Vibration Analysis of a Floating Roof Subjected to Radial Second Mode of Sloshing

In a previous paper, a cost-efficient modal analysis method for the vibration of a floating roof coupled with nonlinear sloshing in a circular cylindrical oil storage tank is presented. This method is extended to the case in which the out-of-plane deformation of the roof-deck caused by the radial second mode of sloshing induces an elliptical deformation of the pontoon around the deck. First, the radial contraction of the deck is calculated from the slope of the out-of-plane deformation of the deck, and the following two points are confirmed: (i) the circumferential variation in this radial contraction results in the elliptical deformation of the pontoon, and (ii) the present theoretical prediction for the radial contraction is in good agreement with a numerical result obtained by LS-DYNA. Based on these points, the stresses arising in the pontoon are calculated by considering the contraction of the deck as an enforced displacement of the pontoon. Numerical results show that (a) the elliptical deformation of the pontoon causes a large circumferential in-plane stress, (b) reduction achieved by the increase in the thickness of the deck is larger for the radial contraction of the deck and the stresses in the pontoon than for the out-of-plane deformation of the deck, and (c) the radial contraction of the deck for a fixed value of the out-of-plane deformation of the deck increases with the decrease in the radius of the deck.