The cables comprising the delta module of the SEACON array had uniform currents incident over 

 their lengths and were found to be subject to cable strumming. As shown in Section 4.3, the strum- 

 ming vibrations lead to increases in the effective steady drag coefficients. Since the steady drag 

 coefficient is a basic parameter in all array motion computations, an accurate knowledge of its value is . 

 required in order to validate the various models for the analysis of cable structures. 



The calculated steady drag coefficient was frequently 150 to 230% greater than the value of the 

 nominal stationary-cable drag coefficient C^q because of strumming of the SEACON II array. Large 

 increases in the resulting drag loads would be expected to have a significant effect on the magnitude of 

 the predicted array motions. In Fig. 5.3 the measured motions of a point near the intersection of two- 

 cables on the horizontal delta of the SEACON II array are compared to the predicted motions during a- 

 semi-diurnal tidal cycle. The calculated motions were obtained using the computer code DESADE. 

 The magnitude and direction (predominantly from the southeast) of the current during the cycle can be 

 inferred from the movement of the measurement point from its zero current location designated by 

 "X" The North direction is shown on the plot as a reference. 



The measured motion of the point is shown in Fig. 5.3 by the solid line, and the predicted motion 

 using a constant stationary-cable drag coefficient Coo = 1-55 (measured in water (57) on a sample of 

 the SEACON II cable) is given by the dashed line with circles (—0—). Finally, the predicted motion 

 using Cd = (Cd/C[)o)Cdo^ with Cd/Cdo calculated from equation (4.3.3) and Coo = 1-55, is shown 



by the dashed line ( ). Excellent agreement was obtained between the predicted and measured 



motions when the strumming-amplified steady drag coefficients were employed. It was also apparent, 

 for the larger current magnitudes, that the displacement amplitudes predicted using the nominal 

 stationary-cable drag coefficient (Cdo = 1-55) were considerably smaller than the measured displace- 

 ments. As noted in Section 3.4, the delta cables of the SEACON II array did not undergo a pure mode, 

 resonant lock-on response to the strumming forces. However, the strumming-amplified drag 

 coefficients measured during well-controlled laboratory experiments that were characterized by resonant 

 lock-on have been validated for applications in practice by the SEACON 11 computations and measured 

 array motions. 



