Hydrodynamic coefficients of a free floating barrier in regular waves

Abstract

The Ocean Cleanup plans to deploy a barrier array, to concentrate and capture plastic in the North Pacific gyre by 2020. Both shape and behaviour of the barrier are quite different from the most marine structures.
To get an estimation of the behaviour and loads on the system, the system has been modelled by The Ocean Cleanup in Orcaflex. This software uses the Morison equation.
The hydrodynamic coefficients used in this equation are obtained from experiments with cylinders far from the free surface. Using these coefficients with the Morison equation is a generally accepted method to estimate the loads on submerged cylinders such as piles. However, the barrier is in the free surface and free to move.
This research focuses on the determination of the hydrodynamic coefficients of an unconstrained floating barrier. A model has been set up to evaluate the hydrodynamic coefficients of a floating barrier in regular waves. The barrier was simulated using a numerical model as wave tank. This numerical wave tank has been preliminarily verified with the linear wave theory, in order to optimise the mesh resolution. The numerical model uses fluid-structure interaction to model the flow around a rigid body, the barrier. The hydrodynamic coefficients were in post-processing determined from the response of the numerical model, with the Morison equation, by means of a least squares method.
The barrier with 2 degrees of freedom, sway and heave, has been compared to model tests performed at MARIN with 3 degrees of freedom, sway, heave and roll. The model with 2 degrees of freedom shows reasonable comparison in waves.
These results found in this thesis indicate that the mass coefficient, C_m of the barrier in both the horizontal and vertical plane is close to zero. The drag coefficient, C_d in the vertical plane is close to zero, in the horizontal plane C_d is close to 0.5, both show resemblance to results found in literature.