Modeling the dynamics of a shallow-water oceanographic surface mooring using full-scale data

Full-scale measurements of the buoy motions and mooring-line tensions of a shallow-water, catenary-type oceanographic surface mooring are presented. The data were analyzed by fitting the tension spectra with a one-dimensional, mass-damper-spring model that takes into account only heave dynamics. The calculated mass coefficient of the model varied linearly with the mean tension at the top of the mooring line. For moderate to high sea states where mooring-line drag was important, the drag coefficient varied linearly with the product of the mean tension and the standard deviation of the heave velocity of the buoy. The spring coefficient had a small effect on dynamic tensions and was taken to be a constant equal to 1.7 times the weight-in-water per length of the bottom mooring chain. An analytical model for predicting the standard deviation of the tension at the top of the mooring line in irregular seas is presented. The analytical model, based on the same mass-damper-spring system used to analyze the experimental data, uses 5 parameters to predict dynamic tensions over a wide range of environmental forcing. Comparison between predictions and experimental measurements gave average absolute errors of less than 2%. The effect of surge and sway buoy motion on the results is discussed.