An Experimental Study of Mooring Line Damping and Snap Load in Shallow Water

Abstract

The aim of this paper is to establish a simple approach to experimentally study the mooring line damping in shallow water, where snap loading may occur more frequently. 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 gages. Moreover, the line geometry, touchdown point speed, and mooring line velocity were derived from image processing techniques. Surge motions at fairlead were driven from a programmable wavemaker. Regular surge motions with different frequencies and pretensions were tested in this system in order to investigate the quasi-static and dynamic behaviors of the mooring chain. In the quasi-static test, the 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 semitaut 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. Two criteria for snap event were examined, and both of them were verified by the experiments.