Damping Coefficient Analyses for Floating Offshore Structures

Abstract

The damping evaluation of floating offshore systems is based on the viscous effects that are not considered in numerical models using the potential theory. Usually, different techniques for different systems are used to evaluate these hydrodynamic coefficients. The total damping is separated by potential and viscous damping, the first one is evaluated numerically and the second through experiments at reduced scale model. Common techniques considering linear motion equations cannot be applied to all degrees of freedom. Some methods were compared for results of decay test, such as: exponential and quadratic fit. Fourier transform (FT) spectral analysis and Hilbert Huang transform (HHT) can be used to evaluate the signal natural frequency and with HHT this can be done during the time domain. Also, analysis through the Random Decrement Technique (RDT) is presented to demonstrate the damping evaluation for irregular waves. The method to obtain external damping was presented for the different techniques in an ITTC semi-submersible model. The linear method is not sufficient to predict the damping coefficient for all the cases, because in most of them, the viscous damping was better represented by a quadratic fit. The HHT showed to be a good alternative to evaluate damping in non-linear systems.

Seakeeping Tests of a FOWT in Wind and Waves: An Analysis of Dynamic Coupling Effects and Their Impact on the Predictions of Pitch Motion Response

“…The linear and linearized dampings are expressed as a percentage of the critical damping for each dof, while the quadratic damping coefficients are presented as a percentage of the mass/inertia of the respective dof. The estimation of the damping levels followed the procedures presented in [31]. The thrust force notably increases the surge and pitch damping ratios.…”

Mooring-Configurations Induced Decay Motions of a Buoy

“…Previously [19], we used an improved P-Q method to estimate linear and quadratic damping of a moored buoy and found the results to be reliable although they were sensitive to data input. Alternately, the equivalent linear damping approach [32] was adopted in this study, as it is relatively simple and robust. The nonlinear motion equation of a floating unit can be written as follows:…”

Seakeeping Tests of a Fowt in Wind and Waves: An Analysis of Dynamic Coupling Effects and Their Impact on the Predictions of Pitch Motion Response

“…The linear and linearized dampings are expressed as a percentage of the critical damping for each dof, while the quadratic damping coefficients are presented as a percentage of the mass/inertia of the respective dof. The estimation of the damping levels followed the procedures presented in [22]. It is notable that thrust force increases the surge and pitch damping ratios.…”