MODEL STUDY OF EFFECTS OF DAMAGE ON THE VIBRATION PROPERTIES OF STEEL OFFSHORE PLATFORMS

Changes in the vibration frequencies and mode shapes of fixed offshore platforms can be used to detect damage. The vibratory properties of a, 1/50th scale, three dimensional model possessing the key features of a typical, eight legged, k-braced steel offshore platform were studied considering both damaged and undamaged conditions. In this study, for the first time, quantitative information on mode shapes were utilized leading to improved damage detection capabilities.; The scaling considerations, the model and the experimental method are described. The experimental results are complemented with analytical results showing excellent correlation. In general, emphasis is placed on the vibration frequencies and deck modal displacements of the first two translational modes and the first torsional mode. The effects of a variation in deck mass are assessed both experimentally and analytically. Also, the effects of shifts in the position of the deck center of mass are assessed analytically.; Both experimental and analytical results support the potential application of mode shape monitoring to detect structural damage. Results demonstrate that modal displacements of the deck and the vibration frequencies for the first two translational modes and the first torsional mode of the structure are sufficient for integrity monitoring purposes. Findings show that frequency changes of only a few percent are accompanied by changes of more than 30% in the normalized modal displacements of the deck when certain members are damaged. Further, the characteristics of the changes in the vibratory properties caused by damage are different from the characteristics of non-damage related variations when both frequencies and mode shapes are considered and this provides latitude to distinguish significant non-damage related variations from damage related ones.