Hydrodynamic interaction of wave driven icebergs in close proximity with a fixed offshore structure

Sayeed, Tanvir Mehedi (2017) Hydrodynamic interaction of wave driven icebergs in close proximity with a fixed offshore structure. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Fixed or floating offshore structures and supply vessels in ice prone regions are subject to environmental loading from various forms of glacial ice fragments. Iceberg/bergy bit impact load with offshore structures is an important design consideration but a research gap exists in the study of the viscosity dominated, very near field region, where phenomena such as negative wave drift force (against the direction of propagation of the waves), shadowing, change in added mass, hydrodynamic damping, eccentric impact etc. have been observed in previous studies. In order to better understand and quantify the hydrodynamic effects on small ice masses, a two phase, experimental and numerical, study has been conducted. Physical model experiments were conducted in the Ocean Engineering Research Center (OERC) at Memorial University of Newfoundland (MUN). In the first phase, experiments were conducted to investigate changes in wave loads on ice masses at different separation distances from the structure. The experimental results show that the distance to wavelength ratio dictates the corresponding wave loads in horizontal and vertical directions. The mean drift force in the horizontal direction becomes negative (against the direction of wave propagation) for most cases, when the body is close to the structure. As the body is positioned closer to the structure, the non-dimensional RMS forces in the horizontal direction decrease, and the non-dimensional RMS forces in the vertical direction increase. In the second phase, experiments were conducted to investigate the change in wave induced motions for different sizes of free floating ice masses approaching a fixed structure. The experimental results of motion data show excellent correlation with the force data gathered in the first phase. Similar to previous studies, the separation distance to wavelength ratio is shown to dictate the corresponding wave induced motions. As the body gets close to the structure, the surge motion slows and at the same time the heave motion is increased. Some experiments are also conducted to understand the motion behaviour in irregular waves. The significant wave heights showed a standing wave pattern generated by the superposition of incident and reflected peak frequency wave. Further analysis showed that the significant heave forces and motions will increase and significant surge forces and motions will decrease as the body gets close to the structure. Numerical simulations were conducted using RANS based commercial CFD code Flow3D. Flow3D showed promising results when compared against the force measurements demonstrating the highest and lowest forces at different locations from the structure. For the motion simulations, the velocity changes at node and antinode locations in front of the structure are well captured by the numerical simulations. The challenges lie in the proper modeling of geometry and mass properties of the physical model considering the limitations of computational resources. The simulation results for irregular waves show the capability to simulate random waves and force and motion results in irregular waves are also reasonable showing the expected trends.

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