Validation of computer models of cable system dynamics

Comparisons are made between measurements taken during four series of dynamic cable experiments and simulations of the experimental events using two computer models, SEADYN and SNAPLOAD. Three of the experiments were conducted in laboratory water tanks using an elastically stiff nylon cord and a soft silicon rubber cable. The first experiment used cables 6 feet long. Five destinct geometries were measured, including simulation of the anchor-last deployment of a single mooring leg, the relaxation of a subsurface mooring from a displaced condition, and the response of a load suspended along a cable fixed at one end and moved around a circle at the other. The deployment and relaxation simulation were repeated in the second experimental series, using cables 60 feet long. The tension was measured during the third series while a weight suspended from the nylon or rubber cable was paid out or reeled in from a small winch. The winch base could be oscillated vertically to simulate wave action. The fourth experiment was the deployment of a full-size instrumented subsurface mooring in 2,500 feet of water. SEADYN is a general three-dimensional model of the dynamic response of cable networks to environmental changes using a nonlinear finite element technique in the time domain. SNAPLOAD uses the lumped parameter method to model the dynamics of serially connected cables suspended in a vertical plane. Both models are shown to reproduce all the significant motions and forces in the modeled events, but tension is introduced in both models by the inaccurate hydrodynamic drag coefficients that they use. Material damping caused by elastic hysteresis in the cable material is found to play a significant part in smoothing the computation of cable tension, even though it has little overt relation to the overall event.