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Operational Modal Analysis of Existing Floating Structure

[+] Author Affiliations
Keisuke Saito, Shigeyuki Naruta, Hiroaki Eto, Osamu Saijo

Nihon University, Chiba, Japan

Kiyotaka Ohki

Terrada Warehouse Company, Tokyo, Japan

Paper No. OMAE2009-79829, pp. 731-738; 8 pages
doi:10.1115/OMAE2009-79829
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

The oceanic architectural buildings were designed in consideration of structural safety based on various structural calculations. However, there is not conclusive proof such as constructed buildings satisfy strength according to the expectation or the buildings built in 1970’s still satisfies specified concrete strength. Therefore, in this study, vibration characteristics of existing floating structure were measured by mode analysis. Generally mode analysis is performed by giving a measurement object a shock with the impulse hammer. But it is hard to be allowed to strike the existing oceanic architectural buildings with a big impulse hammer for a vibration experiment. Therefore, operational modal analysis (OMA) which does not give a shock to measurement object was adopted in this study. In the OMA, natural frequency, mode shape and damping ratio are calculated from the measurement results of ordinary vibration by ground vibration, winds and waves. OMA is used in a field of mechanical engineering. However, it is not easy to apply a vibration experiment by OMA to floating structure. It is because the mode peaks of rigid motions such as heaving and pitching motion in low frequency domain is prominent, and the mode peaks of elastic vibration are difficult to be confirmed. Firstly, we carried out a water tank experiment with floating elastic plate model. Concretely, elastic plate model in floating condition on the water was analyzed by OMA. Because mode peak of the elastic vibration in floating condition was detected by examining frequency response function carefully, it was confirmed that we could apply OMA to the vibration experiment of existing floating structure. Secondly, vibration characteristic of the floating lounge named “WATERLINE” in Tokyo Bay was investigated by OMA. Ordinary vibrations of the base and the building of “WATERLINE” were measured, and natural frequency, mode shape and damping ratio were calculated. And it was reported about vibration characteristic that is given by measurement of existing floating structure by OMA.

Copyright © 2009 by ASME

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