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MONITORING STRUCTURAL INTEGRITY OF NORTH SEA PLATFORMS BY ACOUSTIC EMISSION

By J.S.Mitchell (Chevron (UK) Ltd.), L.M.Rogers, Subsea (Offshore Ltd).
Published in Proceedings of Offshore Technology Conference, Houston, 1992 (Paper No. 6957)

This paper discusses Acoustic Emission (AE) monitoring of sub-sea welds in the Ninian Southern Platform, which located and monitored crack growth between 1989 and 1992. Evidence is provided to show that measured defect sizes and defect locations were in good agreement with previous data obtained by ultrasonic and radiographic testing.

The advantages listed in the paper for AE include:

  • The ability to distinguish between actual cracks and other less serious weld defects (such as porosity, lack of root penetration)
  • The ability to differentiate between active (propagating) and inactive (dormant) defects.
  • Provides fully automated monitoring from a remote (shore based) location without intervention from offshore personnel.

    • A problem had been discovered in the 1980's on the Ninian Southern Platform with fatigue failure at access windows, provided for welding purposes. A number of cracks had been repaired using saturation divers. There was a need to monitor some remaining cracks, to determine whether they were active. Two such cracks were instrumented, each with four AE transducers. Monitoring of these locations was continuous between September 1989 and April 1991. The paper provides details of the lay-out of the subsea installation of the AE equipment, which included a taut wire system to relay signals through the splash zone, to a modem for transmission to shore.

    Results are given in the paper for two winter periods. There is good agreement between the locations of the cracks as determined by AE and the locations determined by ultrasonics. Evidence for fatigue crack growth was determined for one crack, and from previous work it was concluded that crack growth was less than 30% of through wall thickness. This information enabled decisions to be taken on repair options.

    The costs of installing AE instrumentation are given, amounting to £210K. Monthly monitoring and reporting costs are about £1500. These costs are compared with those of conventional inspection.

    The paper points out that AE transducers, when located near areas of interest, have accurately located, sized and quantified growth activity of cracks in steel welds. This information has been very valuable in supporting repair decisions, which have much greater cost implications.

    The paper also reports on future developments in AE instrumentation. For example a marine battery powered sensor array has been on test, serviced from an ROV. In particular such a system avoids the need for hardwiring through the splash zone. It is claimed that this development would enable a storm by storm record of the location and severity of all active AE sources within a sensor array to be stored, and the data accessed at the end of the monitoring period, which is a advantage of systems operating at that time.

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