determine which configuration is most suitable for use as a tether for a particular mission 

 scenario. The cable scope and required standoff will dictate tiie magnitude of the binding 

 force desired on the payout reel. The available vehicle thrust must then be checked to ensure 

 that it is sufficient to overcome vehicle drag and cable puUout forces. 



The cost of Outer Continental Shelf inspection of pipelines and structures is an 

 important consideration. Since the link is meant to be expendable, it is desirable to pay out 

 a minimum length of tether, lest the mission become too costly. The total amount of deployed 

 fiber-optic link is therefore an important parameter. However, there is a minimum length that 

 must be payed out. This is the straight-line distance to the payload. The ratio of the straight- 

 hne distance to the total amount of deployed cable is called the effective length ratio (ELR). 



It was found that for different vehicle paths and different currents, there was a wide 

 variation in the ELR. In the worst case tested, it was 0.29 and in the best case 1.00. This 

 means that, under some conditions, there could be payed out more than three times as much 

 cable as the straight-line distance, while at other times no such penalty would occur. 



The results of testing three vehicle paths will be given here. In the tests, important 

 cable properties as well as current speed were varied. The weight and diameter were varied to 

 simulate the different types of optical fiber tethers under study, and the current was varied 

 to simulate different environmental conditions. The properties of the different tether cables 

 are listed below. 



OD, inches 



Unarmored Optical 

 Fiber 



0.018 



Ruggedized S-Glass 

 0.027 



Almost Neutrally 

 Buoyant Fiber 



0.027 



Weight per Unit 

 Length in Water, 

 lb/ft 



Present cost 



-5 



3.27 X 10 



$0.25-2.50 



1.75 X I0~4 



$0.75 



.75 



1 .00 X 1 0' 



r7 



(in seawater) 

 Unavailable 



In all the mission trajectories presented in this report, the vehicle ends up approxi- 

 mately 1077 feet from the mother ship. This is accomplished by diving for 10 minutes at 

 0.25 knot and moving horizontally at I knot for 10 minutes. On the first three graphs that 

 follow, the vehicle dives first and then heads into the current. The next three have the 

 vehicle diving and then heading with the current. In the final three graphs, the vehicle first 

 heads into the current and then dives. In these graphs, the current ranged from 0.1 knot to 

 0.5 knot and was varied in steps of 0. 1 knot. 



It was found that very few generalizations could be made about which paths and 

 which cables proved superior. Most of the time, better ELRs were noted for weaker currents 

 (as might be expected), but this was not always the case. Heavier cables tended to be inferior 

 to light cables in weak currents (0. 1 or 0.2 knot), but they would be superior in strong currents 

 (0.4 or 0.5 knot). The reason is that it is harder to push heavier cables out into their catenary- 

 Uke shape. Stronger cables might tend to permit higher deployment tensions, thus inhibiting 

 catenary formation. Neutrally buoyant designs do not appear desirable in either case. One 

 important general result is that substantial benefits can be enjoyed if the vehicle travels with 

 the current, instead of into it (a novel mode in the case of conventional tethered vehicles). 

 For example, in the case of a vehicle deploying an S-glass-type tether heading into a 0.5-knot 



40 



