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Research Papers: Pipeline Systems

Applicability of Equivalent Constant Amplitude Loading for Assessing the Fatigue Life of Pipelines and Risers and the Influence of Compressive Stress Cycles

[+] Author and Article Information
Farid Taheri

e-mail: farid.taheri@dal.ca
Department of Civil and Resource Engineering,
Dalhousie University,
1360 Barrington Street,
Halifax, NS, B3J 1Z1, Canada

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the Journal of Pressure Vessel Technology. Manuscript received August 20, 2012; final manuscript received September 6, 2012; published online March 18, 2013. Assoc. Editor: Saeid Mokhatab.

J. Pressure Vessel Technol 135(2), 021703 (Mar 18, 2013) (10 pages) Paper No: PVT-12-1134; doi: 10.1115/1.4007647 History: Received August 20, 2012; Revised September 06, 2012

Fatigue life assessment of pipelines and risers is a complex process, involving various uncertainties. The selection of an appropriate fatigue model is important for establishing the inspection intervals and maintenance criteria. In offshore structures, the vortex-induced vibration (VIV) could cause severe fatigue damage in risers and pipelines, resulting in leakage or even catastrophic failure. The industry has customarily used simple fatigue models for fatigue life assessment of pipelines and risers (such as the Paris or Walker models); however, these models were developed based on constant amplitude loading scenarios. In contrast, VIV-induced stress-time history has a variable amplitude nature. The use of the simplified approach (which is inherently non conservative), has necessitated the implementation of large safety factors for fatigue design of pipelines and risers. Moreover, most of the experimental investigations conducted to date with the aim of characterizing the fatigue response of pipelines and risers have been done based on incorporation of constant amplitude loading (CAL) scenarios (which is unrealistic), or converting the variable amplitude loading (VAL) scenarios to an equivalent CAL. This study demonstrates that the use of such approaches would not be lead to accurate assessment of the fatigue response of risers subject to VIV-induced VAL. The experimental investigation performed in this study will also clarify the underlying reasons for the use of large safety factors by the industry when assessing the fatigue life of pipelines and risers. In addition, an experimental investigation was also conducted to highlight the influence of the compressive portion of VIV stress-time history on the fatigue life of such components. It is shown that the compressive stress cycles significantly influence the fatigue crack growth response of risers, and their presence should not be ignored.

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References

Figures

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Fig. 1

Cross-flow acceleration-time history of the 8.5 m long model riser subject to 600 N pre-tension force

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Fig. 2

Stress-time history applied to the test specimen

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Fig. 4

Specimen layout and dimensions

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Fig. 9

Comparison of the fatigue crack growth rates under tension-compression and tension-tension VIV-induced stress-time histories

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Fig. 8

Stress-time history applied to the test specimen with the compressive portion omitted

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Fig. 7

Power spectral density of the cross-flow stress-time history

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Fig. 6

Comparison of the experimental and analytical results

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Fig. 5

Crack length versus the number of applied VIV-induced stress blocks

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Fig. 10

Comparison of the fatigue crack growth due to VIV-induced stress-time history and the equivalent CAL

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