The removal of a platform (WE) from the West Sole Field in 1978 provided an unusual opportunity to practically test the effectiveness of structural response measurements to detect damage. The platform was deliberately and progressively damaged prior to removal by cutting members. Measurements of the dynamic response of the structure to ambient excitation were made before damage and after each cut using accelerometers placed above water and below water. The work helped define the capabilities of structural monitoring techniques and thus their potential as an aid to inspection. It was shown that complete severance of a member can be detected by overall measurements above water and that flooding and part severance can be detected by local measurements underwater. The application of the methods to other structures is discussed.
In August 1978 the West Sole Platform ‘WE’ was removed from the North Sea. It was installed in 1966 in the southern sector of the North Sea approximately 45 miles due East of Easington. It was a small unmanned platform with one producing gas well. By 1978 corrosion was discovered in the well-tubing which meant that an expensive workover operation would be necessary. In addition gas production was falling and major strengthening was required to update the jacket structure to meet current standards. British Petroleum (BP), who operate the field, decided to plug the well and remove the platform. It was the first platform to be removed from the North Sea and provided an unique opportunity to mount an extensive programme of inspection and research. Financial support was obtained from the U.K. Department of Energy and it was agreed that the results of the research would be made generally available in a detailed report which was published in 1979(Reference 1).
Part of the research programme included a study to assess the ability of structural response measurements to detect damage and this study is the subject of the paper. There has been considerable interest in recent years in the use of response measurements to determine the behaviour and condition of offshore structures. Several terms including "vibration monitoring", "structural Monitoring" and "integrity monitoring" have been used to describe the subject. The authors will use the general term "structural monitoring" to mean the collection of structural response data to provide information on the behaviour of offshore platforms. The information can be put to a variety of uses including design assurance, fatigue damage assessments, troubleshooting and damage detection. All of these are of interest to platform designers and operators but the investigation described here was specifically concerned with the damage detection objectives.
Some previous papers on the subject of structural monitoring are given in References 2 to 12. These have been mainly descriptive papers and have not given specific results. To the authors' knowledge this paper is the first to describe a study involving damage to a real platform.
Previous measurements had been made on the platform in 1975.
