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Fatigue Design and Analysis of Offshore Pipelines and Risers Subjected to Waves and Currents

[+] Author Affiliations
M. Liu, C. Cross

Aker Solutions, London, UK

Paper No. OMAE2016-54252, pp. V005T04A049; 10 pages
doi:10.1115/OMAE2016-54252
From:
  • ASME 2016 35th International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 5: Pipelines, Risers, and Subsea Systems
  • Busan, South Korea, June 19–24, 2016
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 978-0-7918-4996-5
  • Copyright © 2016 by ASME

abstract

The industry consensus would appear that the effect of currents on wave induced fatigue damage accumulation is assumed as insignificant and can be ignored. Only when dealing with stability, ultimate limit state design and vortex induced vibration, the recommended industry practice is to consider both currents and waves simultaneously, but except for fatigue design.

This paper presents a study on how environmental loads should be considered in terms of currents and waves for the fatigue life design of offshore pipelines and risers. The study is intended as a spur to redress the misapprehension by focusing on the coupling effect of direct waves and currents in the context of fatigue damage assessment.

It is demonstrated unequivocally that waves and currents cannot be decoupled for fatigue design assessments. Wave induced fatigue with the inclusion of currents is manifested twofold, not only the increased mean stress correction effect but also higher total damage accumulation due to elevated stress ranges. The practice of using wave histograms while ignoring currents is shown to result in an unacceptable non-conservative fatigue design. Both effects should be accounted for in the engineering assessment. A first order correction factor involving the ratio of current and wave velocities is introduced to evaluating the environmental load coupling effect. It is recognized that fatigue associated specifically with VIV phenomena is well understood and documented elsewhere, its discussion is thus outwith the aims of this paper.

Copyright © 2016 by ASME

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