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Research Papers: Piper and Riser Technology

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

[+] Author and Article Information
M. Liu, C. Cross

Front End,
Aker Solutions,
London W4 5HR, UK

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received September 16, 2016; final manuscript received August 18, 2017; published online November 16, 2017. Assoc. Editor: Kazuhiro Iijima.

J. Offshore Mech. Arct. Eng 140(2), 021701 (Nov 16, 2017) (9 pages) Paper No: OMAE-16-1116; doi: 10.1115/1.4038325 History: Received September 16, 2016; Revised August 18, 2017

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 (VIV), is the recommended industry practice to consider both currents and waves simultaneously, 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 nonconservative 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 out with the aims of this paper.

FIGURES IN THIS ARTICLE
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Copyright © 2018 by ASME
Topics: Stress , Waves , Fatigue , Currents
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References

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Figures

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Fig. 1

Comparison of loads with and without currents

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Fig. 2

Comparison of load range and approximation

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Fig. 3

Comparison of loads with currents (Kc < 5)

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Fig. 4

Mean stress correction for fatigue

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Fig. 5

A Caisson riser assembly with Subsea Tie-ins

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Fig. 6

Cross section for stress evaluation

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