35 



presence of ferrous (magnetic) structures. Many different models are 

 commercially available, but drift rates in excess of several degrees/hour 

 are not uncommon in the less expensive models. Accuracies of -1 degree and 

 less than l°/hour drift rate are available and used on several ROVs. 



Gyrocompasses are listed as onboard equipment on five ROVs, but details as 

 to their characteristics and capabilities are not available. 



2.1.7.r Depth Sensors 



All vehicles but three are equipped with some form of gage or instrument 

 to measure their depth. The most widely used depth measuring device is the 

 pressure transducer, which consists of a pressure sensing element, such as 

 a bellows, a device to convert its tip motion to an electrical parameter 

 and a device to indicate or record pressure changes. 



There are a wide variety of pressure transducers and other gages available 

 and in use on ROVs. This variety is reflected in the following depth 

 accuracies provided on the vehicles listed. 



Vehicle 



CETUS 



DEEP DRONE 



RECON III 



SCORPIO 



SNURRE 



TROV 



ROWS 



Resolution is in the order of 0.1m to 0.3m 



2.1.7.S Magnetic Sensing Device 



Magnetic sensing devices can be used to determine the thickness of overburden 

 over a pipeline or cable, and to track the pipeline or cable under zero visi- 

 bility conditions. Details of the various devices in use are not available 

 since they are designed by the vehicle operator and their construction is 

 considered proprietary. Essentially they consist of two fluxgate magnetometers 

 which are coupled to act as a magnetic gradiometer which provides peak 

 amplitude signals when the vehicle is directly over the buried pipeline. 



A magnetic gradiometer/echo- sounder system has been designed and tested by 

 Intersub Developpement, Paris, which can detect pipelines buried by 3m (10 ft) 

 of overburden with an accuracy better than 10cm (4 in.). The tracking accuracy 

 is such that the vehicle - in this instance, a manned submersible - is provided 

 information allowing it to stay within ±50cm (20 in.) of the pipe axis. 



2.1.8 Umbilical Cable 



The umbilical cable provides the means of carrying propulsive and instrumentation 

 power to the vehicle and telemetering video and instrument data to the surface. 

 Owing to the wide diversity in vehicle instrument capabilities, operating 



