Fatigue and monitoring of SPM

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 subjected 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 in October 2009. The system is composed of fiber optic strain gauges sensors and biaxial inclinometers that detect rotations of the column and the bridge. The research concerns the method of collecting and interpreting statistical data, to determine the structural behavior under the action of the wind and sea. Through the processing/interpretation of the data it was possible to identify the dynamic response of the system SPM, count the number of fatigue cycles and conduct the structural verification in welded sections of the column and the hinge joints. 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.