Monitoring Structural Integrity of North Sea Production Platforms by Acoustic Emission
Authors: Mitchell J.S. , Rogers L.M. 
Published: October 2008

Monitoring structural integrity of North Sea production platforms has been the source of much attention throughout the last 'decade. Stress, Vibration and Acoustic Emission (AE) methods have been developed to the stage where long-term unattended monitoring using retrofit equipment installed by divers has achieved a good record of reliability. This paper discusses AE monitoring of subsea welds in the Ninian Southern Platform which located and monitored crack growth between 1989-92. The monitoring was fully automated and initially used a taut-wire to relay signals through the wave splash zone and a modem for data transmission to shore. A later installation used smaller transducers which were deployed without use of a taut-wire. Measured defect sizes and defect locations were in good agreement with previous data obtained by ultrasonic and radiographic testing. Defect growth over the 3 year period was found to be minimal, as predicted by fracture mechanics fatigue life calculations. The advantages of AE technology over other NDT practices are shown to include avoidance of nonessential repairs and reduced requirements for conventional inspection. This paper includes reference to new developments aimed at removing completely the need for divers in long-term weld monitoring.

INTRODUCTIC)N

The integrity of large, offshore platforms can often be compromised by relatively small defects which may exist in critical points of steel welds. These defects usually start from flaws in fabrication and propagate through fatigue. Using conventional NDT methods, only small numbers of potential problem areas can be targeted, from which overall structural integrity must be assessed.

When defects are encountered, the cost to repair can range from S20,000 for a small clamp in the air diving range to #2,000,000 for a hyperbaric weld using saturation divers. Information on defect size and rate of growth are essential for use with detailed analysis methods to allow intelligent decisions regarding these repairs.

Use of Acoustic Emission (AE) for detection and monitoring of weld defects is gaining increasing acceptance by North Sea operators. The advantages of using AE for these inspections include:AE can distinguish actual cracks from other less serious weld defects such as porosity, over penetration, burn through and lack of root penetration.AE can differentiate between active (propagating) cracks and inactive (dormant) defects.AE allows fully automated monitoring from remote (shore based) locations without intervention from offshore personnel.

In this paper, the successful use of AE technology is demonstrated from offshore tests. An ongoing installation on Ninian Southern Platform is described in detail and results are given which document cost savings. In addition a planned prograrnme which will further develop AE technology offshore is discussed.

PREVIOUS VALIDATION OF AETECHNOLOGY

Extensive tests of AE monitoring were carried out on full-scale tubular joints at the UK National Engineering Laboratory (NEL), as part of the UK Offshore Steels Research Programme (UKOSRP). It was shown that all cracks had been located and correctly sized from the AE measurements by the time they had reached 10% through wall thickness.