Inspection of tether string welds
In August 2003 FORCE Technology inspected welds on tension legs of Heidrun the Statoil platform placed at 360 metres depth off the Norwegian coast and that is probably the deepest automated ultrasonic inspection done at that time.
The tension legs of the Heidrun platform are 24” diameter pipes constructed by welding together 12 metres long sections with a wall thickness of 38 mm. These tension legs were examined very detailed during the production phase. Now, about eight years after start of platform production these welds should be inspected again with the best possible subsea technology.
Statoil contacted FORCE Technology because the Heidrun maintenance people knew that the major components of the latest version of the ROV operated P-scan Subsea Inspection System had been developed for operation depth of 400 metres.
In August 2003 the FORCE team arrived at Heidrun with the P-scan Subsea Ultrasonic Inspection System. The system includes a PC to be placed topside, the AUS-4 subsea magnetic wheel scanner, the electronic bottle housing the P-scan-4 data acquisition and processing unit and the tool skid to carry the subsea system components and cables.
After arrival on the platform the tool skid was mounted underneath the Heidrun ROV and communication between subsea system and topside PC via ROV umbilical (optical fibre) was established.
The inspection procedure included a full volumetric inspection using pulse-echo technique for detection and TOFD for sizing of defects if any. It was possible to inspect up to 6 welds in a normal 12-hour shift.
During the inspection period set for 2003, inspection of all welds (except for 4) on two tension legs was completed. The inspected welds were located in depth ranging from about 60 metres to 325 metres and this is probably a depth record for automated ultrasonic inspection.
The inspection of the welds have been ongoing annually since 2003, and inspection has also been carried out on the Snorre A TLP offshore Norway and the Jolliet TLP in Gulf of Mexico.