Continuous Dynamic Monitoring of Offshore Structures

Jacket platforms and gravity based structures are located and operating offshore in the Italian sea. The design was implemented to respond to the storms and earthquakes. Different types of analyses have been performed in the design phase: analyses based on a three-step solution where the output describes seabed earthquake motions along with the impedance functions of the rigid foundation, and analyses based on the global (one-step) method including the structures in the computational model of the soil. The output evidences are the time history of earthquake motions (accelerations or displacement) at specified points on the structure or the acceleration response spectra at these specified points. The present paper deals with data collection and the statistical analysis of the structural dynamic response data recorded since 2002; data recorded during storms and seismic events, are used to compare design accelerations of the structures, and to determine the natural frequencies and relevant mode shapes after the events. A new technique based on kernel principal component analysis is developed to eliminate environmental influences on natural frequencies and relevant mode shapes.