Chapter 6 - Cable Dynamics For Offshore Application

• Published in: Developments in Offshore Engineering: Wave Phenomena and Offshore Topics pp. 256 - 294
• Main Author: Triantafyllou, M.S.
• Format: Book Chapter
• Language: English
• Published: Gulf Professional Publishing 1998
• ISBN: 9780884153801; 0884153800
• DOI: 10.1016/B978-088415380-1/50025-X

Mooring lines are used to anchor large structures, i.e., prevent them from drifting away from their average position and from performing large-amplitude, low-frequency resonant oscillations. At the same time, the mooring lines are forced to move at the top with the wave-induced motions of the structure, and are subject to flow-induced vibrations.. Cables have complex mechanics of their own, which interact with fluid forces to create several important mechanisms for dynamic tension amplification and generation of damping forces. Recent progress in developing codes for the simulation of cable structures, conceptual developments in the proper modeling of low-tension cables, and the development of experimentally-based models for fluid forces, allows a comprehensive analysis of mooring line dynamics, suitable for design. Because of the superposition of excitation forces of many time scales, the problem becomes tractable for design applications only by separating the equations through an amplitude and time-scale expansion. Three separate equations describe: slowly-varying large amplitude motions, which are very important for studying the resonant motions of a moored structure, wave-induced motions of moderate amplitude, which are important to evaluate the peak dynamic tensions due to the seaway, as well as the drag coefficient amplification caused by nonlinear superposition of slow and wave-induced motions; and very fast, small-amplitude vortex-induced motions, which affect the drag coefficient. All three sets of equations are coupled parametrically; hence analysis is still computationally intensive. Reliable results can be obtained now, which can assist the practicing engineer in choosing optimal configuration.