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Criteria for pipe girth welds inspected using ultrasonic testing

Connect, no. 121, November/December 2002, p.3

Schematic of transducer arrangement Courtesy of Shaw Pipeline Services Ltd
Schematic of transducer arrangement Courtesy of Shaw Pipeline Services Ltd

Automated ultrasonic testing (AUT) is becoming the method of first choice for inspection of pipe girth welds, due to its speed, sizing ability and health and safety advantages compared with radiography. In addition, due to its ability to size the through-thickness dimension of flaws, AUT is readily integrated into an Engineering Critical Assessment (ECA) approach, in which acceptance of welding flaws is based on fitness-for-purpose rather than on workmanship criteria.

In response to industry needs, TWI has launched a group sponsored project to generate guidance on qualification and reliability of AUT systems, and derivation of ECA-based flaw acceptance criteria for pipe girth welds inspected by AUT.

The project started in June 2001 and is currently supported by eleven sponsors. These are: Advantica Technologies Ltd, Allseas Engineering bv, BP Exploration, Coflexip Stena Offshore Ltd, Health & Safety Executive, Oceaneering International Services Ltd, OIS plc, Saipem SpA, Stolt Offshore Ltd, Shaw Pipeline Services Ltd, and TransCanada PipeLines Ltd. The main benefits of the project are that it can potentially reduce the cost of fabricating offshore and onshore pipelines by:

  • Reducing the number of unnecessary weld repairs (through the use of optimised acceptance criteria based on fitness-for-purpose principles and/or revised workmanship-based criteria).
  • Reducing the cost of qualification of AUT inspection procedures, partly by reducing the requirements for individual inspection qualification.

The deliverables from the project include:

  • A multi-tier (three level) approach to set acceptance criteria dependent on the applied load and including 'ready-made' criteria in the form of minimum required material properties. This will be based on improved treatments ofplastic collapse, strength mismatch and constraint effects.
  • Practical guidance on qualification of AUT systems including: dependence on pipe geometry, flaw type, pipe materials, equipment type and settings; uncertainties in flaw detection and sizing and their interaction with acceptance criteria; use of prior information, physical reasoning, and theoretical modelling.

Although the 'ready-made' acceptance criteria address a range of pipelines installation loading, other deliverables from the project, especially the generic method to derive acceptance criteria and the guidance on qualification of AUT systems, apply equally to girth welds in pipelines, SCRs and tubular tendons for TLPs.

For further information contact Mohamad Cheaitani. Email: mohamad.cheaitani@twi.co.uk

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