A Dynamic Analysis Moored and Free-Floating Cable Systems

Abstract

This paper entails a study and modeling of the dynamic behavior of cable-mass systems suspended in the ocean with a current profile varying linearly with depth. Multi segment, Multipayload (a) continuous, (b) lumped parameter models representing the cable system subject to surface loading and depth varying current drag loading are discussed. The continuous cable element model employs an equivalent linearization scheme to model payload drag loading and is one dimensional. The lumped parameter model is quasi-two dimensional and includes the nonlinear drag and viscoelastic effects. Nonlinear material characteristics can also be included with the latter model. Model predictions compare very well with measured data obtained at the NAVAIR R&D test facility operated by TRACOR/MAS at St. Croix, V.I. Comparisons of the tension as a function of time and frequency and snap loadings were done between the lumped parameter model results and experimental data. A comparison of the system tension response as a function of frequency was made with the continuous model results and experimental data. Both models can be used synergistically to determine optimal cable system designs the continuous model being used to predict the nature of the frequency response for a given cable system configuration. Whereas, the lumped parameter model is used to investigate in detail the snap loading, system displacement, and interesting "resonance" phenomena. Model optimization and restriction considerations led to the conclusion that the models developed were applicable to moored and free-floating cable systems with long cables. Towed cable systems are not within the scope of the capabilities of the models. INTRODUCTION Cable systems find wide application in buoy and ship mooring systems and underwater acoustic sensor systems. These cable systems must reliably satisfy Naval/Offshore requirements in a corrosive seawater environment subject to the vagaries of deployment and retrieval operations, fish bite, and dynamic surface excitation. This paper is directed toward the analysis of the dynamic displacement and tension in multicable, multipayload cable systems (moored or free-floating) subject to wave surface excitation and depth varying current drag loading. Measured peak/snap tensile loads in such systems can exceed six to eight times the predicted static loads at or near the resonance frequencies [1,2,3] The development of validated cable system models is motivated primarily by cost effectiveness considerations. A validated model can be used toInvestigate design tradeoffs;Evaluate system performance under a variety of applications and conditions;Alleviate many testing requirements;Provide immediate results;Identify critical components to provide testing guidelines; andDerive optimal cable system configurations having desired frequency response behavior. Though sometimes costly, exercising such computer models is less expensive and more convenient than building and testing a multiplicity of cable, payload systems.