calculated mode shape for mode number 162 is shown in Fig. 5.2. This mode is excited by current 

 velocities near one knot and thus is likely to occur in practice. The complexity of the response is evi- 

 dent. 



An accurate prediction of the strumming-induced drag amplification depends upon accurate 

 knowledge of the natural frequencies and mode shapes of the cables in their higher modes. When the 

 cable system has large numbers of attached masses, the prediction of the cable modes and frequencies 

 must be done numerically. NATFREQ is ideally suited to this type of analysis. 



5.2 Static Cable Structure Analysis Models. 



5.2.1 The DES4DE Code. DESADE was developed by NRL for computing the current-induced 

 static deflections of cable structures (58). The solution technique is the method of imaginary reactions. 

 This is a powerful method that usually converges rapidly; however, it has been shown to be sensitive to 

 problems where the cable tension is low and/or the current velocity is high. DESADE can accommo- 

 date a complex cable structure with multiple interconnections and a variety of cable materials. An 

 option exists to perform parametric studies to determine the eff"ect of structural changes or various 

 current regimes on the deformation response. Strumming of the cables can be handled by specifying 

 increased drag coefficients obtained from other models. 



A simplified approach to the drag amplification routine in DESADE has been described for appli- 

 cation to mooring system design (77). The code also has been employed recently in a design study 

 (78) of the riser power cable segment that provides the link between a floating OTEC power plant and 

 the bottom-resting cable segment that transmits electric power to shore. The required input to the pro- 

 gram is listed in detail in reference 58, and the code is available to interested users from NRL or CEL. 



The importance of including increased drag due to strumming was made apparent in a comparison 

 between the DESADE model and data from an at-sea cable structure experiment. This was the SEA- 

 CON II structure; it is discussed in reference 57 and in Section 3.4 of this report. Results from the 



comparison with the SEACON II data are shown in Fig. 3.19. 



100 



