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Long Term Analysis of Semi Submersible Offset

[+] Author Affiliations
Sindre S. Sandbakken

NTNU / BW Offshore, Oslo, Norway

Sverre Haver

NTNU / University of Stavanger, Stavanger, Norway

Kjell Larsen

NTNU / Statoil, Trondheim, Norway

Paper No. OMAE2017-61462, pp. V03AT02A053; 12 pages
  • ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 3A: Structures, Safety and Reliability
  • Trondheim, Norway, June 25–30, 2017
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 978-0-7918-5765-6
  • Copyright © 2017 by ASME


During the design of risers, it is necessary to calculate the 100-year (ULS) top-end motion with reasonable accuracy. The ULS offset is usually taken as the most probable 3-hour maximum response in a storm modeled with 100-year wind and 100-year waves when current forces are neglected. This design approach is assumed to be conservative. In this paper, the level of conservatism is considered by performing a Peak Over Threshold (POT) and All Sea States (ASS) stochastic long term analysis.

The 3-hour maximum offset is assumed to be Gumbel distributed for the POT and ASS analysis. Gumbel parameters are estimated using method of moments for all metocean combinations. Response surfaces are established as a function of wind speed, W, significant wave height, Hs, and spectral peak period, Tp, to represent the short term variability of the response problem. The long term distribution of the most probable maximum response is established for the POT analysis. Similarly, a long term probabilistic model for W, Hs and Tp is established for the ASS analysis. Finally, the long term analyses are performed by combining the short term and long term variabilities using IFORM.

Copyright © 2017 by ASME



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