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Avoiding Local Brittle Zones in Offshore Pipeline Girth Welds for Reeling Installation Involving Large Plastic Strain

[+] Author Affiliations
Jens P. Tronskar

Det Norske Veritas Pte. Ltd., Singapore

Vebjørn Andresen

Det Norske Veritas, Hovik, Norway

Paper No. OMAE2013-11094, pp. V003T03A041; 10 pages
doi:10.1115/OMAE2013-11094
From:
  • ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 3: Materials Technology; Ocean Space Utilization
  • Nantes, France, June 9–14, 2013
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 978-0-7918-5535-5
  • Copyright © 2013 by ASME

abstract

Pipelines for reeling are designed to tolerate the large plastic strain associated with the reeling installation process based on widely accepted strain based design principles for subsea pipelines as described in Det Norske Veritas (DNV) Offshore Pipeline Code OS-F101: 2012 [1].

Engineering Critical Assessment (ECA) to develop flaw acceptance criteria for automatic ultrasonic testing (AUT) for girth welds subject to large plastic strain shall according to DNV-OS-F101: 2012 [1] and DNV RP-F108 [2] be carried out in accordance with BS 7910 [3], at assessment Level 3B, with amendments and adjustments described in Appendix A of DNV-OS-F101 for strain-based loading. This is a tearing analysis using the material specific failure assessment diagram (FAD), the material stress-strain curve and the fracture resistance J-R curve (or CTOD-R curve) for the HAZ or WM. It is therefore essential that the pipeline girth welds exhibit maximum load behavior and large tearing capacity to enable development of workable and practical flaw acceptance criteria for the girth welds on the stalks.

Welds in offshore structural steels are known from the early 80s introduction of low carbon-manganese micro-alloyed steels, to occasionally exhibit low fracture toughness associated with so-called local brittle zones (LBZ) in the HAZ. Similarly, in the 90s LBZs were found in pipeline seam welds welded at high arc energies. Presence of such microstructures may have a dramatic effect on the coarse grained HAZ CTOD fracture toughness properties causing unstable fracture in the CTOD tests and CTOD values below 0.1 mm at test temperatures of 0°C and below. Recently low CTOD critical fracture toughness values due to pop-ins and unstable fracture initiation in the HAZ have been experienced for pipeline girth welds for reeling and investigation confirmed these were caused by LBZs. This paper makes a comparison with the situation experienced earlier for welds in structural steels and pipeline seam welds, to understand the factors influencing the LBZ formation, and to show how such problems can be avoided. To avoid LBZs formation in the girth welds is imperative for reeling installation, where the large plastic strain associated with reeling installation affects every girth weld.

Copyright © 2013 by ASME

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