Computational Investigation On Air Gap Response Of The Semi-submersible Platform

The air gap response of the semi-submersible platform under random waves is generally of considerable interest. The semi-submersible platform is a floating offshore structure, which will move along with the waves. Its large volume will result in the wave diffraction, radiation effects and the interaction of the floater and the waves. Therefore, the air gap response is the effect of the platform motion under the wave action, the computational investigation of which is on the coupled motion of the wave, platform and the air gap response in the excitation of the incident wave.Firstly, the six freedom responses of the semi-submersible platform are computed by the boundary element method. According to the linear transformation, the wave spectrum under a certain sea-state can be calculated. The platform motion in time domain can be established by applying the frequency-time transformation. Secondly, combining with the related model test data, the platform motion and the air gap time domain response are evaluated. Three theoretical models are set up: (1) the standard linear diffraction model, in which the first- and second-order incident wave and the first-order diffracted wave effects are considered; (2) the theory-test combining model, in which the first-order incident wave and the model test data are considered; (3) narrow-band model, in which the first- and second-order waves are assumed to attain peak values at the same time.The hybrid model on the wave research is established, in which the first diffraction is considered and the second diffraction is neglected. It is more accurate than the conventional method and its computational time is not increased. The narrow band theory is applied, the result of which is in a good unity with the model test data.