Investigation On Hydrodynamic Interaction Between Multiple Floating Bodies

With the continuous development of global economy, the demand foroil and gas resources escalates. The exploitation of offshore oil and gasgradually moves to deep water, which contributes to the development ofvarious offshore platforms and corresponding offshore operations. Manynewly emerged facilities and operations consist of multiple floating bodies.Examples include two-barge float-over installation of the platform topsideand FLNG (Floating Liquefied Natural Gas) offloading. Hydrodynamicinteraction between multiple bodies is a very complex problem and greatlyaffects the motion responses and wave forces of the bodies, andunderstanding it is particularly significant for safe operations in practicalengineering.In this thesis, hydrodynamic interaction between two side-by-sideidentical barges in a float-over installation is investigated using varioustheories and research methods, including numerical simulation based onthe conventional potential flow theory and viscous flow theory, model teststudy, and the damping lid method based on fairly perfect fluid. Thehydrodynamic resonance and fluid flow around the two barges are mainlydiscussed.The hydrodynamic performances of the two side-by-side barges witha3-m gap are first investigated through the conventional potential flowtheory. Hydrodynamic coefficients, wave exciting forces, motion responses,and mean drift forces of the barges under different incident wave directionsin the frequency domain are calculated; wave elevation distribution around the barges is also concerned. The calculation shows that in thehydrodynamic results of the two barges, new resonant peaks appear atsome wave periods different from the single-barge case. In beam seacondition, motion responses of the barge on the lee side are smaller thanthose of the barge on the weather side because of the shielding effect. Atthe resonance period, the calculated wave elevations in the gap areextremely large and overestimated through comparison with correspondingexperimental results. This discrepancy can be attributed to the neglect offluid viscosity consideration in the potential flow theory. Despite thedifference in peak values, the comparison also demonstrates the accuracyfor resonance period prediction via potential flow computation.Based on the previous studies, hydrodynamic resonance due tomulti-body interaction is discussed in more detail by considering the fixedbarges and barges with constrained motion in each degree of freedom. Thesignificant discrepancy in the resonance period of the free-floating bargeand fixed barge shows that the resonance is a combined result of both thebarge motion and fluid flow. Sway, heave, and yaw motions are the maininfluential factors. In addition, sensitivity analyses of the gap size, bargedimensions and draft are carried out by considering a variety of numericalmodels, and they come to the conclusion that the resonant period (orwavelength) increases with an increase in the barge dimensions or the gapwidth, and that the increasing trend can be fit by an exponential function.To deeply investigate the underlying principle and mechanism ofmulti-body interaction and to clearly observe fluid flow around the barges,a CFD simulation for the two barges with a3-m gap under the resonancecondition is performed by FLOW-3D software. The vertical motions of thetwo barges are dynamically coupled with fluid flow. At first, athree-dimensional numerical wave basin is established and the generatedwave is validated through comparison with theoretical values. On thisbasis, two barge models in side-by side configuration are added into thenumerical wave basin for CFD simulation; wave and multi-body interaction is modeled and simulated. Comparison between the simulationand experimental results demonstrates that the present CFD simulation iscapable of simulating the wave elevation around the two barges. Throughfluid flow observation in a typical wave period, it is revealed that theexcitation of wave amplification in the gap is mainly induced by the fluidflow under the barge bottoms rather than by a standing wave from thesuperposition of incident and diffracted waves.The overestimated hydrodynamic parameters and wave elevations inthe gap can be suppressed using the damping lid method, which is basedon the fairly perfect fluid assumption. By applying a damping lid to thefree surface between the two barges, the peak values at the resonanceperiod are effectively reduced to be more reasonable.In summary, the hydrodynamic interaction between multiple bodiesand fluid flow around the bodies are investigated in great detail in thisthesis, which is of a certain degree of creativity and engineering practicalsignificance. This study provides valuable reference for further elucidationof the mechanism of multi-body interaction.