Experimental Determination of the Dynamics of a Mooring System

A moored body system consists of an anchor, a mooring line, and the moored body itself. If the mechanics of the moored body and the mooring line are known, the motions of the system in the sea and the associated forces can be determined. Most past work on the mechanics of mooring lines has been oriented toward the development of mathematical models for their mechanics. If only the body motions, the forces in the body, and the connection between body and line are to be found, knowledge of all of the mechanics of the line is not necessary. The only required information about the mooring line is the relations between the forces and motions of the termination of the line attached to the moored body. To solve the problem of body motions and forces, an alternative to a mathematical model for the mooring line is a catalog of relations between line termination forces and motions for various mooring line geometries and current distributions. This paper sets out these ideas and reports the results of an experimental program for determining the relations between mooring line endpoint motions and forces for three different lines having different diameters and weights, but otherwise deployed in identical mooring geometries. The experiments were carried out in the absence of a current so as to have the most straightforward situations possible for the first experiments of this type which have ever been done. It was found that the relationship between mooring line endpoint forces and motions could be well approximated by linear relations, so that the concepts of impedances and admittances could be used.