Modal Parameter Estimation for Jacket-Type Platforms Using Free-Vibration Data

Abstract

An accurate estimation of the modal parameters of a structure, including modal frequency and damping ratio, is crucial for many practical engineering problems. This article introduces Prony’s method for identifying multiple modal parameters of a jacket-type platform from the measured free-vibration data. In contrast to the Fourier analysis for the decomposition of a periodic signal, Prony’s method involves the decomposition of a transient signal into a number of damped sinusoidal components. By using a finite-element model of a realistic offshore jacket-type platform, free-vibration data, corresponding to two different initial displacement patterns, were extracted and used in a numerical study. From the sampled response acceleration signals associated with each of the initial displacement patterns, the modal parameters of different participating modes were accurately determined. This numerical study demonstrates that the modal parameters, of at least the first nine modes of the jacket-type platform, including six translation modes and three torsion modes, can be accurately estimated by using the proposed method.