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Fatigue Safety Factors for Deepwater Risers

[+] Author Affiliations
B. Stahl

Consulting Engineer, Houston, TX

H. Banon

BP Upstream Technology, Houston, TX

Paper No. OMAE2002-28405, pp. 349-355; 7 pages
doi:10.1115/OMAE2002-28405
From:
  • ASME 2002 21st International Conference on Offshore Mechanics and Arctic Engineering
  • 21st International Conference on Offshore Mechanics and Arctic Engineering, Volume 2
  • Oslo, Norway, June 23–28, 2002
  • Conference Sponsors: Ocean, Offshore, and Arctic Engineering Division
  • ISBN: 0-7918-3612-6 | eISBN: 0-7918-3599-5
  • Copyright © 2002 by ASME

abstract

Fatigue life is governed by a number of variables that are highly uncertain. The safety factor on fatigue life is used in a deterministic way to account for the estimated fatigue damage uncertainty. High uncertainties lead to high fatigue safety factors, and vice versa. Evaluation of the uncertainties in the variables governing fatigue design provides a grip on what the safety factor should be. This paper addresses riser fatigue using a fatigue reliability model that is relatively simple but still captures the important elements of the fatigue problem. The bias and uncertainty of stress range are extremely important parameters in design against fatigue. This is due not only to the fact that these parameters are highly uncertain, but also to the fact that they are greatly amplified in the fatigue damage equation by the ‘slope’ m of the S-N curve. The Palmgren-Miner fatigue damage index and the intercept value of the S-N curve are additional important variables in fatigue design. A model for combining wave-induced and vortex-induced vibration (VIV) is introduced together with the best available data and reference to industry work in this technology area. A recently completed joint industry project on riser reliability provides good calibration points for the critical fatigue reliability variables. Reliability and sensitivity studies are performed to demonstrate the effect of the uncertainty parameters. An approach to selecting deterministic fatigue design factors that yield specified reliability targets is developed and illustrated. The study provides a rational approach to selecting safety factors for design of deepwater risers, taking into account both wave and VIV-induced fatigue damage.

Copyright © 2002 by ASME

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