The paper covers the particular aspects of testing and analysis in the ECAs (engineering critical assessment) for installation have been performed for thin-walled pipe. Central to the current guidance for conducting ECAs for pipelines is the predominantly stress-based BS 7910. DNVRP-F 108 supplements the main fracture assessment procedures with specific amendments for installation methods that introduce cyclic plastic strain. Nevertheless, the amended guidance remains stress-based. DNV OS-F101 now recognises more clearly the limitations that thin-wall pipelines place upon ECAs and gives unambiguous but limited guidance on how to proceed for wall thickness below 10 mm. The main message from this study is that, although full scale testing will provide verification of a single-valued, selected acceptance criterion, e.g 2 x 50 mm, the skilful use of fracture mechanics has the benefit of giving broader information and a richer understanding of flaw tolerance. Even the exercise of working through the assessment in a rigorous manner provides a better understanding of the effect of input variables. The estimation of crack driving force using FE (finite element) simulations is also discussed where important differences are noted between tolerable flaw dimensions from these and BS7910-based assessments. The authors even tentatively argue for the qualified use of existing fracture mechanics SENT (single edge notched tension) data obtained on similar linepipe grades and weld procedures. Comparative evaluation can be made of material and weld tensile properties to ensure equivalence. Although WPQ testing, notched cross-weld tensile tests and segment testing are all that are required. limited fracture mechanics testing may also be performed to calibrate against the reference R-curves should further confidence be necessary.
Engineering critical assessment of thin-walled offshore pipelines
Technische Gefährungseinschätzung von dünnwandigen Offshore-Pipelines
2008
19 Seiten, 23 Bilder, 20 Quellen
Conference paper
English
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