Data Processing Strategies for Monitoring an Offshore SPM system

Near the offshore production platform Vega A, the tanker FSO Leonis is moored to a steel column with circular section, 130 meters high and 10 meters in diameter, installed in 1988 in the Sicilian Channel, the connection is via a bridge structure with welded steel sections that make up the system SPM (Single Point Mooring). The structural system in steel box girders and column, with its cylindrical hinges, is calls-to the actions of the sea that induce cyclic stresses and fatigue. The paper presents the monitoring system of the structure connecting the ship to the column, installed on your system by structural in October 2009, is composed of strain gauges sensors, in fiber optic, and inclinometers biaxial that detect rotations of the column and the bridge. The Structural Health Monitoring (SHM) relies on the repeated observation of damage-sensitive features such as natural frequencies and mode shapes. The problem is that changes in temperature, relative humidity, operational loading, and so on also influence those features. This influence is in general nonlinear and also nonlinear is the system SPM-Yoke-Vessel. In this work, a technique based on kernel principal component analysis is used with both the sensor measurement (strain gauge data and inclinometer data); this method is used for eliminating environmental and operational influences on the damage-sensitive features. The results allowed us to evaluate the conditions assumed in the project and made it possible to define a program of inspection and maintenance of steel structures.