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Numerical study of nonlinear sloshing and its coupling with vessel motions

Zhang, C; (2016) Numerical study of nonlinear sloshing and its coupling with vessel motions. Doctoral thesis , UCL (University College London). Green open access

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Abstract

The FLNG (floating liquefied natural gas) facility is a new ocean engineering application, proposed to exploit natural gas reserves under the sea bed. It is considered as a giant floating vessel with several huge tanks. This study aims to investigate two hydrodynamic problems related to the FLNG facility, i.e. nonlinear sloshing in liquid tanks and its coupling with vessel motions, within the potential-flow theory. The main body of the research contains three parts. The first part considers the sloshing in wall-sided FLNG tanks. Nonlinear boundary conditions satisfied on the instantaneous free surface are considered. In-house FORTRAN codes based on paralleled BEM are developed for sloshing simulations. The numerical scheme is verified through the simulation of a series of classical sloshing problems. Particularly, a distinct resonance (i.e. second-order resonance) is observed in 3D rectangular tanks, which suggests that the resonant sloshing does not have to be triggered at the natural frequency. In the second part, a design of FLNG vessel with wedged tanks is proposed, aiming to avoid the dangerous simultaneous occurrence of sloshing resonance in all tanks. A BEM-based approach is developed to numerically predict resonance conditions of these wedged tanks. Then, two types of sloshing resonances, i.e. the classical resonance and Faraday waves, in wedged tanks are investigated. In particular, an improved semi-Lagrangian procedure is introduced to accurately track the nonlinear free-surface motion near non-vertical tank walls. In the third part, an analytical solution for the coupling between the vessel motion and liquid sloshing in multiple tanks is firstly derived. A method to determine natural frequencies of the coupling system is also given. The dynamic properties of the system are studied systematically. Then, time-domain simulations of floating vessels with nonlinear sloshing tanks are performed. An extended modal decomposition method is derived to decouple the motions of the floating vessel, external waves and sloshing liquids. The dynamic properties of the FLNG model is also investigated.

Type: Thesis (Doctoral)
Title: Numerical study of nonlinear sloshing and its coupling with vessel motions
Event: UCL (University College London)
Open access status: An open access version is available from UCL Discovery
Language: English
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Mechanical Engineering
URI: https://discovery.ucl.ac.uk/id/eprint/1522358
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