curvature. A typical comparison between the VORTOS prediction and experimental measurements 

 taken with a long, flexible circular beam is plotted in Fig. 5.4. 



The flexible cylinder of diameter D = 25 mm (1 in.) employed in this example was very much 

 like a cable since it had an aspect ratio of LI D = 240 and a low value of structural damping. The 

 important features of the code are described in more detail in reference 80, which also includes a 

 worked example problem with output and a listing of the input data required to exercise the code. 



A computer code, MARISE, for the analysis of marine riser dynamics has been developed by the 

 Shell Development Company. This code recently was modified to accommodate a wake-oscillator type 

 of vortex shedding analysis (40). Predictions have been made of the oscillatory behavior of Cognac 

 platform piles in various configurations during lowering and driving operations (16,40). Fair agreement 

 was obtained between the MARISE predictions and model test results such as those shown in Fig. 2.24. 

 Several additional marine riser dynamics codes that model the resonant vortex/structure interaction are 

 discussed briefly in reference 3. 



5.4 SEADYN. a Dynamic Analysis Model. SEADYN is a nonlinear finite element cable system 

 model being developed by CEL. Both the static and dynamic behavior of cable systems can be simu- 

 lated. A wide variety of situations can be modeled, including: pay-out and reel-in, time varying current 

 fields, point loads and surface excitations. 



The Skop-Griffin strumming model has been incorporated into SEADYN. Strumming calculations 

 are updated in a dynamic simulation whenever the relative velocity of the cable through the water 

 changes by 10%. This is an arbitrary interval and can be changed by the user. To date, the strumming 

 calculation option has been used infrequently because of the disparity between the relatively small 

 number of nodes required for adequate hydrodynamic modeling as compared to the large number 

 required to obtain an adequate description of mode shape for the strumming model. Modeling with a 

 large number of nodes results in a large cost penalty in computing the gross response of the cable. A 



103 



