Procedure for the identification and analysis of vibration characteristics and mode shapes of semi-submersible platforms

Journal article, 2013

The fundamental vibration characteristics and mode shapes of a semi-submersible platform determine its operation conditions and long-term structural response. The current investigation presents a procedure for the identification of global natural frequencies and 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 models 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 details and model complexity. It is shown that both beam and shell element models 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/spectrum of global resonance frequencies compared to 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 vibration in air. The fundamental frequency was 1.9 Hz for the beam element model and 1.5 Hz for the shell element model. When weights corresponding to a submerged structure in operation mode are considered, including the effects of added mass, the fundamental frequency for the first mode using the beam element model was decreased to 0.7 Hz, and to 0.6 Hz when using the shell element model. When compared to the DNV world wave spectrum’s highest frequency of 0.29 Hz it is concluded that the natural frequencies of the semi-submersible are at a sufficient distance from the exciting wave spectrum.