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Should Residual Stresses Be Taken Into Account in Structural Integrity Assessment of Offshore Monopiles?

[+] Author Affiliations
Anais Jacob, Ali Mehmanparast

Cranfield University, Cranfield, UK

Jeferson Araujo de Oliveira, Foroogh Hosseinzadeh

Open University, Milton Keynes, UK

Filippo Berto

Norwegian University of Science and Technology (NTNU), Trondheim, Norway

Paper No. OMAE2018-78519, pp. V003T02A051; 7 pages
doi:10.1115/OMAE2018-78519
From:
  • ASME 2018 37th International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 3: Structures, Safety, and Reliability
  • Madrid, Spain, June 17–22, 2018
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 978-0-7918-5122-7
  • Copyright © 2018 by ASME

abstract

A key challenge in the Offshore Wind industry is assuring the life-cycle structural integrity of wind turbine foundation monopiles. This is due to harsh environmental aspects as well as the loading regime (i.e. constant exposure to wave and wind forces introducing both fatigue and corrosion damage). Welding is a widely used joining technique for the manufacturing of offshore monopile structures. However, this is an aggressive process that introduces high levels of residual stress, which in turn may lead to reduced fatigue life, corrosion cracking resistance and accelerated degradation mechanisms. This study presents evidence that a measurement-informed strategy could be used towards developing a more reliable structural integrity assessment procedure for offshore monopile structures by taking into account the effect of residual stresses. A welded mock-up, 90 mm thick, 2600 mm wide and 800 mm long plate, was fabricated using a typical double-V welding procedure following current industrial practice. The contour method of residual stress measurement was employed to map residual stresses in the welded mock-up as well as in the CT specimens extracted from the weld region of the plate for future fatigue tests. Residual stress measurement results show that the mock-up plate contained tensile residual stresses above yield in the core of the weld, while the extracted CT specimens had lower though still significant residual stress levels. These results indicate that if the initial residual stresses are not carefully considered during fatigue or corrosion cracking tests, the results from the CT specimens alone will likely result in misleading structural life estimations.

Copyright © 2018 by ASME

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