Vibration-Based Techniques for Measuring the Elastic Properties of Ropes and the Added Mass of Submerged Objects

Abstract

The magnitude of peak tensions during the deployment of subsurface moorings is dependent on the elastic stretch of the mooring line and the added masses of the mooring buoyancy and instruments. Measured transient tensions, which are twice the mooring anchor weight, highlight the importance of being able to quantify these critical parameters so that these dynamic tensions can be predicted. In this paper, the theory of the mass-spring oscillator is applied in two simple experiments to demonstrate practical techniques for measuring the elastic properties of ropes and the added mass of submerged objects. In the first procedure, a known mass is suspended from a rope, and the tension is monitored when the ?mass-spring? system is set into free longitudinal oscillation. The spring constant is calculated from the measured frequency of this oscillation. Trials with ¼" diameter synthetic and wire ropes demonstrate that satisfactory results are attainable with rope lengths of less than 2 m. Results using different rope lengths vary by less than 1%. The method is shown also to provide a means of measuring the internal friction of ropes using the rate of vibration decay. In a second experiment, the added mass of a sphere is determined by monitoring the line tension during the deployment of a subsurface oceanographic mooring that consists of a spherical float and a mooring line of known spring constant. The frequency of the observed oscillation in the tension record just after anchor impact is used to compute the added mass of the sphere. The value obtained is consistent with determinations made by other methods.