Numerical and experimental studies of two-body hydrodynamic interaction in waves

Meng, Wei (2020) Numerical and experimental studies of two-body hydrodynamic interaction in waves. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Challenges remain in the prediction of hydrodynamic interactions of multiple floating bodies in close proximity, such as side-by-side offloading and ship replenishment. During such operations, large free-surface elevations in the gap and body motions may occur, impacting operation and crew safety. In this thesis, numerical and experimental studies are presented, focusing on the two-body interactions in waves. Linear potential-flow based seakeeping programs have been widely employed to solve hydrodynamic interaction problems due to their high efficiency. However, these methods over-predict body motions, free surface elevations in the gap, and hence low-frequency loadings on the bodies. To suppress the over-predictions, artificial damping is required as input, which is typically obtained from model tests. With objectives of investigating the effects of viscosity and dynamic gap changes in the two-body interaction problem and developing a systematic approach to estimate the artificial damping for use in potential-flow tools, an immersed-boundary method based finite volume method solver has been implemented in the OpenFOAM framework. The pressure implicit with splitting of operators (PISO) algorithm is applied for velocity-pressure coupling. Free surface is captured using the geometrical volume of fluid method. The relaxation zone method is utilized for wave generation and absorption. To provide high-quality experimental data and to validate the numerical method, model tests on two identical box-like FPSO models arranged side-by-side in head waves at zero forward speed have been conducted in the towing tank of Memorial University. Besides, sources of uncertainties in the model test were identified, and comprehensive uncertainty analysis on the test results was conducted. A combined experimental and numerical approach has been developed to estimate uncertainties due to model geometry, model mass properties, and test set-up. Validation studies on the present flow solver were conducted by firstly simulating the present experiment for two-body interactions in head seas without forward speed. Further, the solver was validated by simulating the underway replenishment of a frigate and a supply vessel at a moderate speed. Simulations were also performed using a panel-free method based potential-flow program in the frequency domain. The numerical results from both methods were compared with each other and with the experimental data to identify sources of the discrepancies in potential-flow predictions. A quasi-steady approach, which accounts for the gap changes due to transverse drift forces at zero speed, was adopted to improve the potential-flow simulations.

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