6. SUMMARY 



6. / Findings and Conclusions. Problems associated with tiie shedding of vortices often have been 

 overloolced or crudely approached on an ad hoc basis in the past in relation to the design of marine 

 structures and cable systems, largely because reliable experimental data and design procedures have not 

 been available. However, the dynamic analysis of marine structures and cable systems has become 

 increasingly important and sophisticated in order to accurately predict stress distributions and opera- 

 tional lifetimes in the ocean environment. The strumming of marine cables has serious consequences 

 because these vibrations are a potential cause of fatigue for system components and they are a cause of 

 increased hydrodynamic drag. Strumming vibrations also introduce acoustic noise in sensor com- 

 ponents attached to the cable and they cause abrasion and wear of fittings and of the cables themselves. 



This report has summarized the present state-of-the-art concerning the strumming vibrations of 

 marine cables. Reliable data now are in hand for the dynamic response of and hydrodynamic forces on 

 model-scale structures and cables, and based upon these findings empirical and semi-empirical predic- 

 tion models have been developed and calibrated for use in practice. Many, if not most, of these 

 findings have come from the marine cable dynamics program of the Naval Facilities Engineering Com- 

 mand. The results of this program are reported in detail in the various sections and appendices of this 

 report and others (1,12). 



Detailed information now is available for the resonant vortex-excited response of model cylindri- 

 cal structures and cables that oscillate at subcritical Reynolds numbers, i.e. Re < 2(10^). There also is 

 reasonably detailed knowledge of the steady drag amplification that accompanies vortex-excited oscilla- 

 tions. This force amplification causes increased static deflections of the structure or cable and practical 

 design methods have been developed to predict this static deflection, as shown in this report. Unsteady 

 hydrodynamic strumming force coefficients have been measured at moderate Reynolds numbers. Re = 

 10^ to 10**, and these coefficients have been employed in the development of the practical design pro- 

 cedures that are described here. Virtually all of the measurements of cylinder and cable dynamic 



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