1.2 Background. It is often found that bluff, or unstreamlined, structures display some form of 

 undesirable oscillatory instability arising from motion relative to a surrounding fluid. A common 

 mechanism for resonant, flow-excited oscillations is the organized and periodic shedding of vortices as 

 the flow separates alternately from opposite sides of a long, bluff body. The flow field exhibits a 

 dominant periodicity and the body is acted upon by time-varying pressure loads. These result in steady 

 and unsteady drag forces in line with the flow and unsteady lift or side forces perpendicular to the flow 

 direction. If the structure is flexible and lightly damped internally as in the case of a cable, then 

 resonant oscillations can be excited normal or parallel to the incident flow direction. For the more 

 common cross flow oscillations, the body and the wake have the same frequency of oscillation which is 

 near one of the characteristic frequencies of the structure. The shedding meanwhile is shifted away 

 from the natural, or Strouhal, frequency at which pairs of vortices would be shed if the structure were 

 restrained from oscillating. This phenomenon is known as "lock-on" or "wake capture." 



The vortex-excited oscillations of marine cables, commonly termed strumming, result in early 

 fatigue, increased hydrodynamic forces and amplified acoustic flow noise; they sometimes lead to struc- 

 tural damage and possibly to failure. Flow-excited oscillations very often are a critical factor in the 

 design of underwater cable arrays, mooring systems, drilling risers, and offshore platforms, since these 

 complex structures usually have bluff cylindrical shapes which are conducive to vortex shedding when 

 they are placed in a flow. An understanding of the basic nature of the fluid-structure interaction which 

 produces vortex-excited oscillations is an important consideration in the reliable design of offshore 

 structures and cable systems. 



Problems associated with the shedding of vortices often have been neglected in the past in rela- 

 tion to the design of offshore platforms and cable structures, largely because reliable experimental data 

 and design methods have not been available. However, the dynamic analysis of ocean structures and 

 cable systems has become increasingly important in the prediction of stress distributions and fatigue life 

 in the offshore environment. These factors are particularly relevant as new and more complex systems 



