical mechanisms associated with the wave–cylinder interaction were analysed using system identification and modelling techniques. A third-order frequency domain Volterra model and its orthogonalized counterpart were used to analyse the relationships between wave elevations at various locations in the vicinity of the cylinder and cylinder acceleration data at various cylinder longitudinal locations. It was found that linear mechanisms dominate, particularly at the frequency band where the majority of the wave energy is located. At higher frequencies, the cubic component of the Volterra model is the main contributor to the total model coherence, i.e. the fraction of the measured output power that can be approximated by the model output, whereas the quadratic components contribution to the total model coherence was in general quite small. This process of identification and quantification of the non-linear mechanisms of the unknown physical system can lead to the design of improved parametric models for the cylinder response, which should by design simulate non-linearities such as the ones identified by the Volterra model. The estimated linear and non-linear Volterra transfer functions were also used to predict the cylinder acceleration under excitation inputs not used in the estimation of the model transfer functions. The good match between predicted and measured output auto-power spectra suggests that the estimated transfer functions are indeed true models of the underlying physical mechanisms of the interaction. However, the latter can only be achieved if a minimum number of data segments, as determined by an error analysis involving modelling and prediction errors, is used in the estimation of the Volterra transfer functions.

A Numerical Study of the Hydrodynamics of Reversing Flows Around a Cylinder

A numerical study of the nonlinear and random behavior of flow-induced forces on offshore structures and experimental verification of the results are presented. The numerical study is based on a finite-element method for the unsteady incompressible Navier-Stokes equations in two dimensions. The momentum equations combined with a pressure correction equation are solved employing fourth-order artificial dissipation with a nonstaggered grid, instead of the more commonly used staggered meshes. The solution is advanced in time with a combined explicit and implicit marching scheme. Emphasis is placed on study of reversing flows around a cylinder. Comparisons with experimental data evaluate accuracy and robustness of the method.

Analysis of Wave Run-Up Measurements on a Mini-TLP

Wave run-up on deepwater offshore structures may contribute to wave overtopping of the platform deck. It may also cause undesirable loads including impact loads on the underside of the deck when combined with other hydrodynamic phenomena beneath the platform deck. In this study a 1:40 scale model of an unmanned mini-TLP design was subjected to a series of design sea environments for the Gulf of Mexico. Complimentary testing of compliant and fixed hull configurations was performed and the wave run-up was measured at several locations around the hull in both head and quartering sea orientations of the platform. The wave elevation data reported here was obtained at three critical points on the hull and a reference wave gage well upstream of the model. Analysis of the data was performed using an orthogonal third order Volterra series system analysis technique. The wave run-up measurements were compared and correlated to the incident wave elevation measurements. The results indicate that the run-up is more pronounced in the rigid hull model configuration than for the compliant model configuration and this was the case for both headings. Further, the analysis revealed a strong linear relationship between wave run-up and the incident wave field over the high wave energy frequency band. This result adds credence to the previously proposed simplified computational procedures for estimating the second order wave run-up on large cylinders in random waves.© 2005 ASME