EXECUTIVE SUMMARY 



Vortex-excited oscillations of marine cable systems and structures are caused by the resonant, 

 nonlinear interaction between the flowing water and the structure, which takes place as a result of vor- 

 tex shedding. These oscillations usually are characterized by a resonant wake capture or "lock-on" 

 phenomenon in which the wake flow and the body oscillate in unison at the same frequency. In the 

 case of a cable these relatively high frequency oscillations, which are predominantly in a direction nor- 

 mal to the incident flow, are called strumming. Reduced fatigue life, large hydrodynamic forces (partic- 

 ularly drag) and induced stresses, and high acoustic noise levels often accompany vortex-excited oscilla- 

 tions. The reliability of a cable system's performance depends on the ability to predict this dynamic 

 behavior for conditions that are commonly found in the ocean environment. 



The objective of this report is to present an overview of the state of knowledge concerned with 

 strumming oscillations and to apply these findings to the development of design methods for cable sys- 

 tems that are likely to undergo these oscillations. The report emphasizes recent findings from the 

 marine cable dynamics research program conducted by the Naval Facilities Engineering Command. 

 Although this report emphasizes marine cable systems, results of the NAVFAC/CEL research effort are 

 applicable to other marine structures as well. These applications are discussed in the report as appropri- 

 ate. 



This report is limited in scope to the problems caused by vortex shedding from bluff, flexible 

 structures and cables in steady currents, and the resulting vortex-excited oscillations. Emphasis is 

 placed on an understanding of various aspects of the basic phenomena, the development of design pro- 

 cedures, the definition of hydrodynamic force coefficients applicable in practice, and the specification of 

 structural response parameters relevant to marine cable design. 



