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Engineering critical assessment carried out on ageing oil platform

TWI worked with a member company to prepare a study into the assessment of an ageing oil production platform.

A particular problem in assessing the integrity of ageing structures using engineering critical assessment (ECA) lies in the availability of materials data, especially fracture toughness. Historic data were available from the platform, supplemented by new data from a similarly designed platform. The older tests were based on specimens with a range of different flaw depths.

Bringing scientific advances to bear

Flaw depth (a/W) is now known to influence fracture toughness – as measured by the crack tip opening displacement (CTOD) test – but at the time that the original tests were carried out, the relationship between the two was not fully understood. The question, then, was how to generate a sufficient quantity of consistent toughness data to form the basis of a credible safety case.

TWI’s analysis used the concept of crack tip constraint, as codified in the latest (2013) edition of BS 7910 ('Guide to methods for assessing the acceptability of flaws in metallic structures') to address this question. Essentially, test specimens and structures under predominantly tensile loads (and/or containing shallow flaws) have an effective toughness that is higher than that determined from standard fracture toughness tests on deeply notched specimens in bending. This difference can, in principle, be taken into account by testing a variety of specimens with both high and low crack tip constraint, and generating a relationship between toughness and constraint. A schematic of the method, described in Annex N of BS 7910:2013, is shown below.

By calculating constraint parameters for both the small-scale specimens and the relevant parts of the platform, it proved possible to demonstrate enhanced fracture toughness for selected structural details, which in turn contributed significantly to the overall safety case.

For more information about our work to support life extension of oil and gas assets, please email contactus@twi.co.uk.

Avatar Dr Isabel Hadley CEng, Eur Ing, FWeldI Technology Fellow – Materials and Structural Integrity Technology Group, TWI

Isabel is a Technology Fellow, recently semi-retired from TWI. Her work focuses on the development of analytical flaw assessment techniques, and their application to safety-critical welded structures and pressure equipment. She has a particular involvement in the development and validation of structural integrity procedures and standards such as:

  • BS 7910 (UK flaw assessment procedure); Chair of the committee, 2008-2021
  • R6 (UK nuclear assessment procedure); member of the panel
  • FITNET (European fitness-for-service procedure); co-author.

She is also Royal Academy of Engineering Visiting Professor in Integrity Management at the University of Bristol, as part of an initiative to transfer industrial know-how to engineering undergraduates.

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