T 2 

 F = —2— (18a) 



c 



4EI 



Example of Kinking Prediction 



A 1/2-inch, 3x19, torque-free cable is selected to deploy an instrument 

 package to the seafloor 10,000 feet below the surface. The package weighs 

 5,000 pounds in seawater. Evaluate the possibility of kinking when the 

 lift line becomes slack. 



From Figure 11, the free-end rotation rate with a 5,000-pound dead 

 weight is -40 degrees per 72 inches of cable length, or -6.67 degrees 

 per foot. The total rotation during the free -end deployment to 10,000 feet 

 is the -185 revolutions. From Figure 10, the corresponding torque is about 

 -5 in. -lb at near slack conditions. It will now be determined whether this 

 amount of torque is sufficient to form a kink and, if required, the tension 

 needed to prevent kinking. The value of the cable-bending stiffness is 

 found to be 1,200 lb-in.2 and according to Equation 18a, the critical tension 

 for a -5 in. -lb torque is 0.0078 pound. This small torque is not suffi- 

 cient to cause any wire bending or twisting failures. The lower end of 

 the cable may form large radius loops on the seafloor but it will not form 

 kinks that cannot be pulled out by increasing the tension. No wire failures 

 are expected. 



FINDINGS AND CONCLUSIONS: 



1. The extensional and rotational response of contrahelically wound, 

 double-armored electromechanical cable to external loadings and deformations 

 can be satisfactorily simulated by an analytical model such as the one 

 presented in this report. 



2. The extensional response of multistrand cables can be satisfactorily 

 predicted by the analytical model presented in this report but the rotational 

 response for a nearly non-rotational multistrand cable can not be predicted 

 accurately without precise knowledge of the cable's internal dimensions. 

 Because the amount of rotation for such cables will be small, precise 

 prediction of such rotation may not be required. 



3. A relationship between critical tension and critical torque, as derived 

 from the cylinder buckling criterion and verified by laboratory test data, 

 can be expressed by the following formula: 



F = T 2 / 4EI 

 c c 



This formula shows that the critical tension can be minimized by maximizing 

 the bending stiffness and minimizing the external torque. The magnitude 

 of external torque can be minimized by minimizing the tension-induced 

 rotation. 



13 



