An Experimental Study of Mooring Line Damping in Shallow Water

The aim of this paper is to establish a simple approach to experimentally study the mooring line damping in shallow water. Experimental measurements were conducted in a wave basin at a scale of 1:50, which corresponds to a full scale of 28 m water depth. A chain made by stainless steel was used and the tension force at the fairlead was measured by tension gauges. Moreover, the mooring line geometry and velocity were computed from image processing technique. Series of horizontal motions at fairlead were driven from a programmable wavemaker. Regular surges with different frequency and pretensions were tested in this system to investigate the quasi-static and dynamic behaviors of the mooring chain. In the quasi-static test, mooring line keeps a typical catenary shape and its indicator diagram exhibits a smooth-closed curve. In the dynamic test, the mooring line is fully lifted from the seabed and it cyclically goes through the stage of semi-taut and fully taut. We successfully reproduced a snap event in the laboratory scale and the resulting mooring line damping can considerably increase in this manner. The mechanism of snap load is investigated by the measured tension, surge and mooring line velocities.