A simple model for heave-induced dynamic tension in catenary moorings

An empirical model for the dynamic tension due to vertical motions at the t op of a catenary mooring is presented. The model is applicable for forcing at ocean wave frequencies (as opposed to slow drift frequencies). The model calculates the standard deviation of the tension as the sum of an inertial term proportional to heave acceleration and a drag term proportional to qu adratic heave velocity. Using numerical simulations, the model is shown to capture coupling between inertia and damping effects. The proportionality p arameters of the model are expressed as effective mass and drag constants t imes a linear function of the mean tension at the top of the mooring. Formu lae are derived for calculating these constants in terms of the hydrodynami c and material properties of the mooring. Comparison of model results to me asurements from an instrumented oceanographic mooring and simulations of a lazy wave riser under ranges of conditions yield maximum errors of 8 and 11 % and root mean square errors of 2 and 3%, respectively. The greatest error s occur in situations where there is high mean tension and large dynamic fo rcing and when horizontal motion produces significant tension effects. (C) 2001 Elsevier Science Ltd. All rights reserved.