Comparison of a Floating Cylinder with Solid and Water Ballast

Abstract

Modelling and understanding the motion of water filled floating objects is important for a wide range of applications including the behaviour of ships and floating platforms. Previous studies either investigated only small movements or applied a very specific (ship) geometry. The presented experiments are conducted using the simplified geometry of an open topped hollow cylinder ballasted to different displacements. Regular waves are used to excite the floating structure, which exhibits rotation angles of over 20 degrees and a heave motion double that of the wave amplitude. Four different drafts are investigated, each with two different ballast options: with (water) and without (solid) a free surface. The comparison shows a small difference in the body’s three translational motions as well as the rotation around the normal axis to the water surface. Significant differences are observed in the rotation about the wave direction comparable to parametric rolling as seen in ships. The three bigger drafts with free surface switch the dominant global rotation direction from pitch to roll, which can clearly be attributed to the sloshing of the internal water. The presented study provides a new dataset and comparison of varying ballast types on device motions, which may be used for future validation experiments.

Experimental Data of a Floating Cylinder in a Wave Tank: Comparison Solid and Water Ballast

“…Three different solids (two crosses and a cylinder) were used to give a mass distribution in all three axes comparable to the water-filled case. This simplification has only a very small influence on the results as shown by the comparison presented in Gabl et al [13]. Table 1 lists ballast weights as well as the four combinations as tested.…”

“…A hollow cylinder was chosen with an outer diameter of 0.5 m and a total height of 0.5 m. The transparent plastic structure had a wall thickness of 7 mm for the ground plate and 5 mm for the wall. All edges were sharp, and the mooring is described and investigated in Gabl et al [13]. The cylinder incorporates copper tape wave gauges on the wall and floating balls measured by the motion capturing system for additional measurement of the inside free surface [17].…”

“…The provided six-degree-of-freedom motions were measured in relation to this local body coordinate system. Further information is provided by Gabl et al [13].…”

“…The spacing between the WG was initially defined by a reference value re f of 1.27 m (available length in the tank for WG installation divided by 17, which would allow the installation of 16 WG with the same distance). The expected maximum movement of the the floating cylinder in the main wave direction, which is relatively large due to the soft moorings [13], was limiting for the exact location of the WG close to the cylinder. WG 1 was relatively close to the generating wave-makers, therefore the typical working range is in a radius of 7.5 m from the centre of the tank with a diameter of 25 m. With this chosen set-up of wave gauges, the incoming wave including the reflection at the floating cylinder was documented in a cross section.…”

“…Therefore, the floating cylinder is investigated and the water-filled set-up is compared with a solid case of the same mass distribution. The analysis of the study is presented in Gabl et al [13] and the full data is available in the DataShare repository of the University of Edinburgh [14]. The study is designed as a validation experiment to asses the capability of a numerical model for fluid-structure-fluid interaction, which will help to design comparable applications (storing facilities, platforms or wave energy converter).…”