Dynamic tension in risers and mooring lines: an algebraic approximation for harmonic excitation

A riser or mooring line, when excited dynamically at its upper end, resists the imposed displacement by increasing its tension. Viscous damping in the lateral motion is known to be crucial and the resulting problem is thus in trinsically nonlinear. In this paper, an algebraic expression for the dynam ic tension, formerly obtained [Polar Engineers, ISOPE' 93 (1993)], is revis ed and enlarged, once the variation of the tension along the suspended leng th is specifically focused here. The obtained expression is systematically compared with results from usual nonlinear time domain programs and with ex periments, showing a fair agreement. This algebraic expression is used then , in two accompanying papers, to address relevant problems from a more prac tical point of view: in the first one, the question of the dynamical compre ssion of risers, with a proper estimative of the related critical load, is analyzed in conjunction with the results here derived; in the second one, t he algebraic expression is used to obtain an analytic approximation for the probability density function of the dynamic tension in random waves. (C) 2 001 Elsevier Science Ltd. All rights reserved.