Journal of Ocean Engineering and Science

Volume 1, Issue 1

(8)Nonlinear roll damping of a barge with and without liquid cargo in spherical tanks

Wenhua Zhao ^a,∗, Mike Efthymiou ^a, Finlay McPhail ^b, Sjoerd Wille ^b
^a Faculty of Engineering, Computing and Mathematics, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
^b Shell Global Solutions BV (Shell), Kessler Park 1, 2280 AB, Rijswijk, The Netherlands
Received 1 June 2015; received in revised form 3 July 2015; accepted 19 July 2015
Available online 29 January 2016

Abstract
Damping plays a significant role on the maximum amplitude of a vessel’s roll motion, in particular near the resonant frequency. It is a common practice to predict roll damping using a linear radiation–diffraction code and add that to a linearized viscous damping component, which can be obtained through empirical, semi-empirical equations or free decay tests in calm water. However, it is evident that the viscous roll damping is nonlinear with roll velocity and amplitude. Nonlinear liquid cargo motions inside cargo tanks also contribute to roll damping, which when ignored impedes the accurate prediction of maximum roll motions. In this study, a series of free decay model tests is conducted on a barge-like vessel with two spherical tanks, which allows a better understanding of the nonlinear roll damping components considering the effects of the liquid cargo motion. To examine the effects of the cargo motion on the damping levels, a nonlinear model is adopted to calculate the damping coefficients. The liquid cargo motion is observed to affect both the linear and the quadratic components of the roll damping. The flow memory effect on the roll damping is also studied. The nonlinear damping coefficients of the vessel with liquid cargo motions in spherical tanks are obtained, which are expected to contribute in configurations involving spherical tanks.
© 2016 Shanghai Jiaotong University. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: LNG carrier; Nonlinear roll damping; Liquid cargo motion; Spherical tanks; Model test.