In this paper, the modal strain energy index for detecting damaged diagonal member(s) of an offshore jacket structure using partial modal information is investigated. Only the first two mode shapes, with merely horizontal translational degree of freedoms (DOF) in each mode, are utilized in the damage detection procedure. Meanwhile, the damage detection with mass change on the deck of the platform model is also considered. First a numerical model of an offshore jacket structure as baseline (pre-damaged) is developed by a finite element method. Several damages are simulated by reducing the elastic modulus of the structural element(s). Then the modal strain energy method is applied to detect the damaged location(s). Numerical results demonstrated that this damage detection method only uses a few lower modes and partial DOF of the whole structure, and can detect the damaged location(s) when the deck mass is changed.
Many civil structures — such as bridges, buildings, and offshore platforms — subjected to dynamic loading continuously accumulate damage during their service life. For example, offshore platforms, which work in hostile environment, are continuously exposed to wave loads and may result in critical damage from fatigue or ship collision. Commonly used traditional inspection method is the visual inspection techniques. However, both the poor visibility condition in deep water and concealment of damage by marine growth make the visual inspection unrealistic. Other Non-Destructive Testing (NDT) techniques, like ultrasonic, radiograph, etc., are usual ways to inspect the potential damages. 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.