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in Section 4.12 since it also affects the vehicle's station-keeping capabilities. 

 The difficulty was encountered when thrust to the horizontal propulsion is 

 inadvertantly reduced by activation of the vertical propulsion. The result 

 being to make smooth vehicle control difficult, if not impossible. 



.In another instance maneuvering of the vehicle (i.e., course changes) was not 

 possible if the vehicle was required to work broadside to a current of greater 

 than 1 knot. In order to turn, the vehicle thrust to one of the two stern- 

 mounted propulsers had to be reduced. When this reduction took place the vehicle 

 lacked the power required to maintain its course in the current and was set 

 with the current accordingly. A solution to this problem was to be attained 

 by addition of a lateral thruster to assist in holding station under such 

 current conditions. 



4.19 WATER VISIBILITY 



Water visibility or clarity acts as a major factor in the successful and safe 

 deployment of an ROV. Since well over 95 percent of all tasks undertaken to 

 date depend upon remote viewing, the ability to see the object of interest 

 is, obviously, critical. As an example, one operator was forced to abandon a 

 project in the mouth of Cook Inlet, Alaska, because there was no underwater 

 visibility available owing to fine-grained, suspended material in the water 

 column. 



The effects of reduced visibility are varied. In two instances the operators 

 inflicted damage to their umbilicals when the cables fouled on an object on 

 the bottom, the fouling was ascribed to lack of visibility. In another operation 

 the vehicle collided with a wreck and suffered minor damage owing to reduced 

 visibility and a malfunctioning forward looking sonar. To reduce the amount 

 of backscatter in turbid water, one operator has mounted his photographic 

 (strobe) lights on a 2 to 3m (5 to 10 ft) long boom, inadvertently the boom 

 lights collided with a structure outside of the viewing range and were broken. 



The visibility conditions under which industrial ROVs operate depend upon the 

 nature of the work and the accuracy and reliability of the vehicle's positioning 

 system. Several operators have successfully operated in Im (3 ft) visibility, 

 in one instance the minimal requirements were only that visibility allowed 

 looking off about Im to either side of the vehicle to safely maneuver. Conven- 

 tional obstacle avoidance sonars provide assistance in detecting and acquiring 

 or avoiding targets or structures under reduced visibility, but they are of 

 no assistance when the vehicle is 1 or 2m from the structure since the outgoing 

 pulse cannot be differentiated from the return. 



4. 20 TELEVISION 



Section 2. 1.7. a described the types of closed circuit television systems currently 

 employed on ROVs. Overall the performance of these systems is considered 

 satisfactory and in only one instance was an operation cancelled owing to poor 

 TV performance. The following comments summarize the ROV operator's experience 

 and opinions of currently used television systems. 



Reliability - Satisfactory 



Resolution - Adequate for the work now being performed. Some operators 

 felt that greater resolution (800 lines was suggested) would be helpful. 



