TWI was asked by Phillips Petroleum Company (United Kingdom) Ltd to investigate the use of automated ultrasonic inspection and engineering critical assessment (ECA) as a viable alternative to radiography for inspection of riser tie-in welds offshore. In response to this request, TWI carried out a programme of work that included ultrasonic inspection on the Judy platform in the North Sea.
A specific riser tie-in weld was ultrasonically inspected using a P-scan computerised flaw detector and mechanised scanner, backed up with time-of-flight diffraction (TOFD) for accurate flaw sizing. In parallel with this, an ECA was carried out to PD 6493 procedures to calculate tolerable flaw sizes for the weld inspected. Results were also compared with flaw acceptance criteria based on 'good workmanship' as contained in BS4515 and BS 5500.
This project provided the client with essential information to put forward a case to the regulatory authority for using this approach in the future. Having a suitable alternative to radiography means that problems of radiation safety offshore, including evacuation of work areas and disruption to nearby work, can be avoided. Further, this approach enhances the quality of inspection results whilst still retaining traceability and repeatability records, plus a more reliable assessment of weld flaws.