Structural Integrity Monitoring in Real Seas

ABSTRACT 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 monitoring of global condition of an offshore platform. INTRODUCTION Vibration monitoring for the purposes of assessing the structural integrity of offshore platforms has received considerable interest in recent years. It is based on the principle that shifts in the characteristic frequencies in the vibration response spectra or the response 'signatures' are indicative of changes in the structural stiffness or mass of the structure. If, through regular monitoring, such changes could be successfully identified as those due to structural failure it could prove a valuable complement to current inspection techniques and could lead to a reduction in conventional underwater inspection. Recognising the potential of such a system, the U.K. Department of Energy drafted a comprehensive research program in 1976. The general objectives of this investigation have been documented elsewhere (1). Accordingly, a vibration monitoring system based on ambient excitation was proposed in view of the severe environment in the North-Sea. Further, such a system would impose a minimum of interference with the normal operations of a platform. In the absence of any precedence of a permanent vibration monitoring system, the Department of Energy selected three contractors to independently design, install and operate monitoring systems on three platforms in the North-Sea and appointed a Certification Authority to review and assess these systems at all stages of this project. The assessment of performance and reliability of the monitoring systems was to be based on two years of regular data gathering. The initial project ended in December 1981 and an extension to the project covers data for a further winter until March 1982. This paper is limited to the work carried out by the Certification Authority based on a selected sample of 'raw' data.