Historically, undewater visual examination by divers has been employed to monitor the structural integrity of fixed offshore platforms. Alternative approaches for inspection are being sought to lessen the cost and hazard of diver operations, especially in deep and hostile waters. An approach being applied in the North Sea makes use of ambient vibration monitoring with above-water accelerometers to identify the lower global modes of platform vibration. Changes in the natural frequencies and mode shapes are indicators of possible below-water structural breakage. The United States Geological Survey (USGS) has sponsored studies at The Aerospace Corporation to examine the feasibility of this approach. As part of the study, the producing Shell Platform SP-62C in the Gulf of Mexico was instrumented at 17 locations between decks and at the boat landing level for detection of its ambient vibrations during normal producing operation of the platform. Some vertical and angular accelerations were measured, in addition to lateral accelerations. The platform is an eight-leg diagonally braced jacket construction consisting of eight ungrouted main piles and eight grouted skirt piles, and stands in a 327-ft (100-m) water depth. The interpretation of the random vibration data via spectral analysis already has been reported, and includes a favorable comparison of the modal parameters derived with subsequent results from forced shaking of this platform. The overall conclusion from this field experiment was that the data acquisition and analysis aspects of the ambient monitoring method are practical, with use of readily available hardware and software.

Media Info

  • Features: References;
  • Pagination: p. 101-110

Subject/Index Terms

Filing Info

  • Accession Number: 00329925
  • Record Type: Publication
  • Report/Paper Numbers: Volume 4 Proceeding
  • Files: TRIS
  • Created Date: Apr 15 1981 12:00AM