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course, when this did occur the reaction time of the compass was so slow 

 that the operator intentionally bottomed the vehicle and waited until the compass 

 settled on a particular heading. Vehicle heading was then incrementally 

 adjusted until the correct compass heading was attained. The survey was then 

 resumed along the proper course. 



Compass unreliability around metallic structures is a problem encountered 

 by several operators. The distance from the structure at which the compass 

 begins to be affected varies, in some instances it was unaffected until the 

 vehicle made actual physical contact with the structure, in others the distance 

 was Im (3 ft) or less. The problem is treated differently from operator-to- 

 operator. Some have added directional gyros to the vehicle and rely upon 

 these as primary systems and the magnetic compass as a backup source. Other 

 operators treat the problem as one they can work around and do not think it 

 serious. Obviously, the nature of the work is a controlling factor. If the 

 job requires, for example, inspection of a pipeline, then relatively broad 

 compass inaccuracies are tolerable since the potential for entanglement 

 is minimal. If, on the other hand, the task requires penetration and inspection 

 of a steel jacket structure, where the chance for entanglement is high, then 

 any compass inaccuracies are serious potential defects. 



4.8 POWER SUPPLY SURGES 



ROV power is supplied by either the support ship or from a generator dedicated 

 to the system. There have been several incidents where interruptions or surges 

 in ship supplied power has occurred which caused a delay and, ultimately, 

 termination of the operation. 



As a general practice., most commercial operators provide their own power 

 generator which is dedicated to ROV utilization. Ship's power is considered 

 as a backup or emergency source. This arrangement evolved through problems 

 created by surges in ship's voltage or by complete loss of ship's power. 

 The problems created by voltage surges can be substantial. In one instance 

 power to the ROV was lost for four hours; during which time the vehicle surfaced 

 and was subsequently pounded against the platform resulting in $100,000 worth 

 of repairs. Another power failure ended with the vehicle surfacing under 

 its support ship resulting in damage to it and its umbilical. In other instances 

 components were sensitive to power surges and ran the danger of being destroyed 

 by unregulated voltages. One manufacturer revealed that the lights they were 

 using (supplied by a sub-contractor) were sensitive to changes of -5 volts 

 and could be seriously damaged by such small power changes. As a consequence 

 of such realized and potential experiences the employment of a power generator 

 dedicated to the ROV has become a relatively standard procedure. 



4.9 CURRENTS 



The major problems created by water currents are cable drag, reduced vehicle 

 maneuverability and cable entanglement. While only a few operators have 

 been forced off the job by currents, virtually all are restricted by currents 

 to when, where and at what depth they can operate. Typical current speeds 

 under which ROVs can operate range from 1 to 2k knots (2 to 4km/hr) . One 



