Mooring chains are critical components of off-shore installations. The fatigue assessment of these components often requires complex calculations to determine the loadings in the mooring chains. Traditionally the loadings can be converted into fatigue lives using S-N curves such as the DnV Posmoor curve, or the API RP2 SK curve. Deep water SPARs undergoing vortex-induced-motion (VIM) in loop current conditions may be subject to higher mean/cyclic loadings with considerably lower fatigue life estimates — compared with earlier installations in which fatigue life estimates were so large that fine tuning fatigue prediction methodologies was only of academic interest. In this case a more accurate evaluation of the fatigue performance of mooring chains is needed. In this study the stress concentration factors (SCFs) of a studless 5.25” (133mm) mooring chain were examined in a seven-pocket fairlead. The chain-fairlead system analyzed in this study had a very tight fit (i.e., was not designed for passing connector links), and the results of this study will, in general, not be applicable to other chain-fairlead combinations without additional study. The computed SCFs of the stud-less link interacting with the fairlead pocket were compared to the corresponding SCFs in a chain link away from the fairlead. The study shows that the maximum SCF ratio is 1.15, significantly less than the upper bound 2.5 value recommended by Det Norske Veritas in OS-E301 in lieu of detailed analyses. This has a significant impact, nearly an order of magnitude, on fatigue life prediction of the chain, justifying the analytical effort. The study also found that the SCF of the chain link in the fairlead is a function of the geometry of the chain and the fairlead. As a result some guidance is provided in this paper with respect to the implications of minimum SCFs on other link and fairlead geometries. This study combines computational efforts from NEL and ChevronTexaco in a two-pronged approach where: 1) NEL provided calculations addressing parametric variations of the chain link angles of the mooring line leaving the fairlead and the chain tension levels, and 2) ChevronTexaco validated simplified modelling assumptions done by NEL to make the parametric problem tractable.
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ASME 2004 23rd International Conference on Offshore Mechanics and Arctic Engineering
June 20–25, 2004
Vancouver, British Columbia, Canada
Conference Sponsors:
- Ocean, Offshore, and Arctic Engineering Division
ISBN:
0-7918-3744-0
PROCEEDINGS PAPER
Stress Concentration Factors for Stud-Less Mooring Chain Links in Fairleads
Pedro M. Vargas,
Pedro M. Vargas
ChevronTexaco, Richmond, CA
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Wai Kong Lee
Wai Kong Lee
NEL, East Kilbride, Glasgow, UK
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Pedro M. Vargas
ChevronTexaco, Richmond, CA
Teh-Min Hsu
ChevronTexaco, San Ramon, CA
Wai Kong Lee
NEL, East Kilbride, Glasgow, UK
Paper No:
OMAE2004-51376, pp. 909-917; 9 pages
Published Online:
December 22, 2008
Citation
Vargas, PM, Hsu, T, & Lee, WK. "Stress Concentration Factors for Stud-Less Mooring Chain Links in Fairleads." Proceedings of the ASME 2004 23rd International Conference on Offshore Mechanics and Arctic Engineering. 23rd International Conference on Offshore Mechanics and Arctic Engineering, Volume 2. Vancouver, British Columbia, Canada. June 20–25, 2004. pp. 909-917. ASME. https://doi.org/10.1115/OMAE2004-51376
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