Full-scale dynamic measurements of a shallow-water oceanographic surface mooring

Full-scale measurements of the buoy motions and mooring-line tensions of a shallow-water, catenary-type oceanographic surface mooring are presented. The data was analyzed by fitting the tension spectra with a one-dimensional, mass-spring-damper model that took into account only heave dynamics. The coefficients of the model were found to be strongly dependent on the geometric shape of the mooring as measured by the mean tension. The mass coefficient increases with tension at a rate slightly less than the ratio of the mass per length of the bottom chain and its weight-in-water per length. The value of the spring coefficient under slack-current conditions was estimated to be about twice the weight-in-water per length of the bottom chain. The value increased by a factor four for the highest recorded mean tensions. The drag coefficient was found to be linearly proportional to the standard deviation of the heave velocity for small sea states. In large sea states, the drag coefficient was proportional to the mean tension but independent of the heave velocity. The values of the drag coefficient were much large than those estimated by summing the profile drag and frictional drag effects of the mooring-line components.