Drag Coefficients For Mooring Line Hydrodynamic Damping

For catenary moored systems low frequency excitation caused by random waves and wind loading results in resonant motion responses in the horizontal plane leading to high mooring line forces. Many of the loading mechanisms associated with this are well understood. However until relatively recently the fluid induced forces acting on the moorings were assumed to have little influence on the vessel dynamic response. It is now known that the catenary mooring system may under certain circumstances provide up to 80% of the total damping which may significantly reduce the peak line tensions. Most of the previous experimental work performed to measure line damping has been associated with the relatively small geometric scale at which the model tests were performed (typically 1/100th). This and the fact that it is difficult to apply the results to non-specific field scenarios has led to severe difficulties in system design. This paper presents results at significantly larger scale model testing work at University College London as part of a JIP study. This provides the added mass and drag contributions of sections of mooring line. The tests were carded out at large scale on sections of line to ensure that the Reynolds number flow regime was as close as possible to that encountered at full scale. A chain line segment was tested at various amplitudes and frequencies to represent a section of single catenary mooring line. Because of the complex nature of the flow around oscillating lines it is essential to consider interaction between the low mooring frequency and wave frequency flows as it is known that simply superposing the two effects gives erroneous results. To this end a computer controlled biplanar carriage was designed and installed in the wave tank at UCL to simulate these motions with the line specimens.