5.0 NUMERICAL MODELS FOR STRUMMING ANALYSIS AND ASSOCIATED 

 STRUCTURAL MODELING 



As the state of the ocean engineering art steadily progresses, more and more stringent demands 

 are being placed upon the performance of cable structures. In particular, displacement tolerances and 

 constraints in response to currents are ever tightening; fatigue is becoming an important design con- 

 sideration; and the sensitivity of acoustic sensors has become such that they cannot differentiate 

 between legitimate acoustic targets and slight variations in their vertical position. All of these are prob- 

 lems that are aggravated by strumming. 



In order for an engineer to be able to design a structure to meet the constraints imposed by his 

 own and other disciplines, he must be able to assess the effect of strumming on the structure in ques- 

 tion. Numerical techniques to predict strumming have been developed using the models described in 

 this report as well as other models which account for the effect of strumming on cable structures. For 

 the most part, the strumming and structural analysis models are separate; however, a few codes have 

 integrated the two types of analyses. The earliest codes that accounted for strumming were static 

 models that allowed the user to specify drag coefficients; other codes performed the strumming analysis 

 and supplied the drag coefficients. Recently, the capability to do strumming calculations continuously 

 has been incorporated into a dynamic model. This allows strumming effects to be modeled and updated 

 virtually continuously as a cable system changes geometry. 



5.7 NATFREQ, a Strumming Prediction Model. NATFREQ is being developed by CEL for calcu- 

 lating natural frequencies, mode shapes, and drag amplification factors for taut cables with attached 

 masses. Drag amplification factors calculated by NATFREQ using the Skop-Griffin strumming model 

 are used as inputs to the DESADE and DECELl structural analysis models. The solution technique is 

 based on a new, efficient iterative algorithm (76). The computed results have been compared to simple 

 laboratory experiments with good agreement. Mode shapes generated by the algorithm for modes 5 and 

 7 of the experimental cable with six unequally spaced attached masses are given in Fig. 5.1. One of the 



cases analyzed using the algorithm was a 4700 m (15,400 ft) cable with 380 attached bodies. The 



99 



