Numerical simulation of fluid resonance in the narrow gap of twin bodies in close proximity

Nima Moradi

    Research output: ThesisDoctoral Thesis

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    Abstract

    Recent oil & gas explorations have moved to remote deep water locations as soon the conventional reserves would not be able to provide sufficient energy for the global market. In order to meet the growing demand for deepwater operations, the need for new ocean developments such as huge FPSO (floating production, storage and offloading) and floating LNG (liquefied natural gas) terminals would be inevitable. Nowadays, various types of multi-body systems are being used in remote offshore locations where the marine environment could be hostile. Hydrodynamic interaction between multiple bodies in close proximity is one of the features that can affect the safety and performance of such systems. This interaction may result in unfavourable responses or increase the risk of collision in a multi-body system. The fluid resonance phenomena may occur when multi-floating structures with narrow gaps in between are subjected to water waves. This can lead to vertical oscillation of the free surface in the narrow gap with amplitudes substantially larger than the incident wave and increase of wave forces on the structures around the resonant frequencies. The resonant wave frequency, resonant wave height and wave forces are the main aspects of gap resonance phenomena and are of great engineering significance. There are many factors influencing the resonant characteristics of fluid trapped in the narrow gap. Some of these are including but not limited to body draft, gap width and body breadth. The main objective here is to predict reliably the resonance response of water in the narrow gap of two side-by-side bodies and to examine the sensitivity of the results to each of these parameters using computational fluid dynamics (CFD) method. Furthermore, the influence of some new parameters, such as gap inlet configuration, water depth, individual body draft as well as three-dimensionality on resonance characteristics is also examined in a systematic manner.

    The numerical wave flume used in this thesis is modelled in OpenFOAM software, a Finite Volume Method (FVM) based simulation package which can consider the effects of viscosity and free surface nonlinearity as well as fluid separation especially around the sharp corner of the bodies.

    Through comparison of sharp and rounded corner body simulations, substantial differences in the resonant wave heights and frequencies are captured. These discrepancies are further explained by a newly defined roundness factor and an empirical correlation based on the curved corner simulation results, which is obtained via modification of an existing theoretical gap resonance formulation. The influence of water depth, gap length, and three-dimensionality on gap resonant frequency and wave height are then shown for different side-by-side setups. The results demonstrate that in a side-by-side offloading configuration, increasing water depth will result in the reduction of wave height and increase of the resonant frequency. It is also shown that for three-dimensional simulations, secondary peaks corresponding to higher modes of harmonic resonant motion are observed at frequencies higher than the primary piston-mode frequency. Comparing the results obtained in different stages of the present simulations, it can be concluded that the resonant frequency and the resonant wave height are not only governed by some well-known geometrical parameters such as draft, gap width, and breadth, but also depend on the shape of the gap inlet, water depth, ratio of the multi-body drafts and three-dimensionality effects such as gap length and the relative length of side-by-side bodies.

    The governing parameters for gap resonance investigated in this thesis can be applied to similar side-by-side offloading operations and will help reduce the level of uncertainty in predicting their wave resonant response. This may help extend their operational conditions, say into deeper waters and harsher sea-states, or reduce their downtime.

    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • The University of Western Australia
    Supervisors/Advisors
    • Zhou, Tongming, Supervisor
    • Cheng, Liang, Supervisor
    Award date13 Jul 2016
    Publication statusUnpublished - 2015

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