The fundamental vibration characteristics and the mode shapes of a semi-submersible platform determine its operation conditions and long-term structural response. The current investigation presents a procedure for identification of the global natural frequencies and the mode shapes of a semi-submersible platform. The purpose is to evaluate the separation in frequency between the semi-submersible’s global natural frequencies and the exciting wave spectrum. Two types of finite element model are developed and compared: a beam element model and a shell element model. The main differences in the models are the level of resolution in the details and the model complexity. It is shown that both the beam element model and the shell element model can be used for the analysis. However, the beam element model is recommended for a first approximate assessment of the fundamental natural frequency and the interval and spectrum of the global resonance frequencies compared with the wave spectrum. The shell element model is recommended when a more thorough analysis is required. In addition, the natural frequencies of the semi-submersible are calculated for free vibrations in air. The fundamental frequency was 1.9 Hz for the beam element model and 1.5 Hz for the shell element model. When the masses corresponding to a submerged structure in operation mode are considered, including the effects of the added mass, the fundamental frequency for the first mode was decreased to 0.7 Hz when using the beam element model, and to 0.6 Hz when using the shell element model. When compared with the world wave spectrum’s highest frequency of 0.29 Hz reported by Det Norske Veritas, it is concluded that the natural frequencies of the semi-submersible are at a sufficient distance from the exciting wave spectrum.
Procedure for identification and analysis of the vibration characteristics and the mode shapes of semi-submersible platforms
2013-11-01
12 pages
Article (Journal)
Electronic Resource
English
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