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 intrinsically nonlinear. In this paper, an algebraic expression for the dynamic tension, formerly obtained [Polar Engineers, ISOPE' 93 (1993)], is revised and enlarged, once the variation of the tension along the suspended length is specifically focused here. The obtained expression is systematically compared with results from usual nonlinear time domain programs and with experiments, showing a fair agreement. This algebraic expression is used then, in two accompanying papers, to address relevant problems from a more practical point of view: in the first one, the question of the dynamical compression of risers, with a proper estimative of the related critical load, is analyzed in conjunction with the results here derived; in the second one, the algebraic expression is used to obtain an analytic approximation for the probability density function of the dynamic tension in random waves. (C) 2001 Elsevier Science Ltd. All rights reserved.