Session: 01-03-01 Hydrodynamics

Paper Number: 78644

78644 - Importance of the Inertial Components in Modal State Covariances 

In order to cross wide and deep fjords or straits, long-span bridges usually have to be supported by floating supports. However, traditional time and frequency domain analysis methods actually struggle to evaluate the extreme response to the hydrodynamic actions of such large floating structures in a reasonable amount of time because the timescales associated with the motions of either the structure, either the loading, are most often clearly separated.   

Indeed, in the time domain, it relies on the statistical treatment of response signals which thus have to be simulated long enough to capture the slow dynamics of the floaters, but with a sufficiently short time step to capture the fast dynamics of the waves. Similarly, in the frequency domain, sharp and distant peaks appear in the response spectra whose numerical integration, therefore, requires using many closely spaced points spread over a wide domain to provide a correct estimation of the response statistics that are used for determining the extreme distribution.

In an attempt to accelerate the power spectral density method evoked just before, recent efforts have first focused on computing more efficiently the loading spectrum which is subsequently multiplied by a transfer function to get the respective response spectrum that needs to be integrated. This paper tackles the problem from another but complementary angle, by trying to reduce the computational time it takes to integrate the response spectrum. 

To do so, the Multiple Timescale Spectral Analysis method is used since it has been developed for the same purpose when dealing with wind-loaded structures and has already allowed deriving semi-analytical approximations for the background and the resonant components of their response statistics. It is just specialized in this paper to provide rapid and accurate estimations for the variances and the covariances of the modal responses of wave-loaded structures, the main difference being that they might respond in the inertial regime as well. 

These results are then recombined to get the variances and covariances of nodal responses which are further analyzed in an analytical and a numerical way in order to better understand the conditions under which the influence of a given modal covariance is significant or not. Illustrations are finally provided for the Bergøysund Bridge in Norway, which is a floating pontoon bridge.

Presenting Author: Margaux Geuzaine University of Liège

Authors:

Margaux Geuzaine University of Liège
Aksel Fenerci Norwegian University of Science and Technology
Øiseth Ole Norwegian University of Science and Technology
Vincent Denoël University of Liège