Dynamic Monitoring of Steel and Concrete Offshore Structures

Steel jacket platform and gravity based structure located and operating in Italian offshore sea are monitored with a continuous dynamic system to investigate the structural response to the sea storms and earthquakes. In order to make a long-term dynamic monitoring, are located on the structures some acceleration units positioned at different elevations, each unit can contains linear or angular accelerometers. The numerical elaborations are performed through the experience gained over 10 years of continuous dynamic monitoring, and compared the behavior of one steel structure, tubular jacket type with pile foundation, and one gravity reinforced concrete structure. The paper deals with data collection and the statistical analysis of the structural dynamic response data recorded during the normal work, during the storms and seismic events; the accelerometer data are used to compare design accelerations of the structures, and to determine the natural frequencies and relevant modal shapes after the events. For this purpose, methods of Operational Modal Analysis (OPA) and methods of the principal components (PCA) can be used to highlight structural damage states. The identification methods are applied to two offshore structures that have different frequencies and different dynamic behaviors.