ABSTRACT

In this paper, the modal strain energy method for damage detection using partial modal information is investigated. This method only uses a few lower modes and partial DOFs of the whole structure, and doesn't rely on accurate baseline. For validating the damage detection method, a laboratory model of an offshore jacket structure was manufactured and modal testing was conducted on the pre-damaged and damaged structure by impact hammer. Several damage scenarios including damaged diagonal braces without and with the mass change of the deck of the structure are simulated. Modal identification is applied to extract the modal parameters of the pre-damaged and damaged structure. Modal strain energy index is employed to detect the damage(s). Results demonstrate that this damage detection method can detect the damage(s).

INTRODUCTION

Offshore platforms, which work in hostile environment, are continuously exposed to wave loads and may result in critical damage from fatigue or ship collision. Visual inspection techniques and localized ultrasonic, radiograph, etc., are usual ways to inspect the potential damages. However, many adverse conditions can limit, both technically and economically, the effectiveness of the inspection. For example, both the poor visibility condition in deep water and concealment of damage by marine growth make the visual inspection unrealistic. And techniques such as ultrasonic, radiograph, etc., are designed to detect localized damage, requiring that approximate damage location be known as a priori.

Because of those disadvantages mentioned above, vibration based global damage detection has received many researchers' close attention (Doebling et. al., 1996). It is well known that damage would alter the stiffness, damping or mass of the structure, and so as to change the modal parameters such as modal frequencies and mode shapes. That is to say, changes of modal parameters can be used as a damage indicator.

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