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limited. However, when the vehicle is equipped with an array of instrumentation 

 (e.g., scanning sonar, side scan sonar, sub-bottom profiler, echo-sounder, etc.) 

 which is operating concurrently, the interference problem may become more serious. 

 One operator, who had no interference problems with the vehicle as an observation 

 platform, experienced a variety of interference problems when a side scan 

 sonar and sub-bottom profiler were added to its capabilities. 



4.6 SUPPORT SHIP STATION-KEEPING 



Several ROV operators have had missions abruptly terminated when the support 

 ship could not maintain station above the vehicle. In such circumstances the 

 vehicle has been pulled off-station (intentionally and unintentionally) which 

 required subsequent repositioning with an attendant loss of time to regain the 

 station. The loss of time and the inconvenience can be substantial as the 

 following excerpts from one operator's daily log demonstrates: 





1050 

 1145 

 1220 

 1245 

 1610 

 1720 

 1815 



Vehicle on bottom. 



Vehicle pulled off station by ship. Depth 855m. 



Back on station. Resumed operation. 



Lost holding position. 



Resumed position. 



Lost holding position. 



Start recovery of vehicle. 



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What is merely an inconvenience can turn more serious. In the same operation 



the vehicle fouled in an anchor and the surface ship dragged it and the anchor some 



150m (492ft) untilthe vehicle broke free. Fortunately, the cable did not part 

 before the vehicle freed itself. 



In almost every incident of this nature the cause v^as ascribed to the surface 

 ship's lack of a bow thruster. (The incident from which the above logs were 

 quoted involved a single-screw, sea-going tug.) All open ocean ROV operators 

 agree that twin screws would be best, but such vessels are not readily 

 available. A support ship with a dynamic positioning system is considered 

 as the ideal platform, but the cost is prohibitive. One negative aspect of 

 dynamic positioning was indicated by the possibility of the support ship 

 inadvertently moving or activating a particular thruster at a critical juncture 

 in the operation which could foul the cable and/or the vehicle. 



The above comments apply to "live boat" type operations. Vessels which deploy 

 the ROV when they are moored or anchored do not, obviously, require a bow 

 thruster. 



4.7 COMPASS PERFORMANCE 



The performance of magnetic compasses on ROVs is not always satisfactory. 

 Two problems were revealed by vehicle operators: 1) the compass does not 

 react quickly enough to course changes, and 2) and compass is affected by 

 close proximity to metallic structures. 



In the first instance only one operator was affected by this problem. During 

 survey runs along pre-planned tracks it was occasionally required to change 



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