5.2.2 Variations of DESADE. CEL has made modifications to its version of DESADE resulting in 

 the re-named program DECELl. The modifications include: user conveniences; plotting of structure 

 shape and current field; iteration limits to prevent unexpected high execution costs; and three dimen- 

 sional current field specification using data from up to four current meter strings. A new users manual 

 (79) has been prepared that includes experience gained from using the program. 



5.3 Other Computer Codes. DESADE is one of two existing cable structure models that explicitly 

 takes account of strumming-induced hydrodynamic force amplifications of marine cables. However, 

 other codes to predict vortex-excited oscillations are being developed because of the importance of vor- 

 tex shedding-related problems in marine applications. VORTOS is a computer code developed by 

 Atkins Research and Development in the the United Kingdom. The essential features of the code are 

 described in a recently published report (80). This program predicts the dynamic response of a flexible 

 cylinder to vortex-excited oscillations in steady flow. The vibration amplitude and frequency response 

 in a steady flow may be calculated for flexible cylindrical members of a variety of marine structures. 

 The calculation is based upon experimental measurements of the cross flow response and the excitation 

 forces using spring-mounted rigid cylinders and flexible cylinders (5). 



The program is based upon the well-founded assumption that the lift force at each position along 

 the length of a cylinder in steady flow is a sinusoidal function of time and is dependent upon the local 

 incident flow velocity and the displacement amplitude. This point is discussed in Section 2 of this 

 report. The structure is represented by simple finite elements (at this stage up to eleven in number) 

 and the appropriate mass and stiffness matrices. The vortex shedding frequency is determined from the 

 reduced velocity V, for each element and is assumed to lock-on close to the natural frequency of the 

 structure at the critical velocities described in Sections 2, 3 and 4 of this report. More specifically, 

 lock-on is assumed to occur if the Strouhal frequency Wj is between 0.8 w„ and 1.6 w„ (80). The 

 resonant, vortex-excited lift forces as a function of displacement amplitude for each vibrating element 

 are derived from experimental data. An iterative procedure is employed to calculate the steady-state 

 deflected shape of the cylindrical member and the maximum bending stress are determined from the 



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