Dynamic Monitoring of GBS Terminal

A Gravity Based Structure (GBS) is operating in the Adriatic sea, offshore Italy. The GBS contains two LNG tanks and supports a number of topside steel structures. The design was carried out for two earthquake scenarios: Operating Basis Earthquake (OBE) and Safe Shutdown Earthquake (SSE). Two types of analyses have been performed in the design phase: an analysis based on a three-step solution where the output describes seabed earthquake motions along with the impedance functions of the rigid foundation, and an analysis based on the global (one-step) method including the GBS in the computational model of the soil. The output is the time histories of earthquake motions (accelerations or displacement) at specified points on the structure or the acceleration response spectra at these points. There are three acceleration units positioned at two different elevations on the GBS, one on the East Side (top and bottom) and the other at the West Side (top). Each unit contains 3 linear accelerometers. Relating to the GBS concrete structure, this paper deals with data collection and the statistical analysis of the structural dynamic response data recorded since October 2009. Data recorded during storms and four seismic events, that took place on 20th May-3rd June, 2012 are compared to design accelerations of the GBS, computed for OBE and SSE earthquakes, and are used to determine the GBS’s natural frequencies and relevant mode shapes.