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Experimental Study of Slow-Drift Ship Motions in Shallow Water Random Waves

[+] Author Affiliations
Carl Trygve Stansberg

MARINTEK, Trondheim, Norway

Trygve Kristiansen

Norwegian Institute of Science and Technology, Trondheim, Norway

Paper No. OMAE2011-50221, pp. 815-823; 9 pages
doi:10.1115/OMAE2011-50221
From:
  • ASME 2011 30th International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 3: Materials Technology; Jan Vugts Symposium on Design Methodology of Offshore Structures; Jo Pinkster Symposium on Second Order Wave Drift Forces on Floating Structures; Johan Wichers Symposium on Mooring of Floating Structures in Waves
  • Rotterdam, The Netherlands, June 19–24, 2011
  • ISBN: 978-0-7918-4435-9
  • Copyright © 2011 by ASME

abstract

Slowly varying motions and drift forces of a large moored ship in random waves at 35m water depth are investigated by an experimental wave basin study in scale 1:50. A simple horizontal mooring set-up is used. A second-order wave correction is applied to minimize “parasitic” long waves. The effect on the ship motion from the correction is clearly seen, although less in random wave spectra than in pure bi-chromatic waves. Empirical quadratic transfer functions (QTFs) of the surge drift force are found by use of cross-bi-spectral analysis, in two different spectra have been obtained. The QTF levels increase significantly with lower wave frequencies (except at the diagonal), which is special for finite and shallow water. Furthermore, the QTF levels frequencies at low frequencies increase significantly out from the QTF diagonal. Thus Newman’s approximation should preferrably not be used in these cases. Using the LF waves as a direct excitation in a “linear” ship force analysis gives random records that compare reasonably well with those from the cross-bi-spectral analysis. This confirms the idea that the drift forces in shallow water are closely correlated to the second-order potential, and thereby by the second-order LF waves.

Copyright © 2011 by ASME
Topics: Motion , Waves , Ships , Water

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