Roll Motion of a Water Filled Floating Cylinder—Additional Experimental Verification

Roman Gabl,Thomas Davey,David M Ingram

doi:10.3390/w12082219

Abstract

Understanding the behaviour of water filled bodies is important from an applied engineering perspective when understanding the sea-keeping performance of certain floating platforms and vessels. Even by assuming that the deformation is negligible small in relation to the motion of the structure, these fluid-structure-fluid interactions are challenging to model, both physically and numerically, and there is a notable lack of reference data sets and studies to support the validation of this work. Most of the existing information is highly specific to certain hulls forms, or is limited to small motions. A previous study addressed this by modelling a floating cylinder (giving a more generic case) with roll and pitch motions in excess of 20°. The presented experiment expands on that work to further investigate the previously observed switch between pitch and roll in the cylinder under wave action as induced by the sloshing of the internal water volume. An additional experimental investigation, focused on a single draft, was conducted to test open research questions from the previous study. Here we show that the roll response of the water filled cylinder is repeatable, independent of the tank position and wave amplitude, provided the observation time is long enough to capture the fully developed motion response of the floating object. The mooring system used comprised four soft lines connected on two points on the cylinder. This arrangement resulted in slightly different restoring forces in different wave directions. A relative change of the wave direction by 90° led to a larger wave frequency band in which the roll motion occurred. These cases were, again, also conducted with the solid ballast. Both sets of data provide an interesting validation case for future work on water ballast inside a floating object.

Highlights

Fluid-Structure-Fluid-Interactions influence the behaviour of a number of floating platforms, vessels and devices under the assumption of a negligible small deformation in relation to the large motions.Motivated by the paucity of studies and data relating to generic vessel shapes with large motions, which in turn limited numerical model validation, a previous study by Gabl et al [1] examined a water filled floating cylinder
The observations from the previous experimental programme are used to narrow down the experimental scope in terms of cylinder draft and ballast variables, allowing this study to concentrate to the interesting behaviours observed in pitch/roll response switching and its connection to outlined research questions
The solid ballast option was investigated to provide a direct comparison between the two options for this additional wave directions

Summary

Introduction

Motivated by the paucity of studies and data relating to generic vessel shapes with large motions, which in turn limited numerical model validation, a previous study by Gabl et al [1] examined a water filled floating cylinder. This experimental investigation explored four different drafts and two different ballast options, namely water and solid, under regular waves with a variable wave frequency, with the full dataset available for modelling validation purposes [2]. The observations from the previous experimental programme are used to narrow down the experimental scope in terms of cylinder draft and ballast variables, allowing this study to concentrate to the interesting behaviours observed in pitch/roll response switching and its connection to outlined research questions

Objectives

Results

Discussion

Conclusion