Bag and floater motions of a fabric membrane cage

Abstract

We investigate bag and floater motions of a scaled fabric membrane cage model. Model tests with an assumed model scale of 1:27 were performed at SINTEF Ocean’s towing tank, where the cage model consisted of an enclosing fabric (or a bag), an elastic floater, a mooring system, and an internal water mass. The model was moored in a towing tank and subjected to regular waves with different periods and steepnesses. The first harmonic component of the floater and bag motions time series were analyzed, and image analysis was performed for the latter. Several interesting responses of the fabric membrane cage are discussed: cage set-down, wavelike bag motion, and floater motions. The magnitude of the cage set-down is dependent on the incident wave period and steepness; the magnitude increases in response to a longer and steeper wave. The bag also exhibits a wavelike motion that propagates from the floater to the bottom part of the cage and has apparent implications for the dynamics of the bag bottom and the snap loads on the fabric material. Simplified numerical simulations were performed using WAMIT, which assumed a rigid cage model. A floater motions comparison between model tests and WAMIT results shows the importance of taking the bag and floater flexible modes into account when performing hydrodynamic analysis of a fabric membrane cage.