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Specialist NDT services move offshore

Case Study

Carrying out a P-scan inspection of riser welds
Carrying out a P-scan inspection of riser welds

In the wake of the Piper Alpha disaster offshore, and heightened concern about the integrity and safety of gas pipeline risers, Industrial Member Conoco UK Ltd carried out a detailed assessment of the condition and fitness-for-purpose of critical riser welds on the Viking platforms in the southern North Sea.

As part of the exercise, TWI was asked to carry out automated ultrasonic inspections of particular accessible riser welds above sea level. Bryan Kenzie and Mark Kirby of TWI's NDT Department were mobilised to carry out the work.

The welds to be inspected were 325mm diameter circumferential welds where the surfaces to be scanned were vertically oriented. To gain access to these welds, scaffolding was erected at the platform spider deck level some six metres above mean sea level. For protection from sea spray, the ultrasonic data collection and storage equipment was remotely sited 12 metres above the weld area just under the cellar deck, and connected via umbilical cables to the mechanised scanner and probes below.

The inspections were time-of-flight diffraction (TOFD) ultrasonics using Zipscan equipment, backed up with conventional pulse-echo ultrasonics using P-scan equipment. The mechanised scanner used for these inspections was of the magnetic wheeled variety, guided around the weld circumference by a magnetic strip.

Ultrasonic scanning of the riser welds was successfully carried out and the acquired data stored on magnetic disk to provide a permanent record of the inspection results. An on-the-spot initial interpretation of the TOFD D-scan and P-scan images was provided for Conoco offshore, and a full analysis was later carried out at TWI. Conoco benefited from the knowledge that the welds examined did not contain any serious defects and were in good condition.

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