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Reliability Assessment for TLP Tether Overload in Nonlinear Waves

[+] Author Affiliations
O̸istein Hagen

Det Norske Veritas, Ho̸vik, Norway

Paper No. OMAE2011-50112, pp. 771-782; 12 pages
doi:10.1115/OMAE2011-50112
From:
  • ASME 2011 30th International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 2: Structures, Safety and Reliability
  • Rotterdam, The Netherlands, June 19–24, 2011
  • ISBN: 978-0-7918-4434-2
  • Copyright © 2011 by ASME

abstract

Non-linear effects caused by extreme crest loading might be governing for important structural limit states. For TLP’s, non-linear effects caused by extreme crest might lead to tether overload and tether slack. Simple scaling of ULS results is not necessarily adequate if wave in deck events (or other NL phenomena) might occur with annual probability >10−4 . In this case ALS analysis should be performed, and detailed knowledge of the physics and statistics of extreme crest events, and of the loading caused by these waves, is required. Ignoring the possibility for wave in deck events and establishing the characteristic tension from data where there are no wave in deck events, might lead to unrealistic low estimates for the extreme tether tension. The present study demonstrates effects of the CresT JIP (Cooperative Research on Extreme Seas and their impacT) findings regarding extreme and rogue waves through a simplified reliability analysis of a generic TLP defined by the project. In this paper, SRA for tether overload is discussed, with emphasize on the effect of tension Twid caused by wave in deck events. Data from TLP basin test undertaken as part of the CresT JIP are applied, and parametric models for tether tension from measured crests and measured tether tension data are established. The model for Twid is used in the SRA for analysis of tether overloads for design sea states and for long term analysis for different geographic locations. Special attention is given to assessment of the effect of higher order crest non-linearities beyond 2nd order. The crest models are based on results from other work packages of the Crest JIP. Recommendations for future research needs are given.

Copyright © 2011 by ASME

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