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Contribution of the Mooring System to the Low-Frequency Motions of a Semisubmersible in Combined Wave and Current

[+] Author Affiliations
Amany M. A. Hassan

Newcastle University, Newcastle upon Tyne, UK; Alexandria University, Egypt

Martin J. Downie

Newcastle University, Newcastle upon Tyne, UK

Atilla Incecik

Universities of Glasgow and Strathclyde, Glasgow, UK

R. Baarholm, P. A. Berthelsen, C. Pakozdi, C. T. Stansberg

MARINTEK, Trondheim, Norway

Paper No. OMAE2009-79074, pp. 55-62; 8 pages
doi:10.1115/OMAE2009-79074
From:
  • ASME 2009 28th International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 1: Offshore Technology
  • Honolulu, Hawaii, USA, May 31–June 5, 2009
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 978-0-7918-4341-3 | eISBN: 978-0-7918-3844-0
  • Copyright © 2009 by ASME

abstract

This paper presents the results of an experiment carried out on a semi-submersible model to measure the steady drift force and low frequency surge motions. In the experiments, the influence of mooring systems was also investigated in different combinations of current and sea state. The measurements were carried out with a 1/50 scale model which was moored using horizontal springs and catenary mooring lines. A comparative study of the mean values of steady drift motions and the standard deviation of the low frequency motion amplitudes is presented. In addition, the effect of current on the damping ratio is discussed. It is found that for both horizontal and catenary moorings, the presence of a current increases the damping ratio of the system. For the catenary mooring system, as expected, the presence of mooring lines and their interaction with waves and current increases the damping compared to the damping of the horizontal mooring system. The measured mean values of the surge motions in a wave–current field are compared to the superposed values of those obtained from waves and current separately. For the horizontal mooring, it is found that there is good agreement in moderate sea states, while in higher sea states the measured motion responses are larger. In the wave-current field, the standard deviation of the surge motion amplitudes is found to be less than that obtained in waves alone. This can be explained by the increased magnitude of the damping ratio. Only in the cases of high sea states with the horizontal mooring system, was it found that the standard deviation of the surge motions is slightly larger than those obtained for waves and current separately. This may be explained by the absence of catenary mooring line damping.

Copyright © 2009 by ASME

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