Structural Integrity Monitoring in Real Seas

A programme of structural integrity monitoring through vibration measurements on three fixed steel platforms in the North-Sea was sponsored by the Department of Energy. The objectives were to develop monitoring systems to operate continuously over a period of two years and to demonstrate the feasibility of such systems to detect structural damage. The vibration response due to ambient excitation was measured through instrumentation consisting mainly of accelerometers and data was collected until March 1982. This paper describes the work carried out in analysing selected records from the three platforms. Spectral analysis techniques together with the results from finite element analyses were used to identify the natural frequencies. The results indicate that only the lowest natural frequencies could be identified without ambiguity and these frequencies are stable throughout the monitoring period. Temporary reduction of these frequencies due to increase in deck mass associated with drilling activities was detected. It is essential, therefore, to monitor the deck mass in order to distinguish the frequency variations associated with primary structural damage and foundation stiffness changes. It is concluded that vibration monitoring systems comprising accelerometers at the 'surface' and deck can only detect global changes. Nevertheless, ambient monitoring systems are recommended for continuous monitoringmore » of global condition of an offshore platform.« less