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Non-Linear Air Gap Analyses of a Semi-Submersible Compared With Linear Analyses and Model Tests

[+] Author Affiliations
Jørgen Kvaleid, Volkert Oosterlaak

TDA, Oslo, Norway

Tor Kvillum

Agility Group, Sandefjord, Norway

Paper No. OMAE2014-24044, pp. V01BT01A012; 10 pages
  • ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 1B: Offshore Technology
  • San Francisco, California, USA, June 8–13, 2014
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 978-0-7918-4538-7
  • Copyright © 2014 by ASME


For semi-submersible units, the magnitude of air gap or local wave impact in the survival condition is a key design driver. Linear analyses are widely used in the industry to predict survival air gap for semi-subs. Large relative motions, leading to large changes in shape of the submerged hull and large changes in water plane area make this approach questionable.

In this paper, the GG5000 [1], a twin pontoon four legged semi-sub is considered. Both linear analyses and model tests had been performed, but the results were diverging. It was decided to investigate further, using non-linear hydrodynamic analyses.

Initially, the model test setup is reproduced in the numerical model. The simulation model is verified for both response power spectra and extreme response distributions. In the non-linear simulations, the wetted surface of the hull is updated for each time step. Both excitation and restoring forces are based on the instantaneous wetted surface. This proves essential for the prediction of large motions. Later, the verified simulation model is run with realistic full scale setup including elastic catenary moorings with coupled cable dynamics, thruster assist, irregular waves and irregular wind.

Highly non-linear effects proven to be vital to accurate air gap prediction are investigated and their representation in the non-linear analyses is validated against model tests.

Copyright © 2014 by ASME



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