Analytical and Experimental Study of the Dynamic Response of Cable Systems

Abstract

ABSTRACT Segmented cables of steel and nylon are frequently used in oceanographic applications to support a suspended payload. This report contains an experimental and analytical study of the dynamic response of nylon rope and stranded steel cables with a suspended payload under simulated conditions of ocean wave motion. A generalized distributed mass model was developed for a segmented cable made up of two viscoelastic materials including internal damping and linear external damping of the payload and cable. Viscoelastic behavior was simulated by a two-parameter Voigt model. Linear viscous damping was used for the payload and the cable, but an approximate method was developed for taking into account nonlinear damping effects. The phenomenon of impact loading due to initiation of slack in the cable was modeled on the analog computer for elastic cables, and by a digital computer for segmented cables of two viscoelastic materials. Nonlinear water damping was included in these models. Theoretical calculations of force in the 1/4-in. nylon rope during simulated ocean wave motion agreed reasonably well with experimental results for both air and water tests. Internal damping of the nylon rope was determined experimentally from free oscillation tests. Tests on stranded steel cables indicated snap loads due to cable slack are extremely important. Snap forces as high as nine times the static payload force were encountered. It was shown that these forces can be significantly mitigated by the addition of a short length of nylon rope added to the bottom of the cable. The predicted force response agreed very well with the experimental value. INTRODUCTION Nylon rope and stranded steel cable are frequently used in oceanographic cable systems. Nylon is highly desirable for buoy systems, mooring systems and other oceanographic applications because of its light weight, relatively good strength and ease of handling. However, in cases where fishbite is a problem1, steel stranded cable is used in the upper portion of the cable while nylon is used in the lower portion. Many applications involve the suspension of a payload by a segmented cable of steel and nylon where the upper end is excited by longitudinal oscillation due to ocean wave motion. This problem was studied analytically2 by approximating the wave motion as a sinusoidal displacement function. The cable segments were considered as distributed masses with viscoelastic behavior including internal damping and also external damping due to fluid viscosity. The purpose of this paper is to compare experimental data with theoretical prediction using this model and a second analytical model which approximates viscoelastic behavior of nylon rope by a three-parameter model frequently referred to as a stranded linear solid. Lumped parameter models were also developed to predict the cable force during a snap condition caused by slack in the cable.