Wave-structure dynamic interaction of the VEGA platform

Abstract eng:
The offshore platform VEGA-A is a fixed eight-legged structure that has been operating from 1988 in the Sicily channel. In the platform, there is an environmental and structural monitoring system installed within. In September 2015 the processing mode of the acquired data on the platform was changed, in order to establish their representativeness in relation to the structural control of the platform. The present work presents the results of the new data processing provided by the monitoring system, which performs the spectral analysis and dynamic identifications, in particular, will be obtained by the frequencies of oscillation in order to find a variation of them during the life of the platform. The variation of forms and periods of oscillation on its own can be related to damage to the structure or a variation of own loads. The scope includes a statistical analysis of the accelerometric signals in order to identify important events and for the warning thresholds and alarm. These results are then compared with data from the project in accordance with the storms that have affected the structural system. 1  INTRODUCTON The control plan for off-shore structures, or the verification of such structures for use beyond their initial life, requires a design of an inspection plan aimed to constantly check-up the structural elements (joints members). The inspections, their frequency, and their typology (i.e. the proper selection of the elements and/or joints to check-up) is a critical issue (since, for instance, it may not be feasible to inspect all critical components), and inspection planning was for the last decade, and still is, based mainly on probabilistic analysis (Risk Based Inspection, RBI). In addition, the visual inspection of structural damage is in most cases hard to perform (taking into account both the water depth and the marine growth plants that hide the structural member) and evermore economically demanding. To overcome these problems, monitoring techniques for damage identification through the analysis of the changes in the model properties of offshore structures have been developed. To deepen these aspects the paper investigates the ability of monitoring systems and damage measures to assess possible structural damage. To obtain this features that are the mainly sensitive to damage, but insensitive to environmental and operational variations, the Principal Components Analysis (PCA) and Novelty Detection are used for this scope. This output-only, black-box technique has been applied for eliminating environmental influences on features such as natural frequencies, which boils down to estimating a relationship between the

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